Hot Weather Concreting Starts Long Before the First Truck Arrives

Every concrete placement begins long before the first truck arrives on site. During hot weather, that planning becomes even more critical. Elevated temperatures, low relative humidity, solar radiation, and wind can dramatically change the behaviour of fresh concrete, increasing the risk of rapid moisture loss, reduced workability, plastic shrinkage cracking, cold joints, and finishing challenges. At the same time, these conditions place additional physical demands on the people responsible for delivering a successful placement.

It bears repeating that we all have a responsibility to watch out for one another. Make sure you and your teams stay hydrated, take breaks, and do what you can to stay cool throughout the pour.

Planning for hot weather concreting isn't just about protecting the concrete; it's about protecting the people placing it as well. The two go hand in hand.

Hot weather concreting is not simply a contractor's concern or a requirement buried within CSA A23.1. It is a shared responsibility that requires owners, designers, suppliers, contractors, testing personnel, and inspectors to understand both the risks and the measures needed to mitigate them. Success depends on planning, communication, and execution before and during the placement.

This article explores the requirements of CSA A23.1:24 for hot weather concreting, explains why those requirements exist, and provides a few practical recommendations to help improve concrete performance.

Planning Begins Before the Forecast – When should we start thinking about hot weather concreting?

The weather forecast is the obvious starting point. If the forecast calls for temperatures approaching 27 °C, there is a realistic possibility that the hot weather concreting requirements of CSA A23.1 will apply during placement. Since none of us can control the weather, the best time to begin planning is before the forecast demands it.

The pre-pour meeting is often the first opportunity for all stakeholders to discuss hot weather concreting. Most of us have sat through meetings where concrete protection is mentioned, a checkbox gets ticked, and the discussion moves on. But how often does that conversation result in a practical, executable plan?

This is the time to get SMART.

• Specific – What factors need to be controlled? What protection methods will be used? Who is responsible for implementing them? Who has the authority to adjust the protection plan?

• Measurable – How will responsibilities be confirmed? What observations or measurements will demonstrate that the protection plan is working?

• Achievable – Nobody should be asked to control the weather. Develop a plan that works with the conditions expected on site.

• Relevant – Does every task contribute to protecting the concrete and supporting a successful placement?

• Time-bound – When will decisions be made if conditions change?

A successful hot weather concreting plan is one that the entire team understands before the first truck arrives.

Understanding the Risk

Once the plan is in place, attention shifts to understanding the environmental conditions on the day of the placement.

The weather forecast provides much of the information needed to evaluate the risk. CSA A23.1 includes a nomograph that estimates the evaporation rate using air temperature, concrete temperature, relative humidity, and wind speed. Your concrete supplier can often provide an expected delivery temperature based on recent production, while the remaining information is readily available from the weather forecast.

If the estimated evaporation rate exceeds 0.5 kg/m² per hour, the concrete is at increased risk of drying out.

Concrete Alberta's Tech Tip #12 provides a copy of the nomograph. Online evaporation calculators are also available, and commercial instruments can calculate the evaporation rate directly in the field using the actual site conditions.

Protecting Fresh Concrete

If the evaporation rate indicates elevated risk, concrete protection must begin as soon as the concrete is deposited into the forms.

High temperatures, low humidity, and increased wind speed all accelerate evaporation. Concrete surfaces that are allowed to dry can fail to develop the intended surface quality, become susceptible to plastic shrinkage cracking, and may be more vulnerable to deterioration throughout their service life.

Early-age drying can have a compounding effect on durability. For structures designed for long service lives, protecting the concrete during its plastic state is one of the most important steps in achieving the intended performance. Once the concrete has dried while still plastic, it is highly unlikely that the intended performance can be fully restored.

Evaporation Retarders

Evaporation retarders are often the first line of defence against rapid moisture loss. Applied as a thin film, they reduce evaporation from the concrete surface while remaining compatible with finishing operations once the carrier water has evaporated.

Several common misconceptions are worth addressing.

• "It's a finishing aid." Evaporation retarders are dispersed primarily in water. If that water is finished into the concrete surface, there is a risk of future deterioration.

• Application matters. Always use the manufacturer's recommended spray nozzle. Large droplets, uneven application, or clogged nozzles reduce effectiveness. Multiple clean nozzles should be available throughout the placement.

• Timing matters. Evaporation retarders may be applied immediately after placement, after screeding, during finishing, or multiple times throughout the placement whenever drying conditions warrant additional protection.

Water Misting

One of my preferred protection methods is water misting.

Using a pressure washer to produce a fine mist upwind of the placement creates two beneficial conditions. First, evaporation cools the surrounding air before it passes over the concrete. Second, the evaporated water increases the relative humidity around the placement. Together, these effects significantly reduce the rate of evaporation while also creating a more comfortable working environment for the crew.

At this point you may be wondering: didn't I just say that finishing water into concrete is a bad idea?

The difference is that properly applied misting should never allow liquid water to accumulate on the concrete surface. If water begins collecting on the surface, misting should stop until any accumulated water evaporates. As long as liquid water is not incorporated into the concrete during finishing, the risk of deleterious effects remains low.

Wind Breaks and Sunshades

Wind breaks and sunshades can also play an important role in reducing evaporation.

Wind is often one of the largest contributors to moisture loss from fresh concrete. Reducing wind exposure can substantially decrease evaporation rates. Likewise, limiting direct solar radiation helps reduce concrete temperature and the associated increase in evaporation.

Like every protection method, however, these measures require planning. Wind direction can change throughout the day, often accompanied by stronger gusts. The angle of the sun also changes continuously, and some placements are simply too large for practical shading. These realities should be considered during the planning stage rather than after placement begins.

Transitioning to Curing

Protection continues until curing begins.

Curing should begin as soon as practical for the specific placement. Once curing controls the temperature and moisture conditions surrounding the concrete, the concrete can continue developing the properties expected by the designer.

It is important to remember that curing cannot undo damage caused by early-age drying. Even effective wet curing cannot restore concrete that was allowed to dry while still in its plastic state.

Successful hot weather concreting is not the result of one product or one decision. It is the result of planning ahead, understanding the risks, selecting appropriate protection methods, and ensuring that every stakeholder understands their role before the first truck arrives.

If your team is looking to strengthen its approach to hot weather concreting, RCJohnson Engineering Ltd. can help develop practical protection plans that move beyond checklists and prepare your team for the realities of field placements.