Bringing Multiphysics Simulation to Construction Sites
Time, temperature, material selections, weather conditions, and casting technique can all affect the early-age performance of concrete used to construct buildings. To help its customers make informed decisions about how key variables will affect their projects, Heidelberg Materials provides access to predictive multiphysics modeling capabilities with a compiled simulation application.
By Alan Petrillo
Concrete is a fundamental part of countless structures, from modest homes to soaring bridges and skyscrapers. But for concrete to fulfill its promise of longevity, contractors must make the right decisions during the construction process. Their choices will affect the rate at which concrete hardens, or matures, which helps determine its long-term strength and durability.
Contractors can predict the potential outcome of a concrete casting process by using the maturity method, but it can be difficult to apply this technique in the field. This is why Heidelberg Materials, one of the world's largest suppliers of cement, aggregates, and precast and ready-mixed concrete, provides its customers in Sweden and Norway with the computer program they call HETT. (Ref. 1) They have participated in the development of multiple generations of HETT, but the new version is different: HETT22 is a compiled simulation app that provides timely access to forecasts based on multiphysics models that account for site conditions, ambient temperatures, material selections, and other relevant variables. HETT22 and its associated models were created for Heidelberg Materials by Deflexional, a COMSOL Certified Consultant that specializes in using the COMSOL Multiphysics® software to build multiphysics models and simulation apps (Ref. 2). After using the Application Builder to turn the models into a custom app, Deflexional used COMSOL Compiler™ to deploy it. In just six months after launch, HETT22 was downloaded more than 1100 times.
"HETT22 helps you evaluate options from different perspectives," said Mikael Westerholm, project manager for the HETT program at Heidelberg Materials' Cement Sverige (formerly the company Cementa). By using simulation to predict potential outcomes of the early-age maturity process, contractors can be more confident in the choices they make regarding construction alternatives — before their decisions are effectively set in stone.
Time, Temperature, and the Tradeoffs of Cement Hydration
While multiple factors can affect the chemical processes that control the maturity and strength development of concrete, temperature plays an especially significant role.
"Cement hydration, which is the chemical reaction between cement and water, develops a lot of heat," explains Tom Fredvik, technical manager at Heidelberg Materials' Sement Norge (formerly the company Norcem). "This leads to temperatures rising during the hardening process, and the rate of cement hydration is very temperature-dependent. Higher temperatures lead to faster hydration and strength development."
Rapid hydration is not necessarily desirable. Concrete that cures quickly in hot weather is likely to be weaker than concrete that matures more slowly under cooler conditions. Conversely, below-freezing temperatures can also impair strength development. "It is very important to account for these effects, especially when casting in the wintertime," said Fredvik. "In the worst case, the concrete may suffer permanent frost damage if it freezes before gaining sufficient strength."
Contractors can insulate the formwork and cover free concrete surfaces with insulating materials to mitigate freezing risks, or even add heat from external sources. Such techniques must be applied judiciously to avoid overheating, premature drying, or significantly increasing construction project costs.
Estimating Strength Development with the Maturity Method
Before committing to a thermal management strategy, contractors can use the maturity method to predict the potential outcome of a particular project. "The maturity method [...] has been used for more than 50 years to estimate how temperature affects concrete strength development," said Westerholm. "It is a nondestructive way to predict strength, which can otherwise only be determined by analyzing core samples after the concrete is cast."
The maturity method combines known metrics with site- and project-specific data. The values for the maturity function and the reference strength of a concrete mix can be obtained in advance but the temperature to which the concrete will be exposed must be estimated. This estimated temperature curve should account for ambient temperatures and the internal heat generated by cement hydration. Actual temperature levels will not evolve uniformly throughout the entire volume of a particular concrete casting, which means that strength can develop unevenly as well.
Putting Multiphysics Simulation into the Hands of Contractors
To support wider access to the predictive potential of simulation, Heidelberg Materials engaged Deflexional to create the latest version of HETT. "When the team from Heidelberg explained their goals, we saw a great opportunity to expand HETT's usefulness," said Daniel Ericsson, CEO at Deflexional. HETT22 is the first generation of the program to be developed using the Application Builder in the COMSOL Multiphysics software and compiled using COMSOL Compiler™.
