CONCRETE hno o lo g y PAVING Te c hn Co n c r e t e In t e r s e c t io n s A Gu i d e f o r D e s i g n a n d C o n s t r u c t i o n Introduction Traffic caus ca uses es d am ag e to p avem ave m en t o f atat-g rad e street ee t and an d ro ad inter ntersec sect tio ns p erha erhap p s m o re tha than n any an y ot o ther he r lo cat ca tio n . H eavy ea vy veh ve h icle sto p p ing an d tu rn ing can ca n stress th e p avem ave m en t sur su rface ac e sever seve rely al a lo ng the ap p ro ache ac hes s to an a n in tersec ersect tio n . T he p avem ave m en t w ithin th e ju n ctio n (p hysi ysicalarea) o f an in tersec se ctio n also m ay receive n early tw ice th the tr traffic as a s th e p avem ve m en t o n th e appr ap proac oachi hing road w ays. A t b usy in terse ersect cti io ns, the ad d ed lo ad an d stress es s fro m heavy he avy veh ve h icles o ften caus ca use e asp a sp hal ha lt p avem ave m en ts to d eterio rate p rem atu rely. A sp h alt sur su rfaces ten d to to rut and an d sho ve u nd er the strain of b usses usse s and a nd trucks uc ks sto p p ing and an d tur turni ning . T hese he se d efo rm ed surf surfaces ace s b eco ec o m e a safety ety co ncer nc ern n fo r d rivers vers and a nd a co c o stly m aintena ntenance nce p rob lem for the roa road d w ay agenc ag ency y. C o ncr nc rete p avem ent en ts b etter w ithstan hstand d the lo ad ing and an d turni urnin g m o vem ent en ts of o f h eavy ea vy vehi veh icles. A s a resu resu lt, city,co un ty and an d state ate roa road d w ay age a genc nci ies h ave b egun eg un reb u ild in g d eterio rated asp h alt in tersec se ctio ns w ith co c on cret crete e pavem p avem ent en t. T hese he se ag enci en cies are are exten extend d ing road oa d and an d street ee t system m ain tenan en ance ce fu nd s b y elim inat na ting th e exp e xp ense o f in tersec se ctio n s th at req u ire freq u en t m aintenance. enan ce. A t-gra -grad e in tersec se ctio ns alo n g b u sin ess, ss, in d ustr strialan d arterialco rrid o r ro u tes ar a re so m e o f th e b usiest an d m ost vitalp avem ent en ts in an a n u rb an ro ad net ne tw ork. ork. C lo sing these he se ro ro ad s an d inter ntersec sect tio ns fo r p avem ave m ent en t rep air creates co c o stly traffic d elays an d d isrup srup tio n to lo cal ca lb usi us in esses. ess es. C o ncr nc rete ete pave p avem m ent en ts pro pro vid e a lo ng ser se rvice life fo r th ese m ajo r co rrid o rs and in tersec se ctio ns.
C o ncr nc rete ete pave p avem m ent en ts also o ffer o ther he r ad vant van tag es fo r
A s a ru le, it is im p o rtant to eval va lu ate th e existi stin g p aveve -
in tersec se ctio ns:
m en t co n d itio n b efo re ch o o sing lim its fo r th e new co ncr nc rete ete pavem p avem ent en t. O n b usy ro ro utes, utes, it m ay be b e desi d esir-
1. Low long -term m aintenance co sts. 2. N o sof so ften in g o r d eterio ratio n ca c au sed se d b y oi o ild rip p in g s. 3. G o o d lig ht reflect ec tivity th that enh en h anc an ces p ed est es trian and an d veh veh icle saf sa fety at a t n ig h t and an d in in clem ent en t w eat ea th er. 4 . A d u rab le an d ski sk id res resi istan t sur su rface. ac e.
ab le to to ext ex ten d th e lim its fo r th e n ew co ncrete p ave m ent b eyon yo nd th e in tersec se ctio n ’ s fu n ctio n alb o u n d aries. Traf Traffic co n g est es tio n at a b usy in tersect ersec tio n m ay ext ex ten d th e d istan ce w here ere veh ve h icles start art an d sto p , w h ich m ay ext e xtend en d the leng en g th o f d istressed esse d p avem ent en t. Th e
len g th tha t p avem ave m en t d istress exten extend d s b eyo nd the S u ccessf ssfulco nstr stru ctio n o f co n crete in tersec se ctio ns is
in te rsec se c tio n ’ s fu n c tio n alb o u n d aries w illd ep en d u p o n
ch allen g in g , esp es p eci ec ially in ur u rb an area areas, s, w here acco ac co m -
the nu m b er, er, spee sp eed d , an d type yp e o f vehi veh icles tha t use us e the in tersec se ctin g ro ro a d w ays. ys. A sim ilar exte xten sio n of o f d is-
m o d ati ating traffic an a nd ad jacen ac en t b usiness es s nee need d s of o ften m ust us t sup su p ersed ersed e o ther he r eng en g in eer ee ring o r co n struct uc tio n fact ac to rs. H o w ever ever, , m o d ern ern tech ec h n o lo g y, in clu d ing fast as t-
tress is p o ssib le w he re tr truc ks cau c ause se d am ag e w hile acce ac cel lerat erati ing slo w ly up a stee steep p g rad e aw a w ay fro m an
track co c o n str stru ctio n, sim p lifies th ese ch allen g es.
in te rsectio n .
Design Considerations
If sig n ifican t ch an g es to to an in tersec se ctio n are req u ired , it
W h en b u ild ing o r reb u ild in g an in tersec se ctio n, the n ew co n cret crete e p avem avem en t sho sh o u ld co ver at least ea st th e en e n tire fu n ctio n alarea o f th e in tersec se ctio n. T h e fu n ctio n alarea in clu d es the lo n g itu d in allim its o f any auxiliary lan es (F ig u re 1 ).(1 ,2 ) N o rm ally, the d istress cau ca u sed b y hea h eavy vy veh ve h icles b raking and tu rn ing w illo ccur w ith in an in tersec se ctio n ’ s fu n ctio n alb o u n d aries.
aries of o fthe in tersectio n ’ s ne n ew fu n ctio n alarea. T raffic
is id eal ea lto ext e xtend en d the new ne w p avem ent en t to the b o un d p att attern ern s ch c han g e w ith m o d ificat ca tio n s to to an in terse ersecctio n ’ s th ro u g h-lan es, au xiliary lan es, an d acceleratio n and d eceleratio n ta tap ers. T h erefo re, th e lo catio n w her he re vehicles cause cau se dam da m age ag e also m ay chan ge from th e lo catio n in the existing in tersec se ctio n co n fig u ratio n . A s a stand ard, som e ag encies extend the n ew con cret crete p avem ent, ent, from 30 -60 m (10 0-200 ft) on each eac h leg o f the in tersectio n fo r alltraffic lan es. O th ers extend en d the the n ew p avem ent en t ap p ro achi ac hing the inter ntersecsectio n fart arther than the n ew p avem avem en t leavi ea ving the in tersect se cti io n. In th these es e cas c ases es, , th e co c o ncret crete e lan es ap p ro ach ing the inter ntersec sect tio n m ay b eg in 6 0-12 0 m (2 0 0 -40 -40 0 ft)fro m the in tersec se ctio n ’ s p h ysi ysicalarea, w hile lan es leaving th e in tersec se ctio n term in ate ab o ut 1 5 m (5 0 ft) b eyo nd th e ph p h ysi ysicalarea. F o r in tersec se ctio n s car ca rrying m o d erat erate e traffic vol vo lum es an d a lo w p erce ercent ntag e o f h eavy ea vy vehi veh icles, 1 5 -3 0 m (5 0 -1 0 0 ft) o f n ew p avem ve m ent is usu u su ally su fficien t to rep lace the d istr stresse ss ed pavem ent.
C oncrete Slab Slab Thickne Thickne ss — B ecau se an in tersec se ctio n ’ s p h ysi ysicalarea carries traffic fro m b o th ro ad w ays, the co n cret crete e slab thickne ck ness ss in th e p h ysical ca larea area o f th e in terse ersect cti io n m ay n eed ee d to b e g reat ea ter than th e th th ickn ck ness es s on o n either app ap p ro ach ac h in g ro ad w ay. T he need ne ed fo r extr extra thi thickne ck ness ss w illd ep end en d u p o n the ro ad w ay o r street ee t classi as sificat ca tio n an d th e averFigure gure 1.
2
Phys Physiical and funct functiional onal area areas s of an inte inters rsect ectiion.
ag e d aily tru ck traffic (A D T T )th at each ro ute carries.
R eference 3 defines six roadw ay (street) classifications. These classifications depend upon traffic vo lum e, vehicle type(s) and m axim um axle load ing . Table 1 describes these classifications. D esigners should co nsider increasing the slab thick ness for at-grad e intersections of industrialand arterial road w ays. The physica larea w illlikely req uire 12-25 m m (0.5-1.0 in.)of additionalthickn ess (see Table 2 next page). W hen traffic w arrants extra concrete thickn ess in the intersection’ s p hysicalarea, it is g enerally easier to change the thickn ess at a location before the rad iifor the intersection. The slabs n ear the intersection’ s rad ii are built using fixed form s and separate han d-pours in m ost cases. A tran sition len gth o f ab out 1-2 m (3-6 ft) for ch an ging the thickness is usually ad eq uate. The decision on precisely w here to change thickness should be left to the contractor. R equiring the transi-
tion to be at a sp ecific location m ay co m plicate construction and conflict w ith other job site factors, such as providing access to ad jacent businesses. A t-grade intersections o f light residential, residential, co llector, an d business road w ays should not req uire an y extra concrete thickness in the p hysicalarea. The intersection thickness should be the sam e as the thicker ofthe tw o ap proaching road w ays.
J ointing — Jo int design is arguab ly the m ost im portan t design aspect for co ncrete pavem en t intersections. A t-grad e intersections o ften introduce jointing challenges that do not exist along tan gen t sections of co ncrete road w ay- or street-pavem ents. H ow ever, these com plica tions can b e overco m e b y ap plying sim ple jointing fun dam entals.
(3) Table 1. Street Classifications .
Street Class
Description
Two-way Average Daily Traffic (ADT)
Light R esidential
S hort streets in subdivisions and sim ilar residentialareas often not through-streets.
Less than 200
R esidential
C ollector
B usiness
Industrial
A rterial
Two-way Average Daily Truck Traffic (ADTT) 2-4
Typical Range of Slab Thickness
100-125 m m (4.0-5.0 in.)
Through-streets in subdivisions and sim ilar residentialareas that occasionally carry a heavy vehicle (truck or bus).
200-1,000
S treets that collect traffic from severalresidentialsu bdivisions, and that m ay serve buses and trucks.
1,000-8,000
10-50
125-175 m m (5.0-7.0 in.)
50-500
135-225 m m (5.5-9.0 in.)
S treets that provide access to shopping and urban central business districts.
11,000-17,000
400-700
150-225 m m
S treets that provide access to industrialareas o r parks, and typically carry heavier trucks than the b usiness class.
2,000-4,000
S treets that serve traffic from m ajor expressw ays and carry traffic through m etropolitan areas. Truck and bus routes are prim arily on these roads.
4,000-15,000 (m inor)
300-600
150-225 m m (6.0-9.0 in.)
4,000-30,000 (m ajor)
700-1,500
175-275 m m (7.0-11.0 in.)
(6.0-9.0 in.) 300-800
175-260 m m (7.0-10.5 in.)
3
Table 2.
Slab thickness considerations for the physical area of at-grade intersections. Note that in this table, thickness T3 is greater than T2, and T2 is greater than T1.
Intersecting Roadway 1
Intersecting Roadway 2
Physical Area Thickness*
Low A D TT (T1)
Low A D TT (T2)
T2
Low A D TT (T1)
H igh A D TT (T3)
T3
H igh A D TT (T3)
H igh A D TT (T3)
T3 + 25-50 m m **
*
Assumes thickness (T1, T2 or T3) for intersecting roadw ays ba sed on a nticipated traffic and ca lculated in a rationa l design procedure s uch a s that o f AASHTO (4) o r P C A(3,5).
