Direct compressive strength and Elastic modulus of ... · Direct compressive strength and Elastic...

INTERNATIONAL JOURNAL OF CIVIL AND STRUCTURAL ENGINEERING

Volume 2, No 1, 2011

© Copyright 2010 All rights reserved Integrated Publishing services

Research article ISSN 0976 – 4399

Received on September, 2011 Published on November 2011 292

Direct compressive strength and Elastic modulus of recycled aggregate

concrete Gupta Arundeb

1, Mandal Saroj

2, Ghosh Somnath

3

1- Research Scholar, Department of Civil Engineering, Jadavpur University, Kolkata 700032,

2, 3- Professor, Department of Civil Engineering, Jadavpur University, Kolkata 700032

[email protected]

doi:10.6088/ijcser.00202010110

ABSTRACT

An experimental investigation was conducted to study the change in direct compressive

strength and elastic modulus of recycled aggregate concrete in presence of fly ash (as

replacement of cement). Concrete cubes, cylinder and prisms were prepared as test specimens

with varying percentage of fly ash using natural aggregate and recycled aggregate. These

specimens were tested at different age of concrete to have compressive strength, elastic

modulus, split tensile strength as well as flexural strength and their changes were noticed .

MIP ( Mercury intrusion porosimetry ) test was conducted to estimate the percentage of

micro voids in recycled aggregate concrete and also to appreciate the change of these micro

voids due to presence of fly ash .Empirical formulas involving major parameters such as fly

ash content, water cement ratio, age of concrete etc., have been developed to predict elastic

modulus and compressive strength of recycled aggregate concrete.

Keywords: Recycled Aggregate Concrete, Natural Aggregate Concrete, Fly ash, Mercury

intrusion porosimetry, Compressive strength, Elastic modulus

1. Introduction

Recycling of waste concrete is now quite popular in the construction industry. Recycled

coarse aggregate are widely used as sub base in road project and also for making lean

concrete. Research works are in progress to establish it to be fit for structural concrete [all the

research works mentioned under reference]. Successful use of recycled aggregates in regular

concrete works would lead to a considerable reduction in demand of natural aggregates.

Recycled aggregate may be considered as a green material to some extent. It is quite well

known that the porosity of recycled aggregate concrete is more than conventional natural

aggregate concrete. It is also known to us that is strength of concrete reduces with increase in

porosity. Addition of fly ash to some extent will provide better micro structure as well as

strength.

This paper deals with an experimental investigation on recycled aggregate concrete made in

presence of fly ash (as cement replacement). MIP (Mercury intrusion porosimetry) test was

conducted to estimate the percentage of voids in recycled aggregate concrete and also to

study the change of micro voids due to presence of fly ash. Empirical formulas involving

major parameters such as fly ash content, water cement ratio, age of concrete etc., have been

developed to predict elastic modulus and compressive strength of recycled aggregate concrete.

2. Materials and Method

2.1 Materials

Direct compressive strength and Elastic modulus of recycled aggregate concrete

Gupta Arundeb, Mandal Saroj, Ghosh Somnath

International Journal of Civil and Structural Engineering

Volume 2 Issue 1 2011

293

Cement

Ordinary Portland cement of Grade 53 Conforming to IS 12269-1987.

Fine aggregate

Locally available natural sand of Zone III as per IS 383-1970.

Coarse aggregate

Two types of coarse aggregate were used

1. Natural stone aggregate of Basalt variety ii) Recycled aggregate processed from

Laboratory waste concrete cubes of 150mm x 150mm x150mm of mean compressive

strength 23.5 MPa.

The recycled coarse aggregates were designated as Type-I and Type-II. The coarse aggregate

passing through 40mm sieve and retained on 20mm sieve was designated as Type-I and the

fraction of coarse aggregate passing through 20mm sieve and retained on 4.75mm sieve was

designated as Type-II.

Fly ash: The fly ash was directly obtained from Bandel thermal power station near Kolkata,

India. Chemical composition of fly ash is shown in Table I and compared with standards as

prescribed by IS 3812 part-I.

