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Changes in long-term performance of fibre reinforced shotcrete due to corrosion and embrittlement
Tunnelling and Underground Space Technology 98 (2020) 103335
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Changes in long-term performance of fibre reinforced shotcrete due to corrosion and embrittlement
T
Erik Stefan Bernard TSE P/L, U4, 1-3 Burns Rd, Wahroonga, NSW 2076, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Fibre reinforcement Degradation Corrosion Long-term performance Aging
The post-crack performance of fibre reinforced shotcrete (FRS) differs from that of conventionally reinforced concrete in that the post-crack pull-out characteristics of fibres depend on the properties of the concrete matrix. Changes in the concrete matrix with time can effect substantial changes in the post-crack performance of the fibres in a way that is independent of external exposure conditions. When cracks occur in the FRS matrix, fibres may also be exposed to aggressive agents that can reduce their already small cross-sectional area at the crack and thereby diminish their post-crack performance, particularly at larger levels of deformation. The present investigation has examined how the post-crack energy absorption capacity of fibre reinforced shotcrete is affected by the interaction of aging of the concrete matrix and corrosion at cracks. The findings show that changes in performance characteristics are related to crack width and fibre type. The majority of fibre types show negligible changes in performance at low deformations over time. However, following aging, some fibre types exhibit significantly lower post-crack performance at high deformations than is evident at 28 days. The use of fibre reinforcement in shotcrete therefore needs to address aging effects, the exposure conditions regarding potential corrosion, as well as the in-service conditions of use in order to specify an appropriate maximum crack width limit that will enable the chosen fibre type to perform without risk of corrosion or embrittlement.
1. Introduction In recent years the issue of durability and long-term performance of Fibre Reinforced Shotcrete (FRS) and concrete (FRC) has become more significant to the owners of underground infrastructure. In many tunnel linings, fibres are the only form of reinforcement present and thus satisfactory performance of fibres as the lining ages is important to continued ductility. There has been a concurrent trend toward specification of a very long design life for structures such as tunnels. Fibre Reinforced Shotcrete is also the dominant form of surface support in underground mines in many countries. Performance can be assessed in many ways, but the ability of FRS to rapidly achieve and then retain ductility (commonly measured as ‘toughness’) over time is generally recognized as being important to the successful use of this material. This is because ductility is critical to the re-distribution of load when, for example, ground movement causes localized cracking of the concrete matrix within a shotcrete lining. Ground movement can occur over the first few months after excavation, so it is expected that some FRS linings will suffer cracking either while under construction or in service, particularly in areas of weak ground. Cracks may also arise as a result of sloughing of freshly sprayed shotcrete, or drying shrinkage. If ductility diminishes with age
or exposure to aggressive agents at these cracks, then the ability of the structure to maintain function may be compromised. This is why minimum levels of ductility are considered mandatory in conventional above ground structures, made of, for example, reinforced concrete (ACI, 2005; AS3600, 2018). The ability of FRS infrastructure to achieve and then maintain a satisfactory level of toughness over the life of a structure is a critical aspect of ‘durability’. The capacity of fibres within uncracked FRC/FRS to resist corrosion under conditions of normal atmospheric and coastal exposure has been demonstrated through several long-term exposure trials (Schupack, 1985; Hara et al., 1992, Mangat and Gurusamy, 1985). Steel fibres that corrode due to proximity to a concrete surface have been shown to exert insufficient expansive pressure to disrupt the enveloping concrete (Hoff, 1987; Lankard and Walker, 1978). While surface staining due to corrosion is regarded as unsightly, localized surface corrosion does not develop into structurally-threatening through-corrosion of the kind that is commonly observed in conventionally-reinforced concrete (Phan et al., 2001). In contrast to the relatively good durability of uncracked FRC/FRS, the presence of cracks can degrade the long-term post-crack performance of this material, particularly when reinforced with steel fibres, but this will depend on the width of the cracks and exposure conditions.
E-mail address: [email protected]. https://doi.org/10.1016/j.tust.2020.103335 Received 13 May 2019; Received in revised form 12 November 2019; Accepted 2 February 2020 0886-7798/ © 2020 Published by Elsevier Ltd.
Tunnelling and Underground Space Technology 98 (2020) 103335
E.S. Bernard
Many laboratory and field tests have shown that exposure of cracked steel FRC surfaces to aggressive environments can degrade post-crack performance (Kosa and Naaman, 1990; Marcos-Meson et al., 2018). Nordström (2005, 2016) and Bernard (2015a) found that the rate of corrosion of steel FRS increased with crack width but that late-age hydration may have the effect of overcoming some of the deterioration in performance at small crack widths caused by corrosion of fibres. Nordström also noted that negligible corrosion and deterioration occurred for steel FRS exhibiting very narrow (< 0.1 mm) cracks. The effect of cracking on the performance of macro-synthetic fibres is not as well documented as for steel fibres, but the absence of corrosion and immunity to salt ingress characteristic of these fibres have been noted by several researchers (Marcos-Meson et al., 2018; Chernov et al., 2006; Zhang et al., 1999). Maximum in-service crack width limits of about 0.15–0.20 mm have been recommended in guidelines for steel FRC in aggressive exposure conditions (AFTES, 2013; MC2010, 2012). In shotcrete-lined tunnels, cracks equal to or wider than this can arise as a result of ground movement, shrinkage, or slumping during spraying. While durability issues in FRS related to matrix degradation and corrosion have been relatively widely addressed, changes in ductility with age have only recently been investigated. Several early laboratorybased studies noted the evolution of post-crack performance characteristics for FRC with the passage of time (Naaman and Najm, 1991; Banthia and Trottier, 1994) but generally did not examine behavior beyond 56 days age. A field-based study of the age-dependent behavior of FRS on the M5 motorway tunnel in Sydney, Australia (Bernard and Hanke, 2002) revealed a loss of toughness at ages up to two years (termed ‘embrittlement’) for shotcrete reinforced with some types of fibre. This was due to the development of high strength and hardness in the enveloping concrete matrix resulting in a change from the highenergy pull-out mode of post-crack fibre behavior to the low-energy rupture mode of fibre failure. This change in failure mode leads to a dramatic fall in post-crack performance that is unrelated to corrosion or any mechanism of deterioration in the concrete matrix. Indeed, it is usually associated with high strength and rigidity in concrete, and