Expert: Dan Griffin - 11/30/2007

Question
We recently built a garage not connected to our house in northern arkansas.  concrete was poured in july of this year and the block wall is seven block high at the back.  my concern is last night we located about 10 different cracks in the garage floor.  they all look like hairline cracks, but some of them span wall to wall and most are several feet long.  is this normal shrikage cracks or something more serious.  please help asap as the concrete contractor will be coming out to look at it next week and i would like to be informed before he comes back.e

thank you

Answer
Concrete gets hard and cracks - it's an old saying/joke, but it as true as blue skies and green grass.

I don't know why your concrete cracked and you've not given me enough information to even hazard a good guess.  The best I can do is offer you some professional and verifiable information about concrete that you might discuss with the contractor.  You are both looking for a reasonable solution/resolution to a cosmetic (?) situation.  From what you have told me, I don't think the car is going through the garage floor.

The most important and the most overlooked issue with any concrete work is the compaction of the subsoil.  Commercial work requires a test, often to 95% Proctor density or more, of the subsoil.  Insufficient or non existent compaction leads to patios and sidewalks tipped the wrong way, sidewalks sunken below curb tops, and cracked and broken concrete slabs.  You mentioned 9 block - 72" of fill.  Fill must be put in in 6" lifts and compacted.  Filling it up and beating on the top doesn't cut it.

Concrete can be reinforced.  Fiber reinforcement is mixed in the truck to a requisite density.  The fibers interlock and, thus, reinforce the concrete, help prevent displacement, and reduce shrinkage.  6/6x10/10 remesh is NOT a reinforcement even when properly placed (seldom, if ever, happens), though it does help keep the cracked concrete from spreading.  Rebar works on slabs over 4" thick.  It probably causes more harm than good on most slabs, especially 4" and under.  It is meant to be placed at the bottom 1/3 point of the slab with sufficient cover to protect the steel.  Steel helps the concrete in tension at the bottom of the slab as concrete excels at compression, the force at the top of the slab.  Steel placed too high in the slab, exposed to the summer heat before the pour without getting the steel cooled completely through will cause shrinkage cracks over the tops of the steel bars.

Concrete can be purchased at different strengths depending on how much Portland cement is used per cubic yard.  I would have used 3500# concrete without air entrainment at a 4" slump.  Each gallon of water added to concrete beyond the minimum required for hydration reduces the compressive strength by 500 pounds.  Many finishers like to add additional water, especially when it is hot, to make the concrete easier to spread and finish.  Excess water will cause shrinkage cracking.

Concrete should be cured.  The best cure possible would be to build a dirt dam around the new slab, flood it, and keep it underwater for a minimum of seven days.  Other methods:  continuous lawn sprinkler, plastic sheeting with water reapplied as necessay, chemical curing compound.  Without curing, concrete loses too much moisture much too fast which contributes to cracking and shrinkage.  The Hoover dam will continue to cure for the next 500 years and they invented the chiller to create the ice to cool the water that is still circulating in the poured concrete to help cool it and prevent the heat of hydration from destroying the dam.  Your garage floor sure doesn't need this much help, but I use it to indicate the problem.

Concrete needs construction, isolation/expansion, and contraction joints.  Construction joints are structurally sound ways to  pour a portion today and continue tomorrow.  Isolation/expansion joints are those usually black strips along the foundation or garage slab that provide the ability for the concrete to move and expand in the heat without breaking from the stress.  Contraction joints can be sawed or tooled into the concrete a minimum of 1/4 the thickness to create a thin section to encourage the concrete to crack in a straight line.  These are typical on a sidewalk, driveway, or interstate highway.  Sawed joints should happen the same day as the pour.  Concrete does not like to be a rectangle, it wants to be square; and it does not like to be much more than 12 feet before it really thinks about cracking.  Many jointer trowels do not have 1" or more keels.

Structural, moving cracks we care about.  Hairline, shrinkage cracks are annoying, cosmetic, and a sign of some one of concrete's gotchas (there are many).  The worry would be that bad stuff enters the cracks or the cracks  got bigger.  Bigger cracks can be caulked or epoxied, hairline cracks don't lend themselves to these technologies.  A good silane or siloxane application will not hide the cracks, but will seal and water proof the concrete

Again, I don't know why you see hairline cracks.  Hairline cracks are NOT typically structural cracks and are more often stress and shrinkage related.