C4301 Unit 2

1 REINFORCED CONCRETE STRUCTURAL DESIGN

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UNIT 2 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 the properties of concrete.

2.

list the constituent materials of concrete.

3.

list the characteristics of hardened concrete.

4.

describe the method of concrete mix design methods.

5.

design concrete mix using DOE and ACI design 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 strength for the limit state requirements.

b)

durability

c)

any other overriding characteristics

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 durability of concrete is concerned, structures exposed to corrosive 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 8110.

2.3

Concrete mix Concrete mix is categorized as ‘design’ or ‘specified’ mix. The 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 properties of reinforcement that are of interest to the reinforced concrete designer are welding ability (potential loss of strength when welded), fatigue performance and behaviour under force conditions.

In reinforced concrete construction special ensuring that the reinforcement used covers considerations complies with the requirements of the appropriate British 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 of hardened concrete. a)………………………………………………… b)…………………………………………………. c)………………………………………………… d)………………………………………………… e)…………………………………………………

2.3

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

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

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2.5

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

2.6

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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, aggregate, sand and water

2.2

i)

very strong in compression

ii)

very weak in tension

iii)

very durable; i.e long lasting relatively cheaper than other material fire resistance.

2.3

i)

design mix

ii)

specified mix

2.4

grade 25 , strength 25 N/mm2

2.5

i)

rapid hardening 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

INPUT 2

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2.7 Principles of concrete mix design

The aim in mix design is to select the optimum proportion of cement, water and aggregates to produce a concrete that satisfies the requirements of strength, 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 free w/c ratio is the single most important factor that influences the strength of the concrete.

b)

The water content is the single most important factor that influences 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 water less than the absorbed water. It is available for the hydration and the lubrication of the mix.

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

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maximum size, the shape, grading and surface texture of the aggregate 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)

Department of Environment ( DOE method )

b)

American Concrete Institute ( ACI method )

c)

Road Note No. 4 Method

(Please note that only DOE Method will be described in this unit)

ACTIVITY 2b

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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 proportion of the constituents of concrete. 2.8 w/c refers to water/cost ratio

Y/N Y/N Y/N

2.9 w/c refers to water/cement ratio.

2.10 w/c ratio influences the strength of concrete.

Y/N Y/N Y/N

2.11Water content affects the workability of concrete.

Y/N Y/N

2.12Total water = free water + absorbed water.

2.13Water content is expressed in KN/m2.

2.14DOE stands for Department of Energy.

2.15DOE is the one of the mix design methods used in proportioning

Y/N

Y/N

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

FEEDBACK 2b

Please check your answers here. Award 10 marks for every correct answer. Congratulations if you have got more than 80% correct!.

ANSWERS: 2.7 2.8 N 2.9 Y 2.10Y 2.11Y 2.12Y 2.13N 2.14N 2.15Y 2.16Y

Y

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INPUT 3

2.17DOE mix design method

The principle objectives of DOE mix design method is to obtain a preliminary 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 the required characteristic strength at a specified age, use equation (1) to obtain the target mean strength at that age, which is of compressive strength to be used in the mix design.

…………………..equation 1 f m = f k + 1.64σ

Where: fm = Target Mean Strength fk = Characteristic Strength σ = Deviation

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This is similar to the formula given below;

Target Mean Strength = Characteristic 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/mm 2 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 3 aggregate Ordinary Uncrushed 18 Crushed 23 Portland RapidUncrushed 25 crushed 30 Hardening Portland

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

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c)

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In Figure (1), follow the ‘starting line’ to locate the curve which passes through the point (40N/mm2, w/c ratio 0.5), in this particular case, it is the 4th 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 of 0.47. Note that in 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 Good control with weight batching, use of graded aggregates, etc. Constant supervision. Fair control with weight batching. Use of two sizes of aggregates. Occasional supervision. Poor control. Inaccurate volume batching of all-in aggregates. No supervision.

Standard deviation, (N/mm2)

σ

4 -5

5–7

7 – 8 and above

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

…...equation (2)

The value given by equation (2) should be checked against any maximum and minimum cement contents that may have been 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 the =1aggregate

….equation (3) cement content water content − γc γm

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

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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 Grading Grading Grading sieve zone1 zone2 zone3 10 mm 100 100 100

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% 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.

