EFFECTS OF FINE AND COARSE AGGREGATE CONTENT ON PASSABILITY BETWEEN REBARS OF CONCRETE

Abstract

 In present concrete work, basic measures to reduce segregation in terms of design, mix proportion and construction are routinely taken, and they are useful to prevent severe defects such as rock pocket and lost part. However, it is not clear if they prevent the blocking of coarse aggregate while passing between rebars and the settling of coarse aggregate during compaction with vibration, as well. In case these types of segregation phenomenon occur, the distribution of ratio of coarse aggregate to matrix mortar becomes non-uniform in a concrete structure. The authors previously reported that though this makes little difference in compressive strength and surface quality of concrete in a member, it makes much difference in amount of rise in temperature caused by heat of hydration, Young’s modulus and drying shrinkage strain.

 In this study, ordinary concrete and high fluidity concrete were placed into wall forms with rebars. Coarse aggregate content which seems to have a major effect on passability and fine aggregate content in matrix mortar which seems to have a large effect on ability entraining coarse aggregate were changed in the experiment. Moreover, these effects on the two performances concerning passing between rebars, namely, passability between rebars (i.e. the ability that concrete can just pass between rebars), and segregation resistance while passing between rebars (i.e. the resistance to segregation of coarse aggregate arising from blocking by rebars), were investigated. The following findings were obtained as the results.

 (1) When ordinary concrete passes the narrow space between rebars, the passability degrades and the segregation resistance slightly degrades with increasing coarse aggregate content.

 (2) In case of ordinary concrete, the segregation resistance while passing narrow space between rebars degrades with decreasing fine aggregate content in matrix mortar, because yield value and viscosity of matrix mortar decrease and thereby the ability entraining coarse aggregate decreases.

 (3) In case of high fluidity concrete with high performance AE water reducing agent containing viscosity modifying agent, as is well known, the passability between rebars without compaction by vibrator is higher than that of ordinary concrete. On the other hand, the segregation resistance while passing between rebars is lower than that of ordinary concrete. Furthermore, coarse aggregate content has little effect on these fact within the limits of bulk volume of 0.48 - 0.62 m³/m³.

 (4) The relationship between the passability between rebars and the segregation resistance passing between rebars varies with types of concrete and fine aggregate content in matrix mortal. Namely, these are different performance.