c4301 Unit 2

REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT2/

UNIT 2

CHARACTERISTICS OF MATERIALS

GENERAL OBJECTIVE

To be able to gain knowledge regarding the properties of concrete and concrete mix

design.

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.

OBJECTIVES

SPECIFIC OBJECTIVES

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.

INPUT 1

2


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.

3


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.

4


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)………………………………………………..

ACTIVITY 2a

5


2.4 What is the lowest concrete grade that should be used with manual 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)…………………………………………………….

6


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.

2.6 i) mild steel ( R )

ii) high-yield steel (T)

iii) fabric reinforcement (BRC)

FEEDBACK 2a

7


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.

INPUT 2

8


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

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)

9

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


Answer the following questions by indicating ‘Y, for Yes or ‘N’ for No for the

statements given.

ACTIVITY 2b

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

2.9 w/c refers to water/cement ratio.

2.10 w/c ratio influences the strength of concrete.

2.11 Water content affects the workability of concrete.

2.12 Total water = free water + absorbed water.

2.13 Water content is expressed in KN/m2.

2.14 DOE stands for Department of Energy.

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

proportioning concrete constituent materials.

2.16 The water content required to produce a certain

workability in concrete mix will depend on the size,

shape, texture and grading of aggregate.

Y / N

Y / N

Y / N

Y / N

Y / N

Y / N

Y / N

Y / N

Y / N

Y / N

10


Please check your answers here. Award 10 marks for every correct answer.

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

FEEDBACK 2b

11


2.17 DOE 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.

f m=f k+1 . 64σ …………………..equation 1

Where: fm = Target Mean Strength

fk = Characteristic Strength

σ = Deviation

INPUT 3

12


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/mm 2 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

Compressive strength (N/mm2)Age (Days)

3 7 28 91Ordinary Portland

Uncrushed 18 27 40 48Crushed 23 33 47 55

Rapid-Hardening Portland

Uncrushed 25 34 46 53crushed 30 40 53 60

13


c) 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

14

W=23W f+

13W c


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, σ (N/mm2)

Good control with weight batching, use of graded aggregates, etc. Constant supervision.

4 - 5

Fair control with weight batching. Use of two sizes of aggregates. Occasional supervision.

5 – 7

Poor control. Inaccurate volume batching of all-in aggregates. No supervision.

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.

15

= 1 -

cement contentγ c

−water contentγm ….equation (3)

Volume occupied by the aggregate


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:

Where,

γc (3150 kg/m3) is the density of the cement particles and

γm(1000 kg/m3) is density of water.

Cement content (kg/m3) = water content ( from step2)w /c ratio ( from step1 )

16


Therefore,

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) 0 - 10 10 - 30 30 - 60 60 - 180VB time (seconds) > 2 12 - 6 6 - 3 3 - 0

Max. size of aggregate (mm)

Type of aggregate

10 Uncrushed 150 180 205 225Crushed 180 205 230 250

20 Uncrushed 135 160 180 195Crushed 170 190 210 225

40 Uncrushed 115 140 160 175crushed 155 175 190 205

Total aggregate content (kg/m3)

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

17


Table 4: The grading zones

Percentage by weight passing standard sieves

Standard sieve

Grading zone1

Grading zone2

Grading zone3

Grading zone4

10 mm 100 100 100 100

5 mm 90 - 100 90 - 100 90 - 100 95 - 100

No.7 (2.36mm)

60 - 95 75 - 100 85 - 100 95 - 100

No.14 (1.18 m)

30 - 70 55 - 90 75- 100 90 – 100

No.25 (600 μm)

15 - 34 35 - 59 60 - 79 80 – 100

No.52 (300 μm)

5 - 20 8 - 30 12 - 40 15 – 50

No.100 (150 μm)

0 - 10 0 - 10 0 - 10 0 - 15

Figure 2: Proportion of fine aggregate (per cent)

18

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. Please go through this example

thoroughly. Good Luck!


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.

19


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) 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/mm 2 (28 days)

Target Mean Strength (given) = 43 N/mm 2

From Figure 2: w/c ratio = 0.47

Step 2: From Table 2,

Water content = 180 kg/m 3

Step 3: From equation 2,

Cement content =

1800 . 47

= 385 kg/m 3

300 kg/m3 and < 550 kg/m3

20


Step 4: From equation 3,

Total aggregate content = (2600 )[ 1−385

3150−180

1000]

= 1815 kg/m 3

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/m 3

Coarse aggregate content = (1-0.30) × 1815 = 1270 kg/m 3

Your answer may be summarized as follow:

The required mix proportions are:

Cement content: 385 kg/m 3

Water content : 180 kg/m 3

Fine aggregate content: 545 kg/m 3

Coarse aggregate content: 1275 kg/m 3

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

21


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.

2.18 In 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 _____________ and the

________________, but excludes the compacting factor.

2.20 The mixes are designed for the cube ___________________ strength.

2.21 The mix proportions are expressed in _____________________ and the unit

is___________________.

ACTIVITY 2c

22


2.22 There 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 2

STEP 3

STEP 4

STEP 5

STEP 1 fine and coarse aggregate

free w/c ratio

aggregate content

water content

cement content

23


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 weights of materials per unit volume of fully compacted fresh concrete ,

kg/m2

FEEDBACK 2c

24


2.22

STEP 2

STEP 3

STEP 4

STEP 5

STEP 1 fine and coarse aggregate

free w/c ratio

aggregate content

water content

cement content

25


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.

SUMMARY

26


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

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

27


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 …

A. 0.24 kN/m3

B. 2.4 kN/m3

C. 24.0 kN/m3

D. 240 kN/m3

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

cube strength.

A. 5

B. 7

C. 14

D. 28

SELF-ASSESSMENT

28


3. The following tests are to be performed if DOE method is used. Which of the

following is not required?

A. Compacting factor

B. VB Consistometer

C. Slump

D. Setting time

4. The water content required for a specific workability depends on a number of

factors. Which of the following is not one of them?

A. Maximum aggregates size

B. Cement content

C. The shape of aggregate used

D. Grading of the aggregates

5. The final mix proportions are expressed in ….

A. kg/m3

B. g/m2

C. g/m3

D. mg/m2

29


6. 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

C. 32.0 N/mm2

D. 324.0 N/mm2

7. The quantity 1.64σ represents the current margin by which the target mean

strength must ….

A. be less than

B. be exceeded

C. be equal to

D. all of the above

8. The standard deviation, σ depends on various factors. Which of the following

is not one of them?

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.

30


9. 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

10. 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

31


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

FEEDBACK OF SELF-ASSESSMENT

32


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

33

REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT2/ 34

c4301 Unit 2

Documents

Transcript of c4301 Unit 2