Abstract
Ultra-high performance concrete (UHPC) has a potential cracking risk due to its large autogenous shrinkage. The use of an expansive agent is an effective approach to reduce autogenous shrinkage of UHPC. However, different kinds of expansive agents show different expansive efficiency in UHPC. To study the cause for the difference in expansive efficiency, this study selected three expansive agents, namely highly reactive MgO-based, medium reactive MgO-based, and CaO-based expansive agents, and carried out the following experiments: autogenous shrinkage, hydration heat, hydration process of expansive agent under different relative humidity (RH), and micrographs. The results showed that the CaO-based expansive agent has high hydration activity at RH of more than 44.0%, while the hydration activity of two kinds of MgO-based expansive agents, especially a medium reactive MgO-based expansive agent, decreases significantly when RH drops below 80%. Meanwhile, the CaO-based expansive agent had higher expansive efficiency in UHPC than the MgO-based expansive agent. This study suggested that the CaO-based expansive agent is more suitable for compensating autogenous shrinkage of UHPC due to its low humidity sensitivity compared to the MgO-based expansive agent.
Highlights
Ultra-high performance concrete (UHPC) is a kind of cement-based composite with excellent properties such as ultra-high compressive strength (≥120 MPa) and extremely low porosity [1,2,3]
This study focuses on the humidity sensitivity of the hydration of expansive agent to study mechanisms for expansive efficiency of expansive agent in UHPC
Magnesite powder is calcined in electric furnace at 750–850 ◦C for 1 h to prepare highly reactive MgO-based expansive agent (H-MEA), and calcined at 950–1050 ◦C for 1 h to prepare medium reactive MgO-based expansive agent (M-MEA)
Summary
Ultra-high performance concrete (UHPC) is a kind of cement-based composite with excellent properties such as ultra-high compressive strength (≥120 MPa) and extremely low porosity [1,2,3]. UHPC has large early age shrinkage, especially autogenous shrinkage, due to factors such as extremely low water to binder ratio (w/b, typically ≤ 0.20), incorporation of ultra-fine powder, and almost no coarse aggregate [2,3]. The increase of capillary tension in pores due to the decrease of internal relative humidity (RH) is the main driving force that results in autogenous shrinkage of UHPC. Studies have shown that autogenous shrinkage of UHPC can reach approximately 1000 με [4,5]. When this large shrinkage is constrained, cracks may occur and lead to negative impacts on the durability and safety of UHPC structures [6]
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