USE OF SBR LATEXES TO MITIGATE INFERIOR CONCRETE PROPERTIES RESULTING FROM RECYCLED COARSE AGGREGATES

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International Journal of Civil Engineering and Technology (IJCIET)
Volume 7, Issue 3, May–June 2016, pp. 67–80, Article ID: IJCIET_07_03_007
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USE OF SBR LATEXES TO MITIGATE
INFERIOR CONCRETE PROPERTIES
RESULTING FROM RECYCLED COARSE
AGGREGATES
Yehia Daou
Professor and Chair, Lebanese University, Dept. of Civil Eng, Lebanon
Joseph J. Assaad
Professor and R&D Manager, Holderchem Building Chemicals, Lebanon
ABSTRACT
In civil engineering works, the partial or complete replacement of natural
coarse aggregate (NCA) by recycled concrete aggregate (RCA) is most often
associated with inferior mechanical properties including bond to embedded
steel bars. The main objective of this paper is to evaluate the efficiency of
styrene-butadiene rubber (SBR) polymeric latexes to compensate such losses.
Two RCA quality types were tested in different concrete mixtures prepared
with 320 or 440 kg/m3
cement; the SBR addition rates varied from 1% to 4%
of cement mass. Test results have shown that SBR can remarkably improve
RCA concrete compressive and splitting tensile strengths, particularly when
curing is realized in air conditions at 23 3 °C and 50% relative humidity.
The bond stress vs. slip curves remained fundamentally similar to those
observed with NCA concrete. Yet, the initial stiffness was considerably
accentuated with SBR additions together with improved responses of
ascending curves and increased ultimate bond strengths.
Keywords: Adhesion, Bond stress, Concrete, Polymer, Recycled aggregate,
SBR latex.
Cite this Article: Yehia Daou and Joseph J. Assaad, Use of SBR Latexes To
Mitigate Inferior Concrete Properties Resulting From Recycled Coarse
Aggregates, International Journal of Civil Engineering and Technology, 7(3),
2016, pp. 67–80.
http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=7&IType=3

Yehia Daou and Joseph J. Assaad
1. INTRODUCTION
The partial or complete replacement of natural coarse aggregate (NCA) by recycled
concrete aggregate (RCA) in concrete production has considerably increased over the
last years. The use of RCA has the potential of diverting construction and demolition
debris from landfills while promoting a sustainable building approach.
Generally, the RCA concrete properties are inferior from equivalent NCA
mixtures, given the poorer quality of recycled aggregates including greater water
absorption and lower density [1,2]. In fact, RCAs are composed of NCA with
approximately 30% of adhered mortar that gives the RCA a rough surface with
numerous pores and micro-cracks. In the fresh state, it has been shown that concrete
workability containing RCA is lower than equivalent NCA mixture, given the more
angular shape and roughened surface texture of recycled aggregates [1,3]. Hence, to
produce similar workability, approximately 5% more water is required for RCA
concrete, while 15% more water is needed when both fine and coarse recycled
aggregates are used. On the hardened state, the use of RCA affects the interfacial
transition zone (ITZ) between aggregates and cement paste, which in its turn, reduces
the strength development. Reductions varying from 5% to 25% are reported for
compressive strength, while the splitting tensile strength remained the same or, at
most, 10% lower [1,4,5,6]. For the bond behavior with embedded steel, the majority
of findings have shown that the stages of load vs. slip curves are similar to NCA
mixtures; yet, the ultimate bond strengths were lower, depending on the concentration
and quality of RCA. For example, Kim et al. [7] reported that bond strength of RCA
concrete decreases gradually when RCA replacement rates increased from 0% to
30%, 60%, and 100%; the highest drop was about 18% from equivalent NCA
concrete. Butler et al. [8] found that ultimate bond strength is directly affected by
RCA quality; on average, RCA concrete developed around 10% to 21% lower bond
strength than equivalent NCA mixture.