Table 1: Chemical composition of Fly ash

Chemical

Composition

Fly ash

(% weight)

Specified requirement

weight (%) IS –3812 part-I

SiO2 60.0 35.0 minimum

Al2O3 20.0

CaO 8.0

MgO 1.0 5.0 maximum

TiO2 0.5

Loss of

ignition

8.0 12.0 maximum

Na2O/K2O 1.0

2.2 Concrete Mix Proportion

There is no standard mix design procedure for recycled aggregate concrete. Hence, trial

mixes as per ACI 14 for natural aggregate concrete were prepared and tested. Same

proportion was used for recycled aggregate concrete also.

The mixture proportion by weight for natural aggregate concrete (NAC) and recycled

aggregate concrete (RAC) are furnished in Table 2.

Table 2: Mix Proportion of NAC and RAC specimens

Sl no.

Mix

Mix Proportion (by weight )

Cement Fly ash Sand Coarse

aggregates

Water

cement





294

designation Type I Type

II

ratio

1 NAC 1.00 - 1.60 1.32 1.98 0.4

2 NAC1 0.9 0.1 1.6 1.32 1.98 0.4

3 RAC 1.00 - 1.60 1.32 1.98 0.4

4 RAC1 0.90 0.10 1.6 1.32 1.98 0.4

5 RAC2 0.80 0.20 1.60 1.32 1.98 0.4

6 NACa 1.00 - 1.60 1.32 1.98 0.5

7 NAC1a 0.9 0.1 1.6 1.32 1.98 0.5

8 RACa 1.00 - 1.60 1.32 1.98 0.5

9 RAC1a 0.90 0.10 1.6 1.32 1.98 0.5

10 RAC2a 0.80 0.20 1.60 1.32 1.98 0.5

11 NACb 1.00 - 1.60 1.32 1.98 0.6

12 NAC1b 0.9 0.1 1.6 1.32 1.98 0.6

13 RACb 1.00 - 1.60 1.32 1.98 0.6

14 RAC1b 0.90 0.10 1.6 1.32 1.98 0.6

15 RAC2b 0.80 0.20 1.60 1.32 1.98 0.6

2.3 Test Specimens

Cubes of 150x150x150 mm, cylinders 150 dia x300 mm height and prisms 100x100x500mm

long, were casted from each mix to have compressive strength, tensile strength, flexural

strength and elastic modulus of recycled aggregate concrete. Elastic modulus in direct

compression has been determined as per DIN 1048 Part 1. It may be noted here that the

specimens were cured under water for 28 days and then left in air till the date of testing.

3. Results and discussion

3.1 Workability, wet density and yield of concrete

Detailed results of workability, density and yield for NAC and RAC are presented in Table 3

It is noticed that workability of RAC is less than that of NAC. Similar results have been

reported by other researchers [Topcu, 1997, Oliviea, 1996] also. Addition of fly ash to both

RAC and NAC marginally reduces the compacting factor values.

Table 3: Result of fresh Concrete mix

MIX NO Mix

ID Compacting

FACTOR

Unit Weight.

(KG/ M3)

Yield / 50Kg

of Cement

1 NAC 0.776 2321 0.138

2 NAC1 0.767 2396 0.134

3 RAC 0.753 2228 0.144

4 RAC1 0.722 2254 0.142

5 RAC2 0.715 2220 0.144

6 NACa 0.88 2349 0.136

7 NAC1a 0.82 2377 0.135

8 RACa 0.849 2245 0.143

9 RAC1a 0.848 2265 0.141

10 RAC2a 0.835 2207 0.145

11 NACb 0.96 2443 0.131





295

12 NAC1b 0.94 2452 0.13

13 RACb 0.984 2264 0.141

14 RAC1b 0.98 2292 0.14

15 RAC2b 0.945 2264 0.141

It is also observed that wet densities of recycled aggregate concrete are less than that of

natural aggregate concrete. This is due to presence of low density old mortar attached on

recycled aggregates. Results indicate that 10% addition of fly ash as replacement of cement

in RAC and NAC marginally increases the unit weight. However, 20% addition of fly ash as

replacement of cement in RAC and NAC marginally decreases the unit weight. Addition of

fly ash reduces workability of concrete as observed in compaction factor test.

