International Concrete Abstracts Portal

Concrete structures that are expected to last a long time in a severe environment must be built with proper attention to design, materials selection and proportioning, and construction practices. This paper addresses the construction issues, including placement and curing of concrete. In placement, the importance of cover depth, effects of pumping on air content, need for properly functioning vibrators and screed, and the consequences of improper consolidation are described. The necessity of proper curing is addressed by explaining cracking that result from loss of moisture. The variation in strength between that of the test samples and that of the member is described for concrete subjected to steam curing.

High performance concrete (HPC) with its improved service under load and improved resistance to environmental conditions represents a promising material to assist with the rehabilitation of the crumbling infrastructure. Although HPC has found widespread application within the building industry in certain pockets of the country, its incorporation into transportation structures has been very recent. To demonstrate the suitability of transportation structures has been very recent. To demonstrate the suitability of transportation structures has been very recent. To demonstrate the suitability of HPC for use in highway structures, the Federal Highway Administration (FHWA) initiated a series of projects that included the complete incorporation of HPC from design to long-term monitoring of the bridges in service. The design and construction of Louetta Road Overpass in Houston, Texas and the North Concho River US 87 & S.O. RR Overpass in San Angelo, Texas were conducted as a joint effort by The University of Texas at Austin and the Texas Department of Transportation (TxDOT). The Louetta Road Overpass project incorporated the use of a newly developed pretensioned precast U-Beam. The high initial prestressing forces required high early release strengths of 63.4 MPa (9,200 psi) and design strengths of 91.0 Mpa (13,000 psi) at 56 days. The designers (TxDOT) also required a high initial modulus of elasticity of 41.3 kPa (6,000 ksi) at release and long-term to satisfy the serviceability requirements for the beams. The North Concho River US 87 & S.O. RR Overpass project incorporated the use of pretensioned AASHTO Type IV beams. This is the most widely used bridge system in the state of Texas. These members also required high initial release strengths of 74.5 MPa (14,700 psi) at 56 days. In order to satisfy these design requirements, but also result in an economical mix design. The following paper discusses the evolution and optimization of the mix design and it's subsequent use in the field. In addition, the selection process of the aggregate determined to be most suitable for the production of high performance concrete beams is discussed. A brief description of each project is also presented.

Based on four strength parameters testing of three high-performance concrete (HPC) design mixes and parametric studies, the following conclusions have been made. Creep and shrinkage strains of HPC are lower than those in conventional concrete. Amount and type of coarse aggregates affect the value of modulus of elasticity. The modulus of elasticity of HPC should be determined through experiments with local materials. Beam sections that have large bottom flange are efficient for HPC application. The most significant property of HPC prestressed beam is compressive strength at release. Allowable compression at release has the most impact on span capacity, while allowable tension at service has minor impact. Prestress loss can be reasonably predicted by either the proposed method or AASHTO LRFD Lump Sum method. PCI deflection multipliers at final time are not accurate. The proposed multipliers which are the functions of creep coefficient can be used for conventional and HPC members.

The AASHTO “Task Force on Strategic Highway Research Program (SHRP) Implementation” developed and instituted the Lead State Program in 1996. The mission of the Task Force was to optimize ways in which SHRP technologies could be implemented at the state level. The Task Force understood the benefits of mutual cooperation in sharing resources, working as teams, and collectively implementing the technologies. In order to achieve their mission the Task Force developed the concept of “Lead States Teams”. A “Lead States Team” is a group of states that are willing to take the lead and assist in the implementation of specific, targeted SHRP technologies in which they have interest and have gained some practical experience.

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.