Reasons of Concrete Cracks

When installed properly, concrete is one of the most durable and long lasting products you can use around your home. But it is important that concrete contractors follow well-established guidelines with respect to concrete placement. Durable, high strength, and crack resistant concrete does not happen by accident.

Why Concrete Cracks

Reason #1 - Excess water in the mix

Concrete does not require much water to achieve maximum strength. But a wide majority of concrete used in residential work has too much water added to the concrete on the job site. This water is added to make the concrete easier to install. This excess water also greatly reduces the strength of the concrete.

Shrinkage is a main cause of cracking. As concrete hardens and dries it shrinks. This is due to the evaporation of excess mixing water. The wetter or soupier the concrete mix, the greater the shrinkage will be. Concrete slabs can shrink as much as 1/2 inch per 100 feet. This shrinkage causes forces in the concrete which literally pull the slab apart. Cracks are the end result of these forces.

The bottom line is a low water to cement ratio is the number one issue effecting concrete quality(Use a maximum .50 water t)o cement ratio when concrete is exposed to freezing and thawing in a moist condition - and excess water reduces this ratio.

What you can do about it:
Know the allowable water for the mix the contractor is pouring- or be very sure you have chosen a reputable contractor who will make sure the proper mix is poured. It is more expensive to do it right- it simply takes more manpower to pour stiffer mixes
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Reason #2 - Rapid Drying of the concrete

Also, rapid drying of the slab will significantly increase the possibility of cracking. The chemical reaction, which causes concrete to go from the liquid or plastic state to a solid state, requires water. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete.

You can make sure that the necessary water is available for this reaction by adequately curing the slab.

What you can do about it:

Properly Curing Concrete Slabs

 

Why cure concrete. Curing serves two main purposes.

• It retains moisture in the slab so that the concrete continues to gain strength
• It delays drying shrinkage until the concrete is strong enough to resist shrinkage cracking.

Properly curing concrete improves strength, durability, water tightness, and wear resistance.
How to cure concrete.

• Water cure:The concrete is flooded, ponded, or mist sprayed. This is the most effective curing method for preventing mix water evaporation. Make sure you allow proper time for water curing.
• Water retaining methods:Use coverings such as sand, canvas, burlap, or straw that are kept continuously wet. The material used must be kept damp during the curing period.
• Waterproof paper or plastic film seal:Are applied as soon as the concrete is hard enough to resist surface damage. Plastic films may cause discoloration of the concrete-do not apply to concrete where appearance is important.
• Chemical Membranes:The chemical application should be made as soon as the concrete is finished. Note that curing compounds can effect adherence of resilient flooring, your flooring contractor and/or chemical membrane manufacturer should be consulted.

All the desirable properties of concrete are improved by proper curing!
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Reason #3- Improper strength concrete poured on the job

Concrete is available in many different strengths. Verify what strength the concrete you are pouring should be poured at.

Talk to the ready mix supplier

Consult with the Ready Mix Concrete Association in your area.

Reason #4 - Lack of control joints.

Control joints help concrete crack where you want it to. The joints should be of the depth of the slab and no more than 2-3 times (in feet) of the thickness of the concrete (in inches). So 4"concrete should have joints 8-12' apart.

Be Active in Deciding Where Control Joints Will Be Placed

Correctly laid out joints. Note: Inside corners, where cracks would typically occur, have correctly placed joints.

Control joints are planned cracks which allow for movements caused by temperature changes and drying shrinkage. In other words, if the concrete does crack-you want to have an active role in deciding where it will crack and that it will crack in a straight line instead of randomly.
Space joints properly. Space joints (in feet) no more than 2-3 times the slab thickness (in inches). A 4" slab should have joints 8-12 feet apart.
Cut joints deep enough. Cut joints 25% of the depth of the slab. A 4" thick slab should have joints 1" deep.
How to cut joints. Grooving tools cut joints in fresh concrete. Saw cutting cuts joints as soon as the concrete is hard enough that the edges abutting the cut don't chip from the saw blade.
Cutting joints soon enough. In hot weather, concrete might crack if joints are not cut within 6-12 hours after finishing concrete. In this condition, if you don't want to use a grooving tool to cut joints, there are early-entry dry-cut lightweight saws that can be used almost immediately after finishing. These saws cut 1" to 3" deep, depending on the model.
Place joints under walls or under carpet areas.Under walls they won't be seen. Under carpet areas the joints won't have a chance to telegraph through vinyl areas.
Avoid re-entrant corners. Planning the joint pattern can sometimes eliminate re-entrant corners.
Most plans don't have joint spacing marked on them. So don't leave this important part of concrete construction to chance.
Jointing is often not taken seriously enoughand the "sawcutter" comes to your job and puts the cuts where he feels they belong or where it is convenient for him.
Be active in deciding where control joints will be placed!

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Other reasons:

Never pour concrete on frozen ground.

The ground upon which the concrete will be placed must be compacted:

Make Sure The Subgrade is Compacted

Excavations beneath the slab that are not to be filled with concrete (usually plumbing or other mechanical trenches) should be brought back to grade in compacted lifts. This means a 24" trench would be backfilled 6" at a time, each "lift" being mechanically vibrated so it is left compact. Learn more about subgrades and subbases for concrete slabs.
If these excavations are not compacted when backfilled, this loose soil will settle over time leaving the concrete over that area with no earth under it. This becomes a prime place for concrete to settle. Since the soil next to these excavations is native soil, the uncompacted trench can literally become a thoroughfare for water.
Most rental yards have compacting equipment and it is worth the investment to use it. Learn how to compact soil before pouring concrete.
Excavations from the house to the street for utilities should also be backfilled and compacted in the same manner so areas under the driveway concrete don't sink. Uncompacted areas under lawns can be identified by areas of sunken grass-so it is good idea to compact trenches even under areas not receiving concrete.

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The sub grade must be prepared according to your soil conditions. Some flatwork can be poured right on native grade. In other areas 6"of base fill is required along with steel rebar installed in the slab

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Understand what you contractor is doing about each of the above listed items and you will get a good concrete job

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