The climate in Florida is usually beneficial for crack free concrete placements.  However, in spring and fall we have periods where the temperature differential between the high and low as well as the humidity and wind speed create the potential for plastic shrinkage cracks.  Plastic Shrinkage cracks form when the rate of evaporation from the surface of the concrete exceeds the concrete’s set-time.  In other words, the longer a concrete slab is exposed to the environment the higher the potential for cracking.  I would like to share some technical guidance on how you can mitigate cracking.  I will focus on methods that will cost you nothing or very little. 

Slump’s Effect on Set-time

Each unique concrete mix is designed at a given slump and also includes a slump range, generally plus or minus 1 inch.  This is the “Designed” slump, and at this slump the mix should set normally and achieve the designed strength.  If you add water to concrete to increase slump you will also decrease strength and increase set-time.  For every gallon of water you add you will increase set-time by ½-1 hour.  If your mix is designed at a 4 +/-1 inch and you add 3 gallons of water per yard to achieve an 8 inch slump, you will thereby increase the set-time by up to 3 hours.  In this example, instead of achieving final set in 5 hours you would achieve final set in approximately 8 hours.  If you do this on a day with a high evaporation rate then you are essentially exposing the concrete to the conditions for an extra 3 hours.

Timing of Jointing

When you cut your joints is completely dependent on set-time.  You should be cutting your joints shortly after the concrete achieves final set.  In concrete placed at a 3-5 inch slump this should occur at about 7-8 hours old.  In concrete with water added this could be 10-12 hours old or older depending on how much water you added.  Your joints should be at least .25 x the thickness of the slab.  The purpose of jointing is that when concrete begins to set it creates tension and that tension must be released.  The joint creates a weakened plane that theoretically should cause the concrete to crack at the joint (the weakest place).  If you wait long enough the concrete will relieve these tension forces on its own, by cracking.  In these cases it is no longer necessary to cut joints as the concrete has already completely relieved itself and cracked generally where you would have put the joint.

How to Prevent Surface Evaporation

If you read ACI you will see there are several methods for preventing evaporation, however they are usually not practical in real life.  Setting up foggers and wind breaks is generally not something that contractors can allocate the time and effort to do.  This leaves one last approved method, which is the use of a “Surface Evaporation Reducer”.  This is a chemical that is mixed with water at a 1:9 ratio and then applied to the concretes surface after each stage of finishing.  You would apply it after bull-floating, after troweling, after edging, etc.  It is ideally applied with an airless sprayer although it can be applied by a pump sprayer as well.  It creates a barrier that prevents evaporation.  Its applied cost is roughly 3 cents per square foot per application.

Using Microfibers

Microfibers serve two primary purposes.  First, they help to displace some of those tension forces that occur in concrete as it sets.  Second, they bridge a crack and help to keep the crack tight when cracking does occur.  Microfibers are much more efficient at controlling cracking versus welded wire mesh because the fibers are throughout the entire depth of the concrete versus the welded wire mesh just residing at the center of the slab.  The use of microfibers is also referred to as “continuous reinforcement”.  Microfibers can be used as a single or double dosage, you may opt for the double dosage on a day where conditions are right for cracking.

The Most Cost Effective Way to Reduce Cracks

If you put the four topics mentioned above (Slump, Joints, Evaporation, and fibers) together you will achieve a significant decrease in the potential for cracking!  The first two factors, slump and joints, are ones that cost you nothing!  We are partnered with Euclid Chemical and we sell their evaporation reducer Euco-Bar, it is $60.00/gallon.  Remember, that one gallon will make 10 gallons once diluted with water.  We also sell Euclid’s microfiber for $8.00/0.75 lb (single dose) and $12.00/1.5 lb (double dose).

1 Gallon of Euco-Bar will cover approx. 3000 square feet.  At a cost of $60.00 per gallon this equates to 2 cents per application.  If you apply it twice, once after bull-floating, and once after troweling this yields a cost of 4 cents per square foot.  The Technical Data Sheet for Euco-Bar is attached.

Serious Crack Protection

If you have a slab where crack control is very important then you should consider the use of a “Shrinkage Reducing Admixture”.  These admixtures come at a slightly higher cost than the above mentioned technologies, however they have a significant impact on crack reduction.  We suggest the use of Euclid Chemical’s Conex.  Conex can be used at 3% or 6% of the mixes cement factor, by weight.  The cost of a dosage of Conex is dependent on how much total cementitious material there is in the mix.  In a mix with a cement factor of 500 lbs/yard a Conex dose would cost about $32.00 per yard for 3%  Conex should always be used in addition to proper slump, evaporation reduction, and the use of fibers in order to give you the best potential for a crack-free slab.