thus tends to occur in shotcrete within long-life tunnel linings due to the high cementitious content and low water/binder ratio that is typically required of these concretes to satisfy matrix durability requirements. More recent studies by Bjøntegaard et al. (2014) and Kaufmann (2014) confirmed the results by Bernard and Hanke (2002) by demonstrating that late-age performance losses in FRS appeared to be a consequence of a transition in post-crack behavior from pull-out to rupture of fibres. Bjøntegaard et al. also found that increasing the tensile strength of a fibre improved its resistance to rupture at late-age. A second study by Bernard (2015b) detailed the changes in post-crack performance that occurred in FRS as a consequence of embrittlement, and confirmed that this phenomenon primarily occurred in concrete reinforced with steel fibres rather than macro-synthetic fibres. This paper constitutes an extension to this earlier work. As a result of the tests conducted as part of the M5 motorway project (Bernard and Hanke, 2002), the Roads and Maritime Service of New South Wales became concerned about the capacity of FRS linings in motorway tunnels to maintain ground stability in the long term. The present investigation was therefore initiated to examine how corrosion at cracks can interact with aging effects to change the long-term postcrack performance of FRS reinforced with steel and macro-synthetic fibres. The investigation was intended to quantify deterioration of cracked and uncracked FRS exposed to typical environmental conditions in a motorway tunnel over a period of 10 years, but performance in other underground environments is also of relevance In the process of designing a shotcrete lining for ground support, lining performance can be expressed as either a post-crack residual strength based on beam tests such as EN14561 (2007) or ASTM C1609/ C1609M (2012), or energy absorption based on panel tests such as EN14488 (2006) or ASTM C1550 (2012). The relationship between ground stability and minimum requirement for energy absorption has
been incorporated into several widely used design guidelines for shotcrete linings such as the Q-system (Barton and Grimstad, 2004; Papworth, 2002), and the Australian Recommended Practice for Shotcrete (2010). In each of these documents the minimum energy absorption requirement for FRS is related to the expected degree of ground movement. In Australian and North American practice (Bernard, 2013; Decker et al., 2012), energy absorption is commonly specified on the basis of FRS performance using ASTM C1550 round panels (2012). ASTM C1550 panels have therefore been used to assess FRS performance throughout this investigation. A minimum of 360–400 Joules energy absorption at 40 mm in the ASTM C1550 panel test is commonly regarded as suitable for temporary and permanent support in the majority of ground conditions in tunnels (Papworth, 2002). This is also widely specified for ground support in mines (Concrete institute of Australia, 2010). If in-service deformations are expected to be minor, then energy absorption at 5 mm deflection is the most appropriate measure of performance relevant to the serviceability limit state. Typical serviceability performance levels specified in recent tunnels in Australia lie in the range 60–100 Joules at 5 mm for ASTM C1550 panels for permanent sprayed linings.
2. Experimental program To assess the interaction of aging and corrosion effects on FRS performance, an experimental investigation consisting of a field exposure trial was undertaken. Eight sets of FRS specimens were produced by spraying and one set of specimens reinforced with SL62 steel mesh was produced by casting (see Tables 1 and 2 for reinforcement and mix data). Each set included 105 ASTM C-1550 round panels that, if sprayed, included a set accelerator at a dosage rate of about 4% bwc. Cores were also produced and tested in parallel with the panels. Uncracked control specimens from each set were kept continuously wet and tested at approximately 1, 2, 3, 7, 14, 28, 56, 91, and 180 days, and then 1, 2, 3, 5, and 10 years age. The remaining panels within each set were pre-cracked at 56 days age, and subsequently exposed in the field, before re-testing at 180 days, 1, 2, 3, 5, or 10 years. All the sprayed specimens were produced by manually spraying shotcrete into round steel and plywood forms that were propped at 45° against a rack. The specimens were moved to a flat surface immediately after spraying and screeded so as to achieve a flat surface and uniform thickness. They were then left outside under plastic sheeting to harden overnight before being stripped and transferred to curing tanks for 56 days of immersed curing. About 34–36 uncracked panels from each set were cured in water continuously for up to 10 years and tested at the required ages starting at 1 day age to examine the effect of aging independently of any corrosion effects. The remaining 66 panels from Table 1 Mix details for shotcrete sets examined. Reinforcement
Barchip Kyodo SL62 mesh* Dramix RC 65/ 35 BN Dramix RC 65/ 35 BN Dramix RC 65/ 35 BN Novotex 0730 Enduro 600 Enduro 600 Barchip BC54
Quantity
Grade
UCS Cyl. (MPa)
UCS Cores (MPa)
(kg/m3)
(MPa)
28d
10 yr
28d
56d
91d
10 yr
10 – 50
32 50 50
42 58 57
58 72 77
41 – 60
46 – 66
53 – 70
63 – 76
50
40
45
74
45
51
57
88
50
25
29
44
38
41
44
52
50 7 7 7
40 40 50 50
48 43 53 55
63 64 63 90
44 45 48 45
52 49 52 53
54 57 55 59
62 68 64 84
* Welded steel mesh comprising 6 mm deformed bars on a 200 mm orthogonal grid. 2
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Load resistance increased with age at small deflections (up to 5 mm, or 1.5 mm crack width) typical of service conditions, but fell with age at large deformations (20–40 mm, or 7–13 mm crack width) typical of ultimate load conditions. A contrasting series of load-deflection curves are shown for two of the macro-synthetic fibre reinforced mixtures in Fig. 2; these reveal a very consistent pattern of post-crack behavior that steadily increased in energy absorption but otherwise did not change much in shape with the passage of time. The energy absorption results for the uncracked controls up to 10 years age indicate that the influence of aging on energy absorption can be substantial for some types of steel fibre reinforcement. Aging appears to have less of an effect on energy absorption for the currently examined macro-synthetic fibre types. The change in energy absorption for each specimen set as a function of age is shown for the uncracked controls in Figs. 3–5. Each set of curves includes actual data points and curve-fits for energy absorption at 5 and 40 mm central deflection (representing maximum crack widths of 1.5 and 13 mm). Energy absorption has also been included at 10 and 20 mm central deflection (representing maximum crack widths of 3.5 and 7 mm, respectively) for the mixtures reinforced with hooked-end steel fibres because these sets demonstrated the most dramatic changes in performance with age. The energy absorption characteristics of the shotcrete reinforced with Dramix steel fibres clearly changed with the passage of time, but the magnitude of this change varied with the compressive strength of the concrete matrix and the degree of deformation at which performance was assessed. A transition from fibre pull-out to rupture occurred in the 50 and 40 MPa mixes before 90 days age (Fig. 3), but little change occurred in the 25 MPa mix even after 10 years aging. At 5 mm central deflection, energy absorption topped out at 140 Joules in the 50 and 40 MPa mixes and