Mixture Design for Internal Curing

Except for the lightweight aggregate, internally cured concrete mixture design generally is identical to that of conventional concrete with similar air content, water content, and coarse aggregate content. Currently, internal curing in North America is typically achieved by replacing a portion of the conventional fine aggregate (i.e., sand) with a pre-wetted lightweight fine aggregate. This practice occurs mainly due to lightweight aggregate availability, economics, and the fact that the fine lightweight aggregates already appear on the approved materials lists of many state highway agencies. Fine lightweight aggregate is generally preferable to coarse lightweight aggregate because:

• The fine lightweight aggregate can provide sufficient distribution of the curing water across the cross section to enable the internal curing water to reach the matrix.
• The use of conventional coarse aggregate provides conventional stiffness and fracture properties for the concrete.
• The fine lightweight aggregate is generally less expensive than the coarse lightweight aggregate.
• The fine lightweight aggregate can be evaluated using ASTM C1761, although expanded clay, shale, slate, and slag have all been used effectively for internal curing.

Figure 4 provides an example of the volume proportions of conventional and internally cured concrete. The figure illustrates the similarities and differences between the design of a conventional six-bag mixture (water-to-cement ratio of 0.36 and 6 percent air) and an internally cured mixture using the same volume proportions of the constituents. The example assumes 15 percent absorption of the fine lightweight aggregate and a volume of internal curing water that is equivalent to the volume of chemical shrinkage of a typical cement (7 lb of water for every 100 lb of cementititous materials).

The volume of sand that is replaced with fine lightweight aggregate will vary depending on the properties (porosity and desorption) of the fine lightweight aggregate and the design assumption (providing internal curing water to equal chemical shrinkage or providing some other amount of internal curing water). Using the mixture design from Figure 4 and assuming that the internal curing water is designed to replace the chemical shrinkage water, an example to illustrate the mixture design for the wide range of expanded lightweight aggregates in North America (considering the properties of the lightweight aggregate such as specific gravity, 24-hour absorption, desorption) is shown in Figure 5.

There is a wide range of tools that can be used to proportion a specific mixture; however, a simple spreadsheet is available to convert a conventional mixture design to an internally cured mixture design in a simple step using the properties of the lightweight aggregates (5). This spreadsheet, along with the majority of internal curing conversion tools, has been designed with a volume of internal curing water that is equal to the volume of chemical shrinkage to negate the effects of self-desiccation. It is possible to design at differing levels of internal curing water, which may be done to account for the majority of internal curing benefits for a lower-cost mixture or to compensate for drying shrinkage or other sources of water loss.