Sustainability has become a major focus of the building industry as well as society in general. In addressing the sustainability issue, an essential step is evaluating how a given product interfaces with adjacent products and what affect this has on the performance of the combined material. In this case, how does the interface between ESCS lightweight aggregate used for IC and the hydrating cementitious material affect the performance of concrete?
The following four sources address how internal curing contributes to sustainability and show how ESCS lightweight aggregate can enhance hydration, substantially reduce transport properties (such as diffusion and sorptivity), reduce curling/warping and increase the service life of concrete in a cost effective way.
Internal Curing: Constructing More Robust Concrete by Jason Weiss, Dale Bentz, Anton Schindler and Pietro Lura, published in the January 2012 issue of STRUCTURE Magazine:
“Implications on Practices and Sustainable Mixtures: Internal curing may also provide sustainability benefits. Replacing cement with supplementary cementitious materials (SCMs i.e., fly ash, slag) is suggested as a way to use substantially less clinker, resulting in a lower carbon footprint for in-place concrete. SCMs take longer to hydrate, thereby requiring water to be present for a longer time. While research has shown improved long term durability performance, recent work has shown that internal curing is particularly well suited to be used in mixtures with larger volumes of SCMs. Internal curing enables the SCMs in these mixtures to react for a longer time, since the higher water content needed to support the reaction of the SCMs can be maintained.
While there are many benefits associated with internal curing, one needs to remember that these materials require quality control assessment at the plant to insure proper aggregate prewetting and often have a relatively small increase in costs associated with materials purchase, handling, and prewetting. The authors are not recommending that contractors stop providing conventional (external) curing that minimizes the evaporation of water. Rather, experience indicates that internal curing provides the construction community with a new approach for producing concrete that is more robust during the often variable construction phase. As a result, by using internal curing it may be possible to greatly reduce the risk of unwanted cracking.
ACI 213R-14 Guide for Structural Lightweight-Aggregate Concrete, Section 9.6 Internal Curing Summary and Potential Impact on Sustainability:
“Mixtures with internal curing show similar or improved mechanical properties, reduced risk of cracking, and the reduced chloride ingress. The additional costs of concrete with internal curing are estimated to be between 0 and 14 percent of the materials cost. Internal curing may require additional quality control and aggregate management. With time and increased familiarity with internal curing, it is expected that new opportunities will rise to use internal curing.
Internal curing is just one of many tools that might increase the sustainability of concrete elements. Internal curing has the potential to improve the durability and reduce the life-cycle costs of concrete structures. Cusson et al. (2010) compared the service lives of theoretical high performance concrete bridge decks with and without internal curing. The high-performance concrete deck without internal curing was assumed to exhibit early-age autogenous and thermal cracking. The high-performance concrete with internal curing was assumed not to exhibit such early-age cracking and provided a further 25 percent reduction in the expected diffusion coefficient. Based on these and other assumptions, service life estimates of 22 years for conventional concrete, 40 years for high-performance concrete without internal curing, and 63 years for high-performance concrete with internal curing were reached. In this case, internal curing should produce a bridge deck with an increased service life and a significantly reduced life cycle cost. Recent work with the use of supplementary cementitious materials has suggested that substantially less cement clinker can be used in a mixture, resulting in a lower carbon footprint (De la Varga et al. 2011). This may also be true for mixtures with increased limestone powder replacement for cement (Bentz et al. 2009).”
ACI (308-213)R-13 Report on Internally Cured Concrete Using Prewetted Absorptive Lightweight Aggregate:
“CHAPTER 6—SUSTAINABLE CONCRETE CONSTRUCTION USING INTERNALLY CURED CONCRETE (ICC)
A great contributor to sustainability is the service life of the concrete. Achieving and even exceeding the design life depends critically on the mixture proportioning, placement, and curing. The importance of curing concrete is often misunderstood, sometimes not enforced, and in some cases even left as an afterthought. To reach optimum sustainability, curing should be emphasized along with the water-cement ratio (w/c). All other methodologies, including proper preparation and placement, can be negatively affected if the curing procedure is not performed with diligence. Without proper curing, strength can suffer 10 to 20 percent (Mack 2006), permeability 20 percent (Hoff 2003), warping 60 percent (Ya and Hansen 2008), and together with cracking could suffer from weaker interfacial transition zone and reduced durability.
Concrete is the most widely used construction material in the world because of its utility and durability. It can meet the criteria of sustainability because its characteristics as a construction material can “meet the needs of the present without compromising the ability of future generations to meet their needs” (United Nations World Commission on Environment and Development 1987). Concrete and its ingredients need to include higher-quality and more-durable construction components that will be more sustainable. The key is to build with an appropriate combination of materials and construction technologies to achieve the targeted results. Curing is an important part of that key. Surface curing is important with high-w/cm concrete, and traditional curing (along with absorptive materials) is important with low-w/cm concrete. In a higher-w/cm mixture, there is a degree of drying shrinkage, regardless of how the concrete is cured. Moving to a lower w/cm reduces drying shrinkage by decreasing the water content of the mixture and increasing the modulus of elasticity of the concrete. This also increases the autogenous shrinkage dramatically, the mitigation of which was the initial impetus for using preconditioned absorptive materials. To overcome autogenous shrinkage and cracking, internal moisture for hydration of the cement is provided by the substitution of a portion of the normalweight sand in a concrete mixture with a prewetted lightweight fine aggregate (PLFA). Any extra water left from the prewetted lightweight aggregate (PLA) will help to reduce drying shrinkage.”
Evaluation of Internally Cured Concrete for Paving Applications, By Chetana Rao & Michael I. Darter, September 2013:
“POTENTIAL BENEFITS OF ICC
ICC used in both JPCP [Jointed Plain Concrete Pavement] and CRCP [Continuously Reinforced Concrete Pavement] provides benefits in terms of structural and durability longevity as well as sustainability:
This source also offers the following:
“Two projects, previously analyzed with the AASHTO ME Design procedure, were used for the LCCA. The optimized designs for a design life of 30 years with both conventional and ICC alternatives were used in the LCCA for an analysis period of 60 years. M&R activities and schedules were established based on AASHTO ME Design performance predictions for both the alternatives. The results of the LCCA are as follows:
These pavement design and life cycle cost analyses indicate that using ICC in JPCP and CRCP can provide:
For additional information on Internal Curing, see ESCSI publication 4362.1, Internal Curing: Helping Concrete Realize its Maximum Potential.