Mountain Grout staff occasionally receive inquiries about the compressive strength of polyurethane. The short answer is that compressive strength can vary immensely. In the past, many manufacturers, including Mountain Grout, listed compressive strength values on data sheets. However, due to the highly manipulable nature of that measurement due to specific site conditions, most prudent companies have removed the value from TDS(s). It didn’t make sense to list a value from a confined sample when the result on the job would be a free rise foam (or vice versa).
The two major factors involved in compressive strength of polyurethane grouts are the polymer strength and the density of the expanded foam.
The polymer strength is determined by the formulation (composition). In general, the more cross-linking within the polymer chain the stronger (stiffer) the cured polymer. Mountain Grout Flexible, for example, is best classified as a semi-rigid product. It has more resilience than a typical rigid foam, but much less than a flexible foam.
The foam density is very critical in what the compressive strength will be for a particular formulation. We have test results showing compressive strengths from 7 psi to 71 psi when the density ranged from 2.75 to 7.7 PCF. If one were to extrapolate to estimate the compressive strength at higher densities, you would expect a strength of over 100 psi at densities of about 12 to15 pounds per cubic foot.
If we use the Dupont graph for the compressive strength vs density for a typical rigid foam, we see that Flexible provides compressive strengths of about one half those values. This would also point to compressive strengths over 100 psi at densities of 10 to 12 pounds per cubic foot. If one could confine Flexible to produce a cured product of very high density (~50 PCF) the compressive strength would be over 800 psi. Obviously, confining the expanding grout is difficult in normal applications. However, it could be achieved in special circumstances such as our work with Sandia National Labs in wells over 400 feet deep. I would not expect Flexible to be capable of compressive strengths exceeding 1,000 psi.
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