SHEAR BEHAVIOR OF REINFORCED RECYCLED CONCRETE BEAMS

Transcription

1 SHEAR BEHAVIOR OF REINFORCED RECYCLED CONCRETE BEAMS Masaru Sogo (1), Takahisa Sogabe (1), Ippei Maruyama (1) Ryoichi Sato (1), Kenji Kawai (1) (1) Department of Social and Environmental Engineering, Hiroshima University, Japan Abstract Shear behavior of reinforced recycled concrete (RRC) beams are investigated experimentally in order to contribute to establishing design method for shear by comparing with that of reinforced concrete beam made of virgin aggregate. Ten reinforced concrete beams are prepared using manmade recycled fine and coarse aggregates as well as those from site. The major factors are combination of aggregates, water to cement ratios, effective depth and usage of expansive additive. Experimental results indicate that RRC beams without stirrups is decreased in shear strength by 2% and empirical design code overestimates the shear strength of recycled aggregate concrete. On the contrary, shear strength of RRC beams with stirrup is almost the same as those of RC beams in experimental results, while modified truss theory underestimates the ultimate shear strength of beams by 3% on the condition that shear failure is defined by yielding of stirrup. Keywords: Recycled aggregate concrete beam, Size effect, Modified truss theory, Shear properties 1. INTRODUCTION According to the white report of 2 by ministry of the environment [1], concrete waste amounted to 35Mt and 1.3Mt out of them are disposed. This indicates that the 96% of concrete waste is recycled in Japan. But almost all the recycled use of concrete waste is for pavement base, back filling for retaining wall and so on, which does not necessarily require high performance compared with structural concrete. The limited usage for recycled aggregate is attributable to not only unclear quality of the original concrete but also low and scattering quality due to high porosity of attached mortar and impurity included in the aggregate. On the other hand, reuse of recycled aggregate to pavement base is will decrease in near future. Consequently utilization to structural concrete should be indispensable in order to enhance the rate of reuse as well as to compensate for lack of natural aggregate. In utilizing recycled aggregate as a structural aggregate, the inferiority in concrete quality is a major concern. Some studies concerning concrete and reinforced concrete (RC) made of recycled aggregate have been performed [2-4]. However, those are not sufficient for making clear behavior of structural members leading to establishing design method. 61

2 The aim of the present study is to investigate the shear behavior of reinforced recycled aggregate concrete (RRC) beams containing high amount of mortar and cement paste in order to contribute to establishing design method for shear by comparing with that of RC beam made of virgin aggregate, in which water to cement ratio (W/B) is adopted as a parameter, and size effect as well as the validity of modified truss theory are discussed. 2. OUTLINE OF EXPERIMENT 2.1 Materials and mixture proportion Recycled aggregates were produced from demolished concrete structures. The original concrete structure of recycled aggregate of series A is 4-year-old building and located under the ground, series B is 1-year-old concrete foundation. Details of recycled aggregate as well as virgin aggregate of both series A and series B are shown in Table 1. Mixture proportions of concrete are listed in Table 2., and denote concrete Table 1: Properties of aggregate Series A Series B Coarse aggregate Density (g/cm 3 ) Absorption (%) Fineness Modulus Fine aggregate Density (g/cm 3 ) Absorption (%) Fineness Modulus JIS A JIS A Virgin Virgin Recycled Recycled Virgin Virgin Recycled Recycled Table 2: Mixture proportions of concrete (Series A) Name W/C s/a Unit content (kg/m 3 ) (W/B) % W C S G EX Pigment SP AE AEWR Antiformer EX EX EX EX: Expanssive additive, Sp:Super plastisizer, AE:Air-entraining agent AEWR:Air-entraining agent and water reducer agent 611

3 (Series B) Name W/C s/a Unit content (kg/m 3 ) % W C S G AE AEWR 55L L H H Table 3: Details of reinforced concrete beams L h b d a/d a Shear reinforcment p (%) (mm) (mm) (mm) (mm) (mm) s (mm) p w (%) Series A (2D19) (2D25) Series B (2D25) (4D25) (4D25) L:Full length of beam, h :Hight of beam, b :Width of beam, d:effective dipth, a :Shear span length P=As/(bd):Ratio of tension reinforcement, As:Cross sectional area of tension reinforcement, p w =A w /(bs):ratio of shear reinforcement, A w :Cross sectional area of tension reinforcement, s :Distance of shear reinforcements Figure 1: Details of reinforced concrete beams with virgin coarse and fine aggregates, that with recycled coarse aggregate and virgin fine aggregate, that with recycled coarse and fine aggregates, respectively. As is listed in the same table, W/C =.3,.45 and.6 in series A. Additionally, lime type expansive additive is added to concrete of W/C=.45 to improve structural performance of RC beam. W/C is fixed to be.55 in series B as is tabulated in the same table. 2.2 Specimens Table 3 and Fig. 1 show size of specimens and loading condition. The effects of W/C, recycled aggregate and usage of expansive additive are investigated in series A, and size 612

