USE OF SBR LATEXES TO MITIGATE INFERIOR CONCRETE ......USE OF SBR LATEXES TO MITIGATE INFERIOR...

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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.

Key words: 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

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

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


Coarse Aggregates


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

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 cross

linking.

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/m

3 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

y=13.79x-7.07R²=0.69

y=20.88x-22.36R²=0.84

-30

-20

-10

0

10

20

30

Δ(),%

RCA1(ACV=23.1%)

RCA2(ACV=28.2%)

Dataobtainedunderair-curingregime

y=6.81x-9.11R²=0.26

y=12.35x-23.93R²=0.75

-30

-20

-10

0

10

20

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1

Δ(f'c),%

Polymer/Cementra o(p/c),%


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

0

3

6

9

12

15

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Bondstress,M

Pa

Slipatfree-end,mm

320-NCA

320-RCA1

320-RCA1-1%SBR

320-RCA1-2%SBR

320-RCA1-3%SBR



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

polymeric latexes can effectively compensate the loss in RCA concrete performance.

The improvement in strength was further accentuated when curing was realized for 24

y=2.38x2-1.93x+3.01(R²=0.83)

y=1.82x2-0.22x+7.77(R²=0.66)

y=2.83x2-2.48x+13.57(R²=0.58)

2

6

10

14

18

22

0 0.5 1 1.5 2 2.5

Bondstress,M

Pa

Polymer/Cementra o(p/c),%

τ0.01mm

τ0.1mm

τu


Coarse Aggregates


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.

REFERENCES

[1] Rilem TC 217. Progress in recycling in the built environment. Springer

publication, Ed. E. Vanquez. ISBN 978-94-007-4907-8, (2013), 400 p.

[2] ACI 555R-01. Removal and reuse of hardened concrete. ACI Committee 555,

(2001), 26 p.

[3] Rao, G.A., Prasad B.K.R. Influence of the roughness of aggregate surface on the

interface bond strength, Cement and Concrete Research 32, (2002), 253-257.

[4] Hansen, T.C., and Narud, H. (1983). Strength of recycled concrete made from

crushed concrete coarse aggregate. Concrete International, 5(1), 79-83.

[5] Patel, P.J., Mukesh A. P., Patel, H.S. Effect of coarse aggregate characteristics

on strength properties of high performance concrete using mineral and chemical

admixtures, International Journal of Civil Engineering & Technology, 4(2),

(2013), 89-95.

[6] Reddy, M.M., Naidu K. S. D, Rayudu, S. E. Studies on recycled aggregate

concrete by using local quarry dust and recycled aggregates. International

Journal of Civil Engineering & Technology, 3(2), (2012), 322-326.

[7] Kim, Y., Sim, J., Park, C. Mechanical properties of recycled aggregate concrete

with deformed steel re-bar. Jr. of Marine Science and Technology, 20(3), (2012),

274-280.

[8] Butler, L., West, J. S., and Tighe, S. L. The effect of recycled concrete aggregate

properties on the bond strength between RCA concrete and steel reinforcement.

Cement and Concrete Research, 41(10), (2011), 1037-1049.

[9] Ohama, Y. Handbook of polymer-modified concrete and mortars. Nihon Univ.

Koriyama, Japan, (1995), 245 p.

[10] Gomes, C.E.M., Ferreira, O.P., Fernandes, M.R. Influence of vinyl acetate–

versatic vinyl ester copolymer on the microstructural characteristics of cement

pastes. Material Research, 8(1), (2005), 51-6.

[11] Chen, P.W., Fu, X., Chung, D.D.L. Microstructural and mechanical effects of

latex, methylcellulose, and silica fume on carbon fiber reinforced cement. ACI

Materials Journal, 94(2), (1997), 147-155.



[12] Issa, C., Assaad, J.J. Stability and bond properties of polymer-modified self-

consolidating concrete for repair applications. Materials and Structures, in press,

(2016). 15 p.

[13] ASTM C33 / C33M-16. Standard specification for concrete aggregates. Annual

Book of ASTM Standards, West Conshohocken, Pennsylvania, (2016), USA.

[14] BS 812–110. Testing Aggregates, British Standards Institute, London, England,

(1990), 18 p.

[15] Assaad, J.J. Disposing waste latex paints in cement-based materials - Effect on

flow and rheological properties, Journal of Building Engineering, (2016), 6, 75-

85

[16] Yasser R. Tawfic and Wael Abdelmoez, The Influence of “Water

Magnetization” On Fresh And Hardened Concrete Properties, International

Journal of Civil Engineering and Technology, 4(6), 2013, pp. 31–43.

[17] M.V.S.S.Sastri, Dr. K.Jagannadha Rao and Dr. V. Bhiksma, Studies on

Compression and Flexural Strength Characteristics of Triple Blended High

Strength Recycled Aggregate Concrete, International Journal of Civil

Engineering and Technology, 5(3), 2014, pp. 268–274.

[18] 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

[19] ACI 548.3R-03. Polymer-modified concrete. ACI Committee 555, (2003), 40 p.

[20] Issa, C., Assaad, J.J. Bond of tension bars in underwater concrete – Effect of bar

diameter and cover. Materials and Structures 48(11), (2014), 3457-3471.

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