Sulfate's effect on concrete foundations has been the source of litigation and large settlements in California and has raised questions in other parts of the United States. To learn what's going on with this, we asked two experts: Adam Neville, an honorary member of the American Concrete Institute and author of Properties of Concrete (Wiley & Sons, now in its 4th edition), who has investigated numerous foundations and slabs in southern California as an expert witness; and Robert E. Tobin, who has 70 years' experience as a structural engineer, much of it in California. Here's what they have to say about the sulfate issue:
In August 2004, I (Neville) published a long article in the journal Cement and Concrete Research titled “The Confused World of Sulfate Attack on Concrete.” That article showed that our knowledge of sulfate attack on concrete in the field is woefully inadequate. Moreover, the various rules in the building codes and their interpretation, especially by lawyers, are not soundly based. Is it reasonable to take legal proceedings against contractors in the absence of reliable knowledge? For example, do we really know the minimum sulfate content that can reliably be deemed to lead to attack, damage, need for repairs, and a claim for compensation?
We feel that the objective of research should be to achieve good concrete in practice—to benefit those who mix, sell, place, and consolidate concrete, and who do not want to be hounded without reason. Towards that end, here are our main points:
- With the exception of a handful of problematic homes, we found no damage caused by, or consequent upon, sulfate attack. We define sulfate attack as destructive chemical action and damage; the presence of sulfates by itself is not proof of anything untoward as far as the concrete in the field is concerned. So, the situation is that, in homes 10 ro 20 years old, no structural damage has occurred and no concrete needs to be replaced.
- What is even more remarkable is what has happed in those lawsuits that were settled with payment, sometimes large, made to homeowners and, of course, to their attorneys. To our knowledge, the money has not been used to repair, let alone replace, the foundations or slabs. The homes continue to be occupied and fit for the purpose; in a number of cases, the property has changed hands, the purchaser seemingly not being worried about safety. The absence of replacement or significant repairs in the “lawsuit homes” is indirect proof that attack and damage has not occurred. Not a single letter reporting such events was written in response to the article “The Confused World of Sulfate Attack.”
- The homeowners' lawyers have won some of the lawsuits because the court held that there was a violation of the building code insofar as the maximum water/cement ratio of the concrete was concerned. The applicability of some tables in the building code to slabs on grade and foundations is a matter for discussion, but we do not believe they are applicable. The ACI Building Code ACI 318 states explicitly that it does not apply to slabs on grade. It would therefore be best for the engineer in charge of a project to specify the mix. This would do away with some lawyers' arguments that the contractor or the ready-mixed-concrete supplier should ensure conformance with the design code.
- I (Tobin) have observed surface sulfate incrustations on concrete in arid regions of California, Montana, and Wyoming. In these areas of low relative humidity and low rainfall, the natural moisture in the soil dissolves the available sulfates, and when the water evaporates at the surface, a white deposit of sulfate minerals remains on the ground surface. These are referred to locally as “alkali flats.” Similar behavior can be observed in concrete that was placed on the ground without a protective plastic moisture barrier, but this is not damage.
- There is currently a large gap between laboratory research and field construction. Laboratory studies typically focus on calcium sulfate, but in the field there may be other sulfate compounds as well. The action of the various sulfates is not the same. There is also a wide gap between the findings of the laboratory studies and their usefulness on the jobsite.
- There is no proven justification for those values of sulfate content in the groundwater which are classified as mild, moderate, or severe. If we do not know that the description is correct, we should not use it to select the mix.
- Likewise, there is no agreed water extraction ratio of sulfate from a soil sample. Each party to a dispute can choose the ratio, from 1:1 to 1:20, so as to “prove” its point.
All of this does great harm to the satisfactory use of concrete in foundations and slabs on grade. It also exposes the concrete providers (both producers and contractors) to unnecessary and unjustified litigation. An honest and unbiased review of field behavior is needed without delay.
A considerable amount of verbiage has been published on the subject of sulfate reactions, but the amount of disagreement that prevails after many years of research is considerable. Sir Walter Scott wrote at the beginning of the 19th century: “O what a tangled web we weave, when first we practise to deceive.” Although these researchers may not seek to deceive, their results have certainly become tangled.