Time-dependent Accelerated ageing

Before strategies for improving the durability of frc can be discussed, it is necessary to outline the methods by which much of the relevant data has been obtained. As frc is a relatively young material, a body of `real-time’ weathering data did not exist at the time efforts were made to improve frc durability; there is still a lack of long-term natural weathering data. As new formulations designed for durability were advanced, accelerated ageing procedures were used to attempt to validate the purported improvements and these procedures are still in widespread use today. Two technical types of test can be identified; continuous ageing and cyclic ageing. The former aims to accelerate the degradation either by immersing samples of the composite in water at elevated temperature (typically =>50ëC) and thus thermally accelerating the fibre-matrix reaction, or in aggressive solutions (i.e., solutions of de-icing salts or sea-water) where the diffusion of ions into the composite is accelerated by increasing the concen- tration gradient. Hot aggressive solutions are sometimes used for extreme tests.

Hot water is used for two reasons: it provides thermal inertia and also many frcs do not appear to suffer degradation owing to `dry’ elevated temperature alone; indoor stored grc seems to suffer no degradation. The degradation mechanism appears to involve water in some way, chemical or physical. Aggressive solutions, generally of chloride ions, are usually only used in studies of steel-frc; hot water is used for glass-frc and others. Cyclic tests attempt to replicate the cycles of temperature and/or moisture frc components are likely to be exposed to during service. These cyclic tests normally involve alternate and repeated exposure to hot/wet/cold/dry environments or freeze/thaw cycles. For steel-frc, `splash’ exposure to salts to simulate, e.g. tidal zone marine exposure, is also used.

It should be noted that these tests are not interchangeable; certain tests are more appropriate for certain frc formulations, e.g. hot water for glass-frc, and cyclic wet-dry for natural frc; for ageing of cellulose fibre-reinforced concrete, it is also considered important that at least one of the steps should promote carbonation of the matrix (Bentur and Mindess, 1990, pp. 161±163); ageing of steel-frc almost always involves salts.

Tests can also be divided into two other categories: `deemed-to-satisfy’ (DS) tests and predictive tests. DS tests are intended as a quality control measure; if, for example, a coupon of composite is tested after a fixed ageing period or number of cycles and is found to have properties that exceed some pre- determined value or percentage of the unaged properties, then it is deemed to have `passed’ and the material is fit for purpose. No prediction of the long-term behaviour of the composite in service is advanced or inferred, only that it is in some way `durable’ since it has passed the test. Predictive testing, on the other hand, attempts to correlate periods or numbers of cycles of accelerated ageing with longer periods of in-service weathering and thus goes some way towards quantifying the lifetime that can be expected from the material. Although in theory either cyclic or hot water testing can fall into either category, in practice continuous ageing, especially hot water ageing tends to be used for predictive testing and cyclic ageing for DS testing with few exceptions.

Investigators have also frequently subjected fibres (as opposed to composites) to accelerated ageing regimes and thus inferred their likely behaviour in a cement matrix. These tests generally involve immersion of samples of fibre in hot alkaline solutions, often of analogous composition to cement pore solution, for various lengths of time and measurement of resultant reduction in strength. Such tests, although useful for helping to assess new fibres, should be treated with caution with respect to their use in predicting durability, since the action of the cement matrix on the fibre is not just controlled by its alkalinity. Sisal and coconut fibres were shown to completely lose their strength after 300 days in Ca(OH)2 solution of pH 12 (ToleÃdo Filho et al., 2000). Aramid (Twaron) fibres have been subjected to immersion in concentrated NaOH for 24 hours in both stressed and unstressed conditions, with no ill effect (Vilkner, 2003) but no longer term data is evident. Such tests have long been used with prospective glass compositions and results are reviewed by Majumdar and Laws (1991, pp. 15±25) but new glass or ceramic formulations now tend to be tested in actual frc samples (e.g., Cheng et al., 2003, Ma et al., 2005) although some fundamental work continues to use just fibres (Gao et al., 2003, Orlowsky et al., 2005).

The best known (and most frequently abused) accelerated ageing rationale for frc is probably that derived for OPC matrix AR-glass grc by Proctor and co- workers in the late 1970s (Litherland et al., 1981, Proctor et al., 1982). A combination of tests on composites aged by hot water immersion and com- posites exposed at various locations around the world were combined with data from a specially developed durability test for fibre strands known as the `strand- in-cement’ (SIC) test. The SIC test involves a specimen comprising a strand of fibres (for grc, typically about 200 filaments, each of about 14 um diameter) being partially encased in a small cylinder of cementitious matrix. The specimen is aged by immersion in hot water for a specified time and the strand sub- sequently tested in tension with the small cylinder of matrix still in place (Fig. 9.5). They concluded that a single degradation mechanism with an activation energy of about 90 kJ/mol controlled strength loss in all these cases and the data was pooled into an Arrhenius-type relationship correlating the accelerated ageing tests with longer periods of in-service weathering. This allowed `accele- ration factors’ to be advanced, e.g. that 1 day of hot water immersion ageing at 50, 60 and 80ëC induced the same strength loss as 101, 272 and 506 days of weathering in a UK climate respectively. This predictive rationale lead to hot water ageing being accepted as the de facto standard method for validating new grc formulations with respect to durability, which it remains (e.g. Orlowsky and Raupach, 2003, Hempel et al., 2003, Brockmann and Raupach, 2002). However, the method was only derived with OPC composites in mind. Concern over its application by other investigators to non-standard grc (and even frp) with evidently different ageing characteristics and mechanisms, together with shortcomings of the model regarding service life prediction and the lack of a micromechanical model, lead to the rationale being updated by Purnell and co- workers (Purnell et al., 2001a). The basis and implications of their model is discussed in more detail in Section 9.2.5, but it is clear that a unique set of acceleration factors applies to each matrix formulation used with AR glass (Beddows and Purnell, 2003, Purnell and Beddows, 2005); by implication, acceleration factors derived for grc should not automatically be applied to other types of frc.

