CONCRETE PAVEMENT CHARACTERISTICS AND BEHAVIOR The behavior of concrete pavement that is expose to loading and environmental effect entirely depends upon the:
Quality of concrete Underlying sub-grade Base course
Concrete
-
-
-
-
strong in resisting compression load acting on it, but considerably weak in resisting tensile stress. also expand and contract due to temperature changes. It expands when wet and contracts when dried. After pouring, concrete shrinks as the mortar hardens and the cement hydrates Concrete pavement changes in length with time of day for being exposed to different elements of weather changes A curl curl tendency is very likely due to the effect of daily and seasonal temperature and moisture differences between the top and bottom of slab.
CONCRETE PAVEMENT DESIGN ASSUMES THE FOLLOWING CONSIDERATIONS CONSIDERATIONS 1. That the pavement slab was designed as plain concrete beams. 2. That, transverse cracks on the concrete pavement cannot be avoided. The designer however, presumes that the pavement cracks could be controlled, by providing reinforcement to the slab joints with the following assumptions. a.) With reinforcement, cracks on the slab will be confined to a weekend plain joints spaced at 4.50 to 6.00 meters distance. b.)Vertical b.) Vertical offsetting across the narrow cracks will be prevented by aggregate interlock, or by dowel bars. c.) With simply reinforced slab, cracks will only appear at weakened plane joints spaced at 12-20 meters interval. Hair cracks that can be held tightly by the steel between joints.
d.) With continuous reinforcement, transverse joints omitted. Hair cracks are checked by the steel and developed at close intervals. e.) Faulting is countered by aggregate interlock and steel bars. 3. The longitudinal cracks on the pavement slab more than one lane wide are inevitable. 4. Pavement slab is supported by foundation that deflects when loaded but recover when the load is removed, assuming that the foundation materials are elastic or like a dense liquid. DEFINITION OF TERMS Deterioration Deterioration of concrete pavement is due to stress brought about by 1 rad, moisture and temperature. Distress of concrete is concrete is generally grouped into the following categories: a. Distortion b. Cracking c. Disintegration Distortion Distortion is a vertical displacement of concrete slab at the joint of the cracks. Distortion is due to failure or weakness of concrete joints. Faulting Faulting is the result of pumping tremendous force or load that developed under the pavement. For faulting to occur, there must be free water on the top of the base course and pavement deflection across the joint due to heavy axle loads. Causes of faulting : a. Loss of slab support b. Erosion of sub base Cracking Cracking can take many forms in concrete pavement that could be the result from; applied load, temperature or moisture changes The most common type of cracks : a. Corner cracks associated with excessive corner deflection.
b. Transverse cracks associated with mixture or temperature stress, or poor construction methods. Disintegration appears in the form of durability cracking, scaling or spalling, as the result of mix design or construction related problems like: a. Durability cracking. Result from freezethaw action b. Scaling. A network of shallow fine hairline cracks which extend through the upper surface of the concrete. This is the result from deicing salts, improper construction, freeze-thaw cycle, or steel reinforcement too closed to the surface c. Spalling is the breaking or chipping of the joint edges. It is the result from excessive stresses at joint, weak concrete, poorly designed or constructed joints Changes in temperature and moisture content create slab curling, flexure stresses and overall lengthening and shortening of the slabs. The tendency of the slab to shorten is due to temperature drop or dying that create tensile stresses. On the other hand, the tendency to lengthen is due to temperature rise or increased in moisture that creates compression stresses. TRANSVERSE EXPANSION JOINTS Expansion Joints provide space allowance for the lengthening of slab due to expansion. Because of the many buckling upward of concrete pavement, Engineers have come up with a conclusion that these blowups serves as conclusive evidence that expansion joint is necessary.
