Concrete temperature monitoring is the practice of recording the temperature inside a concrete element continuously from the moment it is placed, so the team can confirm the concrete is curing within specified limits and act before a problem becomes permanent. It is done with a temperature probe embedded in the concrete before the pour, either a wired thermocouple connected to an external data logger or a fully embedded wireless sensor that transmits to a phone or a hub. On ordinary elements the concern is that the concrete is warm enough to gain strength and cool enough not to lose durability. On mass concrete elements a third concern is added: the temperature difference between the hot core and the cooler surface, which is what causes thermal cracking, and which is why mass pours are monitored at more than one depth.
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What is a Mass Concrete Element?
Mass concrete is categorized as any volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat hydration of the cement and attendant volume changes to minimize cracking. When it comes to large bodies of concrete, one of the most important characteristics is thermal behavior. For mass concrete projects such as buildings, dams, bridges, tunnels, etc., the rate at which the heat is generated is higher than that at which it is released causing the overall temperature to increase over the duration of the curing stage.
Due to the slow release of heat from concrete during this stage, and concrete’s low conductivity, mass concrete can experience gradients in temperature and a portion of heat can become trapped in the center of the mass concrete element, which can cause a slow release of the heat and lead to cracking.
Curious About Determining the Allowable Variation in Concrete Temperature? Learn more here!
What is Mass Concrete Temperature Monitoring?
Mass concrete temperature monitoring is the action of recording and reading the temperature of freshly poured mass concrete elements. This requires a temperature recording sensor (also known as a thermocouple for mass concrete) to be placed near the center of the element.
Why Is It Important to Monitor Mass Concrete Temperature?
In order to ensure the quality and durability of larger projects, it is important to monitor and record the temperature of your concrete. It is also necessary to control the mix temperature, ambient temperature, and differential temperature in mass concrete elements. Temperature differences can cause stress that leads to thermal cracks and loss of structural integrity, thus shortening the life and decreasing the strength of the mass concrete element. If the concrete falls below roughly 40°F (5°C), hydration slows sharply and strength gain can stall until it warms again. When concrete temperatures are properly monitored, appropriate adjustments are able to be made when needed.
Heat generation depends on many factors such as the compound composition, the initial temperature of the concrete, the ambient temperature, the shape of the mass concrete element, the volume-to-surface ratio, as well as other surrounding conditions. Generally, the higher the cement content, the more heat will be produced.
There are a number of ways in which you can regulate the temperature of mass concrete elements during a project starting with an initial cooling of the concrete mixture. This can be done with the use of chilled water, ice, or liquid nitrogen. Temperature regulation can also be made throughout the project by running cool water through cooling pipes installed before the concrete placement. In order to know when it is appropriate to undertake cooling processes, temperatures must be monitored in real-time with a thermocouple for concrete.
Read More About Monitoring Concrete Temperature in Cold Weather Here!
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The Temperature Monitoring Process
Thermocouple versus embedded wireless sensor:
| Wired thermocouple with external logger | Fully embedded wireless sensor | |
|---|---|---|
| How data gets out | Wire runs from the probe to a logger outside the concrete, data is downloaded on site | Sensor transmits from inside the concrete to a phone, tablet or hub, no wires leave the pour |
| Data access | Manual download, then export and analysis, usually by one person on site | Continuous, viewable by anyone on the project with access, no site visit needed |
| Unit cost | Lower per probe | Higher per sensor |
| Total cost | Rises with every site visit, download and manual report | Front loaded, little ongoing labour |
| Site risk | Wires are exposed to placement, finishing and traffic damage, and loggers can be stolen or damaged | Nothing protrudes, nothing to trip over or damage |
| Reuse | Logger is reusable, the probe usually is not | Sensor is sacrificial and stays in the concrete |
| Best suited to | Sites already staffed for manual data collection, short duration pours, tight budgets | Multi-pour schedules, restricted access, remote or long duration monitoring, decisions made off site |
| Main limitation | Data is only as current as the last download | Transmission range depends on cover depth and obstruction, sensor placement must account for it |
The initial process for heat monitoring of concrete mixtures is based on predictions made by taking into account the design of the concrete mix, the shape of the project, the ambient temperature, and more. Once the concrete is in place, sensors for temperature recording concrete and thermocouples are used to report changing temperatures.
