International Concrete Abstracts Portal

Rheological properties of fresh concrete are important both for the requirements for various production methods and for designing the desirable properties for a particular concrete. The research reported in this paper provides more basic information about factors controlling rheological properties of concrete. The results of the traditional slump test and Tattersall's approach (two-point test) are discussed. The relationship between paste and concrete rheological properties is evaluated. Rheological properties can play an important role in improving the engineering properties of concrete and provide valuable information for design innovations.

According to the trade literature, the determinations of location, direction, and the cover thickness over a single steel bar in concrete are relatively easy and reliable from a magnetic measurement if the bar diameter is known. The estimation of the bar diameter is also possible if the cover thickness is known, although these results are less reliable. Only recent publications suggest double measurements from which both the cover thickness and the bar diameter can be estimated without previous knowledge of either of them. Unfortunately, the accuracy of diameter determination remains unimproved even with these methods. This paper attempts to present the further improvement of the magnetic determination of bar characteristics. The basic idea is to combine a magnetic device with a computer that calculates, without any previous knowledge about the bar: 1) the thickness of the concrete cover above the bar and, 2) the diameter of the bar. Preliminary data also indicate that distinction can be made whether the tested area is above a single bar or multiple bars, although this is not discussed in this paper. Experimental data obtained on laboratory specimens illustrate the new method.

Presents the results of radar tests conducted in a controlled laboratory environment to provide an independent assessment of the usefulness of the technique for investigating in situ concrete structures. Parameters studied include size and thickness of concrete elements, the presence of voids and steel reinforcement, and the influence of moisture. The use of a commercially available computerized signal enhancement system has also been examined to assess the potential benefits. Results are presented that include typical signal response traces and show the degree of precision that can reasonably be expected from radar measurements under a range of practical circumstances. Results are considered in the light of the basic theoretical factors and a number of important limitations are identified. It is concluded that the method offers considerable potential in a range of structural applications but is subject to a number of practical limitations, and guidance is offered concerning experimental technique and interpretation of results.

Although prestressed concrete beams reinforced with fiber reinforced plastic (FRP) rods have a high potential of applicability to structures in corrosive or electromagnetic circumstances, most prestressed concrete beams showed brittle flexural failure in past experiments due to elastic rupture of the FRP used. From the viewpoint of the maintenance and hazard anticipation of structures, the brittle failure mode is undesirable. To improve the ductility, a series of flexural tests were carried out on beams with bonded and unbonded tendons, or with additional nontensioned tendons. Test results showed that the ultimate deformation in the beam with unbonded tendons was 250 percent that of the beam with bonded tendons, while the loading capacity was 90 percent, and additional placement of nontensioned tendons allowed a 400 percent larger deformation at ultimate load. Appropriate combinations of bonded and unbonded tendons with different amounts of tensioning force will satisfy the required ductility for each structure that is designed with each design concept.

A new method to enhance the moment-curvature relations of reinforced concrete sections in post-yield range of deformation was developed. The point of the idea is to introduce the sequential yielding of different grade longitudinal bars in a section with the progress of section curvature. The section then does not have any negative stiffness up to the yielding of highest grade bars and results in very ductile behavior. To confirm the advantages of the new method, flexural analyses and loading tests were carried out on reinforced concrete columns with different grade longitudinal bars. The results of theoretical calculations and loading tests indicated that the combined use of different grade longitudinal bars can considerably improve the flexural behavior of reinforced concrete sections.