INFLUENCE OF THE DIAMETER OF THE BAR AND CDW CONTENT ON THE BOND BEHAVIOR OF RECYCLED REINFORCED CONCRETE

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1 INFLUENCE OF THE DIAMETER OF THE BAR AND CDW CONTENT ON THE BOND BEHAVIOR OF RECYCLED REINFORCED CONCRETE Reis, C. N. S. (1), Leite, M. B. (2) and Lima, P. R. L. (2) (1) CAPES scholarship, Post-graduation Program of Civil end Environmental Engineering, State University of Feira de Santana, Bahia, Brazil. (2) Doctor, Department of Technology, Post-graduation Program of Civil and Environmental Engineering, State University of Feira de Santana, Bahia, Brazil. ABSTRACT The bond behavior between the concrete and the steel rebars is a fundamental parameter for perfect operation of the reinforced concrete, which happens due to three aspects: physical-chemistry adhesion; mechanical anchorage, and friction resistance. The recycled concrete, containing construction and demolition waste (CDW), presents physical, mineralogical and mechanical behavior characteristics different from the conventional concrete which can affect its bond behavior with the steel rebars. To use recycled reinforced concrete in a safe way it becomes necessary, therefore, the evaluation of the bond behavior of recycled concrete and steel rebars. In the present work it was evaluated the influence of the recycled fine aggregate (RFA) in steelconcrete bond through experimental analyses based on the pullout tests prescribed by RILEM. RFA replacement used at this study was 0, 25 and 50% by weight of natural aggregate. Two steel rebars diameters of 10.0 and 16.0mm of a CA-60 were still evaluated. The concrete mixture was designed to achieve 38MPa at 28 days age. The results show that the bond between recycled concrete and steel rebars decrease with an increase of the RFA replacement rate and rebar diameter. Keywords: Construction and Demolition Waste (CDW), recycled concrete, steelconcrete bond behavior. 1. INTRODUCTION The use of recycled concrete in the production of structural elements has been tested by researches in many countries of the world [1, 2, 3, 4, 5] and represents an promising alternative of reduction of environmental impact of construction industry since the use as structural material represents the major consumption of concrete and, consequently, of cement and aggregates. To application of recycled aggregate in reinforced concrete, however, it is necessary to evaluate the interaction between steel and surrounding concrete and the behavior of recycled concrete in structural elements. 207

2 Mukai et al [6] evaluated that the shear strength of reinforced concrete beams is reduced in 10% when recycled aggregate is used. The load correspondent to the first crack is lower, which was attributed to reduction of bond strength between reinforcement bar and recycled concrete. Some researchers have conducted studies on bond failure of recycled concrete, but the results are not conclusive. No variation was observed in bond behaviour [7, 8] and reduction of bond strength has been attributed to addition of recycled aggregate [9]. This contradiction can be explained by variation of studied parameters as type and content of recycled aggregate, diameter of bar or strength of concrete. In this study the effect of content of fine recycled aggregate on bond behaviour is investigated using pullout test. The research program was conducted for concrete with substitution of 25 and 50%, by weight, of natural aggregate. Two steel bars diameters of 10.0 and 16.0mm of a CA-60 were still evaluated. The concrete mixture was designed to achieve 38MPa at 28 days age. 2. MATERIALS AND METHODS 2.1 Materials Portland cement CP II Z 32 with specific gravity of 2.89 g/cm³ was used. The natural fine aggregates used were one medium river quartz sand (NFA2) and one fine quartz sand (NFA1). Both are regularly commercialized in Feira de Santana City (Northeast Brazil) and their properties are presented in Table 1. Tap water supplied by the local water supply company was used on the concrete mixtures. The natural coarse aggregate (NCA) used was originated from granitic rock, with specific gravity of 2.69 g/cm³ and maximum size aggregate of 9.5 mm. Recycled aggregate sourced from a demolition of religious temple sites of Feira de Santana City, Brazil, was crushed into pieces using a jaw crusher and screened in a 4.8 mm sieve. It was verified that recycled material is basically composed by inert materials such mortar (29%), ceramic (27%) and concrete (44%). The characterization results of recycled fine aggregate (RFA) is presented in Table 1. Deformed (i.e., crescent ribbed) low-carbon steel rebar with diameter size of 10 mm and 16 mm and yield strengths of 800 MPa, determined according NBR ISO 6892, were used for this investigation. 2.2 Concretes Mix Proportions Once the mixture proportions for the control mixture REF were established, two series of mixtures with recycled aggregates were cast. Each series had a designated amount of fine recycled aggregate (RFA), either 25% or 50%, substituted volumetrically for the fine natural aggregate. Increasing the amount of recycled material in the mixture required additional amounts of water to compensate of water absorption of aggregate. To keep the w/c ratio constant, the amount of free water was adjusted based in absorption rate to time of mixture of ten minutes. Superplasticizer (SP), based on modified polycarboxylic ether polymers, was used to maintain a target slump of 100 ± 10 mm. The mixture proportions for all batches are detailed in Table

