Aluminum tools

Concrete Construction, August, 2004 by Patrick Watson

In the Concrete Basics article (Feb. 2004, p. 40) Striking of Concrete by Hand and Bull Floating" you seem to be recommending aluminum tools. To the best of my knowledge, no manufacturer offers bull floats made of aluminum. Strikeoffs made from aluminum studs should never be used.

The friction of an aluminum tool in contact with portland cement concrete will abrade microscopic particles of aluminum onto the concrete surface. The aluminum particles may be worked into the surface of the concrete, react, and create hydrogen gas. The gas will cause bubbles or blisters and the finisher will he accused of closing the surface too early. The floor will require repair.

On one occasion some years ago I was called to investigate a blistered slab, unique because the blisters were in a pattern 1 foot wide by 250 feet long. I found that the vibratory screed had an aluminum frame with steel strike surfaces. The frame had broken and been welded with aluminum. The blister pattern exactly matched the location of the weld on the screed.

On another occasion, I was called to look at a metallic aggregate topping that was forming blisters as it was bullfloated. The cause was the use of an aluminum stud as a strikeoff. It takes only a microscopic particle to react. In fact, when aluminum powder is used as a gas forming agent to create expansion during the plastic stage in concretes and grouts, the normal ratio is one part aluminum powder to 50,000,000 parts cement!

Please inform your readers (and I am a loyal reader) of the potential dangers of allowing an aluminum tool to be in direct contact with plastic concrete and portland cement materials.

--Patrick "Doc" Watson

Materials Specialist

Degussa, ChemRex

Editor's response

Mr. Watson is certainly correct that pure aluminum mixed into concrete can and has led to the problems he mentions. The word aluminum is often used in connection with various concrete finishing tools--an aluminum bull float, an aluminum hand float, and so forth. The same is true for the word magnesium; we refer to a hand float as a "mag float," for example. We checked with a few manufacturers of concrete finishing tools and there are, in fact, "aluminum" hand tools and screeds on the market, but they are all alloys that include iron and other metals and are manufactured for hard use and long service lives. There aren't any tools on the market that are 100% aluminum or magnesium. Aluminum or magnesium alone is too soft, and tools made from them would wear out quickly. The same is true for the aluminum formwork industry, by the way. None of the manufacturers we contacted were aware of any problems resulting from aluminum tools reacting with portland cement.