The foundation of your new house connects your man-made structure to Mother Earth. Every foundation is uniquely designed to minimize the effects of nature on that man-made structure, and those effects can vary greatly from region to region, and even from one lot to the next. If your foundation is not designed & built correctly you could have serious problems with your home - doors won't open and close right, sheetrock will crack, etc. Our Project House will be nearly 6000 square feet - that's a big house that will need a big foundation. The subcontractor estimates it will take 15 or 16 cement trucks to fill our foundation - that's about 150 cubic yards of concrete. Let's take a closer look at why your foundation's such a crucial part of your house.

Effects of Nature
Nature's largest threat is shifting soil. It usually shifts for one of two reasons.

1. High clay content - this region (North Central Texas) is among the worst in the country for foundation damage caused by soil shifting due to high clay content.
2. Changing water content in the soil - when soil gets wet (from rain, plumbing leaks or subsurface water sources) it expands. When it dries out it contracts. Changes in water content don't have to happen very deep beneath the surface at all to have a big effect on the foundation.

Resulting Foundation Problems

1. Slab Upheaval - more common than settlement, caused by moisture increase, or even a frost (water expands as it freezes). Even a slow leak can build up pressure over time and heave the foundation. The building pressure underneath can be immense, easily two or three times greater than the weight of the house itself.
2. Slab Settlement - caused by moisture loss (as rain evaporates, or a drought condition builds). Soil shrinks as it dries out, so the foundation's support hollows out & disappears, causing the foundation to settle.

Soil Reports determine the design of the slab - Even though our project house is in a heavy clay region, our particular lot offers an ideal foundation site, since the ground gets very hard just beneath the surface. The soil report on your lot will help determine the natural conditions your foundation will have to deal with over the lifetime of your house, so it will influence the design and the cost of that foundation.

Concrete Defined - Concrete is a mixture of rock, sand, water and cement. The cement starts as a gray powder that forms a paste that fills the space between all the pieces of rock and sand. The concrete dries, or "cures" over time. That means the mixture undergoes a chemical reaction as the water evaporates from the mix. Builders will start building the frame on this slab within a few days, but it can take a month or longer for the concrete to fully cure. This cure time varies with the temperature and the amount of moisture in the air. Concrete strength is measured in pounds per square inch. Most residential foundations are rated to bear a compressive force of 3000 psi. The psi rating of concrete has to be higher when you're designing parking lots, roadways, and structural pieces that have to support heavier loads.

Consumer Tip - Some builders will add more water to the concrete mix than necessary during the actual pour because it makes it easier for the crew to spread and finish the pour job faster. This creates a weaker mix that could cause major problems years down the road. Homebuyers should consider hiring an independent engineer to supervise and oversee the concrete pour to ensure a proper mix.

Floating Slab Defined - This is a slab which sits or floats on the ground. If the ground heaves beneath it, the foundation will float up with the ground and settle back into place when the ground moisture recedes and the ground settles.

Once the holes for the beams are formed out of soil, crews pour sand on top to make everything level and to distribute the weight of the slab uniformly over the ground. Then the beams are covered in thick plastic. If wet concrete is poured directly over dry sand, the sand would suck all the moisture out of the concrete. This would leave a lot of little air pockets in the concrete, causing it to break up later on.

Because of the size of our house, almost six thousand square feet, we must use a pump truck to ensure the even distribution of the concrete. The pump truck will suspend a tube overhead and 'pump' the concrete using air pressure to easily and efficiently distribute the concrete. This is essential to making sure the foundation is poured correctly to achieve a good even surface, and a strong foundation.

Our foundation has a waffle design. As the trucks pour concrete into the hole, it will fill a grid-work of beams that will make up the bottom of the slab. This will be a single-piece foundation; we'll be watching a monolithic (or one-time) pour. So that one piece of concrete will take a bite out of the ground in such a way that it will resist lateral motion.

Post-tension design in the slab - The trucks will pour concrete over a latticework of tension cables. The cables are wrapped in plastic to avoid a chemical reaction (electrolysis) between the concrete and the metal. Otherwise, the cables would corrode in a few years and be worthless. The cables are suspended on little plastic "chairs" so that they sit in the middle of the slab as the concrete cures.

About 7 to 10 days after the pour, after the concrete has cured some, engineers will bring in machines to apply about 4500 pounds of pressure on the cables from all four sides of the slab. The cables try to resist this tension by contracting back into a state of rest. This creates a compressive force on the entire slab. Since concrete is stronger when it's compressed, this design stiffens the slab so it will be stronger if soil expansion ever applies force to it.

Consumer tip - If you ever have to dig up a piece of your foundation for any reason, be extremely careful that you don't hit one of those post-tension cables. If you cut one of those you'll release the tension and they'll recoil like artillery shells

Other slab design - Pier & Beam - When the soil is not as solid as at our site, engineers will design a slab with piers dug deep into the ground (deep enough to hit bedrock or solid soil) and filled with concrete before the concrete for the foundation is poured. The piers help the surrounding soil support the weight of the slab. The foundation beams sit on those piers. If the ground swells under the foundation, the slab will float off the piers, and resettle once the ground shrinks again.

Here are three important tips for you to maintain the health of your foundation:

1. Make sure all your landscaping slopes away from your house, so water never wants to pool towards the foundation. Uneven moisture in the soil around your house could cause the ground to shift.
2. Don't let the ground dry out around the foundation. You want to maintain even moisture around the house so the ground around the foundation is evenly saturated, especially during the dry summers. Use a soaker hose turned down real low, about 18 inches away from the house to give it an even soaking at least every other day so the ground around your concrete foundation will remain stable and won't shift.
3. Make sure you have your foundation designed by a professional engineer. It doesn't make sense to spend a lot of money putting finishing touches on a home that could crack or shift because it's built on a poorly-designed foundation. Spend the money now on a professional engineer for the design of your foundation so you won't run into trouble years from now. That way you're guaranteed to get off the ground with a healthy start as you're building your new house.

Contact Information:
Post Tension & Steel - Post-tension Cable Engineering Firm
214-398-8465