22.5
32
.45
.30
25.0
32
.45
.30
27.5
32
.45
.30
30.0
32
.45
.30
Source : AASHTO Interior Guide & Proposed Revision 1980
FIGURE 8-1 BASIC TYPE OF CONCRETE PAVEMENT JOINTS LONGITUDINAL JOINTS Longitudinal joints are provided between adjacent traffic lanes. It is considered as hinges to provide edge support, but allows rotation between the slabs. By this joint, flexural stresses that might cause irregular cracks along the length of the road are relieved or neutralized. Longitudinal joints cannot be considered as a major problem under the following assumptions: 1. That there is no big load transfer across it. 2. That, the expansion and contraction movement developed across the pavement width is very small.
TABLE 8-2 RECOMMENDED DMENSIONS OF DOWELS FOR CONCRETE PAVEMENT JOINTS Pavement Thickness cm.
Dowel Diameter mm
Dowel Length meter
Dowel Spacing meter
15.0
20
.45
.30
17.5
25
.45
.30
20.0
25
.45
.30
FIGURE 8-2 LONGITUDINAL JOINT LOAD TRANSFER 3. When lanes are constructed at different time using side forms, the joints are provided with
key way in the first slab to accept load transfer. 4. For longitudinal joint, deformed tie bars are used because the purpose is to hold the slabs tightly together, rather to allow the joints to open and close. 5. The diameter and spacing of tie bars are based on the forced needed to pull the narrow pavement slab over the sub-grade to the joint. 6. The length of tie bar is determined from the embedment inside the concrete necessary to develop the strength of the bar.
CONSTRUCTION JOINT If concrete pouring will be interrupted for quite some time that cold joint will be inevitable, the practice is to provide a transverse construction joints. Deformed tie bars are used to hold the joint tightly closed together. However, if the construction joint replaces a contraction joint, the use of dowels is the alternative. Construction joints and cracks should be cleaned and sealed to prevent infiltration of water to the sub-grade and to keep dirt out of the joints. Materials for such purpose includes harder paving and air blown asphalt sometimes mixed with mineral filler, rubber asphalt, and various rubber compounds. They are poured hot and stiff, then cooled, and others are placed cold. There are some pre-formed sealant made of strips of extruded neoprene compressed for insertion into the groove joints, the sealant will expand and fill the space completely. REINFORCEMENT OF JOINTS Steel reinforcement for concrete pavement joints is specified in the design to prevent the widening of cracks produced by shrinkage or thermal contraction.
The reinforcement holds the fractured faces on rigid contacts preserving the aggregate interlock and the intrusion of dirt or water. These reinforcing bars, however, is not intended to resist flexural stress being produced by loads or curling. The reinforcing steel bars are mounted in one layer along the mid-depth of the slab. The formula used in designing this reinforcement for concrete slab joint is” =
Where: As = Area of steel cross section per foot of slab. L = Length of slab between joints in feet. f = Coefficient of friction between the slab and the sub-grade called the coefficient of sub-grade resistance ranging from1 to 2 with 1.5 recommended by AASHTO Interim Guide. S = Working stress in the reinforcing steel in pounds per sq. in. AASHTO Interim Guide suggested working stress from 30,000 to 45,000 psi depending upon the type and grade of steel. Wielded fabrics are also used as reinforcements for concrete pavement made from cold drawn steel wired having the following properties: Minimum allowed tensile strength
80,000 psi
Yield Strength
70,000 psi
Reinforcing bars of billet, rail or axle steel yield strength among the from
40,000 to 75,000 psi.
The SUB-GRADE and SUB-BASE for Concrete Pavement The construction standard for sub-grade and embankment are precise and almost typical for all pavement types. Under the concrete pavement literature, the under course of selected material is always imposed upon between the sub-grade or embankment and the concrete slab. SUB-GRADE
the natural ground grade and compacted
-
on which the pavement is built. Preparation of the subgrade includes: 1. Compacting soils at moisture contents and densities that will ensure uniform and stable pavement support. 2. Whenever possible, setting gradelines high enough and making side ditches deep enough to increase the distance between water table and the pavement. 3. Crosshauling and mixing of soils to achieve uniform conditions in areas where there are abrupt horizontal changes in soil type. 4. Using selective grading in cut and fill areas to place the better soils nearer to he top of the final grade elevation. 5. Improving extremely poor soils by treatment with cement or lime, or importing better soils, whichever is more economical. SUB-BASE
-
defined as the layer of material that lies immediately below the concrete pavement.