did not fall with the transition in fibre failure mode. For the 25 MPa mix, the 5 mm energy absorption at 28–56 days was about 110 Joules, and eventually topped out at about 130 Joules after 10 years. Thus, performance at narrow crack widths typical of inservice conditions was not substantially affected by the transition to an embrittled state even when a relatively high strength concrete mixture was used. In contrast, the performance of the Dramix-reinforced FRS at 40 mm central deflection changed dramatically with the transition from fibre pull-out to rupture. For the 50 MPa mix, the specimens absorbed about 500–550 Joules prior to embrittlement (at about 7 days age). For the 40 MPa mix, the maximum performance was around 650 Joules. However, both sets dropped to about 300 Joules after 90–180 days aging, and most of this energy was absorbed in the first 10 mm of central deflection (Fig. 3d). In contrast, the 25 MPa mix did not transition from fibre pull-out to rupture and experienced only a minor change in 40 mm energy absorption with the passage of time, rising to a maximum of about 550 Joules. The post-crack load-deflection curves for this mix showed that toughness at large deflections remained good even after 10 years aging, unlike the 40 and 50 MPa mixes for which residual capacity beyond 20 mm central deflection was minimal (Fig. 1). Intermediate levels of performance occurred for these same specimens when assessed at 10 and 20 mm central deflection. Cumulatively, these results indicate that as the degree of deformation is increased, the effect of embrittlement is more pronounced, hence embrittlement is primarily a problem affecting the ultimate limit state and not in-service performance. The energy absorption data obtained up to 10 years age indicate that none of the macro-synthetic FRS specimen sets exhibited a fall in performance with age. Indeed, each of them revealed a steady rise in performance at late ages, even at 40 mm central deflection (see Figs. 4 and 5). The SL62-reinforced specimens demonstrated a close-to-maximum post-crack performance capacity within one day of casting, and this barely changed with the passage of time. The specimens reinforced with Novotex 0730 steel fibres did not sustain the high level of postcrack load resistance exhibited by the hooked-end steel fibres, but proved better able to retain performance with age and thus appeared to
Table 2 Mix design for shotcrete used in each trial. Component
Coarse aggregate (10/7 mm CRG) Coarse sand (2 mm) Fine sand Cement (type GP) Fly Ash Silica Fume Water reducer (L/m3)
Nominal grade and quantity (kg/m3) 25 MPa
32 MPa
40 MPa
50 MPa
600 372 720 305 80 – 1.0
600 372 680 315 100 10 1.0
610 350 680 335 100 10 1.1
620 330 640 375 100 20 1.2
each set were pre-cracked at 56 days age and then left at a field exposure site for periods of between 180 days and 10 years before being re-tested to assess residual energy absorption. The pre-cracking procedure involved placing the specimen in a standard ASTM C1550 test machine and deflecting the center up to a central deflection corresponding to the required crack width, then reversing the displacement and removing the specimen. By examining both cracked and uncracked specimens at between 180 days and 10 years age, the influence of both corrosion and embrittlement on performance could be distinguished. The exposure site comprised a compound under a wide highway overpass about 15 km from the coast in Sydney, Australia. The site was intended to represent the entrance to a tunnel, and thus was protected from direct rain but not traffic-induced spray or ambient humidity. The panels were oriented in a vertical direction during storage and were therefore not subject to moisture settling into the cracks through the action of gravity. The exposure classification could be considered relatively benign with no contact with either groundwater or atmospheric chlorides. The temperature ranged from 6 to 42 °C seasonally, and averaged 22 °C. The relative humidity averaged 45–50%. No deicing salt was used on the adjacent roadways. According to the Model Code 2010 (2012), this corresponds to an XC3 exposure classification. The fibres used in the trial included both steel and macro-synthetics (Table 1). The Dramix RC65/35 BN fibres were hooked-end and made of cold-drawn steel wire of minimum 1100 MPa tensile strength. These had a diameter of 0.55 mm. The Novotex 0730 steel fibres were flattened-end and made of cold-drawn steel wire of 1400 MPa tensile strength and were 0.70 mm diameter. The Kyodo macro-synthetic fibres were (like all the macro-synthetics) made of polypropylene. They were embossed, 48 mm in length, 1.4 mm equivalent diameter, and 550 MPa tensile strength. The Enduro 600 fibre was a crimped fibre of 50 mm length and 540 MPa tensile strength. The BC54 fibre was embossed, 54 mm in length and 1.4 mm width, and 640 MPa tensile strength. The shotcrete mixtures used in the trials are listed in Table 2. The cement was a General Purpose (GP) type conforming with AS3972 (2010), and the Supplementary Cementitious Materials (Fly Ash and silica fume) conformed with AS3582 (2016).
3. Results The results of the investigation comprised load-deflection curves obtained for each of the C-1550 panel specimens and Unconfined Compressive Strength (UCS) data obtained from the cores and cylinders (Table 1). The results for the pre-cracked panels subject to exposure in the field were divided into a load-deflection curve obtained during the initial cracking test and then a second load-deflection curve obtained following exposure. The post-crack performance of these specimens has been summarized in terms of cumulative energy absorption at 5, 10, 20, and 40 mm total central deflection. Example sets of load-deflection curves for the Dramix RC 65/35 specimens produced using 50 MPa and 25 MPa shotcrete and continuously wet cured in the laboratory are shown in Fig. 1. These curves reveal that a dramatic change in toughness characteristics occurred with age for the higher strength mixture. 3
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Fig. 1. Load-deflection curves for specimens reinforced with Dramix RC65/35 in a) 50 MPa, and b) 25 MPa shotcrete tested at various ages after spraying showing an increase in load resistance at small deflections with aging for the higher strength mix but a fall in resistance at large deformations.
Fig. 2. Load-deflection curves for specimens reinforced with a) Kyodo in 32 MPa shotcrete, and b) Barchip BC54 in 50 MPa shotcrete tested at various ages after spraying showing a steady increase in load resistance both at small and large deflections with aging.
and absence of corrosion products around the fibres exposed within the cracks for the 50 MPa Dramix-reinforced mix suggested that limited corrosion occurred in these specimens, even for an initial crack width of 1.0 mm (Fig. 6). The 40 MPa mix exhibited a steady fall in performance with age, but the inconsistency with crack width and relative absence of corrosion products within the cracks suggested that this was at least partly attributable to the late effects of embrittlement (Fig. 7). For the 25 MPa mix, there was no evidence of embrittlement even after 10 years aging. However, corrosion was evident at the crack surfaces and appeared to have progressed to a greater extent in the 25 MPa mix than in the higher strength mixes despite identical exposure conditions (Fig. 8). Corrosion products were observed around many of the fibres intersecting each crack, particularly near the tensile face of the panels.