3.2 Relation between Elastic modulus of Recycled aggregate concrete and other

parameters of concrete

Elastic modulus in direct compression of recycled aggregate concrete having different ratio of

cement to fly ash are presented in Figure 1 and an equation is obtained by regression analysis.

Similarly, in Figure 2, the relationship between elastic modulus and 28 days compressive

strength are shown.

In Figure 3 the relationship between elastic modulus and water cement ratio has been shown.

Figure 1: Elastic Modulus vs Percentage of fly ash

Figure 2: Elastic Modulus vs 28 days compressive strength





296

Figure 3: Elastic Modulus vs Water-cement ratio

A relation between modulus of elasticity, compressive strength, percentage of fly ash and

water cement ratio has been determined from regression analysis,

Assuming a relation:

E fck fa / w

Or, E = fck fa / w ------- (3.2.1)

Where,

E = Modulus of Elasticity

fck = Compressive strength of concrete

fa = % fly ash replacement in RAC sample

w = Water cement ratio

= Constant of proportionality

From Figure 1,

E = K1 fa2 + K2 fa + K3 ------------ (3.2.2)

K1= -0.0058 , K2 =0.1005, K3= 2 .26

From Figure 2,

E = K4 fCK 2

+ K5 fCK + K6 --------- (3.2.3)

K4= -0.125 , K5 =8.725, K6= -149.54

From Figure 3,

E = K7 w 2

+ K8 w + K9 ------- (3.2.4)





297

K7= -6 , K8 =4, K9= 1.76

Combining above equations

E = (K1 fa2

+ K2 fa + K3 )( K4 fCK 2

+ K5 fCK + K6 ) /

( K7 w 2

+ K8 w + K9 ) -------- (3.2.5)

Or, Ē = Ĕ -------- (3.2.6)

Where,

Ĕ = (K1 fa2

+ K2 fa + K3 )( K4 fCK 2

+ K5 fCK + K6 ) /

( K7 w 2

+ K8 w + K9 ) ---------(3.2.7)

Computing average for water cement ratio 0.4 and 0.5, derived value of modulus of

elasticity from above relation are computed below

Table 4: Comparison between predicted and experimental value of modulus of elasticity

SL w/c = 0.4 w/c = 0.5

Exp. E

value

Calc. E

value

%

deviation

Exp. E

value

Calc. E

value

%

deviation

1 24000 21334 11 22600 21244 6

2 27100 30028 10.8 26800 30765 15

3 23000 23585 2.5 19300 18142 6

It is observed that the value of modulus of elasticity calculated from the derived formula

tallies with the experimental results, so the relation could be used to predict the modulus of

elasticity of recycled aggregate concrete of different mixes.

3.3 Effect of fly ash on Compressive strength at different ages

Table 5 shows the compressive strength at various ages (7, 28, 90 days) for recycled

aggregate concrete and natural aggregate concrete. Compressive strength of RAC specimens

are found lower than that of NAC specimens. The percentage decrease in compressive

strength at all ages and all water cement ratio for the RAC specimens varies from 11% to

31%. However, reduction in compressive strength for recycled aggregate concrete with 10%

fly ash replacement (RAC1) is much lower (4% to 26%).

Table 5: Result of Compressive Strength

Mix

NO.

Mix

Designation

7 days

Strength MPa

28 days

Strength MPa

90 days

Strength MPa

1 N.A.C 43.33 47 49.53

2 N.A.C1 45.2 50.74 54.4

3 R.A.C 31.1 36.88 42.07

4 R.A.C1 35.5 38 43.4

5 R.A.C2 30 36.33 41.19

6 NACa 32 38 40





298

7 NAC1a 36.4 39.5 42

8 RACa 28.9 33 35.33

9 RAC1a 31.2 35.4 38.22

10 RAC2a 25.75 32.4 37

11 NACb 26 34 36

12 NAC1b 27 36 38

13 RACb 19.8 26 27.5

14 RAC1b 20.2 30 32

15 RAC2b 19.7 25.8 26.5

3.4 Effect of fly ash on Split tensile and flexural strength

Table 6 presents the flexural and split tensile strength of specimens at 28 days. Comparison

with NAC specimen reveals a reduction of 6% in RAC specimen. However, significant

improvement can be noticed in the RAC1 specimen which show only 3% reduction in both

flexural strength and split tensile strength.

