Civil Engineering ETDs

Tai Fan

Publication Date

7-3-2012

Abstract

Conventionally, mechanical properties of concrete are attained through experiment by leaving microstructural phases interaction in a black box. To fully understand concrete, it is necessary to bridge the gap between microstructure and macro properties. In this dissertation, with several models being given progressively, an innovative homogenization model of concrete is proposed in which concrete is regarded as cement and aggregate particles connected by interfacial transition zone (ITZ). Defined on a representative volume element (RVE), the relationship between microstructure and macro properties is established. The proposed model is validated by experimental results and then applied in the study of concrete serviceability. The concrete homogenization model includes RVE in two scale levels: cement paste RVE in microscale and concrete RVE in mesoscale. Cement paste RVE is composed by microstructural phases (water, unhydrated cement, calcium hydroxide, calcium silicate hydrate, etc.), which are determined by the validated three-dimensional (3D) cement hydration and microstructural development model HYMOSTRUC® or CEMHYD3D. The developed cement paste RVE at different hydration ages is transferred to a finite element method (FEM) model and upscaled by homogenization as inputs for concrete RVE in the mesoscale. Cement paste homogenization model is validated by the experimental study of nanosilica effects on the mechanical properties of cement paste. Concrete RVE can be generated by converting realistic or (re)constructed concrete material image into finite element environment. In this dissertation, cell operation method is presented to (re)construct concrete. The similarity between (re)constructed image and target image is verified by low-order correlation functions. In the discrete model of concrete, each cement paste element or each aggregate is treated as a discrete particle; and these particles are bonded together by equivalent ITZ. To simulate cracking and particle interaction, ITZ is represented by cohesive zone model (CZM) and contact mechanism. This dissertation will demonstrate that the concrete homogenization technique can capture the relationship between structure and material, and enable us to study concrete serviceability in view of microstructure evolution. As the applications of the proposed homogenization model of concrete, the following studies on concrete serviceability are carried out: deflection variation in reinforced concrete (RC) beams propagated from concrete microstructural variability, and mechanical consequences of concrete subjected to alkali-silica reaction (ASR). Due to the inherent uncertainty in concrete microstructure, variation of RC beam deflection is inevitable. For satisfactory use of RC members, it is necessary to incorporate uncertainty of concrete properties in deflection prediction. With the help of homogenization modeling, microstructural variability in concrete is projected to the deflection variation in RC beams. Alkali-silica reaction (ASR) is a kind of chemical reaction in concrete. Alkali in cement meets with reactive silica in aggregate and expanding gels are produced if there is enough water. The swelling of gels will induce stress and alter concrete microstructure. In some cases, this alteration includes cracking and expansion in concrete member. The condition becomes more complicated when the expansion caused by swelling gels is confined by reinforcement and prestress in concrete. Using the proposed homogenization technique, the mechanical consequence of ASR on concrete is simulated. ASR gels expansion is achieved by aggregate volume increase, which causes internal stress and deteriorates ITZ in concrete RVE model. The proposed mechanical model of concrete subjected to ASR is demonstrated on plain concrete specimens (prism and cylinder). The simulated cases are validated by experimental work by others. It is proved that the proposed ASR model by using concrete microstructure homogenization can stand with ASR chemical and diffusional models given by other researchers to predict the serviceability of concrete structure subjected to ASR.

Keywords

Concrete--Mathematical models, Concrete--Testing, Concrete--Microstructure, Concrete--Chemical resistance, Concrete beams--Testing.

Sponsors

The Defence Threat Reduction Agency (DTRA)

Document Type

Dissertation

Language

English

Degree Name

Civil Engineering

Level of Degree

Doctoral

Department Name

Civil Engineering

First Committee Member (Chair)

Taha, Mahmoud Reda

Second Committee Member

Gerstle, Walter

Third Committee Member

Maji, Arup K.

Fourth Committee Member

Shen, Yu-Lin

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