International Concrete Abstracts Portal

A total of 25 papers are included in this Symposium Publication on HPC. The general topics include HPC bridges, HPC structural lightweight concrete, material science of HPC, and structural safety of HPC. Note: The individual papers are also available as .pdf downloads.. Please click on the following link to view the papers available, or call 248.848.3800 to order. SP189

A new mathematical model to study the problem of fiber pullout in fiber reinforced cementitious composites is briefly introduced. However, with an objective of optimizing fiber-matrix interfacial properties, the main focus of this paper is on the parametric studies carried out using the proposed model. Stresses required to cause initial, partial and complete depending of the fiber-matrix interface are analyzed based on shear lag theory, and closed-form solutions are derived to predict the complete pullout response. Influence of radial stresses (normal contact stress) acting at the interface is considered using the shrink-fit theory of elasticity. Analysis show that the interfacial frictional shear stress decreases with increase in Poisson's contraction of fiber. Furthermore, based on energy considerations, an analytical solution derived to compute interfacial coefficient of friction depicts that interfacial coefficient of friction decreases with increase in pullout distance. Increase in matrix wear resulting with fiber pullout is most likely responsible for the decay of coefficient of friction. Parametric studies are carried out to investigate the influence of fiber-matrix interfacial properties (adhesion bond shear strength, normal contact stress coefficient of friction) and elastic modulus of fiber. Results suggest that for a given set of interfacial properties, initial depending stress, maximum pullout stress, catastrophic debond length, interfacial shear stress distribution, and overall pullout response significantly depend upon fiber elastic modulus. Given the fiber elastic modulus, recommendations are made as to how efficiency of fiber in pullout could be improved by modifying the interfacial properties.

Concrete structures are deteriorating at an alarming rate. While a substantial body of research exists to describe the corrosion process of pristine concrete systems, this paper describes a recent study in which the corrosion process of pristine concrete systems, this paper describes a recent study in which the corrosion and concrete systems, this paper describes a recent study in which the corrosion and mechanical response were assessed for beams exposed to various loading histories. Specifically, different levels of preloading were applied to generate damage while substained loading was also used to investigate the interaction between load level and corrosion rate. Results illustrate that loading history can significantly influence the corrosion and mechanical response of reinforced concrete elements. Corrosion is initiated in cracked beams much aster than in uncracked beams, presumably due to the cracks which facilitate the ingress of aggressive agents to the surface of reinforcing steel. Specimens with high levels of loading illustrated higher corrosion rates. Corrosion of the beams under sustained loading illustrated a similar load deflection history until the point at which significant corrosion was initiated. After significant corrosion occurred, the creep/corrosion behavior resulted I increased deformations which ultimately resulted I a creep/corrosion failure of high load level beams. Beams with higher loading levels were observed to have corrosion imitation sooner and undergo larger deflection. These results indicate that corrosion is accelerated in cracked structures and even further accelerated I structures where the load is maintained over a long-period of time. This suggests the need for models which assess the impact of the loading history in addition to corrosion driving forces, environmental conditions, and material proportions.

The composition of the cement paste system of concrete consisting of cement, water, mineral admixtures and superplasticizers, practically govern its flow behavior, influencing workability, slump loss and other phenomena. Obviously, it also provides the cohesion necessary for the mechanical integrity and durability of concrete. The optimization of the composition of high performance concretes should therefore include the design of the paste phase, which should incorporate the selection of superplasticizer type and dosage. The present work deals with studies of the flow behavior of superplasticized pastes using the Marsh cone test. Saturation superplastcizer dosages and loss of fluidity with time have been determined suing this test. In order to validate the use of data obtained from pastes for proportioning mortar and concrete, fluidity test have been performed on mortars and concrete, and the results compared with those of the corresponding paste phases. It is seen that the superplasticizer saturation dosage may increase slightly due to the incorporation of sands with significant coefficients of absorption and that the loss of fluidity is much higher in mortar than in paste. The behavior of the concrete, characterized by slump and DIN flow test, is similar to that of the mortar. However, when the concrete is maintained in movement (or remixed) as in truck transportation, the slump loss is more dramatic. Never theless, the loss in DIN flow is not very high indicating that some of the loss in workability can be compensated by vibration. The K-slump test data and those of compactability based on DIN flow show peak values that coincide with the paste composition with the superplasticizer saturation dosage. In general, the results demonstrate that for practical purposes the optimization of high performance concrete in terms of its flow behavior can be based on the behavior of its cement paste phase, permitting the selection of superplasticizer type and dosage from simple fluidity tests on pastes.

This paper presents data on the performance of concrete containing ternary combinations of Portland cement, silica fume and fly ash. Porosity measurements show that the individual effects of silica fume and fly ash on pore size distribution are cumulative when both materials are added to Portland cement. Chloride diffusion and rapid chorine permeability test on concrete indicate significant improvements can be achieved through the use of these ternary combinations. There is a synergistic effect attributed to the early-age benefits of incorporating silica fume and the long-term improvements normally associated with fly ash. The final product is concrete with very low initial directivity values that continue to reduce with time. Simplistic service-life calculations indicate that high-performance concrete with ternary cementations blends may provide protection to steel reinforcement way beyond the normal expectations of engineers today. These results are not inconsistent with recent studies of marine exposed concretes, which indicate that the penetration of chlorides may eventually decrease to almost insignificant rates in concretes containing 30% to 50% fly ash.