Table 6: Split tensile and Flexural strength at 28 days

Mix

No.

Mix

Designation

Strength ( MPa)

Split tensile Percentage

change

Flexural Percentage

change

1 NAC 3.58 - 6 -

2 NAC1 3.7 (3.4) 6.2 (3.3)

3 RAC 3.37 (- 5.86) 5.64 (-6)

4 RAC1 3.68 (-2.8) 6.18 (3)

5 RAC2 3.0 (-16.2) 4.78 (-20.3)

Note: The value in the bracket ( ) indicates the percentage change of compressive

strength with respect to Mix No.1

It is noted that optimum replacement of cement by fly ash for RAC recovers the compressive

strength significantly. It shows that 10% fly ash replacement gives the maximum

compressive strength among RAC with normal fine aggregate. This may be due to optimal

gel formation, modified pore structure and improved interface bond between new cement

paste and recycled aggregate with old mortar attached to it.

3.5 Relation between compressive strength, age, percentage replacement of Fly ash and

water cement ratio

The relation between compressive strength of concrete, percent fly ash replacement, water

cement ratio and age of concrete are assumed as follows:

Assuming a relation:

fck fa ǡ / w

Or, fck = fa ǡ / w ---- ( 3.5.1)

Where,

= Constant of proportionality





299

fck= Compressive strength of recycled concrete

fa = % fly ash replacement in RAC sample

ǡ = Age of concrete

w = Reciprocal of Water cement ratio

From Figure 4 to 6, regression equations of different relationships between the above factors

have been obtained.

Figure 4: Strength of concrete vs Age of concrete

Figure 5: Strength of concrete vs Percentage Fly ash





300

Figure 6: Strength of concrete vs Reciprocal of water content

From Figure 4,

fck = K1 ǡ 2 + K2 ǡ + K3 -------- (3.5.2)

Where,

K1 = -0.0027, K2 = 0.3955, K3 = 27.363

From Figure 5,

fck = K4 fa2 + K5 fa + K6 -------- (3.5.3)

Where,

K4 = -0.0139 , K5 = 0.2515

, K6 = 36.88

From Figure 6,

fck =K7w2-K8w+K9 ---------- (3.5.4)

Where,

K7 = -15.888, K8 = 79.256, K9= -61.96

Combining regression equations from Figure 1 , 2 & 3 as per eqn (3.5.1)

fck = (K1 ǡ 2

+ K2 ǡ + K3) (K4 fa2 + K5 fa + K6 ) / (K7w

2-K8w+K9)

For Water content = 0.4

= 1.034576


= 0.823121


= 0.49344

Table 7: Comparison between predicted & experimental compressive strength of RAC mixes

Sl % fly

ash

Age of

conc.

Water

content

Exp. fck

(MPa)

Calc. fck

(MPa)

%

deviation

1 0 7 0.4 31.1 31.03 0.2





301

2 0 28 0.4 36.88 37.57 1.88

3 0 90 0.4 42.07 42.51 1.04

4 10 7 0.4 35.5 31.98 9.9

5 10 28 0.4 38 38.72 1.9

6 10 90 0.4 43.4 43.8 0.93

7 20 7 0.4 30 30.59 1.97

8 20 28 0.4 36.33 37.03 1.94

9 20 90 0.4 41.19 41.89 1.72

10 0 7 0.5 33.52 27.59 4.51

11 0 28 0.5 40.59 33.41 1.24

12 0 90 0.5 45.91 37.79 6.98

13 10 7 0.5 34.54 28.43 8.85

14 10 28 0.5 41.82 34.43 2.73

15 10 90 0.5 47.31 38.94 1.90

16 20 7 0.5 33.04 27.19 5.62

17 20 28 0.5 40.00 32.93 1.63

18 20 90 0.5 45.25 37.25 0.68

19 0 7 0.5 19.8 20.99 6.01

20 0 28 0.5 26 25.41 2.26

21 0 90 0.5 27.5 28.75 4.54

22 10 7 0.5 20.2 21.63 7.08

23 10 28 0.5 30 26.19 12.71

24 10 90 0.5 32 29.63 7.42

25 20 7 0.5 19.7 20.69 5.02

26 20 28 0.5 25.8 25.05 2.92

27 20 90 0.5 26.5 28.34 6.93

From table 7, it is found that, the test results and the calculated values are quite agreeable .

