Temperature monitoring
Table of Contents
Why is it important to monitor concrete temperature?
Concrete temperature has a direct effect on two very important aspects:
- Strength development process
- Risk of cracks and imperfections
Strength development process
The concrete temperature significantly impacts the concrete’s strength development process. A significant heat increase occurs when cement, water, aggregate, and additives are mixed together. This is due to the exothermic process in the reaction between cement and water (called cement hydration). As the temperature increases, the processing speed also does. The internal temperature development will accelerate until the availability of free cement particles becomes a limitation; at that point, it will start decreasing again. The internal temperature of the concrete will depend on the size and shape of the cast, the type of concrete, mix composition, curing techniques, and the ambient conditions at the job site or precast factory.
It is essential to be aware of the crossover effect. This effect shows that if the concrete cures at high temperatures during the early stages, the final strength may be lower than expected compared to curing it at lower temperatures. For example, you can see in the figure below that the strength of two concrete samples cured under low temperatures and high temperatures have slightly different strength development curves.
The strength development in the concrete cured at high temperatures goes faster at the beginning than in the low curing one. However, this fast strength development does not last in the long term. In contrast, if the same concrete is cured at low temperatures, the strength initially develops slower, but the final strength ends up being higher than the high-temperature curing concrete.
Risk of cracks and imperfections
Temperature also plays a vital role in the risk of cracks and defects. During the cement hydration process, the reaction emits heat along the structure. However, the temperature is usually 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 become 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, ending as a costly affair.
So we have seen that temperature plays a key role in the concrete’s strength development and quality. If this is not managed correctly, you may end up with a lower than expected 28-day strength, a concrete with cracks and imperfections, or a concrete that seems to take forever to cure. It is hard to solve the issues created by the temperature if you are not measuring it in any way.
Benefits of concrete temperature measurementa
Temperature plays a significant role in concrete. While monitoring temperature is a requirement in mass pours, these are not the only ones that can benefit from doing it. Some of the potential benefits of monitoring the internal temperature of concrete are:
- Fulfil the thermal plan requirements in mass pours
- Follow the temperature development in extreme ambient conditions (hot- and cold weather conditions)
- Reduce the risks of thermal cracks and drying shrinkage
- Enable concrete maturity and strength calculations
How can you monitor temperature?
The customers that benefit the most from using the system are general contractors. They are many gains to make, and these can be classified into two broad categories:
Temperature monitoring solutions
In broad terms, there are two main types of solutions in the market. The first is traditional data loggers. This type of devices normally consist of:
- A central unit, where the temperatures are displayed and where a USB cable can be connected to download the data.
- And thermocouples of diverse types and lengths, which will be placed into the concrete to measure the temperature.
The second type is concrete sensors. These vary in shape and functionality but normally consist of:
- A transmitter and/or a sensor, that collect the data and either save it locally or send it to the cloud
- An online platform, here the data sent from the sensors is displayed in graphs and reports.
Installation
The concrete sensors can be broadly classified in:
Wired sensor solutions
Wired sensor solutions, either using thermocouple sensors or custom-made sensors with internal memory, are attached to the rebar and connected to a data logger or transmitter. The sensors are then embedded into the concrete while the data logger or transmitter stays outside the concrete.
Wire-free sensor solutions
Wire-free, these sensors combine the sensor, internal storage, and transmitting electronics in one single device. These are usually attached to the rebar and are fully cast into the concrete.
Reusability
Reusable components
When using wired sensors, the data logger or transmitter stays outside the concrete which means that these can then be reused again and again.
Non-reusable components
When using wire-free sensors, all the components, which are contained in a single device, are embedded in the concrete. Therefore, you lose one device in every monitoring.
The major characteristic setting apart the different solutions is the way that the data is collected. This is important because some products will require you to be close to the sensor to collect the data while others can send it directly to the cloud. This is explained below:
Data collection
When using data loggers, you or someone on the team needs to personally get to every sensor and obtain the data manually.
When using wire-free sensors, to get the data you need to be on-site to connect to each sensor via Bluetooth (distance max. 8 meters).
When using wireless transmitters, the data is available from anywhere, as the measurements are sent to the cloud continuously – with no need to be on-site to collect measurements.
How to choose the best solution for you?
The range of options within temperature monitoring solutions has been expanding rapidly in the past few years. This makes it harder to get a clear overview of what is important and which solution will suit your needs better. In general, we advise to keep these basic recommendations in mind:
- The sensor and data transmitter (might be the same unit) needs to be robust, as it needs to operate in a tough, wet and alkaline environment.
