The TREC SOP definition of performance
Achievement of an operation, function or configuration relative to accepted industry standard practices with consideration of age and normal wear and tear from ordinary use.
Applying the TREC definition of “performance” to slab-on-ground foundations
It is not obvious to me how this definition should be applied to a slab-on-ground foundation. There are no code requirements regarding how a slab-on-ground foundation should perform. The codes stipulate how slab-on-ground foundations should be designed, but they say little or nothing about performance. If you doubt this, I would ask a simple question: tell me where in any applicable code does it stipulate how a 54-year old foundation should perform given the TREC definition of “performance?”
If not codes, what?
The TREC SOP definition does not address any code, but it does refer to: accepted industry practices. What, specifically, would these be?
One obvious answer is the TREC SOP itself. In short, it states that foundation performance should be judged by looking for and observing what TREC calls “visible indications of adverse performance.” These indicators are listed below:
(i) binding, out-of-square, or non-latching
(ii) framing or frieze board separations;
(iii) sloping floors;
(iv) window, wall, floor, or ceiling cracks or
(v) rotating, buckling, cracking, or deflecting
How to apply the phrase “achievement of an operation, function or configuration”
The best way to look at this is to focus on “function” as in: what is the function of a slab-on-ground foundation?
From an engineering perspective, a slab-on-ground foundation serves at least three functions: it is a floor, a relatively level, hard surface that can take heavy foot traffic and can be easily cleaned.
Any foundation worthy of the name should be configured so that it does not sink into the earth to any appreciable extent.
Third, a slab-on-ground foundation should be stiff enough that distress/damage due to foundation distortion is reduced.
Note that much of this is subjective and involves tradeoffs. For instance, a slab-on-ground foundation can always be made stiffer, but only at a cost. Deeper stiffening beams and more stiffening beams cost more money. A stiffer foundation will bend less and thereby cause less distress and less damage to the house. Surprisingly, it may also result in a foundation surface that is less level.
How to account for age and normal wear and tear?
As a foundation ages, it is likely to become progressively less level. Droughts come and go progressively causing more distress and damage to the house. Some homeowners will repair any distress soon after it appears. Others never repair or repaint anything. I have seen numerous houses that have had dozens of piers/piles installed and the owner had never repaired a single crack or door after the foundation was underpinned.
Creep is another complicating factor. Reinforced concrete will distort even when the load does not change. Creep distortion is likely to be a significant factor in any slab-on-ground foundation 10 years old or older. Yet distortion due to creep is normally structurally significant due to the phenomenon of stress relaxation.
The bottom line
The bottom line is that any TREC inspector or licensed engineer has to use their best judgment when taking into consideration age and normal wear and tear from ordinary use. The ability to make such judgments comes with experience. There is no magic bullet.
Some examples of failure to provide a performance opinion
In my opinion, the foundation does (or does not) need repair.
This is an opinion concerning repairing the foundation, not how well or poorly the foundation is performing. It is possible to recommend not repairing the foundation even while saying the foundation performance is substandard. Some slab foundations cannot survive the underpinning process.
There were visible cracks.
There is nothing unusual about concrete cracks, brick veneer cracks or drywall cracks. Whether a slab does or does not have visible cracks tells me nothing about how the foundation is performing.
Having said that, cracks are a factor in assessing the performance of a slab foundation.
There were sloping floors.
There is nothing unusual about sloping floors per se. This is more of an observation than a performance opinion.
The foundation was not level.
Again, this is an observation, not an opinion as to foundation performance.
Some examples of a proper performance opinion
Opinion as to foundation performance: in my opinion the foundation is performing in a normally expected manner taking into consideration age and normal wear and tear from ordinary use.
Here is an opinion stating that the foundation is performing below average:
In my opinion, due to the severity and extent of the indications of adverse foundation performance, this foundation is not performing as well as other foundations I have inspected in the past in this area and this age.
TREC allows for the possibility that you might be uncertain about what opinion is or you may be certain as to how reliable your opinion is.
You are allowed to express that uncertainty. Here is what TREC has to say about this.