Avoiding driveway cracks

To the real estate editor: I read your reply regarding a cracked concrete driveway. First, let me explain that I'm an architect. The type of joint you describe -- saw-cutting the concrete at 15- or 20-foot intervals -- is realy what is called a ''control joint.'' The theory behind a control joint is that any cracking that occurs will follow the path of least resistance.

Ideally, cracking will only occur at the control joint; or at least the cracks will only run short distances to the joint, then follow it, leaving one with a resonably aesthetic slab.

As you point out, though, concrete cracks can be very puzzling, and they often ignore the presence of control joints. The best way to avoid cracks is to follow your accentuated recommendations and warning to future driveway owners.

Expansion joints serve a different function, completely separating the slab areas with some type of resilient, moisture-resistant material. This allows adjacent slabs to expand and contract with climate changes without adversely affecting each other.

If differential subsidence between slabs is a problem, then special steel dowels should be used that allow longitudinal movement but not vertical. No steel reinforcing should penetrate the joint. Placing expansion joints in an existing slab would be a costly operation.

The saw-cut joints you recommend may help in the reader's case by containing and localizing cracks, but will do little to mitigate damage from expansion and contraction.

If expansion and contraction are a problem, only an expansion-contraction-type joint will do the job.

I always read and enjoy your column. It fills a real need.

Richard A. Gorman

Scotch Plains, N.J.