Polymeric latexes are widely used in cementitious materials to increase adhesion
and bond strengths to various substrates. Latexes typically include polyvinyl acrylic
(PVA) homo- and copolymer and styrene-butadiene rubber (SBR); these consist of
very small polymer particles (0.05–5 μm) formed by emulsion polymerization and
stabilized in water with the aid of surfactants [9]. Generally, adhesion increases with
the increase in polymer-to-cement ratio (p/c); for example, Gomes et al. [10] reported
3- to 5-folds increase in adhesion for polymer-modified cement pastes at 5% to 10%
p/c on concrete substrates. The microstructural images of failure interfaces showed
distinct diffusion of modified pastes to the bonded substrate, implying the formation
of monolithic bond between both materials [11]. Latexes also found particular
acceptance in reinforced concrete applications due to the resulting improvement in
bond strengths with embedded steel and resistance to corrosion and chloride ion
penetrability [9,12].
Limited attempts have been realized to incorporate polymeric latexes in concrete
containing RCA, including the extent to which such additions would help
compensating the eventual decrease in workability or drop in hardened properties.
The paper is divided in two parts; the first seeks to evaluate the suitability of SBR to
compensate the loss in concrete workability and strength as a result of 100%
replacement of NCA by RCA. Two types of RCA having different qualities were
considered; the effect of curing regime on strength development for polymer-modified
RCA concrete was also evaluated. The second phase presents the experimental load
vs. slip data obtained from beam-end specimens, along with the effect of p/c on

Use of SBR Latexes To Mitigate Inferior Concrete Properties Resulting From Recycled
Coarse Aggregates
variations in bond stresses. This paper can be of interest to environmental
organizations and concrete engineers dealing with composite structures and efficient
re-use of RCA materials in the construction industry.
2. EXPERIMENTAL PROGRAM
2.1. Coarse aggregate characterization
Two RCAs having 20-mm nominal size were used; the RCA1 was obtained by
crushing returned concrete from ready-mixed batching plant, while RCA2 consisted
of crushed concrete recuperated from processing old infrastructure elements such as
manholes, concrete pipes, and culverts. Also, continuously graded crushed limestone
NCA having 20-mm nominal size was employed. The aggregates gradations were
within ASTM C33 limitations, sieve No. 67 [13].
The physical NCA and RCA properties are summarized in Table 1. The freeze-
thaw test procedure was used to determine the adhered mortar portion of RCA. The
materials were immersed in sodium sulphate solution, and subjected to five daily
cycles of freezing and thawing. After the final cycle, the sodium sulphate solution was
drained and aggregates washed and sieved over a 4.75-mm sieve. The aggregate
crushing value (ACV), reflecting the compressive strength of loose aggregate, was
determined by subjecting a measured volume of aggregate to 400-kN load [14]. After
crushing, the sample is sieved over 2.36-mm sieve where the percentage of material
passing the sieve represents the ACV (i.e., higher ACV value reflects weaker
aggregates with lower compressive strength).
2.2. Materials used for concrete production and bond testing
Portland cement conforming to ASTM C150 Type I was used; its surface area,
median particle size, and specific gravity were 355 m2
/kg, 24.7 m, and 3.14,
respectively. The natural fine aggregate consisted of well-graded siliceous sand
complying to ASTM C33 specification [13]; its bulk specific gravity, fineness
modulus, and absorption rate were 2.65, 2.5, and 0.97%, respectively. A naphthalene-
based high-range water-reducing (HRWR) admixture with specific gravity of 1.18
and solid content of 34% was used. This admixture complies with ASTM C494 Type
F; it can be used up to 3.5% of cement mass.