"With HETT22, one of our goals was to be as user-friendly as possible," said Fredvik. "We have also added new features that enable our customers to consider real-world conditions in more detail."
A brief walkthrough of a hypothetical concrete casting project demonstrates HETT22's expanded capabilities (Figure 2). The app user starts by selecting from a list of typical cases representing different construction scenarios and then defines the casting geometry parameters, material mix, strength class of concrete, time frame, and expected weather conditions. The model accounts for how the physical surroundings of a casting may affect its behavior.
"In a situation where concrete is cast on an existing slab, the connection between new and old castings is very critical," said Fredvik. "HETT22 gives us the ability to analyze what is happening around that joint." Other relevant physical attributes that may affect the temperature and strength development of concrete, such as the presence of heating cables or heating/cooling pipes inside a casting, can be incorporated into the model. The geometry of the model and its mesh is shown in Figure 3.
After defining the formwork and geometry, the user can integrate site-specific weather forecasts for the planned casting (Figure 4). Forecasts for the entire world can be automatically downloaded and transformed into appropriate boundary conditions for the model. "Along with selecting a forecast in advance, we can also provide HETT22 with in situ recorded temperatures during the curing period and then make adjustments if measured conditions are significantly different from what was expected," Fredvik said.
Time constraints and desired strength are key influences on a user's choice of materials. "In this example, the strength requirement is set to 15 MPa before we can remove the formwork," said Westerholm, "so we choose a concrete with appropriate strength development from the preinstalled library. A user can also choose supplementary cementitious materials to be mixed with the concrete. These materials may include fly ash, granulated blast furnace slag, and silica fume," he explains.
These materials are by-products from other industries, such as energy production and the processing of iron and ferrosilicon. The use of supplementary materials in cement or as additives to concrete helps lower the overall carbon footprint of concrete construction. "Reducing CO2 emissions is a focus of the concrete industry worldwide," said Westerholm. "But these alternative materials may slow down hydration and have different strength development characteristics. We wanted HETT22 to help users predict the behavior of concrete that they may not be familiar with."
Simulation Results Guide Preemptive Adjustments
With help from the COMSOL models powering the app, HETT22 yields predictive values for determining how long it will take to achieve the desired strength — in this example, 15 MPa — before form removal. "We can track expected temperatures for the surrounding air and soil and forecast a temperature curve for the concrete itself," said Fredvik. (Figure 5) "Based on our temperature development, we can calculate that it will take about 30 hours for the entire casting to reach the strength that we need." If that is too long for this particular job, then HETT22 can show the potential effect of different concrete selections on the estimated time for form removal. Builders can select different options from a menu of Heidelberg Materials' concrete products as well as review details about the performance characteristics of each option directly in the simulation app.
What if actual weather conditions differ from the forecast? In that case, users can adjust temperature values to see how that may affect strength development. "If air and windspeed change significantly, we can potentially remove our formwork sooner than expected," Fredvik said.
Multiphysics Simulation Supports Cost–Benefit Analyses
By predicting the effects of choices related to physical conditions, a construction team can use a compiled simulation app to better manage the economics and carbon footprint of each project. For example, if the weather forecast shows that a casting with a low-carbon concrete will take too long due to cold temperatures, the contractors will be confronted with potential cost–benefit tradeoffs.
"Should you switch to a faster-curing cement or a higher strength class of concrete, even if it is more expensive and potentially results in a higher carbon footprint?" Westerholm questions. "Or is it possible to stick with your initial plan, and instead take steps to insulate or warm up your formwork?"
Heidelberg offers its customers hundreds of potential concrete recipes; from the company's perspective, the simulation app is a necessary complement to a potentially daunting array of options. The predictive modeling capability of the COMSOL Multiphysics software, when presented through the app's custom interface, helps users make informed decisions more efficiently.
"This is why we provide HETT22 to our customers," said Fredvik. "Because of the value it adds for them at each decision point of a concrete casting job, we see it as a core part of our tech support offering."
- "HETT22," Heidelberg Materials; https://www.cement.heidelbergmaterials.se/sv/hett22
- "Applications," Deflexional; https://www.deflexional.com/applications.php