be less sensitive to embrittlement than the more slender hooked-end type of steel fibre. This was probably assisted by the higher tensile strength of the wire used to produce these fibres (1400 MPa). The results for the pre-cracked specimens shown in Figs. 6–11 indicate that the residual post-crack performance of steel FRS (normalized relative to uncracked performance at 180 days age) is quite variable but may decrease appreciably with the passage of time. Performance has been normalized with respect to energy absorption at 180 days because embrittlement was judged to have been largely complete by this age and thus any further change in performance was taken to be attributable primarily to corrosion. The magnitude of the decrease in performance after 180 days age is sensitive to both crack width and strength of the concrete matrix. The stability of the results 4
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Fig. 3. Energy absorption at a) 5 mm, b) 10 mm, c) 20 mm, and d) 40 mm for Dramix RC65/35 as a function of age.
performance even at lesser levels of deformation (Fig. 9a). The cracked macro-synthetic FRS specimens showed no sign of deterioration with the passage of time up to 10 years field exposure (Figs. 10 and 11). While some variation in performance was evident, there was no systematic change in performance with age both for the uncracked and cracked specimens. This indicates that macro-synthetic fibres made of polypropylene are highly resistant to in-field degradation, at least for the exposure conditions presently imposed. The mean energy absorption measured at 10 years of each set of cracked steel FRS panels has been divided by the measured performance of the corresponding uncracked panels at 180 days and is plotted as a function of nominal crack width in Fig. 12. The data show that the Dramix-reinforced set made with 50 MPa shotcrete was largely unaffected by corrosion at any crack width. The Dramix-reinforced set made with 40 MPa shotcrete was somewhat more affected by corrosion for wider cracks, particularly at a deformation of 40 mm (Fig. 12a). These results reflect the relatively low level of rust observed on the
This contrast between the 25 and 50 MPa mixtures suggests that the richer cementitious content typical of the higher strength mixes provided some protection for the exposed fibres within the cracks. It should be noted that most of the deterioration in performance due to corrosion occurred between 5 and 10 years age (up to 40% loss of energy absorption at 40 mm for the present benign exposure condition), so the observed adverse trend raises concerns about the level of performance likely to be retained by steel FRS linings at 50–100 years age, especially in aggressive conditions. The performance of the pre-cracked specimens with Novotex steel fibres in 40 MPa shotcrete appeared similar to that of the Dramix fibres in 25 MPa concrete (Fig. 9). Up to 5 years exposure, there was little variation in performance. However, at 10 years the fibres at the crack faces showed clear signs of corrosion and post-crack performance fell substantially compared to performance at 180 days (Fig. 9b). The loss of performance occurred primarily at 20–40 mm central deflection, but there appeared to be a small systematic downward trend in 5
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Fig. 4. Energy absorption at a) 5 mm, and b) 40 mm for synthetics as a function of age.
Fig. 5. Energy absorption at a) 5 mm, and b) 40 mm for mesh, Novotex, and Enduro in 40 MPa concrete, as a function of age.
exposed fibres in the cracks within these specimen sets, especially for the 50 MPa mixture. In contrast, the Dramix-reinforced set made with 25 MPa shotcrete revealed significant degradation due to corrosion that was more pronounced for both wider cracks and larger levels of deformation (Fig. 12b). A similar phenomenon occurred for the specimens reinforced with Novotex fibres in 40 MPa shotcrete. Both these specimen sets exhibited more substantial rust deposits around the fibres at 10 years age. It is notable that the compressive strength of shotcrete cores extracted for these latter two mixtures was somewhat lower than for the first two (Table 1), suggesting that the in-place water/binder ratio was lower for the Novotex set despite the same nominal strength grade for the two 40 MPa mixes. For all the cracked SFRS specimen sets, corrosion did not have a significant effect on 5 mm energy absorption, but for most had a significant negative effect on performance at larger
levels of deformation. Note that none of the FRS mixes exhibited a fall in UCS with age (Table 1). Inspection of the crack surfaces after testing revealed that for the uncracked control specimens stored in water, the loss of toughness that occurred with age was due to a change in the mechanism of postcrack fibre behavior as the concrete matrix became progressively stronger and harder. The loss of post-crack performance was therefore not attributable to degradation of the concrete matrix For the pre-cracked steel FRS specimens exposed in the field, the change in performance with aging and exposure appeared to be due to a combination of corrosion (evidenced by deposits of rust around the base of each fibre) and embrittlement. As the concrete matrix hardened with age the bond to the fibre and corresponding resistance to pull-out increased. This was manifested as an initial increase in energy absorption.
6
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Fig. 6. Energy absorption normalized relative to 180 day performance, at a) 5 and 10 mm, and b) 20 and 40 mm, for Dramix RC65/35 in 50 MPa shotcrete.
If the strength of the anchorage at the ends of the fibre exceeds the tensile strength of the fibre, the fibre will rupture rather than pull-out as the crack widens. Exposure to aggressive agents at the intersection with the cracks can lead to concurrent corrosion that may reduce the crosssectional area of fibre available to resist rupture. Examination with a field microscope indicated that many of the ruptured steel fibres suffered a loss of cross-section by 5–10 years age as a result of corrosion, thereby exacerbating their propensity to rupture at a widening crack rather than pull-out. The areas of corrosion were surrounded by black and brown deposits on the crack surfaces that appeared to be rust. This suggests that the effects of corrosion and embrittlement are cumulative and must be considered concurrently when assessing the long-term inservice performance of steel FRS infra-structure. All the 40 and 50 MPa steel FRS mixes exhibited an increase in energy absorption at small deformations with age, while concurrently
exhibiting a fall in energy absorption at large deformations. This meant that the residual load resistance at large deformations was doubly diminished with age (see Fig. 1 for typical load-deflection curves). In contrast, the macro-synthetic FRS mixes exhibited a steady increase in energy absorption with age at all levels of deformation. These results suggest that steel fibres may perform better under service loads (represented by energy absorption at 5 mm), but macro-synthetic fibres offer superior performance under ultimate strength conditions (represented by energy absorption at 20–40 mm), especially at late ages and in high strength shotcrete, because they exhibit a high-energy pullout mode of failure regardless of the age and strength of the concrete matrix.
Fig. 7. Energy absorption normalized relative to 180 day performance, at a) 5 and 10 mm, and b) 20 and 40 mm, for Dramix RC65/35 in 40 MPa shotcrete. 7
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Fig. 8. Energy absorption normalized relative to 180 day performance, at a) 5 and 10 mm, and b) 20 and 40 mm, for Dramix RC65/35 in 25 MPa shotcrete.
