Control of cracking in full-scaled columns made of ultra-high-strength concrete

Experiments have shown that autogenous shrinkage and elevated temperature due to hydration in reinforced concrete members composed of ultra-high-strength concrete, such as concrete that has a design strength of over 120 MPa cause surface cracks, internal cracks, and cracks around rebar. Since ultra-high-strength concrete is generally used in Japan as cast-in-place concrete or precast concrete, the mitigation of these cracks is a crucial issue because the concrete is expected to have high durability. In the present study, expansive additive, shrinkage reducing agent, combined use of expansive additive and shrinkage reducing agent, thermal insulation to control temperature differences within a member, and combined use of thermal insulation and expansive additive were evaluated by placing full-scale columns and cutting out specimens in order to observe the crack patterns in the columns. Ways to mitigate the cracks were found and characterized. Although thermal insulation generally suppressed surface cracks, the peak temperature increased and consequently so did the risk of internal cracks due to the higher peak temperature and the resultant increase in autogenous shrinkage. Partial replacement of binder with expansive additive reduced cracks around rebar and surface cracks, but the soundness of the bond increased the risk of internal cracks. Addition of shrinkage reducing agent reduced surface cracks and internal cracks. Combined use of expansive additive and shrinkage reducing agent appeared to give a combination of these advantages. The best way to mitigate cracking was combined use of thermal insulation and expansive additive. This method reduced all types of cracks.