International Concrete Abstracts Portal

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.

The sulfate resistance of concretes with compressive strengths between 60 and 110 MPa was evaluated. The test comprises several soaking/drying cycles of samples in a Na2SO4ù10H20 solution, followed by measurement of mass variation and residual compressive strength. Visual inspection and sulfate recovery by distilled water immersion increased the accuracy of test results. Results reveal significant differences compared to those tests normally used, involving prolonged immersion. The resistance to sulfate attack depends on concrete porosity and capillary absorption and not on permeability, because pozzolanic reactions seem to interrupt pore continuity. The reduced water-cement ratio obtained with the aid of the superplasticizer was much more effective than the chemical characteristics related to the presence of mineral admixtures in concrete regarding its resistance to sulfates.

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.