Monitoring Concrete: Why It’s Important in Extreme Weather Conditions

Extreme weather conditions are the enemy of every construction site, because weather conditions influence, among other things, the maturing process of the concrete. Summer temperatures accelerate curing, so that formwork can be removed much earlier or traffic clearances issued more quickly. Exactly the opposite occurs in winter. When outside temperatures are low, heating equipment has to be used. However, this creates enormous challenges – especially if the determination of the compressive strength of the concrete takes place via test cylinders, as is so often the case. Because if the curing conditions are not one hundred percent identical, the results are inevitably falsified. Intelligent concrete sensors can be a good solution here to prevent time delays and achieve consistent quality.

The strength development of concrete decreases massively at low outside temperatures. For comparison: At +5 degrees Celsius, the process takes about twice as long as at +20 degrees Celsius. At temperatures of -10 degrees Celsius, there is no strength development at all. 

The concrete only becomes freeze-resistant after its protection period has expired, during which it must be kept at a temperature of at least +10 degrees Celsius for three days. Until then, there is a risk of frost damage, the remedial measures for which will be very complex and expensive.

Therefore, it is very important to have preventive protective measures ready in winter. These include:

  • The use of special concrete compositions containing cements with a higher heat of hydration. The use of superplasticizers or additives such as fly ash should be avoided. In addition, the water/cement ratio should be reduced.
  • A correspondingly high placing temperature of the fresh concrete. Recommended guide values: at air temperatures from +3 degrees Celsius, the concrete should have a temperature of at least +5 degrees Celsius, at even lower outside temperatures even at least +10 degrees Celsius
  • Short transport times and avoidance of long waiting times on the construction site
  • The right choice of formwork: wooden formwork dissipates heat more slowly than steel formwork
  • Covering the concrete with thermal mats or heating tarpaulins, which prevent the hydration heat from escaping, or the use of hot air or heating coils
  • Protection from drafts and rain or condensation water, since wind accelerates the evaporation of water in the fresh concrete and moisture increases the water-cement ratio

Deviations of the general conditions for concrete specimens

Conventional strength testing of concrete involves test cylinders that are subjected to fracture tests at five different stages of maturity.  It is obvious that the first three protective measures are also applied here. But what about the other three measures? They are unlikely to be applied to the specimens, especially if they are stored in the laboratory instead of on the construction site. Conversely, however, this also means that the curing process is different despite identical concrete mixes, which has a massive impact on the progress of curing. This is because the comparatively small samples cannot simulate the actual progress of the concrete on the construction site at all.  The graph below shows how differently the curing process develops depending on the temperature. It includes the temperature and strength of the same concrete under different conditions: As poured at the construction site, as well as the specimens when cured in the laboratory and when cured on site.
The discrepancy becomes even clearer in the concrete strength development, here in the example after a curing period of 1.5 days. The original concrete on the construction site already has a strength of around 3,000 pounds per square inch (psi) or 20.7 MPa. However, the specimen curing on site only achieves a strength of around 1,700 psi or 11.7 MPa. So if you, as the project manager, were to follow the specimens, you would spend a lot more time on the curing process and remove the forms much later than is necessary.

Concrete sensors reflect reality - not assumptions

This is where modern sensors for concrete monitoring come into play. They measure the hydration temperature directly in the concrete of the component and can track the maturing process of your concrete in real time. This gives you unerring predictions about its compressive strength and allows you to make your own decisions about further processing. Rely on real data and act instead of waiting for the redeeming call from the lab.  This will not only save you time, but also costs. This is even more important than usual for concreting work in winter, because the heating measures already generate high material and energy costs. That’s when every day counts. After all, perhaps the desired compressive strength has already been achieved on the construction site, but not yet in the office? This means that you can not only continue building earlier, but also stop the heat-retaining measures sooner.  This graphic shows how likely that is. We have set 3,000 psi or 20 MPa as the target value for the required compressive strength. While this is reached after 72 hours for the test specimens, the concrete sensors from the original casting report the desired compressive strength after only 30 hours. That is a time difference of 42 hours! Precious hours in which you could have long since removed the formwork and moved on instead of spending energy and rental costs on heating equipment.
Ultimately, the use of sensors for concrete monitoring is simply a matter of arithmetic. Do the math yourself:  What are the costs for personnel, rental equipment and energy for just under two days?  That quickly adds up to several hundred euros. By contrast, ConcR’s affordable concrete sensor, the reusable model from the ConcR R series, is available for as little as 45 euros a month, including use of the ConcR portal in the cloud.
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