Table 1 Properties of NCA and RCAs used for concrete batching
Specific
gravity
Oven-dry
rodded bulk
density,
kg/m3
Absorption
rate, %
Material
finer than
75-m, %
Fineness
modulus
Adhered
mortar
content,
%
ACV,
%
NCA 2.72 1,763 0.61 0.42 6.71 n/a 17.8
RCA1 2.43 1,505 7.04 0.9 6.77 41.2 23.1
RCA2 2.4 1,497 6.12 1.16 6.84 44 28.2
Commercially available white SBR latex typically used for enhancing flexibility
and water-impermeability of cementitious materials was used. The carboxylated
styrene butadiene dispersion contains 60% of bound styrene without solvents and
stabilized using anionic emulsifying system. Its solid content, specific gravity, pH,
Brookfield viscosity (spindle 4 at 10 rpm), maximum particle size, and minimum film
forming temperature (MFFT) are 56%, 1.05, 8.5, 250 cP, 0.22 m, and -5 °C,
respectively. Relevant research studies examining the effect of SBR latexes on fresh

and hardened properties of cementitious materials can be seen in references 9, 10, 12,
and 15.
2.3. Concrete proportioning and mixing
Two control NCA concrete mixtures (i.e., lean and high strength) commonly used for
residential and repair applications were considered. The lean mix contained 320 kg/m3
cement with 0.56 water-to-cement ratio (w/c), while the high strength one was made
using 440 kg/m3
cement and 0.44 w/c (Table 2); the corresponding 28-days f’c was
31.6 and 54.8 MPa, respectively. The HRWR dosage was adjusted at either 0.85 % or
1.45%, respectively, of cement mass to achieve slump of 220 10 mm. The sand-to-
total aggregate ratio was set at 0.46.
Table 2 Concrete proportions using different aggregate types and SBR additions
Cement,
kg/m3
Water,
kg/m3
Net
w/c
HRWR,
% of
cement
SBR,
% of cement
mass
Coarse aggregate Fine
aggregate,
kg/m3
Type Content,
kg/m3
320 180 0.56 0.85 0 NCA 995 850
320 180 0.56 0.85 0, 1, 2, or 3 RCA1 940 800
320 180 0.56 0.85 0, 1, 2, or 3 RCA2 930 790
440 195 0.44 1.45 0 NCA 925 790
440 195 0.44 1.45 0, 2, 3, or 4 RCA1 875 745
440 195 0.44 1.45 0, 2, 3, or 4 RCA2 855 730
The effect of recycled aggregates on fresh and hardened concrete properties was
evaluated by 100% replacement of natural aggregate by RCA, with or without SBR
additions. The cement content, net w/c (given that water content in SBR was
accounted during concrete batching), HRWR dosage, and sand-to-total aggregate ratio
remained fixed as earlier described for control NCA mixtures. As can be seen in
Table 2, three SBR dosage rates of 1%, 2%, and 3% of cement mass were added in
RCA1 and RCA2 mixtures prepared with 320 kg/m3
cement, while dosages of 2%,
3%, and 4% were used in higher strength mixtures made with 440 kg/m3
cement. This
was decided following preliminary testing showing that the drop in hardened
properties is particularly accentuated in higher strength RCA concrete, making thus
more relevant the incorporation of higher latex concentrations to compensate such
losses. The corresponding p/c varied from 0.56% to 2.06%. It is to be noted that p/c
could be as high as 10% during concrete production [9,12]; however, this was limited
to 2.06% in this study because of the tangible improvement in RCA concrete
performance when compared to equivalent NCA mixtures. Also, practically speaking,
it is generally desirable to limit polymer addition rates to prevent excessive increase
in concrete cost.
Given the high RCA water absorption and eventual effect on concrete workability,
all coarse aggregates (i.e., NCA, RCA1, and RCA2) were pre-soaked for 24 hours in
water and then drained for around 1 hour prior to batching to ensure full saturation at
or above saturated-surface-dry condition [1,2,8]. The batch proportions were then
adjusted for aggregate surface moisture to maintain constant w/c. All mixtures were
prepared in an open-pan mixer of 100-Liters capacity. The mixing sequence consisted
of homogenizing the sand, aggregate, and around 50% of mixing water before
introducing the cement. After one minute of mixing, the other 45% of water was
added, followed by HRWR, and then SBR diluted in the remaining 5% of water. The
concrete was mixed for two additional minutes. The ambient temperature during
mixing and sampling hovered around 21 ±3 °C.