4. Consequences of embrittlement
shotcrete incorporating high cement and silica fume contents. FRS mixes intended for final linings also commonly incorporate high cementitious contents, supplementary cementitious materials, and low water/binder ratios due to requirements for impermeability related to a 100+ year design life. The problem with this type of shotcrete is that the late age strength can be extremely high (80+ MPa), leading to a change in post-crack failure mechanism from a high-energy frictionbased pull-out mode to a low-energy yielding mode for some types of fibre (such as the 1100 MPa steel fibres presently examined). This can reduce the ability of a lining to redistribute loads should ground movement occur at late ages which may lead to a requirement for rehabilitation at late ages. Late-age ground movement may occur, for example, in the event of hydrologically-induced changes in ground stability, roadway widening, nearby underground excavation (particularly in mines), or a change in groundwater level many years after the
The results of this trial indicate that, as far as ductility retention with aging is concerned, the consequences of embrittlement are comparable to the consequences of corrosion for steel FRS when placed in a benign environment. For ground support applications in which deformations are expected to be substantial, ductility is as important to the maintenance of satisfactory load resistance as the maximum strength of materials, hence the loss of ductility with age exhibited by many of the steel FRS mixtures examined in this investigation is cause for concern. The data shown in Figs. 3–5 is particularly important in the context of recent rapid underground excavation practice. Early re-entry during in-cycle shotcreting usually demands high early-age strength development, and this is most commonly achieved by using a high strength
Fig. 9. Energy absorption normalized relative to 180 day performance, at a) 5 and 10 mm, and b) 20 and 40 mm, for Novotex 0730 in 40 MPa shotcrete. 8
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Fig. 10. Energy absorption normalized relative to 180 day performance, at 5 and 40 mm for a) Kyodo, and b) Enduro in 40 MPa shotcrete.
5. Conclusions An investigation was undertaken into long-term changes in postcrack performance of Fibre Reinforced Shotcrete mixtures reinforced with either steel or macro-synthetic fibres. The investigation involved a field exposure trial in which both uncracked and pre-cracked ASTM C1550 panels were aged for up to 10 years to examine the effects of embrittlement and corrosion on post-crack energy absorption. The results demonstrated that post-crack performance at relatively small crack widths is not affected by embrittlement but in-field exposure in even a benign exterior environment can promote substantial corrosion and performance loss for steel fibre reinforced mixtures. Aging-related effects including high strength development and hardening of the cement matrix can change the mechanism of failure for some types of fibre leading to a loss of post-crack performance at large deformations. The effect of corrosion and embrittlement is negligible at small crack widths (here represented by energy absorption at 5 mm in the ASTM C1550 panel test) pertinent to service conditions, but quite pronounced at wide crack widths (represented by energy absorption at 20–40 mm) relevant to ultimate limit state considerations. The current exposure tests, in an environment free of percolating groundwater or chloride ions, revealed a loss of 25–40% energy absorption at 40 mm deflection after 10 years of exposure even for steel fibre reinforced specimens with cracks 0.1–0.2 mm wide. In contrast, the macro-synthetic fibre reinforced shotcrete specimens showed no change in performance over 10 years of exposure regardless of in-service crack width. The present results demonstrate that when designing a shotcrete lining, it is necessary to utilise the most suitable fibre type to guarantee performance over the intended service life time for the exposure conditions expected. The use of some fibre types in unfavourable conditions may lead to significantly lower performance at late age than required and measured at 28 days.
Fig. 11. Energy absorption normalized relative to 180 day performance, at 5 and 40 mm for BC54 in 50 MPa shotcrete.
FRS lining was initially installed. It may also occur as a result of loosening of rock in flat roof or shallow-arched tunnels (Decker et al., 2012). If high toughness is required throughout the life of a FRS structure, then it is necessary to look beyond the performance achieved at 28 days and consider the effects of excessive strength development and embrittlement on late age performance. The present data indicate that fibres intended for long life structures should be selected on the basis of demonstrated performance at late ages. The assumption that excellent toughness obtained in 28 day Quality Control (QC) specimens will automatically translate to similar levels of performance throughout the design life of a structure could lead to serious over-estimates of late-age structural ductility in long-life infrastructure.
Declaration of Competing Interest The author declares that there is no conflict of interest. Acknowledgements This work was supported by funding and material support from the 9
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Fig. 12. Energy absorption at 10 years relative to uncracked 180 day performance, for a) cracked Dramix reinforced panels in 50 MPa and 40 MPa shotcrete, and b) cracked Dramix reinforced panels in 25 MPa shotcrete, and cracked Novotex reinforced panels in 40 MPa shotcrete.
Roads and Maritime Service of New South Wales, Transurban Roads, Barchip Australia P/L, Propex Concrete Systems, and Holcim Concrete Australia Ltd. The sponsors played no role in design of the study or the interpretation or reporting of the results.
concrete: experience with marine applications. Concr. Int., ACI 28 (8), 56–61. Decker, J.B., Madsen, P.H., Gall, V., O’Brien, T.M., 2012. Use of synthetic, fiber-reinforced, initial shotcrete lining at devil’s slide tunnel project in California. Transport. Res. Record: J. Transport. Res. Board, No. 2313 147–154. EN14488, 2006. Testing Sprayed Concrete. British Standards Institute, London. EN14651, 2007. Test method for metallic fibre concrete. Measuring the flexural tensile strength (limit of proportionality (LOP), residual). European Standards. fib, 2012. Model Code 2010, Final Draft, Bulletin 65. Fèdération Internationale du Béton, Lausanne. Hara, T., Shoya, M., Kikuchi, K., 1992. Assessment of steel fibre concrete exposed for 14 years. Fibre Reinforced Cement and Concrete, Ed. R.N. Swamy, pp. 872–882. Hoff, G., 1987. Durability of fiber reinforced concrete in a severe marine environment. Concrete Durability – Katharine and Bryant Mather International Conference SP-100, American Concrete Institute, pp. 997–1041. Kosa, K., Naaman, A.E., 1990. Corrosion of steel fiber reinforced concrete. ACI Mater. 87 (1), 27–37. Kaufmann, J.P., 2014. Durability performance of fiber reinforced shotcrete in aggressive environment. World Tunnelling Congress 2014, (Ed. Negro, Cecilio and Bilfinger), Iguassu Falls Brazil, p. 279. Lankard, D.R., Walker, H.J., 1978. Laboratory and field investigations of the durability of Wirand concrete exposed to various service environments. Battelle Development Corp, Columbus Laboratories, Ohio, pp. 26. Mangat, P.S., Gurusamy, K., 1985. Steel fibre reinforced concrete for marine applications. 4th International Conference on Behaviour of Offshore Structures, Delft, The Netherlands, pp. 867–879. Marcos-Meson, V., Michel, A, Solgaard, A, Fischer, G., Edvardsen, C., Skovhus, T.L., 2018. Corrosion resistance of steel fibre reinforced concrete – a literature review. Cement Concr. Res. 103, 1–20. Naaman, A.E., Najm, H., 1991. Bond-slip mechanisms of steel fibers in concrete. ACI Mater. J. 88 (2), 135–145. Nordström, E., 2005. Durability of Sprayed Concrete - Steel fibre corrosion in cracks, Doctoral thesis. Luleå University of Technology, Sweden. Nordström, E., 2016. “Evaluation after 17 years with field exposures of cracked steel fibre reinforced shotcrete”, BeFo Rapport 153. Rock Engineering Research Foundation, Stockholm. Papworth, F., 2002. Design guidelines for the use of fibre reinforced shotcrete in ground support. Shotcrete, American Shotcrete Association, Spring 2002, 16–21. Phan, Q.H.D., Sakamoto, J., Matsuura, S., Hirama, A., Uomoto, T., 2001. Fundamental study on durability of shotcrete. Third International Conference on Concrete Under Severe Conditions, (Ed. Banthia, Sakia, and Gjørv), Vancouver, 18–20 June, pp. 1481–1488. Schupack, M., 1985. Durability of SFRC exposed to severe environments. Proceedings, U. S.-Sweden Joint Seminar on Steel Fiber Concrete”, Swedish Cement and Concrete Research Institute, Stockholm, pp. 479–496. Shotcreting in Australia: Recommended Practice, 2010. Second Ed., Concrete Institute of Australia & AuSS, Sydney. Zhang, M.H., Mirza, J., Malhotra, V.M., 1999. Mechanical properties and freezing and thawing durability of polypropylene fiber-reinforced shotcrete incorporating silica fume and high-volumes of fly ash. Cem., Concr., Aggregates, ASTM 21 (2).
