Early Age Cracking Risk of High-Strength Lightweight Aggregate Concrete

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Title: Early Age Cracking Risk of High-Strength Lightweight Aggregate Concrete

Author(s): T.A. Hammer

Publication: Special Publication

Volume: 234

Issue:

Appears on pages(s): 641-660

Keywords: aggregate concrete; aggregate type; autogenous shrinkage; lightweight early age cracking; mechanical properties; moisture content; temperature; thermal dilation

Date: 3/22/2006

Abstract:
Early age cracking may be what is commonly known as "plastic shrinkage cracking", which normally is cracking of horizontal surface before and during setting (initial phase), and "thermal cracking" which normally is in the period of cooling following the period of temperature rise due to heat of hydration (thermo phase). In the initial phase, any mix water absorption by the LWA (i.e. like in concretes with relatively dry LWA) may contribute to increased settlement, capillary tension of pore water and shrinkage, and thus, an increased risk of cracking in typically the first hour after finishing. However, in the early hardening age the absorbed water may constitute a reservoir which contributes to swelling of the paste which counteract any plastic shrinkage and/or any contraction due to cooling, and thus, to reduced risk of cracking. In the thermo phase the temperature rise in LWAC is potentially higher due to the lower heat capacity of the LWA, which may result in larger contraction during cooling, which again may generate higher stress, and finally to higher risk of cracking. On the other hand the autogenous shrinkage is significantly reduced or even eliminated, and the E-modulus is lower, which both contributes to lower stress, and thus, to lower risk of cracking. The net result is often reduced risk of early age cracking.