These chemical reactions result in cement particles joining together, creating a stronger, more cohesive substance. While the concrete will harden within a relatively short time after the initial pour, the hydration process will continue for weeks, even months thereafter.
However, the most dangerous aspect of pouring concrete in a cold environment is when the water in the cement mixture freezes and expands. In the worst case scenario, it can eventually cause building foundations to crack under the weight of the superstructure, resulting in total collapse.
Over time, the layers of concrete may shift, causing severe structural damage to any building on top. They may use coarser (larger) aggregate particles in the concrete mix, which will minimize the risk of shrinkage from the heat.
Using heaters or heated mats to thaw frozen ground Mixing in hot water with the cement Using extra cement to make the chemical reactions hotter, and speed up hydration Using additives designed to quicken the hardening process Using heated concrete mixes or special blends designed for use in cold weather Using chemical accelerators to speed up the curing process Keeping the concrete's molds or framework in place until the hardening process is completed (early removal will lead to the concrete's collapse) Once the concrete has been set, it's important for it to remain at a temperature above 50 degrees Fahrenheit for at least the next two days in order to finalize the curing process.
It is vital that such an enclosure has enough ventilation for the space heater inside, to avoid accidental carbonation of the concrete's surface. One wrong move or incorrect calculation could result in a weaker final product, extra work in the short term, and potentially dangerous structural consequences in the long run.
At Empire Parking Lot Services, our concrete professionals have years of training and experience under their belts, and know how to successfully complete a concrete pour no matter how hot or cold it may be. Furthermore, our workers have the required skills to perform repairs on concrete parking lots, sidewalks, and other surfaces as needed.
The necessary chemical reactions that set and strengthen concrete slow significantly below 50 °F and are almost non-existent below 40 °F. However, if the correct measures are taken, concrete can still be successfully placed during even the coldest months of the year.
Before embarking on a cold weather concrete project, it’s important to determine any special strength requirements or considerations. This will help as you schedule your pouring and determine which strategies you will use to keep your surroundings and materials warm.
You might take the following suggestions into consideration as you plan your upcoming project: Use extra cement (typically 100 lb/ cubic yard) to make the reaction hotter and cause concrete to hydrate more rapidly.
Bleeding starts later and takes longer during cold weather ; you can use squeegees or a vacuum to remove water quickly. Wait until concrete has reached desired strength to remove any framework.
If the framework is removed too early, the concrete will be damaged and the surface could collapse. After implementing the above suggestions, It’s important to consider how you will keep concrete at the correct temperature during the curing process.
Concrete must maintain a temperature above 50 °F for approximately 48 hours for the correct chemical reactions to take place. Two popular options used during cold weather concrete curing are heated enclosures and insulated blankets.
Additionally, ensure that there is proper ventilation for the space heater. There are some variables when it comes to the best time of year to mix and pour cement for a concrete patio, path, or other outdoor project.
The Midwest tends to experience cold and snowy winters, short springs, hot and humid summers, and cool autumns. However, due to this short window before frost arrives, it might be difficult to book a professional for your project, especially if you don't give them much notice.
In the Southwest, the mild climate along the coast differs greatly from the inland deserts that have more extremes in temperature. Unlike the rest of the U.S., you can pour cement in these states pretty much year-round, except for during periods of extreme heat.
Late spring, summer, and early fall all can be a good time to pour cement, though this will depend on the specific temperatures of where you're at in the region. Late spring and early fall are typically the best times to pour cement, as the weather generally is mild.
With winter winding down and spring and summer arriving shortly, that means outdoor projects will soon be started as well. This is actually not true, as the best possible conditions for pouring concrete come in spring when the temperature is consistently in the 50s and the skies are cloudy.
This may seem odd, but if you were to pour concrete on a day when it is hot or even cold for that matter it can end up being bad for the concrete and its lifespan. The wind can also play a factor because if it is too high on the day you’re pouring, it can cause too much water to evaporate from the concrete too quickly, leading to shrinkage cracks.
