Shear strength of reinforced recycled concrete

beams without stirrups

G. Fathifazl*, A. G. Razaqpury, O. Burkan Isgor{, A. Abbas}, B. Fournier} andS. Foo**

Adjeleian Allen Rubeli Consulting Structural Engineers; McMaster University; Carleton University; AmecAmericas; Universite Laval; Public Works and Government Services Canada

A new method of mixture proportioning is used for investigating the shear performance of reinforced concrete (RC)

beams made with coarse recycled concrete aggregate (RCA). In this method, RCA is treated as a two-phase

material comprising mortar and natural aggregate therefore to proportion the concrete mixture with RCA, the

relative amount and properties of each phase are considered separately. Using the new mix proportioning method,

several beams were designed and tested to study the effect of a number of parameters including the shear span-to-

depth ratio and beam size on the serviceability and strength of RCA concrete beams without shear reinforcement.

For each beam its load–deflection curve, shear deformations, diagonal cracking load, crack pattern, ultimate shear

strength and failure mode were determined. The results showed that the shear performance of RC beams made with

RCA can be comparable, or even superior, to that of beams made entirely with natural aggregates at both

serviceability and ultimate limit states, provided the proposed mixture proportioning method is used. Furthermore,

the simplified methods of ACI and CSA standards as well as Eurocode 2 were found applicable to all reinforced

RCA-concrete beams.

Introduction

Despite the economical and environmental benefits

of concrete involving recycled concrete aggregates

(RCA),1 the construction industry has not embraced it,

especially for structural applications. This is partly a

consequence of previous findings reported in the litera-

ture and of the prevailing belief that concrete made

with coarse RCA has inherently inferior short- and

long-term properties compared with conventional con-

crete made entirely with natural aggregates.

In the past, the majority of investigations have re-

ported similar shear cracking pattern and failure modes

in conventional reinforced concrete (RC) and rein-

forced recycled concrete (RRC) beams, but lower diag-

onal cracking load and ultimate shear strength have

always been presented.2–4 Furthermore, smoother crack

interface, less effective aggregate interlock mechanism

and consequently less ductile shear behaviour in RRC

beams compared with conventional RC beams have

been reported.2 Consequently, questions have been

raised with respect to the applicability of the existing

empirical relations for calculating the concrete contri-

bution to the shear resistance of conventional RC mem-

bers, commonly denoted as vc, to RRC beams,

especially at larger shear span-to-depth ratios.2,4 This

has been mainly attributed to the less effective aggre-

gate interlock mechanism in RRC beams compared

with conventional RC beams. It is shown in the current

study that the previously reported lower shear strength

of RCA-concrete – that is, concrete made with RCA –

is not an intrinsic property of RCA-concrete; rather, it

is the consequence of an improper method of mixture

proportioning.

Until now the conventional mix proportioning

method for normal concrete has been commonly used

* Adjeleian Allen Rubeli Consulting Structural Engineers, Ottawa,

ON, Canada

y Department of Civil Engineering, McMaster University, Hamilton,

Ontario, Canada

{ Department of Civil and Environmental Engineering, Carleton Uni-

versity, Ottawa, Ontario, Canada

} Amec Americas, Calgary, Alberta, Canada

} Department of Geology and Engineering Geology, Universite Laval,

Quebec, Quebec, Canada

** Public Works and Government Services Canada, Gatineau, Que-

bec, Canada

(MACR 800203) Paper received 14 December 2008; accepted 10

February 2009

Magazine of Concrete Research, 2009, 61, No. 7, September, 477–490

doi: 10.1680/macr.2008.61.7.477

477

www.concrete-research.com 1751-763X (Online) 0024-9831 (Print) # 2009 Thomas Telford Ltd

for RCA-concrete, with some adjustments, such as in-

crease in the cement content, but with no special con-

sideration given to the residual mortar quantity in

RCA.5–10 However, RCA is a two-phase material com-

prising residual mortar and original virgin aggregate;

thus concrete made with coarse RCA, if proportioned

in accordance by conventional methods, would contain

less natural aggregate and more mortar compared with

the concrete containing an equal volume of natural

aggregate only. The reason is that in the conventional

mix design, the residual mortar in RCA is treated as

part of the aggregate rather than mortar. In the current

authors’ opinion, the lower total natural aggregate con-

tent of RCA-concrete is generally responsible for the

observed inferiority of both plain and RRC concrete

compared with conventional concrete. From the shear

resistance perspective, the smaller natural aggregate

content of RCA-concrete results in fewer coarse aggre-

gate particles crossing the cracked shear plane, which

results in reduction of the roughness or asperity of the

crack faces and consequently reduction in the effective-

ness of aggregate interlock mechanism to resist shear

in members made of RCA-concrete. It is suggested

here that the observed inferior properties of RCA-

concrete are not intrinsic, but instead are the conse-

quence of its composition, and the inferiority can be

eschewed by adjusting its composition through applica-

tion of a proper method of mix proportioning.

To test this hypothesis, a new mix proportioning

procedure was developed by the authors of the current

paper,11 based on the fundamental premise that RCA is

a two-phase material comprising residual mortar and

original virgin aggregate; therefore, when proportioning

a concrete mix involving RCA, the volumetric ratio

and relevant properties of each phase must be ac-

counted for separately. In other words, it cannot be

assumed, as is currently customary, that RCA simply

replaces natural aggregate in the mix because it also

modifies the overall mortar content of the mix owing to

the presence of residual mortar in RCA. The main

feature of the proposed method, termed equivalent mor-

tar volume (EMV), is the treatment of residual mortar

in RCA as part of the total mortar volume of concrete.