**
The AASHTO thickness d esign proce dure(4) show s that do ubling the traffic loading req uires abo ut an a dd itiona l 25 mm (1 in.) of co ncrete pa vement thickness . The P CA de sign proce dure(3,5) s h o w s t ha t an extra 12 mm (0.5 in.) of slab thickness is required when doubling traffic.
Jo ints are n ecessary prim arily to controlthe location of cracks that occur from naturalactions on co ncrete pavem ent. W hen design ed correctly,joints accom m odate the exp ansion and co ntraction o f co ncrete slab s caused by tem perature fluctuations, and acco un t for stresses that develop from slab cu rling an d w arping. (6) D uring co nstruction, joints also divide the p avem ent into suitab le placem ent increm en ts o r elem en ts for the co ntractor. C ertain joints also acco m m odate slab m ovem ent ag ainst fixed structures.
an d m ultilan e street pavem en ts are typically about 3.0-4.2 m (10-13 ft) ap art, an d serve the d ual purpose of crack controland lane d elineation. Equation 1 determ ines the m axim um allow ab le joint spacing based on slab thickness and sub base type. S lab s kept to dim ensions shorter than the eq uation determ ines w illhave cu rling an d w arping stresses w ithin safe lim its to ensure m inim al risk of random cracking:
ML= T xC For at-grad e intersections, a designer sho uld co nsider three m ajor joint design elem ents: joint sp acing, joint typ e, and joint layout. E ach factor can influence the long-term perform an ce o f the pavem ent. In addition, other factors to consider include: dow elbars for load transfer, tiebars for tying lanes, an d sealing joints*.
J oint Spacing — Fo r unreinforced co ncrete p avem ent, joint spacing or slab len gth dep en ds upon slab thickness, co ncrete aggreg ate, subbase, an d clim ate.(6,7) In m ost areas, the typicalm axim um tran sverse joint sp acing for unreinforced (plain) pavem ent is about 4.5 m (15 ft). Longitudinaljoints on tw o-lane * If loca l experience indicates that it is nec ess ary, joints s hould be sealed to minimize infiltration of water and incompressible pa rticles. The effectiveness of joint sea ling d epend s upon the sealant type, installation technique, maintenance, concrete properties a nd joint configuration. For pa vements w ith joints s pa ced less than 4.5 m (15 ft), the impac t of the s ea la nt on pa vement performanc e is not a s c ritica l as it is for longer sp ac ing. The impact of joint sealing on pavement performance also may be more noticeable in urban areas and Northern climates if pavements carry heavy traffic and receive sand or salt applications for snow and ice c ontrol. Se e Reference 8 for more deta ils on joint sealing tec hniques a nd ma terials.
4
S
(E q. 1)
w here: M L = M axim um length betw een joints (S ee N otes 1 and 2). T = S lab thickness (E ither m etric or Eng lish units). C S = S up po rt con stant. U se 2 4; for sub grad es o r granular sub bases. U se 21; for stabilized subbases, or existing co ncrete o r asp halt pavem en t (for co nventional overlays). U se 12-15; for ultra-thin overlays of asp halt (S ee N ote 3 ). Notes: 1. The spacing of transverse joints in plain (unreinforced) conc rete pavement should not exceed 6 m (20 ft) for slabs less than 250 mm (10 in.) thick. 2. A general rule-of-t humb requires that the transverse joint spacing should not exceed 150% o f the longitudinal joint spacing. This ratio is difficult to maintain within intersec- tions d ue to islands, m edians, auxiliary lanes and curved areas, and can be disregarded in favor of comm on-sense jo int ing pat te rn s to acc om m od ate th ese elem ent s w it hin the intersection. 3. The spacing of transverse and longitudinal joints in ultra- thin overlays range from 0.6 to 2.0 m (2 to 6 ft) depend ing upon overlay thickness, suppo rt cond itions, and lane width.
The clim ate and concrete agg regate com m on to som e geo grap hic regions m ay allow transverse joints to be further ap art, or req uire them to be closer together than Equation 1 d eterm ines. For exam ple, concrete m ade from granite and lim estone coarse aggregate is m uch less sen sitive to tem perature ch ange than co ncrete m ade from siliceous gravel, ch ert, or slag ag gregate. A less-tem perature-sen sitive concrete d oes not exp and or contract m uch w ith tem perature change, w hich allow s a longer spacing betw een pavem ent co ntraction joints w itho ut any greater chance of random cracking . H ow ever, un less experience w ith localco nditions and co ncrete aggreg ates indicates o therw ise, use E quation 1 to d eterm ine the m axim um allow ab le transverse joint spacing for unreinforced pavem ents. A transverse joint sp acing up to 9 m (30 ft)is allow able for pavem ents reinforced w ith distributed steelreinforcem en t. The p urpose of distributed steelis to hold together any interm ed iate (m id-panel)cracks that w ill develop in the long panels b etw een transverse joints.** D istributed steelneither adds to the load-carrying cap acity of the pavem ent nor com pensates for poo r subgrad e conditions.
J oint Types — There are three b asic joint types for co ncrete pavem ents: co ntraction, co nstruction and isolation. S pecific design requirem ents for each type dep end upon orientation to the direction of the road w ay (transverse or longitudinal). M ost concrete intersections w illrequire each of the three joint typ es in both longitudinal an d tran sverse orien tations. Figure 2 (pag e 6 )provides cross-sections detailing each type. Transverse Jo ints - Transverse contraction joints
run transverse to the pavem ent centerline and are essential to co ntrolcracking from stresses caused by shrinkag e, therm al co ntraction, and m oisture or therm al gradients. Typ ically these joints are at a right angle to the pavem ent centerline and ed ges. H ow ever, som e ag encies skew transverse contraction joints to decrease the dynam ic load ing across the joints b y elim inating the sim ultan eo us crossing of each w heelon a vehicle’ s (6,7) axle. R ight-angle transverse contraction joints are ** P a vements with distributed ste el are often ca lled jointed reinforced c oncrete pavements (J RCP ). In J RCP , the joint spa cing is purpos ely increas ed a nd reinforcing steel is used to hold together intermediate c racks. If there is enough distributed steel within the pavement (0.10 to 0.25% per crosssec tiona l area ), the mid-pa nel cracks d o not d etract from the pavement’s performance. (9) However, if there is not enough steel, the steel ca n corrode or rupture and the cracks ca n start to open and deteriorate.
preferable to skew ed joints for concrete intersections because they d o not create com plex jointing patterns w ithin the intersection’ s p hysicalarea. S kew ing joints is not a substitute for the load transfer provided by dow els. The need for dow els (sm ooth round bars) in transverse co ntraction joints dep ends up on the road w ay or street classification. U ndow eled co ntraction joints (Type A -1, Fig. 2)are usually sufficient for light residential, residential, or collector pavem ents. Industrialand arterial streets that w illcarry heavy truck traffic for long periods require d ow eled contraction joints (Type A -2, Fig. 2).(7) D ow eled co ntraction joints are also necessary in pavem en ts w ith distributed steelreinforcem en t, an d should be considered for slab s longer than 6 m (20 ft). Tab le 3 provides recom m end ed do w elsizes. Transverse construction joints (Typ e B -1, C -1, Fig. 2)are necessary at the end of a p aving segm ent, or at a p lacem ent interruption for a drivew ay or cross road . A dow eled butt joint (Type B -1) is p referable, and should be used w henever the construction joint w illco rrespond to the location of a contraction joint or construction joint in an adjacent lan e. S om etim es it is no t feasible to m atch the location of a transverse joint in the adjacent lane, w hich necessitates use ofa tied co nstruction joint (Type C -1). T he deform ed tieb ars in a Type C -1 joint prevent the joint from opening and causing sym pathy cracking in adjacent lane(s).
Table 3. Dow el sizes for plain pavements and pavements reinforced with distributed steel.
Slab Thickness mm (in.)
Dowel Diameter mm (in.)
Dowel Length mm (in.)
P lain (un reinforce d)P avem en ts* < 200
(< 8)
not necessary
not necessary
200-249
(8-9.9)
32
(1.25)
450
(18)
≥250
(≥10)
38
(1.50)
450
(18)
D istributed S teelR einforced P avem en ts 150
(6)
20
(0.75)
360
(14)
165
(6.5)
22
(0.875)
360
(14)
180
(7)
25
(1.00)
400
(16)
190
(7.5)
28
(1.125)
400
(16)
200-249
(8-9.9)
32
(1.25)
450
(18)
≥250
(≥10)
38
(1.50)
450
(18)
*Assumes thickness is based on anticipated traffic and is calculated in a rational design proc edure such as that o f (4) AASHTO or PCA (3,5) .
5
Fig ur e 2 .
6
C ro ss s ec t io ns o f dif fe re nt jo in t ty pe s.
Longitud inal Joints - Longitudinalcontraction joints
(Type A -3, A -4, Fig. 2)also are necessary to co ntrol cracking from stress caused by concrete volum e ch anges and m oisture or therm al grad ients throug h the concrete. These joints run parallelto the p avem ent centerline and usually co rresp ond to the edge o f a driving lan e. O n tw o-lan e an d m ultilane p avem ents, a spacing of 3.0 to 4.0 m (10 to 13 ft) serves the d ual purpose of crack controlan d lan e d elineation. Fo r unu sual or special locations, such as ram ps and turning areas betw een m ed ian islands, the m axim um recom m ended slab w idth (distance betw een long itud inalcontraction joints) is 4.5 m (15 ft). H ow ever, this m ay b e excessive for thinner slab s, in w hich case Equation 1 sho uld b e used to d eterm ine the m axim um allow ab le longitudinal joint sp acing. The need to tie longitudinalcontraction joints w ill depend upon the degree of lateralrestraint available to p revent the joints from open ing perm anently. M ost longitudinal contraction joints on roadw ay tangent section s con tain #13 M or #1 6M (N o. 4 o r N o. 5) deform ed reinforcing b ars.† The d eform ed bars are usu ally ab ou t 600-750 m m (24-30 in.) lon g and are spaced at 75 0-10 00 m m (30 -40 in.) intervals. W here there are curbs on b oth sides o f the pavem ent, it m ay not be necessary to tie the joints unless localexp erien ce indicates otherw ise.
keyw ays are too large o r too close to the slab surface. S om e co ntractors rep ort that half-round keyw ays are easier to co nstruct an d less prone to problem s than trap ezoidalkeyw ays. W here a keyw ay is deem ed necessary, the dim ensions ind icated in Figure 3 w illafford the optim um load -tran sfer perform an ce. Isolation Jo ints - Iso lation joints (Typ e D , Fig. 2)are
essentialat asym m etricaland T-intersections to iso late the side road from the through street (Figure 4 ). Isolation joints also are need ed w here the pavem ent abuts certain m anholes, drainage fixtures, sidew alks, ap rons, an d structures. C ertain agencies and contractors also prefer to use iso lation joints at crossroad intersections. W here u sed, the iso lation joint w ill
Fig ure 3 .
K eyw ay s ta nd ard dim en sio ns .
Longitudinalconstruction joints (Type B -2, C -2, Fig. 2) join pavem en t lan es that are paved at differen t tim es. C oncrete intersections req uire these joints b ecau se of the n um erous pours necessary to place p avem ent arou nd island s and m ed ians, and betw een the curves connecting the tw o roadw ays. The o ptionalkeyw ay for a tied longitudinalconstruction joint can be difficult to construct correctly in thin pavem ents. Therefore, som e agencies avoid placing keyw ays in slab s less than 250 m m (10 in.) thick. (7) K eyw ay shear failures can occur in thin slab s w hen †
For very wide roa dw ays , there is a limit to w idth of pa vement that c an b e tied to gether without concern for cracking. Current reco mmenda tions (7) limit the tied width of highway pavement to about 14.5 m (48 ft) based on the subgrade drag theory. However, there has been go od field performance of certain intersections with up to 21 m (70 ft) of tied pavement. If local-experience records are not available for intersection des ign, co nsider using a n untied co ntrac tion joint (Type A-3) or co nstruction joint (Type C -2) nea r the ce nterline o f road wa y sections that exceed 14.5 m (48 ft).
