Contractors that handle municipal projects requiring the mass pouring of concrete have their work cut out for them. From construction delays to compliance issues, there are a lot of things that can go wrong, and that’s a big problem for dam construction.
These structures must be perfectly engineered and can’t afford to sustain any kind of damage, which makes managing the concrete pour effectively a must.
Temperature Management to Avoid Damage
There are specifications that limit maximum internal temperatures and temperature differences between the surface and center of mass concrete pours for a reason. Even a minor deviation can cause damage to the concrete that could threaten structural integrity.
The internal temperature can’t exceed 158 degrees Fahrenheit, and the temperature difference must be less than 35 degrees Fahrenheit for contractors to be in compliance with ACI 301.
There are good reasons for having these regulations in place. Limiting the internal temperature prevents issues with delayed ettringite formation (DEF), which can cause internal expansion, cracking, displacement, and an increased risk of reinforcement corrosion and ongoing freeze/thaw damage. Limiting internal temperatures ensures that ettringite can form normally within the first few hours after the concrete is poured.
Keeping the temperature difference between the surface and center to 35 degrees or less minimizes the risk of thermal cracking by reducing tensile stresses in the poured concrete. The reason this is so important is that when the tensile stresses exceed a concrete structure’s tensile strength, it leads to cracks, the severity of which depends on how significant the thermal gradient was when the concrete began to cure.
Advanced Technology for More Precise Pours
A combination of concrete sensors and advanced computer modeling can be used to help contractors manage temperatures throughout the pour. Modeling the results before the pour makes it easier to choose the right thermal control measures, while embedding sensors allows contractors to track the effectiveness of those techniques and ensure that the project is in compliance with all relevant regulations.
Effective Thermal Control Measures
Managing mass-poured concrete projects at dams requires the use of one or more targeted thermal control measures. They can include the following.
Changing the Concrete Mixture
Limiting the amount of cement in a concrete mixture and supplementing it with Class F fly ash, slag cement, or other supplementary cementitious materials (SCM) can be effective. It’s also important to avoid both Type III or HE cements and chemical accelerators.
Reduce Placement Temperature
Most contractors use placement temperatures of between 50 and 70 degrees Fahrenheit because the maximum concrete temperature is affected in part by placement temperature. As a general rule, for each degree of precooling, the concrete temperature reduces by around one degree.
Pre-cooling can reduce internal temperatures and core-to-surface temperature differences. Methods for pre-cooling can include shading, using chilled mix water or shaved ice, or even injecting it with liquid nitrogen.
Post-cooling involves the use of insulation to control maximum temperature differences by slowing the rate of surface heat dissipation. This helps to prevent the threat of thermal cracking but can cause construction delays.
Advanced Solutions for Complex Problems
Contractors working on dams and other large-scale municipal projects need to make use of all the tools in their arsenals to ensure compliance and reduce the risks of structural damage. Those should include both thermal modeling and the embedding of temperature sensors in the concrete itself.