The total mortar volume is considered as the sum

of the residual and fresh mortar volumes in RCA-

concrete. Concrete proportioned based on this method

has been found to have the same or superior fresh and

hardened properties compared with an equivalent con-

ventional concrete with the same volume of fresh mor-

tar as the total volume of mortar in the companion

RCA-concrete.11

Because the RCA-concrete mixes proportioned by

the EMV method do not suffer from the inferiorities of

similar concrete proportioned by the conventional

method, it is expected that RC beams made of RCA-

concrete proportioned by the EMV method will not

experience lower ultimate shear strength compared with

conventional RC beams made of natural-aggregate-

concrete. To verify this hypothesis, an extensive experi-

mental study was carried out. Concrete mixes propor-

tioned by the EMV method were used to cast a large

number of beams. As the key parameters that are

known to affect the concrete contribution to the shear

resistance of RC member, vc, are the shear span-to-

depth ratio and the beam size,12 the effects of these

parameters on vc in reinforced RCA-concrete beams

are investigated. RCA from two different sources are

used in the study, and are designated as RCA-M and

RCA-V, which were obtained from demolition concrete

recycling plants in Montreal (M) and Vancouver (V)

respectively. The original virgin aggregate in RCA-M

is limestone while that in RCA-V is river-bed gravel.

Experimental investigation

Mixture proportions

Two mix types were prepared for each RCA source

using ordinary Portland cement: (a) a control concrete

mix made with coarse natural aggregate of the same

kind as that in the companion RCA and proportioned

according to the American Concrete Institue (ACI)

method for normal concrete;13 and (b) a companion

RCA-concrete mix involving replacement of the coarse

natural aggregate by coarse RCA and proportioned

according to the EMV method.11 The fine aggregate in

all the mixes was natural sand. Note that using the

EMV method ensured equal total mortar volume in the

two mix types.

To compensate for the deficiency in the total natural

aggregate volume of RCA-concrete mix compared with

its companion natural-aggregate-concrete mix, in the

former, which contained both RCA and fresh coarse

natural aggregate, the fresh natural aggregate volume,

VRCA-concreteNA , was set equal to the total residual mortar

volume in RCA-concrete, VRCA-concreteRM . The fresh natur-

al aggregate and the natural aggregate contained in the

RCA in each mix were of the same kind; depending on

the RCA source, that is Vancouver against Montreal,

they were either river gravel or crushed limestone.

The specific gravity and absorption capacity of the

aggregates were determined using the standard testing

procedures of the American Society for Testing and

Materials (ASTM).14 The residual mortar content of

each RCA type was determined based on a new method

that involved the immersion of RCA in sodium sul-

phate solution and exposure to several freeze–thaw

cycles.15 Table 1 shows the weighted average properties

for RCA-M, RCA-V, natural limestone, natural gravel

and river sand. Both the natural and the recycled con-

crete aggregates had a nominal maximum size of

19 mm. These aggregates were presoaked while the

fine aggregate was kept moist for 24 h before mixing.

For each mix, six 100 3 200 mm cylinders were pre-

pared and cured in a moist room for 28 days to deter-

mine the compressive and splitting tensile strengths of

G. Fathifazl et al.

478 Magazine of Concrete Research, 2009, 61, No. 7

the concrete, using three specimens for each test. An-

other three 150 3 300 mm cylinders were similarly pre-

pared to determine the 28-day elastic modulus of each

concrete mix. An additional nine concrete cylinders

were prepared and cured adjacent to and in the same

manner as the test beams to evaluate the compressive

strength (three 100 3 200 cylinders), splitting tensile

strength (three 100 3 200 cylinders) and elastic modu-

lus (three 150 3 300 cylinders) of the pertinent con-

crete at the time of the testing of the beams. Table 2

presents the proportions of the mixes for the beams,

with the mix designations defined in the last row of the

table. Note that the proportions of the fine and coarse

aggregate are based on oven-dried and air-dried states

respectively. The air-dried coarse aggregate was pre-

pared in individual size fractions (35%, 25% and 40%

for 4.75 mm, 9.5 mm and 12.5 mm fraction sizes re-

spectively) and subsequently combined to produce the

desired grading.

All the beams for each RCA source were cast simul-

taneously using a truck mixer, but as the number of

beams made of the control concrete mixes was small, a

pan mixer was used to prepare the mixes for the control

beams. Table 3 shows a summary of the fresh and

hardened properties of the mixes used.

All of the steel used as flexural reinforcement was

Table 1. Average physical properties of coarse and fine aggregates

Aggregate Moisture

content: %

Absorption

content: %

Specific gravity RMC*: %

Bulk SSD Apparent

RCA-M 1.1 5.40 2.31 2.42 2.64 41

RCA-V 1.3 3.30 2.42 2.50 2.64 23

Limestone 0.2 0.34 2.70 2.71 2.73 —

River gravel 0.2 0.89 2.72 2.74 2.79 —

River sandy 4.0 0.54 2.70 2.72 2.76 —

* Residual mortar content ¼ oven-dry weight of residual mortar/oven-dry weight of RCAy Fineness modulus of 2.60

Table 2. Mix proportions of reinforced recycled concrete and control beams

Beam ID RCA content: % Mix proportions: kg/m3

Water Cement Sand Coarse aggregate WRA*:

ml

AEy: ml

RCA Natural aggregate

EM 63.5 151 335 630 720 414 1055 35

CL 0 193 430 808 0 835 None 92

EV 74.3 161 358 645 813 281 1132 38

CG 0 191 424 763 0 900 None 91

Mix designation

nomenclature

E or C: mix proportioned based on EMV (E) or conventional method (C); and 2) M, V, L or G: mix made with RCA-MO

(M), RCA-VA (V), natural limestone (L) or natural gravel (G)

* WRA: Water reducing agent, y AE: Air entraining

Table 3. Fresh and hardened properties of investigated concrete mixes

Mix ID Fresh properties Hardened properties Hardened

ªc: kg/m3

f 9c: MPa Ec: GPa f t: MPa

Slump:

mm

Air content:

