Investigation and comparison of the effect of melting and freezing cycles on the strength and durability of recycled and conventional concrete with different ratios of water to cement

Recycling of concrete has been the focus of researchers owing to the large volume of concrete waste and the limited resources of valuable materials. Recycled aggregate concrete (RAC) contains aggregates of various types of concrete and building materials that have been destroyed due to their shelf life or as a result of war and natural disasters. In order to use different types of recycled aggregates (RA), it is necessary to conduct more detailed and sensitive research on the behavior of RAC in order to reach the highest possible level of quality and cost in the construction process. On the other hand, RA are made of concrete with different properties and its aggregates have been subjected to different form of loading, hence it is important to carefully examine their behavior and properties. Therefore, in this study, the strength and the durability of RAC after freeze and thaw cycles were compared with natural concrete while different water/cement ratios are used.
In this study, RA were extracted from demolished waste concrete specimens and used to produce three mixtures named RC, RN and NC while 100, 50 and 0 percent of RA were replaced respectively. Water to cement ratio were kept as 45 and 55 percent of cement. A total number of 57 samples for flexural, compressive, tensile strength and ultrasonic tests were made. 24 samples were placed in the refrigerator and freeze and thaw cycles were applied on. 18 and 15 specimens were prepared for testing at 28 and 120 days of age, respectively. According to ASTM C666, the relative dynamic modulus of the specimens was tested using ultrasonic test after each 40 cycle’s period. After the test was finished, compressive strength, single point bending, indirect tensile tests and elastic modulus were determined for each specimens. Finally, the behavior of recycled and natural concrete was compared.
The results showed that the freeze and thaw cycles reduced the resistance of both RAC and NAC. Recycled aggregates have 2.5 times more water absorption than Natural aggregates. RAC has lost 41%, 18%, 10% and 28% of its compressive, flexural, tensile strength and modulus of elasticity, after the freeze and thaw cycle, respectively. The 100% replacement of RA reduced even more the mechanical behavior of samples. The relative dynamic modulus at the end of the cycles for RAC was 2% higher than that of NAC, indicating its better durability properties against freeze and thaw cycles. Concrete with 50% replacement of recycled aggregate has almost the same durability and strength as NAC, hence is recommended to be use in similar condition