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Open-Graded Base Courses for Slabs-on-Ground

ASCC Position Statement #37

Some specifications for concrete slabs-on-ground call for concrete to be placed on a layer of granular material containing very few fines. ASTM No. 57 and No. 67 coarse aggregates are examples of such materials. Such open-graded (permeable) base courses are intended to serve as subdrainage layers in concrete pavements and as a capillary break that prevents liquid water from rising to the bottom of interior concrete slabs through capillary action in the base course.
Open-graded bases are not as stable as densely graded, well-compacted, crushed stone bases. Because of the low stability, rutting due to construction traffic is common.

It is also difficult to fine grade an open-graded base to a relatively uniform elevation. This nonuniform surface elevation and the rutting result in restraint to concrete movement as the concrete cools or dries, thus increasing the probability of out-of-joint cracking. A stable construction platform is needed, as indicated by the following quote from Section 4.1.4 of ACI 302.1R04, “Guide for Concrete Floor and Slab Construction”: 

“The base material should be a compactible, easy to trim, granular fill that will remain stable and support construction traffic.” 

Open-graded bases don’t meet these requirements because they are relatively incompactible, difficult to trim, and unstable.

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