Will Hansen

Pullout behavior of steel fibers from cement-based composites

Abstract A comprehensive experimental program on pullout tests of steel fibers from cement based matrices is described. A specially designed single fiber pullout apparatus was used to provide a quantitative determination of interfacial properties that are relevant to toughening brittle materials through fiber reinforcement. The parameters investigated included a specially designed high strength cement based matrix called Densified Small Particles system (DSP), a conventional mortar matrix, fiber embeddment length, and the fiber volume fraction. The mediums from which the fiber was pulled included a control mortar mix without fibers, a mortar mix with 3, and 6 percent fibers by volume. The results indicate that: (1) The dense DSP matrix has significantly improved interfacial properties as compared to the conventional mortar matrix. (2) Increasing the fiber embeddment length and the fiber volume fraction in the cement matrix increase the peak pullout load and the pullout work. (3) The major bond mechanism in both systems is frictional sliding.

CONCRETE HYDRATION AND MECHANICAL PROPERTIES UNDER NONISOTHERMAL CONDITIONS

This paper presents the development of an analytical procedure to determine the heat and degree of hydration due to nonisothermal temperature history using a 3-parameter model. The procedure can also be applied to concrete mechanical properties if they are strongly related to the degree of hydration. To demonstrate this, experiments were conducted to obtain the heat of hydration, chemically bound water, compressive strength, split tensile strength, and Youngs modulus of concrete. Experimental results in the literature were also used. It was found that the measured degree of hydration relates uniquely to measured concrete properties. The analytical procedure was then extended to determine a generalized hydration or aging time. Results from the literature were used to confirm the predictive capability of the proposed aging relation. The proposed maturity relation was found to relate uniquely to mechanical properties such as compressive strength and fracture energy.

EFFECT OF NONLINEAR TEMPERATURE GRADIENT ON CURLING STRESS IN CONCRETE PAVEMENTS

Temperature and moisture gradients can lead to significant tensile stresses at the slab top and bottom. Current techniques for assessing the internal stresses due to such gradients are based on the assumption that temperature and moisture distributions through the slab thickness are linear. However, the actual distributions of such gradients have been found to be highly nonlinear. A new closed form solution technique for calculating the stresses in a pavement slab due to nonlinear gradients is introduced. The analysis is separated into two parts. In the first, an expression is presented for calculating the self-equilibrated stresses within a cross section due to internal restraint (i.e., satisfying equilibrium conditions and continuity of the strain field within the cross section). These stresses are independent of slab dimensions and boundary conditions. In the second, the stresses due to external restraint (i.e., self-weight and subgrade reaction) are calculated using an equivalent linear temperature gradient obtained from the first part and existing closed form solutions by Westergaard or Bradbury. The solution to this step includes slab length and boundary conditions. Total internal stresses due to nonlinear gradients are obtained by using the superposition principle. The methodology has been applied to field data from two studies in which the temperature profiles were recorded throughout a 24-hr period. Linear gradient solution methods cannot accurately predict the curling stresses in concrete pavements. This is especially pronounced during nighttime and early morning hours, during which nonlinear analysis predicts tensile stress in both the slab bottom and top before the application of any traffic loading.

Volume Relationships for C-S-H Formation Based on Hydration Stoichiometries

Volume changes can be calculated from the hydration stoichiometry of C 3 S if the composition of C-S-H is taken as C 1.7 SH 4.0 with a density of 1.85 g/cm 3 . The results are in general agreement with the volume changes determined by Powers. However, the calculated evaporable water content is higher and the space-limiting water-cement ratio is calculated to be 0.42. The calculations can be applied also to the pozzolanic reaction and predict a marked increase in solid volume. In that case the composition of C-S-H is modified to C 1.5 SH 3.8 .

MICROMECHANICAL MODELING OF CRACK-AGGREGATE INTERACTION IN CONCRETE MATERIALS

Abstract A new condition for crack penetration into the aggregate phase in concrete materials is developed based on numerical simulations. The numerical simulation utilizes a newly developed micromechanical model which considers the concrete internal structure as a three-phase material, viz. matrix, aggregate and interfaces between them. The micromechanical model is capable of capturing the entire load-deformation response of a concrete specimen under monotonic loading including softening. The new condition for crack penetration is developed based on a simple specimen configuration where a crack is driven towards an aggregate particle. Results from numerical simulations are implemented to relate the relative properties of both aggregate and matrix phases (represented by the characteristic length ratio) to their tensile strength ratio. It is shown that the tensile strength ratio between the aggregate and the matrix plays the dominant role in determining the penetration condition. Predictions based on this condition agree with direct tensile simulations using different specimen configuration.

