The inclusion of prewetted expanded shale, clay and slate (ESCS) lightweight aggregates (LWAs) in concrete to provide internal curing (IC) has shown numerous benefits in terms of material performance and durability, especially from the shrinkage reduction viewpoint. This has been observed in both laboratory studies [1, 2] and field applications [3]. Numerous documents explaining the mechanism behind IC in concrete have been published [1, 2]. Similar benefits might be expected in other cementitious materials such as cementitious grouts.
So-called “non-shrink” cementitious grouts are widely used in construction industry applications, including joint sealing, flooring, structural repairs, and field-cast connections for prefabricated bridge elements (PBEs). The U.S. Federal Highway Administration (FHWA) has a strong interest in the latter application. These grouts are typically proprietary materials that are prepackaged and ready to mix on site. They are normally designed to provide high fluidity, high early strength, and low shrinkage, among other properties. However, and despite their nomenclature, field-cast cementitious grouts have at times shown serviceability issues mainly associated with dimensional stability (primarily shrinkage), even at very early ages. Early-age shrinkage under restraint can cause the development of tensile stresses within the material which commonly results in shrinkage cracking.
The structural concrete research team at the FHWA Turner-Fairbank Highway Research Center (FHWA-TFHRC) investigated the possibility of including IC using prewetted ESCS LWA in cementitious grouts with the aim of reducing or mitigating most of the shrinkage observed in these materials. In the study, autogenous (sealed) and drying shrinkage were measured in two commercially-available non-shrink cementitious grouts with and without IC provided using a prewetted ESCS LWA (Figure 1). For this study, an expanded shale LWA was added to the grout, rather than the conventional approach to IC for which a portion of the normal-weight aggregate is replaced with prewetted ESCS LWA. More details on the IC mixture design approach can be found in the report on this study [4]. As shown in Figure 1a, both cementitious grouts undergo autogenous shrinkage (preceded by a slight expansion in the case of Grout 2). These same grouts exhibit a considerable amount of drying shrinkage (Figure 1b). When IC is added, the short-term autogenous shrinkage component is eliminated, the long-term autogenous shrinkage component (sealed) is reduced, and the long-term drying shrinkage is significantly reduced.
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Figure 1: Results of shrinkage tests for grouts with and without IC [4]
Additional research performed at the FHWA-TFHRC focused on other grout material properties relevant to PBE applications. It is worth mentioning that if the ESCS LWA particles are much coarser than the grout solid components, the addition of ESCS LWA in cementitious grouts may reduce the fresh workability. This effect is less visible in concrete, where coarser particles (e.g., coarse aggregates) are present. With respect to setting times, internally-cured cementitious grouts exhibited similar setting times to those of the control grout (without IC). In regard to strength development, IC is not expected to reduce the strength of the material since an increased degree of hydration of the cement (and cementitious) particles is normally observed [1]. The effect that IC using prewetted ESCS LWA has on the strength development of the two grouts tested is shown in Table 1. IC clearly increases the strength of Grout 1, while the compressive strength changed only slightly for Grout 2. The reason for the different strength development of the two internally-cured grouts cannot be directly evaluated because the grouts are proprietary materials with unknown formulations. Nevertheless, IC does not seem to negatively affect the strength development to a point that a grout would not fulfill the strength requirements shown in ASTM C1107 [5] for two grouts tested in this study. Other properties relevant to PBE applications have also been studied, such as the bond strength between a grout and a concrete substrate. No negative effects of IC using prewetted ESCS LWA have been observed; in fact, bond enhancements have been observed in some cases [6].
From a practical perspective, non-shrink cementitious grouts are often prepackaged and can be extended using small aggregate for volumetrically large pours. IC can be thought of as an extension of the grouts using ESCS LWA rather than normal weight aggregate. The implementation of the IC technology as a “grout extension” can be helpful not only in reducing shrinkage but also in improving curing conditions in some locations where conventional (external) curing is difficult or impossible to implement. Additionally, IC can bring cost benefits because the cost per yielded volume of ESCS LWA is less than the cost per yielded volume of grout (solid fraction). In other words, when extending a grout with ESCS LWA, the amount of grout required to yield 1 yd3 is reduced, thus decreasing the overall material unit cost (see Table 2).
The research performed at the FHWA-TFHRC shows the potential application of IC using prewetted ESCS LWA in cementitious grouts. This could also be extrapolated to other cementitious materials, as other research groups are currently investigating, for instance, in fiber-reinforced repair materials [7].