Unit 2 ( CHARACTERISTICS OF MATERIALS )

REINFORCED CONCRETE STRUCTURAL DESIGN

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UNIT 2 CHARACTERISTICS CHARACTERISTICS OF MATERIALS

OBJECTIVES

GENERAL OBJECTIVE To be able to gain knowledge regarding the properties of concrete and concrete mix design.

SPECIFIC OBJECTIVES

At the end of this lesson, you should be able to:-

1. list list the the prop propert erties ies of conc concret rete. e. 2. list list the consti constitue tuent nt mate materi rials als of conc concret rete. e. 3. list list the charact characteri eristi stics cs of hardened hardened concr concrete ete.. 4. describe describe the method of concrete concrete mix design design methods. methods. 5. design design concrete concrete mix mix using using DOE DOE and and ACI design design methods. methods.


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INPUT 1

2.1

Introduction

It’s a good idea if you could refer to section 6 of BS 8110: Part 1 before going through this unit. Section 6 of the code gives some guidelines on concrete materials, the specifications and construction. 2.2

Concrete

The selection of concrete grade and type that are to be used in design depends on the strength required. For example, a concrete of higher strength is needed for ground level column rather than increasing the column size, which will in turn decrease the floor area. The grade of concrete appropriate for use should be selected from the preferred grade in BS 5328 taking account of the following factors: a) adequate adequate strength strength for the limit limit state requirement requirements. s. b) b) dura durabi bili litty c) any other other overr overridi iding ng charac character terist istics ics According to clause 3.1.7.2, BS 8110, the lowest grade that could be used is C25 for concrete made with normal – weight aggregates. As for for dura durabi bili lity ty of concr concret etee is conce concern rned ed,, stru struct ctur ures es expos exposed ed to corr corros osiv ivee environment should use a denser and higher grade of concrete. For interior member, such as, in school buildings and office blocks, a lower concrete grade is preferable.


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Generally, ordinary Portland cement is used for most building structures but other types of cement could be used such as rapid hardening, low heat, and sulphate – resisting Portland cement. Refer to clause 6.1.2.1, BS 811 0.

2.3

Concrete mix

Concrete Concrete mix is categorized categorized as ‘design’ ‘design’ or ‘specifie ‘specified’ d’ mix. The differences differences between design and specified mix are as follows: In design mix, the contractor should select the mix proportion in order to get the required strength and workability of concrete. In specified mix, however the engineer should decide the mix proportion strength and workability of concrete. 2.4

Reinforcement

Please refer to clause 3.1.7.4 and Table 3.1, BS 8110 regarding the strength of steel reinforcement to be used. Reinforcements are in the form or round, mild steel bars, high-yield bars and fabric reinforcement bars (BRC). Mast reinforcement is produced by hot rolling process. Considerable advantage is gained from using these types of reinforcement that has a high ductility. Ductility is the ability of a bar to be bent in a small radius without loss of strength. This is of particular importance to tie the main reinforcement by using links.

Fabric reinforcement gives considerable advantage when used in floor slabs, walls and pavement. Construction and labour costs could be cut down drastically when the right type of reinforcement is used.


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Other Other proper properti ties es of reinfo reinforce rcement ment that that are of intere interest st to the reinfo reinforce rced d concret concretee design designer er are weldin welding g abilit ability y (poten (potenti tial al loss loss of strengt strength h when when welded welded), ), fatigu fatiguee performance and behaviour under force conditions.

In reinfo reinforce rced d concre concrete te constru constructi ction on specia speciall ensuri ensuring ng that that the reinfo reinforce rcemen mentt used used covers covers conside considerat ration ionss compli complies es with with the requir requireme ements nts of the approp appropria riate te Briti British sh Standards recommended in BS 8110 and BS 4466.

2.5

Size of reinforcement

It is invariably more economical to use a few but different sizes of reinforcement as possible, even though this may involve using a larger amount of reinforcement that the design actually requires.

The standard sizes of reinforcing bars and the lengths in which they are readily available are given in the manufacturer’s catalogue.


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ACTIVITY 2a

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE TO THE NEXT INPUT!

2.1

State the constituent materials of concrete. ………………………………………………………………………….

