Investigating some mechanical properties of pervious concrete containing micro silica

Pervious concrete is a particular type of concrete with high porosity and often without fine aggregates or with little sand. The sufficient amount of cement paste covers and glues aggregates in a porous matrix together, resulting in a fast drainage system. On the other hand, high porosity and low amount of cement paste lessen the compressive strength of this type of concrete in comparison with normal concretes. Its main application is in the construction of pavements and management of stormwater. Pervious concrete has many economic and environmental benefits. Reducing the costs of surface water drainage, protecting roads from floods during major storms, preventing contamination of rainwater, recharging groundwater resources, and preventing road surface damage from freezing are some of its advantages. In this study, compressive strength, permeability, and freeze-thaw resistance of pervious concrete specimens containing micro silica have been investigated. Fifteen mix designs containing different amounts and sizes of aggregates, and water to cement ratios, and incorporating highly reactive, amorphous micro silica were prepared. The specimens were demolded after 24 hours and were cured in water with a temperature of 23ºC until the test day. The compressive strength of the specimens was evaluated. Then, permeability and freezethaw resistance tests were conducted on selected mixes. According to the results, the compressive strength of the specimens after 28 days of curing was in the range of 101 to 404 kgf/cm2 . Mix designs containing higher cement and micro silica content, and with lower water to cement ratio exhibited high compressive strength. The flow rate of specimens evaluated in the range of 0 to 111 cc/s, and the corresponding permeate velocity was between 0 and 3.43 mm/s. In mix number 11 with the highest cement and micro silica content, the flow rate was equal to zero due to the filling of concrete pores with cement paste, which happened in bottom layers during vibration. It is worth mentioning that the higher content of sand in the mix number 11 was also effective in the sedimentation of cement paste during vibration. Pervious concrete specimens, due to the permeability, are more durable in freezing and thawing cycles than normal concrete. After 10 cycles of freeze-thaw tests, micro cracks appeared on the surface of normal concrete specimens, and they were utterly destroyed after 16 cycles through fracture of cement paste. However, there were no visible cracks in pervious concrete specimens even after 23 cycles. Weight loss of pervious concrete specimens in freezing and thawing cycles was evaluated and compared in different mix designs. Mix number 10 containing medium gravel aggregates displayed better freeze-thaw resistance than mixes numbers 11 to 13, which contain fine gravel aggregates. This can be attributed to the more porous structure of the former specimen in comparison with the latter mentioned ones. In the case of mix number 11, as mentioned earlier, because of low permeability despite high compressive strength, maximum fracture occurred due to expansion in freezing cycles. Some of the studied mixes are appropriate in pavement construction, such as parking lots or sidewalks, including mix number 9, which contains micro silica at an amount of 7 percent of cement weight with compressive strength of 283 kg/cm2 and a flow rate of 111 cc/s.