Figure 4.
Asymmetrical and T-intersections that require isolation joints.
7
Notes: 1. Isolation joints sho uld be at lea st 12 mm (1/2 in.) wide a nd filled with a c ompress ible material. 2. Boxouts should be large enough to provide at least 0.3 m (1 ft) clearance between the fixture and the surrounding isolation joint.
Figure 5.
Details for boxing out fixtures.
allow ind epend ent m ovem ent of the pavem ent and the structure, w ithout any co nnection that could cause dam age. To be effective the p reform ed com pressible filler should m eet the requirem ents o f A S T M † † D 17 51, D 175 2, or D 994 , and m ust cover the en tire dep th of the co ncrete slab . A t asym m etricalor T-intersections, undow eled , thicken ed-edge o r sleeper-slab iso lation joints (Typ e D -1 or D -3, Fig. 2)are preferable to dow eled iso lation joints, because they each perm it ind ep endent lateralm ovem en t of the through-street co ncrete slab s. The sleeper slab and thickened ed ge d esign s each provide im proved sup port to co m pensate for the ab sen ce o f dow elbars. Fo r a thicken ed ed ge joint, the ab utting ed ges o f the concrete slab s should be 2 0% thicker at the joint an d tap er back to the n om inal thickness o ver about 1.5 m (5 ft). A t locations inaccessible to heavy vehicle loads, such as tho se b etw een a p avem ent and a structure, a thickened-edge joint is not necessary. A butt joint w ith a no n-extrud ing , preform ed co m pressible m aterial is adeq uate.
† † AS TM
is now the o fficial na me o f the o rga niza tion originally ca lled the America n So ciety for Tes ting and Mate ria ls. Equivalent Cana dian sta nda rds for all ASTM stand ards and tests addressed in this publication are found on page 28.
8
For utility fixtures such as m anholes, catch basins and drainage inlets, the need for iso lation w illdepend upon the casting design and potentialfor differential m ovem ent. N on -telescoping m anh oles w ith ribbed cylinder w alls usually require a boxo ut w ith a p erim eter iso lation joint to allow independent verticaland horizontalslab m ovem ent. C om m on sq uare bo xouts som etim es cause cracks to form at the boxout’ s co rners. To avoid crack-inducing co rners, co nsider using rounded boxouts o r placing fillets o n the co rners o f square boxouts. It is advantag eo us to p lace w elded w ire fabric or sm all-diam eter reinforcing bars in the co ncrete p avem ent around any interior co rners at sq uare b oxouts to hold cracks tightly should they develop. Figure 5 show s details for boxing out inpavem en t fixtures. In som e circum stances, boxing out fixtures m ay b e un desirab le. Fo r instance, boxouts can im ped e fasttrack co nstruction b ecau se m ore tim e is need ed to place concrete around the casting after the p avem ent gains strength. It is also very difficult to m aintain a uniform joint pattern if there are too m an y m an holes random ly-positioned in an intersection. In these cases it m ay be best to cast the fixtures into the co ncrete. To isolate a fixture w ithout a b oxout, som e contractors an d ag en cies w rap the casting w ith p liab le expan sion joint filler or suitable bond breaker. Ifno differential
m ovem ent is expected the m anho le can b e cast directly into the concrete. Telescoping m anhole fixtures have a tw o-piece casting, w hich allow s vertical m ovem ent.
throu gh em bed ded fixtures such as m anh oles or drainage inlets. It is com m on for the actuallocation
C oncrete pavem ent perform ance suffers if the pavem ent co ntains too m any transverse exp ansion (isolation-typ e) joints. O utdated sp ecifications so m etim es req uire expan sion joints spaced uniform ly along tan gent sections. These joints create m aintenan ce prob lem s becau se n earby tran sverse co ntraction joints open excessively as the exp an sion joint closes grad ually over tim e. The o pen contraction joints then lose load transfer, and develop distresses like faulting and pum ping. Tran sverse expan sion joints at reg ular intervals m ay be needed w hen:
during construction so that they coincide w ith the
1. The pavem en t is divided into long pan els [18 m (60 ft) or m ore] w ithout co ntraction joints in-betw een. 2. The p avem ent is con structed w hile am bient tem peratures are b elow 4°C (40°F). 3. The contraction joints are allow ed to be infiltrated by large inco m pressible m aterials. 4. The pavem ent is constructed of m aterials that in the past have show n high expansion characteristics. In m ost situations, these criteria do not apply. Therefore tran sverse expan sion joints should not norm ally be used .
J oint Layout — A w ell-designed joint layout contributes to good long -term perform an ce of at-grad e intersections. A good jointing plan w illease construction by providing clear guidance to the contractor. It is com m on practice for som e d esigners to leave intersection joint layo ut to the field engineer and contractor. These designers often justify this practice b y citing the m any field ad justm en ts that occur du ring co nstruction, w hich they conten d neg ates the usefulness of a jointing plan. H ow ever, it is not desirable to elim inate the jointing plan entirely, except for very sim ple intersections. A jointing p lan and appropriate field ad justm ents are b oth im portant for m ore com plex intersections, because islands, m edians, and auxiliary turning lan es co m plicate joint layout an d req uire som e forethought before co nstruction. A plan also en ab les co ntractors to bid new projects m ore accurately.
of m an holes or drainag e inlets to vary from the location show n o n the p lans. It w illbe necessary for the construction crew to adjust the location of som e joints actuallocation o f a nearby m anhole or inlet. The designer should co nsider placing a no te on the plan to give the field engineer and contractor the latitude to m ake ap propriate adjustm ents. R eference 10 provides a ten-step m ethod for laying out joints for concrete intersections. A nother im portant asp ect of laying out intersection joints is d eterm ining w here to use dow el bars o r tiebars near the intersection’ s p hysicalarea. Figure 6 (pag e 10) sho w s exam ples of dow eland tieb ar use in intersections.
Phasing Cons truction — P hasing is alm ost alw ays a key elem ent of intersection construction p lan s. The need for a refined phasing plan dep en ds upon the need to m aintain traffic flow through the intersection during construction. There are four basic co nstruction stag ing options: com plete closure w ith detours, partialclosure w ith detours, co m plete closure d uring tim e-w ind ow s, and construction under traffic. Intersections o f ruralor other low -traffic roadw ays do not usually require the sam e levelof consideration as is necessary for intersections that carry high volum es of traffic. C losing low -traffic intersections for the d uration of construction is o ften the optim also lution and sho uld alw ays be co nsidered . In som e cases, the availability o f convenient alternate routes (e.g., frontag e road s) m ay even perm it closing an intersection that carries a high traffic volum e w ithout significant concern for traffic flow or business disrup tion. For the co ntractor, com plete closure is ideal. C om plete closure elim inates co m plex w ork-zone lane co nfigurations, w hich increases the safety of the construction w ork zone. C om plete closure also allow s the co ntractor to p lace m ore p avem ent in a continuous operation, generally increasing pavem ent sm oothn ess, im proving quality, and reducing co nstruction tim e.
D uring construction, it is likely that location changes w illbe necessary for so m e joints w ithin an intersection. The p rim ary reason is to en sure that joints pass
C om pletely closing an intersection for construction req uires developing a detour plan . C lear an d under9
Figure 6.
10
Use of dow el bars and tiebars in intersections
stan dab le signing along the detour route w illm ake the detour m ore accep tab le to m otorists. A sign indicating the date w hen the intersection w illre-op en also can im prove public relations. U nfortunately, closing intersections for the entire construction period is o ften not viable along urban arterial or corridor routes. For exam ple, the lack o f traffic over an extend ed period m igh t cause b usinesses near the closure to lose custom ers. In these circu m stances one option is to lim it com plete intersection closu re to non-business hours. If it is feasible to divert traffic around the intersection, even for a few hours, the contractor can com plete criticalconstruction phases quickly an d exp edite the entire p roject. S om e ag encies develop phasing plans that allow com plete intersection closure during sp ecific periods (w indow s). U sually the w indow w illbeg in at ab out 6 p.m . an d last untilab out 6 a.m . the follow ing m orning. The starting and ending tim e d ep ends up on the local rush-hour traffic pattern. W ithin the w indow the co ntractor m ay close an d occupy the en tire intersection. A t the end of the w indow public traffic m ust be able to use the intersection. In this m anner, the closu re w ill
Figure 7.
not hinder m orning, evening, or daytim e traffic flow . U nd er tim e-w ind ow phasing plans, contractors p erform each sequen tial co nstruction operation during successive tim e-w ind ow s. Fo r exam ple, if the p roject includ es rem ovalof an existing pavem ent, the co ntractor m ay p lace a tem porary pavem ent after rem oving the existing m aterial during one 12-hour w indow . The tem porary pavem en t carries traffic untilthe co ntractor rem oves it to pave the new co ncrete road w ay in a subsequen t tim e w ind ow . A nother option to avoid closing an entire intersection is to close one leg of the intersection at a tim e. This is often feasible for intersections betw een residential and collector streets. D etours along the closed residential street are usually sh ort and not a burden to localresidents. O n som e roadw ays, it m ay be u naccep tab le to close the entire intersection at any tim e. M any agencies have h ad good success rep lacing busy intersections w ith co ncrete pavem ent w hile m aintaining no rm altraffic volum es. F igure 7 sh ow s possible options for phasing construction under traffic.
Possible options for phasing construction under traffic.
11
These o ptions m ay red uce the num ber of availab le throug h-lanes and m ay som ew hat lim it turning m ovem ents during co nstruction . H ow ever, the degree o f these restrictions dep ends upon the num ber of lanes on the app roaching road w ays. N one m ay be necessary if the approaching road w ays have at least three through lanes in each direction. A detour for one leg of the intersection or specialalternating traffic signals w illbe necessary if one o r both of the ap proaching streets h as just one lane in each direction. C onstruction under traffic can generally start on any leg of an intersection. H ow ever, if an intersection includ es a m ajor road an d a m ino r cross road , the d riving lan es of the m ajor road w ay usually are built before the cross road . C oncen trating on the m ajor roadw ay pavem ent generally prod uces a sm oo therriding intersection. A fter the m ajor road w ay pavem en t lan es are finished , other pavem en t areas are built w ithout affecting the sm oothness through the intersection. This m ethod also is usually m ore p roductive becau se the contractor can place m ore p avem ent in a co ntinuo us operation w itho ut gap s or ch anges in the pavem ent w idth.
Quality C onc rete Mixtures — A suitab le concrete m ixture is necessary to en sure the success of the construction phasing plan . W hether the co ntractor or ag en cy d eterm ines the concrete m ixture p roportions, the co ncrete m ust be cap ab le of m eeting strength requirem ents reliably w ithin any specified tim e w ind ow s for co nstruction, an d m ust have adeq uate long-term durab ility. The co ntractor should have so m e latitude to ad just the m ixture p roportions during construction if the m ixture d oes not w ork properly for the req uired co nstruction phasing plan. B efore co nstruction, contractors also m ay offer valuable su ggestions or value-engineering options to exped ite co nstruction.
load capacity. Flexuralstren gth provides an assessm ent of the ten sile strength at the b ottom of the slab w here w heel load s induce tensile stresses. H ow ever, prob lem s casting and testing beam specim ens d isco urag e m any engineers and contractors from this test m etho d.
