EXPERIMENTAL STUDY ON SHEAR BEHAVIOR OF REINFORCED RECYCLED AGGREGATE CONCRETE BEAMS

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International Journal of Civil Engineering and Technology (IJCIET)

Volume 7, Issue 2, March-April 2016, pp. 128–139, Article ID: IJCIET_07_02_010

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EXPERIMENTAL STUDY ON SHEAR

BEHAVIOR OF REINFORCED RECYCLED

AGGREGATE CONCRETE BEAMS

Pinal C. Khergamwala

PhD Scholar, I. K. G. Punjab Technical University,

Jalandhar, Punjab, India

Dr. Jagbir Singh

Associate Professor, Department of Civil Engineering, GNDEC,

Ludhiana, India

Dr. Rajesh Kumar

Professor and Head, Department of Civil Engineering,

CCET, Chandigarh, India

ABSTRACT

The use of recycled aggregates (RA) for structural concrete in

construction, to the maximum possible limit, is becoming a necessity more

than a desire. One such mechanical property, shear resistance of recycled

aggregate concrete (RAC) beams is an intensive area of research. Three

parameters i.e. compressive strength, percentage of tension steel and shear

span to depth ratio were considered. An attempt has been made to study shear

strength of RA concrete beams of M 20 grade with 25 and 50 % weight

replacement of natural aggregate (NA) with recycled aggregate (RA) for

different shear span to depth ratios a/d = 1.5, 2.5 and 3.5 with 1 % tension

steel without shear reinforcement and compare the test results with the

available shear models. Seven shear models for comparison were considered

namely ACI 318, Canadian Standard, IS Code, CEB-FIP Model, Zsutty

Equation, Bazant Equation and Okamura and Higai equation. The results

revealed that Shear capacity of a RAC beams with 25 and 50 % RA is

comparable, or sometimes superior, to that of a controlled beam made of

conventional concrete. Equations proposed by Zsutty and Bazant gave

relatively more accurate results in terms of the similar pattern as compared to

other models but still considerably lower values as compared to experimental

results and hence these models can be used effectively for recycled aggregare

concrete also.

Key words: Recycled aggregates, Parameters, Recycled aggregate concrete,

Shear resistance, Shear models, Shear span to depth Ratio (a/d).

Experimental Study on Shear Behavior of Reinforced Recycled Aggregate Concrete

Beams


Cite this Article: Pinal C. Khergamwala, Dr. Jagbir Singh and Dr. Rajesh

Kumar, Experimental Study on Shear Behavior of Reinforced Recycled

Aggregate Concrete Beams, International Journal of Civil Engineering and

Technology, 7(2), 2016, pp. 128–139.

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=2

1. INTRODUCTION

Under the goal of sustainability, the use of recycled aggregate concrete (RAC) has

become an important issue in the field of civil engineering. Continuous efforts are

being made to improve the mechanical properties of RAC as compared to normal

aggregate concrete. There are several modes of failure in concrete structural members.

Due to the fragility of concrete structures, shear failure is one of the most important

and undesirable modes of failure. Shear strength of concrete depends significantly on

the ability of the coarse aggregate to resist shearing stresses. RA used is relatively

weaker than NA in most cases and yielded reduced shear strength. Shear force is

present in beams at sections where there is a change in bending moment along the

span. It is equal to the rate of change of bending moment. An exact analysis of shear

strength in reinforced concrete beam is quite complex.

The reuse of hardened concrete as aggregate is a proven technology - it can be

crushed and reused as a partial replacement for natural aggregate in new concrete

construction. The use of 100% recycled coarse aggregate in concrete, unless carefully

managed and controlled, is likely to have a negative influence on most concrete

properties but literature shows that the compressive strength of concrete up to 50 %

RA have strength in close proximity to that of normal concrete.

