Microstructural Investigation of Heat-Treated Ultra-High Performance Concrete for Optimum Production.

Sung-Hoon Kang,Sung-Hoon Kang,Juhyuk Moon,Sung-Gul Hong,Ji-Hyung Lee

doi:10.3390/ma10091106

Sung-Hoon Kang, Sung-Hoon Kang + Show 3 more

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https://doi.org/10.3390/ma10091106

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Abstract

For optimum production of ultra-high performance concrete (UHPC), the material and microstructural properties of UHPC cured under various heat treatment (HT) conditions are studied. The effects of HT temperature and duration on the hydration reaction, microstructure, and mechanical properties of UHPC are investigated. Increasing HT temperature accelerates both cement hydration and pozzolanic reaction, but the latter is more significantly affected. This accelerated pozzolanic reaction in UHPC clearly enhances compressive strength. However, strength after the HT becomes stable as most of the hydration finishes during the HT period. Particularly, it was concluded that the mechanical benefit of the increased temperature and duration on the 28 day-strength is not noticeable when the HT temperature is above 60 °C (with a 48 h duration) or the HT duration is longer than 12 h (with 90 °C temperature). On the other hand, even with a minimal HT condition such as 1 day at 60 °C or 12 h at 90 °C, outstanding compressive strength of 179 MPa and flexural tensile strength of 49 MPa are achieved at 28 days. Microstructural investigation conducted herein suggests that portlandite content can be a good indicator for the mechanical performance of UHPC regardless of its HT curing conditions. These findings can contribute to reducing manufacturing energy consumption, cost, and environmental impact in the production of UHPC and be helpful for practitioners to better understand the effect of HT on UHPC and optimize its production.

Highlights

One of the advantages of ultra-high performance concrete (UHPC) is outstanding mechanical performance
Each sample was cured at 20 ◦ C for 24 h before the HT, and the hydration heat during the designated HT period was measured by inserting the sample into the calorimeter that had reached the target HT temperature (60 ◦ C or 90 ◦ C)
As soon as the curing temperature increased at 24 h from 20 ◦ C to 60 ◦ C or 90 ◦ C, the hydration heat sharply increased and the second peak was formed

Summary

Introduction

One of the advantages of ultra-high performance concrete (UHPC) is outstanding mechanical performance. This includes ultra-high compressive strength (150–250 MPa), non-brittleness, and good load-carrying capacity under tension by incorporating fibers [1,2,3,4]. The increase in curing temperature is known to promote a pozzolanic reaction among silica fume (SF), portlandite, and water [6,9]. This enables the concrete to achieve high early strength that is useful in precast concrete production [8]

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