International Concrete Abstracts Portal

Presents results of an experimental investigation to determine the flexural fatigue strength of lightweight concretes made with expanded shale aggregates. Six mixtures were investigated. A total of 120 prisms (20 prisms measuring 76 x 102 x 406 mm for each mixture) were tested in flexural loading of 20 cycles per sec, Hz. The prisms that survived 2 million cycles of fatigue loading were tested in static flexure to determine their residual strength (modulus of rupture). The static flexural strength (modulus of rupture) ranged from 3.04 to 4.91 MPa. The fatigue strength varied from 2.2 to 3.0 MPa. The endurance limit (ratio of the fatigue strength to modulus of rupture) ranged from 0.55 to 0.72. The wet specimens tested at earlier ages had higher strength values (both fatigue strength and modulus of rupture), whereas the endurance limit was higher for dry specimens tested at later ages. There was an increase in the residual static flexural strength for the prisms previously subjected to 2 million cycles of fatigue stress.

The problem of concrete deterioration and its durability has become a matter of great concern to everyone involved in the construction industry. Carbonation and chloride ingress are the two major sources of deterioration, and the penetration of both is influenced by the pore structure of the concrete. Paper presents data on pore structure, carbonation depths, and the interrelationship between the two in structural lightweight concrete after 10 years' outdoor exposure in an industrially polluted area. The concrete was made with expanded slate aggregate using either all lightweight aggregates or with part of the lightweight fines replaced by sand. Both cement content and water-cement ratios were varied. The results showed that the total pore volume was influenced by both the water-cement ratio and fine aggregate content of the concrete. The total pore volume was higher for concretes containing all lightweight fines than for concrete with part replacement of fines by sand. However, for a given pore volume, carbonation was higher for the concretes containing sand than for concrete containing all lightweight aggregates. This phenomenon is explained in terms of the pore structure of the concrete, and a pore structure characteristics parameter is introduced to correlate carbonation with pore volume.

This experimental investigation was aimed at gathering information on flexural properties, including ductility, of high-strength lightweight concrete members (concrete with a dry unit weight of approximately 120 lb/ft 3 and with compressive strength approaching 9 ksi at 56 days) under reversed cyclic loading. Two sets of six specimens each were manufactured using lightweight aggregate concrete having compressive strengths of 5 ksi at 28 days and 9 ksi at 56 days. The test variables were concrete strength, amount of longitudinal reinforcement, and spacing of ties. The test results, including hysteretic load-deflection curves, for specimens representing columns under zero axial load are reported.

The effect of stress on the nitrogen gas permeability of structural lightweight concrete was determined using cylindrical hollow concrete specimens loaded in axial compression at the same time that a nitrogen pressure differential was maintained across the cylinder wall. The nitrogen gas flow rate across the cylinder wall was noted and concrete permeability was measuredas the load increased. Flow rates tended to remain constant up to a critical stress corresponding to the onset of unstable crack propagation, at which time the flow rate increased rapidly. Rapid increases in permeability occurred at lower levels of applied stress-to-strength ratio with normal weight concrete than with lightweight concrete.

Presents information regarding highly confined, high-strength lightweight aggregate (LWA) concrete specimens, tested as part of a proprietary research program for which Phase I results have recently been released. The program specifically investigated the ultimate and post-ultimate behavior of members designed to resist high-intensity bending/punching shear loads, such as those imparted by massive ice features against offshore oil/gas platforms. Two special steel confining systems were utilized to confine the high-strength (compressive strengths nominally between 8000 and 9000 psi) LWA concrete; these were T-headed stirrup bars for use in reinforced concrete, and overlapping button-headed studs for use in plate-steel/concrete/plate-steel sandwich composites. These two confining systems both allowed the LWA concrete to exhibit extreme ductility prior to failure. Flexural, deflection, and ductility factors of over 40, and axial compressive strains of over 8 percent, were achieved, while maintaining essentially 100 percent of the ultimate capacity of the test specimens The tests were performed on 1- to 3.5-scale specimens, using a 4 million-lb capacity testing machine. Three approximately 16 x 16 x 42-in. prisms--two of reinforced concrete and one of sandwich composite concrete--were tested in axial compression. Also, four continuous beam specimens (one reinforced concrete and three sandwich composite concrete) were tested in bending/punching shear. These beam specimens were approximately 153 in. long, 36 in. wide, and had effective depths of approximately 13 in. Nonlinear finite element analyses of the beam specimens were also performed as part of the study.

During the past three decades, lightweight aggregate concrete has emerged as an important sector of the structural concrete industry. It possesses unique properties, similar in some ways to those of normal weight concrete, but differing in significant aspects. Difficulties experienced with lightweight concrete in some projects appears to be caused by a lack of understanding of the differences between normal weight concrete and lightweight concrete as materials and differences in production technologies. It is most wisely used when treated as a material in its own right, with its special properties fully considered in design and construction. A three-phase model of lightweight concrete and its effect on durability are discussed as they relate to selection of materials, concreting and curing technology, control of in-service distress due to freezing and thawing, and corrosion of reinforcing steel. ased on the observed performance of bridge and marine structures built over the past four decades, the author presents a series of generalized observations applying to durability of lightweight concrete that provide a fair cross section of the entire experience. Paper concludes that, with proper selection of materials and design, and good construction practices, lightweight concrete offers an excellent solution to the problem of durability in severe environment.

Structural Lightweight Aggregate Concrete

Describes the use of expanded shale lightweight concrete for both older bridge widenings and new bridge construction on the California State Highway System in the past 30 years. Examples of major projects illustrate the durability and reliability of a properly designed and constructed lightweight aggregate bridge. Cost comparisons of lightweight aggregate structures bid in competition with structural steel and normal weight concrete alternative structures highlight the economic viability of this material. The outstanding performance of these lightweight bridges under heavy traffic and the close competition in bidding suggests that lightweight aggregate is a material that should be considered in future bridge designs, especially in earthquake country, where dead load is such an important factor in seismic design. The known consistent creep, shrinkage, and modulus properties of lightweight aggregate remove any doubts about performance, as certain structures have demonstrated. Industry advances in controlling moisture content have reduced considerably the handling and finishing problems of earlier years.

Project consists of a 15-story office tower and a 4-level parking structure. The advantages of lightweight concrete over other structural materials for this particular project, and the process followed for its selections, as well as different types of structural systems, are evaluated. The length of spans in both structures was a determining factor in the selection of the floor system. Lightweight concrete, 4000 psi, was chosen for the floors, and 6000-psi normal weight concrete was selected for the columns. As a first step of the design process, economic comparisons were made between concrete and structural steel. After determining that concrete was more economical, alternate floor systems were studied for constructability, function, economy, and availability of materials. Lightweight concrete was preferable for all floor systems, even though a premium cost of nearly 10 dollars per yd 3 is common for this geographic location.

Final part of a three-part paper presents the results of a joint industry project to develop high-strength lightweight aggregate concretes for use in the Arctic and describes the determination of selected structural parameters for those concretes. Both crushed and pelletized lightweight aggregates were used with supplementary cementing materials and high-range water reducers to produce concretes with compressive strengths from 8000 to 11,000 psi (55 to 76 Mpa). Structural parameters evaluated were the stress-versus-strain behavior of concrete, multiaxial stress behavior, beam shear strength, shear friction capacity, bearing strength for post-tensioning operations, and reinforcing bar development length. Where possible, the test results were compared to ACI 318 provisions. In almost all instances, the ACI code requirements were satisfactory for use with these types of concrete.