2. EXPERIMENTAL PROGRAMME

Nine reinforced concrete beams were cast and tested, under two point loading for

varying shear span to effective depth ratio (a/d). The section of all the beams (width

thickness) was kept constant at 150 300 mm. To investigate the effect of shear span-

to-depth ratio, a/d values of 1.5, 2.5, and 3.5 were selected to cover short,

intermediate, and long beams. Accordingly the overall length of the beam specimens

was varied in the range 1.60 m, 2.20 m and 2.70 m. The percentage of tension

reinforcement,bd

Ast100was kept constant 1.1%. Concrete of grade M 20 having

nominal crushing strength of 20 N/ mm2 was used for investigation. Keeping in view

the lower compressive strength of concrete with more than 50 % of recycled

aggregates, concrete mix with more than 50 % recycled aggregates were not taken in

to account for shear investigations and only 25 and 50 % weight replacement of

natural aggregate with recycled aggregate for M 20 grade was considered. Controlled

beams with 100 % natural aggregates (0 % RA) were also cast and tested to compare

the results. The details of the specimens for shear test are listed in the Table 1 below:

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Pinal C. Khergamwala, Dr. Jagbir Singh and Dr. Rajesh Kumar


Table 1: Details of the specimens for shear tests

Specimen

Name

Overall

length

L (mm)

Effective

Depth

d (mm)

Pt

of

steel

%

Ast

(mm2)

Shear

span-to-depth

Ratio a/d

No. of

specimens

Mix M20

M20R25A1.5P1 1600 265 1.1 452.16 1.5 01

M20R50A1.5P1 1600 265 1.1 452.16 1.5 01

M20R25A2.5P1 2200 265 1.1 452.16 2.5 01

M20R50A2.5P1 2200 265 1.1 452.16 2.5 01

M20R25A3.5P1 2700 265 1.1 452.16 3.5 01

M20R50A3.5P1 2700 265 1.1 452.16 3.5 01

Controlled beams with 100 % NA

M20A1.5P1 1600 265 1.1 452.16 1.5 01

M20A2.5P1 2200 265 1.1 452.16 2.5 01

M20A3.5P1 2700 265 1.1 452.16 3.5 01

Note: In Colum. (1), ‘M’ stands for Conventional mix type, ‘R’ indicates % of

recycled aggregate, ‘A’ stands for a/d ratio and ‘P’ indicates % of tension

reinforcement.

2.1. Test Materials

The concrete test specimens were cast using cement, fly ash, fine aggregate, natural

coarse aggregate, recycled coarse aggregate, water and steel. The materials, in

general, confirmed to the specification laid down in the relevant Indian Standard

Codes. Ordinary Portland Cement of 53 Grade from a single source with specific

gravity 3.14, confirming to IS: 8112-1989 was used. A low-calcium fly ash obtained

from the combined fields of the electrostatic precipitator of the thermal power plants

with specific gravity 2.24 was used. Locally available natural river sand having a

specific gravity of 2.58, water absorption of 1.10% and a fineness modulus of 2.68

was used as fine aggregate. Portable water free from any harmful amounts of oils,

alkalis, sugars, salts and organic materials was used for proportioning and curing of


Beams


concrete. Deformed steel bars of 10 mm and 12 mm nominal diameters and with

nominal yield strength of 423 MPa were used as tension reinforcement in the beams.

Shear reinforcement in the form of stirrups were not provided. All the steel

reinforcement bars confirmed to IS 1786: 1985.

Two types of coarse aggregates named Natural Aggregate (NA) and recycled

aggregates (RA) were used in the RAC mixes. Locally available crushed granite

having a specific gravity of 2.70 was used as NA. RA was derived from the tested

concrete cubes in the laboratory that contained well-graded crushed granite stone.

Specific gravity of RA was found 2.48, which is lower than NA. The concrete cubes

were crushed manually to the specified size using a hammer and gradation was

achieved through sieving of RA. The maximum size of coarse aggregate used was 20

mm in both recycled and natural aggregate concrete.

2.2. Concrete Mix Design

The concrete mix M 20 of characteristic strength of 20 N/ mm2 with constant water to

cement ratio (w/c) 0.5 was used in this investigation which is commonly used in

construction of structural members. The mix design was done according to the IS:

10262- 2009 and numerous trial mixes were conducted to obtain the optimum mix.

