We're designing a retaining wall that consists of precast concrete units stacked on each other. What's the coefficient of friction for concrete on concrete? The manufacturer of the units says it's 0.75, but that sounds high to me.
The manufacturer's number is slightly less than the values given by the Portland Cement Association (PCA) and Precast/Prestressed Concrete Institute (PCI). PCI's Design Handbook says the concrete-to-concrete friction factor for dry conditions is 0.80. The latest edition of PCA's Concrete Masonry Handbook, Appendix A, gives a precast concrete-to-concrete masonry friction coefficient of 0.4 based on a safety factor of two.