Experimental Investigation of the Effect of Stiffening of Fresh Concrete on Workability

Initial defects may impair the durability of concrete structures. One cause of this is thought to be “stiffening,” a change in flow behavior that reduces concrete’s workability. Every engineer involved in concrete production and construction knows this phenomenon and that it establishes a static condition in the concrete. However, the effect of stiffness on construction has not been empirically elucidated. Without a quantitative evaluation method for stiffness, it is only possible to estimate its effect based on experimental investigations under limited conditions and then to set up countermeasures. To provide an engineering basis for countermeasures, this study targeted “stiffness” specifically the phenomenon in which the external force required to deform stationary concrete increases over time before the cement starts to set. Experiments were performed with the assumption that the “required force” is related to the apparent yield value of concrete. This study also confirmed with mortar experiments that it is possible to quantify concrete stiffening under static conditions by measuring the maximum shear stress with a simple vane shear test. Moreover, this study used U-shaped filling, deformation with vibration, and deformation performance tests to confirm that reducing the stiffening of fresh concrete in the static state by using a stiffening reduction agent contributes to increased workability. Furthermore, it was also confirmed that the maximum shear stress obtained from the vane shear test of static concrete decreased when a stiffening reduction agent was used. These results show that the workability of concrete can be improved by using a stiffening reduction agent.