Guide for Internally Curing of Concrete with Lightweight Aggregate Sand

     In order to cure concrete, not only at its surface with water applied externally after it is placed, but also throughout the mass, internal curing should be used to enhance its characteristics by improving the hydration of the cement (c) and cementitious materials (cm).  This is accomplished by replacing a portion of the natural or manufactured normal weight sand with an absorbent preconditioned lightweight aggregate sand (LWAS).  The ACI TAC Concrete Terminology Task Group provided the definition in August 2010 as follows:
            curing, internal------supplying water throughout a freshly placed cementitious mixture
            using reservoirs, via pre-wetted lightweight aggregates, that readily release water as
            needed for hydration or to replace moisture lost through evaporation or self-desiccation.

Although medium range water cement ratio concretes are improved, it is the lower range (<0.43 ) that vitally need it.

     Specification writers using this guide should achieve the concrete characteristics they desire by choosing the methods, materials, and procedures outlined herein. Those characteristics that internal curing can improve are:  shrinkage (including autogenous), cracking, early age and later age compressive strength, lower permeability, resistance to freeze-thaw, minimization of carbonation, densification of the interfacial transition zone, improved mortar strength, and reduced warping. Certain of these characteristics require immediate release from the reservoirs of the needed water.

Methods, Materials, and Procedures

     The lightweight aggregate sand (LWAS) substitution is to be on an equal volumetric basis and in accordance with the following:

LWAS shall be expanded shale meeting the following standards and criteria:

*	"ASTM C330 Standard Specifications for Lightweight Aggregates for Structural Concrete"
*	"ASTM C33 Standard Specifications for Concrete Aggregates" (under revision)
*	"ASTM C109C 109M Standard Test Method for Compressive Strength of Hydraulic Mortars"
	LWAS shall be capable of meeting 100% of the control strength.
*	"ASTM C128 Standard Test Method fror Density, Relative Density (Specific Gravity) and Absorption of Fine Aggregate".
	LWAS shall be capable of having an absorption of 15 %  at 24 hours and a minimum of 9 to 10 % in 30 minutes.
ON-THE-JOB PROCEDURE
*	Lightweight aggregate sand shall be shipped to the batching plant at saturated surface dry (SSD) and be sprinkled with water while in storage for 24 hours to maintain a 24 hour absorption of a minimum of 15 %.  It shall be tested before using to be sure that the 30 minute absorption is at least 9 or 10 %.
PRECONDITIONING OF LWAS
*	LWAS shall be pre-saturated by either sprinkling or soaking to ensure its 30 minute and 24 hour absorption.
BATCHING
*	Introduce SSD LWAS into the mixer drum first, along with the mixing water, or at least a sufficient quantity of it to be sure that it remains SSD as dryer materials are sequentially batched after the introduction of the LWAS.
SAND REPLACEMENT
*	The number of pounds of LWAS substituted shall be determined by the following formula:
Cf x CS x αmax MLWA =          S x ΦLWA

     Depending on the characteristics desired, a volume of replacement of 5% to 25% of natural/manufactured normal weight sand by LWAS will yield desired benefits.  For confirmation of the amount to use to obtain the desired results, a 3-point test results curve of varying amounts of LWAS is recommended.