Internal curing in cementitious systems made using saturated lightweight aggregate

Low water-cement ratio concrete mixtures have been increasingly promoted for use in civil engineering infrastructure due to potential improvements in strength and durability. Despite their increased strength and reduced permeability, these mixtures are susceptible to early-age cracking caused by autogenous shrinkage. Methods have been developed to reduce the cracking susceptibility of these mixtures. One such method is internal curing. Internal curing uses water-filled lightweight aggregate to supply water to counteract the effect of self-desiccation after the initial pore structure of the paste has formed. This project evaluated the shrinkage cracking performance and fluid transport properties of mixtures containing internal curing. This thesis describes how chemical shrinkage results in internal vapor-filled void creation which leads to self-desiccation. The use of internal curing agents can provide a sufficient volume of water in the lightweight aggregate (LWA) to counteract the effects of self-desiccation. In addition to the volume of water provided by the LWA, the distribution of the LWA plays an integral role in the effectiveness of internal curing. X-ray absorption measurements indicate that the water can travel up to 1.8 mm during the first 75 hours. These results are combined with a hard core soft shell model developed at NIST to determine the spacing between lightweight aggregate particles to indicate an efficient aggregate size and grading. Measurements of unrestrained length change were performed for mortars made using different volume replacements of LWA under sealed and unsealed curing conditions. The cracking potential of sealed and unsealed mixtures is presented when the mixtures are restrained from shrinking freely using the restrained ring test. The results indicate that if a sufficient volume and an appropriate spacing of lightweight aggregate are used, shrinkage cracking can be significantly reduced or eliminated for both sealed and unsealed curing conditions. The results of this study indicate that when internal curing is used, the overall water absorption and electrical conductivity is reduced. This is shown to be due to the increased degree of hydration and a depercolation of the interfacial zones around the aggregate. As a result, mixtures with internal curing may have a w/c of 0.30 but exhibit the performance of a concrete with a lower w/c (e.g., 0.23)

Development of Internally Cured Concrete for Increased Service Life

Higher strength, lower water to cement ratio (w/c) concrete has been advocated over the last two decades due to its increased strength and reduced permeability. The lower w/c of these concretes makes them susceptible to autogenous shrinkage. This autogenous shrinkage can be significant and can be a contributing factor to early age cracking. Internal curing was investigated as a potential method to improve the durability of concrete pavements and bridge decks. Prewetted lightweight aggregate was used to supply water to the hydrating cement paste. This additional water can counteract the hindered strength development, suspended hydration, autogenous shrinkage, and early age cracking. An overview of the concepts behind internal curing was presented. It is important the internal curing agent (lightweight aggregate (LWA) in this case): be able to provide a sufficient volume of water, has a structure that allows the water to be released to the paste as needed, and is small enough so that they can be appropriately spaced in the matrix. Local materials were used. Before concrete could be prepared the locally produced LWA was characterized to determine absorption and desorption properties. Concrete mixtures were prepared for concrete with and without internal curing. A constant aggregate volume was maintained. Tests performed on these mixtures were designed to measure: autogenous shrinkage, drying shrinkage, plastic shrinkage cracking, drying shrinkage cracking, autogenous shrinkage cracking, water absorption, compressive strength, elastic modulus, tensile strength, thermal cracking and freeze-thaw resistance. Internally cured mixtures showed less autogenous shrinkage. In addition they were less likely to crack due to plastic, autogenous, and drying effects. Internal curing reduced the water absorption and potential for freeze-thaw damage. Further, internal curing allowed a greater temperature swing in the concrete before cracking would occur. Internally cured concrete mixtures could enable INDOT to produce more durable concrete pavements and structures that are less susceptible to cracking and have improved transport properties thereby providing great potential for more sustainable, cost-effective construction

The Effects of Internal Curing on the Properties of Concrete

There are many factors affecting the properties of concrete. This project focused on the effects of using different water-retaining fine aggregates on the properties of concrete, while keeping all other factors controlled. The main objective of this project is to determine what effects water retaining aggregates have on concrete, as opposed to traditional fine aggregates (sand) which have negligible water retention capabilities. Particular emphasis is placed on the shrinkage properties of concrete. All methods, materials and equipment used for testing conformed to the American Society of Testing Materials (ASTM) standards or to the American Association of State Highway and Transportation Officials (AASHTO) standards. Testing has indicated a positive result for shrinkage with significantly lower shrinkage over time when using water retaining fine aggregates to replace small proportions of traditional fine aggregates. Testing has also indicated, however, that there is a corresponding negative impact on the strength properties of the concrete when using water-retaining fine aggregates. The significance of either depends upon the intended use of the concrete. If strength but not shrinkage is a concern, water-retaining aggregates may not be warranted, however if shrinkage is a major concern such as in transportation construction, water retaining aggregates may produce a positive impact on the quality of the system