Temperature Effect on Concrete Creep Modeled by Microprestress-Solidification Theory

Abstract

The previously developed microprestress-solidification theory for concrete creep and shrinkage is generalized for the effect of temperature ͑not exceeding 100°C͒. The solidification model separates the viscoelasticity of the solid constituent, the cement gel, from the chemical aging of material caused by solidification of cement and characterized by the growth of volume fraction of hydration products. This permits considering the viscoelastic constituent as non-aging. The temperature dependence of the rates of creep and of volume growth is characterized by two transformed time variables based on the activation energies of hydration and creep. The concept of microprestress achieves a grand unification of theory in which the long-term aging and all transient hygrothermal effects simply become different consequences of one and the same physical phenomenon. The microprestress, which is independent of the applied load, is initially produced by incompatible volume changes in the microstructure during hydration, and later builds up when changes of moisture content and temperature create a thermodynamic imbalance between the chemical potentials of vapor and adsorbed water in the nanopores of cement gel. As recently shown, this simultaneously captures two basic effects: First, the creep decreases with increasing age at loading after the growth of the volume fraction of hydrated cement has ceased; and, second, the drying creep, i.e., the transient creep increases due to drying ͑Pickett effect͒ which overpowers the effect of steady-state moisture content ͑i.e., less moisture-less creep͒. Now it is demonstrated that the microprestress buildup and relaxation also captures a third effect: The transitional thermal creep, i.e., the transient creep increase due to temperature change. For computations, an efficient ͑exponential-type͒ integration algorithm is developed. Finite element simulations, in which the apparent creep due to microcracking is taken into account separately, are used to identify the constitutive parameters and a satisfactory agreement with typical test data is achieved.

Characterization and Modeling of Pores and Surfaces in Cement Paste

“…Although the rate of creep under different conditions has been modeled extensively (see for example Bazant 2004), little has been published on the possible microstructural changes associated with either irreversible drying shrinkage or creep (Bentur et al 1978). Recently, (Jennings 2008) the debate about porosity has been framed in a slightly different way by the suggestion that the C-S-H gel is granular at the nanoscale (Constantinides and Ulm 2007).…”

“…Some mathematical models of shrinkage and creep, like that of Bazant et al (2004) incorporate aging to account for long term shrinkage and creep over a wide variety of environmental conditions. A few theories of aging have been advanced, and these generally rely on the observation that the degree of polymerization of C-S-H continues to increase over time after it forms, making it stiffer.…”

Spalling tests on embedded cores and slabs: A comparative study

“…The first is thermo-mechanical and is essentially based on the differential dimensional constraints developed during exposure to high temperature [8,9]. When considering this mechanism, the spalling risk is then largely dependent on the thermal gradients in the first centimetres of the concrete where spalling occurs and then on the samples geometry.…”

Modeling Time-Dependent Behavior of Concrete Affected by Alkali Silica Reaction in Variable Environmental Conditions

“…ξ 1 and n α are model parameters. To account for the effect of change in relative humidity and temperature the reduced time concept is used [96], where t r (t) = t 0 ψ(τ)dτ and Kelvin) at time t, R is the universal gas constant and Q v is the activation energy for the creep processes. For typical concrete mixes Q v /R ≈ 5000 K [93].…”

“…In this differential equation, the initial value S 0 at time t = t 0 must be defined and it is assumed to be a model parameter [96]. However, if one assumes, as verified by experiments, that the purely viscous strain is a logarithmic function of time in the case of basic creep, one has S 0 κ 0 t 0 = 1 where t 0 = 1 day can be assumed without loss of generality.…”