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2.5.1 Example: Using DOE method, design a mix if the target mean strength is 43 N/mm 2 at 28 days and the required slump is 10 – 30 mm. The following data are given : Cement : ordinary Portland Aggregate type : a)

Coarse : uncrushed, max. Size 10 mm

b)

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

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300 kg/m3 and < 550 kg/m3 Step 4: From equation 3, Total aggregate content =

( 2600)[1 −

385 180 − ] 3150 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 The final mix proportions should satisfy the strength, workability and______________ requirements.

2.18In the DOE mix design method, only two (2) types of aggregates are considered. They are a)___________________________ b)___________________________ 2.19

The

data

for

workability

include

the

_____________

________________, but excludes the compacting factor.

2.20The mixes are designed for the cube ___________________ strength.

and

the

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2.21The mix proportions are expressed in _____________________ and the unit is___________________.

2.22There are five (5) steps to follow in the preparation of the concrete mix 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 2.18a)

uncrushed b)

crushed

2.19a) slump b)

VB time

2.20 compressive 2.21 weights of materials per unit volume of fully compacted fresh concrete , 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 degree of workability of fresh concrete is measured by performing the slump test, compacting factor test and VB Consistometer test.

2.

Workability is the ease with which concrete can be mixed, placed, compacted and finished.

3.

For simple to normal reinforced concrete work, a slump between 0 to 5 mm 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 design method is based on free water/ cement ratio and the water content of the concrete mix.

6.

The average strength of concrete is called the target mean strength and is statistically related to the required characteristic strength.

7.

DOE mix design method has three features. They are as follows:

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a)

Mixes designed for the cube compressive strength.

b)

Data for workability include the slump and VB time but not the compacting factor.

c)

Only crushed and uncrushed aggregate are considered.

d)

The final mix proportions are expressed in term of weights of material per unit volume of fully compacted fresh concrete.

8.

The DOE mix design method involves five steps in the design

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SELF-ASSESSMENT

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 structural design purposes, the unit weight of concrete made with normal aggregates is usually taken as …

2.

A.

0.24 kN/m3

B.

2.4 kN/m3

C.

24.0 kN/m3

D.

240 kN/m3

The characteristic strength of concrete is based on __________________ day cube strength.

3.

A.

5

B.

7

C.

14

D.

28

The following tests are to be performed if DOE method is used. Which of the following is not required?

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4.

A.

Compacting factor

B.

VB Consistometer

C.

Slump

D.

Setting time

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The water content required for a specific workability depends on a number of factors. Which of the following is not one of them?

5.

6.

A.

Maximum aggregates size

B.

Cement content

C.

The shape of aggregate used

D.

Grading of the aggregates

The final mix proportions are expressed in …. A.

kg/m3

B.

g/m2

C.

g/m3

D.

mg/m2

Given that fcu is equal to 25 N/mm2 and σ = 4.5 N/mm2. The target mean strength is …. A.

0.32 N/mm2

B.

3.2 N/mm2

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7.

C.

32.0 N/mm2

D.

324.0 N/mm2

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The quantity 1.64σ represents the current margin by which the target mean strength must ….

8.

A.

be less than

B.

be exceeded

C.

be equal to

D.

all of the above

The standard deviation, σ depends on various factors. Which of the following is not one of them?

9.

A.

Degree of supervision

B.

How weight batching is controlled

C.

Aggregate being graded or not

D.

Weather condition on the day of making concrete.

One of the listed items need not be specified in the final mix proportion. A.

Admixture content

B.

Aggregate content

C.

Water content

D.

cement content

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10.

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If w/c = 0.58 and the water content is 200 kg/m3, what is the cement content needed in the concrete mix? A.

3.45 kg/m3

B.

34.5 kg/m3

C.

345.0 kg/m3

D.

3450 kg/m3

FEEDBACK OF SELF-ASSESSMENT

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

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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. Do not give up! Malaysia Boleh! See you in UNIT 3

END OF UNIT 2