2.2

List five properties properties of hardened hardened concrete. concrete. a)………………………………………………… b)…………………………………………………. c)………………………………………………… d)………………………………………………… e)…………………………………………………

2.3

State State 2 methods methods on how concrete concrete mix is prepared prepared a)……………………………………………….. b)………………………………………………..


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2.4 What What is the lowest lowest concrete concrete grade that should should be used used with with manual manual weight weight aggregate? ……………………………………………………………………………

2.5

State 3 other types of cement other than Ordinary Portland Cement. a)…………………………………………………… b)……………………………………………………. c)……………………………………………………

2.6

State 3 types of reinforcement specified by BS 8110. a)…………………………………………………….. b)……………………………………………………… c)…………………………………………………….


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FEEDBACK 2a

The answers are as follows. Check whether your answers are correct before you proceed to the next unit. 2.1

Cement, Cement, aggregate, aggregate, sand and water

2.2

i)

very strong strong in compressio compression n

ii)

very weak in tension iii iii)

ver very dur durable able;; i.e long ong las lasting ting relat elativ ivel ely y chea cheape perr than than othe other r

material fire resistance. 2.3

i)

design design mix

ii)

specified mix

2.4

grade 25 , strength strength 25 N/mm2

2.5

i)

rapid hardening hardening Portland Portland Cement

ii)

sulphate-resistance Portland cement

iii)

low heat Portland cement.

i)

mild steel ( R )

ii)

high-yield steel (T)

iii)

fabric reinforcement (BRC)

2.6


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INPUT 2

2.7

Principles of of concrete mix design

The aim in mix design is to select the optimum proportion of cement, water and aggr aggreg egat ates es to produ produce ce a concr concret etee that that sati satisf sfie iess the the requi require reme ment ntss of stre streng ngth th,, workability, durability and economy. Mix design methods are useful as guides in the initial selection of these proportions. The final proportion to be adopted should be established by actual trials and adjustments on site.

Mix design methods are based on the following two sample observations: a)

The The free ree w/c rat ratio is the the sing singlle most ost impo imporrtant ant fact factor or that that inf influenc uences es the the strength of the concrete.

b) b)

The The wate waterr cont conten entt is the the singl inglee most most impo imporrtant ant facto actorr that that inf influen luence cess the workability of the fresh concrete mix.

In calculating w/c ratio in (a) above, only the weight of the free water is used. The total water in the concrete mix consists of the water absorbed by aggregate and the free water, which is the total total water less than the absorbed water. It is available for the hydration and the lubrication of the mix.


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The water content in (b) is expressed as the weight of the free water per unit volume of concrete. The water content required for a specified workability depends on the maxi maximu mum m size size,, the the shap shape, e, grad gradin ing g and and surf surfac acee text textur uree of the the aggr aggreg egat atee but but is relatively independent of the cement content. (i.e. the weight of cement per unit volume of concrete.) There are 3 mix design methods. They are: a) Depart Departmen mentt of Enviro Environme nment nt ( DOE DOE method method ) b) American American Concret Concretee Instit Institute ute ( ACI method method ) c) Road Road Note Note No. No. 4 Meth Method od (Please note that only DOE Method will be described in this unit)


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ACTIVITY 2b

Answer the following questions by indicating ‘Y, for Yes or ‘N’ for No for the statements given. 2.7 The aim of mix design is to select the optimum optimum proportion proportion of the constituents of concrete.

Y/N

2.8 w/c refers refers to water/cost water/cost ratio ratio

Y/N

2.9 w/c refers to water/ceme water/cement nt ratio.

Y/N

2.10 2.10

w/c w/c rati ratio o infl influen uence cess the str streng ength th of of concr concret ete. e.

Y/N

2.11 2.11

Water Water content content affect affectss the the work workabi abili lity ty of concret concrete. e.

Y/N

2.12 2.12

Tota Totall wate waterr = fre freee wate waterr + abs absor orbe bed d wate water. r.

Y/N

2.13 2.13

Wate Wa terr cont conten entt is is expre express ssed ed in KN/m KN/m2.

Y/N

2.14 2.14

DOE DOE sta stands nds for for Dep Depar artm tment ent of Ener Energy gy..

