The first 45 minutes

The first 45 minutes refers to the mixing of the concrete at the concrete plant and its transportation from the plant to the building site. Almost all concrete leaves the plant with the proper mix of ingredients. One of those ingredients is water. As soon as water is added at the concrete plant, the water acts as a catalyst to start a chemical process called hydration. For first 45 minutes os so, the mix is rapidly becoming concrete but remains in a plastic state. Once the 45 minute mark is reached, the hydration slows significantly.

The next few hours

After about 45 minutes, the concrete will begin to get noticeably harder although it may be still in a plastic state.

If the slab is post-tension, there will be no crack control at this time. Any cracks that form will be free to widen and grow in any way that they wish. If the slab is conventionally reinforced the concrete will immediately bond to the steel reinforcement. So there will be some degree of crack control almost immediately.

Slab-on-ground foundations for the next 30-days

Over the next 30 days, the concrete will continue to harden and cracks will continue to show up especially if the slab is a post-tension slab.

By the 28th day, the concrete should have reached its design strength. If the house is north of I 10, the concrete should be around 3000 psi or more. If the house is south of I 10, 2500 psi concrete is the norm.

During this time the concrete slab is likely to curl. It will normally curl around the perimeter of the slab. By “curl”, I mean that the perimeter curls or bends upward. This is normal and cannot be controlled by the foundation contractor. It is simply a result of the fact that the exposed top surface of the concrete will cure faster than the concrete below. As a service concrete shrinks because it is curing faster the slab tends to curl upward around the perimeter. One reason I mention this is that some people cannot understand why/labs are not cast level. What they fail to understand is that no matter how the contractor works the concrete the slab will always curl in it simply beyond anybody’s control.

Slab-on-ground foundations for the next 10-years

For the first 10 years of the life of the slab it will see seasonal loadings from the ground swelling and shrinking. In Houston and surrounding areas, the typical scenario is that our winners and spring will typically be somewhat wet, but our summers and fall are usually pretty dry. This results in a seasonal variation that goes on every year.

As a general rule, the central area of the slab will lift due to the fact that the soul below this area will absorb moisture and begin to swell. This is normal and it will not create any significant problems as long as the foundation was properly constructed and built to withstand the swelling loads from the soil below the foundation.

I hasten to add something here this important. Most people look at a house being built on a slab and in their minds the only load on the slab is the frame structure their brick veneer the other loads inside the house. It is true that these loads are significant and important, but they pale in insignificance compared to the pressure of the soil pushing up in the central area of the slab due to center-lift.

Every summer as we enter into relatively dry weather it is the perimeter of the slab that will typically go down as the soil around the perimeter shrinks. This causes the mole on the mound in the middle area of the slab to become exaggerated. It is for this reason, that most people only see significant foundation issues in July, August, September, and October.

Once we get into the fall any distress caused by foundation movement typically becomes mitigated to a significant degree.

Here is something else to keep in mind: every summer when the perimeter soils ranks in the perimeter grade beams come down then in the wet winter time the soul swells and pushes the perimeter grade beams upward. It is easy to think that when the slab is pushed upward around the perimeter that goes back to where it was before the soil dried out. However that is not the case. In most cases it only goes back up a little bit not the full amount that came down in this cycle is repeated every year. As a result the slab tends to get more and more unlevel as time passes. It is not unusual in a house that is 10 to 20 years old or older, that when you walk into the outside corners of the house, you will notice a perceptible slope to the floor. One of the causes of this is the fact that the perimeter grade beam in effect is wearing a rut into the soil as the soil shrinks and swells.

Slab-on-ground foundations for the next 20-years

At the end of the 30-year mark, of not before there will be even more unlevelness.. The floor slope as you walk into an outside corner will be even more noticeable. There will also be a new form of floor sloping: As you walk the house going from side to side or from front to back, there may be a noticeable up and down slope.. The main cause of this is that the ground between the grade beams will have settled leaving an air gap between the floor concrete and the ground. This is actually very common in older slab-on-ground foundations.

In a standard design of a slab-on-ground foundation, the concrete between the grade beams is assumed to be fully supported by the ground. This assumption is initially true, but it becomes less and less true as the house gets older.

Slab-on-ground foundations after 30-years

Once the 30-year mark is met, the foundation is very close to as unlevel as it is likely to ever be. A new maintenance issue will raise it’s head if it has not already: post-tensioned slabs will almost certainly have exposed cable ends and conventionally reinforced slabs will have some exposed rebar.