One activity that helps engineers, contractors, and project managers minimize cracking in mass concrete is measuring its concrete temperature gradient (or temperature differential). This specification is the difference between the temperature of the core and the outside of a mass concrete element. The higher the concrete temperature gradient, the more the structural integrity and lifespan of a concrete structure will be jeopardized.
According to ACI 301-16, Specifications for Structural Concrete, the maximum concrete temperature gradient should not exceed 35°F (19°C) during curing. In most situations, this approach is highly conservative; in other cases, it may overestimate the allowable temperature gradient.
How does temperature differential cause cracking?
A concrete pour generates a considerable amount of heat (exothermic reaction) that raises the overall temperature of the element. Generally speaking, the higher the cement content, the more heat will be produced.
However, when the outside of the mass concrete is exposed to an environment that is at a lower temperature, it cools while the temperature of its core remains the same. This is aggravated if exposed to extreme weather conditions.
Because the core is at a higher temperature, it expands in volume while, conversely, the outside contracts due to its cooler temperature. As a result, these opposing forces create tension. If that tension exceeds the tensile strength of the
concrete, thermal cracking will occur.
How to measure the concrete temperature gradient?
On construction sites, temperature gradients are typically measured using thermocouples or data loggers. The use of these primitive tools to collect data and subsequently analyze it on a computer takes quite some time, which could negatively affect a project’s schedule.
Fortunately, contractors and engineers can take advantage of advanced technology that uses the maturity method to monitor internal concrete temperatures, which includes gradient analysis where thresholds can be programmed, graphs can be easily exported, and notifications can be set up to alert you when the concrete temperature gradient limit is crossed.
