Guide for Calculating the Quantity of Prewetted ESCS Lightweight Aggregates for Internal Curing
The following table is used to determine the minimum amount of prewetted expanded shale clay and slate (ESCS) lightweight aggregate needed to provide the internal moisture for internal curing of cementitious material in concrete mixtures. Trial batches should be conducted to confirm desired results.
- (1) Seven pounds of internal moisture per 100 lbs of cementitious material has been found to be the desired amount of internal moisture to achieve internal curing of concrete mixtures. Additional internal moisture may be needed to address adverse climate or curing conditions that cause rapid moisture evaporation from concrete.
- (2) Aggregate Absorption: ESCSI recommends using the New York test method NY 703-19E Moisture Content of Lightweight Fine Aggregate (Aug 2008). This method gives the most consistent and accurate results with ESCS lightweight aggregates, due to their unique physical properties. Due to their cellular structure, ESCS lightweight aggregates absorb more water than their normal weight aggregate counterparts. Based upon absorption test conducted in accordance with the procedures of NY 703-19E, ASTM C 127 or AASHTO T 84 as well as ASTM C 128 or AASHTO T85, ESCS lightweight aggregates will absorb from 5 to more than 25 percent moisture by mass of dry aggregate. By contrast, normal weight aggregates generally absorb less than 2 percent moisture. The rate of absorption and the time needed to reach saturation, is unique to each ESCS lightweight aggregate, and is dependent on the characteristics of pore size, continuity, and distribution, particularly for those pores close to the surface.When the ESCS lightweight aggregate is used in concrete the internally absorbed water within the particle is not immediately available for chemical interaction with cement as mixing water, and as such, does not enter into water-cement ratio (W/CM) calculations. However, it is extremely beneficial in maintaining longer periods of hydration (Internal Curing) essential to improve the aggregate/matrix interfacial transition zone, reduce shrinkage, reduced cracks, narrower cracks and improve durability. Internal curing will also bring about a significant reduction of permeability by extending the period during which additional products of hydration are formed in the pores and capillaries of the binder. (Ref. ESCSI Reference Manual for the Properties and Applications of Expanded Shale Clay and Slate Lightweight Aggregate, Chapter 4 Section 4.1 Page 4-5)
- (3) Aggregate Desorption: Desorption studies (Bentz, Lura, & Roberts, 2005) (Radlinska, Rajabipour, Bucher, Henkensiefken, Sant, & Weiss, 2008), (Castro, Keiser, Golias, & Weiss, 2011) of prewetted ESCS lightweight aggregates using the dynamic vapor desorption were conducted to measure its desorption isotherm. In these tests the samples were placed in a high-resolution balance in an air stream within a carefully controlled relative humidity environment. These measurements indicated that approximately 90% of the absorbed water is readily released at high relative humidity (> 93%) from nearly all of the examined prewetted ESCS lightweight aggregates currently produced in the U.S.