Coarse Aggregates
2.4. Specimen preparation and experimental testing
Part 1-Effect of SBR concentration on RCA1 and RCA2 concrete strength
Following the end of mixing, the slump, unit weight, and air content were determined
as per ASTM C143, C138, and C231 Test Methods, respectively. The fresh concrete
mixtures were then filled in 100×200 mm steel cylinders and covered by wet burlap
for 24 hours. After demolding, all concrete specimens prepared without SBR were
placed in a moist-curing room at 23 3 °C and more than 95% relative humidity (RH).
For mixtures containing SBR, the specimens were divided into two groups: the first
was wet-cured in similar manner as earlier described, while the second group was
cured for 24 hours in 95% RH, followed by air curing at 23 3 °C at 50% 5% RH, as
per ACI 548 recommendations [16]. By subjecting the specimens to various curing
conditions, the intention was to capture the effect of SBR on strength variations under
different curing conditions.
At 28 days, the compressive strength (f’c) and splitting tensile strength (ft) were
determined as per ASTM C39 and C496 Test Methods, respectively. Averages of 3
measurements are considered in this paper; the failure planes of concrete cylinders
were examined visually after crushing using a magnifying glass and classified as
being mainly around or mainly through the aggregate skeleton [8].
Part 2 Effect of SBR on bond stress vs. slip behavior
The effect of SBR additions on bond to steel behavior was determined using beam-
end specimens, as per ASTM A944 Test Method (only the RCA1 was considered in
this part). The specimen dimensions measured 220 mm in width, 250 mm in length,
and 220 mm in height, as shown in Fig. 1 [17]. Deformed steel bars complying to
ASTM A615 No. 13 with nominal diameter (db) of 12.7 mm was used; the Young’s
modulus and yield strength (fy) were 203 GPa and 428 MPa, respectively. The
specimen is positioned in a test rig so that the bar can be pulled slowly from concrete;
it is restrained from translation through a compression reaction plate and restrained
from rotation through a tie-down, thus approximating boundary conditions of simply
supported beams.
All bars were embedded inside the specimens at fixed lengths of 5db (i.e., 60 mm)
to prevent yielding of steel; similar rib orientation with respect to pullout load was
maintained in all specimens. The concrete clear cover was kept constant at 40 mm; a
size typically used in the design of flexural beams. Two stirrups with 9.5-mm nominal
diameter were placed on each side were provided for shear resistance, but were
oriented parallel to the “pull” direction to avoid confining the test bar along its bonded
length. The concrete samples were placed in two consecutive lifts in the beam-end
specimen molds, and internally vibrated using 150-Hz frequency vibrator. Before
testing, the specimen was shimmed and aligned so that the test bar is parallel to the
loading frame. The tensile load was gradually applied at a rate of 25 ±4 kN per minute
until bond failure occurred. The bar’s relative slips to concrete were monitored from
measurements of two LVDTs placed at the free and loaded bar surfaces (Fig. 1).

Figure 1 Set-up used for determining bond using beam-end specimens
3. TEST RESULTS AND DISCUSSION
3.1. Part 1: Effect of SBR concentration and curing regime on concrete
properties
3.1.1 Air content and workability
The fresh and hardened properties of tested concrete mixtures are summarized in
Table 3. Generally, the air content of fresh concrete prepared with RCA1 or RCA2
did not vary considerably with respect to corresponding NCA mixture (Table 3).
Nevertheless, this followed an increasing trend with SBR additions; for example, such
increase was from 2.8% for 440-RCA1 mix prepared without polymer to 3.5% and
4.1% when the SBR was added at 3% and 4% rates, respectively. This could be
related to the surfactants added to stabilize the polymer including their inherent
surface activity (i.e., ability to reduce surface tension of water) and solubility in the
high cement pH solution [9].