When the use of a sub-base is considered appropriate, the best results are obtained by: 1. Selecting sub-base materials that meet minimum requirements for preventing mudpumping of sub-grade soils. 2. Specifying gradation controls that will ensure a reasonably constant sub-base gradation for individual projects. 3. Specifying a minimum sub-base depth of 4 in. 4. Specifying a minimum density for untreated sub-bases of 105 percent of AASHTO T99 for heavily travelled projects. 5. Specifying a cement-treated or lean concrete sub-base that provides a strong and uniform support for the pavements and joints; provides an all-weather working platform; and contributes a smoother pavements by giving firm support to the forms or paver during construction.
6. Specifying a permeable sub-base for pavements carrying high volumes of heavy trucks for which past experience indicates the potential for pavement faulting and pumping. PUMPING
-
the ejection of water and sub-grade soil through the joints and cracks along the edges of the concrete pavement.
CONCRETE PROPORTIONS The fundamental rule to obtain good concrete is the proper selection of cement aggregate and water thus: 1. Type 1 or II cement is specified for concrete pavement 2. Water for concrete must be clean, free from acids, alkali and oil. Water that is suited for drinking purposes is acceptable for mixing cement except water containing large amount of sulfate. 3. If concrete is to be strong, sound and durable, the aggregate must have similar properties. 4. The mineral aggregate of concrete is about 75% of the volume or about 80% of the weight of normal pavement 5. The maximum size of coarse aggregate is 2 inches. The use of larger aggregates, according to some experienced highway engineers, increases its length and durability. With larger aggregate, less water is needed thereby, increasing the ratio of cement to water. Under the cement-ratio principle, concrete is stronger when water content is less. ADMIXTURE A substance added in mixing to change the characteristic of concrete mixture. There are varieties of admixtures available – the air- entraining admixture, which is very common. Others, like water reducer, retarder, accelerator, Pozzolan ad plasticizer are also used. AIR ENTRAINMENT The entrapment of air in the concrete mixture in the form of evenly distributed small
bubbles. It is used to increase the concrete resistance to surface scaling caused by increase the concrete resistance to surface scaling caused by deicing with calcium or sodium chloride. Another working advantage of air entrainment is improving its workability and reduces bleeding in fresh concrete.
aggregate and a less fluid cement-water paste can be used.
The effectiveness of air entrainment is to increase the concrete durability that is influenced by:
2. Ascertain the aggregate is uniformly graded
1. Percentage of air present in the mixture. 2. Grading of aggregates. 3. Size and distribution of air bubbles. CEMENT and WATER RATIO The strength and other desirable properties if concrete mixture varies depending upon the ratio of concrete to mixing water. A non-airentrained concrete with a water cement ratio by weight of 5 gallons of water per bag of cement may have a compressive strength of about 5,300 psi in 28 days. On the other hand, a mixture of concrete with 7 gallons of water per bag of cement has developed strength of 3.700 psi only for 28 days. Durability is dependent on the water-cement ratio that should be properly controlled to obtain the richness of the paste. AASHTO Guide specification for highway construction established the max. water cement ratio at 6 gallons per bag of cement on normal conditions and 5 ½ gallons per bag of cement fir severe atmospheric conditions.
On how to reduce the amount of cementwater paste and the cost of the mixture we have to: 1. Allow the larger size of aggregate that can be accommodated in the pavement slab. from coarse to fine. 3. Avail of the biggest quantity of coarse aggregate consistent with proper workability. 4. Adopt the lowest slump consistent with the proper pouring and finishing. Slump Test - is the old traditional and most widely used method in determining the consistency of concrete. - A Truncated Cone of metal sheet 12 inches high with base and top diameters 8 inches and 4 inches is filled 3 layers with fresh concrete. - Each layer is rodded 25 times. Then the cone is lifted off vertically, allowing the concrete to subside. The slump is the height in inches that the top of the specimen falls.
