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

Transcription

1 International Journal of Civil Engineering and Technology (IJCIET) Volume 7, Issue 3, May June 2016, pp , Article ID: IJCIET_07_03_007 Available online at Journal Impact Factor (2016): (Calculated by GISI) ISSN Print: and ISSN Online: IAEME Publication 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/m 3 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 editor@iaeme.com

2 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 ( 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 68 editor@iaeme.com

3 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 freezethaw 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) 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 m 2 /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 naphthalenebased 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. Specific gravity Table 1 Properties of NCA and RCAs used for concrete batching Oven-dry rodded bulk density, kg/m 3 Absorption rate, % Material finer than 75- m, % Fineness modulus Adhered mortar content, % ACV, % NCA , n/a 17.8 RCA , RCA , 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 69 editor@iaeme.com

4 Yehia Daou and Joseph J. Assaad and hardened properties of cementitious materials can be seen in references 9, 10, 12, and 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/m 3 cement with 0.56 water-to-cement ratio (w/c), while the high strength one was made using 440 kg/m 3 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 mm. The sand-tototal aggregate ratio was set at Cement, kg/m 3 Table 2 Concrete proportions using different aggregate types and SBR additions Water, kg/m 3 Net w/c HRWR, % of cement SBR, % of cement mass Coarse aggregate Type Content, kg/m 3 Fine aggregate, kg/m NCA , 1, 2, or 3 RCA , 1, 2, or 3 RCA NCA , 2, 3, or 4 RCA , 2, 3, or 4 RCA 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/m 3 cement, while dosages of 2%, 3%, and 4% were used in higher strength mixtures made with 440 kg/m 3 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 editor@iaeme.com

5 Use of SBR Latexes To Mitigate Inferior Concrete Properties Resulting From Recycled 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 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 (f t ) 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 beamend 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 (d b ) of 12.7 mm was used; the Young s modulus and yield strength (f y ) 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 5d b (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) editor@iaeme.com

6 Yehia Daou and Joseph J. Assaad 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 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/m 3 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 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 f t 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 72 editor@iaeme.com

7 Use of SBR Latexes To Mitigate Inferior Concrete Properties Resulting From Recycled 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. Mixture codification Table 3 Effect of SBR concentration and curing regime on concrete properties p/c, % Slump, mm Air content, % Unit weight, kg/m 3 Wet-cured at 95% RH for 28 days Cured at 95% RH for 24 hours, then air-cured for 27 days f c, MPa f t, MPa f c, MPa f t, MPa 320-NCA n/a n/a 320-RCA n/a n/a 320-RCA1-1%SBR RCA1-2%SBR RCA1-3%SBR RCA n/a n/a 320-RCA2-1%SBR RCA2-2%SBR RCA2-3%SBR NCA n/a n/a 440-RCA n/a n/a 440-RCA1-2%SBR RCA1-3%SBR RCA1-4%SBR RCA n/a n/a 440-RCA2-2%SBR RCA2-3%SBR RCA2-4%SBR n/a refers to not applicable 73 editor@iaeme.com

8 Yehia Daou and Joseph J. Assaad Figure 2 Effect of curing regime on f c and f t of SBR-modified RCA concrete Effect of SBR on variations in f c and f t 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/m 3 cement. Hence, this varied from -5.7% to -15.3% for concrete made with 320 or 440 kg/m 3 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/m 3 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/m 3, 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/m 3 cement was 74 editor@iaeme.com

9 Use of SBR Latexes To Mitigate Inferior Concrete Properties Resulting From Recycled 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 f t for concrete containing RCA1 B Splitting tensile strength Unlike (f c ), the incorporation of SBR polymers led to gradually increased f t values, even much higher than corresponding NCA concrete. For example, (f t ) reached +14.1% and +8.9% for RCA1 mixtures made with 320 or 440 kg/m 3 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 hydrateaggregate 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] Effect of p/c and comparison between RCA1 vs. RCA2 The relationships between p/c with respect to (f c ) and (f t ) 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 f t, such decrease was from around 1.4% to 0.9%, respectively. Practically speaking, this 75 editor@iaeme.com

10 Yehia Daou and Joseph J. Assaad 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 freeend (δ u ) 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] Bond stress vs. slip curves of tested mixtures The vs. δ curves determined for control NCA concrete prepared with 320 kg/m 3 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/m 3 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/m 3 cement, respectively (Table 4). This can be directly attributed to the reduced RCA1 concrete hardened properties including f c and f t, 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 f t 76 editor@iaeme.com

11 Use of SBR Latexes To Mitigate Inferior Concrete Properties Resulting From Recycled Coarse Aggregates Table 4 Effect of SBR on bond stress vs. slip concrete properties Mixture codification 0.01mm, 0.1mm, u, δ u, MPa MPa MPa mm u / (f c ) NCA RCA RCA1-1%SBR RCA1-2%SBR RCA1-3%SBR NCA RCA RCA1-2%SBR RCA1-3%SBR RCA1-4%SBR Figure 5 Typical bond stress vs. slip curves for mixtures made with 320 kg/m 3 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, δ u increased from 0.55 mm for 320-RCA1 concrete prepared without SBR to 0.88 mm with 3% 77 editor@iaeme.com

12 Yehia Daou and Joseph J. Assaad 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 f t 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 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 = (p/c, %) , with R 2 of 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 = (p/c, %) , with R 2 of 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 f t, particularly for lean mixtures made with 320 kg/m 3 cement, which practically implies that 78 editor@iaeme.com

13 Use of SBR Latexes To Mitigate Inferior Concrete Properties Resulting From Recycled 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. REFERENCES [1] Rilem TC 217. Progress in recycling in the built environment. Springer publication, Ed. E. Vanquez. ISBN , (2013), 400p. [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), [4] Hansen, T.C., and Narud, H. (1983). Strength of recycled concrete made from crushed concrete coarse aggregate. Concrete International, 5(1), [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), [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), [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), [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), [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), [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), editor@iaeme.com

14 Yehia Daou and Joseph J. Assaad [12] Issa, C., Assaad, J.J. Stability and bond properties of polymer-modified selfconsolidating 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 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, [16] ACI 548.3R-03. Polymer-modified concrete. ACI Committee 555, (2003), 40 p. [17] Issa, C., Assaad, J.J. Bond of tension bars in underwater concrete Effect of bar diameter and cover. Materials and Structures 48(11), (2014), editor@iaeme.com