Effect of Internal Curing on Early Shrinkage and Crack Resistance of UHPC by SAP and Ceramsite

TL;DR

This study compares how SAP and ceramsite affect the shrinkage and crack resistance of ultra-high-performance concrete.

Contribution

The study systematically compares SAP and ceramsite as internal curing materials in UHPC under consistent mixture proportions.

Findings

Increasing w/b ratio reduces autogenous shrinkage but compromises mechanical properties.

Pre-saturated ceramsite and SAP effectively mitigate shrinkage and enhance crack resistance.

Exceeding ceramsite volume dosage or using dry addition reduces crack resistance.

Abstract

This study investigated the effects of varying water–binder (w/b) ratios and internal curing materials—superabsorbent polymer (SAP) and ceramsite—on the shrinkage behavior and crack resistance of ultra-high-performance concrete (UHPC). Although internal curing has been extensively studied, the comparative effectiveness of different internal curing materials on early-age shrinkage and restrained cracking behavior of UHPC under consistent mixture proportions remains unclear. To address this gap, a systematic experimental comparison of SAP and ceramsite was conducted. The influences of w/b ratio and different amounts and addition methods (dry and pre-absorbed addition) of SAP and ceramsite on the flowability, mechanical properties, early autogenous shrinkage, drying shrinkage, and early crack resistance of UHPC were discussed. Findings indicate that increasing the w/b ratio reduces autogenous shrinkage but compromises mechanical properties, altering the cracking mode from primary microcracks to a few wider cracks. Pre-saturated ceramsite (less than 10% volume) and SAP effectively mitigate autogenous and drying shrinkage, enhancing crack resistance without significantly reducing mechanical properties. However, exceeding a ceramsite volume dosage of 10% or using the dry addition method increased the flowability of UHPC, while decreasing crack resistance. Microstructural analysis reveals that internal curing materials facilitate hydration and enhance structural density through the formation of ettringite and calcium silicate hydrate. To optimize shrinkage reduction while maintaining mechanical properties, SAP should be incorporated in a dry state, with a dosage limited to 0.4% of the mass of the cementitious material; ceramsite needs to be pre-saturated and limited to 5% of the total volume.

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