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.tust.2020.103335. References AFTES, 2013. Design, dimensioning, and execution of precast steel fibre reinforced concrete arch segments. French Tunnelling and Underground Space Association. American Concrete Institute, 2005. Building Code Requirements for Structural Concrete, Document 318–05. ACI, Farmington Hills. ASTM International, 2012. Standard C-1550, Standard Test Method for Flexural Toughness of Fiber Reinforced Concrete (Using Centrally Loaded Round Panel). ASTM, West Conshohocken. ASTM International, Standard C-1609/C-1609 M, 2012. Standard Test Method for Flexural Toughness of Fiber-Reinforced Concrete (Using Beam with Third-point Loading). ASTM, West Conshohoken, PA. Australian Standard AS3600 Concrete Structures, 2018. Standards Australia, Sydney. Australian Standard AS3582 Supplementary cementitious materials, 2016. Standards Australia, Sydney. Australian Standard AS3972 General purpose and blended cements, 2010. Standards Australia, Sydney. Banthia, N., Trottier, J.-F., 1994. Concrete reinforced with deformed steel fibers, Part 1: Bond-slip mechanisms. ACI Mater. J. 91 (5), 435–446. Barton, N., Grimstad, E., 2004. The Q-system following thirty years of development and application in tunnelling projects. Rock Engineering - Theory and Practice, Proceedings of the ISRM Regional Symposium EUROCK 2004. Salzburg, Austria, pp. 15–18. Bernard, E.S., 2013. Safe support with fibre reinforced shotcrete. Tunnell. J. September, 44–47. Bernard, E.S., 2015a. Effect of Exposure on Post-crack Performance of FRC for Tunnel Segments, WTC 2015 May 22-28, Dubrovnik, Croatia. Bernard, E.S., 2015b. Age-dependent changes in post-crack performance of fibre reinforced shotcrete linings. Tunn. Undergr. Space Technol. 49, 241–248. Bernard, E.S., Hanke, S.A., 2002. Age-dependent behaviour in fibre reinforced shotcrete. Fourth International Symposium on Sprayed Concrete, Davos, Switzerland, 22-26 September, pp. 11–25. Bjøntegaard, O., Myren, S.A., Klemtsrud, K., Kompen, R., 2014. Fibre reinforced sprayed concrete (FRSC): energy absorption capacity from 2 days age to one year. In: Seventh International Symposium on Sprayed Concrete, pp. 88–97. Chernov, V., Zlotnikov, H., Shandalov, M., 2006. Structural synthetic fiber reinforced
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Changes in long-term performance of fibre reinforced shotcrete due to corrosion and embrittlement
T
Erik Stefan Bernard TSE P/L, U4, 1-3 Burns Rd, Wahroonga, NSW 2076, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Fibre reinforcement Degradation Corrosion Long-term performance Aging
The post-crack performance of fibre reinforced shotcrete (FRS) differs from that of conventionally reinforced concrete in that the post-crack pull-out characteristics of fibres depend on the properties of the concrete matrix. Changes in the concrete matrix with time can effect substantial changes in the post-crack performance of the fibres in a way that is independent of external exposure conditions. When cracks occur in the FRS matrix, fibres may also be exposed to aggressive agents that can reduce their already small cross-sectional area at the crack and thereby diminish their post-crack performance, particularly at larger levels of deformation. The present investigation has examined how the post-crack energy absorption capacity of fibre reinforced shotcrete is affected by the interaction of aging of the concrete matrix and corrosion at cracks. The findings show that changes in performance characteristics are related to crack width and fibre type. The majority of fibre types show negligible changes in performance at low deformations over time. However, following aging, some fibre types exhibit significantly lower post-crack performance at high deformations than is evident at 28 days. The use of fibre reinforcement in shotcrete therefore needs to address aging effects, the exposure conditions regarding potential corrosion, as well as the in-service conditions of use in order to specify an appropriate maximum crack width limit that will enable the chosen fibre type to perform without risk of corrosion or embrittlement.
1. Introduction In recent years the issue of durability and long-term performance of Fibre Reinforced Shotcrete (FRS) and concrete (FRC) has become more significant to the owners of underground infrastructure. In many tunnel linings, fibres are the only form of reinforcement present and thus satisfactory performance of fibres as the lining ages is important to continued ductility. There has been a concurrent trend toward specification of a very long design life for structures such as tunnels. Fibre Reinforced Shotcrete is also the dominant form of surface support in underground mines in many countries. Performance can be assessed in many ways, but the ability of FRS to rapidly achieve and then retain ductility (commonly measured as ‘toughness’) over time is generally recognized as being important to the successful use of this material. This is because ductility is critical to the re-distribution of load when, for example, ground movement causes localized cracking of the concrete matrix within a shotcrete lining. Ground movement can occur over the first few months after excavation, so it is expected that some FRS linings will suffer cracking either while under construction or in service, particularly in areas of weak ground. Cracks may also arise as a result of sloughing of freshly sprayed shotcrete, or drying shrinkage. If ductility diminishes with age
or exposure to aggressive agents at these cracks, then the ability of the structure to maintain function may be compromised. This is why minimum levels of ductility are considered mandatory in conventional above ground structures, made of, for example, reinforced concrete (ACI, 2005; AS3600, 2018). The ability of FRS infrastructure to achieve and then maintain a satisfactory level of toughness over the life of a structure is a critical aspect of ‘durability’. The capacity of fibres within uncracked FRC/FRS to resist corrosion under conditions of normal atmospheric and coastal exposure has been demonstrated through several long-term exposure trials (Schupack, 1985; Hara et al., 1992, Mangat and Gurusamy, 1985). Steel fibres that corrode due to proximity to a concrete surface have been shown to exert insufficient expansive pressure to disrupt the enveloping concrete (Hoff, 1987; Lankard and Walker, 1978). While surface staining due to corrosion is regarded as unsightly, localized surface corrosion does not develop into structurally-threatening through-corrosion of the kind that is commonly observed in conventionally-reinforced concrete (Phan et al., 2001). In contrast to the relatively good durability of uncracked FRC/FRS, the presence of cracks can degrade the long-term post-crack performance of this material, particularly when reinforced with steel fibres, but this will depend on the width of the cracks and exposure conditions.