Y/N

2.15 2.15

Y/N

DOE DOE is the the one one of the the mix mix desig design n meth method odss used used in in

proportioning concrete constituent materials. 2.16 2.16

The The wate waterr cont conten entt requi equire red d to prod produc ucee a cer certain ain

workability in concrete mix will depend on the size, shape, texture and grading of aggregate.

It is not so hard to find the right answers is it? Try to do your best. Best of luck!

Y/N


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FEEDBACK 2b

Pleas Pleasee chec check k your your answ answer erss here here.. Awar Award d 10 mark markss for for ever every y corr correc ectt answ answer er.. Congratulations if you have got more than 80% correct!.

ANSWERS: 2.7

Y

2 .8 N 2 .9 Y 2.10

Y

2.11

Y

2.12

Y

2.13

N

2.14

N

2.15

Y

2.16

Y


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INPUT 3

2.17

DOE mix design method

The princi principle ple object objective ivess of DOE mix design design method method is to obtain obtain a prelim prelimina inary ry estimate of the mix proportions as a basis to make trial mixes to arrive at the final mix proportions that satisfy the strength , workability and durability requirements. The DOE mix design procedure is summarized as follows:

Step 1: Determining the free w/c ratio

a)

Given Given the the requ requir ired ed char charac acte teri rist stic ic str stren engt gth h at a spec specif ifie ied d age, age, use use equ equat atio ion n (1) (1) to obtain the target mean strength at that age, which is of compressive strength to be used in the mix design.

f m

=

f k

1.64 σ

+

Where: f m = Target Mean Strength f k k = Characteristic Strength σ = Deviation

…………………..equation 1


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This is similar to the formula given below; Target

M ean

Strength

C haracte teri

stic

=

Strength

1.64ó

+

In the following example, we shall suppose that the target mean strength obtained as 43 N/mm2 for 28 days.

b)

Given the type of cement and aggregate, use Table (1) to obtain the compressive strength, at the specified age that corresponds to a free w/c ratio of 0.5. Ordinary Portland cement and uncrushed aggregate are used. Then Table (1) shows that the compressive strength is 40N/mm2 at 28 days. (and 27N/mm2 at 7 days and so on). This pair of data (40N/mm2, w/c ratio 0.5) will now be used to locate the appropriate strength – w/c ratio curve in Figure (1), as explained below.

Table 1: Compressive strength Type of cement Type of coarse aggregate 3 Ordinary Uncrushed 18 Portland Crushed 23 RapidUncrushed 25 Hardening crushed 30 Portland

Compressive strength (N/mm2) Age (Days) 7 28 91 27 40 48 33 47 55 34 46 53 40 53 60


c)

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In Figu Figurre (1), (1), fol follow the the ‘star startting line line’’ to loca locatte the the curv curvee whi which pas passes ses through the point (40N/mm2, w/c ratio 0.5), in this particular case, it is the 4 th curve from the top of the figure. This curve shows that to obtain our target mean strength of 43 N/mm2, we need a w/c ratio ratio of 0.47. 0.47. Note that that in figure figure (1) a curve happens to pass almost exactly through the point (40 N/mm2, w/c ratio = 0.5), this does not always happen, so that in practice it is usually necessary to interpolate between two curves in the figure.

Figure 1: curve of target mean strength


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Step 2: Determining the water content Given the slump or VB time, determine the water content from Table (2). In using Table (2), when coarse and fine aggregates of different types are used, the water content W is estimated as follows: Table 2: Standard Deviation (σ)

Conditions

Standard deviation, (N/mm2) Good control with weight batching, use of 4 -5 graded aggregates, etc. Constant supervision. Fair control with weight batching. Use of 5–7 two two sizes izes of aggr aggreg egat ates es.. Occa Occasi sion onal al supervision. Poor control. Inaccurate volume batching of 7 – 8 and above all-in aggregates. No supervision.

W

=

2 3

W f

+

1 3

σ

W c

Where, Wf = water content appropriate to the type of fine aggregate; Wc = water content appropriate to the type of coarse aggregate.

The aggregate type in Table (2) refers to all the aggregates used and not just the coarse aggregate.


