Why Today’s Slab Foundations Are So Much Better Than Earlier Slabs

Early slab-on-ground foundation design

A slab foundation is nothing more than a slab of concrete that supports a load. If the slab of concrete is totally ground supported, the portion that is not supported by the ground is structural. 

Early slab design consisted of estimating the weight of the outside walls and then, design the perimeter grade beam wide enough so that it could support the weight of the grade beam and the weight of the exterior wall assuming it was brick veneer. The idea was that the weight per square foot would no exceed the shear capacity of the supporting soil. The perimeter grade beam also had to be deep enough so that it was supported on virgin undisturbed soil or on compacted fill. 

Even today, the standard design methodology for slab on ground design (in areas where expansive soils do not exist) is focused on making sure that the weight of the slab plus the weight of the house does not overload the load capacity of the supporting soil.

Stiffened slab-on-ground design 

Traditional slab foundation design focused on what the weight of the slab and the house did to the supporting soil. At some point, it began to be understood that what the soil did to the house was much more important. There was not much that could be done to prevent the supporting soil from pushing upward against the slab. What could be controlled to some degree was the stiffness of the slab. Perimeter grade beams were of some value but given the pressure swelling soils were capable of, designing the slab for stiffness was obviously a possible solution.

Today the term slab-on-ground foundation is a misnomer. They are really stiffened slab-on-ground foundations. The normal design called for only perimeter grade beams and possibly some footings to support load-bearing walls. A stiffened slab-on-ground foundation was designed to 1. transfer loads to the supporting soil without overloading the soil and 2. be stiff enough to reduce the distress in the house to the brick veneer, the drywall and the doors.     

Modern post-tensioned slab design

The next step in the evolution of slab foundation design was the use of post-tensioning in the design of slab foundations.

The use of post-tensioning meant that grade beams and the stiffening beams did not have to be so deep. Thus the trenches did not have to be as deep as they would need to be if it were a conventionally reinforced slab. Shallower beams meant less concrete. 

The savings was not much, but if a builder built enough homes over, say, a year, the savings would add up. 

In post-tensioned design, all perimeter grade beams and interior stiffening beams had to be continuous, unless there was a good reason not to be continuous.

Improved understanding of expansive soils

Research into expansive soils, especially how to estimate how these soils shrink and swell and how they interact with a slab foundation continues to be an active area of research. To give just one example: for years it was thought that the active soil zone was no more than 8-feet. Today, it is not uncommon for a soil report to state that the active soil one is as deep as 14-feet.  

Improved concrete

For decades after WWII, oyster shell dredged from Galveston Bay was used as aggregate in concrete. It was inexpensive and would almost certainly outlive the builder’s warranty. Many builders would use oyster shell concrete for flatwork such as sidewalks and driveways. 

I would never recommend the use of oyster shell concrete for a slab-on-ground foundation. Mixed by someone who knows what they are doing, using crushed oyster shell as aggregate can be stronger than hard rock aggregate. Used as a filler, it can result in an inferior product.    

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