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Heading: 1.1.1.2RE:Clay film on coarse aggregate?
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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


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David Alexander


How was it proved that the aggregate had a Clay film? Have any tests been taken on the aggregate to see if there is a film? Also, has another aggregate been substituted for your aggregate and cast in cylinders to see if the concrete acts differently?

Jay Yantosh
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joseph yantosh


David,
Natural aggs can present problems regarding the bond achieved with mortar. Clays are very fine and can remain in the microstructure of the aggregate surface (in the valleys between the peaks of the texture)even under quite abrasive conditions. However this is a double edged sword, as surface texture become smoother there is less surface ares for the paste to bond with, but as surface texture becomes rougher there are more areas for the clay to "hide" in the texture.
Regarding the logic of your statement about mortar and aggregate strength. It is almost allways the case that the compressive strength of the mortar (in your case 4000psi)will be less than the aggregate strength (typically over 20,000psi). What is important is the ability of the aggregate and mortar to transfer stress without failure. This is what Bond is. Partly crushing the aggregate can improve bond charateristics by increasing faces with more surface area due to texture. However some of the smooth natural worn faces will still exist. This is a difficult issue to resolve, with your customers. My suggestion would be to conduct some parallel trials with an aggregate of accepted quality. ie make a batch of concrete with your aggregate to a given mix design, then do another batch with the control aggregate substituted on a volumetric basis. The relevant performance should show if there is a problem or not, and might satisfy your doubting customer.
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Kevin Galvin


David,

the University of Wisconson has done a study onthe effects of clay on aggreagates. The Innovative Pavement Research Foundation website IPRF.org has information which I have copied and pasted below:

" A study accomplished by the University of Wisconsin at Madison (Reference IPRF Project 01-G-002-01-4.2, Effects of Coarse Aggregate Clay Coatings on Concrete Performance) provides affirmation that clays disbursed in cement reduce compressive strength and increase shrinkage of concrete. The study also identified that the presence of clays in plastic concrete impacts hydration rates. But, ancillary discovery recognized that during the hydration phase, there were detectable spikes in the formation of Calcium Hydroxide and that dirty aggregates themselves are sources of alkaline chemicals. IPRF Project 03-9 identified that when a pavement deicer (Potassium Acetate) is introduced into a solution of Calcium Hydroxide there is a significant increase in the pore solution pH.

A by product of the work at the University of Wisconsin at Madison is a protocol which can be used in the laboratory to control the volume of the fine materials present on dirty aggregates that is dispersed into the water phase as a result of aggregate mixing and before cement is added. Studies of the dispersed materials in suspension at the concentrations determined from the disbursement protocol allowed for determination of properties of the cement."

Robert Flynn


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Robert Flynn
 
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