CONCRETE MIXTURE The objective in mixing concrete is to use more aggregates and as a little cement as possible, while maintaining the workability necessary for a successful pouring and consolidation. For structural concrete poured in inaccessible small areas around reinforcing bars, it is necessary to over-fill the voids or spaces around the aggregate and reinforcement using free flowing cement in water paste form. On the contrary, pavement where the slab is open and thin with an access to manipulate from the surface, a drier mixture can be poured inside the form with ease and success. Thus, a higher percentage of
Kelly Ball Method - a metal cylinder 15 cm diameter and 12 cm high with hemisphere bottom shaped weighing 14 kg. - a graduated handle rising from the top of the ball passes through a metal frame that is 30 cm apart. The ball is placed on the surface of the fresh concrete. Its penetration is measure by comparing its position with that of the frame. - The advantage is that reading could be taken immediately on the concrete being poured on the roadway. On the contrary, the slump test requires
more time but could be conducted only on selected samples.
constructed on the prepared base in accordance with the plans and specifications . “ MATERIAL REQUIREMENTS PORTLAND CEMENT - shall conform to the applicable requirements of Item 0-700, hydraulic cement. Only type 1 or the normal or common Portland cement should be used. Different brands or the same brands from different mills shall not be mixed nor shall they be used alternately unless approved by the supervising engineer.
CURING of CONCRETE PAVEMENT Newly placed concrete pavement needs curing. Curing may be accomplished by several methods but basically; all the methods could be categorized into two: 1. Those that keep the surface constantly wet or cover it with water absorbent material that is rewetted from time to time. 2. Those that pavement evaporated from the water already in the concrete which is retained is sufficient enough for hydration. The chemical action between cement and water produces strength of concrete. If the concrete dries out quickly, hydration and strength processes will stop, but when moisture becomes available during hydration, strength gain will continue. The shortest period of curing for normal concrete is 5 days according to AASHTO recommendation.
DPWH SPECIFICATIONS PAVEMENT
ON
CONCRETE
Concrete pavement is categorized under item 311 of DPWH standard specifications, which provides that: “ This item shall consist of Portland cement Concrete pavement with or without reinforcement,
FINE AGGREGATE 1. Fine aggregate shall consist of natural sand, stone screening or other inert materials with similar characteristics or combination thereof, having hard, strong and durable particles. It shall be free from injurious amounts of organic impurities. 2. Fine aggregates from different sources of supply should not be mixed or stored in the same pile nor used alternately. 3. Fine aggregates should not contain more than 3 mass percent of materials passing the 0.075mm (No. 200) sieve by washing nor more than one mass percent each of clay lumps or shale. 4. The use of beach sand will not be allowed without the approval of the supervising engineer. 5. If the fine aggregate is subjected to 5 cycles of the sodium sulfate soundness test, the weighed loss should not exceed 10 mass percent. 6. If fine aggregate is subjected to test for organic impurities and a color darker than the standard is produced, it should be rejected. However, when tested for the effect of organic impurities on the strength of mortar by AASHTO T-71, the fine aggregate may be used if the relative strength at 7 and 28 days is not less than 95 mass percent.
Table 8-4 aggregates
grading
Sieve Designation
requirements
for
Mass Percent
fine
10.0 mm 3/8”
100
4.75 mm No. 4
95-100
1.18 mm No. 16
45-80
0.30 mm No. 50
5-30
0.15 mm No.
