Estimation and Mitigation of Stresses in Mass Concrete Structures Containing Ground Granulated Blast FurnaceSlag and Fly Ash

Abstract

In large concrete structures, due to concrete’s low thermal conductivity, the interior temperature rise can approach the concrete’s adiabatic temperature rise. As the concrete’s surface losses heat to the environment and cools rapidly, the interior of the concrete attempts to expand while the exterior provides an internal restraint. The phenomenon causes thermal stresses at the concrete’s surface. If the thermal stresses exceed the concrete’s tensile strength, there is a high probability of cracking. The cracks usually appear during the first few days after construction while the concrete’s tensile strength is low. These early-age cracks can reduce the service life of the structure by allowing the entry of detrimental chemicals such as deicer chemicals and chloride salts that corrode the concrete’s steel reinforcement. The risk of early-age cracking can be alleviated by lessening the temperature gradients inside the concrete structure. This can be accomplished by reducing the cement quantity in the mix design, reducing the placement temperature, adding supplementary cementitious materials, or using insulation blankets. Structures that are at risk of experiencing thermal cracks are usually termed as “mass concrete”. However, the definition of mass concrete is non-uniform and varies from state to state. Thus, a uniform definition is needed. In this study, a methodology to estimate the early-age tensile stresses was developed and used to create definition tables for three common concrete pier stems. The mechanical and thermal properties of two thermally friendly “Class M” mix designs with supplemental cementitious materials (using either 50% replacement of the cement with ground granulated blast furnace slag (GGBFS) or 30% replacement of the cement with Class F fly ash) were experimentally measured and incorporated into a finite-element analysis. These properties include heat of hydration, adiabatic temperature rise, activation energy, thermal conductivity, static elastic modulus, compressive and tensile strength, creep, and thermal expansion coefficient. A methodology to incorporate the time-dependent material properties of concrete containing GGBFS and fly ash was developed, and a viscoelastic analysis was used to estimate the early-age thermal stresses. A two-term exponential degree of hydration was proposed to better capture the hydration behavior of cement binders with GGBFS and Class F fly ash. The heat rate was derived for the two-term degree of hydration function and incorporated into the finite-element model. The derived heat rate was found to model the hydration of blended binders better than those found in literature. The effect of creep was considered in a viscoelastic analysis