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Why cracks show up in a new driveway

Three years ago I had a new concrete driveway installed. The next spring the concrete showed cracks which now have enlarged. The concrete contractor tells me he cannot find a filler. Do you know of a material which might be used to fill the cracks? The contractor admits he should have installed expansion joints.

A reader Ellensburg, Wash.

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The Mrs. Wiggs of the concrete patch is Compat, made by Hartline Products Company, 2186 Noble Road, Cleveland, Ohio 44112. There are other products as well at your local building-supply house. Follow the labeled instructions completely.

Why did the concrete crack? Your contractor should surely have installed expansion joints since fresh concrete during the drying process shrinks as much as three-quarters of an inch per 100 lineal feet. Expansion joints should be installed at intervals of no more than 20 lineal feet.

Why certain slabs sometimes crack stumps many a veteran concrete expert. Among other means suggested to forestall concrete cracks are:

* Careful preparation and compaction of the base.

* Use of reinforcement in the slab.

* Avoidance of laying concrete on expansive soil.

* Protection against undermining by water.

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* Careful handling of the ''mud'' during the pouring process.

* Use of sound, clean aggregates in proper proportion with the cement.

* Use of a low slump concrete mixture.

* Absolute avoidance of a sloppy mix.

Even with all these precautionary measures, some concrete will still crack. That is why even the experts sometimes scratch their heads in perplexity.

Likely, the cracks now are enlarging due to moisture penetration into the subgrade. Frozen moisture below the slab heaves it. Filling of the cracks will help prevent that incursion in the future.

If it were my property, I'd saw-cut the driveway into 15- or 20-foot horizontal increments so that no surface area exceeds 400 square feet. Saw-cut more than halfway through the slab. This provides an expansion joint to take the place of existing crack enlargement.

See your telephone Yellow Pages for a saw-cutter under ''Concrete Breaking, Cutting, Sawing, Etc.''

To other readers planning to lay concrete as a driveway, sidewalk, or slab, hire only experienced concrete people who install expansion joints as a matter of course. Veteran concrete men surely know that there have been crack fillers on the market for decades.

Should the filled cracks provide an unacceptable driveway surface appearance, and if you can afford the expense, lay a thin surfacing material over the concrete. Suitable overlay materials are made by Sealwall Products Inc., Eastlake, Ohio (216) 951-3445; Okun Company, West Haverstraw, N.Y., (914) 947- 1505; and Hartline Products, Cleveland, Ohio, (216) 451-6573.

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