Micromechanical Modeling of Concrete Response under Static Loading—Part 1: Model Development and Validation

A new micromechanical model has been developed that considers tensile cracking as the only fracture criterion at the microlevel. The model utilizes the finite element method as a numerical tool where the truss element is used as the basic element in the finite element mesh. The internal structure of concrete is considered as a three-phase material: matrix, aggregate, and interfaces between the matrix and the aggregate phases. The model considers the randomness of the aggregate phase, as well as the probabilistic nature of the properties of the three phases. The constitutive relations of the elements for all phases at the microlevel are described according to the smeared cracking concept that is based on the fictitious crack model. The proposed model is able to predict the complete mechanical response of concrete materials under monotonic loading: the crack patterns associated with different loading stages, localization of deformation, and the effect of size on nominal strength. Model predictions are in agreement with documented experimental work.

Ultimate Drying Shrinkage of Concrete--Influence of Major Parameters

The effect of aggregate content and elastic modulus on ultimate drying shrinkage of concrete was investigated. A shrinkage model developed by one of the authors that includes these parameters was tested. The model showed excellent agreement with experimental results. Further, the effect of water-cement (w/c) ratio, curing time, and aggregate content on the drying shrinkage versus relative humidity curves in the relative humidity range of 0 to 100 percent was investigated for miniature paste and mortar specimens. For these specimens the relative humidity correction factor was found to be independent of w/c ratio, curing time, and aggregate content in the relative humidity range of 11 to 100 percent and was best described by two linear relationships. The two relationships for the relative humidity correction factor in the relative humidity range of 11 to 50 percent and 50 to 100 percent was characterized by different slopes. Using the relative humidity correction factor and the shrinkage model, ultimate shrinkage of concrete can be estimated at any relative humidity between 11 and 100 percent.

TENSILE STRAIN HARDENING AND MULTIPLE CRACKING IN HIGH-PERFORMANCE CEMENT-BASED COMPOSITES CONTAINING DISCONTINUOUS FIBERS

From experiments using fiber reinforced densified small particles containing high volume fraction of fine and short steel fibers, it was found that the formation of multiple cracking which occurs between the point of first cracking and the peak load, was the main cause for the improvement in total strain capacity. This region, which is the inelastic strain region due to microcracking, is a unique property of high-performance fiber reinforced concrete (FRC). Multiple microcracking is a property of the bulk material, since no strain localization occurs. The details of the study are described, in which an analytical approach, based on the energy changes associated with cracking, was used in expanding the occurrence of multiple microcracking. The analysis predicts that the major energy term determining cracking behavior is the fiber debonding energy. The study also derived predictions of the number of microcracks and the minimum fiber volume fraction needed for the occurrence of multiple microcracking.

Three-Dimensional Finite Element Study on Effects of Nonlinear Temperature Gradients in Concrete Pavements

A three-dimensional (3D) finite element (FE) model is developed to investigate whether the condition of plane sections remaining plane exists in concrete pavements subjected to nonlinear temperature gradients. This model is utilized to validate the analytical method proposed by Mohamed and Hansen. The 3D brick element is chosen so that the plane section condition is not imposed in the model, as compared with the model using the flat plate element. Furthermore, the possibility of loss of contact between the pavement slab and the subgrade is studied. The condition of full contact is investigated for a nonlinear temperature gradient that produces the maximum tensile stress in the slab according to the data used. Two slab lengths and two radii of relative stiffness are considered. It is found that plane sections remain plane for the entire slab except for a region very close to the free edges, which also establishes the boundary where solutions by Mohamed and Hansen are applicable. In both cases of the contact condition, the 3D FE model predicts no loss of contact between the slab and the subgrade.

Pore structure and frost durability of clay bricks

The interrelationships among pore structure, saturation coefficient and moisture saturation behaviour of burnt clay brick specimens made from two different raw materials and fired to different temperatures are discussed. From these relationships an evaluation method for the forst resistance of clay bricks based on the continuous and the total porosity obtained from mercury intrusion porosimetry is presented.