International Concrete Abstracts Portal

SP-154 In 1995, The Canadian Centre for Mineral and Energy Technology (CANMET), in association with the American Concrete Institute and other organizations sponsored a second conference on Advances in Concrete Technology. The objectives of this conference was to bring together representatives from industry, universities, and government agencies to present the latest information and explore new areas of needed research and development. Thirty two papers from 20 countries were reviewed and accepted for inclusion in this new publication based on the symposium subject, advances in concrete technology. The range of subjects is varied due to the wide range of experts involved in this project.

Presents the results of laboratory and field tests conducted by the authors over a twelve year period on the long-term durability performance of silica fume based concretes, shotcretes, grouts, and concrete slab overlays. The silica fume mixture proportions varied, from 2 to 22 percent of silica fume by weight of cement, and included normal portland cement, shrinkage compensating cement, normal weight and lightweight aggregates. Laboratory and field specimens were tested for compressive strength, bonding strength, chloride permeability, electrical resistivity, and freezing and thawing durability. The exhibited long-term performance characteristics indicate that silica fume based concretes, shotcretes, and grouts provide excellent protection to embedded reinforcing steel in chloride environments. As the materials age, they become stronger, lower in permeability, and higher in resistivity. Silica fume modified materials produce one of the best cementitious products available for adverse concrete environments.

Silica fume has been available commercially in the United States for over 10 years. Until recently, there has not been a well-accepted consensus specification for it. This paper deals with the question of a specification for silica fume for use in concrete by reviewing the following areas. 1. ASTM and AASHTO efforts to develop a specification for silica fume are described. The author's comments on these documents are presented. 2. The status of international efforts to develop silica fume specifications is reviewed. Work from Australia, Canada, Norway, RILEM, South Africa, and other European countries is reviewed to indicate the direction that is being taken outside the United States. 3. Recommendations, based upon the author's experience with silica fume since its introduction to the United States, for what a specification for silica fume for use in concrete should include are presented. The use of silica fume is increasing every year. Some engineers, particularly those in public agencies, have been hesitant to use the material because of the lack of a standard specification. Most engineers wanting to use silica fume have developed their own specification for silica fume with provisions that may have little or no relationship to the performance of the material in concrete. A consensus needs to be established to increase the confidence of specifiers wanting to use silica fume.

For years, the concrete industry has used ultimate compressive strength and elastic modulus as principal design and analysis tools. This can be very misleading when cracking and failure are evaluated. With modern concrete that include roller-compacted concrete (RCC) and lower strength mass applications, cracking that is serious may not occur until the concrete is strained well beyond the elastic region. Two things are needed to resolve this problem. First, a new property called the "ultimate modulus" should be determined, along with the elastic modulus. If these values are nearly the same, the concrete is brittle and may have a low strain capacity, even if it has a high strength. If the ultimate modulus is much lower than the elastic modulus, the material is "tough" and may have a high strain capacity despite a low strength. Examples are given in which deliberately designing a lower strength concrete has resulted in a much higher strain capacity. In one case with RCC, a mixture with five times less strength resulted in a tensile strain capacity (and resistance to thermal cracking) that was three times greater. Second, there should be a better understanding of the relationships between strain capacity, strength, and modulus (ultimate and elastic) in compression as compared to those material properties in tension. With the broader range of concrete mixtures possible in today's concretes (RCC being an example), the ratio between split cylinder tensile strength and compressive strength may be twice as high for a lower strength mixture than it is for a higher strength mixture. Somewhat offsetting this is the fact that the conversion factors from split tensile strength or flexural strength to direct tensile strength are substantially smaller for low strength concretes and greater (exponentially) for high-strength concretes. When only concretes in the compressive strength range of about 20 to 50 MPa are considered, the adjustment factor happens to be about one, so this phenomenon has not been obvious or very important in the past.

Examines the relationship between deterioration of concrete and the structural performance of bridge structures. Case 1: A 37-year-old, three-span concrete slab bridge was decommissioned due to heavy deterioration. Modal testing was used to detect the mos