International Concrete Abstracts Portal

The concrete bridge structural members, called "skew members" (SM), which are positioned from 1.5 m above the sea level to about 20 m down in the sea, and are among the most important elements in bridge construction, were investigated for maintenance purposes after 11 years of service. The underwater arch foundation concrete was also tested. The compressive strength, determined as the average value of 10 concrete cores drilled out from each of two skew members--SM-St. Marko and SM-Mainland--was 62.3 Mpa and 57.4 MPa, respectively. Chlorides had penetrated through the high-alkaline composite by over 20 mm in the splash zone concrete and by over 45 mm in the fully submerged concrete, where {Cl-}-penetration was probably enhanced by hydrostatic pressure. The lack of corrosion of the steel in the concrete, even in the presence of high chloride concentration, could be explained by the absence of oxygen. The gas permeability coefficients Kg determined on the concrete core slices varied in the inner concrete layers of SM-St. Marko from 5.58 to 20.10 x 10-13 cmý and from 0.55 to 2.84 x 10-13 cmý in the concrete at SM-Mainland.

The results of investigation into the erosion-abrasion resistance according to CRD-C 63-80 test method and abrasion resistance according to Bohme test method of steel fiber reinforced concrete specimens are discussed in the paper. Nine mix proportions were used. The water-cement ratios (w/c) were varied from 0.30 to 0.65. The volumetric percentage of hooked steel fibers were varied from 0.25 to 2.0 volume percent at the w/c of 0.30 and at the others the quantity of fibers was constant. In addition, mixes without fibers were made at each w/c. The results show that adding steel fibers to the concrete improves the resistances as measured by both test methods. The erosion-abrasion resistance is improved by an increase of compressive strength and an increase in fiber content. It can be correlated to improvements of abrasion resistance from the Bohme test method but only at constant w/c and different content of fibers.

Fiber cement composites developed commercially as replacements for asbestos cement have been under investigation at the Building Research Establishment for a number of years. These include composites containing fibers of glass, polyvinyl alcohol, and cellulose. The suitability of other fibers, including some natural vegetable fibers for cement reinforcement, has also been examined in the past. A summary of the results obtained in some of these studies is presented. Among the composites just mentioned, glass fiber reinforced cement has received the most attention at BRE for historical reasons. In this study, different types of cement--portland, high-alumina, supersulfated, etc.--as well as portland cement modified by pozzolans and polymers--were used. For some of these composites, important mechanical properties after natural weathering for up to 20 years have been determined recently. A review of this work is given, and the results are analyzed in terms of mechanisms that are considered to be important in determining the long-term durability of these materials. It has not been possible yet to devise realistic accelerated aging tests for many of the new fiber cement composites to predict their long-term properties.

Natural weathering, dry air storage, and water curing for long periods of time will have different effects on matrix properties in most fiber reinforced cements. Changes in matrix properties are shown to effect the cracking stress of the composite that will change with time and curing conditions, regardless of changes in fiber properties. The reasons for the differences in the critical fiber volume in uniaxial tension and flexure are explained, and examples are given of 10-year tests on thin cement-based sheets containing networks of fibrillated polypropylene film in which the effects are demonstrated. It is shown that the manufacturer of the composite needs to have an understanding of these problems if the component is to remain ductile for many years in natural weathering conditions.

The long-term durability of glass fiber reinforced (GFR) mortars and concretes manufactured by the premixing method was investigated. Microhardness measurements and the quantitative back-scattered electron image (BSE) analysis were made in the regions around glass fiber strands embedded in the cement paste. Changes of flexural strength and toughness in the GFR mortars with age were found to be related to the features of microstructure in the interfacial regions. The toughness of the GFR mortars decreased with age in response to the increase in microhardness at the immediate vicinity of strands and around 70 to 100 æm from the interface. The solidification in the regions around 70 to 100 æm from interface, as well as the formation of the hydration products in the spaces among the glass filaments, appear to relate to reduction in toughness in GFRC composites.