Study On Size Effects Of Mechanical Properities Of Recycled Concrete Beams

A large number of studies have been carried out for recycled concrete beams in recent years.However,there is little research on the size effect of strength of the recycled concrete beam.In view of this,this study will focuses on the size effects on mechanical prosperities of recycled concrete beams.Four recycled concrete beams without web reinforcement subjected to static loading were tested by the four point bending experiment to obtain their shear performances.The effects of beam length and cross section height on the shear performance were investigated.From the experimental results,we discussed the failure modes,midspan deflections,cracking loads,strains of longitudinal reinforcements,strains of concrete in compression zone,ultimate shear loads,and shear strengths in detail.It has been shown that the shear performance of recycled concrete beams has a significant size effect.The main contents of this study are as follows.(1)For these specimens,the diagonal cracks occurrence and their development processes are similar to those of ordinary concrete beams.The oblique section failure often occurs in the segment where the shear force and bending moment work together.And the failure surface of the specimen is in the section of the shear span near the support.(2)According to the comparison of the tested values and the calculated values by three existing methods,it was found that the existing calculation methods on the cracking loads are only suitable for small-sized beam specimens.It is very necessary to correct the calculation error of the existing method for large-size beam specimens.The main reason for the error is that the existing calculation method does not consider the size effect of tensile strength of recycled concrete.Therefore,this study presents an improved formula for the cracking load calculation by considering the size effect.The calculated results obtained by the proposed method are closer to the test values than those obtained by the original method.It has been shown that it is necessary and reasonable to consider the size effect in the calculation of the cracking load of the recycled concrete beam.(3)It was found that the ultimate shear loads,shear strengths,and maximum shear stresses of recycled concrete beams all have obvious size effects.In general,the values of these parameters for the recycled concrete specimens decrease with the increase of the specimen size.But sometimes the shear strength abnormally increases for the large size specimen.It was found that the existing calculation method on ultimate shear load by considering the size effect is suitable for these beam specimens.In the end,several key issues related to the size effect for recycled concrete beam are emphasized and future study aspects are pointed out.