%

Fresh

ªc: kg/m3

28

days

Test

date

28

days

Test

date

28

days

Test

date

EM 96 6.4 2341 41.6 36.9 29.8 24.6 3.4 2.8 2333

EV 60 4.5 2398 49.1 43.5 31.8 27.1 3.7 3.4 2364

CL Batch-1 185 5.9 2333 37.1 38 30.3 24.5 3.2 3.0 2308

Batch-2 160 6.4 2330 38.8 38.3 31.7 25.2 3.8 2.7 2324

CG Batch-1 220 6.4 2347 33.8 35.9 30.5 27.9 3.3 3.2 2308

Batch-2 200 6.2 2358 34.4 32.8 31.3 27.1 3.3 3.2 2322

Shear strength of reinforced recycled concrete beams without stirrups

Magazine of Concrete Research, 2009, 61, No. 7 479

grade 400 bars in accordance with the requirements of

CAN/CSA G30.18.16 Nominal bar diameters varied

from j8 mm to j30 mm; the j8 mm bar was smooth

while all the other bar sizes were deformed. In the

coupon tests, the deformed bars exhibited a clear yield

plateau with their yield strength varying between 407

and 473 MPa and their ultimate strength varying be-

tween 572 and 733 MPa. The round smooth bar had a

yield strength of 530 MPa and ultimate strength of

596 MPa. The elastic modulus for all the bars was

approximately 180 GPa, which seems smaller than the

expected value of 200 GPa.

Details of test beams

The test programme comprised 20 longitudinally re-

inforced beams without shear reinforcement. In addi-

tion to the type of concrete, the other test parameters

included beam shear span/depth ratio, a/d, and beam

size.12 Four a/d ratios (1.5, 2, 2.7 and 4) were selected

to cover the shear behaviour of short, intermediate and

slender beams, which are known to exhibit different

shear strength even if they are otherwise identical.17

For each RCA-concrete type proportioned by the EMV

method, four beams were tested – that is, one beam for

each a/d ratio. All the beams were prismatic with

200 mm wide rectangular cross-section, and with an

overall depth ranging from 350 to 375 mm (effective

depth of 305 � 5 mm). For each RCA type, a control

beam with a/d ratio of 2.70 was made of the compa-

nion natural-aggregate-concrete. Therefore, in total, ten

beams were tested to investigate the effect of a/d ratio

on the shear behaviour and strength of RRC beams

without shear reinforcement. All the beams were long-

itudinally reinforced, with their reinforcement ratios

given in Table 4. To facilitate tracking of the shear

deformations and diagonal crack movements, the west

shear span of each beam was instrumented with a

rosette of linear variable differential transducers

(LVDTs) as described below; consequently, to ensure

shear failure occurrence in the instrumented half of the

span, the other half was reinforced with j10 mm closed

steel stirrups as shown in Figure 1(a). There was only

one exception, in which the whole beam was inadver-

tently reinforced with stirrups, and this case will be

further discussed in a subsequent section.

To study the size effect, four 200 mm wide rectangu-

lar beams with depths of 250, 375, 450 or 550 mm, and

with a constant a/d ratio of 2.70 were built for each

RCA type. Once again, for each RCA type a 200 mm

wide by 375 mm deep control beam was made of

Table 4. Test beams details

Effect of a/d ratio

Beam

ID

a/d Dimensions Stirrup

spacing(s)

Longitudinal bottom bars

(r, %)

D L

EM-1.5N 1.50 300 1900 150 2 No. 20 (1.00)

EM-2N 2.00 300 2200 200 3 No. 20 (1.5)

EM-2.7N 2.59 309 2600 200 3 No. 15 + 2 No. 15 (1.62)

CL-2.7N 2.59 309 2600 200 3 No. 15 + 2 No. 15 (1.62)

EM-4N 3.93 305 3400 200 3 No. 20 + 2 No. 20 (2.46)

EV-1.5N 1.50 300 1900 150 2 No. 20 (1.00)

EV-2N 2.00 300 2200 200 3 No. 20 (1.5)

EV-2.7N 2.59 309 2600 200 3 No. 15 + 2 No. 15 (1.62)

CG-2.7N 2.59 309 2600 200 3 No. 15 + 2 No. 15 (1.62)

EV-4N 3.93 305 3400 200 3 No. 20 + 2 No. 20 (2.46)

Size effect

Beam ID a/d Dimensions: mm Stirrup

spacing(s)

Longitudinal bottom bars

(r, %)

A L

EM-L 2.69 201 2080 135 2 No. 20 + 1 No. 15 (1.99)

EM-M 2.59 309 2600 200 3 No. 15 + 2 No. 15 (1.62)

CL-M 2.59 309 2600 200 3 No. 15 + 2 No. 15 (1.62)

EM-H 2.73 381 3180 200 2 No. 25 + 2 No. 15 (1.83)

EM-HH 2.73 476 3700 200 2 No. 25 + 2 No. 20 (1.68)

EV-L 2.69 201 2080 135 2 No. 20 + 1 No. 15 (1.99)

EV-M 2.59 309 2600 200 3 No. 15 + 2 No. 15 (1.62)

CG-M 2.59 309 2600 200 3 No. 15 + 2 No. 15 (1.62)

EV-H 2.73 381 3180 200 2 No. 25 + 2 No. 15 (1.83)

EV-HH 2.73 476 3700 200 2 No. 25 + 2 No. 20 (1.68)

G. Fathifazl et al.

480 Magazine of Concrete Research, 2009, 61, No. 7

natural aggregate concrete to compare its shear strength

with that of an otherwise identical beam made of RCA-

concrete. Therefore, a total of ten beams were tested to

investigate the size effect on the shear strength of re-

inforced RCA-concrete beams; Table 4 gives the details

of these beams. The following notation is used to desig-

nate them: symbols 1.5N, 2N, 2.7N or 4N refer to the

nominal a/d ratio of 1.5, 2, 2.7 or 4 and N signifies no

shear reinforcement; L, M, H or HH characterises the

beam depth as 250 mm (low-L), 375 mm (medium-M),

450 mm (high-H) and 550 mm (very high-HH), respec-

tively. These beams were similarly reinforced as those

previously described.