Another accepted accelerated ageing test method for grc is codified in European standard DD EN 1170-8:1997 Precast concrete products. Test method for glass-fibre reinforced cement. Cyclic weathering type test. This involves cyclic ageing, where each cycle involves 24 hours immersion in water at 20ëC, 30 minutes of forced drying in air at 70ëC and 1msÿ1 airflow, 23 hours in air at 70ëC and 30 minutes of forced cooling in air at 20ëC and 1msÿ1 airflow, with samples tested in bending after 10, 25 and 50 cycles. The test is notoriously difficult to apply as it requires specialist equipment and contains no predictive component, only serving as a comparison. As such, it is unpopular with researchers but has some adherents in industry (e.g., Cian and Della Bella, 2001).

Some attention has been paid to the accelerated ageing of cellulose and/or natural fibre based-frc. In early work, the hot water ageing rationales developed by Proctor and co-workers (Litherland et al., 1981, Proctor et al., 1982) for glass-frc were used for sisal-frc (BergstroÈm and Gram, 1984) but found to be inappropriate. Bentur and Akers subsequently established methods for accelerated ageing of cellulose-frc in a series of papers (Bentur and Akers, 1989a,b; Akers and Studinka, 1989). Two 24 hour/cycle cyclic regimes were proposed; ambient/elevated temperature (AE) and CO2 rich/elevated tempera- ture (CE), both followed by three-point bend testing based on ISO 39611 ± 1980 E. The AE cycle was 9 h in water at 20ëC; 3 h in air at 20ëC; 9 h infra-red radiation 80ëC in air; and 3 h cooling to 20ëC in air. The CE cycle was 8 h in water at 20ëC; 1 h in oven at 80ëC; 5 h in oven at 20ëC in an atmosphere saturated with CO2; 9h in oven at 80ëC; and 1 h cooling to 20ëC. Cycling was continued for 3 months. The CE cycle was found to most closely approximate a 5-year period of natural weathering and was thus considered the most appro- priate method. This regime continues to be used to age cellulose frc (Kim et al., 1999) although other investigators have used slightly modified carbonation tests in addition to freeze-thaw methods (MacVicar et al., 1999). All investigators lean towards the same conclusions: the strength and modulus of the material increases by about 20±50% (as a result of the carbonation of the matrix increasing matrix strength and possibly bonding) but the strain to failure is reduced to values similar to that of the unreinforced matrix i.e. <0.1%. The same regime has been used for PVA-frc (Akers et al., 1989) but again at very low Vf . Some application-specific tests are also used. Cellulose-frc is used for sewer pipes (an application previously filled by asbestos-frc) and as such is exposed to external deleterious agents that can cause acidic and/or biological attack on the fibres and/or matrix. Fisher et al. (2001) exposed cellulose-frc samples to sewage in aerobic and anaerobic treatment plants, and also immersed samples in sulphuric acid (pH ~5). It was concluded that there might be advantages in using cellulose-frc over steel-frc in these applications. For frc with natural fibres, simple wetting and drying cycles (e.g. 1 day immersion at room temperature followed by 6 days drying in ambient lab conditions) seems to be considered appropriate, although since such composites suffer degradation of properties in relatively short time under normal weathering, it is debatable whether accelerated ageing is necessary (ToleÃdo Filho et al., 2000, 2003).

Since ingress of aggressive salts rather than fibre-matrix interactions are the controlling factor, tests used for steel-frc tend to have a different focus compared with other frc types and performance is often compared to RC. Durability in the splash zone of marine structures seems to have attracted the most attention, since this is deemed the most severe possible environment, and testing attempts to simulate this. Wetting and drying cycles using 3.5% NaCl solution at 20, 50 and 80ëC for up to 10 months with or without enhanced carbonation induces significant degradation in steel-frc. Strength and toughness are reduced by 20 and 60% respectively for samples carbonated prior to exposure; only toughness is reduced significantly in uncarbonated samples, by about 35% (Kosa and Naaman, 1990). Mangat and Gurusamy (1988) compared a laboratory marine spray chamber with natural tidal exposure and showed that, with respect to chloride ingress into the concrete, the chamber was about 10 times more `aggressive’ than natural exposure owing to the increased salt content. Recent investigators have used a year of 2- week cycles, each of 1 week of 3.5 g/l NaCl salted fog and 1 week `dryness’ (Granju and Balouch, 2005) or up to 1500 freeze-thaw cycles in fresh water or 3.5 g/l NaCl saline solution in accordance with ASTM C666A (Mu et al., 2002).

Accelerated ageing of other frc types does not appear to have been afforded the same degree of investigation, with a variety of different ad hoc methods extant in the literature. Polypropylene-frc has been assessed using 50 wet/dry cycles followed by impact testing, and 50 freeze/thaw cycles followed by flexural/compressive testing (Puertas et al., 2003), which caused some decrease in flexural strength but left compressive strength unchanged. Recently, Al2O3- based ceramic fibre reinforced concrete has been immersion aged at 70ëC (Ma et al., 2005). Carbon-frc has been subjected to freeze-thaw ageing (based on ASTM C666) for between 30 cycles (Chen and Chung, 1996) and 300 cycles (reported in Ohama, 1989) and water immersion at 75ëC for five months (reported in Ohama, 1989) with little if any effect on strength. Bentur and Mindess (1990, p. 355) discuss how carbon-frc does not show any long-term response to hot water or cyclic accelerated ageing and conclude that it is unlikely to suffer any durability problems.