Question
we installed a concrete driveway and within 3 weeks we see hairline cracks in a
portion of the drive. Is this something that can be repaired for good or must  be
completely replaced.  If it can be repaired, HOW? We live in South Texas where it
is hot and humid. I'm afraid a repair will only crack later, after the warranty is   
is off.

Answer
Settlement and shrinkage cracks are normal in concrete within a few days of pouring a driveway and can be excessive if good construction practices are not followed. To avoid excessive cracking or in order to control the locations of the cracks, you must install the concrete very carefully. The ground must be dry and well compacted, apply a moisture barrier (plastic sheet), then cover it with 2-3inches of gravel, lay down welded wire fabric (steel reinforcing, then pour the concrete, 4 inches minimum thick and be sure it is not too wet, when concrete has set up for roughly 12 to 24 hours, but before it is fully cured (48 hours) cut it with a saw at intervals of not less than 10 to 12 feet in any one direction. The cuts should be 1 inch min. depth. You can also create expansion and contraction joints using a "joint trowel" while working and finishing the concrete. But these are often not deep enough to regulate where the cracks form. It is also good to continually wet the concrete several times a day starting about 24 hours after the pour. This cools and slows the process making it stronger. Some people even seal the concrete with a clear penetrating sealant within hours of finishing it. This slows the evaporation of water and makes the concrete much harder.

In any case, for your situation a repair may help the situation form getting worse but probably won't look very good. You have to saw cut the crack to about 1" depth and 1/4 inch width so it will receive caulk/crack sealant. This will stop future water intrusion and reduce future problems. You may also cut out the bad section and re-pour it.

Best of Luck
Sean J Murphy
www.seengineering.com
www.amenityarchitects.com
Southeastern Engineering
Amenity Architects