 |
A concrete truck mixes its load as another truck delivers concrete during a foundation pour
at Trinity Falls in McKinney on Nov. 18, 2019. (Mark M. Hancock / © DFWmark.com)
|
By Mark M. Hancock / DFWmark.com
The Foundation
We’ll literally start this series at the foundation. Foundation technology has improved dramatically since the 1980s. From the 1970s to now, the science behind foundation and energy compliance has improved dramatically. All other aspects of home construction have remained relatively constant through the last half-century.
 |
James
Wrigg, project manager with Perry Homes, stands
on
a foundation pad at Trinity Falls in McKinney.
|
James Wrigg is a project manager with Perry Homes. Britton Homes is the luxury brand of Perry Homes. He oversees construction at Trinity Falls in McKinney where a 60’ lot with a greenbelt view retails for about $450,000 to $550,000 at the start of 2020. This series of posts is based on the Construction 101 class he teaches to real estate professionals.
Perry Homes does phase building. This means the homes are built and inspected in three distinct construction phases: foundation, mechanical (framing, electrical, HVAC, etc.) and finish (complete with drywall, flooring, lighting, appliances, etc.).
There’s always a project manager on site during the concrete pour because nobody wants a foundation failure. It must be correct, or it will be ripped up and done again at the builder’s expense.
The word “cement” is often used interchangeably with “concrete.” For accuracy, concrete is the completed formula or product. It contains an aggregate (such as gravel, rocks and sand), water and cement, which has a base of lime or other binder.
Slab-on-grade foundation
Many tract housing communities in North Texas have slab-on-grade foundations. These are also known as floating foundations. This process involves digging a mold into the soil, preparing the mold and essential infrastructure then pouring concrete into the mold for maximum efficiency of the concrete.
 |
A
foundation mold complete with tendons awaits a
concrete
pour. PVC plastic sheets trap moisture in the pad.
(Mark M. Hancock / © DFWmark.com) |
Developers have conditioned and built up the soil to make it as stable as possible. It’s the most important step due to the expansive clay soil of North Texas. These soils will expand and contract depending on the amount of water in the soil.
In North Texas, untouched soil is volatile. Builders want to trap moisture in the pad before the pour. Once the soil is completely swollen with water, a mold for the concrete beams is dug. PVC (Polyvinyl chloride) synthetic plastic is layered over the molded pad to keep soil swollen and avoid movement. The plastic is likely to outlive the house itself.
Once the house is built, it sits on the slab, which has a raft or grid of beams below it. As the soil dries in the summer, it pulls away from the beams. In the wetter months, the soil fills toward the beams but has already reached its maximum swell before the beams were poured. This safeguards the beams from being crushed by the expansive soil during wetter months.
Technical system
 |
Post
tension tendons are comprised of a plastic sheath, a
steel
cable and grease. An anchor is nailed to the inside of
the
slab mold to allow tension to be applied to the cable later.
(Mark M. Hancock / © DFWmark.com) |
Post-tension slab technology was developed in the 1930s and is the main system now. It’s commonly used in areas with expansive soil or poor load-bearing soil. A post-tension slab spreads the weight of the structure over a wider area by using the slab itself. This weight distribution creates a smaller and less expensive foundation.
Post tension means tension is applied to steel cables in the foundation after (post) the concrete pour. A “tendon” is a plastic tube or sheath with a steel cable and grease inside. These are laid out in a specific pattern throughout the foundation.
The plastic sheath protects the metal from corrosive concrete and allows metal to be pulled while the grease lubricates the process and assists with rust and protection within the plastic tube. Anchors at each end of the cable hold it in place.
 |
A
boom concrete pump with an articulated robotic arm delivers
liquid
concrete precisely during a pour in McKinney. A 3,600
square
foot slab requires about 110 yards of concrete.
(Mark M. Hancock / © DFWmark.com) |
A hydraulic tool applies 33,000 pounds of force per cable stretch. The allowance of exposed cable is 1.5 inches at each end. It’s vital the exposed cable is later protected from the elements to avoid corrosion and possible tension failure.
Builders must get everything right before the pour
Builders put minimal plumbing in the foundation. Ideally, only one line runs under the slab. All other lines are in the walls.
Repairs and renovations are inevitable. If repairs are encased in concrete, the costs explode. Often, builders feed metal pipes and electrical lines through PVC conduits (plastic tubes) to protect them from the corrosive cement and make future repairs simpler.
The drains and such are set by engineers before the mold was made. Drains, which are made from PVC, should outlive the house.
 |
Metal
post straps and hurricane clips ensure
structural
integrity up to 90 MPH wind speeds.
|
Metal post straps provide an anchor to tie onto the frame. The metal post straps are placed before the pour. Homes are built for 90 MPH wind speeds in DFW area with metal post straps that go down to the foundation.
A normal foundation pour takes about an hour to 90 minutes. However, the pour could take up to six hours depending on plan and size.
It takes about 110 yards of concrete to pour a 3,600 square foot slab. A yard of concrete costs about $103. Therefore, the concrete cost for a 3,600 sq. ft. slab would be about $11,330. The completed foundation costs about $30,000 to $40,000.
The foundation is one object, but it is made of both the beams and a slab. The beams are laid out in a grid pattern to support the slab. The slab is the flat, 4-inch top of the foundation. The slab has a minimum of 4 inches of concrete everywhere.
While the builder only needs to maintain 4” of slab, the remainder is for the support structure. The beams are each 10” wide and 30” deep. The grid of beams makes the foundation affordable and strong.