E-mail address: [email protected]. https://doi.org/10.1016/j.tust.2020.103335 Received 13 May 2019; Received in revised form 12 November 2019; Accepted 2 February 2020 0886-7798/ © 2020 Published by Elsevier Ltd.
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Many laboratory and field tests have shown that exposure of cracked steel FRC surfaces to aggressive environments can degrade post-crack performance (Kosa and Naaman, 1990; Marcos-Meson et al., 2018). Nordström (2005, 2016) and Bernard (2015a) found that the rate of corrosion of steel FRS increased with crack width but that late-age hydration may have the effect of overcoming some of the deterioration in performance at small crack widths caused by corrosion of fibres. Nordström also noted that negligible corrosion and deterioration occurred for steel FRS exhibiting very narrow (< 0.1 mm) cracks. The effect of cracking on the performance of macro-synthetic fibres is not as well documented as for steel fibres, but the absence of corrosion and immunity to salt ingress characteristic of these fibres have been noted by several researchers (Marcos-Meson et al., 2018; Chernov et al., 2006; Zhang et al., 1999). Maximum in-service crack width limits of about 0.15–0.20 mm have been recommended in guidelines for steel FRC in aggressive exposure conditions (AFTES, 2013; MC2010, 2012). In shotcrete-lined tunnels, cracks equal to or wider than this can arise as a result of ground movement, shrinkage, or slumping during spraying. While durability issues in FRS related to matrix degradation and corrosion have been relatively widely addressed, changes in ductility with age have only recently been investigated. Several early laboratorybased studies noted the evolution of post-crack performance characteristics for FRC with the passage of time (Naaman and Najm, 1991; Banthia and Trottier, 1994) but generally did not examine behavior beyond 56 days age. A field-based study of the age-dependent behavior of FRS on the M5 motorway tunnel in Sydney, Australia (Bernard and Hanke, 2002) revealed a loss of toughness at ages up to two years (termed ‘embrittlement’) for shotcrete reinforced with some types of fibre. This was due to the development of high strength and hardness in the enveloping concrete matrix resulting in a change from the highenergy pull-out mode of post-crack fibre behavior to the low-energy rupture mode of fibre failure. This change in failure mode leads to a dramatic fall in post-crack performance that is unrelated to corrosion or any mechanism of deterioration in the concrete matrix. Indeed, it is usually associated with high strength and rigidity in concrete, and thus tends to occur in shotcrete within long-life tunnel linings due to the high cementitious content and low water/binder ratio that is typically required of these concretes to satisfy matrix durability requirements. More recent studies by Bjøntegaard et al. (2014) and Kaufmann (2014) confirmed the results by Bernard and Hanke (2002) by demonstrating that late-age performance losses in FRS appeared to be a consequence of a transition in post-crack behavior from pull-out to rupture of fibres. Bjøntegaard et al. also found that increasing the tensile strength of a fibre improved its resistance to rupture at late-age. A second study by Bernard (2015b) detailed the changes in post-crack performance that occurred in FRS as a consequence of embrittlement, and confirmed that this phenomenon primarily occurred in concrete reinforced with steel fibres rather than macro-synthetic fibres. This paper constitutes an extension to this earlier work. As a result of the tests conducted as part of the M5 motorway project (Bernard and Hanke, 2002), the Roads and Maritime Service of New South Wales became concerned about the capacity of FRS linings in motorway tunnels to maintain ground stability in the long term. The present investigation was therefore initiated to examine how corrosion at cracks can interact with aging effects to change the long-term postcrack performance of FRS reinforced with steel and macro-synthetic fibres. The investigation was intended to quantify deterioration of cracked and uncracked FRS exposed to typical environmental conditions in a motorway tunnel over a period of 10 years, but performance in other underground environments is also of relevance In the process of designing a shotcrete lining for ground support, lining performance can be expressed as either a post-crack residual strength based on beam tests such as EN14561 (2007) or ASTM C1609/ C1609M (2012), or energy absorption based on panel tests such as EN14488 (2006) or ASTM C1550 (2012). The relationship between ground stability and minimum requirement for energy absorption has
been incorporated into several widely used design guidelines for shotcrete linings such as the Q-system (Barton and Grimstad, 2004; Papworth, 2002), and the Australian Recommended Practice for Shotcrete (2010). In each of these documents the minimum energy absorption requirement for FRS is related to the expected degree of ground movement. In Australian and North American practice (Bernard, 2013; Decker et al., 2012), energy absorption is commonly specified on the basis of FRS performance using ASTM C1550 round panels (2012). ASTM C1550 panels have therefore been used to assess FRS performance throughout this investigation. A minimum of 360–400 Joules energy absorption at 40 mm in the ASTM C1550 panel test is commonly regarded as suitable for temporary and permanent support in the majority of ground conditions in tunnels (Papworth, 2002). This is also widely specified for ground support in mines (Concrete institute of Australia, 2010). If in-service deformations are expected to be minor, then energy absorption at 5 mm deflection is the most appropriate measure of performance relevant to the serviceability limit state. Typical serviceability performance levels specified in recent tunnels in Australia lie in the range 60–100 Joules at 5 mm for ASTM C1550 panels for permanent sprayed linings.