0-10
50.0
2”
-
90 100
90 100
37.5
1 – ½”
25 - 60
35 - 70
-
25.0
1”
-
0 - 15
35 - 70
20.0
¾”
0
-
-
12.0
½”
0-5
0-5
10 - 30
4.75
No. 4
-
-
0-5
100
COARSE AGGREGATE 1. Coarse aggregate shall consist of crushed stone, gravel, blast furnace slag, or other approved inert materials of similar character or combination thereof, having hard, strong durable pieces, free from any adherent coatings 2. Coarse aggregate should contain not more than one mass percent of material passing the 0.075mm (No. 200) sieve nor more than 0.25 mass percent of clay lumps, not more than 3.5 mass percent of soft fragments. 3. If the coarse aggregate was subjected to 5 cycles of the sodium sulfate soundness test, the lost weight should not exceed 12 mass percent. It should have a mass percent of wear not exceeding 40 when tested by AASHTO T-96. 4. If slag is used, its density should not be less than 1120 kilogram per cubic meter. The gradation of the coarse aggregate should conform to Table 8-5. Only one grading specification should be used from any one source.
Table 8-5 grading requirements for coarse aggregate
1 0
WATER Water to be used in concrete mixing or curing or other designated applications should be clean and free from oil, salt, acid, alkali, grass or other substances injurious to the finished product. Drinking water if used needs to be tested. REINFORCING STEEL The specifications state that: dowels and tie bars to be used in concrete pavement shall conform to the requirements of AASHTO M-31 or M-42 except that rail steel shall not be used for tie bars to be bended and re-straightened during construction. Specifications further provide that: 1. Tie bars shall be deformed bars. 2. Dowels shall be plain round bars delivered to the site with one half of each dowel length painted with one coat of approved lead or tar paint. 3. The sleeves of dowels shall be metal of approved design to cover 50mm plus or minus 5mm of the dowels, with a closed end, with a suitable stop to hold the end of the sleeve at least 25mm from the end of the dowel. 4. Sleeve shall be of such design that they do not collapse during construction.
Mass Percent Passing Siev e mm 75.0
Designatio n Inch 3”
63.0
2 – ½”
Grading A
Gradin g B
Gradin g C
100
-
-
90 - 100
100
100
JOINT FILLERS - Joint fillers should be mixed asphalt and mineral or rubber filler. The pre-formed joint fillers are punched to admit the dowels. Filler for each joint should be furnished in a single place for the full depth and
width of the joint
the cement content and the proportions of aggregate and water that will produce a
STORAGE OF CEMENT AND AGGREGATE - The storage house for cement should be waterproof with raised floor from the ground to protect the cement from rain or dampness.
1. The provisions for storage should be ample enough and the shipment of cement as received is separately stored in such a manner as to allow the earliest deliveries to be used first and to provide easy access for identification and inspection of each shipment. 2. Storage house must have the capacity to accommodate sufficient quantity of cement to allow sampling at least 12 days before the cement is used. 3. To secure uniformity of concrete mixture, the coarse aggregate are separated into two or more sizes. Different sizes of aggregates are stored in separate bins or in separate stockpiles sufficiently remote from each other to prevent the materials at the edge on the piles from becoming intermixed
PROPORTIONING, CONSISTENCY AND STRENGTH OF CONCRETE
workable concrete having a slump of between 40 and 70 mm (11/2” to 3”) if not vibrated, or between 10 to 40 mm if vibrated and flexural strength of not less than 3.8 Mpa (550 psi) when tested by the third-point method or 4.5 Mpa (650 psi) when tested by the mid-point method or a compressive strength of 24.1 Mpa (3,500 psi) when tested at fourteen days.
3. The designer should consider the use of lean concrete (econo-concrete) mixture using local materials or specifically modified conventional concrete mixture in base course and in the lower course of composite, monolithic concrete pavements using a minimum of 75 mm of conventional concrete as the surface course.
