Why cracks in concrete

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. 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 or a solid state , requires water. This chemical reaction, or hydration, continues to occur for days and weeks after you pour the concrete. In a concrete slab, control joints help concrete crack where you want it to. It is more expensive to do it right- it simply takes more manpower to pour stiffer mixes. 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.

Read here about the methods to cure concrete and understand how your contractor will cure the concrete. Concrete is available in many different strengths. Verify what strength the concrete you are pouring should be poured at. Control joints help concrete crack where you want it to. The joints should be of the depth of the slab and no more than times in feet of the thickness of the concrete in inches. So 4"concrete should have joints ' apart.

The ground upon which the concrete will be placed must be compacted. 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. Understand what you contractor is doing about each of the above listed items and you will get a good concrete job.

Some cracks are more concerning than others. Here's an overview of crack types and what to do about them. Not all cracks in concrete are serious enough to require repair. The hairline cracks in this floor were air brushed with brown dye to achieve a beautiful crackle pattern. At what width does a crack in concrete become a problem? If neither has the ability to flex, the resulting force will cause something to crack.

An expansion joint is a point of separation, or isolation joint, between two static surfaces. Its entire depth is filled with some type of compressible material such as tar-impregnated cellulose fibre, closed-cell poly foam, or even timber See Figure 6. This relieves stress on the concrete and can prevent cracking. Expansion joint material can also prevent the slab from grinding against the abutting rigid object during periods of vertical movement.

During these times of heaving or settling, expansion joint material prevents the top surface of the slab from binding up against the adjacent surface and flaking off See Figure 7. Figure 7: Expansion joint between these slabs would have prevented chipping. During such cycles, the frozen ground can lift as much as several inches, and then settle again when the ground thaws.

If the slab is not free to move with the soil, the slab will crack. The presence of large tree roots can also cause concrete to heave. If a tree is located too close to a concrete slab, the growing roots can lift and crack the concrete See Figure 8. Figure 8: Tree roots lifted and cracked this sidewalk. Conversely, if a large tree is removed from near a concrete slab the buried roots will decompose. The resulting void can cause the ground to settle and the concrete to crack.

Settling is also called subsidence. Subsidence is very common over trenches where utility lines and plumbing pipes are buried. Often times, the utility trench is not compacted when it is refilled. If concrete is placed atop a poorly compacted trench, the void created by subsidence can cause a crack across the unsupported concrete slab See Figure 9. Figure 9: Crack across the unsupported concrete slab. Another place where concrete commonly subsides is near a house.

Whether the home is built on a basement or crawlspace, the over-dig is subsequently back filled. Unless the back fill material is compacted in lifts as the over-dig is filled, it will settle over time.

This settling will cause any concrete poured atop it to settle along with it. Many times this settling will cause the concrete to crack and tilt back toward the house, creating negative slope See Figure Another factor which contributes to cracking is placing excessive weight on top of the slab.

Although it is a very strong material, concrete still has load limits. When you hear someone speak of 4, psi concrete, they are referring to the fact that it would take 4, pounds per square inch of pressure to crush it.

Residential concrete, however, is rarely overloaded as far as compressive strength is concerned. What is more common is that the excessive weight is too much for the ground underneath the concrete. This is especially true after periods of heavy rain or snow melt when the ground is saturated and soft. When groundwater migrates under the concrete it causes the underlying soil to become soft or spongy.

Excessive weight on the slab at this point can press the concrete down. Since the flexural strength of concrete is less than its compressive strength, the concrete bends to its breaking point.

Homeowners who place large recreational vehicles or dumpsters on their driveways are more likely to see this type of cracking.

Driving heavy vehicles off the edge of a slab creates a similar type of crack. Figure A heavy truck drove over this sidewalk, cracking the edge.

Crazing cracks are very fine surface cracks that resemble spider webs or shattered glass. They can happen on any concrete slab when the top loses moisture too quickly. Crazing cracks can be unsightly, but are not a structural problem.