Instrumentation and test set-up

Electrical resistance strain gauges were used to mea-

sure the strain in the longitudinal reinforcement and the

concrete. Beam deflection was measured using linear

potentiometers placed along the beam. All the beams

were simply supported and tested in four-point bending.

The load was applied by a 1000 kN servo-controlled

hydraulic actuator, attached to a rigid frame. The actua-

tor applied the load by stroke control to a steel spreader

beam supported by two heavy-duty rocker-and-roller

assemblies symmetrically located 300 mm from the

midspan of the beam. The load was applied using a

displacement rate of 0.01 mm/s, and the automatic data

acquisition system recorded data every 10 s.

As the majority of previous research has reported

wider cracks, smoother crack interface and less ductile

behaviour for RRC beams compared with conventional

concrete beams,2–4 to verify the validity of these find-

ings with regard to the beams in this investigation, the

beams were outfitted with a rosette of three LVDTs

(ST1, ST2 and ST3), arranged as illustrated in Figure

1(b). The length of the LVDTs and the location of the

rosette were chosen based on the expected location of

the major diagonal crack. The LVDTs were placed to

measure both diagonal tension and diagonal compres-

sion deformations in the instrument part of the beam,

with the former giving an indication of crack width and

the latter providing information about the extent of

deformation sustained by the concrete in the diagonal

strut.

Experimental results

The results are presented with focus on the effects of

the a/d ratio and beam size on the shear behaviour and

strength of RCA-concrete beams without shear rein-

forcement.

Effect of a/d ratio

Failure modes. Figure 2 illustrates the cracking

patterns of the beams with different a/d ratios. In this

figure, the dark lines represent the major diagonal

cracks leading to shear failure, the grey lines the

minor shear and flexure-related cracks, and the dark

zones the crushed concrete areas. All the beams

failed in shear, except beams EV-1.5N and EV-2.7N,

which failed in flexure prior to shear failure. The

flexural failure of beam EV-2.7N was caused by a

fabrication mistake that resulted in the reinforcement

of this beam with stirrups throughout its length and

thus to an inadvertent increase in its shear strength.

After inclined cracking, the RCA-concrete beams

West East

600L

a a

2 No. 10

No. stirrup (west)No. 10 @ S (east)

As200

dh

(a)

West East

x

sx

h/2ST2

ST3

ST1Sy

(b)

Figure 1. Beam details: (a) details of shear beams without

shear reinforcement; (b) arrangement of LVDT rosette for

sheared deformation measurements

EM

-1·5

N

EM

-2·0

N

EM

-2·7

NC

L-2·

7N

EM

-4·0

N

(a)

EV

-1·5

N

EV

-2·0

N

EV

-2·7

NG

G-2

·7N

EV

-4·0

N

(b)

Figure 2. Typical crack patterns of test beam with different a/d

ratios at failure: (a) EM and CL beams; (b) EVand CG beams

Shear strength of reinforced recycled concrete beams without stirrups

Magazine of Concrete Research, 2009, 61, No. 7 481

with a/d ratio of 1.5 or 2 behaved akin to a tied arch

carrying the load by direct compression by way of

struts extending from the loading plates to the supports

and by the longitudinal tension reinforcement acting as

tie. Consequently, they exhibited considerable shear

capacity. On the other hand, the beams with a/d ratio of

2.7 or 4.0 did not develop the same shear resistance

mechanism; therefore, they failed shortly after the for-

mation of the major diagonal crack. These observations

are consistent with the known behaviour of conven-

tional concrete beams with similar a/d ratios.17

Considering the ratio of the failure load to the in-

clined cracking load as an indicator of ductility, or

ductility index, beam EM-1.5N had the highest ductility

index of 2.59 compared with 2.07, 2.00, and 1.0 for

beams EM-2.0N, EM-2.7N and EM-4.0N. Similarly,

beam EV-1.5N had a ductility index of at least 3.06

compared with 2.89 and 1.56 for beams EM-2.0N and

EM-4.0N. Beam EM-2.7N had a ductility index of

2.00, which is much higher than the value of 1.38 for

the companion control beam CL-2.7N made of conven-

tional concrete. The latter ductility index values contra-

dict the reported findings of other researchers with

regard to the ductility of RCA-concrete members.

Figures 3(a) and (b) illustrate the effect of a/d ratio

on the longitudinal steel strain variation at a distance d

from the west support face with shear load for both EM

and EV beams. It can be seen that the longitudinal steel

reinforcement near the west support in beams EM-1.5N

and EV-1.5N yielded. This is because of the high shear

resistance of these beams that was sustained by the arch

mechanism, the maintenance of which requires the

development of a substantial tensile force in the tension

reinforcement acting as the arch tie. Consequently, the

resistance of these beams was limited by the tensile

capacity of the longitudinal reinforcement rather than

the shear capacity of the RCA-concrete.

Ultimate shear strength. Figure 4 illustrates the

normalised shear resistance (vc=ffiffiffiffiffif 9c

p) of the EM and

EV test beams with different a/d ratios. For conveni-

ence, the vc value for each beam is normalised by

the square root of the compressive strength of its

concrete.