As expected, the use of RCA1 and RCA2 led to reduced workability when
compared to equivalent NCA mixture (Table 3). For example, such reduction reached
155 and 160 mm for RCA1 concrete prepared with 320 or 440 kg/m3
cement,
respectively. This can mostly be attributed to the angular and roughened surface
texture of recycled aggregates that increased internal friction in fresh concrete [1,3].
Nevertheless, the incorporation of increased polymer additions led to improved
workability, which can be related to the ball bearing and plasticizing effects resulting
from the polymer spherical shapes [9,15]. For example, the slump increased from 160
mm for 440-RCA1 concrete prepared without polymer to 205 and 210 mm with 3%
and 4% SBR additions, respectively. Additionally, it is to be noted that the
improvement in flow can partly be attributed to increased amounts of air entrainment
in the polymer-modified concrete.
3.1.2 Effect of curing regime on hardened polymer-modified RCA concrete strength
The ratios of f’c determined from specimens cured in air conditions with respect to
those wet-cured in 95% RH are plotted in Fig. 2 for all RCA concrete mixtures
containing various percentages of SBR polymers. The figure also plots the ratios of ft
values. With some few exceptions, it is clear that the development of strengths is
affected by the curing regime, whereby specimens cured in air conditions exhibiting

Coarse Aggregates
up to 13% more strength. Such results are in agreement with other findings reported
in literature [16]. In fact, after initial moist curing for 24 hours, the latex particles
coalesce into films that become adsorbed onto the surfaces of hydrated cement
compounds, preventing further moisture loss (i.e., entrapped moisture promotes
cement hydration processes). Concurrently, as the latex films develop, reactive groups
in the polymer crosslink within the internal structure, forming continuous and
impermeable coating film layers (i.e., such films are destabilized under wet curing)
[9,17]. Hence, the increase in strength encountered under air curing conditions can be
associated to an increase in both cement hydration reactions as well as polymer
crosslinking.
Table 3 Effect of SBR concentration and curing regime on concrete properties
Mixture
codification
p/c,
%
Slump,
mm
Air
content,
%
Unit
weight,
kg/m3
Wet-cured at
95% RH for 28
days
Cured at 95% RH
for 24 hours, then
air-cured for 27
days
f’c,
MPa
ft,
MPa
f’c,
MPa
ft,
MPa
320-NCA 0 220 2.6 2335 31.6 3.77 n/a n/a
320-RCA1 0 155 2.35 2315 29.8 3.5 n/a n/a
320-RCA1-1%SBR 0.56 160 2.8 2345 31.3 3.87 30.9 3.92
320-RCA1-2%SBR 1.12 180 3.2 2310 33.9 4.17 35 4.51
320-RCA1-3%SBR 1.68 195 3.3 2290 34.2 4.3 35.8 4.73
320-RCA2 0 135 2.8 2290 23.9 2.97 n/a n/a
320-RCA2-1%SBR 0.56 135 2.75 2310 26.1 3.36 28 3.6
320-RCA2-2%SBR 1.12 165 3.15 2310 28.4 3.97 30 4.11
320-RCA2-3%SBR 1.68 190 3.5 2285 30.1 4.06 33.1 4.6
440-NCA 0 225 2.4 2380 54.8 6.2 n/a n/a
440-RCA1 0 160 2.8 2355 46.4 5.4 n/a n/a
440-RCA1-2%SBR 1.12 185 2.8 2345 47.8 6.5 47.5 6.42
440-RCA1-3%SBR 1.68 205 3.5 2315 49.5 6.76 51.2 6.8
440-RCA1-4%SBR 2.06 210 4.1 2290 53 7.03 56 7.1
440-RCA2 0 125 2.7 2350 39.6 4.46 n/a n/a
440-RCA2-2%SBR 1.12 135 3 2310 47.2 5.5 46.5 5.66
440-RCA2-3%SBR 1.68 170 3.8 2285 46 6.12 49.2 6.7
440-RCA2-4%SBR 2.06 185 4.3 2270 50.8 6.63 54.4 7.2
n/a refers to not applicable

Figure 2 Effect of curing regime on f’c and ft of SBR-modified RCA concrete
3.1.3 Effect of SBR on variations in f’c and ft
The variations in hardened properties for concrete mixtures containing RCA1 with or