3.6 Mercury Intrusion Porosimetry (MIP) test

A typical plot between intruded mercury volume and pore diameter for recycled aggregate

concrete specimens is shown in Figure 7. Total pore volume of RAC is 8.4 cc/gm. However,

addition of 10% fly ash results in significant decrease of pore volume ( 5.2 cc/gm) .Major

portion of pores in RAC lies between 0.10 µm to 4 µm while in case of RAC1, most of the

pores lies between 0.03 µm to 1 µm. Results indicate reduced diameter of pore with addition

of fly ash ( 10% ) which explains in favour of getting higher strength of RAC1 over that of

RAC specimens.Incremental intrusion of mercury of RAC and RAC1 specimen is presented

in Figure 8. It can be clearly observed that RAC specimen exhibit noticeably high

incremental intrusion volume when compared to that of RAC1 specimen.





302

Figure 7: Comparison of Intruded Hg volume Vs pore diameter curve for RAC & RAC1

Figure 8: Comparison of Incremental intrusion of Hg volume vs Pore diameter

4. Conclusion

Based on the results obtained in the present study, following conclusions have been drawn:

1. Partial replacement of cement by fly ash leads to a encouraging result for the

Recycled aggregate concrete, which advocates the utilization of fly ash and Recycled





303

aggregates (both are waste materials and endanger the environment) for making

concrete.

2. Recycled aggregate concrete with 10% fly ash gives higher compressive strength over

Recycled aggregate concrete.

3. Though split tensile and flexural strength are lower in recycled aggregate concrete, it

could be improved to an extent by adding 10% by replacement of cement by fly ash.

4. Pore structure of recycled aggregate concrete could be improved by addition of fly

ash with respect of total pore volume as well as pore diameter.

5. Developed equations could predict the value of elastic modulus in direct compression

and compressive strength of recycled aggregate with reasonable accuracy.

5. References

1. Yamato TakeshiI, Emoto Yukio, Soeda Masashi and Sakamoto Yoshifumi (1988),

“Some Properties Of Recycled Aggregate Concrete”. Proceedings of the Second

International Symposium held by RILEM on “Demolition and Reuse of Concrete

and Masonry”, pp-643-651.

2. Ravindrararajah R. and Tam C.T. (1988), “Methods of Improving the Quality of

Recycled Aggregate Concrete”. Proceedings of the Second International

Symposium held by RILEM on “Demolition and Reuse of Concrete and Masonry”.

pp 575-584.

3. Topcu I.B. (1997), “Physical and Mechanical Properties of Concretes Produced

With Waste Concrete”. Cement and Concrete Research, 27(12), pp 1817-1823.

4. Bairagi N.K., Ravande Kishore and Pareek V.K. (1992), “Properties of Recycled

Aggregate Concrete”. Resources, Conservation and Recycling, 9 (1993) pp 109-

126.

5. Mondal S and Ghosh A Study on properties of recycled aggregate concrete, ICI

bulletin No.68, Indian Concrete Institute, July-September 1999, pp 21-23

6. M. Barra De Oliviea, E Vasquz (1996), The influence of retained moisture in

aggregates from recycling on the properties of new hardened concrete, Waste

manage, 16(1-3), pp113-117

7. A.Katz (2003), Properties of concrete made with recycled aggregate from partially

hydrated old concrete Cement and Concrete Research 33, pp 703-711

8. H.J Chen, T Yen, K.H.Chen (2003), Use of building nibbles as recycled aggregate

Cement and Concrete Research 33, pp 125-132

9. J.M.Khatib (2005), Properties of concrete incorporating fine recycled aggregate

Cement and Concrete Research, 35, pp 763-769

10. IS 12269:1987, Specification for 53 grade ordinary Portland cement, , Bureau of

Indian Standards , New Delhi





304

11. IS: 383: 1970, Specification for Coarse and Fine Aggregates from Natural Sources

for Concrete (Second Revision). Bureau of Indian Standards , New Delhi

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