Once the optimum mix was determined, it was used to produce concrete with 25%

and 50% recycled coarse aggregate by weight replacement of natural coarse

aggregate. Due to the higher water absorption capacity of RA as compared to natural

aggregate, both the aggregates are maintained at saturated surface dry (SSD)

conditions before mixing operations. Fly ash was used as 25% by weight replacement

of cement to achieve proper workability of the mix. The details of optimum mix are

given in Table 2.

Table 2: Mix proportion for optimum mix

Mix Mix

proportion

by weight

Fly

Ash

%

Constituents (kg/m3) W/C

ratio

Cement Fly

Ash

Sand Aggregates

M 20 1:1.5:3.4 25 289 96 578 1310 0.5

2.3. Instrumentation and Testing Procedure

In the present study beams were cast in steel forms with the tension reinforcement

near the bottom. No stirrups (shear reinforcement) were provided in the beams.

Lifting lugs were also provided for transporting the finished specimen to the test

platform. The concrete was compacted with needle vibrator. Form work was removed

after 48 hours. The beams were cured with wet hessian and sand for 28 days. To

facilitate the tracing of cracks, the beams were distempered white prior to testing.

For investigation of the shear behavior, beams designed only for adequate flexural

strength and without any web reinforcement were tested under monotonically

increasing loads in a four point loading configuration to study the shear failure mechanism. The beam specimens were tested as simply supported beam by using a

manually operated hydraulic Jack that applied load gradually on the mid-span of the

beam specimens until shear failure which pre-empted flexural failure. Diagonal

cracking along with the formation of a dominant inclined crack is indicative of shear



failure. Seven deflection gauges were employed to record deflection. The arrangement

of 4 LVDT’s attached diagonally in pairs on the side-face of the beams in the shear

zone were done to detect diagonal cracking. The test setup configuration for the shear

tests is shown in Figure. 1.

Figure 1 Test setup configuration for the shear tests

Figure 2 Shear failure of actual beam specimen

LVDT

Roller Hinge

Spreader beam

Effective span

All dimensions are in

mm

300 300

300

LVDT

Steel sleeve


Beams


3. DISCUSSION ON TEST RESULTS

The cracking and ultimate shear stress calculations of RAC beams are presented in

Table 3. Graphical representation of comparison of cracking and ultimate shear

strength of RAC beams with controlled (NAC) beams is shown in Figs. 3 and 4. In the

case of short beams (a/d < 2.5), a very significant amount of additional loading can be

resisted by the reinforced recycled aggregate concrete beams beyond the formation of

a first diagonal crack before ultimate failure in shear-compression occurs. This

redistribution of stresses in short beams takes place because of the relatively short

distance between the supports and the applied loads, and is evidenced by the large

spread in magnitude that exists between Vcr and Vu (in short beams, the redistribution

of stresses is due to the transferring of the applied loads directly to the supports by

arch action). Conversely, the failure mode in long beams (a/d ≥ 2.5) is in diagonal

tension with the formation and propagation of the first fully developed inclined crack.

As a/d increases from 2.5 to 3.5, this failure mode becomes very sudden in RAC

beams as total shear failure occurs almost immediately after the formation of a first

major diagonal cracking.

Table 3 Shear test results of the RAC beams

Specimen

ID

Area of

tension

steel,

Ast

(mm2)

Reinforcement

ratio,

ρ=Ast/bd

a/d

Measured

characteristics

strength,

fck

(MPa)

Diagonal

cracking

shear,

Vcr

(kN)

Ultimate

shear,

Vu

(kN)

Cracking

shear

stress,

vcr=Vcr/bd

(MPa)

Ultimate

shear

stress,

vu=Vu/bd

(MPa)