4 Figure 2: Typical Crack patterns of RC beams with W/C=.45 effect as well as the validity of modified truss theory are also investigated in series B. Total number of RC beams are 12 in series A and 8 in series B. 2.3 Loading and measurements All specimens were loaded at two points symmetrically about mid-span. In every RC member, magnitude of load was measured by a load cell with 5kN capacity and deflection at the mid-span was measured by a displacement transducer with a minimum graduation of 1/1mm. 3. RESULTS AND DISCUSSION 3.1 Cracking behavior Fig. 2 shows typical crack patterns of RC beams whose a/d is 3.1 and water-cement ratio is.45. This figure indicates that there is no noticeable difference in cracking patterns depending on usage of recycled aggregate and expansive additive. 3.2 Deflection Figs. 3 and 4 show load-deflection relationship of RC beams without and with expansive additive which have no shear reinforcement and whose W/C is.45. As are shown in both figures, using recycled aggregate decreases the load at diagonal cracking which is defined as first sudden increase of deflection independent of containing expansive additive or not. However, expansive additive increased the diagonal cracking load by nearly 1% irrespective of the difference of aggregate, compared with the results without expansive additive obtained from companion beams. All the beams also showed the formation of arch-action after diagonal cracking and subsequently failed, in which no difference in failure modes were observed. Load-deflection relationships of RC beams in series B are shown in Figs. 5 for no arrangement of stirrup and 5 for arrangement of stirrup, in which L and H of specimens name denote 16mm and 335mm in effective depths, respectively. In Fig. 6, RC beams with stirrups whose effective depth are 16 mm showed yielding of stirrup followed by yield of tension reinforcement and subsequent gradual fracture of concrete in compressive zone. On 613

5 1 1 -EX -EX -EX Load (kn) 5 Load (kn) Deflection (mm) Figure 3: Load-deflection relationship of RC beams of W/C= Deflection (mm) Figure 4: Load-deflection relationship of RC beams of W/C=.45 with expansive additive Load (kn) L 55L 55H 55H Load (kn) L-str 55L-str 55H-str 55H-str Deflection (mm) Figure 5: Load-deflection relationship without shear reinforcement 1 2 Deflection (mm) Figure 6: Load-deflection relationship with shear reinforcement the other hand, RC beams with the depth of 335mm failed resulting from yielding of stirrup in which tension reinforcement was in elastic stress state. These figures also indicate that there is little difference in failure mode by the usage of recycled aggregate. 3.3 Shear strength Shear strengths on RC beams without stirrup are calculated by the empirical equation (1) proposed by Niwa et.al [2], which is assumed to be the same as nominal shear stress at diagonal cracking eliminating the increase of shear stress due to arch action. The effect of expansive additive is taken into account as an axial force by the factor M o / M d. M 1/3 1/3 1/4 τ c= f ' c (1 Pw) (1/ d) ( /( a/ d)) M + (1) d Where, f ' c :Compressive strength, P w :ratio of tension reinforcing bars, d :effective depth a :Shear span, M : moment to cancel the stress produced by axial force in tension fiber, M d :design moment. o 614

6 Fig. 7 shows the effect of water-cement ratio and aggregate type on shear strength of RC beams without shear reinforcement which is obtained by V/(bd), where V denotes shear force at diagonal cracking. Compared with shear strength of, the shear strength of indicated 2% decrease at most, and that of indicated 3% decrease in the worst case. This decrease can be explained mainly by the weakness of aggregate without the roundabout crack on the surface of the aggregate, which should reduce fracture energy and interlocking effect, while the effect of the difference of compressive strength on the shear stress is included. Reducing water-cement ratio increases the shear strength and it can be concluded that reducing water-cement ratio improve the shear property of RC beams with recycled aggregate. Fig. 8 shows effect of expansive additive on shear strength. Addition of expansive additive increases the shear strength by 1% in any cases of aggregate type. This can be explained by that the axial force which is produced by expansion of concrete makes wider compressive zone and narrower cracking width. The wider compressive strength zone improve the shear resistance and the narrower cracking width improve the interlocking performance. The comparison of experiment and calculation by Eq. (1) for shear strength is demonstrated in Fig. 9. The experimental strength of is greater than or equal to calculated values. But experimental strength of decrease by 1% on average and 15 % in the worst case. Averaged ratios of measured shear strength of,, and without stirrup to calculated results by Eq. (1) are 1.4,.93, and.92, respectively. Fig. 1 present the contribution of concrete to the shear strength in the RC beam without stirrup, which is calculated by the elimination of the effect of tension reinforcement ratio, size, 13 the ratio of shear span to effective depth from experimental shear strength, i.e. fvc =.2 f ' c in Eq. (1), plotted as a function of compressive strength of concrete. Experimental f vc is calculated from the experimental shear strength divided by the term of Eq. (1) except for 13.2 f ' c. The contributions of all shear resisting components without stirrup of and to the shear strength are 1% smaller than that of. But the dependency of this contribution on compressive strength is similar and parallel to those of ordinary concrete. Figure 7: Effect of recycled aggregate and water-cement ratio on shear strength without stirrup Figure 8: Effect of recycled aggregate and expansive additive on shear strength without stirrup 615

