As an example, if you assume the aggregate gives you a modulus of subgrade reaction (k) of 225 you can determine the slab thickness. The graded aggregate subbase you propose will have little affect on the slab thickness. Subsequent research has validated the road test many times over and some adjustments to design criteria have evolved over the years. It has been the primary design and performance impetus for essentially all of the AASHTO pavement design criteria since. Okiryu.the road test to which OG refers is the original AASHO (American Association of State Highway Officials, which is now AASHTO, the American Association of State Highway and Transportation Officials) Road Test done in 1959. I was just wondering if there is any guidance for subgrade preparation for slab-on-grades. This has been my approach for subgrade preparation for slab-on-grades in locations where the subgrade is weak. This will facilitate compaction operations and ensure high compaction grades for the surface underneath the floor slab. For instance, the bottom 50mm~100mm of the base is acting as a "sacrificial" layer to "rectify" low compacted subgrade. In my case, with a weak subgrade (high compaction grade cannot be achieved for these soils), the recommended thickness of granular base under floor slabs takes into consideration the low compacted subgrade. I do not have that specific Ottawa Road Test information, but I am aware that for rigid pavement design, the thickness of the base course does not affect significantly the thickness of the pavement (this is perhaps most of the load is taken by the pavement and high stresses do not get transmitted to the subgrade).
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