International Concrete Abstracts Portal

Cellulose fiber reinforced cement can provide the highest performance-to-cost ratio among fibrous cement composites considered for the replacement of asbestos cement. Such composites can find applications in the production of thin flat and corrugated cement sheets, nonpressure pipes, and many other thin-sheet cement products. There are, however, concerns regarding the moisture-resistance of cellulose fiber-cement composites. Considerable differences in flexural strength and fracture toughness values are observed when the specimens are tested at different moisture contents. This paper presents the results of a comprehensive experimental study concerned with the effects of moisture content on flexural performance characteristics of cellulose fiber reinforced cement. The cement composites considered in this investigation incorporated 0 percent, 1 percent, and 2 percent mass fractions of kraft pulp. The moisture conditions investigated included oven-dried, air-dried, and saturated. Comprehensive sets of replicated flexural test data were generated and analyzed statistically. Analysis of variance techniques were employed to derive reliable conclusions regarding the moisture-sensitivity of the flexural strength and toughness characteristics of cellulose fiber reinforced cement composites. The results generated in this study indicate signficant effects of moisture content on flexural performance of cellulose fiber reinforced cement. There is a tendency for flexural strength to decrease and flexural toughness to increase with increasing moisture content of the composite material. Microstructural studies indicate that high moisture contents tend to damage the fiber-to-matrix bond strength, leading to changes in failure mechanism that can describe the trends observed in moisture effects on flexural performance of cellulose-cement composites.

The strength and toughness durability of carbon fiber reinforced cements (CFRC) in inorganic acidic environments was investigated by subjected prismatic flexural specimens (15 x 15 x 150 mm) to two acidic environments (H2 and HNO4) at the age of 28 days for up to 90 days. The pH of the two acids was maintained at 4.0. Eight CFRC mixes and three volume fractions of pitch-based carbon fibers were investigated. It was concluded that while plain unreinforced cements had considerable retrogression in their mechanical properties, carbon fiber reinforced cements had no appreciable effect on either the strength or the toughness, at least for the duration of exposure investigated.

Durability of fibers in concrete is a concern for nonmetallic fibers. This paper presents the results of durability studies conducted for synthetic fibers made of Nylon 6, polypropylene, and polyester. Long-term durability was estimated using an accelerated aging process. In this process, the specimens were stored in lime-saturated water maintained at 50 C. The integrity and the effectiveness of the fibers were studied using flexural toughness of 100 x 100 x 360-mm prisms tested under four-point loading. The concrete was designed for a 28-day compressive strength of 20 MPa, which is commonly used for field applications. A higher-than-normal fiber content of 4.75 kg/m3 (approximately 0.5 volume percent) was used to obtain consistently measurable toughness index values. All the fibers were 19 mm long. Nylon 6 and polyester fibers were made of single filaments, whereas polypropylene fibers were fibrillated. The results indicate that, at a fiber loading of 4.75 kg/m3, all three fibers provide post-crack resistance. Nylon 6 and polypropylene fibers are durable in alkaline environment present in concrete. This is demonstrated by the effectiveness of fibers measured in terms of flexural toughness values and the general load-deflection response. Specimens with polyester fibers had some loss of ductility when subjected to accelerated aging.

Fiber reinforced shotcretes have been used in numerous external exposure applications where the shotcrete is subjected to cycles of freezing and thawing, often in a saturated condition. This paper summarizes the results of several laboratory studies in which both wet and dry-mix fiber reinforced shotcretes have been tested to ASTM C 666 Procedure A (Freezing and Thawing in Water). It is shown that both steel and high-volume polypropylene fiber reinforced wet-mix shotcretes can be made freeze-thaw durable, provided the shotcrete is properly air entrained. Nonair-entrained fiber reinforced wet-mix shotcrete deteriorates very rapidly in the ASTM C 666 Procedure A test. In the dry-mix shotcrete process, it does not appear possible to effectively use air-entraining admixtures; in spite of this, it is shown that properly designed and applied steel fiber reinforced dry-mix shotcrete can be made freeze-thaw durable. The important criteria for making such steel fiber reinforced dry-mix shotcretes freeze-thaw durable are discussed. It is currently not possible to practically produce high-volume polypropylene fiber reinforced shotcrete using the dry-mix process, and so the inherent freeze-thaw durability of such a system is not known.

A number of seven-year-old, externally stored 500 x 100 x 100 mm concrete beams, some of which had suffered severe cracking due to alkali-silica reaction, have been examined. The concretes were produced using pulverized fuel ash (PFA) at a range of addition levels and contained a fixed proportion of a known reactive sand. Following 7 years of exposure, severe cracking was observed in the specimens without PFA or, with 5 percent PFA, surface crack widths were often in excess of 1 mm and examination of sawn surfaces indicated that the depth of visible cracks was up to 20 mm. Specimens containing 20 percent or more PFA did not exhibit any visible cracking. Expansion measurements, USPV, dynamic modulus of elasticity, and modulus of rupture tests were undertaken, and the results broadly confirm the visual condition of the specimens, with cracked specimens displaying significantly reduced engineering performance. Average carbonation depths were less than 3 mm for all the concrete specimens. However, depths of up to 20 mm were observed at the location of some of the wider cracks. Petrographic examination of thin sections showed evidence that alkali-silica reaction had occurred in all the concretes, but had only led to cracking in the concretes with no PFA or 5 percent PFA. In the concretes containing higher levels of PFA, the sites of gel were rare and there was no evidence of associated damage. Examination of polished sections by quantitative electron probe microanalysis showed differences between ordinary portland cement and PFA concrete in the composition of the alkali-silica gel and the cement hydrates. The gel in pores in the PFA concrete was lower in calcium than that in cracks in the ordinary portland cement concrete. In addition, hydrate rims around alite grains had lower Ca/Si ratios and higher K/Si ratios in PFA concrete. The lower quantity of available calcium in PFA concrete and the increased absorption of potassium by its contributions to the suppression of damaging alkali-silica reaction.