QUALITY CONTROL OF CONCRETE For quality control of concrete in general, the DPWH specifications provides that:
leaner or richer mixture may be used in
“The contractor shall be responsible for the quality control of all materials during the handling, blending, mixing and placement operations. The contractor shall furnish the engineer a Quality Control Plan detailing the production control procedures and the type and frequency of sampling and testing to insure that the concrete produced complies with the specifications. The supervising Engineer shall be provided free access to recent plant production records, and if requested informational copies of design, materials certifications and sampling and resting reports. “
order to meet the minimum strength
REQUIRED QUALIFICATION OF WORKMEN
Prepare a design mixture based on the absolute volume method as specified in the American Concrete Institution (ACI) standard “Recommended Practice for selecting Proportion for normal and heavyweight concrete”. 1. The intent of this specification is to require approximately 9.0 bags of cement per cubic meter of concrete based on a 40 kg weight per bag of cement. However,
requirements. 2. The engineer will determine from the laboratory tests. The materials to be used,
Concrete Butcher – The person performing the batching of mixing operation, capable of accurately conducting aggregate surface moisture
determinations and establishing correct scale weight for concrete materials. Concrete Technician – The person responsible for concrete production control and sampling and testing for quality control proficient in concrete technology having a sound knowledge of the specifications as they relate to concrete production. He shall be:
Capable of conducting test on concrete and concrete materials in accordance with the specifications.
Capable of adjusting concrete mix designs for improving
workability
and
specification
compliance and preparing trial mix design.
He shall be qualified to act as the concrete batcher in the absence of the batcher.
and thoroughly re-rolled or tamped. Imperfections or variations above the grade should be corrected by tamping or by cutting as necessary. 2. Grading and Alignment – form shall be set sufficiently advance from the point where the concrete is being placed. After setting to correct grade, the base is thoroughly tamped, mechanically or by hand, at both edge of the form base inside and outside. The forms should not deviate from the true line by more than one centimeter at any point. 3. Grading and Alignment – the alignment and grade elevation of the forms should be checked
and
corrections
be
made
immediately before the placing of concrete. Prior to the placing of concrete, the crown
PREPARATION OF GRADE
and elevation are verified by holding an
After the base or sub-grade have been placed and compacted to the required density, the areas that will support the paving machine and the grade on which the pavement is to be constructed should be trimmed to the proper elevation by means of a properly designed machine extending the work at least 60 cm beyond each edge of the proposed concrete pavement.
approved template in a vertical position
If loss of density results from the trimming operations, it should be restored by additional compaction before concrete is placed. If any traffic is allowed to use the prepared sub-grade or the surface, it should be checked and corrected immediately ahead of the placing of concrete. The sub-grade or base should be uniformly moistened when the concrete is placed. Setting of Forms 1. Base Support – The foundation under the forms should be hard and true to grade, so that the form when set will be firmly in contact with its whole length at the specified grade. Any roadbed, which is below the established grade, should be filled with approved granular materials to grade in lifts of three centimeters or less,
moving backward and forward on the forms. HANDLING, MEASURING AND BATCHING OF MATERIALS The batching plant and equivalent layout must provide a smooth and flow of continuous supply and transport of materials to the work. Stockpiles are built up in layers of not more than one meter in thickness with each layer completely in place before beginning the next that should not be allowed to “cone” down over the next lower layer. All washed aggregates and aggregate produced or handled by hydraulic methods are stockpiled or binned for draining at least twelve hours before being batched. The mixer should be charged without loss of cement and batched material should be charged without loss of cement and should be weighed for each material required within a tolerance of 1% for cement and 2% for aggregates. Water may be measured by volume or by weight and the accuracy of measuring water shall be within a range of x > 1% error.
