Concrete
Frequently Asked Questions

Concrete Frequently Asked Questions

Concrete curiosities.

Curious about concrete? Here are a few questions we often get. If you’ve got a different concrete question, please contact us.
What makes concrete harden?

When cement, sand, stone and water are combined, a chemical process called “hydration” occurs in the cement particles. The cement particles develop finger-like extensions which become interlocked with other nearby cement particles, much like Velcro balls sticking together. These hydrating particles also bond to the sand and stone, increasing strength. The amount of water and cement in the mix determines how close these cement particles are, which is a good indicator of the strength (water/cement ratio). As these components bond together, moisture is pushed out, leaving hard concrete in its place.

What is plastic shrinkage cracking?

The most common type of cracking, plastic shrinkage cracking (PSC) appears on the surface of fresh concrete, while it’s still in a plastic or non-hardened state. These shallow cracks are typically parallel to each other, 1-to-3 feet apart, and generally don’t run the entire length of the slab. While PSC may be unsightly, it rarely impacts the strength or durability of the concrete.

What causes plastic shrinkage cracking?

Plastic shrinkage cracking (PSC) occurs when the top surface layer of the concrete dries out before the lower portions of the plastic concrete. On an ideal concrete slab, the concrete hardens from the bottom to the top, causing the moisture—or bleed water—to rise to the top surface of the concrete. When the rate of surface moisture evaporation exceeds the rate at which the bleed water can replace it, PSC occurs. Weather conditions can play a critical factor in PSC. Days with wind velocity above 5 mph, low humidity and/or high temperatures are more likely to cause PSC.

How can I minimize plastic shrinkage cracking?

Because PSC is highly related to weather conditions, several precautions can be taken to minimize it:

  • Erect a temporary wind break, sunshade or fog spray to keep the surface from drying out too fast.
  • Dampen the subgrade, formwork and reinforcement.
  • After the initial float or trowel, use a spray-on evaporation retardant, such as MasterKure® ER 50 (formerly Confilm), to minimize surface evaporation.
  • Start curing the concrete as soon as possible with a liquid membrane, or cover with wet burlap.
  • Use synthetic fibers to help resist PSC.
  • If PSC occurs during finishing, the finisher may be able to close them by refinishing the surface.
How does hot weather impact concrete placement?

Higher temperatures affect concrete in several ways:

  • Increased water demand, which may lead to higher water/cement ratios and lower strengths.
  • Accelerated slump loss and loss of entrained air.
  • Faster set time.
  • Increased potential for plastic shrinkage cracking and other types of cracking.
  • Decreased strength if concrete is exposed to continuous high temperatures without proper curing.
How do I handle hot-weather concrete placement?
Hot-weather placement requires the finisher to take certain precautions to minimize the impact of higher temperatures:
  • Use a set retarder or order concrete containing some percentage of fly ash to slow set time.
  • Avoid pouring concrete during the hottest times of the day.
  • Make sure to properly cure the concrete.
  • Make sure you have enough manpower and equipment to place the concrete without delays.
  • Schedule trucks properly, and make sure the job site is accessible so trucks can unload and leave quickly. The quicker you get the concrete in place, the more time you have to work with it.
  • See the precautions above to avoid plastic shrinkage cracking, including setting up windbreaks or sunshades and using surface evaporation retardant or fibers.
How does cold weather impact concrete placement?

Cold weather is considered any period in which the average daily temperature falls below 40 degrees for three consecutive days. When concrete is in its plastic state, it will freeze if its temperature falls below 32 degrees. If it does freeze, the potential strength can be reduced by half, and durability will be severely undermined. To avoid this, concrete should be protected from freezing until it attains a minimum compressive strength of 500 psi, which is approximately two days for concrete maintained at 50 degrees. Because temperature adversely affects set time, the colder the temperatures, the longer the time it will take for the concrete to achieve this threshold.

How can I protect concrete from freezing?

Several precautions can be taken:

  • In cold temperatures, be sure to order concrete with heated water and/or aggregates, so the temperature of the delivered concrete is at least 50 degrees.
  • Use an accelerator, such as calcium chloride, or non-calcium chloride if reinforcing steel is used. This will speed up set time, so the concrete achieves 500 psi faster. Use concrete with a higher percentage of portland cement.
  • Pour concrete at a lower slump to speed up set time.
  • Be sure to use insulation on the finished concrete to maintain temperature and protect against freezing.
  • Use insulated blankets, or plastic covered with straw, to insulate the concrete.

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