Frequently Asked Questions

Why does concrete crack?

Concrete is extremely strong and durable, thus there is minimal flexibility in concrete. Because of this, it is expected that concrete will crack. There are basically two types of cracks that can occur in concrete:

  • The irregular cracks that occur in the surface of fresh concrete soon after it is placed and while it is still plastic are called “plastic shrinkage cracks.” These are caused by the shrinkage that occurs as concrete dries and hardens at the surface.
  • Over time, temperature changes and ground movement under concrete slabs due to moisture, and/or poor compaction, can also cause concrete to crack.

What can be done to help reduce cracking in concrete?

Because concrete may crack for the above stated reasons, there are guidelines set to control the cracking of concrete with the use of “control joints.” These are the hand cut grooves you see in concrete flatwork that are tooled or saw cut into the concrete slab in precise locations to regulate where the concrete will crack. If the concrete does crack, it should do so along these control joints, making cracks less noticeable.

It is recommended to caulk cracks as soon as they appear. This will help to eliminate moisture from getting down below the surface, which could in turn cause settlement and increased cracking. For more information please see our Taking Care of Your Concrete page.

Why does my concrete look like it’s flaking off in layers?

The flaking or breaking away of the hardened concrete surface is called “scaling or “spalling.”

One of the causes is the freeze/thaw cycles that are common here in Colorado winters. Concrete is porous and will absorb moisture from snow that melts during the day. This moisture will then freeze and expand overnight causing pressure to the surface of the concrete. PTL Concrete, Inc. recommends shoveling snow completely off all concrete surfaces as soon as possible to help eliminate the effects of freeze/thaw cycles.

Deicing chemicals can also cause scaling. The salts that are used on the city streets and then transferred to your driveways, sidewalks, and curbs from the plows or from dripping off your vehicle, as well as the de-icing salts you can purchase to sprinkle on surfaces, will deteriorate the surface of concrete. It is recommended not to use de-icing salts on any concrete surface, and to sweep or shovel off any chemicals that are transferred to your concrete surface from the city streets.

How can stains be removed from concrete?

Stains can be removed from concrete with dry or mechanical methods, or by wet methods using chemicals or water.

Common dry methods include sandblasting, flame cleaning , shot blasting, grinding, scabbing, planing, and scouring. Steel-wire brushes should be used with care because they can leave metal particles on the surface that later may rust and stain the concrete.

Wet methods involve the application of water or specific chemicals according to the nature of the stain. The chemical treatment either dissolves the stain so it can be blotted up from the surface, or it bleaches the stain so it will not show.

What is PSI?

PSI is an abbreviation for “Pounds per Square Inch.” It is a unit of measure to specify the strength of the concrete. It is recommended to use a 3000 psi concrete mix for interior slabs. For all exterior slabs, PTL Concrete, Inc. recommends using a 4000 psi concrete mix. This denser and stronger mix is better able to withstand the freeze/thaw cycles of winters here in Colorado, and well worth the comparably small increase in price.