Mixing Concrete The concrete may be mixed at the site, in a central plant or by truck mixers of approved type and capacity. Mixing time will be measured from the time when all materials except water are already inside the drum. 1. Ready mix concrete shall be mixed and delivered in accordance with ASSHTO M-157 requirements, except that the minimum required revolutions at the mixing speed for transit mixed concrete may be reduced to not less than that recommended by the mixers manufacturer. The number of revolutions recommended by the mixer manufacturer should be indicated on a serial plate attached to the mixer. 2. When mixing is done at the site or in a central mixing plant, the mixing time should not be less than 50 seconds nor more than 90 seconds, unless mixing performance tests provide adequate mixing of the concrete in a shorter time period. Mixing time ends when the discharge chute of the mixer opens. The contents of the individual mixer drum shall be removed completely before a succeeding batch is loaded therein. 3. The volume of concrete mix per batch should not exceed the mixers nominal capacity in cubic meter, as indicated on the manufacturers standard rating plate attached on the mixer except that an overload up to 10 % above the mixers normal capacity may be permitted provided that concrete test data for strength, segregation and uniform consistency are satisfied and no spoilage of concrete should take place. 4. The batches shall be charged into the drum with a portion of the mixing water enter in advance of the cement and aggregates. The flow of water should be uniform that all water shall be inside the drum at the end of the first 15 seconds of the mixing period. 5. The throat of the drum shall be kept free of concrete accumulation that may restrict the free flow of materials into the drum. 6. Mixed concrete from the central mixing plant shall be transported in truck mixers, truck
agitators, or non-agitating trucks. The time elapsed from the time water is added to the mix until the concrete is deposited in place at the site shall not exceed 45 minutes when the concrete is hauled in non-agitating trucks, nor 90 minutes when hauled in truck mixers or truck agitators, except that in hot weather or under conditions contributing to quick hardening of concrete by the supervising Engineer. 7. Re-tempering concrete by adding water or by any other means shall not be permitted, except that when concrete is delivered in truck mixers, additional water may be added to the batch materials and additional mixing is perform increasing the slump to meet the requirements if permitted by the Engineer, provided that all these operations are performed within 45 minutes after the initial mixing operations and the water cement ration is not exceeded. Limitation of Mixing No concrete should be mixed, placed or finished, when natural light is insufficient, unless an adequate and approved artificial lighting system is operated. PLACING OR DEPOSITING OF CONCRETE Concrete is deposited in such a manner requiring minimal re-handling. Unless truck mixers or non-agitating hauling equipment are equipped with a means to discharge concrete without segregation of the materials, the concrete should be unloaded inside the form in a manner to prevent segregation of the particles. 1. Placing of concrete between transverse joints without the use of intermediate bulkheads. Necessary hand spreading shall be done with shovels, not rakes. Workers are not allowed to walk on. 2. Where concrete is to be placed adjoining a previously constructed lane, and mechanical equipment will be operated upon the existing lane, that previously constructed lane must have attained the strength for 14 day concrete. If finishing equipment is carried on the existing lane paving in adjoining lanes may be permitted only after 3 days.
3. Vibrators should not be allowed to come in contact with a joint assembly, grade or side form. In no case it be operated longer than 15 seconds in any one location. 4. Concrete should be deposited as near as possible to the expansion and contraction joints without disturbing them, but should not be dumped from the discharge bucket of hopper into a joint assembly unless the hopper is well centered on the joint assembly. Concrete Joint Concrete joints are constructed according to the type, dimensions and at the locations as indicated on plans or special provisions. All joints should be protected from the intrusion of injurious foreign materials until after sealed. Concrete Pavement Joints are classified into: 1. 2. 3. 4. 5.
