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Internal Curing Specifications

Suggested Internal Curing Specifications
 

SUGGESTED INTERNAL CURING OF CONCRETE SPECIFICATIONS
(Guide for Internally Curing of Concrete with Lightweight Aggregate Sand (LWAS)

     In order to cure concrete, not only at its surface with water applied externally after it is placed, but also throughout the mass, which external curing is unable to do, internal curing should be used to enhance the characteristics of the concrete by improving the hydration of the cement (c) and cementitious material (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:
            Internal Curing------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 (<0.42) that vitally need it.

     Specification writers using the guide should achieve the concrete characteristics they desire by choosing the methods, materials, and procedures outlined herein. Those characteristics that initial curing can improve are: shrinkage (including autogenous), cracking, early age and later age compressive strength, early age flexural strength, lower permeability, resistance to freeze-thaw, minimization of carbonation, densification of the interfacial transition zone (ITZ), 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 shall 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/C330M-13 Standard Specifications for Lightweight Aggregates for Structural Concrete."

* "ASTM C33/C33M-12 Standard Specifications for Concrete Aggregates, except for gradation, which shall comply with ASTM C330."

* " ASTM C109C 109M-02 Standard Test Method for Compressive Strength of Hydraulic Mortars"
 
  • LWAS shall be capable of meeting the compressive strength of the Control Strength
* "ASTM C128-12 Standard Test Method for 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 8-10 % in 30 minutes.
ON-THE-JOB PROCEDURE
* Lightweight aggregate sand (LWAS) shall be shipped to the batching plant at saturated surface dry (SSD) or more, and be sprinkled with water while in storage 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 8 to 10 %.

PRECONDITIONING OF LWAS
* LWAS shall be pre-saturated by either sprinkling or soaking to ensure its 30 minute and 24 hour absorption rates.

BATCHING
* Introduce SSD LWAS into the mixer drum first, along with some of the mixing water, or at least a sufficient quantity of it to be sure that it remains SSD as the dryer materials are sequentially batched after the introduction of the LWAS.

MIXTURE DESIGN

SAND REPLACEMENT for GENERAL APPLICATIONS
* The number of pounds of LWAS substituted shall be determined by the following formula:

              Cf x CS x αmax
MLWA =
         S x ΦLWA
Where:

MLWA   =   mass of (dry) LWAS needed per unit volume of concrete (kg/m3 or lbs/yd3)
Cf   =   cement factor (content) for concrete mixture (kg/m3 or lbs/yd3)
CS   =   chemical shrinkage of cement (g of water/g of cement of lb/lb)
αmax   =  

maximum expected degree of cement hydration.  For ordinary Portland cement, the maximum expected degree of hydration of cement can be assumed to be 1 for w/c >0.36 and to be given by [(w/c)/0.36] for w/c<0.36   
S   =   degree of saturation of LWAS (0 to 1)
ΦLWA   =   absorption of lightweight aggregate (kg water/kg dry LWA or lb/lb).
   
SAND REPLACEMENT FOR PAVEMENTS and SLABS ON GRADE
* For applications such as Pavements, Slabs on Grade, White Topping, Slip Forming and Pervious Concrete, the amount of sand replacement should be ascertained to meet the lowest life cycle cost (LCC) or first cost, depending on design needs. The number of pounds of LWAS substituted shall be determined by the design engineer with a 3-point results curve for deciding the degree of improvements to be required.

SAND REPLACEMENT for DRY-WINDY DAYS

*

The number of pounds of LWAS substituted shall be determined by the evaporation rate of the unprotected concrete depending on the relative humidity and wind velocity (ACPA Webinar August 29, 2013 on Concrete Pavement Curing).


     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.