M20A1.5P1 452.16 0.011 1.5 24.17 106.69 191.36 2.684 4.814

M20R25A1.5P1 452.16 0.011 1.5 23.43 88.83 181.97 2.235 4.578

M20R50A1.5P1 452.16 0.011 1.5 24.93 92.43 189.84 2.325 4.776

M20A2.5P1 452.16 0.011 2.5 24.17 50.15 88.16 1.262 2.218

M20R25A2.5P1 452.16 0.011 2.5 23.43 43.95 80.82 1.106 2.033

M20R50A2.5P1 452.16 0.011 2.5 22.67 48.15 89.57 1.211 2.253

M20A3.5P1 452.16 0.011 3.5 23.80 44.95 61.79 1.131 1.554

M20R25A3.5P1 452.16 0.011 3.5 23.43 40.26 58.46 1.013 1.471

M20R50A3.5P1 452.16 0.011 3.5 22.67 43.84 54.78 1.103 1.378



5. COMPARISON OF SHEAR STRENGTH OF RAC BEAMS

WITH CONTROLLED (NAC) BEAM

Figure 3 Cracking shear strength Vcr versus a/d

Figure 4 Ultimate shear strength Vu versus a/d

Results indicated that with the increase in a/d ratio, there is sharp decrease in the

shear capacity of the beam. At a/d ratio 1.5, the first cracking load as well as the

ultimate diagonal shear load was observed to be almost double than that at a/d ratio

2.5 and 3.5. At a/d ratios 3.5 and concrete with 50 % of RA, the failure was observed

to be sudden as compared to failure pattern observed for lower a/d ratio 1.5. Crack

width for RAC beams was wider as compared to control beam due to weak bonding

of RA with new concrete. Results also showed that the shear capacity of a RAC

beams with 25 and 50 % RA is comparable, or sometimes superior, to that of a

controlled beam made of conventional concrete.

0

20

40

60

80

100

120

1.5 2.5 3.5

Crack

ing s

hear

stre

ngth

, k

N

Shear span to depth ratio, a/d

0 % RA

25% RA

50% RA

0

20

40

60

80

100

120

140

160

180

200

1.5 2.5 3.5

Ult

imate

sh

ear

stre

ngth

, k

N


0 % RA

25% RA

50% RA


Beams


4. COMPARISON OF EXPERIMENTAL SHEAR RESULTS OF

RAC BEAMS WITH AVAILABLE SHEAR MODELS

Based on experimental observations, different researchers have developed different

equations for the prediction of shear capacity of the NAC rectangular beams. Shear

models used for NAC beams have been used here to predict shear strength of RAC

beams because till date no specific code and models for RAC is formulated.

Table 4 presents the experimental and predicted shear strength values of all the

test beams with different percentage of recycled aggregate and with different a/d

ratios. It can be observed that regardless of the a/d ratio, all the empirical equations

gave a conservative estimate of the actual ultimate shear strength for all the

replacement level of recycled aggregate of RAC beams. For all the models used to

calculate Vc, the calculated strength values became more conservative as the a/d ratio

decreases. T C Zsutty collected the test data of about 200 beams from different

responsible sources and developed equations by combining the techniques of

dimensional and statistical regression analysis for the prediction of shear strength of

longitudinally reinforced beams.

Table 4 Experimental and Predicted Results of shear strength

Specimen ID

Experime

ntal

Failure

Load

Vexp

(kN)

Vpredicted

(kN)

ACI

code

Equatio

n

Canadia

n Code

Equatio

n

IS: 456-

2000

code

Equatio

n

CEB-

FIP

Mode

l

Zsutty

Equatio

n

Bazant

Equatio

n

Okamur

a &Higai

Equatio

n

M20A1.5P1 106.69 32.46 39.01 25.91 40.71 79.91 78.71 42.69

M20R25A1.5P1 88.83 32.13 38.40 25.81 40.31 78.72 78.26 42.28

M20R50A1.5P1 92.43 33.02 39.62 26.00 41.16 80.37 79.17 43.15

M20A2.5P1 50.15 30.76 39.01 25.91 34.33 40.44 43.01 39.27

M20R25A2.5P1 43.95 30.44 38.40 25.81 34.01 39.83 42.56 38.87

M20R50A2.5P1 48.15 29.98 37.78 25.70 33.63 39.41 42.10 38.44

M20A3.5P1 44.95 29.82 38.70 25.86 30.53 35.96 34.93 37.41

M20R25A3.5P1 40.26 29.71 38.40 25.81 30.42 35.61 34.71 37.21

M20R50A3.5P1 43.84 29.25 37.78 25.70 30.12 35.23 34.24 36.81



(a) For M 20 (NAC beams)