without SBR are plotted in Fig. 3. The (Property) was normalized with respect to
corresponding control NCA concrete made with either 320 or 440 kg/m3
cement, as
follows:
A. Compressive strength
Concrete made without polymers – As can be seen, the complete NCA substitution by
RCA1 led to reduced f’c, particularly for high strength mixtures prepared with 440
kg/m3
cement. Hence, this varied from -5.7% to -15.3% for concrete made with 320
or 440 kg/m3
cement, respectively. Such results are in agreement with those reported
by Butler et al. [8] who associated the drop in f’c of lean and high strength RCA
concrete to different failure planes occurring around or through the coarse aggregate
skeleton. In fact, the visual examination of crushed concrete cylinders made with 320
kg/m3
cement showed distinct failure planes occurring mainly around the aggregate
particles, suggesting that the ITZ between mortar-aggregate is the limiting strength
factor. In contrast, the failure planes become less distinct and mostly passing through
the aggregate particles for concrete prepared with 440 kg/m3
, implying that the
strength of RCA itself is the limiting factor [1,8].
Effect of SBR on (f’c) – Clearly, the use of increased SBR concentration
improved f’c of RCA1 mixtures, albeit this varied depending on cement content and
strength of original concrete (Fig. 3). For example, (f’c) increased significantly from
-5.7% for lean 320-RCA1 concrete made without SBR to +7.3% and +8.2% for
equivalent mixtures containing 2% or 3% SBR, respectively. This can be attributed to
the polymer particles that strengthen the mortar-aggregate interface, especially
knowing that f’c of lean concrete is mostly governed by the ITZ behavior. In contrast,
the (f’c) increase in high strength concrete prepared with 440 kg/m3
cement was

Coarse Aggregates
much less pronounced, and remained in the negative region. Hence, (f’c) varied from
-15.3% for 440-RCA1 concrete made without SBR to -9.7% and -3.3% for mixtures
containing 3% and 4% SBR, respectively. This practically suggests that the beneficial
polymer effect on compressive strength of recycled aggregate concrete is directly
affected by the mixture proportioning.
Figure 3 Effect of SBR on variations in f’c and ft for concrete containing RCA1
B Splitting tensile strength
Unlike (f’c), the incorporation of SBR polymers led to gradually increased ft values,
even much higher than corresponding NCA concrete. For example, (ft) reached
+14.1% and +8.9% for RCA1 mixtures made with 320 or 440 kg/m3
cement,
respectively, containing 3% SBR. In fact, it is well accepted that structure of hardened
cement paste is mainly composed of agglomerated calcium silicate hydrates and
calcium hydroxide bound together by weak van der Waals forces, whereby
microcracks could occur easily under stress leading to poor tensile strength [9].
Hence, in latex-modified systems, the microcracks are bridged by the polymer films
that prevent crack propagation in hardened ITZ, resulting in stronger cement hydrate-
aggregate bond. Additionally, the improved smoothness and flowability of modified
RCA concrete are expected to reduce porosity of ITZ and develop increased bond by
micro-mechanical interlocking mechanisms [11].
3.1.4 Effect of p/c and comparison between RCA1 vs. RCA2
The relationships between p/c with respect to (f’c) and (ft) determined on concrete
mixtures prepared using RCA1 and RCA2 cured under air conditions are plotted in
Fig. 4. Clearly, the incorporation of SBR (i.e., higher p/c) led increased strength
development, albeit this appears to be significantly affected by the type of RCA used.