Notice that generally the vc=ffiffiffiffiffif 9c

pvalue increased as

the a/d ratio decreased. As stated earlier, this is mainly

owing to the arch mechanism resistance which depends

on the magnitude of the diagonal compression and on

the inclination of the thrust line of the arch, which is a

function of the a/d ratio. According to Figure 4(a), the

vc=ffiffiffiffiffif 9c

pvalues of EM-1.50N, EM-2.0N and EM-2.7N

beams were, respectively, 128%, 102% and 9%, higher

than that of EM-4.0N. Furthermore, the vc=ffiffiffiffiffif 9c

pvalues

of EV-1.50N and EV-2.0N beams were 88% and 68%

higher than that of EV-4.0N (Figure 4(b)). Note that the

actual vc=ffiffiffiffiffif 9c

pvalue for EV-1.5N may be higher than

that given in Figure 4(b) because it failed in flexure210

180

150

120

90

60

30

0

She

ar: k

N

0 1000 2000 3000 4000 5000

Microstrain(a)

EM-1·5NEM-2·0N

EM-4NCL-2·7N

EM-2·7N

210

180

150

120

90

60

30

0

She

ar: k

N

0 3000 6000 9000Microstrain

(b)

EV-1·5N

EV-2·0N

EV-4N

CG-2·7N

EV-2·7N

Figure 3. Effect of a/d ratio on longitudinal steel strain near

the support: (a) EM and CL beams; (b) EV and CG beams

0·6

0·5

0·4

0·3

0·2

0·1

0·0

Vf

cc

/�

(a)

EM

-4·0

N

CL-

2·7N

EM

-2·7

N EM

-2N

EM

-1·5

N

0·6

0·5

0·4

0·3

0·2

0·1

0·0

Vf

cc

/�

(b)

EV

-4·0

N

CG

-2·7

N

EV

-2·0

N

EV

-1·5

N

Figure 4. Effect of a/d ratio on the ultimate shear resistance

of RCA-concrete beams: (a) EM and control CL beams;

(b) EV and CG beams

G. Fathifazl et al.

482 Magazine of Concrete Research, 2009, 61, No. 7

rather than shear. This is indicated by the arrow at the

top of the EV-1.5N bar chart.

According to Figure 4(a), the vc=ffiffiffiffiffif 9c

pvalue of EM-

2.7N was 14% higher than that of control beam CL-2.7N.

This finding is contrary to reported findings by others

who have indicated the shear strength of RRC beams to

be lower than that of comparable conventional concrete

beams.2–4 The reason for the previously observed lower

strength can be ascribed to the use of the conventional

mix proportioning method in previous studies against the

EMV method used in this study. The conventional meth-

od leads to lower coarse aggregate content in the RCA-

concrete and thus fewer coarse aggregate particles are

expected to cross the cracked shear plane, which would

reduce crack roughness and weaken the aggregate inter-

lock mechanism of shear resistance.

To ascertain whether existing codes expressions for

estimating the concrete contribution to the shear resis-

tance of concrete beams can be applied to RCA-

concrete members with their concrete mixes propor-

tioned by the EMV method, the vc values for the tested

beams are compared with the values calculated using

three well-known concrete design codes. The bar

graphs in Figure 5 show that none of the tested beams

had smaller shear strength than the calculated values

according to the simplified methods of the Canadian

Standard CSA A23.3-0418 and the American Code

ACI-31819 (equation (11.3)), and Eurocode 2,20 regard-

less of the a/d ratio or the RCA source. It can be

observed that in many cases the calculated values are

rather conservative, particularly for the beams with a/d

ratio of 2 or less, mainly due to the higher contribution

of the arch mechanism to the shear resistance at lower

a/d ratios. Consequently, for RCA-concrete designed by

200

160

120

80

40

0

Vc:

kN

EV-1·5N EM-1·5N(a)

40

30

20

10

0

Vc:

kip

s

200

160

120

80

40

0

Vc:

kN

EV-2N EM-2N(b)

40

30

20

10

0

Vc:

kip

s

120

80

40

0

Vc:

kN

CL-2·7N EM-2·7N(c)

27

18

9

0

Vc:

kip

s

160

120

80

40

0

Vc:

kN

CG-2·7N EV-2·7N(d)

36

27

18

9

0

Vc:

kip

s

120

80

40

0

Vc:

kN

EV-4N EM-4·0N(e)

36

27

18

9

0

Vc:

kip

s

Experimental

EC-2

ACI-318 (simplified)

CSA (simplified)

Figure 5. Experimental and predicted ultimate shear strength of RRC beams with different a/d ratios: (a) a/d ¼ 1.5; (b) a/d ¼ 2;

(c) a/d ¼ 2.7; (d) a/d ¼ 2.7; (e) a/d = 4

Shear strength of reinforced recycled concrete beams without stirrups

Magazine of Concrete Research, 2009, 61, No. 7 483

the EMV method, one can safely use the existing codes

expressions for calculating vc.

Shear performance. Figure 6(a) illustrates the ef-

fect of a/d ratio on the variation of vc=ffiffiffiffiffif 9c

pagainst

midspan deflection in the EM and EV series of

beams. Notice that as the a/d ratio increases, the

post-cracking stiffness decreases irrespective of the

RCA type. This can be mainly attributed to the maxi-

mum moment to the maximum shear (M/V) ratio in

the beam, viz. for larger a/d ratio, for the same shear

level the moment would be larger and consequently

the effective moment of inertia of the section would

be smaller after the formation of the cracks, leading

to a noticeable drop in beam stiffness.

Figure 6(b) shows for the EM and EV beams the

variation of vc=ffiffiffiffiffif 9c

pwith the deformation measured by

the LVDT bridging across the inclined crack in the west

shear span (ST1). Observe that all of the curves exhibit

an initial linear elastic portion, followed by a descending

branch, a longer ascending part and finally another

descending part after the peak load. The first descending

branch is due to the formation of the first major crack and

the fact that the member is under displacement control.

The load at the beginning of the first descending

branch corresponds to the diagonal cracking load, and

its magnitude is mainly a function of the concrete

strength. The length of the horizontal projection of the

first descending branch, which is an indicator of the

reduction of stiffness of the beam, is generally a func-

tion of the a/d ratio of the beam. The deeper beams with

lower a/d ratio have a shorter descending branch than

the slender beams with higher a/d ratio. This is mainly

due to the moment-to-shear ratio and the ratio of the

cracked to the gross moment of inertia of the beam.