- The embedded part of the sensor should have a low heat capacity and thermal conductivity so it doesn’t affect the temperature locally inside the concrete structure where you measure
- The measurement intervals need to be short enough to monitor any temperature variations
- The sensor needs to be easily and securely mounted before the concrete is poured on top.
- The sensor monitoring needs to be reliable and has the required measurement tolerances.
- You should be able to easily repeat the same method every day with a high degree of certainty
- The system should be affordable, so you are able to use it multiple times and on most of your castings, so you gather data and learnings across your concrete mix designs and cast structures
After you find a solution that satisfies the recommendations above, you may still have plenty of available choices. You can narrow these down by considering:
- What is the aim of my monitoring (documentation, formwork removal, temperature variations in cross-sections, etc.)?
- Where do I want to measure?
- How often do I want temperature readings?
- How do I want my data to be collected (manually vs. wireless vs. remote wireless)?
- How do I want to process my data (Excel, automatic online platform etc.)?
- What is the maximum cost for monitoring that I am willing to pay?
Having a clear answer to these questions will make it a lot easier for you to make the right choice of system. This way you will avoid choosing a system that doesn’t fit your requirements or which might be too big and expensive.
Why choose Maturix?
Maturix is a combined sensor and software solution enabling easy, affordable, and efficient monitoring of concrete curing. A cheap, thin temperature probe (thermocouple) is embedded in the concrete. The thermocouple is connected to a transmitter placed outside the concrete. The transmitter sends data directly to the cloud using long-range technology called Sigfox. The data is stored in the cloud, accessible by all internet-enabled devices. Moreover, it is possible to use the already available temperature readings to calculate concrete maturity and strength.
Having a wireless and remote sensor solution is an efficient solution to keep track of:
- Thermal differences
- Ambient temperatures
- Estimations of compressive strength
Having this information available in real-time enables you to:
- Save project time (as you know when to remove formwork, add the next floor
- Save man hours (automatic documentation, collecting data, improved daily
- Save materials (cement content, avoid waste)
- Avoid concrete failures (thermal cracks)
How many temperature sensors do I need?
The number of sensors required depends on the type of project, structure characteristics, and the requirements you may have. Suppose it is the first time that you are using sensors. In that case, the best idea is to contact the solution provider and send them the structure plans, concreting schedule, and other essential requirements, and they can recommend a certain amount. With more experience working with sensors, it will become easier to estimate the amount you need.
Where should you measure concrete temperature?
The best recommendation is to measure the temperature in the spots where you expect the most extreme temperatures, both cold and warm:
Measuring lowest temperature in the concrete
The spots with the lowest temperature development will have slower hydration and, thereby, slower strength development. This means that these spots are at increased risk of early frost, and even if they reach more or less equal strength at the end (studies do show slight variations ), it will take longer. Therefore, monitoring the temperature and strength development at these spots is a good idea. It is important to remember these spots both in your planning (simulation) and in your control (sensor readings), as you need to make sure that they reach the strength before removing the formwork.
You will often be able to find these spots in:
- Corners or nearby surfaces and edges, where you have a large area exposed to the surroundings (heat conduction).
- Areas with limited or no insulation
- Areas casted against a cold surface (could be an existing concrete structure in the winter time)
- Areas with potential thermal bridges that can easily transfer the internal heat energy to the surroundings.
Measuring highest temperature in the concrete
The largest temperature stress occurs when you have a cold spot (described above) and a warm core temperature. You normally don’t want more than 10-20C differences in a cross-section of your concrete. If you exceed these, the risk of thermal cracks will increase. The cracks also result from much shorter exposure to sudden temperature variations, where stresses can suddenly occur.
The temperature stresses are often highest in connection with:
- Internal temperature development in large structures
- Formwork removals
- Casting against “old” existing concrete
In addition to monitoring to reduce the risk of cracks, you might also take the following aspects into consideration, as you could benefit from additional monitoring to cover aspects like:
- Differences in casting time; parts of the structure might be casted several hours before the other part. The curing of the concrete will therefore start much later
- Control of heating; In winter concreting applications you might have heaters to make sure to avoid early frost and start the internal hydration process (temperature development). Financial and environmental cost of running these heaters under limited insulation will be significant. Managing these might be a good way to save costs.
- In projects with post tensioning jobs, you might have specific spots inside the structure with a special interest in knowing exactly what the temperature and strength is, so you can do the job as fast as possible.