Durability — S tren gth is not a reliab le m easure of concrete’ s d urability. In frost-affected areas, a concrete p avem ent m ust be ab le to w ithstand m any cycles of freezing an d thaw ing an d the effects o f deicing salts. This requires q uality aggregate, a low w ater-cem entitious m aterialratio, an adequate cem ent factor, and a sufficient quantity of entrained air bubbles. The p ercentag es o f total air co nten t necessary for w eather-resistant co ncrete are show n in Tab le 4. These recom m end ations vary dep end ing up on the exp osure cond ition o f the co ncrete. A deq uate curing
m easures also are necessary for developing durab le con crete pavem ent. In ad dition to m aking the hardened co ncrete pavem en t w eather resistan t, en trained air bubbles im prove the concrete w hile it is stillin a plastic state by: 1. R ed ucing w ater required for satisfactory w orkability. 2. P reventing segregation . 3. R educing bleeding.
Table 4. Recommended* total air contents. (11)
Nominal Maximum Aggregate Size
Target** Percentage Air Content for Exposure
mm (in.)
Severe***
Moderate***
Mild***
50 (2)
5
4
2
37.5 (1-1/2)
5.5
4.5
2.5
25 (1)
6
4.5
3
Strength — C om pressive strength testing (A S TM C
19 (3/4)
6
5
3.5
39 ) is the m ost com m on and easiest w ay to evaluate concrete strength. C on crete w ith a 28-day com pressive stren gth averag ing 20-30 M P a (3000-4000 psi) is ad eq uate for m ost intersections. D uring co nstruction the pavem en t m ay b e o pened to traffic at a stren gth som ew hat less than the 28-day target value (see page 22 ).
12.5 (1/2)
7
5.5
4
9.5 (3/8)
7.5
6
4.5
S om e highw ay agencies u se flexuralstrength (A S TM C 78) as the structuralstrength criterion to evaluate 12
*
Canadian standards differ, refer to CSA Standard A23.1.
**
A tolerance of -1% to +2% is typical for paving concrete.
***
Severe exposure is an environment where concrete pavement is exposed to w et freeze-thaw c onditions or deicers. Moderate exposure is an environment w here concrete pavement is exposed to freezing but will not be c ontinually moist, exposed to water for long periods b efore freezing, or in contact w ith deicers. Mild exposure is an environment w here concrete pavement is not exposed to freezing conditions or deicers.
B ecau se of these beneficialand essential effects in both plastic and hardened concrete, it is w ise to co nsider using entrained air even in m ild exp osure conditions.
The quantity o f w ater in the m ixture also has a critical influence o n the d urability and w eather resistance o f hardened concrete.(11 ) For a g iven quantity of cem entitious m aterials, a low er quantity o f w ater w illproduce a m ore durab le m ixture in m ost cases. H ow ever, an ad eq uate q uantity of w ater is necessary to produce a w orkable co ncrete. S atisfactory pavem en t durab ility is generally achieved w ith: 1. A w ater-cem entitious m aterialratio not excee ding 0.53 w ith a m inim um cem en titious m aterial co nten t of 310 kg /m 3 (520 lb/yd 3) for m ild exp osure conditions. 2. A w ater-cem entitious m aterialratio no t exceeding 0.49 w ith a m inim um cem entitious m aterialcontent of330 kg/m 3 (560 lb/yd 3)for m oderate-tosevere exposure co nditions (freq uen t freezing an d thaw ing, and application o f deicing agents). C arefulaggregate selection is im portant to avoid future problem s w ith alkaliag greg ate reactions o r D cracking . C oarse o r fine aggreg ates that are su scep tible to alkali-silica or alkali-carbonate reactivity require specialm ixture proportions to produce durab le con crete. M any agen cies sp ecify specialm ixtures w hen using locally availab le aggreg ates know n to have reactivity p otential. It is also possible to test a proposed co ncrete m ixture to determ ine if there is reactivity p otential. For m ore inform ation refer to R eferences 12 and 13 .
Fast-track Conc rete Mixtures — Fast-track co ncrete m ixtures develop stren gth rap idly an d are ben eficial w hen early open ing of the pavem en t is necessary. For intersections, there are severalpractical options availab le to produce co ncrete that gains strength rap idly. The m ixture com ponents can be selected or proportioned for rapid strength gain, and the m ixture w ater can b e heated so cem ent hyd ration beg ins quickly. A lthough proprietary cem ents are available, fast-track m ixtures do not necessarily req uire these sp ecial m aterials. R ap id stren gth developm en t is possible by using greater-than -norm alquan tities ofordinary A S TM C 150 T ype Iand Typ e IIcem ents. H igh-earlystreng th, A S TM C 15 0 Typ e III, cem ent is also com m only availab le. M ost ag greg ates and ad m ixtures
available locally also can be used in fast-track m ixtures if co m bined in the proper proportions. (14 ,15 ) Table 5 show s typ icalfast-track m ixture p roportions. The sooner the concrete tem perature rises, the faster it w illdevelop stren gth. O ne w ay to raise the tem perature of plastic concrete is to heat the m ix w ater. This m ay b e practicalfor intersection projects that do not require a large quantity o f concrete. S everal factors influen ce the w ater tem perature needed to p roduce a desirab le m ixture tem perature at placem ent. The criticalfactors are: am bient air tem perature, ag greg ate tem peratures, ag greg ate freem oisture co ntent, and p ortland cem ent type. W hen necessary, ready-m ix concrete producers heat w ater to 6 0-66°C (140-150°F) to elevate m ixture tem perature sufficiently for cool-w eather construction. The sam e practice w illaccelerate stren gth developm en t in w arm er am bient tem peratures. H ow ever, to avoid a flash set using this m etho d, co m bine the ho t w ater and aggregates b efore adding the cem ent to the concrete m ixer.(14 ) Tho ug h ho t w ater do es facilitate early cem ent hydration, its benefits m ay be short-lived. S everalhours of heat co ntainm en t w ith insulating blan kets m ay be necessary to achieve the desirab le stren gth gain, particularly w hen coolw eather conditions p revail.
Table 5. Typical fast-track mix prop ortions.(14)
Material
Type
Quantity*
C em e nt
A S TM C 150 Type I
415-475 kg/m 3 (700-800 lb/yd 3 )
AS TM C 150 Type II
415-475 kg/m 3 (700-800 lb/yd 3 )
AS TM C 150 Type III
360-450 kg/m 3 (600-750 lb/yd 3 )
Fly ash
A S TM C 618
10-20% b y w eight of cem ent
W ater
A S TM C 94
(S ee note below )
A ir-entraining ad m ixture
A S TM C 260
A s necessary
A ccelerating A dm ixture
A S TM C 494
A s necessary
W ater-reducing ad m ixture
A S TM C 494
A s necessary
* Use quantity of water appropriate to produce sufficient workability and maintain desired strength gain. Water-cementitious material ratio should not exceed 0.37 - 0.43 und er most circumstances.
13
Mixtures for Thin Overlays — Th e concrete m ix-
eq uipm ent such as a front-end load er or bulldozer.
ture for thin overlays is o ften selected based on requirem ents for open ing to traffic. A norm althin-
C old m illing offers p roductivity and suitable grade co ntrol. C old m illing eq uipm en t uses carbide teeth
overlay m ixture includes: cem en t, co arse an d fine
m ou nted o n a rotary drum . T he teeth chip aw ay
aggregate, air-entraining agent, adm ixtures (w aterreducers or plasticizers), fibers (as specified), and a
existing asp halt as the d rum rotates. The size o f the broken m aterialdep ends upon the too th configura-
low w ater-cem en titious m aterials ratio. C om pared to
tion, drum rotation sp eed, forw ard m achine-speed,
ag greg ate u sed for thicker co ncrete p avem ents, the top-size of coarse aggregate for ultra-thin w hitetop-
and rem ovaldepth. P article size also varies w ith the tem perature, condition, an d asphalt co nten t of the old
ping (16 ) is red uced ap propriately for the thin pavem en t.
hot-m ix asp halt. The ability to controlparticle size is
W hen fibers are used in an ultra-thin m ixture, the fiber contents are usually in the range appropriate for the
helpfulw hen the asphalt m illings are reu sed on the project for fillor su bbase.
sp ecific fiber typ e, although o n so m e p rojects higherthan-norm aldo sages have been used.
Construction
A ttaining the desired rem ovaldep th m ay req uire severalm illing p asses. C om m only availab le m achines can rem ove 150 m m (6 in.) of asp halt m aterial in o ne pass.
Variou s m etho ds and m achines are used to build concrete pavem ent intersections, including slipform an d fixed-form construction eq uipm ent. U nlike m ainline road w ay paving, intersection co nstruction w ork usually necessitates som e u se o f fixed -form placem ent. C ontractors m ay elect to use slipform eq uipm ent in an intersection if the paving area is large enough to w arrant its u se, or if staging allow s the contractor to build the driving lan es of the m ajor road w ay through the physicalarea of the intersection. D esp ite the variety of possible equipm ent, the follow ing construction steps are typ icalfor nearly alltyp es o f
S carifying or ripping is also co m m on for rem oving thin layers of existing asp halt. This m ethod uses m otorgrad ers or bulldozers eq uipped w ith scarifying eq uipm ent. W hile scarifying is less exp ensive than cold m illing, there is also less ability to controlrem oval dep th o r grad e. N evertheless, scarifying eq uipm en t is adequate w hen the rem ovalgo es below the d epth o f allasphalt layers. There also are three m ethods for rem oving existing concrete or com posite pavem ents:
con crete pavem ent.
1. B reak the co ncrete into sm allfrag m en ts for rem oval by backho e and han d too ls.
1. R em oving or planing an existing pavem ent (w here
2. Lift the concrete o ut of place in large segm en ts.
necessary). 2. P reparing the grade. 3. S etting form s (w here used ). 4. P lacing in-pavem ent objects (dow els, tieb ars and boxouts, w here used). 5. P lacing an d finishing the co ncrete. 6. Texturing the p avem ent surface. 7. C uring the concrete. 8. Jointing the p avem ent.
Removing or Planing Existing Pavement —
3. S carify the co ncrete w ith large m illing m ach ines. A t urban intersections, the optim al m etho d dep ends upon the size of the intersection, the allow able tim e for rem oval, the lan d use in the surrounding area, an d concerns about no ise and dust generation. Th e presence of sensitive utility p ipes, conduits, or cables beneath the p avem ent also m ay discourage use of eq uipm en t that im parts im pact vibrations. In gen eral, the selection of the m ost productive rem ovalm etho d should be left to the contractor based on exp erien ce and availab le equipm en t.
The first step in the com plete reconstruction of an
If an existing asp halt intersection w illreceive a con-
intersection is to rem ove the existing pavem en t. The
crete pavem en t overlay, rem ovalofthe existing asp halt should stop short of the subbase or sub-
options for rem oving existing asp halt include: cold m illing, scarifying, and excavating the m aterialw ith 14
grade. B ecause cold m illing o ffers excellent grade
control, it is the best choice for rem oving controlled
Flow ab le-fillm aterials do not need co m paction and
layers of existing asp halt pavem en ts. The rough su rface from m illing also provides an excellent bonding
flow easily to filla trench. The m ixtures contain p ortlan d cem en t, sand, fly ash an d w ater an d typically
surface for the overlay.(16 ,17 ) For ultra-thin w hitetop -
develop 28-day com pressive strengths o f ab out 0.35-
ping, an overlay less than 100 m m (4 in.) thick, cu rrent reco m m end ation s (16 )suggest that at least 75 m m
0.70 M P a (50-100 psi). Flow able-fillm aterials p rovide enoug h strength to prevent settlem ent, but are easy
(3 in.) of asp halt thickness rem ain after m illing to get
to rem ove using a b ucket on a b ackho e or front-end
the ben efits o f co m posite action.
loader if future excavation is necessary.
Subbase — A subbase is a thin layer of gran ular
Preparing the Grade —
m aterialplaced on top of the prep ared subgrad e.
A reasonably uniform sub grad e o r sub base, w ith n o
S ub bases p rovide uniform sup port to the pavem ent
ab rupt ch an ges in support, is idealfor an y concrete
an d a stab le p latform for co nstruction equipm en t. S ub bases also help prevent m ud -pum ping of fine-
pavem ent. A chieving this co nd ition after pavem en t rem ovaloperations w illrequire so m e effort even in the
grained subgrad e soils at tran sverse pavem en t joints
relatively co nfined w ork area of an intersection. The
in roads su bject to a large volum e o f unidirectional truck traffic. Intersections at residentialstreets and
first step is to ensure that the subgrade so ils are of uniform m aterialan d den sity.
even som e streets that m ay carry heavier vehicles usually do no t req uire a subbase.
C om pacting the sub grad e su rface adeq uately req uires a co m pactor heavy eno ugh to achieve 95 percent of A S TM D 698 d ensity. H ow ever, it m ay b e d ifficu lt to m aneuver large com pactors in a trench created by
W here u sed , the g ranular subbase thickness gen erally sho uld n ot exceed 1 00 to 1 50 m m (4 to 6 in.). A thicker sub base is not necessary or econo m icalunder
rem oving an older pavem en t for an intersection. A m ore effective strateg y in a confined area m ay b e to ap ply m ore com paction effort using sm aller eq uipm ent. The soilm oisture co ntent should be reasonab ly uniform during co m paction; excessively w et or dry spots require correction to produce reaso nable uniform ity. Fo r m ost soils, co m paction should be d one at m oisture conten ts at or slightly ab ove o ptim um . S oft sp ots in the subgrad e o ften becom e visible after rem oving an old pavem ent. It is not acceptab le to m erely p lace a gran ular layer over these soft areas; excavation is necessary to rem ove the suspect soils. Ideally, the rep lacem en t soilshould be of the sam e type as in the surroun ding subgrad e to develop un iform sup port. C ontractors m ust pay particular attention to sections of the su bgrade o verlying any utility installations such as sew ers, telep ho ne and pow er cond uits and w ater lines. C areless com paction of fillm aterials in these trenches often causes soft spots in the sub grad e. C ontrolled low -strength fill(flow able-fill)m aterials are an econom icalalternative for these areas.
Comp acting subbase against existing curb and gutter after removing existing pavement .
m ost co nd ition s. G ood d ense-grad ed , granular-sub base m aterials h ave a plasticity index o f 6 or less, and co ntain a m axim um of 15 percent fine particles that pass the 7 5 µ (N o. 200)sieve. For stability, the subbase req uires com paction to 1 00 percent of A S TM D 698 den sity. P erm eab le sub bases w ith d rainag e system s are generally unn ecessary for urban pavem ents, because in m any cases, the presence o f curbs an d gutters w ith 15
inlets to a m unicipal storm sew er system w illad equately rem ove surface w ater. P erm eable subbases have becom e pop ular am ong state highw ay dep artm ents for draining concrete highw ay pavem ents. These su bbases either m ay b e untreated or stab ilized w ith p ortland cem en t or asphalt. To be effective, a perm eable subbase req uires a co llector pipe and outlet system to d ischarge w ater aw ay from the p avem ent.
E ach straight m etal form m ust be clean, and in accep tab le co nd ition to prod uce a sm oo th p avem ent. C ontractors should exam ine form s w ith a straighted ge or stringline before using the form s o n a project. S traight form sections that deviate by m ore than 3 m m (0.125 in.) along the top, or 6 m m (0.25 in.) along the inside ed ge sho uld be rep laced .
Trimming — The m etho d for trim m ing or shaping
The quality of the sup port beneath the form dep ends up on the trueness of the sub grad e or sub base su rface.
the grad e varies by project and m ay dep end up on intersection size. Typ icalspecifications (18 ) require:
The base o f the form sho uld bear against the sub base or sub grad e surface com pletely and no t lie on any
1. A subgrad e surface that does no t vary from the design elevation b y m ore than 12 m m (0.5 in.). 2. A gran ular subbase surface w ith deviations that do not exceed 12 m m (0.5 in.), longitudinal or transverse, by a 3 m (10 ft) straightedge. O n large intersections, co ntractors m ay use au tom atic trim m ing eq uipm ent to shap e the subbase or sub grad e and dep osit an y excess m aterialoutside the paving area. For fixed-form paving, the autom atic trim m ing m achine rides o n the form s after they are fastened into place. For slipform paving, the trim m ing m achine references the stringline(s) for the slipform paving m achine. O n sm allprojects an d in confined w ork zo nes it m ay no t be p racticalto use autom atic trim m ing eq uipm ent, an d the co ntractor w illprobab ly trim the grad e w ith a m otor grad er or sm allload er.
clum ps of dirt or large rocks. A fter setting the form s, the form crew should visually check to ensure the form s are aligned and fully supported , and also to be sure the form ends are locked together securely. A deq uately securing form s also is crucialbecause the form s m ust sup port eq uipm ent and rem ain in p lace untilthe concrete has hardened . Fo r ease of rem ovaland cleaning , form s require a thin application of oilbefore paving.
S tan dard 3 m (10 ft) straight form s are accep tab le for form ing co m poun d-rad ius curves and curve rad ii exceeding 30 m (100 ft), but sm aller radiirequire cu rved steelor flexible w ooden form s. (19 ) S ho rt, 1.5-m (5-ft), straight form s also produce acceptable results o n cu rves less than 30 m (100 ft). (20 ) C urved sections req uire a tighter stringline staking intervalthan straight sections. To ensure the form s m eet the design location an d surface elevation, a stringline staking intervalof 1.5 m (5 ft) is idealfor curve radiiless than 15 m (50 ft). A dditionalbracing
B ecause finaltrim m ing disturbs the su bgrad e o r subbase su rface slightly, additionalcom paction rolling is usually necessary.
Placing Forms — Fixed-form paving is alm ost alw ays necessary for the short paving seg m ents, varying paving w idths, an d curved paving areas co m m on to intersections. Fo rm placem ent at intersection s d oes not vary m uch from form placem ent along straight pavem ent sections. S traight sections req uire standard 3 m (10 ft)steel form s that fasten to the su bgrade w ith three pins or stakes. A stringline set to the top elevation of the pavem ent determ ines the location and height for the form s. A stake sp acing for the stringline of about 7.5 m (25 ft)w illproduce good results for straight sections.(19 )
16
Forms should rest on a level surface, and shou ld be fastened securely and pinned in place.
is also som etim es n ecessary to secure form s around sm aller curves; w here necessary a bracing intervalof 0.6 m (2 ft) is usually sufficient.
Placing In-Pavement Objects — Ideally, in-pavem ent objects should be in position before p lacing the concrete. This includes utility boxouts, cast-in-place fixtures, traffic signalhandholds, dow el assem blies (baskets), tieb ars, an d w elded w ire fab ric. H ow ever, in som e cases it is necessary for the co ntractor to use the prep ared grad e to haul co ncrete to the paving eq uipm en t, req uiring placem en t of fixtures as w ork p rogresses. C ontraction-joint dow el assem blies should be fasten ed to the subbase using steelstaking pins for granular m aterials or nailing clips for stab ilized m aterials. C are in p ositioning the b askets is necessary so that the d ow els align w ith the p avem en t centerline. A perm anent m ark ind icating the location o f the d ow el baskets is necessary for referen ce w hen later saw ing the contraction joints. In so m e cases for longitudinaljoints, contractors elect to place tiebars into position ahead of paving. S traight deform ed bars on supporting ch airs fasten to the sub base o r sub grad e in a m anner sim ilar to d ow el baskets. In fixed-form construction, standard deform ed tieb ars or tw o-piece b ars w ith a threaded co up ling m ay b e inserted throug h holes in side form s for longitudinalconstruction joints.
secured before p aving. For either fixed-form or slip form paving , the boxout’ s top surface m ust be about 12 m m (0.5 in.) below the finished height of the slab . This allow s a paver or screed to pass o ver the boxou t w itho ut problem s, and eases surface shaping to provide proper drainage. The tw o-piece casting of a telescoping m anho le has severalheight positions. A position that places the casting’ s su rface below the pavem en t surface also allow s the p aver or screed to pass. Just after the paving eq uipm ent passes over the fixture, w orkers m ust raise the casting into finalposition from a construction bridge that span s the pavem ent. Large-diam eter [up to 1270-m m (50-in)] co ring eq uipm en t is ano ther availab le option, w hich red uces co nstruction p rep aration tim e. The eq uipm ent can core co ncrete aroun d existing or planned m anho les and elim inate the n eed to place u tility boxo uts b efore paving . In N orthern reg ions, co nsideration should be given to leaving m an ho les 6-12 m m (0.25-0.5 in.) below the pavem ent elevation to ensure that snow plow s do no t catch on the m anho le lids.
Plac ing the C oncrete — R eg ardless of placing eq uipm ent, the paving steadiness im pacts the finished pavem ent sm oo thness and
C ast-in-place utility fixtures and boxo ut form s that are w ithin the paving area should be in position an d
quality.(19 ) C onsistent delivery o f concrete to an intersection project site is an im portant elem ent. D ense urban areas require carefulevaluation to predeterm ine w hether traffic delays w illham per concrete delivery. C onsideration of the concrete m ixture is also necessary,w ith norm al-setting m ixtures allow ing longer traveltim es than fast-track m ixtures. G ood b atch-to-batch consistency of the co ncrete also im proves the quality of the finished pavem en t. B atch-to batch co nsisten cy allow s the paving m achine operator to m aintain the paver at a steady forw ard speed , and prod uces u niform extrusion pres sure. B oth stationary (ready m ix) plants and on-site batching and m ixing plants can produce concrete w ith consisten t properties. B efore p lacing co ncrete, m oisten the sub base or sub -
Dow els, tiebars and utility fixtures in po sition for fixed- form p aving.
grad e surface. A dry surface m ay ab sorb w ater from 17
the concrete and lead to unw anted shrinkage cracking in the pavem ent. Fo r larger paving areas, a w ater truck is generally available for this purpose. R eady-m ix trucks also have a tank that can sup ply the w ater necessary to m oisten the subgrad e in sm allpaving areas. W hen p lacing a concrete o verlay on a m illed asp halt surface, no m oistening is norm ally necessary. U ltrathin co ncrete o verlays [overlays less than 100 m m (4 in.)] w hich rely o n bond to the asp halt, require a dry surface. H ow ever, thick overlays, w hich do not rely
A
on bond , m ay req uire w hitew ashing to coola d ark asp halt surface. The need for w hitew ash d ep ends up on the am bient and asp halt-surface tem perature. M ore inform ation on w hitew ash is availab le in R eference 17.
Fixed-Form — There are a variety o ffixed-form paving m ach ines. Th e less com plex equ ipm ent such as han d-operated an d self-propelled vibratory screeds, single-tube finish ers and revo lving triple tubes — are usefulfor alm ost allco m plex paving areas. The external(su rface) vibration that this equipm ent prod uces is ad eq uate to conso lidate m ost pavem ent slab s. H ow ever, supplem en tary internalvibra tion w ith han d-operated sp ud vibrators is usually necessary for ad eq uate co nsolidation of non-reinforced co ncrete slab s thicker than 2 50 m m (10 in.). A co m bination of internal- and su rface-vibration is p referable for reinforced slabs at any thickn ess.(11 ) B ecause surface vibration of concrete is least effective near the form s, it is b eneficialto conso lidate concrete along the form s w ith a spud vibrator. Larger, form -riding m achines can place and co nsolidate the co ncrete betw een form s in one p ass. These m achines either ride o n the form s o r pipes laid outside the form s. S ince form -riding paving eq uipm en t cann ot produce accep tab le results riding on w ooden form s, m ost of the curved areas joining intersecting pavem ents req uire use of han d-placem ent eq uipm ent, such as vibratory o r roller screeds.