Builders don’t want the cables to weave. They need to be separated from each other in a continuous pattern. If they weave, they will put unwanted stress on the other tendons, and something will eventually break.
 |
The
10-inch gap between the plastic-covered pad and the frame
mold
will become a 30-inch high concrete beam. The concrete
slab
will be 4 inches high atop of the beams and plastic.
|
Sheets of PVC plastic covers the soil. This method started in the 1990s. The plastic functions to keep the soil swollen with water until the foundation is poured and acts as a vapor barrier from the soil to the curing foundation.
Once poured, concrete cures for the rest of its life. The plastic keeps concrete from sucking water from below and causing mold under the flooring.
The grid of beams supports the slab as summer soil pulls away from the foundation. The foundation then rests on the beams with minimal flexing until moisture returns to the expansive soil.
The foundation benefits from the density of concrete compaction with the tensile strength of steel.
Expect corners to break off. It’s a cosmetic issue rather than structural.
When water is added to cement, a molecular change begins the cure process. The water initially fills the gaps between the cement molecules, which has an expansive reaction. As concrete cures, water leaves the capillaries, which can result in concrete being pulled in multiple directions. This evacuation leads to shrinkage cracks.
 |
Workers
spread liquid concrete quickly because cement
begins a chemical reaction once it contacts water.
As part of the cure process, cracks in the concrete are
expected. Those cracks close when 3,000 PSI of pressure
is applied to the post tension cables.
(Mark M. Hancock / © DFWmark.com) |
From a homeowner’s perspective, the chemical nature of concrete necessitates cracks. Builders need enough water for the concrete to be workable and fully activated but not too much water that it weakens the chemical bonds. However that window of enough versus too much water is measured in many gallons rather than a few cups.
The owners will freak after the pour because there will be huge cracks in the concrete. However, the “shrinkage cracks” will heal when stressed by the post tension process. Cracks can’t be open afterward.
There are only two types of concrete: cracked concrete and soon-to-be-cracked concrete. Assess which concrete is cracked and how seriously before reacting. A crack on the sidewalk is easily filled with silicone caulk, a hairline crack in the garage isn’t an issue. However, if a crack appears in the foundation that is 1/8th of an inch or more, it needs to be seen by engineers. A crack that is a nickel’s width is a problem.
If you see a crack from the ground up – it’s a problem. If there is major erosion of soil under the house, it is a big problem. Old foundations were flat and about 12” thick with no beams below the slab. If you can see the bottoms of those foundations, it’s a major problem.
 |
Untreated
redwood separates concrete sections. Driveways and
sidewalks
use traditional steel rebar to reinforce the concrete.
|
Untreated redwood separates the concrete sections. The driveway is 4” thick. Near the street, it is 6” deep.
Builders shoot for a slab that is as flat as possible rather than trying to build a drain angle. Tile wants a perfectly flat surface, but it’s unreasonable to expect a perfectly flat surface. Instead, a six-foot span should be within 1” of elevation change.
There are hundreds of gallons of water in a foundation. Rain on the foundation concrete is just cosmetic because the foundation will be protected by the structure. But the driveway needs to be poured on a dry day. If too much rain falls on the driveway concrete, spalling can occur.
Spalling is a condition where the concrete surface peels or flakes off. It’s caused when too much water is introduced during the curing process.
Once the concrete pour starts, it must keep pouring. The maximum allowable time between trucks is 45 minutes. Anything longer can create a cold joint. A cold joint is where new concrete is poured on cured concrete. Cold joints are failures and must be torn out and redone.
Hot weather is the biggest problem because it makes the concrete settle and cure faster. Foundation workers will effectively disturb and mix the poured concrete to keep it from curing until the next truck starts its pour. This ensures a homogeneous foundation.
Once poured, concrete cures at the following rates:
4 hours – can walk on it.
12 hours – can drive a nail into it.
45 days – can handle 2,000 PSI of tension (the Perry standard is 3,000 PSI).
 |
For
many decades, foundations consisted of steel rebar and
about
12 inches of concrete on unprepared soil. Post
tension foundation techniques were developed in
the 1930s.
However, the steel available at the time wasn't up to the task.
Modern post tension foundations became the
industry
standard in areas with expansive soil or poor
load-bearing
soil during the 1990s. (Mark
M. Hancock / © DFWmark.com)
|
Cylinder tests involve leaving concrete in a cylinder to cure on site. These are broken at different points to test the strength of the concrete.
A hairline crack is cosmetic. The post tension cable system ensures it won’t get wider.
If concrete sweats, it pulls moisture from below. It remains warm when the air above it is cool. When cool air contacts a warm surface in the presence of humidity, it creates condensation. The condensation creates the right environment for mold.
The post-tension pockets should be covered and maintained. Corroded cables can snap and severely damage foundations.
If the tendon cap falls off, it should be covered and sealed with hydraulic cement, which is a kind of grout. It’s suitable to repair masonry surfaces both above and below grade.
If you’re ready to make the move to a new-built house, you have the right to have your best interests represented by a licensed REALTOR. The builder’s sales agent represents the builder – not you. The builder planned to pay a REALTOR fee. This cost is built into the purchase price. Don’t PAY for services that you don’t get! Contact a qualified REALTOR before you sign anything at the builder’s office. Otherwise, you possibly waive your right to no-cost representation.
DFWmark is certified as a New Home Sales Agent, Green Home Sales Agent in addition to having the Military Relocation Professional (MRP) and Graduate, REALTOR Institute (GRI) designations. I can ease the strain of finding your new-built home with my proprietary research information as well as help you liquidate your current house.
If you plan to purchase a new-construction home, please read these other important posts:
Want to Buy a New-Built House?
New-Build Home Basics
