Hot weather concreting
Table of Contents
Why use concrete sensors for hot weather concreting?
High temperatures, low humidity, and a lot of wind can make pouring concrete challenging. However, with good planning and appropriate actions, there is the potential to achieve high-quality concrete and benefit from faster curing times and increased margins. In this article, we will explain what hot weather concreting is, its challenges, and some measures you can take to obtain the best possible concrete.
What is hot weather concreting?
The definition of hot weather concreting by the American Concrete Institute (ACI) is: “One or a combination of the following conditions that tend to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and rate of cement hydration, or otherwise causing detrimental results: high ambient temperature; high concrete temperature; low relative humidity; and high wind speed”. Humiditydity and a lot of wind can make pouring concrete a challenge. However, with good planning and appropriate actions, there is the potential to achieve high-quality concrete and benefit from faster curing times and increased margins. In this article, we will explain what hot weather is concreting, its challenges, and some measures you can take to obtain the best possible concrete.
Benefits of using concrete sensors for hot weather concreting
Challenges with hot weather concreting
High temperatures greatly influence the concrete curing process, so there are many challenges when concreting in hot weather conditions.
1. Reduced concrete strength
The high temperatures found during hot weather concreting accelerate the cement hydration process, which may seem advantageous as the concrete gains strength quickly. However, if the temperatures are too high and the reaction goes too fast, the final 28-day strength will be lower than expected. This can be very frustrating as you may think everything is going according to plan until you get the low compression test result from the 28-day tests.
The graph above shows the strength development of the same concrete mix. One was cured at a temperature of 20 °C while the other was cured at 40 °C. You can see that the 40 °C, indicated with a red line, started gaining strength quickly, but around the 14 days, the strength development almost stopped. On the other hand, the 20 °C one gained strength slower but ended up with higher-end final strength. This phenomenon is called the crossover effect.
2. Increased demand for water
Sometimes to counteract the loss of moisture and improve the workability of the concrete during hot weather, additional water is added to the concrete mix. This might help to balance out the increased water demand, but if overdone, you create a high water/cement ratio. This can result in lower final strength and a porous surface which will affect the overall durability of the concrete.
3. Higher risk of cracks
During hot weather, the risk of cracking increases. The causes of these cracks are:
In many cases, the hot temperatures and high humidity increase water evaporation. If too much water evaporates, the concrete volume shrinks, creating small cracks on the concrete’s surface. These cracks may look small, but they can become channels for water and chemicals to enter the concrete in the long run. This compromises the concrete durability and creates concerns for the contractor.
During the cement hydration process, the reaction emits heat along the structure. However, in most scenarios, the temperature is not the same in different parts of the structure. This is because the heat dissipates more easily from the concrete’s surface due to its location and the cooling effect of the wind than in the concrete’s core. You can see this in the illustration below:
In the illustration, you can see that the temperature is higher at the core, and then it decreases on the sides. This means that while the surface is contracting and hardening due to the lower temperatures, the core is still warm and expanded. If the temperatures are too different, the internal tensions inside the concrete will increase and cause cracks. These cracks vary in size and severity; some will be simply aesthetic, while others may present a structural problem, which can end as a costly affair.
How to succeed in the heat?
The best plan for hot weather concreting is to balance quality, costs, and your available resources.
As weather conditions and curing behaviour are hard to predict, continuous checking and monitoring of the concrete temperature and strength is key to ensure efficient curing and take action when needed. An easy way to observe the concrete curing is using wireless concrete sensors. Those can measure and monitor the curing over the whole project time and give real-time insight into the actual conditions.
In the steps below, we have gathered our tips for each phase of the concrete process:
To succeed under warm conditions, the planning and coordination should start in the initial phases of the concrete manufacturing process. Planning for these can save a lot of time and money later on in the project.
A good starting point is to consider the temperature conditions on-site. Questions such as:
Once you have the answers to these questions, it will be much easier to understand your project’s equipment and personnel requirements.
Get necessary equipment
Hot weather concreting might require additional equipment such as water pumps, different electric generators, lighting equipment (if working before the sun rises or when the sun sets) and other tools. One of the key issues in this condition is that an equipment breakdown or delay may cause huge problems as the concrete will lose fluidity (also known as slump) quickly and harden much faster than under normal conditions. As discussed earlier, this can lead to a reduced quality of the cast when the concrete is not curing effectively.
2. Batching and mixing
After knowing the expected weather conditions on-site, the concrete mix design can help to reduce the associated risks with hot-weather concreting:
Reduce the cement content
When designing a concrete mix for hot weather concreting, it is recommended to reduce the use of cement within a reasonable margin while still satisfying the strength and durability requirements. This will reduce the heat emission, slow down the rate of cement hydration, and minimize the thermal expansion and the risk of cracks. Another benefit of reducing the cement content is lower CO2 emissions for the concrete mix used for a more eco-friendly construction project.
Some admixtures, such as fly ash or GGBS (Ground Granulated Blast-furnace Slag), may be used to delay the setting time and reduce the heat emission from the cement hydration process.
Reduce concrete temperature
One of the most successful techniques in warm weather is to reduce the temperature of the concrete mix by cooling it down. This can be done in multiple ways, and ACI recommends the following techniques:
These techniques have the potential to reduce the concrete’s temperature by 0,5 °C to 11 °C depending on the chosen technique. It is worth noting that the cost of the different techniques presented above differ considerably depending on the required equipment, personnel expertise, project location, etc.
Minimize transport time
It is important to minimize the transportation time as much as possible and reduce the risk of delays to prevent early concrete setting problems. These could range from the concrete mix hardening in the mixing truck to the added difficulty of pumping the concrete due to high rigidity.
3. Placing and finishing
Warm weather conditions may increase the moisture evaporation rate and reduce the setting time. This means that the concrete mix starts to solidify quicker and becomes harder to handle. Therefore, the period to place, finish and cure the concrete will be shorter than under other conditions.
Avoid high temperatures
To avoid the high temperatures that can negatively influence your concrete strength, move the pouring time tolater at night or early in the morning. This way the concrete can cure for some time without the influence of the hot temperatures. For this strategy to work, it is important to coordinate well with the batching plant and the subcontractors.
The main goal during the curing phase is to guarantee that the cement hydration proceeds successfully in order to achieve the maximum strength potential of the concrete mix. The two most important things to avoid are moisture loss and excessive temperatures.
Prevent moisture loss
There are many curing techniques to maintain water content and prevent moisture loss in casted concrete:
These help to preserve the moisture within the concrete and assure that the cement will hydrate fully. Cured concrete has a higher final strength, reduced risk of surface cracking and higher durability than concrete that has not been cured.
Monitor concrete temperature
To be able to check if the measures are effective, continuous checks of the curing and temperature are important. This can be done with data loggers or, an even better option, using wireless concrete sensors. These can be used to measure and monitor the concrete temperature, maturity and strength development continuously. Doing this will allow you to take the required actions to prevent big temperature differentials, surface drying or accelerated rate of hydration. This will minimize the risk of thermal cracking, drying shrinkage and surface flaking all of which can affect the durability and strength of your concrete.
Control your hot weather concreting with Maturix
There are many possible actions to mitigate the negative consequences of concreting in hot weather and sometimes it is hard to know exactly how these measures are performing.
Using a system like Maturix can help you to get the answers that you need. The Maturix system is able to monitor both temperature and concrete strength. Some of the benefits of using the Maturix system are: