Compaction under Concrete Paving Section 1

[theonlynamenottaken]

I'm designing repair specs for sections of concrete paving to be replaced. The slab will be 8" thick and carry tractor-trailer wheel loads. Any basic recommendations on compaction of a Georgia red clay and a thickness of crushed stone for under these slabs? Being a rookie structural engineer I have no idea of how to specify compaction.

 

[atomicman]

A good place to start on this is to look at the standard specifications for the state highway department where you're working.

From my experience, which is limited, you would compact the top 6 inches of subgrade to 95% maximum density. You would have to specify to which test the density would conform and any methods of dealing with soft spots. Then you would compact the crushed stone as well.

 

[jgailla]

theonlyname...,
98 percent modified Proctor can be used under pavements. Run the numbers, but 12" 98 percent mod Proc.(do tests to make sure clay isn't excessively plastic or expansive, which it shouldn't be if it's "Georgia red clay") and 8" inches of crushed stone at a CBR of 100 is generally reasonable. Make sure you've given thought to drainage, as high groundwater or improper rain drainage can quickly destroy a pavement. Ask your geotech, or retain one, for this information, it is usually included in the report.

 

[theonlynamenottaken]

Thanks for the info... what is CBR?

 

[eric1037]

CBR is California Bearing Ratio. It is used in pavement design. You should use AASHTO design methods.

As for compaction, 95% should be sufficient. However, we usually recommend that it be compacted with moisture content 2% below optimum.

 

[Ron]

theonly...compaction should be 98% as jgailla noted. Truck loads and the vibration from traffic can provide additional compaction if 95% is used and it will be non-uniform compaction from the traffic, thus increasing the potential for cracking in the concrete.

"Georgia Red Clay" ranges from a slightly clayey to silty fine sand, to a sandy/silty clay or clayey silt...find out where it sits in the classification spectrum.

Control the grade on the subbase very closely. This will help control the thickness consistency of the concrete. Pay attention to the jointing in the concrete (SEE FAQ in Concrete Engineering, General Discussion Forum). Pay attention to the mix design of the concrete. Use the largest coarse aggregate size you can use for the application, and the least amount of cement to achieve the strength you need...these two parameters will help control shrinkage and enhance load transfer at control joints. Further, the timing of sawcutting the joints is critical. Make sure it is done THE DAY OF PLACEMENT, not the next day.

Also, consider load tranfer across all joints. The larger aggregate and lower shrinkage will help this, but you'll need to provide dowels at construction joints (dowels are preferable to keying).

 

[casimmons]

You really need to run a CBR on the subgrade soil. The CBR value will vary with degree of compaction- and the CBR will be an important factor in your pavement design.

 

[RCEJD]

The responses in this thread are raising issues that seem to continue to be questions in many areas. It appears that the Georgia Clay is a low plasticity clay that permits the higher compaction and a lower water content. However, in the high expansive clays, if you compacted the soils to a higher comapction, with a lower water content, as the clays are subject to increasing wetting and drying cycles (recent papers indicate larger movements occur after 6 to 8 cycles) the pavement sections may experience movement related to expansive clays, and increased loss of strength (subgrade softening).

Local knowledge and specific tests on the specific soil subgrade are essential.