Additional Concerns

Naturally, other physical properties of the LWA such as mechanical strength, particle shape, and particle gradation will influence the fresh and ultimate properties of the concrete. To optimize the performance of the LWA in the concrete system, these properties should be as similar as possible to the normal weight sand being replaced by the LWA.²

In addition to providing sufficient water via the internal curing design, another important consideration is that the supplied water be distributed uniformly throughout the concrete mixture. This will be especially important for curing at later ages, when the distance the water can travel may become limited to hundreds of micrometers, due to the ever-decreasing permeability of the hydrating cement paste.⁶ A more uniform distribution of internal curing water is best achieved by the use of fine (as opposed to coarse) lightweight aggregates (or SAP particles which are on the order of hundreds of micrometers in diameter³). The actual projected distribution of water availability for a given concrete mixture with saturated lightweight aggregates, based on a hard core-soft shell (HCSS) microstructural model developed at NIST,²⁰ can be computed and viewed using a web site available at http://ciks.cbt.nist.gov/lwagg.html. At this site, the user provides the aggregate gradation, lightweight aggregate replacement fraction, and overall aggregate volume fraction and the system returns a representative color-coded two-dimensional image from the "virtual" concrete along with a table indicating the protected paste volume fraction⁶ as a function of distance from the surfaces of the LWA particles. An example of the model output for a concrete with 70 % aggregates by volume and replacement of 20 % of the fine aggregates by LWA is provided in Figure 3 and Table IV. In this example, while 100 % of the cement paste is within 1.0 mm of a LWA surface (a relevant distance for early age curing), only 56 % of the cement paste is within 0.2 mm of a LWA surface (a more relevant distance for later age curing).²¹

The above methodology has been developed for concrete mixtures based on ordinary portland cement. The use of blended cements (with silica fume, slag, fly ash, etc.) will require further modifications to the computation of chemical shrinkage and "internal water demand." These blending components influence both the kinetics of hydration and the absolute volume of chemical shrinkage.¹⁰ Further research on these materials is needed to provide a quantitative basis for extending the presented methodology to cover them.

Figure 3- Example 2-D Image (3 cm x 3 cm) from Internal Curing Simulation

Table IV- "Protected Paste" Volume as a Function of Distance from the LWA Surfaces

*Certain commercial products are identified in this paper to specify the materials used and procedures employed. In no case does such identification imply endorsement by the National Institute of Standards and Technology, nor does it indicate that the products are necessarily the best available for the purpose.