دانلود رایگان مقاله تحلیل رفتار بتن با عملکرد بالا در سالهای اولیه

Abstract

Ultra high performance concretes (UHPCs) are cementitious composite materials with high level of performance characterized by high compressive strength, high tensile strength and superior durability. These are reached by a low water-to-binder ratio, optimized aggregate size distribution, thermal activation, and fiber reinforcement. In the past couple of decades, more and more UHPCs have been developed and found their ways into practice. Thus, the demand for computational models capable of describing and predicting relevant aging phenomena to assist design and planning is increasing. This paper presents the early age experimental characterization as well as the results of subsequent simulations of a typical UHPC matrix. Performed and simulated tests include unconfined compression, splitting (Brazilian), and three-point-bending tests. The computational framework is constructed by coupling a hygro-thermo-chemical (HTC) theory and a comprehensive mesoscale discrete model with formulated aging functions. The HTC component allows taking into account various types of curing conditions with varying temperature and relative humidity and predicting the level of concrete aging. The mechanical component, the Lattice Discrete Particle Model (LDPM), permits the simulation of the failure behavior of concrete at the length scale of major heterogeneities. The aging functions relate the mesoscale LDPM mechanical properties in terms of aging degree, defined in this work as the ratio between the quasi-static elastic modulus at a certain age and its asymptotic value. The obtained results provide insights into UHPC early age mechanisms yielding a computational model for the analysis of aging UHPC structures.

5. Summary and conclusions

In recent years, ultra high performance concretes have shown not only a significant rise in popularity but also practical relevance. Yet, a thorough understanding of the evolution of material properties at early age is still lacking in spite of its significance for structural design. In this paper a comprehensive numerical and experimental investigation of the early age behavior of a typical UHPC is presented. The study is based on a large experimental campaign entailing uniaxial compression tests, tensile splitting tests, and three-point-bending tests at different ages and following different curing protocols, complemented by measurements of the evolution of internal humidity in sealed and unsealed samples. In order to shed light on the evolution of macroscopic material properties, an early age model is formulated within the framework of mesoscale discrete models. The coupled processes of moisture transport, heat transfer, cement hydration, and silica fume reaction are captured by a hygro-thermo-chemical (HTC) model yielding reaction degrees, which are then used to formulate the aging degree describing the local maturity of the UHPC. The local mesoscale material properties of the constitutive model, the Lattice Discrete Particle Model (LDPM), are obtained through rather simple aging functions, formulated in terms of aging degree. Based on the experimental characterization clear trends in the evolution of material properties, namely unconfined compressive strength, tensile splitting strength, flexural strength as well as elastic modulus for the investigated UHPC are noted. These increasing trends are distorted by some of the investigated curing protocols. In particular, the thermal activation in the hot water bath not only accelerates the curing but likely also has adverse affects on the material properties stemming from damage associated with rapid cooling. Additional distortions in the trends may be attributed to shrinkage damage.