The slope of the ascending part of the curve after the

first descending part is also a function of the a/d ratio

of the beam. This slope is another indicator of the shear

stiffness of the beam, which is higher for the deeper

beams with lower a/d ratio. At higher a/d ratios, after

the inclined crack formation, the load dropped slightly,

but owing to the aggregate interlock mechanism, it

increased again until diagonal tension failure occurred.

Figures 7 and 8 illustrate the effect of coarse aggre-

gate type – that is, limestone against river gravel – on

the vc=ffiffiffiffiffif 9c

pvariation with midspan deflection and with

the diagonal tensile deformation, respectively. As it can

be seen, generally the type of aggregate has a negligible

effect on the shear strength and stiffness, irrespective of

the a/d ratio.

0·6

0·5

0·4

0·3

0·2

0·1

00 2 4 6 8 10

Side deformation: mmEM beams

Vf

cc

/�

EM-1·5N

EM-2·0N

EM-2·7N

EM-4·0N

0·6

0·5

0·4

0·3

0·2

0·1

0

Side deformation: mmEV beams

Vf

cc

/�

EV-1·5N

EV-2·0N

EV-4·0N

0 2 4 6 8 10 12 14

0·6

0·5

0·4

0·3

0·2

0·1

00 4 8 12 16 20

Midspan deflection: mmEM beams

Vf

cc

/�

EM-1·5N

EM-2·0N

EM-2·7N

EM-4·0N

0·6

0·5

0·4

0·3

0·2

0·1

00 10 20 30 40 50

Midspan deflection: mmEV beams

Vf

cc

/�

EV-1·5N

EV-2·0N

EV-4·0N

(a)

(b)

Figure 6. Effect of a/d ratio on shear behaviour of beams: (a) normalised shear stress–midspan deflection response;

(b) normalised shear stress stress–diagonal deformation response

G. Fathifazl et al.

484 Magazine of Concrete Research, 2009, 61, No. 7

According to Figures 7(c) and 8(c), beam EM-2.7N

is less stiff compared with beam CL-2.7N, but it is

more ductile in the post-inclined cracking stages. This

finding is again contrary to the results previously re-

ported by others2 where RCA-concrete beams were

reported to be less ductile. The reason for this differ-

ence may be ascribed to the concrete mix proportioning

method. By using the EMV method, both crack inter-

face roughness and aggregate interlock mechanism

were enhanced. This is evident by comparing in Figure

7(c) the results for beam EM-2.7N with those of the

companion control beam CL-2.7N

Serviceability. Assuming the service load to be

40% of the failure load, the diagonal crack width at

service load level was estimated for these beams

using the diagonal deformation measured by the

LVDT ST1 parallel to the diagonal tension field. The

LVDT measures the total deformation, which includes

the crack opening and the deformation of concrete,

0·6

0·5

0·4

0·3

0·2

0·1

00 10 20 30 40 50

Midspan deflection: mm(a)

Vf

cc

/�� EM-1·5N

EV-1·5N 0·6

0·5

0·4

0·3

0·2

0·1

00 3 6 9 12 15

Midspan deflection: mm(b)

Vf

cc

/�

�

EM-2·0N

EV-2·0N

0·4

0·3

0·2

0·1

00 3 6 9 12 15

Midspan deflection: mm(c)

Vf

cc

/�

�

EM-2·7N

0·3

0·2

0·1

0

Midspan deflection: mm(d)

Vf

cc

/�

�

EV-4·0N

EM-4·0N

0 5 10 15 20

CL-2·7N

Figure 7. Coarse aggregate type effect on normalised shear stress resistance–midspan deflection response of beams with

different a/d ratios: (a) a/d ¼ 1.5; (b) a/d ¼ 2; (c) a/d ¼ 2.7; (d) a/d ¼ 4

654321 1512963

Side deformation: mm(b)

Vf

cc

/��

8765432187654321

Side deformation: mm(d)

Side deformation: mm(c)

Vf

cc

/��

Vf

cc

/��

EV-1·5N EM-1·5N

EV-4·0N

EM-4·0NCL-2·7N

EM-2·7N

0

0·1

0·2

0·3

0·4

0·5

0·6

0Side deformation: mm

(a)

0

0·1

0·2

0·3

0·4

0·5

0·6

0

0

0·1

0·2

0·3

0·4

0·5

00

0·1

0·2

0·3

0

Vf

cc

/��

EV-2·0N

EM-2·0N

Figure 8. Aggregate effect on normalized shear stress resistance diagonal deformation response of beams at different a/d ratios:

(a) a/d ¼ 1.5; (b) a/d ¼ 2; (c) a/d ¼ 2.7; (d) a/d ¼ 4

Shear strength of reinforced recycled concrete beams without stirrups

Magazine of Concrete Research, 2009, 61, No. 7 485

but the latter is generally much smaller than the

former. By ignoring the concrete deformation, crack

width values of 0.19, 0.05, 0.06, 0.01 and 0.03 mm

were found for beams EM-1.5N, EM-2.0N, EM-2.7N,

CL-2.7N and EM-4.0N, respectively. Similarly, crack

width values of 0.22, 0.00, 0.04 and 0.02 mm were

found for EV-1.5N, EV-2.0N, CG-2.7N and EV-4.0N

beams respectively. Note that the higher crack width

in beams with lower a/d ratio is mainly attributable

to their higher ultimate shear strength, and corre-

spondingly higher service load. Similarly, the higher

crack width in EM-2.7N compared with control beam

CL-2.7N is partially due to the higher failure load

and therefore higher service load of the former beam.

Practically all these crack widths are well below the

Canadian Standard CSA A23.3-0418 recommended

maximum crack width of 0.4 and 0.33 mm for inter-

ior and exterior exposures respectively.

Size effect

In conventional RC beams the size effect on vc is

recognised by a number of design codes, including the

Eurocode. In this section the results of beam size effect

on the shear strength of RRC beams without shear rein-

forcement are discussed.

