Introduction
Ultra-high performance concrete (UHPC), as an advanced cementitious fiber-reinforced composite material, is often widely used as a load-bearing member in the critical parts of various types of large-span and high-rise structures. However, the increasingly frequent fire disasters have posed a great challenge to the safe serviceability of UHPC. Steel fiber, as an important factor for UHPC to achieve ultra-high toughness, the cracking mechanism of steel fibers exposed to high-temperature will directly affect the high-temperature residual properties of UHPC. Although some scholars have carried out studies on the residual properties of UHPC after high temperature, there are less systematic studies on the influence of steel fibers on the mechanical and thermal properties of UHPC, especially on the deterioration mechanism of steel fibers exposed to high temperature. In view of this, this work systematically investigates the influence of steel fiber on the mechanical and thermal properties of UHPC exposed to high temperature. The mathematical model between steel fiber dosage-temperature-mechanical properties/thermal properties was established. In addition, the changes in the pull-out behavior of steel fibers from the UHPC matrix at different temperatures were investigated for the first time, and the deterioration mechanism of steel fibers exposed to high temperature was elucidated based on SEM tests.
Methods
The Onoda P.II 52.5 cement and the blending material produced by Jiangsu Sobute New Material Co. were used, respectively. At the same time, an appropriate amount of polypropylene fiber was mixed to reduce the risk of explosive spalling of UHPC. UHPC with 0%, 1.0%, 2.0% and 2.5% steel fiber volume mixing were prepared. The concrete molding size was 100 mm×100 mm×100 mm, and 28 d standard curing was carried out after the casting was completed. Then the mechanical and thermal properties were tested after high temperature. The changes in the pull-out mechanism of steel fibers after high temperature were also investigated.
Results and discussion
The compressive strength of UHPC after high temperature with different steel fiber dosage increases and then decreases with temperature. The addition of steel fibers can improve its overall compressive strength to different degrees. The tensile strength of UHPC shows almost monotonically decreasing with the increase of temperature, and the degree of loss rate is greater. Among them, the enhancement of steel fiber addition can obviously improve the tensile strength of UHPC at room temperature as well as at 200 ℃, but the degree of its enhancement with the increase in temperature gradually weakened. The elasticity modulus of UHPC with the increase in temperature first increased and then decreased, and the overall rule of change is similar to the compressive strength. The thermal conductivity of UHPC shows a monotonically decreasing law with temperature, which is mainly because the high temperature makes the organic fiber in UHPC melt, free water continues to evaporate, and the hydration products continue to decompose and so on to reduce the apparent density of the system. The addition of the steel fibers effectively improves the thermal conductivity of UHPC. The specific heat capacity showed the opposite law to the thermal conductivity. In addition, the maximum pull-out load, pull-out work and bond strength of steel fibers at 200 ℃ increased to varying degrees, compared to room temperature. At 400 ℃, relevant parameters began to have a substantial decline. When the temperature reached 600 ℃, the failure mode of steel fibers changed from pull-out to pull-off mode.
Conclusions
The compressive strength of UHPC increases at the beginning and then decreases with the increase of temperature, and reaches the peak value at 200 ℃. The addition of steel fiber does not affect the trend of UHPC residual compressive strength exposed to high temperature, but improve absolute value of its residual strength. Temperature changes pull-out mode of steel fiber. At 25–200 ℃, the surface of the steel fibers was smooth when they were pulled out, and the tensile strength of UHPC did not change much. At 400 ℃, steel fibers were attached to part of the matrix when they were pulled-out, UHPC tensile strength began to drop significantly. At 600 ℃, the failure mode of steel fibers changed from pull-out to pull-off mode. The tensile strength of UHPC with different steel fibers continues to decline, and the difference gradually narrowed. The elasticity modulus of UHPC reached its peak at 200 ℃ and began to decline rapidly, and its sensitivity to high temperature is much greater than the compressive and tensile strength changes, the value of the proposal can be used as an important indicator for the assessment of the safety of the relevant building after the fire, and the incorporation of steel fibers does not affect the overall development of its law. Steel fiber can significantly increase the thermal conductivity of UHPC, but will not change the overall trend of thermal conductivity with the increase in temperature. The thermal conductivity of UHPC decrease with the increase of temperature and the gradual decrease in the overall trend of change; specific heat capacity shows the opposite development law with the thermal conductivity. Copper-plated steel fiber as UHPC commonly used metal fiber, can maintain good mechanical properties and play a toughening crack resistance before 400 ℃. When the temperature reached at 600 ℃ and later, the degree of deterioration of copper-plated steel fiber itself is greater than the fiber-matrix interface transition zone, which leads to UHPC tensile and compressive drop in coordination, it is recommended that the steel fiber's own high-temperature performance enhancement should be focused on under this condition.