Longitudinal Joint Transverse Joint Transverse Contraction Joint Longitudinal Contraction Joint Load Transfer Device
Longitudinal Joint 1. Deformed steel bars or special length, size, spacing and materials are placed perpendicular to the longitudinal joints. Tie bar should not be painted or coated with asphalt or other materials or enclosed in tubes or sleeves. 2. Except those made of rail steel, tie bars maybe bent at right angles against the form of the first lane constructed and straightened into final position before the concrete of the adjacent lane is placed. 3. The longitudinal joints are sawed before the end of the curing period or shortly thereafter and before any equipment or vehicles are allowed on the pavement. The sawed area should be thoroughly cleaned and if required, the joint should be filled immediately with sealer. 4. Longitudinal pavement insert type joints should be formed by placing a continuous strip – a plastic material which will react adversely with the chemical constituent of the concrete. Transverse Expansion Joint
1. The expansion joint filler should be continuous from form to form shaped to the sub-grade and to the key-way along the form. 2. Pre-formed joint filler should be furnished in lengths equal to the pavement width or to the width of the lane. 3. Finished joint should not deviate more than 6mm from a straight line. If joint fillers are assembled in sections, there should be no offsets between adjacent units. 4. No plugs of concrete should be permitted anywhere within the expansion space. Transverse Contraction Joint 1. Transverse contraction joint is provided when there is an interruption of more than 30 minutes in the concreting operations. 2. No transverse should be located within 1.50 meters of an expansion joint, contraction joint, plane or weakness. 3. If sufficient concrete has been mixed at the right time of interruption to form a slab of at least 1.50 meters long the excess concrete from the last preceding joint should be removed and disposed of as directed. Classification of Transverse Contraction Joint a. Transverse Strip Contraction Joint is installing as parting strip to be left in place as specified. b. Formed Groove is installed by depressing an approved tool or device into the plastic concrete. The tool or device remain in place at least until the concrete has attained its initial set, and to be removed without disturbing the adjacent concrete. c. Sawed Contraction Joint is made by sawing groove in the surface of the pavement. Sawing is done as soon as the concrete hardened sufficiently to permit sawing without excessive raveling, and the time is usually within 24 hours i. Joints are sewed before uncontrolled shrinkage cracking takes place. ii. If necessary, the sawing operations should be carried during the day or night regardless of weather conditions.
iii. The sawing of any joint is omitted if crack occurs at or near the joint prior to the time of sawing. iv. Sawing should be discontinued when crack develops ahead of the saw. In General, all joints should be sawed in sequence. v. If extreme condition exists making it impractical to prevent erratic cracking by early sawing, the contraction joint groove is formed prior to the initial setting of concrete. Load Transfer Device Load transfer device is provided along the longitudinal centerline of the pavement either by tongue and groove concrete or by steel dowels under the following considerations: 1. When dowel is used, it should be held in position parallel to the surface and centerline of the slab by a metal device that is left embedded in the pavement. 2. The portion of each dowel painted with one coat of lead or tar should be thoroughly coated with an approved bituminous material or an approved lubricant, to prevent the concrete from binding to the portion of dowel. 3. The sleeves for dowel should be metal, design to cover 50 mm plus or minus 5mm of the dowels with a water tight closed end with a suitable stop to hold the end of the sleeves at least 25 mm from the end of the dowel. 4. In lieu of using dowel assemblies at contraction joints, dowels may be place in the full thickness of the pavement by a mechanical device approved by the Engineer. RECOMMENDED DIMENSION OF DOWEL FOR CONCRETE PAVEMENT Pavement Thickness (cm) 15.0
Dowel Diameter (mm) 20
Dowel Length (cm) 45
Dowel Spacing (cm) 30
17.5
25
45
30
20.0
25
45
30
22.5
32
45
30
25.0
32
45
30
27.5
32
45
30
30.0
32
45
30
Source: AASHTO Interim Guide and proposed revision 1980
REMOVAL OF FORMS Forms for concrete pavement should remain in place undisturbed within 24 hours after pouring. The removal could be done as follows: 1. Crowbars are used in removing forms, pulling out nails and pins but care should be exercised not to break the pavement edges. 2. In case a portion of the concrete slab is spelled off, it should be repaired immediately with fresh mortar mixture of 1:2. 3. Major honeycombed area will be considered as defective work, to be removed and replaced.
Protection of Pavement Concrete pavement and its appurtenances should be protected against public traffic, and traffic caused by the workers . Protection of the pavement includes the posting of watchmen to direct traffic and the posting and maintenance of warning signs, lights, pavement bridges or crossovers, etc. Any damage to the pavement prior to the final acceptance of the work shall be repaired or replaced depending upon the extent of the damaged.
