COMPARISON OF DRYING SHRINKAGE AND DRYING CREEP KINETICS IN CONCRETE

The aim of this paper is to show and compare the time evolution of drying shrinkage and drying creep in concrete from three different perspectives. The first one analyzes the basic equations defined in the most common design codes and prediction models for the description of the long-term behavior of concrete (ACI 209, EC2, Model Code 2010, B3, B4). Next, the evolution of drying creep and shrinkage is examined by processing suitable experimental data available from the database developed at the Northwestern University. Finally, the last point of view investigates the results obtained from the finite element simulations employing the material point approach, in particular, the material model based on the Microprestress-Solidification theory.

A nanoscale perspective on the effects of transverse microprestress on drying creep of nanoporous solids

The Pickett effect describes the excess non-additive strain developed during drying of a nanoporous solid material under creep. One explanation for its origins, developed using micromechanical models, is the progressive relaxation of internally developed microprestress. However, these models have not explicitly considered the effects of this microprestress on nanoscale energy barriers that govern the relative motion and displacement between nanopore walls during deformation. Here, we evaluate the nanoscale effects of transverse microprestresses on the drying creep behaviour of a nanoscale slit pore using coarse-grained molecular dynamics. We find that the underlying energy barrier depends exponentially on the transverse microprestress, which is attributed to changes in the effective viscosity and degree of nanoconfinement of molecules in the water interlayer. Specifically, as the transverse microprestress is relaxed (i.e. its magnitude decreases), the activation energy barrier is reduced, thereby leading to an acceleration of the creep behaviour and a stronger Pickett effect. Based on our simulation results, we introduce a new microprestress-dependent energy term into our existing Arrhenius model, which describes the relative displacement of pore walls as a function of the underlying activation energy barriers. Our findings further verify the existing micromechanical theories for the origin of the Pickett effect and establish a quantitative relationship between the transverse microprestress and the intensity of the Pickett effect.

Effects of Differential Shrinkage in Concrete Bridges

The effects of the external environment significantly affect the distribution of stress in concrete bridges. Diffusion of humidity and temperature fluctuations, as a result of the effects of the external environment, result in the emergence of a very complex state of stress and strain in the elements of concrete bridges and the creation of acceptable or unacceptable cracks. In order to predict these correctly, one must use a creep and shrinkage model that realistically describes the moisture diffusion process, which causes that the shrinkage and drying creep of the bulky parts of the box cross section are greatly delayed compared to the thin parts. This delay cannot be predicted with the classical approach, in which either the shrinkage strain and the creep coefficient (or compliance function) are considered as uniform throughout the cross section, or the thickness effect is simply described by a multiplicative factor on shrinkage strain. With full respect for these phenomena the bridge of a double T section segmental bridge is analyzed.

Transient effects of drying creep in nanoporous solids: understanding the effects of nanoscale energy barriers

The Pickett effect is the phenomenon of creep enhancement during transient drying. It has been observed for many nanoporous solids, including concrete, wood and Kevlar. While the existing micromechanical models can partially explain this effect, they have yet to consider nanoscale dynamic effects of water in nanopores, which are believed to be of paramount importance. Here, we examine how creep deformations in a slit pore are accelerated by the motion of water due to drying forces using coarse-grained molecular dynamics simulations. We find that the drying that drives water flow in the nanopores lowers both the activation energy of pore walls sliding past one another and the apparent viscosity of confined water molecules. This lowering can be captured with an analytical Arrhenius relationship accounting for the role of water flow in overcoming the energy barriers. Notably, we use this model and simulation results to demonstrate that the drying creep strain is not linearly dependent on the applied creep stress at the nanopore level. Our findings establish the scaling relationships that explain how the creep driving force, drying force and fluid properties are related. Thus, we establish the nanoscale origins of the Pickett effect and provide strategies for minimizing the additional displacements arising from this effect.

Numerical Analysis for Drying Creep Behavior of Self-Compacting Concrete

Drying creep of self-compacting concrete (SCC) has two different sources: microcracking and stress-induced shrinkage. Based on theory for pore water and microcracking theory of concrete, the model for drying creep caused by constant stress is derived from rheological equations. The proposed model is coupled with FEM analysis by recursive calculation, so that the creep caused by changing load can be calculated without considering stress history. Verification of the model is conducted through comparison with experimental result and Bazant’s empirical model. The comparing results validate the model for drying creep of SCC.

Effects of Humidity Field on Creep of Reinforced Concrete Beams

Creep, as one of the most important properties of concrete, has important influences on concrete structures. In present domestic researches, studies on humidity diffusion in concrete are relative inadequate; and few models can take comprehensive consideration on humidity factors. According to humidity diffusion theory of concrete, this paper studied the distribution of humidity field within concrete at constant humidity condition by using finite element method. Based on the drying creep of the Model B3, this paper analyzes the effects of humidity field on creep of RC beams.