(b) For M 20 R 25

0

20

40

60

80

100

120

1.5 2.5 3.5

Sh

ear

stre

ngth

, k

N


M 20 R 0- 1 %

Experimental

ACI code

Canadian

IS: 456- 2000

CEB- FIP

Zsutty

Bazant

Okamura &Higai

0

20

40

60

80

100

1.5 2.5 3.5

Sh

ear

stre

ngth

, k

N


M20 R25- 1 %

Experimental

ACI code

Canadian

IS: 456- 2000

CEB- FIP

Zsutty

Bazant

Okamura &Higai


Beams


(c) For M 20 R 50

Figure 5: Comparison between predicted and Experimental Results

The comparison of the experimental results with predicted values for all the seven

models are presented in Fig. 5 (a), (b) and (c) for M 20 with 0, 25 and 50 % RA.

Almost similar trend of normal aggregate concrete members is followed by RAC

beams. There is no negative impact of the replacement of 25 and 50 % RA. Analytical

values and experimental results revealed that a/d ratio significantly affects the shear

capacity of recycled aggregate concrete beams. Most of the equations are under

estimating the shear capacity at lower a/d ratios. When the a/d ratio is less than 1.5,

strut action prevails and the shear resistance is very high. For a/d ratio 1.5 the

experimental values showed remarkable increase in shear strength compared to

various design models. Only predicted shear capacity using Zsutty and Bazant

Equation had followed the same pattern for all the three a/d ratios but the values were

still lower than experimental values for all concrete mixes. For a/d ratios 2.5 and 3.5

almost all the models followed the same trend but with quite lower values.

5. CONCLUSION

Shear capacity of a RAC beams with 25 and 50 % RA is comparable, or sometimes

superior, to that of a controlled beam made of conventional concrete.

For a/d ratio 1.5, there is sharp increase (almost double) in shear capacity of RAC

beams as compared to a/d ratio 2.5 and 3.5. There is not much difference in the shear

capacity of RAC beams for a/d ratio 2.5 and 3.5.

For higher a/d ratio 3.5, sudden shear failure of RAC beams were observed as

compared with a/d ratio 1.5. There is less difference between first crack load and

ultimate shear load for a/d ratio 3.5.

Crack width for RAC beams was wider as compared to control beam due to weak

bonding of RA with new concrete.

ACI as well as IS code give overly conservative shear capacity predictions of

recycled aggregate concrete beams without web reinforcement at all a/d ratios

because ACI code presented a formula for the prediction of shear cracking load in

1963, which was developed by the linear regression based on thousands of beam test

results subjected to UDL only.

0

20

40

60

80

100

1.5 2.5 3.5

Sh

ear

stre

ngth

, k

N


M20 R50- 1 %

Experimental

ACI code

Canadian

IS: 456- 2000

CEB- FIP

Zsutty

Bazant

Okamura &Higai



The Canadian code considered only compressive strength of concrete. It has not taken

into account the effect of shear span to depth ratio and longitudinal tension

reinforcement on shear strength of beams. The shear resistance of RAC member

predicted based on Canadian code underestimates the actual shear capacity of

member at all a/d ratios.

Shear capacity of the RAC members predicted based on CEB-FIP model and

Okamura- Higai equation showed conservative values at all a/d ratios.

Zsutty equation is more appropriate and simple to predict the shear strength of both

shorter and long beams as it takes into account size effect and longitudinal steel effect

for RAC beams also.

The Bazant equation has better agreement with the test data. In this equation five

parameters (fc΄, ρ, a,d, d and da) are correlated with ultimate shear strength of

rectangular beams, especially the effect of aggregate size, which plays very important

role in the shear strength.

ACKNOWLEDGMENTS

I express my sincere thanks to I. K. G. Punjab Technical University, Kapurthala,

India for providing strong platform for pursuing Ph.D. Authors acknowledge the

help received from Head and faculty members of the Civil Engineering Department,

Guru Nanak Dev Engineering College, Ludhiana, Punjab, for making testing

facilities available to them. The invaluable cooperation of the laboratory staff of

Heavy Testing Laboratory and Concrete Testing Laboratory of Civil Engineering

Department is gratefully acknowledged.

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