Using and placing concrete during the hot summer months present far different challenges than use and placement during cold weather. The summer month effects of temperature, wind, and air humidity can all have a negative impact on the performance of concrete.
This occurs when a concrete slab or wall is placed on a very hot day and which is immediately followed by a cool night. High temperature also accelerates cement hydration and contributes to the potential for cracking in massive concrete structures.
Other hot weather problems include increased water demand, which raises the water-cement ratio and yield lower potential strength, accelerated slump loss that can cause loss of entrained air, fast setting times requiring more rapid finishing or just lost productivity Work with the concrete producer on the mix design to ensure they deliver a product with the correct slump, strength, and performance properties to meet the job requirements.
Schedule cement mixer trucks to avoid waiting time so the concrete will not begin to set. Consider modifying the concrete mixture to include set retarders and water reducers, and the lowest practical cement factor.
Cover placed slabs on grade with a damp bed of sand rather than polyethylene sheeting if a vapor barrier is required Delay or extend setting times by using retarding admixtures. Protect exposed concrete surfaces from drying out by setting up wind breaks, sunshades, water misting equipment or fog sprays while using approved curing membranes.
Consider adding white pigment to membrane curing compounds to reflect heat away from the concrete surface. It must be ensured through proper surface preparation, material choice and use, and curing.
All damaged, loosened, or unbounded portions of existing concrete should be removed by chipping hammers or other mechanical methods. This removes any distance, soft mortar, dirt, wood chips, form oil, or other foreign materials that may interfere with proper bonding of the new concrete.
2) recommends that all equipment supplying compressed air be equipped with efficient oil and water traps to prevent surface contamination from the compressed air supply. Patches are easier to make and more successful if they are made as soon as practical, preferably when the concrete is still green.
This helps define the scope of work for the laborers and provides a right angle surface cut. The feathered edges of the top drawing will break down under traffic or will weather off.
The chipped area should be at least 34 inch deep (see bottom drawing) with the edges at right angles or undercut to the surface. Workers can prepare the surface of construction joints during the first concrete placement.
For horizontal construction joints, the top surface of fresh concrete can be roughened while still plastic. Green concrete, 12 to 24 hours old, can be easily cut or wire brushed to create a roughened surface.
Sometimes workers sprinkle or mix retarders into the top layer of concrete. For vertical construction joints placed against bulkheads, the concrete surface is generally too smooth to permit proper bonding.
If the slab is being repaired with a bonded overlay, then techniques developed for the pavement industry are typically appropriate. Slabs on grade and pavements now can be cleaned fast with high production, self-propelled cold milling equipment and improved blasting techniques.
The type of coarse aggregate in the existing pavement usually dictates the least costly way to prepare a surface. The US Army Corps of Engineers requires removal of at least 1/4 inch from the surface by scarification followed by high-pressure water flushing and air blowing.
The Portland Cement Association (PCA) recommends that the surface be scarified to remove unsound concrete and cleaned by sandblasting or other means. Epoxy resin grouts specially formulated for each application also are on the market.
ACI 503 details the use and specification of epoxies for bonding fresh to hardened concrete. Some project specifications permit a water-cement grout with a water cement ratio of 0.62.
The proportioning of the resin and hardener is extremely important; they must be mixed thoroughly to produce a homogenous mixture. Epoxy resins can be formulated for different temperatures and for dry or damp surfaces.
After preparing the surface, the contractor need only decide if the concrete should be dry or damp before booming or brushing the bonding medium into place. Most agencies recommend a damp surface free of water, especially in hot, windy weather.
From below, porous aggregates or concrete can absorb enough water to prevent complete hydration. This produces a weak bond interface or the porous surface can absorb enough epoxy to starve the glue line.
Differential shrinkage, thermal movements, or moisture gradients can cause enough stress to break the bond during the curing period. Hutchinson, R. L., “Resurfacing with Portland Cement Concrete,” Chip 99, Transportation Research Board, December 1982.
Osaka, S. and W. Please, “Design and Control of Concrete Mixtures,” 14th Edition, Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077.