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Step 3: Determining the cement content

Cement content (kg/m3) =

water w/c

content ( from ratio

( from

step 2) step1) …...equation (2)

The value given by equation equation (2) should should be checked checked against against any maximum maximum and minimum cement contents that may have been be en specified.

If the cement content calculated from equation (2) below is a specified minimum, this minimum must be used. If the calculated cement content is higher than a specified maximum, try changing the type of cement and the type and maximum size of the aggregate.

Step 4: Determining the aggregate content

The total aggregate content is calculated as follows:

Volume occupied by cement the aggregate =1-

content γ c

water

−

content γ m

Where, γc (3150 kg/m3) is the density of the cement particles and γm(1000 kg/m3) is density of water.

….equation (3)


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Therefore,

Total aggregate content (kg/m3)

= γa × [Volume occupied by aggregate]…….equation 4

Where, γa is the density of the aggregate particles. γa should be taken as 2600 kg/m3 for crushed aggregate.

Step 5: Determining of the fine and coarse aggregate contents .

Refer to Table (3) to determine the fine and coarse aggregate content. In this table, fine aggregate is classified into grading zones. For given slump and w/c ratio, the proportion of fine aggregate can be determined from Figure (2) in which the grading zones are these of Table (4).

Table 3: The fine and coarse aggregate content Slump (mm) VB time (seconds) Max. size of Type of aggregate (mm) aggregate 10 Uncrushed Crushed 20 Uncrushed Crushed 40 Uncrushed crushed

0 - 10 >2

10 - 30 12 - 6

30 - 60 6-3

60 - 180 3-0

150 180 135 170 115 155

180 205 160 190 140 175

205 230 180 210 160 190

225 250 195 225 175 205


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Table 4: The grading zones

Percentage by weight passing standard sieves Standard sieve 10 mm

Grading zone1 100

Grading zone2 100

Grading zone3 100

Grading zone4 100

5 mm

90 - 100

90 - 100

90 - 100

95 - 100

No.7 (2.36mm) No.14 (1.18 m) No.25 (600 μm) No.52 (300 μm) No.100 (150 μm)

60 - 95

75 - 100

85 - 100

95 - 100

30 - 70

55 - 90

75- 100

90 – 100

15 - 34

35 - 59

60 - 79

80 – 100

5 - 20

8 - 30

12 - 40

15 – 50

0 - 10

0 - 10

0 - 10

0 - 15

Figure 2: Proportion of fine aggregate (per cent)


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For example, suppose the slump is 10-30 mm. The w/c ratio is 0.47, and the fine aggregate is in grading zone 3, then Figure (2) gives the proportion of fine aggregate as between 32% and 38% 3 8% by weight, say 35%. Therefore, for this particular example,

Fine aggregate content = 35% of total aggregate content Coarse aggregate content = (100 – 35) % of total aggregate content.

Note that Figure (2) is for use where the nominal maximum size of the coarse aggregate is 10mm. The DOE document contains similar design charts for 20mm and 40mm maximum sizes. How are you doing so far? Do not worry if you are confused. The following example will help you to understand more about the DOE method. Plea Please se go thro through ugh this this examp example le thor thoroug oughl hly. y. Good Luck!


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2.5.1 Example: Using DOE method, design a mix if the target mean strength is 43 N/mm2 at 28 days and the required slump is 10 – 30 mm. The following data are given : Cement : ordinary Portland Aggregate type : a) b)

Coarse : uncrushed, max. Size 10 mm Fine : uncrushed, grading zone 4

Maximum w/c ratio : 0.60 Maximum cement content : 550 kg/m3 (From Table 3.4, BS 8110 : Part 1) Minimum cement content : 300 kg/m3 (From Table 3.4, BS 8110 : Part 1) Solution: Follow step 1, you should get the answer as shown below: W/c ratio = 0.5 Strength = 40 N/mm2 (28 days) Target Mean Strength (given) = 43 N/mm2 From Figure 2: w/c ratio = 0.47

Step 2: From Table 2, Water content = 180 kg/m3 Step 3: From equation 2, Cement content =

180 0.47

= 385 kg/m3 •

300 kg/m3 and < 550 kg/m3


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Step 4: From equation 3, Total aggregate content = ( 2600 )[1 −