Failure mode. Figure 9 illustrates the cracking

patterns of the EM and EV series of beams with

different depths as well as those of the control beams

CL-M and CG-M, which have depth of 375 mm. All

the beams failed in shear with the mode of failure

being diagonal tension, except beam EV-L and EV-L,

which failed in flexure. After the formation of the

major diagonal crack, it propagated towards the com-

pression face of the beam. In the RRC beams with

small and medium depths (effective depth of 200 and

300 mm), the aggregate interlock mechanism and

dowel action were capable of sustaining the load in

the post-inclined cracking stage. The increase in load

eventually led to the failure of the beam in shear

owing to diagonal tension. On the other hand, the

RRC beams with larger depth values (effective depth

of 400 and 500 mm) were not capable of load redis-

tribution after the formation of inclined crack and

they failed shortly thereafter.

Ultimate shear strength. Figure 10 illustrates the

normalised shear stress resistance vc=ffiffiffiffiffif 9c

pof EM and

EV series of beams having different effective depth.

Generally, the quantity vc=ffiffiffiffiffif 9c

pdecreased as d in-

creased. This may be attributed to less effective

aggregate interlock resistance in larger size beams.

The aggregate interlock contribution to the shear re-

sistance depends on the maximum distance between

the layers of distributed longitudinal reinforcement as

stipulated in the Canadian Standard CSA A23.3-04.18

While the size effect on shear resistance of conven-

tional concrete members is recognised by many de-

EM

-L

EM

-M

EM

-H

CL-

M

EM

-HH

(a)

EV

-L

EV

-2M

EV

-H

GG

-M

EV

-HH

(b)

Figure 9. Typical crack patterns of the test beams with

different depths at failure: (a) EM and CL beams; (b) EV and

CG beams

0·0

0·1

0·2

0·3

0·4

0·5

Vf

cc

/��

(b)

EV

-HH

CG

-M

EV

-H

EV

-L

0·0

0·1

0·2

0·3

0·4

Vf

cc

/��

(a)

EM

-HH

CL-

M

EM

-H EM

-M EM

-L

Figure 10. Experimental nominal shear strength of RCC

beams with different sizes: (a) EM and control beams; (b) EV

and control CG beams

G. Fathifazl et al.

486 Magazine of Concrete Research, 2009, 61, No. 7

sign standards, it is clear from the present results that

the same effect exists in RRC beams.

According to Figure 10(a), vc=ffiffiffiffiffif 9c

pof beams EM-H,

EM-M and EM-L are 19%, 53% and 102% higher than

that of beam EM-HH. Furthermore, the vc=ffiffiffiffiffif 9c

pof EV-

H and EV-L beams are 17% and 143% higher than that

of EV-HH (Figure 10(b)). Note that the actual vc=ffiffiffiffiffif 9c

p

value for EV-L may be higher than that given in Figure

10(b) because it failed in flexure rather than shear. This

is indicated by the arrow at the top of the EV-L bar

chart. It can also be observed that the vc=ffiffiffiffiffif 9c

pvalue for

beam EM-M is 14% higher than that of the control beam

CL-M, which is made entirely with natural limestone.

Once again, if the experimentally observed shear

resistance of these beams is compared with the corre-

sponding values predicted by the CSA A23.3-04,18

ACI-31819 (equation (11.3)), and Eurocode 2,20 as in

Figure 11, it is shown that practically all the predictions

are conservative. The only exception is the slightly

higher predicted value by Eurocode 2 for beam EM-

HH. The degree of conservatism tends to increase with

decreasing depth owing to the greater contribution of

the aggregate interlock mechanism to the shear resis-

tance of beams with smaller depth.

Shear performance. Figure 12(a) shows the varia-

tion of vc=ffiffiffiffiffif 9c

pwith midspan deflection in the beam

series EM and EV with different sizes. Generally, the

slope of the curves tends to decrease as the beam

size increases. This is again attributed to the greater

effectiveness of the aggregate interlock mechanism in

smaller size beams. In the larger size beams – that

is, EM-HH and EV-HH – after the formation of the

inclined crack, the stiffness of the beam drops drama-

tically.

Figure 12(b) shows the variation of the vc=ffiffiffiffiffif 9c

pwith

the nominal size of the diagonal crack as measured by

the diagonally oriented LVDT bridging over the in-

clined crack in the west shear span. These curves ex-

hibit a response similar to that described earlier with

reference to Figure 12(a).

As the first descending part of each curve is due to

the formation of the diagonal crack, it is clear that the

larger-size beams experience significant loss of stiff-

Experimental

EC-2

ACI-318 ( )simplified

CSA (simplified)

Vc: k

N

(e)

Vc: k

ips

0

40

80

120

160

EM-LEV-L0

9

18

27

36

0

40

80

120

EM-MCL-M0

9

18

27

0

40

80

120

160

EV-MCG-M0

9

18

27

36

0

40

80

120

160

EM-HEV-H0

9

18

27

36

0

40

80

120

160

EM-HHEV-HH0

9

18

27

36

Vc: k

N

(a)

Vc: k

ips

(b)

Vc: k

N

Vc: k

ips

Vc: k

N

(c)

Vc: k

ips

(d)

Vc: k

N

Vc: k

ips

Figure 11. Experimental and predicted ultimate shear strength of RRC beams with different sizes: (a) d ¼ 201 mm;

(b) d ¼ 309 mm; (c) d ¼ 309 mm; (d) d ¼ 381mm; (e) d ¼ 476 mm

Shear strength of reinforced recycled concrete beams without stirrups

Magazine of Concrete Research, 2009, 61, No. 7 487

ness and strength compared with the smaller-size

beams after the advent of this crack. Furthermore, the

magnitude of the change in the diagonal deformation

between the initial and terminal points of the descend-

ing part is indicative of the size of the diagonal crack.

Accordingly, the larger-size beams experience wider

diagonal cracks and they reach their maximum shear

capacity just before the formation of the diagonal

crack. Conversely, the smaller-size beams carry signifi-

cantly higher shear than their diagonal cracking load.

