Creep and Shrinkage of High-Performance Fiber-Reinforced Cementitious Composites

A class of high-performance fiber-reinforced cementitious composites (HPFRCC), referred to as engineered cementitious composites (ECC), was studied for its time-dependent properties. The material exhibits a pseudo strain-hardening response with multiple fine cracking in uniaxial tension. A series of experiments on ECC specimens, as well as similar specimens without fibers, was conducted to provide information about shrinkage, basic creep, drying creep, and creep recovery of the material. Comparisons with established predictive models for creep and shrinkage of concrete were made. It was found that the ECC material developed greater creep strains than a similar cementitious mixture without fiber reinforcement. Surface cracking was observed to effect shrinkage strain measurements and estimates of material shrinkage behavior. Existing predictive models, while not developed for such materials, can give a reasonable estimate of creep and shrinkage behavior.

Keywords: cement paste; composites; creep; mortar; shrinkage.

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INTRODUCTION

The high-performance fiber-reinforced cementitious composite (HPFRCC) investigated in this paper is one that was designed to exhibit a pseudo strain-hardening response and multiple fine cracking in uniaxial tension. The material investigated herein is also referred to as engineered cementitious composites (ECC). The mixture design investigated is composed of cement, silica fume, water, fine sand, and roughly 2% by volume of short, randomly distributed polymeric fibers. Typical mixture proportions are given in Table 1. One potential application for ECC that has been investigated by the authors is for use in hinge regions of segmentally precast, post-tensioned concrete bridge piers.1,2 In this application, the ECC segments would be subjected to considerable long-term compressive loads. Large creep strains are likely to occur due to the post-tensioning and because the ECC contains no coarse aggregate. Furthermore, a 1.5 to 2% by volume addition of fibers having no chemical bond with the matrix material may change the time-dependent response of the material. At the outset of this research, it was unclear if the fibers would create paths for easier flow of water or if they would control microcracking within the matrix to reduce the flow of water through the composite. Therefore, a testing program to investigate the creep and shrinkage response was developed. In addition, an ability to predict the long-term response of ECC is needed for implementation of this material in structural applications, particularly in prestressed concrete applications where sustained loads can significantly influence global and local structural response.

RESEARCH SIGNIFICANCE

ECC materials are being studied for several structural-scale applications such as in low-rise shearwalls,3 damping devices in frame structures,4 hinge regions of post-tensioned bridge piers,1,2 and various frame systems for seismic resistance.5,6 For many of these applications, and in particular for applications involving prestressing, it is anticipated that large creep strains may develop in ECC materials because they contain no coarse aggregate. The time-dependent behavior of ECC materials has been studied very little and creep experiments on other fiber-reinforced materials have led to conflicting results. Furthermore, currently accepted (for example, codified) creep and shrinkage models have not been calibrated for cementitious materials with or without fibers that do not contain coarse aggregate. The experiments reported herein provide needed information to estimate time-dependent response of these ECC materials for potential application to structures as well as to assess the applicability of currently accepted concrete creep and shrinkage models to ECC materials.

BACKGROUND

Creep and shrinkage of fiber-reinforced concrete is a topic of considerable current research and is not yet fully understood. Experimental studies on fiber-reinforced concrete have reported conflicting findings on the effect of the fibers on creep and shrinkage. For example, Balaguru and Ramakrishnan7 and Houde et al.8 concluded that the creep strains of concrete specimens reinforced with small amounts of steel or polypropylene fibers were consistently higher than strains measured in specimens without fibers. On the other hand, Mangat and Azari9 and Chern and Young10 reported that steel fibers are effective in reducing both creep and shrinkage in concrete. Mangat and Azari explained this result with a model in which the steel fibers aligned with the applied load act as compressive reinforcement for a cylinder of the idealized surrounding matrix.11 Zhang and Li12 used a similar model to make predictions for long-term shrinkage of ECC. No information on the experimental behavior on long-term shrinkage of ECC was provided in that study.

In the ECC materials studied herein, the fibers are considerably finer than steel fibers and traditional polymeric fibers used in fiber-reinforced concrete. The fibers have a diameter between 8 and 38 µm. This size lies between typical capillary pore sizes (0.1 µm and entrapped air [1 mm] and is on the order of the aggregated C-S-H particles (1 to 5 µm).13 As such, the long-term response of ECC is expected to behave differently than traditional fiber-reinforced cement-based composites under compressive loads. For example, Mangat and Azari's model assumed the mortar surrounding the steel fiber to be a uniform, homogeneous, and isotropic material.11 This assumption would not hold at the roughly 10 µm length scale diameter of the polymeric fibers used in ECC.