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Featured Articles
  August 1999  
Does Core Size Affect Strength Testing?
This article presents the results of an experimental investigation on the effect of core size on the compressive strength of core. The size of a core was demonstrated to have a significant effect on the strength of the core, with larger cores testing at higher compressive strengths. The results of tests on cores of different sizes obtained from slabs, columns, and beams showed that the 100 x 200 mm core may be considered as the most suitable core size. Based on the results, equations relating the strength of cores to that of standard cylinders are proposed.

Corrosion-inhibiting Systems in Reconstructed Bridge Barrier Walls
Although many different corrosion inhibitors have been available for the past few years, the mechanism of inhibition is not yet fully understood. Furthermore, there is much debate about their effectiveness under field conditions. Monitoring techniques used in an ongoing field evaluation of the long-term performance of proprietary corrosion-inhibiting systems in a reconstructed concrete bridge barrier wall are described. The field monitoring consists of periodic surface corrosion surveys and continuous remote sensing using embedded sensors. It is also corroborated by a laboratory study on concrete cores and field-cast reinforced concrete prisms. It is anticipated that the investigation will provide an improved understanding of product performance in the field and enable bridge owners to make informed decisions in the selection of corrosion-inhibiting systems.

Evaluation of Existing Concrete Bridges in Spain
The article details the practical application of evaluation techniques for three different concrete bridges. The first is a masonry and plain concrete arch bridge, where the most critical limit state was the ultimate limit state of axial and bending forces. The second is a reinforced concrete portal frame bridge, where the assessment is concentrated on the limit states of bending and shear. The last is a post-tensioned concrete bridge, where the limit states of bending and fatigue are of interest. These examples demonstrate the use of reliability-based assessment methods to obtain a measurement of the actual structural performance that can be used in a decision-making scheme regarding bridge upgrading and maintenance.

Testing of Steel-Free Bridge Decks
The Ontario Highway Bridge Design Code, in its three editions (1979, 1983 and 1992), has incorporated an empirical design method for deck slabs of girder bridges, which takes into account their inherent arching action. Hundreds of deck slabs designed by this empirical method now exist around the world. Researchers at Dalhousie University, Halifax, Canada, and at the Ministry of Transportation of Ontario have successfully enhanced the strength of deck slabs by further exploiting the arching action. With the help of tests on both large- and small-scale laboratory models, they have concluded that a deck slab supported by parallel longitudinal beams does not require any reinforcement, provided that the slab is suitably confined in both the longitudinal and transverse directions of the bridge.

How Tough is Fiber Reinforced Shotcrete? Part 2, Plate Tests
Characterizing toughness or energy absorption capability of fiber reinforced shotcrete has been a challenge. Both beam and plate specimens have been proposed for this purpose. Since it is becoming clearer that shotcrete in many applications is loaded dynamically, slow rate static tests performed on beams or plates are not sufficient for a comprehensive characterization of in-place performance. Some of these issues were addressed in Part 1. In this article, the same five fiber types were investigated using plate specimens. Plate tests show similar toughness enhancement due to fiber reinforcement, but the relative improvements between fiber types are not necessarily in agreement with those indicated in the beam tests. Plate specimens are also less sensitive to changes in the rate of loading from static to dynamic. Considering these issues, and realizing that in practice shotcrete is often loaded in biaxial bending, the use of plate specimens rather than beam specimens for the characterization of toughness of fiber reinforced shotcrete is recommended.

Compression Testing of HSC: Latest Technology
Concrete is now available commercially having a compressive strength approaching 20,000 psi (138 MPa). Researchers predict that even higher strengths can eventually be achieved without the use of exotic materials, special processing under high temperatures, or special atmospheres and high casting pressures. As concrete strengths continue to increase the significance of good quality control procedures becomes increasingly more important and is a critical factor influencing the overall success of a project. An interlaboratory test program was conducted to determine the effect of selected variables on the measured compressive strength of high-strength concrete and conclusions and recommendations are presented.

Introduction: Understanding Materials Science
In an age where “high-performance concrete” is common and enormous sums of money are spent on concrete repair, engineers are becoming aware that there is more to concrete than just its 28-day strength. Specifications for concrete are beginning to include requirements covering the ability to resist chemical attack or to survive for a given period in a given environment. Contractors are seeking ways of moving concrete from mixer to final position more efficiently. Models are being developed to predict the performance of different concrete mixture proportions and materials for use in life cycle costing calculations. All of these are driving a need for a more fundamental understanding of concrete as a material.

Why Rheology Matters
This article is aimed at convincing engineers that a rheological approach to fresh concrete is needed. Rheological concepts, including the Bingham model and the rheometer, are described. Then, an example of high-performance concrete (HPC) devoted to bridge construction is emphasized. It is shown that prevention of slump losses, pumpability and deck slope stability can be controlled with the help of a rheometer. Typical rheological specifications are given, and directions are proposed to adjust a HPC mixture to match those specifications.

Concrete in Compression
The factors that control the behavior of concrete in compression remain controversial. The debate has shifted to the roles played by cement paste, the interfacial transition zone between paste and aggregate, and the relative stiffness of the components. While all three play significant roles, the properties of cement paste and the heterogeneous nature of the material appear to be the key factors in the response of concrete in compression. The balance of this article highlights some of the research that demonstrates the roles played by the various constituents, with emphasis on microcracking, interfacial bond strength, and models of concrete.

Why Engineers Need Materials Science
A discussion of the relative importance of the nano- and microstructure of concrete for civil engineers is presented. The significance of nanostructure on the engineering properties of hydrated cement paste with specific reference to the role of water is highlighted. Relationships between microstructure and volume stability, porosity, crystallinity, and strength of cement systems are described. The utility of micro- and nanostructural probes is outlined to facilitate understanding of engineering behavior such as creep.


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