Frequently Asked Questions
What makes concrete harden?
When cement, sand, stone, and water are combined, a chemical process called hydration occurs in the cement particles. In hydration, 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 together which gives a good indicator of the strength (water/cement ratio). As these components all bond together, moisture is pushed out and evaporates leaving hard concrete in its place.
What is plastic shrinkage cracking?
Plastic shrinkage cracking (PSC) are cracks which appear on the surface of fresh concrete while it is still in a plastic or non-hardened state. This type of cracking is probably the most common type of cracking. These cracks are typically parallel to each other approximately 1 to 3 feet apart, are shallow, and generally do not run the entire length of the slab. While this type of cracking may be unsightly, PSC rarely impacts the strength or durability of the concrete.
What causes plastic shrinkage cracking?
On simple terms, 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. PSC occurs when the rate of surface moisture evaporation exceeds the rate at which the bleed water can replace it. Weather conditions play a critical factor in PSC and you are more likely to have PSC on days with wind velocity above 5 mph, low humidity, and/or high temperatures.
Ways to minimize plastic shrinkage cracking?
Because PSC is highly related to weather conditions, several precautions can be taken to minimize its occurrence:
- 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 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 does occur 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, including:
- 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
- Increase potential for plastic shrinkage cracking (see Plastic Shrinkage Cracking) and other types of cracking
- Decreased strengths if concrete is exposed to continuous high temperatures without proper curing techniques
Coping with hot weather placement of concrete.
Hot weather placement of concrete requires the finisher to take certain precautions to minimize the impact of the higher temperatures. Such precautions may include:
- 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 any delays
- Take care to schedule the trucks properly and make sure the job site is accessible so trucks can unload and leave faster
- Do the precautions set out in Plastic Shrinkage Cracking, including setting up windbreaks or sunshades and the use of surface evaporation retardant or fibers
What impact does cold weather have on concrete?
Cold weather is defined as any period in which the average daily temperature falls below 40 degrees for three successive days. When concrete is in a plastic state, it will freeze if its temperature falls below 32 degrees. If concrete does freeze, the potential strength can be reduced by half and the durability of the concrete will be severely undermined. In order to avoid these problems, 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 to protect concrete from freezing?
Several precautions can be taken to protect your concrete from freezing, such as:
- When placing concrete in cold temperatures, make sure that you order concrete with heated water and/or aggregates so that the temperature of the concrete delivered 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 at a faster pace. Use concrete with higher percentages of Portland cement
- Pour the concrete at a lower slump to speed up set time
- Use of insulation on the finished concrete is a must to maintain concrete temperature and to protect against freezing
- Use insulated blankets or plastic covered with straw to insulate the concrete
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