Introduction
In recent years, the concept of internal curing of concrete has gained popularity and is steadily progressing from the laboratory to field practice. 1, 2 According to the terminology under consideration by the ACI 308 committee, "internal curing refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing water." Typically, the additional internal water is supplied via the incorporation of saturated lightweight fine aggregates (LWA) or superabsorbent polymer (SAP)3 particles into the concrete mixture.4 The benefits of internal curing are numerous and include increased hydration and strength development, reduced autogenous shrinkage and cracking, reduced permeability, and increased durability.2, 4 The impact of internal curing begins immediately with the initial hydration of the cement, so that its benefits are observed at ages as early as 2 days or 3 days.2
Internal curing is beneficial in low water-to-cement ratio (w/c) concretes because of the chemical shrinkage that accompanies portland cement hydration and the low permeability of these materials. Because the water incorporated into and absorbed by the cement hydration products has a specific volume less than that of bulk water, a hydrating cement paste will imbibe water (about 0.07 g water/g cement) from an available source.5 While in higher w/c concretes, this water can be and often is supplied by external (surface) curing, in low w/c concretes, the permeability of the concrete quickly becomes too low to allow the effective transfer of water from the external surface to the concrete interior. 6 Hence, one has the justification for internal curing. If additional water can be distributed somewhat uniformly throughout the concrete, it will be readily available to migrate to the nearby cement paste and participate in the hydration process as needed.
From an engineering viewpoint, one would like to calculate how much internal water (or saturated LWA) is needed for internal curing for any given concrete mixture. Bentz and Snyder6 have previously published an equation for this that is equivalent to:
(1)where: MLWFA = mass of (dry) LWA needed per unit volume of concrete (kg/m3 or lb/yd3),
Cf = cement factor (content) for concrete mixture (kg/m3 or lb/yd3),
CS = chemical shrinkage of cement (grams of water/gram of cement or lb/lb),
αmax = maximum expected degree of hydration of cement,
S = degree of saturation of aggregate (0-1), and
LWA = absorption of lightweight aggregate (kg
water/kg dry LWA or lb/lb).
For w/c below 0.36, the maximum expected degree of hydration of the cement under saturated conditions can be estimated as ((w/c)/0.36) and should not vary significantly with curing temperature. 7 For w/c higher than 0.36, the maximum expected degree of hydration of the cement can be estimated as 1. Because the densities of the dry lightweight aggregates and the conventional aggregates are substantially different, the ultimate substitution in the concrete mixture should be performed on a volume basis with the determined mass of LWA from equation (1) replacing the same volume of conventional aggregates. Knowing the dry densities of the two types of aggregates, a simple calculation can be employed to determine the mass of conventional aggregates that must be removed from the mixture (which will be more than the mass of the LWA determined by equation (1)). As an example of applying equation 1, a concrete mixture with a cement factor of 450 kg/m3, a chemical shrinkage of 0.07 g water/g cement, and an aggregate absorption of 15 % at complete saturation would require 193 kg/m3 and 210 kg/m3 of LWA for w/c of 0.33 and 0.40, respectively.
Substituting the relationship between w/c and maximum expected degree of hydration into equation 1 yields a bilinear dependence of the internal water demand of the cement (CS*αmax in equation 1) on w/c, as illustrated in Figure 1. For w/c<0.36, the dependence is the same as that proposed by Jensen and Hansen. 3 or w/c>0.36, the internal water demand reaches a plateau value equivalent to the chemical shrinkage of the cement (CS=0.065 in Figure 1). For these higher w/c, this relation differs substantially from that proposed by Jensen and Hansen.3 For w/c between 0.36 and 0.42, they propose adding sufficient water only to complete the hydration of the cement, while equation 1 proposes adding sufficient water to maintain the pores in the cement paste completely saturated. These represent two extreme views and the actual optimum in terms of performance may lie somewhere between the two lines shown in Figure 1. We note that equation (1) always estimates the needed internal curing water as that amount needed to exactly compensate for the chemical shrinkage of the hydrating cement paste in the concrete mixture, at the maximum expected degree of hydration. While it can be applied to higher w/c (>0.45) concretes, it does not address the potential problems in rheology and bleeding that might result in such an application due to having an extremely wet mix. For these higher w/c concretes, it may be much more efficient and practical to supply curing water via conventional means such as misting or the use of wet burlap. Even when internal curing is used, however, loss of water from the surface must be minimized to allow for dense cover concrete to be obtained. This applies for all types of concrete.
Figure 1- Internal Water Needed to Maintain Saturated Conditions in Cement Paste
Our goal in this paper is to refine the way in which the parameters in equation (1) are estimated, to provide a readily recognized means of choosing the proper amount of LWA and to increase the accuracy of mixture proportioning via this method. The two major revisions to be considered are: 1) the variation of chemical shrinkage (CS) with portland cement phase composition and curing temperature, and 2) the relevant value for the absorption (or more appropriately desorption) of the lightweight aggregate. After addressing these concerns, a procedure for mixture proportioning for internal curing will be recommended.
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