3 Table 1. Results of the characterization of aggregates Properties (test method) Fine aggregates NFA1 NFA2 RFA Sieve opening (mm) %passing Granulometric analysis Maximum size aggregate (mm) (NBR NM 248/03) Fineness (NBR NM 248/03) Specific gravity (kg/dm³) Bulk density (kg/dm³) (NBR 7251/82) Water absorption rate (%) Fines content (%) (NBR NM 46/03) Experimental Method: NBR NM 30/03 2 Experimental Method: Leite [10] Table 2. Composition of concrete Mix %CD W Mix proportions (kg/m 3 ) C NCA NFA1 NFA2 RFA water compensation water Total water SP (%) Slump (mm) REF CDW CDW Preparation of specimens Figure 1a shows the sketch of pullout test specimens. The samples were cast with the steel rebar in a vertical position, as showed in Figure 1b. Two diameters of 10.0 and 16.0mm were evaluated. The ratio between concrete diameter and rebar diameter was

4 Figure 1. Pullout specimens. For each concrete batch, nine cylinders (100 mm in diameter and 200 mm in height) were cast to determination of compressive strength, splitting strength and elastic modulus. All the specimens were demoulded one day after pouring and transferred to the moist curing room under 95% RH for 28 days. A handheld needle vibrator was used to consolidate fresh concrete in samples. 2.4 Test setup The setup for the pullout test is shown in Figure 2, which is in accordance with RILEM-CEP-FIP (1973). The load (P) and the slip (s) at the free end of steel rebar anchored in the test specimen were measured in order to determine a load slip relationship. The monotonically increased load was applied by 100ton testing machine. High precision linear variable differential transducer (LVDT) was used in steel bar to measure the relative displacement between the steel rebar and top concrete surface of the specimen. The load was read by a load cell of 30ton positioned under sample. A computer was used to collect all test data automatically. 210

5 Figure 2. Pullout test setup. 2.5 Analysis method Under the conditions of these pullout tests, the bond stress along the whole anchorage length of the steel rebar can be considered to be uniformly distributed. The bond stress τ can be expressed by (1): P τ = π.φ. l b (1) where P is the load; l b is the embedded length of the steel rebar and φ is the diameter of the steel rebar. The bond strength τ m, expressed by (2), is obtained by media of three bond stresses relatives to slip of 0.01 mm (τ 0.01 ), 0.1 mm (τ 0.1 ) e 1.0 mm (τ 1.0 ): τ τ τ 1. 0 (2) τ m = 3 If the slip correspondent of maxima load is less than 1.0 mm then τ 1.0 is equal to τ max, that is defined as bond stress correspondent to maxima load of pullout. 3. RESULTS AND DISCUSSION 3.1 Mechanical properties The results of compressive strength (fc), splitting strength (ftd) and elastic modulus (Ec) are presented in Table

