Trained Eye

 
 
 

New Driveway Cracks

Question:

Last fall we purchased a new home in Winnipeg . Last fall the concrete driveway was fine and free of cracks, but when I looked at it this spring I found a crack extending the width of the driveway. Now I know that concrete cracks and that surface cracks will happen, but this crack is all the way through the depth of the concrete. When I looked at the neighbour’s driveway I found that their control joints were spaced about 8 feet apart; on my driveway they are twice as far. I am wondering what is the acceptable spacing for control joints for this region and climate zone? Also what are some possible fixes for this problem, best and worst case?

Answer:

Unfortunately, cracks in your newly poured concrete driveway are all too common in our area. There may be little you can do, now that the cracks have appeared, but I will provide some possible causes and answer your questions about worst case repairs.

Preparation of a surface for pouring & curing of concrete is a very tricky business and may be affected by numerous variables or factors. Condition and age of the soil underneath the new slab, weather, consistency of the concrete mix and steel reinforcement are some of the factors that can affect how the concrete will cure after installation. If the concrete is poured on a day that is too hot or too cold, the concrete may not cure at an optimum rate. If the weather was unusually hot & dry after finishing, the concrete should have been periodically wetted to prevent fast curing. Conversely, if the weather dropped near or below the freezing mark in the first few days of curing, the concrete may have been affected. There may be several things that can be done by the contractor to minimize the affects of these environmental factors, but once the job is complete and the forms removed, there is often no follow-up, unless done by the homeowner. Control joints are installed to help allow for some expansion & contraction of the new concrete, but often only provide a nice straight line for the concrete to crack, which is cosmetically desirable.

Perhaps the most probable cause of your larger-than-normal crack is poor compaction of the base underneath the concrete. There should be a solid base of well compacted crushed rock underneath the new driveway slab. This should be in the minimum range of five to six inches thick, but often only three to four inches is installed. This compacted gravel may also be affected by the amount of water used during compaction or the weather after completion.

Even with properly a compacted granular base, the composition of the soil below this area will ultimately affect the overall performance of the concrete, over time. Many driveways are poured on newly installed clay fill that is made up of excavated material from the building site. This is often unavoidable to ensure proper slope and grading of the driveway to meet the garage floor slab. Many yards of freshly piled clay may have been built up to provide the proper level for the driveway. Just as with the crushed stone base, the degree of compaction of this fill may greatly affect movement of the concrete driveway after installation. If this soil is freshly installed, without several weeks of heavy vehicle traffic to pack it down, it will inevitably settle, causing the cracks.

Despite the minimal control joints, and the obvious cracking, there should be sufficient steel reinforcement in the new concrete slab to prevent major damage due to movement. This re-bar is actually the main component giving strength to the concrete. If this is improperly installed, or undersized, then further cracking & shifting is more likely. As with the compacted fill, determination of this may be impossible, after the fact.

As for solutions to the cracking, there is little you currently can do to fix it, without complete demotion & removal of the slab. It is primarily due to our expansive clay soil and may occur despite the best intentions & careful attention to details of the installer. One thing you can do to help prevent further deterioration is to fill in the crack. Installation of a high quality, flexible caulking designed for concrete is the way to go. This sealant will fill the current gap and prevent water from entering the area. This is critical to prevent erosion of the soil beneath this area and expansion of the crack from ice during freeze and thaw cycles.

In my experience, concrete driveways and sidewalks are not well suited to our climate and soil conditions, despite their widespread installation in homes. It is somewhat of a crap shoot to see if the concrete remains in good shape or heaves and cracks uncontrollably after installation. A better product for this application is interlocking paving stone. While the same factors may affect the interlocking stone, because of its nature, it can easily be removed and reinstalled if excessive movement is seen. Since the pavers are only held in place on a base of sand and by borders and compaction, they can easily be replaced. Installation of more sand along with replacement and compaction of the paving stone will provide a smooth surface.

The disheartening this about concrete driveways and sidewalks is that there is little that can be done if they crack & shift, even shortly after installation. In extreme cases, mudjacking may be an option to close the cracks in new concrete, but should only be done if major shifting is seen. This involves injection of a special slurry under the slab to lift and level uneven surfaces and can be costly. In most cases, sealing the cracks with caulking and hoping for little further movement is all that you can do.