7 τc by exp. (N//mm 2 ) % τc by cal. (N/mm 2 ) -EX -EX -EX f vc (N/mm 2 ) f vc =.2 f 'c 1/3 -EX -EX -EX Compressive strength (N/mm 2 ) Figure 9: Comparison of experimental result and calculation of τ c Figure 1: Relationship between f vc and compressive strength 3.4 Size effect and effect of stirrups Fig. 11 shows the size effect on nominal shear stress at diagonal cracking, which is obtained by measuring stirrup strain when beams are reinforced for shear force. The value of experimental nominal diagonal cracking stress is divided by f c 1/3 and 1P w 1/3 in order to eliminate the effect of compressive strength of concrete and the ratio of shear reinforcement. The lines shown in the figure are the curves fitting experimental results. According to the figure, the size effect is expressed by 1/4 power low of effective depth for virgin concrete beams and, however, is sensitive more than -1/3 power low of effective depth for recycled concrete beams, which must be verified with more data. Fig. 12 shows the shear force Vs measured in stirrup intersecting the dominant shear crack which brought the shear failure [3]. The comparison of measurement and calculation for shear force in stirrup indicates that modified truss theory is valid in estimating shear resisting force of stirrup covering recycled as well as normal concrete beams, while calculated shear forces τc / (f'c 1/3 1P w 1/3 ) /3 power low str str -1/4 power low Effective dipth (mm) Shear Force (kn) L-st r Vc 4 55L-str 55H-st r 55H-str 2 Vs Modified Truss theory Modified Truss theory Shear Force of Stirrups (kn) Figure 11: Nominal shear stress as a function of effective depth Figure 12: Shear force in stirrups versus shear force 616

8 Table 4: Diagonal cracking strength and ultimate shear strength of beams with calculation results by modified truss theory are larger than those of experiment especially in larger sized beams. Shear strength of RRC beams with stirrup is almost the same as those of RC beams in experimental results, while modified truss theory underestimates the ultimate shear strength of beams by 3% on the condition that shear failure is defined by yielding of stirrup. 4 CONCLUSIONS Vc (N/mm 2 ) Vs(N/mm 2 ) Vu (N/mm 2 ) Cal. Exp. Cal. Cal. Exp. (Vu.cal.) /(Vu.exp ) 55L-str L-str H-str H-str Shear behavior of wet-cured reinforced recycled concrete beams was investigated experimentally compared with that of ordinary reinforced concrete beams. The following conclusions are drawn within the limit of the present study. - RC beams using recycled aggregate concrete shows the same cracking patterns and failure mode as those of ordinary RC beams. - Shear strength of beams without stirrup decreased by 1-2% when coarse recycled and virgin fine aggregates were used and by 1-3% when both coarse and fine recycled aggregates were used. - Reducing water-cement ratio increases the shear strength by 25% in case of W/C=.3 and by 1% in case of W/C=.45 compared with that of W/C=.6. - Expansive additive improves it by 1 % in any aggregate types. - The size effect is expressed by 1/4 power low of effective depth for virgin concrete beams and, however, is sensitive more than -1/3 power low of effective depth for recycled concrete beams. - Modified truss theory is valid in estimating shear resisting force of stirrup covering recycled as well as normal concrete beams, while calculated shear forces are larger than those of experiment especially in larger sized beams. - Modified truss theory overestimated experimentally obtained ultimate shear strength of beams irrespective of usage of recycled aggregate. - Shear strength of RRC beams with stirrup is almost the same as those of RC beams in experimental results, while modified truss theory underestimates the ultimate shear strength of beams by 3% on the condition that shear failure is defined by yielding of stirrup. 617

9 REFERENCES [1] White reports of the environment, Ministry of the environment, 2 [2] Niwa, J., Yamada, K., Yokozawa, K. and Okamura, H., Revaluation of the equation for shear strength of reinforced concrete beams without web reinforcement, Proceedings of JSCE, No.372, pp , 1986 [3] Hayakawa, T., Fujita, M., Mise, A. and Sato, R. Effect of shrinkage of high strength concrete on the strain behavior in shear reinforcement, Proceedings of JCI, Vol.22, No.3, pp , 2 618