As can be seen, the tendency curves obtained from concrete containing RCA1 are
consistently higher than those resulting from RCA2 concrete. This could be directly
related to the quality of recycled aggregates (i.e., note that RCA1 was of better quality
than RCA2, the resulting ACV was 23.1% vs. 28.2%, respectively). Hence, the
threshold p/c beyond which f’c becomes equivalent to NCA mixture decreased from
around 2% to 1.2% with the use of RCA2 and RCA1, respectively. For ft, such
decrease was from around 1.4% to 0.9%, respectively. Practically speaking, this

clearly shows the importance of RCA quality on the development of concrete
strength.
3.2 Phase II: Effect of SBR additions on bond stress vs. slip behavior
Table 4 summarizes the bond characteristics of tested concrete including the bond
stresses corresponding to slip of 0.01 and 0.1 mm (0.01mm and 0.1mm, respectively),
ultimate bond stress (u) representing the maximum load at failure, and slip at free-
end (δu) coinciding with the ultimate load. Also, the normalized bond stress calculated
as the ratio of u to the square root of f’c is given. It is to be noted that all tests
exhibited pullout modes of failure characterized by crushing and shearing of the
localized embedded region around the bar. No cracks were observed on their external
surfaces, indicating that the concrete cover provided adequate confinement [17].
3.2.1-Bond stress vs. slip curves of tested mixtures
The  vs. δ curves determined for control NCA concrete prepared with 320 kg/m3
cement as well as those made using RCA1 with or without SBR additions are given in
Fig. 5.
Comparison between NCA vs. RCA1 concrete behavior (without SBR) –
Concurrent with existing literature [8,9,10,12], the substitution of NCA by RCA1 did
not result in considerable changes in τ vs. δ curves. Hence, the three mechanisms
controlling the bond between steel and concrete including adhesion, mechanical
interlock, and friction can be well identified [7]. Nevertheless, it is to be noted that u
at failure for RCA1 concrete was relatively lower than equivalent value determined
using NCA mixture, especially for higher strength concrete prepared with 440 kg/m3
cement. For example, u decreased from 11.8 to 11.4 MPa and from 16.3 to 14.7 MPa
for mixtures made with 320 and 440 kg/m3
cement, respectively (Table 4). This can
be directly attributed to the reduced RCA1 concrete hardened properties including f’c
and ft, thus reducing the material’s bearing strength capacity in front of the bar ribs.
Figure 4 Effect of p/c and RCA1 vs. RCA2 on variations in f’c and ft

Coarse Aggregates
Table 4 Effect of SBR on bond stress vs. slip concrete properties
Mixture codification 0.01mm,
MPa
0.1mm,
MPa
u,
MPa
δu,
mm
u / (f’c)0.5
320-NCA 2.54 7.13 11.8 0.53 2.1
320-RCA1 2.3 6.65 11.4 0.55 2.09
320-RCA1-1%SBR 2.7 7.45 12.8 0.71 2.29
320-RCA1-2%SBR 3.05 9 13 0.9 2.23
320-RCA1-3%SBR 4.6 8.77 14.1 0.88 2.41
440-NCA 3.85 9.6 16.3 0.48 2.2
440-RCA1 3.24 7.8 14.7 0.48 2.16
440-RCA1-2%SBR 5.3 12.7 17.1 0.92 2.47
440-RCA1-3%SBR 7.2 14 18.7 0.9 2.66
440-RCA1-4%SBR 9.7 16.1 21.4 1.32 2.94
Figure 5 Typical bond stress vs. slip curves for mixtures made with 320 kg/m3
cement
Behavior of RCA1 concrete containing SBR – Clearly, the bars free-end of
polymer-modified RCA1 concrete started to slip at bond stresses higher than those of
control mixtures, thus accentuating the initial stiffness of  vs. δ curves (Fig. 5). For
example, at the very small slip of 0.01 mm, 0.01mm increased from 2.3 MPa for 320-
RCA1 concrete prepared without SBR to 3.05 and 4.6 MPa with the addition of 2% or
3% SBR, respectively (Table 4). This can be directly attributed to the latex polymers
that increase the adhesive component in the elastic region and result in increased
interfacial shear stresses between the reinforcing bar and surrounding concrete [12].