385 3150

180

−

1000

]

= 1815 kg/m3 Step 5: From table 3, Proportion of fine aggregate is 27.5 to 32% by weight, say 30%. : - fine aggregate content

= 0.30 × 1815

= 545 kg/m3

Coarse aggregate content = (1-0.30) × 1815 = 1270 kg/m3

Your answer may be summarized as follow: The required mix proportions are: Cement content: 385 kg/m3 Water content : 180 kg/m3 Fine aggregate content: 545 kg/m3 Coarse aggregate content: 1275 kg/m3

GOOD! YOU HAVE DONE A GOOD JOB. KEEP IT UP!


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ACTIVITY 2c

Fill in the blanks with the correct answers. Remember, you must do it on your own.

2.17 2.17

The The fina finall mix prop propor orttions ons shoul hould d sat satisfy isfy the stren trengt gth, h, wor workabi kabillity ity and______________ requirements.

2.18 2.18

In the DOE DOE mix desi design gn met method, hod, onl only two two (2) (2) type typess of aggr aggregat egates es are are considered. They are a)___________________________ b)___________________________

2.19

The

data

for

workability

include

the

_____________

and

the

________________, but excludes the compacting factor.

2.20 2.20

The mixe mixess are are design designed ed for for the the cube ____ ________ ________ ________ _______ ___ stre strengt ngth. h.

2.21 2.21

The mix mix proport proportion ionss are expr express essed ed in ______ __________ ________ ________ _______ ___ and and the unit unit is___________________.


2.22 2.22

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There There are five five (5) (5) steps steps to follo follow w in the prepar preparati ation on of the concr concrete ete mix mix design design according to DOE Method. Match the steps according to the items needed at each stage.

STEP 1

fine and coarse aggregate

STEP 2

free w/c ratio

STEP 3

water content

STEP 4

aggregate content

STEP 5

cement content


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FEEDBACK 2c

Please check your answers here:

2.17

Durability

2.18

a)

uncrushed

b) crushed 2.19

a)

slump b)

VB time

2.20

compressive

2.21 2.21

weig weight htss of materi material alss per unit unit volume volume of fully fully compa compact cted ed fresh fresh concr concret etee , kg/m2


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2.22

STEP 1

fine and coarse aggregate

STEP 2

free w/c ratio

STEP 3

water content

STEP 4

aggregate content

STEP 5

cement content


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SUMMARY

1.

The The degre degreee of work workab abil ilit ity y of fres fresh h conc concre rete te is is meas measur ured ed by by perf perfor ormi ming ng the the slump test, compacting factor test and VB Consistometer test.

2.

Work Workabi abili lity ty is the the ease ease with with which which conc concre rete te can can be be mix mixed ed,, plac placed, ed, comp compac acte ted d and finished.

3.

For For sim simpl plee to to norm normal al rein reinfo forc rced ed con concr cret etee work work,, a slu slump mp betw between een 0 to to 5 mm is is needed.

4.

Mix design method is a guide to the initial selection of the optimum proportion of cement, water and aggregates to produce concrete that satisfies strength, workability, durablity, and economy requirements.

5.

Mix Mix desi design gn met method hod is base based d on free free wat water er// cem cement ent rati ratio o and and the the wat water er cont conten entt of the concrete mix.

6.

The The aver averag agee stren trengt gth h of conc concre rette is cal called the the targe argett mean mean stre streng ngtth and and is statistically related to the required characteristic strength.


7.

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DOE DOE mix mix desi design gn met metho hod d has has thr three ee fea featu ture res. s. They They are are as as fol follo lows ws:: a)

Mixe Mixess desi design gned ed for for the the cube cube comp compre ress ssiv ivee stre streng ngth th..

b)

Data ata for workab kability incl nclude the the slump ump and VB ti time but not not the the compacting factor.

c)

Only Only crus crushe hed d and and uncr uncrus ushe hed d agg aggre rega gate te are are con consi side dere red. d.

d)

The The fina finall mix mix propo proport rtio ions ns are are expr expres esse sed d in in ter term m of of wei weight ghtss of of mat mater eria iall per unit volume of fully compacted fresh concrete. co ncrete.