6 Table 3. Mechanical results Mix %CDW f c f td E c (GPa) REF ± ± ± 3.07 CDW ± ± ± 1.14 CDW ± ± ± 0.06 It is verified a reduction of all mechanical properties studied with addition of fine recycled aggregates. The maximum value of reduction was 14%, 16% and 28% to compressive strength, splitting strength and elastic modulus, respectively. These results already were observed by others researches [2]. 3.2 Stress versus slip curves The measured bond stress versus slip curves for the six series of specimens (i.e., REF, CDW25 and CDW50 mixtures with two rebars diameters) are drawn in Figure 3. Figure 3. Pullout test setup. Observing the stress-slip curves it is verified that behavior on pullout test of recycled concrete is similar to conventional concrete, when bar with diameter of 10 mm is used, as also observed by Xiao e Falkner [7]. To test with bar of 16 mm, two characteristics are different: i) to CDW50 concrete the slope of descendent curve, after maxima load, is smoother than conventional concrete. This fact indicates that the frictional bond stress of recycled concrete is higher; ii) to CDW25 is verified a rupture immediately after maxima load due rupture of recycled concrete while, to others samples, the pullout of the bar was observed. This concrete had same tensile strength of conventional concrete and, consequently, closer bond strength (see Table 4), however the compressive strength is minor (see Table 3). The bond strengths obtained from the test results are summarized in Table

7 Table 4. The summary of the pullout test Mix φ (mm) P max (KN) τ 0.01 τ 0.1 τ 1.0 τ max τ m REF ± CDW ± CDW ± REF ± CDW ± CDW ± The bond strength was influenced by CDW ratio and diameter of bar. The bond strength τ m of all specimens that used CDW were lower than those of reference concrete (REF), however the intensity of this variation depends of the diameter of the bar. To diameter of 10 mm the higher reduction was of 10% while, with the use of bar of 16mm, this reduction was of 30%. The influence of CDW ratio on pullout strength is proportional to your effect on tensile strength, as shown in Figure 4. The strength decrease as the replacement ratio of CDW increases. This behavior was verified by Choi and Kang [9] which verified that the bond strength decreased with the increasing recycled aggregate ratio. Figure 4. Influence of CDW ratio on pullout bond strength and splitting strength when compared with reference mixture (REF). 213

8 4. CONCLUSION Based on the results of this study, with respect to the behavior of the bond between concrete and deformed bar, it is verified that the replacement of natural aggregate by recycled aggregate decrease up to 30% bond strength. The diameter of bar has a great influence on these results. The bond strength is more influenced by recycled aggregate when bars of larger diameters are used. ACKOWLEDGEMENTS The authors would like to thank the CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) which sponsored the first author. REFERENCES [1] Li, X. `Recycling and reuse of waste concrete in china. Part II. Structural behaviour of recycled aggregate concrete and engineering applications`, Resources, Conservation and Recycling 53 (3) (2009) [2] Etxeberria, M., Mari, A. and Vázquez E., Recycled aggregate concrete as structural material. Materials and Structures 40 (2007) [3] Fonteboa, B.G. and Abella, F.M., Shear strength of recycled concrete beams. Construction and Building Materials, 21 (4) (2007) [4] Larrañaga, M. E., Experimental study on microstructure and strutural behaviour of recycled aggregate concrete. Doctoral thesis. Universitat Politécnica de Catalunya. Barcelona, p. [5] Maruyama, I., Sogo M., Sogabe, T., Sato, R. and Kawai, K., Flexural properties of renforced recycled concrete beams, in International RILEM conference on the use recycled materials in buildings and structures. Barcelona, Spain, (2004) 10 pages. [6] Mukai, T. and Kikuchi, M., Properties of reinforced concrete beams consisting recycled aggregate. Reuse of demolition waste In Proceedings of the second international Rilem symposium on demolition and reuse of concrete and masonry, Tokyo, Japan, (1988) [7] Xiao, J. and Falkner, H., Bond behavior between recycled aggregate concrete and steel rebars. Construction and Building Materials, 21 (2007) [8] Ajdukiewicz, A. and Kliszczewicz, A. Infuence of recycled aggregates on mechanical properties of HS/HPC, Cement and Concrete Composites 24 (2) (2002) [9] Choi, H.B. and Kang, K.I., Bond behaviour of deformed bars embedded in RAC, Magazine of Concrete Research 60 (6) (2008) [10] Leite, M.B., Avaliação de propriedades mecânicas de concretos produzidos com agregados reciclados de resíduos de construção e demolição. Tese de Doutorado (Engenharia Civil). Porto Alegre, Universidade Federal do Rio Grande do Sul, p. (in Portuguese). 214