B
C (A) Vibrating sc reed, (B) Roller screed, (C) Form- riding p aver.
from the ready m ix or other co ncrete hau ling truck, so m e d istribution o f the concrete w ith h and tools is usually necessary. S hovels are preferable to other hand tools for this purpose, because they d o n ot cause co ncrete segregation. W hen necessary, sup plem entalvibration w ith hand -
Evenly dep ositing co ncrete o nto the g rad e in front of the fixed -form placem ent m achine eases p aving . P iling too m uch co ncrete in front of the m achine leads to strikeoffdifficulty. The concrete should not overly exceed the height of the form s. H ow ever, piling too little co ncrete in front of the m achine m ay produce low sp ots in the p avem en t surface. A lthough it is idealto distribute the concrete evenly w ith the chute 18
held spud vibrators should preced e the placem ent screed. S tandard p ractice for thicker slabs calls for verticalplunges o f the vibrator head. For thin slabs, it is preferable to insert the vibrator head at an angle or horizontally to keep it com pletely im m ersed in the co ncrete. O perators should neither drag spud vibrators throug h the concrete nor attem pt to m ove the concrete laterally, as either w illsegregate the m ixture.
In general, proper conso lidation of air-entrained concrete takes less tim e than non air-en trained co ncrete, even w hen both m ixtures are p rep ared w ith the sam e co nsisten cy (slum p). The vibration tim e n ecessary to achieve adeq uate conso lidation also dep ends up on the size and type o f vibrator. For m ost eq uipm ent, leaving the vibrator head inserted for 5 to 15 seco nds is usually ad equate.(11 ,19 )
Slipform — U se o f slipform paving eq uipm ent for intersection reconstruction is probably the exception rather than the rule. H ow ever, a co ntractor m ay elect to use slipform equipm ent in an intersection if the paving area is large en ough to w arran t its use. P aving the curb and gu tter is ano ther com m on use o f a slipform m achine for intersection construction. There are m any sizes o f slipform paving m achines, w ith m an y sm aller m odels availab le for urban paving . S lipform paving m achines spread, co nsolidate, screed, and float-finish the concrete in one p ass w ith-
out the n eed for fixed side form s. G enerally, contractors p reset stringlines to establish the line and grade controlfor the p aver. Like fixed-form paving, depositing concrete in front of the paver evenly w illim prove the resulting pavem en t. A slipform paver m ust further spread an d co nsolidate the concrete as it m oves forw ard, and cann ot pro duce ad eq uate results if it m ust push a large pile of co ncrete. W hen operating p roperly, a w ell-co nsolidated and prop erly shaped slab em erges b ehind the slipform paver as it m oves steadily forw ard. C ertain slipform paving eq uipm ent can pave curbs and gutters, and easily pave around curves b etw een intersecting road w ays. S om e slipform paving m achines can place curbs integ rally w ith the driving lanes. In such cases, the co ntractor m ust attach a curb m ule to the paver so that the cu rb section w ill extrude out as the paver m oves forw ard. Integ ral curbs elim inate a separate form ing or placing operation that is otherw ise necessary for m ost urban road w ays.
M ore detailed inform ation on properly se tting up an d operating slipform equipm ent is available in R eference 19 .
Finishing the Surface —
A
A fter the paving eq uipm ent passes, it m ay b e necessary to further finish the concrete surface to rem ove sm allim perfection s and sm oo th any bum ps. T here are a num ber of different au tom atic and hand-operated finish ing tools available for this p urpose. In the tight w ork zones typ icalof intersection construction, m ost contractors w illopt for hand finishing tools. Finish ing is necessary earlier w ith air-entrained concrete than non-air-en trained co ncrete becau se airentrained co ncrete develops m uch less bleed w ater. It is custom ary to w ait untilallbleed w ater leaves the concrete slab surface before finishing non-airen trained co ncrete.
B (A) Slipform curb an d gutt er. (B) Slipform p avement with integ ral curb.
C hecking the surface behind the paving eq uipm ent w ith a 3 - to 4.8-m (10- to 16-ft) han d-operated straigh ted ge is a reco m m end ed proced ure. (19 ) S uccessive straighted ging should overlap by o ne-half the length of the straightedge to ensure that the tool 19
rem oves high sp ots and fills low sp ots in the surface. E xp erien ced finishers can use the straighted ge to rem ove no ticeable b um ps by em ploying a scrap ing m otion. O therw ise, they use a long-han dled float to sm ooth b um ps and disturbed places in the su rface. Edging is necessary for an y co ncrete p laced ag ainst fixed form s. The sm alledging-tooleffectively sm oothes the slab corner and sep arates the concrete from the form . W ithout sep aration, the co ncrete m ay ad here to the top of the form , an d tear or sp allupon form rem oval. P articular attention also m ay be n ecessary for finish ing around boxed-out fixtures and cast-in-place fixtures. Ideally, the height ad justm en t an d supplem en talvibra tion aroun d the object are com plete b efore w orkers need to finish the p avem ent surface. If properly positioned, the object should easily b lend into the surround ing p avem ent. S om e surface w arping m ay be necessary if the object is too high or too low .
Smoothness Requirements — S m oothness or rideability requirem ents can be applied to intersection projects. H ow ever, less stringent requirem ents are necessary than are n orm ally req uired for high-sp eed highw ays. W arping of slab s to m eet fixtures (m an holes, drainage inlets, etc.), existing curb and gutter and cross- or side-road co nn ections, m ake m eeting highw ay-standard sm oothness req uirem ents nearly im possible in m any cases. For C alifornia profilograph testing of intersection projects, the acceptable rideability index should be relaxed , an d certain areas should be excluded from m easurem ent. Tho se areas at intersections w hich should be excluded from testing include: acceleration and deceleration tapers, auxiliary (right and left-turn) lanes, sections less than 15 m (50 ft) and locations that require surface w arping that m ake profile testing irrelevan t. For m ore inform ation see R eference 21. For sm allprojects, exclud ed areas, an d odd-shap ed areas, surface testing w ith a 3-m (10-ft) straightedge [3-5 m m (1/8 - 3/16 in.) allow able deviation] w illprodu ce acceptable sm oo thness.
Texturing the Surface — The surface texture necessary for intersection pavem ents dep ends up on the speed lim it of the ap proach 20
road w ays. For low -sp eed residen tial, m unicipal co llector or urban business streets, a b urlap, turf-drag, or coarse broom surface texture is usu ally su fficient to provide the m icrotexture n ecessary for w et w eather stopping .(22 ) H igh -speed [i.e., + 80 km /h (+ 50 m ph)] arterial road w ays also req uire a good m acrotexture to red uce the w ater film thickness eno ugh to prevent hydroplan ing † † † . The texture chosen for the intersection m ust be applied after finishing and before curing the concrete. Either m echanicalor hand -op erated eq uipm ent can ad eq uately apply the texture, how ever, co nfined intersection w ork zo nes m ay lim it the p racticaluse of m echanicaleq uipm ent.
Curing the Concrete — C uring is the treatm en t or protection given co ncrete during the h ardening period. C uring m easures are necessary to m aintain a satisfactory m oisture an d tem perature co ndition in the co ncrete, becau se internaltem perature and m oisture d irectly influence b oth early and ultim ate concrete p roperties. (11 ,14 ) P rop er cu ring m easures prevent rap id w ater loss from the m ixture an d allow m ore tho roug h cem ent hyd ration. Therefore to m axim ize concrete q uality it is necessary to ap ply cu ring m easures as early as possible after placing concrete.(11 ,14 ) C uring is also criticalto providing a d urable p avem ent surface that w illretain surface texture. A variety o f cu ring m ethods and m aterials are availab le for co ncrete p avem en t, including: w ater sp ray o r fog, w et burlap sheets, plastic sheets, insu lating blankets, and liquid-m em brane-form ing com pou nd s. †††
For concrete pa vement, ma crotexture refers to texture ad ded to the surface of the slabs b y mechanical mea ns. All state agency specifications require concrete pavement to have a surface texture that a ids s topping in wet w ea ther. The specific texture varies greatly among agencies, but the statespec ified texture is usua lly mea nt for high s peed highw ays and is commo nly a trans verse tine texture. One draw ba ck to certain transverse tine textures is that they produce high tireroa d noise levels. Fortunately noise g eneration is not se nsitive to the s urface texture a t low speed s, a nd tined, burlapdrag , turf-drag and coa rse broom textures produce similar nois e levels below 55 km/h (35 mph). Long itudinal tining a lso provides a sa fe, quiet and durable texture. The current recommendation (22) for transverse tine dimensions to optimize noise and s kid resista nce a re as follow s: tine depth: 3-6 mm (1/8-1/4 in.); tine w idth : 3 m m (1/8 in.); tine s pa c ing: 10-40 mm (1/2- 1-1/2 in.) rand om a nd va ria ble w ith no mo re tha n 50% exceeding 25 mm (1 in.).
The ap plication of a liquid-m em brane-form ing co m pou nd to sealthe co ncrete surface is the m ost co m m on curing m etho d for co ncrete pavem ent. A liquidm em brane-form ing com pound that m eets A S TM C 309 m aterial req uirem en ts is ad eq uate for m ost situations w hen applied at the follow ing rates:
purpose of insulating fast-track concrete w ith blankets is to aid early strength gain in coolw eather conditions. The blan kets red uce heat loss an d lessen the influence of both air tem perature and solar radiation on the p avem ent tem perature. The b lankets are no t a substitute for curing com pound, w hich is stillneeded to co ntain m oisture for thorough hydration. Tab le 6
1. 5.0 m 2 /L (200 ft2 /gal)for norm alpaving applications. 2. 3.75 m 2/L (150 ft2 /gal)for fast-track concrete. 3. 2.5 m 2/L (100 ft2/gal)for thin overlays. W hite-pigm entation in the co m poun d is p referab le to a clear co m poun d so co verag e is easily seen. The pigm ent also reflects solar radiation that m ay otherw ise heat the co ncrete surface excessively. The first few hours after paving — w hen the concrete rem ains p lastic — are the m ost criticalfor good curing. A s such , the contractor should ap ply a curing com pou nd as soon as p ossible after the w ater sheen has left the surface and texturing is com plete. A variety of sp raying eq uipm en t is availab le, but on m ost intersection projects sim ple h an d sp rayers are the likely choice. The initial ap plication of cu ring co m pound should co at both the top and ed ges of slipform ed concrete. Fo r fixed-form paving, the cu ring co m pound should initially co at the exp osed co ncrete surface. If rem oving form s early, a seco nd co at should be applied to any exp osed verticaled ges o f the slab to provide a co m plete seal. Insulating blan kets also are so m etim es necessary for curing fast-track concrete in intersection w ork. The
Curing blankets moved aside for sawing fast-trac k concrete.
indicates w hen insulation is recom m en ded for fasttrack concrete.(14 ,15 ) N orm al cu ring m easures w ithout insulation are acceptab le w here rapid stren gth gain is not req uired . H ow ever, specialprecautions are n ecessary w hen the intersection is b eing constructed either in very cold or hot w eather. M ore inform ation o n curing, including w et curing, blan ket insulation, an d co ld-w eather and hot-w eather construction techniques, is available in R eferences 1 1, 14 and 15 .
Table 6. Recom mend ed cond itions requiring insulating blankets.(14,15)
Minimum Air Temperature
Opening Time, Hr
During Time Period
8
16
24
36
48
< 10°C (< 50°F)
YES
YES
Y ES
Y ES
NO
10-18°C (50-65°F)
Y ES
YES
YES
NO
NO
18-27°C (65-80°F)
Y ES
NO
NO
NO
NO
> 27°C (> 80°F)
NO
NO
NO
NO
NO
21
J ointing the Paveme nt — A t-grade concrete intersections usually require every joint typ e. The design d etails an d sp ecific p urpose of each typ e are defined in “ Jo inting”on page 4. Typicalco nstruction m etho ds are described below .
