The finishability of a concrete depends on the relative perception of the person on the job, and it is more critical for some types of structures, such as slabs and bridge decks. No standard test characterizing finishability exists. HPC is almost always reported as being hard to finish with descriptions like "sticky" or "rocky". Factors such as high cementitious materials content, large doses of chemical admixtures and air entrainment contribute to the difficulty in finishing these concretes. Rostam [9] reported that HPC has a tendency to stick to everything, including reinforcing steel and finishing trowels. In the survey, 75 percent of the responses reported that the concrete mixtures were sticky. In fact, some contractors [15] do not use HPC for slabs that must be trowelled for smoothness because of the stickiness problem. Malier et al. [10] suggested that the concrete should be finished less than 30 min. after placement to prevent such difficulties.
The stickiness of HPC is related to its high cohesiveness, the viscosity of the mix and lack of bleeding due to the increased amount of fine particles, and the low w/c ratio. HPC is different from regular concrete and finishers should recognize the need for a different sequence of operations. Due to the lack of bleeding, a crust will form relatively quickly preventing the finishing tool from operating correctly. The crust formation can lead to other problems such as cracking and difficulties in blending two successive layers of concrete. Great attention to timing of finishing operations is recommended.
The fact is that the stickiness is related to the viscosity of the outer layer of the concrete, i.e., mortar with fine sand or cement paste, makes the slump test inadequate to determine the finishability. A test method to predict the finishability of HPC is needed.