Ohama [9] related this phenomenon to the presence of electro-chemically active
polymer-cement co-matrixes at the interfaces with reinforcing bars, thus relaxing the
stresses during loading and retarding the friction-controlled slip of rebars.
When the adhesive component of bond fails, the responses of ascending curves of
SBR-modified concrete showed extended non-linear regions together with higher u,
which can be explained by more pronounced compressive strain-softening
phenomenon due to the presence of polymer latexes [7]. For example, δu increased
from 0.55 mm for 320-RCA1 concrete prepared without SBR to 0.88 mm with 3%

SBR; the corresponding u increased from 11.4 MPa to 13 MPa (Table 4). In fact, the
bond resistance in beam-end specimen is achieved by circumferential tension stresses
created in the concrete around the bar; if these forces exceed the tensile concrete
capacity, failure occurs [7,8,12]. Therefore, given that ft of RCA1 concrete is
significantly improved with polymer additions, this can reduce the propagation of
microcracks and result in increased bond resistance with reinforcing bar.
3.2.2 Relationships between p/c and bond properties
The relationships between p/c with respect to 0.01mm, 0.1mm, and u are plotted in Fig.
6. As can be seen, RCA1 concrete incorporating higher SBR additions (i.e., higher
p/c) led to increased bond stresses. Nevertheless, such increase was particularly
accentuated for 0.01mm, suggesting that the adhesive component of bond could be
highly improved by such additions. For example, at the highest p/c of 2.06%,
(0.01mm) reached 152%, while (0.1mm) and (u) reached respectively 68% and
31%.
From the other hand, it is to be noted that the slips at failure shifted gradually
towards higher values with increased p/c. The resulting correlation can be written as:
Slip at free-end, mm = 0.309 (p/c, %) + 0.51, with R2
of 0.88. Practically, this
indicates that the structural ductility of reinforced RCA concrete members tends to
increase with polymer additions [12]. The ratio of u to the square root of f’c followed
an increasing trend with p/c; the relationship can be written as: Ratio = 0.296 (p/c, %)
+ 2.11, with R2
of 0.78.
Figure 6 Relationships between p/c with respect to bond stresses
4. SUMMARY AND CONCLUSIONS
The use of RCA in structural concrete applications is very limited, given the concerns
pertaining to its inferior mechanical properties when compared to natural aggregate
concrete. The main objective of this paper is to evaluate the effect of SBR polymers
on RCA concrete properties and, consequently, bond to embedded steel bars.
Based on foregoing, test results have shown that the incorporation of SBR can
remarkably improve RCA concrete workability due to ball bearing and plasticizing
effects. On the hardened state, such additions led to increased f’c and ft, particularly
for lean mixtures made with 320 kg/m3
cement, which practically implies that

Coarse Aggregates
polymeric latexes can effectively compensate the loss in RCA concrete performance.
The improvement in strength was further accentuated when curing was realized for 24
hours in 95% RH, and then for 27 days in air conditions at 23 3 °C and 50% 5%
RH.
The mechanisms of bond failure in  vs. δ curves recorded using SBR-modified
RCA concrete are fundamentally similar to those observed with natural aggregate
concrete. Yet, the initial stiffness was considerably accentuated with SBR additions,
reflecting increased interfacial shear stresses between the reinforcing bar and
surrounding concrete. Also, the ascending curves showed extended non-linear regions
together with higher u. Good correlations were established between p/c and bond
properties.
ACKNOWLEDGMENTS
This project was funded by the School of Engineering Research Council of the
Lebanese University (LU), Hadath, Lebanon. The authors wish to acknowledge the
experimental support provided by the Laboratory of the Civil Engineering Department
at LU as well as the contributions of research assistants from Finders SAL, Amchit,
Lebanon.
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