Co nstruction J oints — A t intersections, transverse construction joints typ ically are built by h and at predeterm ined locations. This requires a form (header board)that can co ntain the co ncrete, and secure d ow els or tiebars positioned an d aligned properly. V ibration ofco ncrete near the construction joint is im portant to ensure g ood encapsulation of the steelbars. If the construction joint provides a transition from co ncrete to asphalt pavem ent, specialtransition form ing m ay be necessary (see pag e 24, “ C oncrete-to-Asp halt Transition” ).
ap rons and older drivew ay pavem ents, the isolation joint elim inates “ sym pathy”cracking w here it is not possible to m atch the joints in the o ther pavem en t. The contractor m ust position a section of joint filler ag ainst the back o f the curb before placing the co ncrete for new ap rons, drivew ays or w alks. If the new co ncrete pavem en t w illdirectly abut an older concrete pavem ent, the filler m ust rest against the o lder pavem ent before starting construction. A w ider iso lation filler is reco m m end ed betw een the roadw ay pavem ent and an ab utting sidew alk or ap ron, than is recom m end ed betw een sidew alks and apron or drivew ay pavem en t (Figure 8 ).
For either fixed side form s or slipform construction, the slab edge p rovides the longitudinalconstruction joint. The contractor w illpre-position tiebars and keyw ays for fixed-form co nstruction. W hile m ost fixed form s com e w ith pre-drilled holes for the tiebars, the contractor w illprobably have to attach a board to the side form s to m ake a keyw ay. A contractor can equip a slipform paver w ith a toolto form a keyw ay alon g the slab ed ge as the p aver progresses forw ard. W here required, tiebars are inserted into the slipform ed ed ge w hile the co ncrete is p lastic, or after harden ing they can be an ch ored into holes drilled in the pavem en t edge.
Isolation J oints — T- and asym m etricalintersec tions m ay req uire a thickened ed ge or sleeper-slab isolation joint. The thickened edge iso lation joint is usually preferable to a sleeper-slab isolation joint to avoid the ad ditionaltim e necessary to build and cure the sleeper. S pecific site and staging conditions w illdictate w here a contractor positions the isolation joint. The joint filler m aterialm ust set vertically, extend com pletely through the entire slab thickness, and be h eld firm ly in position (usu ally by stakes d riven into the subgrade.) The iso lation joint m aterial is usually a non-ab sorben t foam board or bitum en-treated fiberboard. A w idth from 12-25 m m (0.5-1.0 in.)is adequate. A longitudinaliso lation joint is necessary w herever the pavem ent ab uts sidew alks, drivew ays, or ap rons. The joint w illperm it differentialm ovem ent that m ight otherw ise dam age the pavem ent or curb. A gainst 22
Figure 8 .
Location of isolation joint for curb and gut ter, aprons and driveways near concrete intersections.
Contrac tion J oints — A fter paving an d cu ring the concrete, the finalstep is to place the longitudinaland transverse contraction joints. A lthough there are several m ethods to form these joints in the p lastic concrete, saw ing the concrete after hardening is b y far the m ost com m on m ethod . C ontractors have successfully cut contraction joints u sing w et-,dry-, and early-age-saw ing eq uipm ent. (14 ,23 ) The initial saw cu t provides a plan e o f w eakness w here cracking w illbeg in. U sing co nventionalsaw s, a cut depth of at least one-fourth the slab thickness (T/4) an d 3 m m (1/8 in.)w ide gen erally controls crack form ation for transverse contraction joints. H ow ever, for pavem en t on stab ilized subbases, an increase in the initialsaw cut to a depth equivalent to one-third the slab thickn ess (T/3)is required for transverse con-
traction joints. The extra cut depth accentuates the plan e o f w eakness to overcom e additional frictional restraint and higher curling stresses in the concrete caused by the stab ilized subbase. Lo ngitudinalco ntraction joints req uire a cut depth equivalent to onethird of the slab thickn ess (T/3) regardless of the subbase. The tim e of saw ing is criticalfor proper contraction joint form ation. S aw ing too so on results in spalling and raveling along the joint face. S aw ing too late results in ran dom cracking elsew here in the slab . Joint saw ing w ith co nventionalsaw s should beg in w henever the co ncrete strength is ad eq uate and the saw blades w illnot excessively ravelthe concrete surface. Th is occurs som etim e betw een 4 to 2 4 hou rs after paving, but usu ally w ithin the first 12 hours. W eather (tem perature, w ind, hum idity, an d direct sunlight) has a large influence on concrete strength gain and the optim altim e to b eg in saw ing. The concrete m ixture itself also affects the optim al tim e to beg in saw ing. M ixtures m ad e w ith softer lim estone aggreg ates req uire less stren gth b efore saw ing than d o m ixtures w ith harder co arse aggreg ates. (24 ) Fast-track m ixtures that gain strength q uickly also req uire saw ing to b egin m uch so oner than norm alsetting m ixtures. E arly-ag e saw s allow cu tting after com pressive stren gths reach ab out 1.0 M P a (150 p si)usually ab out an h our or tw o after paving. M ost currently available early-age saw s p rovide a shallow initialcut at about 25 to 3 3 m m (1 to 1 -1/4 in.) deep. T he sh allow cut has been show n to co ntrolcracking effectively at tran sverse joints w hen m ade early, before the finalset ofthe concrete.(23 ) The tim e of saw ing is u su ally not quite as criticalfor longitudinalcontraction joints as it is for transverse contraction joints. H ow ever, longitudinalcontraction joint saw ing should follow closely beh ind saw ing of transverse contraction joints w henever practicable. This w illreduce the possibility of uncontrolled longitudinalcracking.
usually 50-75 m m (2-3 in.) toleran ce on either side of the true center of the d ow els, dep ending up on d ow el length. S aw cuts that are w ithin the tolerance provide the m inim um 15 0 m m (6 in.) of do w elem bed m ent for effective load transfer. The presen ce of tieb ars along the longitudinal co ntraction joint necessitates sim ilar care by the saw operator to cen ter the cut over the steeltiebars. S oon after w et-saw ing , the crew sho uld flush saw ed joints w ith w ater to rem ove saw slurry. If left in place, the slurry w illeven tually harden and becom e m ore d ifficu lt to rem ove. In so m e conditions the hardened slurry m ay even im ped e joint closure during w arm periods.
Opening to Traffic — The basis for deciding w hen to open a concrete intersection to construction or public traffic sh ould be the concrete’ s strength and not an arbitrary tim e from placem ent.(14 ,15 ) S trength directly relates to the pave m en t’ s load bearing capacity. A s slab sup port or pavem ent thickn ess increases, stress in the concrete w illdecrease for a given load. This relationship allow s different opening strength criteria for different pavem ent designs and early traffic load s.(14 ,15 ,24 ) Table 7 provides traffic opening criteria for public vehicles on concrete pavem ent. T he tab le assum es a 0.6-m (2.0-ft) offset of traffic from the lane or pavem ent ed ge. W ide truck lan es, tied co ncrete shoulders, an d curbs and gutters can allserve to red uce load stresses to levels equivalent to a 0.6-m (2.0-ft) traffic offset. If the pavem ent design does not include these features, the contractor can p lace b arricades to prevent ed ge load s. A fter the co ncrete co m pressive stren gth reach es 17 M P a (2500 psi), or flexural strength reach es 3.0 M P a (450 psi), the co ntractor gen erally m ay rem ove the barricades. H ow ever, it m ay b e necessary to w ait for co ncrete to gain full design strength o n thin m unicipalpavem ents.
If the transverse contraction joints contain dow els, the saw operator should reference the m arkers on either side of the slab to determ ine w here the baskets are and w here to position the saw cut. Fo r typicaldow eljointed pavem ents w ith 4.5-m (15-ft)panels, there is 23
Table 7. Strength necessary to open concrete pavement to pub lic traffic (Based on References 14 and 15).
Opening Strength** Flexural (3rd-Point) Compressive*** MPa (psi) MPa (psi)
Slab Thickness
Foundation Support*
150 m m (6.0 in.)
G ranular
3.7 (540)
24.8 (3600)
S tabilized
2.6 (370)
11.7 (1690)
G ranular
2.3 (330)
9.3 (1350)
S tabilized
2.1 (300)
7.6 (1100)
G ranular
2.1 (300)
7.6 (1100)
S tabilized
2.1 (300)
7.6 (1100)
200 m m (8.0 in.)
250 m m (10.0 in.)
*
Granular foundation assumes a Modulus of Subgrade Reaction, k=27 MPa/m (100 psi/in.). Stabilized foundation assumes k=135 M Pa/m (500 psi/in.).
**
Assumes there will be 500 one-way equivalent single axle load (ESAL) repetitions between time of opening and time c oncrete reaches design strength (28-day strength).
***
There was no comp ressive strength criteria in the original research (Reference 14). The values shown here were developed using the cor relation equation desc ribed in this section with C=9.0. It is strongly recom mend ed to develop a unique correlation betw een flexural and com pressive strength for new mixtures.
A co rrelation b etw een co m pressive strength and flex -
im plem ent m aturity on a project, technician s m ust
ural stren gth can b e m ad e in the lab oratory for each
develop a calibration curve in the laboratory. The cali-
un ique m ix. Equation 2 co nverts com pressive
bration cu rve is used to co nvert field co ncrete tem -
strength to third-point flexu ralstrength.
perature m easurem ents to strength values.
fr = C (f’ cr)0.5 •
(25 )
(Eq. 2)
w here: fr = flexu ralstren gth (m odulus of rupture) in thirdpoint loading, M P a (psi). f’ cr = req uired average com pressive streng th, M P a (psi). C = A constant betw een 8 and 10 for norm alm ixtures [for high-strength concrete C ranges from 7.5 to 1 2 (11 .7 reco m m end ed )].
P ulse-velocity is another non-destructive test available for determ ining concrete strength at early ag es. It is a true non-destructive test that m easures the tim e req uired for an ultraso nic w ave to pass throug h co ncrete from one tran sd ucer to another. T he velocity of the w ave correlates to concrete strength or stiffness.(14 ,15 ) Like m aturity testing, pulse-velocity testing requires laboratory calibration to p roduce m eaningful field inform ation. In the laboratory,technicians take
Non-destructive Testing — S om e agencies,con -
pulse-velocity m easurem en ts through a rep resen tative
sultan ts an d co ntractors use non-destructive stren gth
num ber of cast co ncrete sp ecim ens, test the sp eci-
testing to evaluate concrete p avem en t at early ag es.
m ens for strength, and plot the resu lts against the
M aturity an d pulse velocity testing m etho ds are co m -
pulse-velocity readings to create a calibration curve.
m on on fast-track concrete pavem ent projects. (14 ,15 ) N on-destructive test m etho ds m ay b e b etter suited to M aturity testing provides strength evaluation through
evaluate o pening strength o f concrete intersections
m onitoring of internalconcrete tem perature in the
and o ther pavem ent because there is no delay
field. The b asis of m aturity is that each concrete m ix-
betw een sam pling an d testing the concrete. W ith
ture has a unique strength-tim e relationship.
stan dard cylind ers or flexu ralbeam s, sp ecim ens m ust
Therefore, a m ixture w illhave the sam e stren gth at a
be p rep ared , and som etim es tran sported to a testing
given m aturity no m atter w hat co nditions (tim e o r
lab oratory. R eferences 14 and 15 p rovide m ore infor-
tem perature) occur before m easurem ent. (14 ,15 ) To
m ation on no n-destructive test m etho ds.
24
Vehicle Detector Loop Ins tallation — Traffic signaldesign is based largely o n the traffic vo lum es and the geom etrics of the intersection. M ost busily-traveled intersections req uire traffic controlsig nals w ith traffic-sensing d etectors. P resently, the m ost co m m on vehicle d etector is the ind uctive loop detector.(26 ) These detectors installinto saw cuts in the pavem en t surface, or either cast into the co ncrete or fasten to the grad e in preform ed loops.
