Pouring concrete during hot weather presents significant challenges that can compromise the quality, strength, and durability of the finished slab or structure. When ambient temperatures rise above 85 degrees Fahrenheit, the chemical reactions that cause concrete to set and gain strength accelerate dramatically, reducing the available working time and increasing the risk of cracking, reduced strength, and surface defects. Understanding the effects of high temperatures on concrete and implementing appropriate measures to mitigate these effects is essential for achieving quality results in hot weather concreting. A thorough understanding of concrete repair and placement techniques provides context for understanding the importance of proper temperature control during concrete placement.
How Heat Affects Concrete Performance
The setting and curing of concrete is a chemical process called hydration, in which cement particles react with water to form calcium silicate hydrate, the binder that gives concrete its strength. This chemical reaction is temperature-dependent, with higher temperatures accelerating the reaction rate. While this might seem beneficial, the accelerated reaction causes several problems. The concrete sets too quickly, reducing the time available for placing, consolidating, and finishing the concrete. Rapid setting can also prevent proper consolidation, leaving voids and weak spots in the finished concrete.
High concrete temperatures also increase the rate of water evaporation from the surface. When water evaporates faster than it can be replaced by bleed water rising from within the concrete, the surface dries out and shrinks while the underlying concrete is still plastic. This differential shrinkage causes plastic shrinkage cracks, which are shallow cracks that form on the concrete surface within the first few hours after placement. These cracks are unsightly and can allow water and deicing chemicals to penetrate the surface, leading to more serious deterioration over time.
The long-term strength of concrete is also affected by high placement temperatures. While concrete that sets at higher temperatures initially gains strength faster, its ultimate 28-day strength is typically lower than concrete placed at moderate temperatures. Studies have shown that concrete placed at 100 degrees Fahrenheit can have a 28-day strength that is 10 to 20 percent lower than identical concrete placed at 70 degrees Fahrenheit. This strength reduction occurs because the rapid initial hydration forms a less dense and less uniform crystalline structure, reducing the overall strength of the hardened concrete.
| Temperature | Set Time | Workability Window | 28-Day Strength Impact | Cracking Risk |
|---|---|---|---|---|
| 50-70 degrees F | Normal (4-6 hours) | 90-120 minutes | Baseline | Low |
| 70-85 degrees F | Accelerated (3-4 hours) | 60-90 minutes | 0-5% reduction | Moderate |
| 85-95 degrees F | Fast (2-3 hours) | 30-60 minutes | 5-15% reduction | High |
| Above 95 degrees F | Very fast (1-2 hours) | 15-30 minutes | 10-20% reduction | Very high |
ACI Guidelines for Hot Weather Concreting
The American Concrete Institute provides comprehensive guidelines for hot weather concreting in ACI 305, Guide to Hot Weather Concreting. The standard defines hot weather as any combination of high ambient temperature, low relative humidity, solar radiation, and wind speed that impairs the quality of fresh or hardened concrete. The ACI guidelines recommend that concrete temperature at the time of placement not exceed 95 degrees Fahrenheit, and for critical applications such as structural concrete in bridges or high-rise buildings, the maximum placement temperature should be limited to 85 degrees Fahrenheit.
To maintain concrete temperatures within acceptable limits, the ACI recommends several preventive measures. Aggregate stockpiles should be shaded from direct sunlight and sprayed with water to cool them through evaporative cooling. Mixing water can be chilled using ice or liquid nitrogen, with each 1 percent of ice replacing mixing water reducing the concrete temperature by approximately 1 degree Fahrenheit. Cement temperature should be monitored, as cement delivered from the plant can be as hot as 200 degrees Fahrenheit. Using a retarder or set-retarding admixture can extend the working time of concrete placed in hot conditions.
The timing of concrete placement is another important consideration. In hot weather, concrete should be placed early in the morning or late in the evening when ambient temperatures are lower. Nighttime placement is common for large concrete pours in hot climates, as the cooler temperatures and reduced solar radiation provide more favorable conditions for placement and curing. The concrete supplier should be notified in advance of hot weather placements so they can adjust the mix design and delivery schedule to ensure that the concrete arrives at the jobsite at the proper temperature and within the required workability window.
Site Preparation and Placement Techniques
Proper site preparation is essential before placing concrete in hot weather. The forms, reinforcement, and subgrade should be cooled by wetting them with water before the concrete is placed. Dry subgrade materials will absorb water from the concrete, reducing the water available for hydration and accelerating surface drying. The subgrade should be moistened but not saturated, as standing water can dilute the concrete at the interface. Forms and reinforcement should be dampened to prevent them from drawing moisture from the concrete and to reduce the temperature differential between the concrete and the formwork.
The concrete placement process should be planned and executed efficiently to minimize delays. The concrete crew should be large enough to handle the pour within the available working time, and all tools and equipment should be ready before the concrete arrives. The concrete should be placed as close to its final position as possible to minimize the amount of manipulation required. Vibrators should be used to consolidate the concrete thoroughly, as proper consolidation is more difficult to achieve in fast-setting hot-weather concrete.
Finishing operations must be timed carefully in hot weather. The finisher should begin finishing as soon as the concrete has stiffened enough to support the weight of the finisher and equipment, but before the surface has dried out. If the surface begins to dry before finishing is complete, a light fog spray can be used to wet the surface temporarily. However, adding water to the surface should be done carefully, as excessive water can weaken the surface and cause dusting or scaling. Evaporation retarders, which are liquid products applied to the concrete surface to form a temporary barrier that reduces evaporation, can be used to extend the finishing window in hot weather conditions.
Curing Concrete in Hot Weather
Proper curing is the most critical factor in achieving durable concrete in hot weather. Curing maintains the concrete in a moist condition and at a favorable temperature so that hydration can continue and the concrete can gain strength. Without proper curing, the water needed for hydration evaporates from the surface, leaving the concrete with low strength, high permeability, and reduced durability. The ACI recommends that curing begin immediately after finishing is complete and continue for a minimum of 7 days for most applications.
Several curing methods are suitable for hot weather conditions. Water curing, which involves keeping the concrete surface continuously wet by ponding, spraying, or covering with wet burlap, is the most effective method but requires the most attention. The surface must be kept continuously wet, as allowing it to dry and then re-wetting causes thermal shock that can crack the concrete. Curing compounds, which are liquid membranes applied to the concrete surface that seal in moisture, are a practical alternative for large slabs and flatwork. The curing compound should be applied as soon as the surface is hard enough to walk on, typically within 2 to 4 hours after finishing.
Evaporative cooling during curing can help maintain favorable concrete temperatures. In extreme heat, covering the concrete with light-colored reflective sheeting can reduce surface temperatures by reflecting solar radiation. Wet burlap covered with plastic sheeting provides both moisture and temperature control, as the evaporation from the burlap cools the concrete surface. For mass concrete elements such as thick foundations and retaining walls, cooling pipes embedded in the concrete can be used to circulate cool water through the structure, removing heat generated by the hydration process and preventing thermal cracking. Understanding cooling techniques for mass concrete in hot conditions provides additional guidance for managing temperature in large-scale concrete placements during hot weather.