The Effect of HRWR and AEA on the Air System of Vibrated Concrete

Air entrainment in concrete railroad ties is typically included to prevent freeze-thaw damage. Uniformly-distributed microscopic air voids in concrete ties helps to resist against freeze-thaw. The air entrainment system stability with time after mixing is dependent on a number of factors which include chemical, environmental, and construction related parameters. These resulting differences in the air void system can in turn lead to different performances of concrete under freeze-thaw loading. Vibration is mainly used to consolidate concrete but can also alter the air structure of concrete. This study investigates the effect of different chemical admixtures and rheological properties on the stability of the air system inside vibrated concrete. Form vibration of 75 Hz frequency and 3 g or 10 g peak acceleration was applied to the tested concrete after the lapse of several time durations. This was done to investigate the stability of the air system of concrete as time passes on under different vibration conditions. The testing included slump, unit weight, temperature, rheology and fresh air content before vibration and after vibration for each elapsed time duration. Fresh air content tests showed that the air loss increases linearly with time for almost all mixtures. Concrete rheological properties immediately after mixing were not a good indicator of the concrete air system stability after delayed placement. Tall oil based air entraining agents (AEA) was found to produce slightly lower air instability; however, the differences between AEA were small. The results showed that for precast concrete railroad ties, any instability with time of entrained air should not be a significant problem for precast concrete railroad tie plants if the concrete is placed and consolidated within 30 minutes from mixing at room temperatures.