Devices in this category range from simple wired thermocouples paired with a handheld or panel logger, through embedded wireless sensors that transmit by Bluetooth to a phone, to long range sensors that report to a site hub or the cloud for remote viewing. Which one is right depends less on the technology than on who needs the data and how quickly: if the person making the strip or cool down decision is not on site every day, remote access stops being a convenience and becomes the requirement.
What Temperature Limits Are You Monitoring Against?
| Limit | Value commonly specified | What it protects against |
|---|---|---|
| Maximum in-place concrete temperature | 160°F (70°C), per ACI 301, section 8.1.3(a) | Delayed ettringite formation and long term expansion cracking |
| Maximum temperature differential, core to surface | 35°F (19°C), per ACI 301, section 8.1.3(a) | Thermal cracking caused by restrained differential contraction |
| Minimum concrete temperature, cold weather | 50°F to 55°F (10°C to 13°C) depending on section thickness | Hydration stalling and early age freezing damage |
| Maximum concrete temperature as placed, hot weather | 95°F (35°C) | Rapid slump loss, higher water demand, reduced 28 day strength |
The governing values for any pour are the ones in that project’s specification and thermal control plan. The table above is what those documents most often say.
SmartRock Wireless Concrete Temperature Sensor
The SmartRock sensor records the in-place temperature history and shares it through the Android and iOS app, where the maturity method (ASTM C1074) is used to estimate in-place strength once a strength-maturity relationship has been established for the mix. Like other temperature measurement and monitoring devices, the sensors are placed on the rebar before the concrete pour and can be used from the moment the concrete is poured to the completion of the project. This type of sensor or thermocouple for mass concrete measure the temperature development at regular intervals to deliver high-accuracy data and enable contractors to make appropriate decisions regarding temperature control during the curing stages of the project. This ensures the best possible results for the project.
Unlike many other sensors on the market, the SmartRock is completely wireless and can be accessed from a smartphone device or tablet. The wireless capabilities allow for more freedom and security by removing the risk of tripping over wires or protruding devices. Contractors also benefit from less destructive testing methods. All of the hardware used as well as the concrete stays intact throughout the data gathering process and there is no need for last-minute repairs or adjustments. There are several additional features as well such as a database of mix calibrations and easy software upgrades.
The advances in concrete temperature monitoring technology are helping contractors and the construction industry move towards more sustainable, durable, and safe environments.
Learn how mass concrete monitoring can help your concrete projects.
Concrete Temperature Monitoring FAQ
Where should you place a temperature sensor in mass concrete?
At minimum, one probe at the thermal centre of the element and one near the nearest exposed surface, at a depth as set out in the project thermal control plan, on the same vertical line, so that the differential between them is a real measurement rather than an estimate. The centre probe finds the peak temperature and the surface probe finds the differential. Large elements usually need several such pairs, and the locations should be set out in the thermal control plan before the pour, not decided on the day.
How long should you monitor concrete temperature?
Until the peak has passed and the element has cooled to within the specified range of the ambient temperature, which for a large mass pour can be one to three weeks. For ordinary elements, monitoring through the curing or cold weather protection period is usually enough, commonly seven days. Stopping at the peak is a common mistake, because much of the thermal cracking risk sits on the cooling side of the curve, not the heating side.
What is the difference between a thermocouple and a concrete sensor?
A thermocouple is a wired temperature probe that must be connected to an external logger or reader to produce data, so a wire has to leave the concrete. A fully embedded concrete sensor puts the probe, the battery and the radio inside the element and transmits the data out, so nothing protrudes. Both measure temperature to comparable accuracy. The practical difference is how the data reaches the person who needs it.
Can temperature monitoring tell you concrete strength?
Not directly. Sensors record temperature, and the maturity method (ASTM C1074) uses that recorded time and temperature history to compute a maturity index, from which in-place strength is estimated using a strength-maturity relationship built in the lab for that specific mix. So temperature monitoring supports a strength estimate, it does not measure strength, and the estimate is only as good as the calibration behind it.
Do you still need cylinder breaks if you monitor temperature?
Yes. Cylinders are still required to establish the strength-maturity relationship in the first place and for specification acceptance testing. What temperature monitoring changes is the schedule decisions in between, such as formwork removal, post-tensioning and shoring, which no longer have to wait on a lab turnaround.
**Editor’s Note: This post was originally published on July 10 2017 and has been updated for accuracy and comprehensiveness.