(0.050 in.)polyester w ire insulation w ith an additional 0.8 m m (0.032 in.) of polyester co ating provides protection from m elting to 204°C (400°F), an d is su itable for hot-applied sealants. C old-applied sealants and epoxies that are sp ecifically form ulated for installing loop detectors also are readily available. The d etector w ire should be flexible enough to give w ith pavem ent m ovem ent, bu t provide eno ugh tension to rem ain in the bottom of the saw cut. A backer rod placed above the w ires is recom m end ed by
Vehicle-detector loops that installinto saw cuts can last for m any years after proper installation. A 6-m m (0.25-in.) w ide saw cu t to a d ep th of 50 m m (2 in.) is necessary to recess the d etector below the p avem ent surface. F igu re 9A show s three co m m on configu rations. A fter saw ing, detector system m an ufacturers recom m end flush ing the saw cuts w ith w ater to rem ove saw slurry, then u sing co m pressed air to rem ove d eb ris that m ay p uncture the w ire insulation. R ounding the corners of diag onal or rectan gular loops w ith additional saw cu ts or 18-m m (0.75-in.) diam eter core holes w illease insertion of the detector w ire and allow the w ire to rem ain m ore flexible, preventing rupture.
detector system m an ufacturers to ensure the w ires rem ain in place. P reform ed loo ps can be cast into con crete and do no t req uire saw ing. In a p reform ed loop, P V C pipe en capsulates the detector w ires for protection an d provides rigidity to the loop during installation. The loops m ust be fasten ed secu rely into position before paving at a m inim um of 50 m m (2 in.) ab ove any reinforcing steel. If the slab contains w elded w ire fabric or bar m ats, the p ipes should not align w ith the reinforcem ent grid (Figure 10). A ny reinforcing steelthat aligns w ith the pipes w illinterfere w ith the inductive loop.
D etector system m anufacturers (26) recom m end installing 16 A W G stran ded w ire w ith a coating suit-
The d etector w ires o ften break near the conduit that
ab le for the sealan t. A n outer jacket of 1.25 m m
bring s them to a signalhandho ld or the signalco ntroller cabinet. To avoid breakage, it is advantageous to core drillthis location to provide a larger recess (Figure 9B ).
Figure 10. Fig ur e 9 .
Ve hic le d et ec t or lo o p c on fig u ra tio n s.
Aligning preformed vehicle loop detectors above reinforcing steel.
25
Concrete-to-Asphalt Transition — Concrete to Asphalt Transition Details
The transition b etw een a co ncrete p avem ent intersection and an asp halt pavem ent can be troub leso m e if poorly designed . Figure 1 1 show s four tran sition details for different concrete p avem ents. D etails A , B an d C acco unt for im pact load s on the tran sition slab w ith extra thickness (D etails A an d B are for co ncrete overlays of existing asp halt pavem ent). D etailD sho w s an im pact slab and is m eant for pavem en ts that freq uen tly carry heavy trucks and are thicker than 175 m m (7 in.). The im pact slab protects the asp halt pavem ent from deform ation b y providing additionalsupport at the transition. C ontractors can easily create the lip in the im pact slab using a false form header. O ne w ay to keep the slab s near the tran sition from m igrating on g ranular subbases is to place d eform ed tiebars in the first three transverse contraction joints. The tiebars w illhold the slabs tightly together. S lab m igration is not an issue w hen w hitetopping, as the co ncrete w illbond to the asphalt surface.
Adding Lanes to Existing C oncrete Pavement — S om e intersection im provem en ts req uire additional co ncrete lanes next to existing co ncrete pavem en t or curb an d gutter. Fo r these im provem en ts it is essen tialto place transverse contraction joints (Typ e A -1 o r A -2, Fig. 2) to m atch any existing joints or cracks in the existing p avem en t (Figure 12). W ithout a contraction joint, m ovem ent of the old co ncrete p avem en t m ay cause a sym pathy crack in the new lanes. A n alternative m etho d to avo id sym pathy cracking is to place an iso lation joint w ith a separating m edium (Type D -4, Fig. 2)betw een the edge o f the existing pavem ent, or curb and gutter, and the new lane.
Figure 11.
26
Transition details for concrete pavement to asphalt pavement.
Figure 12.
Aligning joints for adding auxiliary turn lanes to existing JRCP conc rete pavements.
References 1.
“ D rivew ay and Street Intersection S pacing,” Transpo rtation Research C ircular , N o. 456,
Tran sp ortation R esearch B oard, N ationa lR ese arch C ou ncil, W ashing ton , D C , M arch 199 6. 2.
A Policy on Geom etric Design of Highw ays and Streets ,
A m erican A ssociation of S tate H igh w ay and Tran sp ortation O fficials, W ashington, D C , 1994. 3.
, IS 184P, Design of Concrete Pavement s for City Streets A m erican C oncrete P avem ent A ssociation , A rling ton H eights, IL, 1992.
4.
Guide for the Design of Pavement Struc tures , A m erican
A sso ciation o f S tate H ighw ay and Tran sp ortation O fficials, W ash ing ton, D C , 1993. 5.
Thickness Design for Conc rete Highway and Street Pavements , E B 10 9P , P ortland C em ent A ssociation ,
S ko kie, IL, 1984. 6.
Design and Construction of Joints for Conc rete Highways , TB 01 0P , A m erican C on crete P avem ent
A ssociation, A rlington H eights, IL, 1991. 7.
Design and Construction of Joints in Concrete Streets ,
IS 06 1P , A m erican C on crete P avem ent Asso ciation , A rlington H eights, IL, 1992. 8.
Joint and Crac k Sealing and Repair for Conc rete , TB 01 2P , A m erican C on crete P avem ent Pavements
A ssociation, A rlington H eights, IL, 1993. 9.
S m ith, K .D ., an d others, “ P erform an ce of C oncrete P avem ents, E valua tion of In-S ervice C onc rete P avem ents,”Volume 1 - Final Report , D TFH -61-91-C 00 053 , FederalH ighw ay A dm inistration , W ashing ton, D C , A pril19 95.
10. Intersection Joint Layout , IS 00 6P , A m erican C on crete P avem en t A sso ciation, S ko kie, IL, 1996. 11. P an arese, B ., K osm atka, S ., “ D esign an d C ontrolof C onc rete M ixtures,”13th E dition, E B 001TC , P ortland C em en t A sso ciation, S ko kie, IL, 1994. 12. Guide Spec ification for Co ncrete Sub ject to Alkali-Silica Reactions , IS 41 5TC , P ortland C em ent A ssociation an d A m erican C on crete P avem ent A ssociation, S kokie, IL, 1995.
14. Fast-Track Concrete Pavements , TB 00 4.02 P , A m erican C oncrete P avem ent A ssociation , S kokie, IL, 199 4. 15. Acc elerated Rigid Paving Techniqu es: State-of-th e-Art Report (Special Project 20 1) , FH W A -S A -94-080, FederalH igh w ay A dm inistration , W ashing ton, D C , D ecem ber 1994. 16. Whitetopping - State of the Practice , E B 21 0P , A m erican C oncrete P avem ent A ssociation, S kokie, IL, 19 97. 17. G rove, J., an d others, “ B ond C ontribution to W hitetop ping P erform ance o n Low V olum e R oad s,” Transpo rtation Research Record 13 82 , Transp ortation R esearch B oard, N ationalR esearch C ou ncil, W ashington, D C , 19 93 , pp . 10 4-11 0. 18. Guide Specifications for Highw ay Const ruction , A m erican A ssociation of S tate H igh w ay and Tran sp ortation O fficials, W ashington, D C , 1993. 19 . Construction of Portland Cement Conc rete Pavements Participant’s Manual , FH W A H I-96 -02 7, N ation al H ighw ay Institute, Fed eralH ighw ay A dm inistration, W ashing ton, D C , 19 96 . 20 . “ Th e 19 96 P ub lic W orks M anual,”Public Works , V ol. 127. N o. 5, P ub lic W orks Jo urnal C orporation, R idg ew oo d, N J, A pril15 , 19 96 . 21. Constructing Smooth Concrete Pavements , TB 006 P, A m erican C on crete P avem ent A ssoc iation , A rling ton H eights, IL, 1990. 22. H ibbs, B ., Larson , R ., “ Tire P ave m en t N oise and S afety P erform ance,”PCC Surface Texture Technical Working Group , FH W A -S A -96 -06 8, Fed eralH igh w ay A dm inistration , W ashing ton , D C , M ay 1 99 6. 23. Z ollinger,D ., an d others,“ S aw cu t D ep th C onsiderations for Jointed C oncrete P avem ent B ased o n Fracture M echanics A nalysis,”Transport ation Research Record 1449 , Tran sportation R ese arch B oard, N ational R esearch C ou ncil, W ashing ton , D C , 19 95 , pp. 91 -10 0. 24. O kam oto, P ., an d others,“ G uidelines for Tim ing Jo int S aw ing and E arliest Lo ad ing for C on crete P avem ent,” Volume 1 - Final Report , FH W A -R D -91 -07 9, Fed eral H ighw ay A dm inistration , W ashing ton , D C , February 1994. 25. “ H igh S tren gth C oncrete,”ACI Manual of Concrete
13. Diagnosis and Con trol of Alkali-Aggreg ate Reactions in Concrete , 1st E dition, IS 413TC , P ortlan d C em ent A ssociation and A m erican C on crete P avem ent A ssoc iation and N ationalR eady M ixed C on crete A ssociation an d N ationa lA ggreg ates A ssociation, S kokie, IL 19 97 .
Practice, Part 1, M aterials and G eneral Prop erties of Concrete , A C I363R -84, A m erican C onc rete Institute,
D etroit, M I, 1990. 26. “ Loop Installation,”Detector Systems Technical Notes , D etector S ystem s, Inc., S tanton , C A , 19 90 .
27
Equivalent Canadian Standards ASTM Designation
ASTM Title
CSA/CAN Equivalent or Specified Test
A S TM C 33
S tandard S pecification for C oncrete A ggregate
A 23.1
A S TM C 39
TestM ethod for C om pressive S trength of C ylindrical C oncrete S pecim ens
A 23.2-9C
A S TM C 78
Test M ethod for FlexuralS trength of C oncrete (U sing S im ple B eam w ith T hird-P oint Lo ad ing )
A 23.2-8C
A S TM C 1 09
Test M ethod for C o m p ressive S trength of H yd raulic C e m ent M ortar
A5
A S TM C 150
S tandard S pecification for P ortland C em ent
A5
AS TM C 260
Standard Specification for Air-Entraining Adm ixtures for C oncrete
AS TM C 260
A S T M C 3 09
S tand ard S p ecificatio n fo r Liq uid M em b rane-Fo rm ing C o m p ound s for C uring C oncrete
A S T M C 3 09
A S TM C 494
S tand ard S pecification for C h em icalA dm ixtures for C oncrete
A S TM C 494
A S TM C 597
Test M ethod for P ulse V elocity through C oncrete
A S TM C 618
S tandard S pecification for Fly A sh and R aw or C alcined N atural P ozzolan for use as a M ineral A dm ixture in P ortlan d C em ent C oncrete
AS TM D 698
Laboratory C om paction C haracteristics ofSoilU sing Standard Effort
AS TM D 698
A S TM C 8 03
Test M ethod for P enetration R esistance of H ard ened C oncrete
A S TM C 8 03
A S TM C 805
Test M ethod for R ebound N um ber of H ardened C oncrete
A S TM C 805
A STM C 900
Test M ethod for P ullout S trength of H ardened C oncrete
A S TM C 900
A 23.2-24C A 23.5
A S T M C 1 0 17
S tand ard S p ecificatio n fo r C h em icalA d m ixtures fo r P ro ducing Flow ing C oncrete
A S T M C 1 0 17
AS TM C 1074
Practice for Estim ating C oncrete Strength by the M aturity N um ber
AS TM C 1074
A S TM C 1150
S tandard Test M ethod for the B reak-O ffN um ber of H ardened C oncrete
A S TM C 1150
A STM D 4791
Test for Flat or Elongated Particles in C oarse A ggregate
C R D -C 119
This publication is ba sed o n the fac ts, tests , and a uthorities state d herein. It is intended fo r the use of professiona l personne l co mpetent to evaluate the significance and limitations of the reported findings and who will accept responsibility for the application of the material it contains. Obviously, the America n Conc rete Pa vement Ass oc iation disc laims any and all respons ibility for ap plica tion of the sta ted principles or for the acc urac y of a ny of the s ources o ther than work performed or information d eveloped by the Asso ciation.
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