Forum Message

Name: David  Jahn
Heading: film on coarse aggregate?
We mine a mixture of sand, gravel and orange clay and then wash and classify it for use in concrete. We've been mining this site for almost 20 years and have supplied millions of tons of materials for concrete production for all types of projects including pre-stress operations. Our materials have always been accepted for use without qualification.

Several years ago, a concrete customer told us that they had experienced a few low cylinders on a bridge project. The construction superintendent (who had some background in materials and testing)blamed the low test results on a clay film coating our aggregate. He "proved" this hypothesis by noting that the cylinder broke around the coarse aggregate rather than through it because the "clay film" didn't let the mortar bond to the rock. The state materials engineer for that district accepted his hypothesis as did our customer who looked to us to repay them for the reduced payment from the state.

Our belief is that the cylinder broke around the aggregate because the mortar strength was less than the aggregate strength (approximately 4000 psi). Furthermore, we believe that any "clay film" would have been quickly worn off by the abrasion of the concrete mix during the mixing cycle.

This is the only time that it's been suggested that our aggregate had a clay film. However, it just won't die because the state materials engineer keeps bringing it up to our customers and potential customers. We're out quite a lot of money in direct costs as well as loss of sales, so it's an important problem to resolve.

I'd like to hear from anyone who might be knowledgeable about this sort of problem.

David Alexander

David Alexander


A lack of aggregate fracture at relatively low strength is not necessarily an indication of clay coating. With our local river quartzites, there is little aggregate fracture below strengths of 3000-3500 psi.

The easiest means of proving you don't have an aggregate issue is to provide statistical analysis indicating good production of higher strength concrete (5000 psi or better). The higher design strength of concrete produced with this aggregate, the better your agument that the aggregate quality is satisfactory.

If you have a real doubter, save some broken cylinder samples from a single load of concrete to show the change in failure mode as the compressive strength increases with time. You won't see much broken rock at 2000 psi, even if the same mix exhibits a high proportion aggregate fracture at 6000 psi.

THe other method of argueing against this problem is to pay for some thin section petrographic analysis. Clay coatings are easily visible on thin section micrographs.

As a final note, this sounds more like a political problem than a technical issue. Get your local representative to demand the state materials engineer provide evidence to back up his opinion. Forcing him to do third party analysis of his samples may be the only way to get an accurate resolution of this issue.

If they won't let facts cloud the issue, deal with the spin doctors.

have fun!

Dave Robson

I find it scary that a State Materials Engineer "keeps bringing it up to our customers and potential customers."
There is such a thing as an individual "acting outside agency authority" depending on your State's laws. I agree with Mr. Robson that your problem may not be purely technical.
John Shoucair

My name is Charlie Sainte-Claire. I have worked for the California Department of Transportation for 38 years, much of it in Materials Engineering. I discussed your problem with one of my fellow engineers who has fifty years of experience here, much of it in Materials Engineering. I hope you are not located in California, because whoever the District Materials Engineer is in your State should be reported to the State Board of Engineering Licensure.

A clay coating on aggregate will not survive the mixing process for PCC. It will end up in the paste matrix and not prevent bonding of the cement paste to the aggregate. It "might" reduce the strength of the concrete minutely.

A caliche clay will cause stripping in asphalt concrete, but that is a whole different subject.

Get an attorney to write a terse letter to that Materials Engineer.

Good Luck from the left coast.

Charlie Sainte Claire


Several years ago the company I was working for experienced a similar situation- low breaks blamed on dirty aggregate. The cylinder broke around the coarse aggregate and coarse aggregate shapes were very obvious. However, a closer inspection revealed that the color on the coarse aggregate was the same as the paste color (our limestone aggregate was very similar to the color of the paste). We had a petrographic analysis performed on the low strength cylinder and found that the entrained air had migrated to the paste/aggregate interface and was calculated at around 17% along this interface.

The customer and admix supplier (firmly on the customer's side) finally had to admit that it was not an aggregate issue and we avoided paying any penalty on the project.

Best of luck with finding a solution. I may be able to locate the petrographic report if you are interested. . .

David Jahn

David Jahn
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