Figures 13 and 14, respectively, illustrate the effect

of aggregate type on the variation of vc=ffiffiffiffiffif 9c

pplotted

1614121086420

0·1

0·2

0·3

0·4

0·5

0

Midspan deflection: mmEM beams

EM-L

EM-M

EM-HEM-HH

Vf

cc

/��

Vf

cc

/��

0

0·1

0·2

0·3

0·4

0·5

0·6

0

Midspan deflection: mmEV beams

EV-L

EV-H

EV-HH

3530252015105

108642 1086420Side deformation: mm

EV beams

Vf

cc

/��

Vf

cc

/��

(b)

0

0·1

0·2

0·3

0·4

0·5

0Side deformation: mm

EM beams

EM-L

EM-M

EM-M

EM-HH

0

0·1

0·2

0·3

0·4

0·5

0·6EV-L

EV-H

EV-HH

(a)

Figure 12. Size effect on shear behaviour or RRC members: (a) normalised shear stress resistance–midspan deflection response:

(b) normalised shear stress variance plotted against diagonal tensile deformation

352821147 1512963

Vf

cc

/��

12963

Vf

cc

/��

Vf

cc

/��

107·55·02·5

0

0·1

0·2

0·3

0·4

0·5

0·6

0

Midspan deflection: mm(a)

EM-L

EV-L

0

0·1

0·2

0·3

0·4

0Midspan deflection: mm

(b)

EM-MCL-M

0

0·1

0·2

0·3

0

Midspan deflection: mm(c)

EV-H

EM-H

0

0·1

0·2

0·3

0

Midspan deflection: mm(d)

EV-HHEM-HH

Vf

cc

/��

Figure 13. Aggregate type effect on normalised shear stress resistance–midspan deflection of beams with different sizes:

(a) d ¼ 201 mm; (b) d ¼ 309 mm; (c) d ¼ 381 mm; (e) d ¼ 476 mm

G. Fathifazl et al.

488 Magazine of Concrete Research, 2009, 61, No. 7

against midspan deflection and diagonal deformation.

These figures show that the type of natural aggregate

in the two kinds of coarse RCA used in this study did

not have any significant effect on the shear strength of

the beams made with these aggregates.

Serviceability. Assuming the serviceability load to

be 40% of the failure load of these beams, under

service load approximate diagonal crack width values

of 0.16, 0.06, 0.01, 0.02 and 0.00 mm were measured

for beams EM-L, EM-M, CL-M, EM-H and EM-HH,

respectively. Similarly, crack width values of 0.28,

0.04, 0.03 and 0.00 mm were measured for beams EV-

L, CG-M, EV-H and EV-HH, respectively. Note that the

larger crack width in smaller-size beams is mainly

attributable to their higher ultimate shear strength, and

consequently their proportionally higher service load.

These crack widths are well below the maximum crack

width of 0.4 and 0.33 mm for interior and exterior

exposures recommended by CSA A23.3-04.18

Conclusions

In this paper, the results of an investigation into the

shear strength and behaviour of RRC beams without

shear reinforcement were presented. The focus of the

study was the effect of the proposed EMV concrete

mix design method on the shear capacity of RCA-

concrete beams. Based on the results of the current

investigation, provided the RCA-concrete mix is de-

signed by the EMV method, the following conclusions

are reached.

(a) There is no major difference between the failure

modes, cracking patterns and shear performance of

RRC beams and conventional RC beams. Gener-

ally, the tested RRC beams had higher shear stress

resistance (vc) and were found to be more ductile

after the formation of diagonal cracking than the

conventional RC beams.

(b) The shear strength of RRC beams increased as the

a/d ratio decreased, irrespective of the source,

mainly owing to the higher contribution of the arch

mechanism to the shear resistance at lower a/d

ratios.

(c) Irrespective of the RCA source, the vc of RRC

beams increased as the overall depth of the beam

decreased, which is a result of the well-known size

effect as in conventional RC beams. This increase

is primarily attributed to the lower effectiveness of

the aggregate interlock mechanism in larger-size

beams.

(d) Despite the slightly larger diagonal crack width in

RRC beams compared with the companion RC

beams, the observed crack widths in all the RRC

beams were below the maximum crack width per-

mitted by the Canadian Standard CSA A23.3-04

and ACI-318 codes.

(e) For the same a/d ratio, concrete compressive

strength and beam depth, the effect of aggregate

type (RCA against natural aggregate) on the shear

strength of RRC beams was found to be negli-

gible.

( f ) The simplified methods of CSA A23.3-04 and

ACI-318 as well that of Eurocode 2 for calculating

vc were found to be conservative when applied

108642 87654321

Vf

cc

/��

963V

fc

c/�

�642

Vf

cc

/��

0

0·1

0·2

0·3

0

Side deformation: mm(c)

EV-H

EM-H

0

0·1

0·2

0·3

0·4

0·5

0·6

0

Side deformation: mm(a)

EV-L

EM-L

0

0·1

0·2

0·3

0·4

0·5

0Side deformation: mm

(b)

EM 2·7N�

CL 2·7N�

0

0·1

0·2

0·3

0Side deformation: mm

(d)

EM-HH

EV-HH

Vf

cc

/��

Figure 14. Effect of aggregate type on normalised shear stress resistance–diagonal tensile deformation of RRC beams with

different sizes

Shear strength of reinforced recycled concrete beams without stirrups

Magazine of Concrete Research, 2009, 61, No. 7 489

to predict the shear of practically all the RCA-

concrete beams tested in this study.

Acknowledgement

The authors wish to express their sincere apprecia-

tion to Public Works and Government Services Canada

and to Natural Sciences and Engineering Research

Council of Canada for their financial support of this

study.

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Discussion contributions on this paper should reach the editor by

1 March 2010

G. Fathifazl et al.

490 Magazine of Concrete Research, 2009, 61, No. 7