8. The DOE DOE mix design method method involves involves five five steps steps in the design design

SELF-ASSESSMENT


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Answer all the questions given by circling the alphabet corresponding to the correct answer of your choice. Award one mark for every correct answer. Time allocation for this test is 30 minutes. You may start now when you are ready. You may refer to BS 8110 if you wish. Good Luck!

1.

For For str struc uctu tura rall desi design gn pur purpos poses es,, the the unit unit weigh weightt of conc concre rete te mad madee with with nor norma mall aggregates is usually taken as … A. 0.24 0.24 kN/m kN/m3 B. 2.4 kN kN/m3 C. 24.0 kN/m3 D. 240 kN kN/m3

2.

The The char charac acte teri rist stic ic stre streng ngth th of of con concr cret etee is is base based d on ____ _______ _____ _____ ______ _____ ___ _ day day cube strength. A. 5 B. 7 C. 14 D. 28


3.

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The The ffol ollo lowi wing ng tes tests ts are are to to be be per perfo form rmed ed if DOE DOE met metho hod d is is use used. d. Whic Which h of of the the following is not required? A. Comp Compac acti ting ng fact factor or B. VB Cons Consis isto tome mete ter r C. Slump D. Sett Settin ing g time time

4.

The The wate waterr ccon onte tent nt req requi uire red d for for a spe speci cifi ficc work workab abil ilit ity y depe depends nds on a numb number er of of factors. Which of the following is not one of them? A. Maximu Maximum m aggre aggregat gates es size size B. Ceme Cement nt con conte tent nt C. The shape shape of of aggr aggrega egate te used used D. Gradin Grading g of the the aggr aggrega egates tes

5.

The The fin final al mix mix pr propor oporttions ions are are exp exprresse essed d in in …. …. A. kg/m3 B. g/m2 C. g/m3 D. mg/m2


6.

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Given that fcu is equal to 25 N/mm2 and σ = 4.5 N/mm2. The target mean strength is …. A. 0.32 0.32 N/mm N/mm2 B. 3.2 N/ N/mm2 C. 32.0 N/mm2 D. 324. 324.0 0 N/m N/mm m2

7.

The The quant quantit ity y 1.64 1.64σ σ repr repres esen ents ts the the curr curren entt marg margin in by whi which ch the the tar targe gett mean mean strength must …. A. be les less tha than n B. be exce exceed eded ed C. be equa equall to D. all all of of the the abo above ve

8.

The The stan standa dard rd dev devia iati tion, on, σ depe depends nds on var vario ious us fact factor ors. s. Whi Which ch of of the the fol follo lowi wing ng is not one of them? A. Degree Degree of superv supervisi ision on B. How weigh weightt batchi batching ng is contro controlle lled d C. Aggrega Aggregate te being being grad graded ed or not D. Weather Weather condition condition on the the day of making making concrete. concrete.


9.

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One One of the the lis liste ted d item itemss need need not not be be spe speci cifi fied ed in in the the fin final al mix mix propo proport rtio ion. n. A. Admi Admixt xtur uree conten contentt B. Aggr Aggreg egat atee con conte tent nt C. Wa Wate terr con conte tent nt D. ceme cement nt cont conten entt

10. 10.

If w/c w/c = 0.5 0.58 8 and and the the wat water er cont conten entt is 200 200 kg/ kg/m3, what is the cement content needed in the concrete mix? A. 3.45 3.45 kg/m kg/m3 B. 34.5 kg/m3 C. 345. 345.0 0 kg/ kg/m3 D. 3450 3450 kg/m kg/m3

FEEDBACK OF SELF-ASSESSMENT


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Now, check your answers below and calculate the percentage you have scored.

Answers:

1.C 2. D 3.A 4. B 5. A 6. C 7. B 8. D 9. A 10. C


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You should score more than 80% to pass this unit. If your score is more than 80%, you may proceed to the next unit. Congratulations! But if you scored less than 80%, you should go through this unit again. You can go through the entire unit or only the sections you found difficult. d ifficult. Do not give up! Malaysia Boleh! See you in UNIT 3

END OF UNIT 2

Unit 2 ( CHARACTERISTICS OF MATERIALS )

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