246 / 2022-06-15 16:50:18

Study on mechanism of basic tensile creep of recycled fine aggregate ultra-high performance concrete

Ultra-high performance concrete,recycled fine aggregate content,saturated surface dry,autogenous shrinkage,basic tensile creep

The traditional UHPC has excellent mechanical properties and durability, but its preparation cost is high and its  autogenous shrinkage is large. Recycled fine aggregate (RFA) is a kind of fine aggregate obtained by crushing waste concrete, which has high water absorption and can play an internal curing role in concrete. In this paper, a saturated surface-dried RFA was used to replace natural river sand to prepare UHPC, so as to reduce the cost and autogenous shrinkage of UHPC.

RFA content is air-dried RFA/ (air-dried RFA+ river sand) (mass ratio). Air-dried (initial moisture content of 5%, AD) RFA was weighed and changed to saturated surface-dried (initial moisture content 11%, SSD ). Saturated surface-dried state RFA was used in all groups. The effect of RFA content (0, 25, 50, 75 and 100%) on the mechanical properties, internal relative humidity, autogenous shrinkage and basic tensile creep of UHPC was studied. The microstructure, interfacial transition zone and pore structure of UHPC were studied by BSE, microhardness and MIP to reveal the mechanism of basic tensile creep of UHPC.

The results show that with the increase of RFA content, the autogenous shrinkage and basic tensile creep of UHPC first decrease and then increase (Fig. 1 and 2), respectively. In Figs.1-4, SSD25 means a UHPC with RFA content of 25%, and so on. When RFA content is 25%, the autogenous shrinkage and basic tensile creep of UHPC are the minimum. There are three main influencing factors of autogenous shrinkage (Fig. 3): (1) static elastic modulus; (2) mesoporous (<50 nm); (3) internal relative humidity. When the RFA content increases from 0 to 25%, the static elastic modulus of UHPC decreases, and the mesoporous increases, the internal relative humidity increases; the internal relative humidity plays a leading role in UHPC autogenous shrinkage, therefore the autogenous shrinkage of UHPC decreases. When the RFA content increases from 25 to 100%, the static elastic modulus of UHPC decreases, the mesoporous increases, and the internal relative humidity increases; the static elastic modulus and the mesoporous play a leading role, hence the autogenous shrinkage of UHPC increases.

There are three main influencing factors of basic tensile creep (Fig. 4): (1) autogenous shrinkage; (2) microhardness of ITZ3 (interface transition zone between old cement paste and UHPC matrix); (3) ink bottle pore volume. Autogenous shrinkage is the macroscopic driving force of basic tensile creep. Therefore, with the increase of autogenous shrinkage, the basic tensile creep of UHPC increases. The water filled in ink bottle pore, which is benefit for the slippage of hydration products. The larger ink bottle pore volume, the higher the basic tensile creep With the increase of RFA content from 0 to 25%, the autogenous shrinkage and ink bottle pore volume of UHPC decreases, so the basic tensile creep of UHPC decreases. With the increase of RFA content from 25 to 100%, the autogenous shrinkage and ink bottle pore volume of UHPC increase, and the microhardness of ITZ3 decreases, and the basic tensile creep of UHPC increases.