QUALITY CONCRETE FOR SMALL JOBS

•We have heard and read a great deal in recent years about "Hi-Tech Concrete." Twenty thousand psi concrete is being used and thirty thousand psi is possible. Accounts of research and practical application of hi-tech concrete are common in our technical journals. We are even progressing toward concrete on the moon. It's exciting stuff.

On the opposite end of the scale from hi-tech are basic uses, as represented by residential concrete. It's a market in which marginal quality and poor performance are far too common. It also typically represents about 20% of the total concrete market in the United States and Canada (from PCA data). That compares to a concrete market share of about 5% for hi-rise structures, where hi-tech concrete is used. The moon's market share hasn't yet appeared in the statistics.

Obviously we have to pay attention to high-tech markets of the (future, limited though they may be. Just as obviously, we have an immediate down-to-earth job to do in improving the standards of residential construction. There is little excuse for the homeowner to have to accept preventable cracking in basement floors, garage slabs, and driveways, or surfaces that scale or dust. Nor should we expect or want people to accept explanations that lead them to believe "you have to expect that from concrete."

Where the Problem Starts

There are several long recognized problems affecting the quality of residential concrete:

1. The owner seldom knows what the standards for construction should be.

2. There is no job specification to observe or enforce.

3. The small concrete contractor is often not a part of concrete industry organizations, and isn't exposed to basic information that he should have.

Recommended Standards

The standards for residential construction aren't hard to compile. The difficulty is in getting them into the right hands. Standard for residential flatwork can be summarized like this:

Concrete Mix

•Subject to deicers - 5 to 8% air entrainment - 3500 to 4000 psi

• Mild climate - 3 to 4% air entrainment - 2500 to 3000 psi (3500-4000 psi for floors)

Air entrainment is an absolute to deicer scaling. In lower strength, low cement content mixes, often used in mild climates, it improves workability and inhibits excessive bleeding.

Subgrade The topsoil should be stripped. The natural soil beneath is suitable as a subgrade if it can be uniformly compacted. If the natural soil is non-uniform or cannot be uniformly compacted, replace it with a 4-inch to 6-inch compacted depth of granular material.

Slab thickness Minimum of 4 inches on a well compacted subgrade. If there is any concern about uniformity of subgrade support, a 5-inch thickness is a good investment.

Jointing Control joints should be spaced at a maximum of 10 feet for a 4-inch slab (2 ½ times in feet, the thickness of the slab in inches). For sidewalks, the maximum spacing of joints should be a minimum of 1/4 the depth of the slab.

Expansion jointExpansion joint material should separate any slab (floor, walk, driveway, patio, garage slab) from any adjacent slab or wall.

FinishFor outdoor work, a troweled surface is easier to clean, and a broomed surface is non-skid. Either is serviceable, but the broomed finish may avoid complaints of dusting, surface checking, or discoloration that often relate to troweling. It also is less slippery.

Curing -A sprayed-on membrane or waterproof paper are best. A polyethylene sheet may cause discoloration. Sprinkling the surface is risky because of the adverse effect on the surface of alternate wetting and drying.

How to Get the Message Out

An enterprising contractor, possibly with the help of his enterprising ready mix supplier who also has a stake in quality work, could boil their recommendations down and put them in a flyer under his name. When he's soliciting a job with a potential customer, he would present the flyer as his specification for the job. He could then sell himself as a knowledgeable, qualified contractor, and could afford to sell quality instead of price. The customer would have a quality specification and would know what he was getting and why. Most important to the concrete industry, the performance of residential concrete could be improved and lead the way to expanded markets and fewer call backs in the future.

Eugene 0. Goeb, Concrete Consultant, Wheaton, III.