2. Experimental program To assess the interaction of aging and corrosion effects on FRS performance, an experimental investigation consisting of a field exposure trial was undertaken. Eight sets of FRS specimens were produced by spraying and one set of specimens reinforced with SL62 steel mesh was produced by casting (see Tables 1 and 2 for reinforcement and mix data). Each set included 105 ASTM C-1550 round panels that, if sprayed, included a set accelerator at a dosage rate of about 4% bwc. Cores were also produced and tested in parallel with the panels. Uncracked control specimens from each set were kept continuously wet and tested at approximately 1, 2, 3, 7, 14, 28, 56, 91, and 180 days, and then 1, 2, 3, 5, and 10 years age. The remaining panels within each set were pre-cracked at 56 days age, and subsequently exposed in the field, before re-testing at 180 days, 1, 2, 3, 5, or 10 years. All the sprayed specimens were produced by manually spraying shotcrete into round steel and plywood forms that were propped at 45° against a rack. The specimens were moved to a flat surface immediately after spraying and screeded so as to achieve a flat surface and uniform thickness. They were then left outside under plastic sheeting to harden overnight before being stripped and transferred to curing tanks for 56 days of immersed curing. About 34–36 uncracked panels from each set were cured in water continuously for up to 10 years and tested at the required ages starting at 1 day age to examine the effect of aging independently of any corrosion effects. The remaining 66 panels from Table 1 Mix details for shotcrete sets examined. Reinforcement
Barchip Kyodo SL62 mesh* Dramix RC 65/ 35 BN Dramix RC 65/ 35 BN Dramix RC 65/ 35 BN Novotex 0730 Enduro 600 Enduro 600 Barchip BC54
Quantity
Grade
UCS Cyl. (MPa)
UCS Cores (MPa)
(kg/m3)
(MPa)
28d
10 yr
28d
56d
91d
10 yr
10 – 50
32 50 50
42 58 57
58 72 77
41 – 60
46 – 66
53 – 70
63 – 76
50
40
45
74
45
51
57
88
50
25
29
44
38
41
44
52
50 7 7 7
40 40 50 50
48 43 53 55
63 64 63 90
44 45 48 45
52 49 52 53
54 57 55 59
62 68 64 84
* Welded steel mesh comprising 6 mm deformed bars on a 200 mm orthogonal grid. 2
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Load resistance increased with age at small deflections (up to 5 mm, or 1.5 mm crack width) typical of service conditions, but fell with age at large deformations (20–40 mm, or 7–13 mm crack width) typical of ultimate load conditions. A contrasting series of load-deflection curves are shown for two of the macro-synthetic fibre reinforced mixtures in Fig. 2; these reveal a very consistent pattern of post-crack behavior that steadily increased in energy absorption but otherwise did not change much in shape with the passage of time. The energy absorption results for the uncracked controls up to 10 years age indicate that the influence of aging on energy absorption can be substantial for some types of steel fibre reinforcement. Aging appears to have less of an effect on energy absorption for the currently examined macro-synthetic fibre types. The change in energy absorption for each specimen set as a function of age is shown for the uncracked controls in Figs. 3–5. Each set of curves includes actual data points and curve-fits for energy absorption at 5 and 40 mm central deflection (representing maximum crack widths of 1.5 and 13 mm). Energy absorption has also been included at 10 and 20 mm central deflection (representing maximum crack widths of 3.5 and 7 mm, respectively) for the mixtures reinforced with hooked-end steel fibres because these sets demonstrated the most dramatic changes in performance with age. The energy absorption characteristics of the shotcrete reinforced with Dramix steel fibres clearly changed with the passage of time, but the magnitude of this change varied with the compressive strength of the concrete matrix and the degree of deformation at which performance was assessed. A transition from fibre pull-out to rupture occurred in the 50 and 40 MPa mixes before 90 days age (Fig. 3), but little change occurred in the 25 MPa mix even after 10 years aging. At 5 mm central deflection, energy absorption topped out at 140 Joules in the 50 and 40 MPa mixes and did not fall with the transition in fibre failure mode. For the 25 MPa mix, the 5 mm energy absorption at 28–56 days was about 110 Joules, and eventually topped out at about 130 Joules after 10 years. Thus, performance at narrow crack widths typical of inservice conditions was not substantially affected by the transition to an embrittled state even when a relatively high strength concrete mixture was used. In contrast, the performance of the Dramix-reinforced FRS at 40 mm central deflection changed dramatically with the transition from fibre pull-out to rupture. For the 50 MPa mix, the specimens absorbed about 500–550 Joules prior to embrittlement (at about 7 days age). For the 40 MPa mix, the maximum performance was around 650 Joules. However, both sets dropped to about 300 Joules after 90–180 days aging, and most of this energy was absorbed in the first 10 mm of central deflection (Fig. 3d). In contrast, the 25 MPa mix did not transition from fibre pull-out to rupture and experienced only a minor change in 40 mm energy absorption with the passage of time, rising to a maximum of about 550 Joules. The post-crack load-deflection curves for this mix showed that toughness at large deflections remained good even after 10 years aging, unlike the 40 and 50 MPa mixes for which residual capacity beyond 20 mm central deflection was minimal (Fig. 1). Intermediate levels of performance occurred for these same specimens when assessed at 10 and 20 mm central deflection. Cumulatively, these results indicate that as the degree of deformation is increased, the effect of embrittlement is more pronounced, hence embrittlement is primarily a problem affecting the ultimate limit state and not in-service performance. The energy absorption data obtained up to 10 years age indicate that none of the macro-synthetic FRS specimen sets exhibited a fall in performance with age. Indeed, each of them revealed a steady rise in performance at late ages, even at 40 mm central deflection (see Figs. 4 and 5). The SL62-reinforced specimens demonstrated a close-to-maximum post-crack performance capacity within one day of casting, and this barely changed with the passage of time. The specimens reinforced with Novotex 0730 steel fibres did not sustain the high level of postcrack load resistance exhibited by the hooked-end steel fibres, but proved better able to retain performance with age and thus appeared to
Table 2 Mix design for shotcrete used in each trial. Component
Coarse aggregate (10/7 mm CRG) Coarse sand (2 mm) Fine sand Cement (type GP) Fly Ash Silica Fume Water reducer (L/m3)
Nominal grade and quantity (kg/m3) 25 MPa
32 MPa
40 MPa
50 MPa
600 372 720 305 80 – 1.0
600 372 680 315 100 10 1.0
610 350 680 335 100 10 1.1
620 330 640 375 100 20 1.2
each set were pre-cracked at 56 days age and then left at a field exposure site for periods of between 180 days and 10 years before being re-tested to assess residual energy absorption. The pre-cracking procedure involved placing the specimen in a standard ASTM C1550 test machine and deflecting the center up to a central deflection corresponding to the required crack width, then reversing the displacement and removing the specimen. By examining both cracked and uncracked specimens at between 180 days and 10 years age, the influence of both corrosion and embrittlement on performance could be distinguished. The exposure site comprised a compound under a wide highway overpass about 15 km from the coast in Sydney, Australia. The site was intended to represent the entrance to a tunnel, and thus was protected from direct rain but not traffic-induced spray or ambient humidity. The panels were oriented in a vertical direction during storage and were therefore not subject to moisture settling into the cracks through the action of gravity. The exposure classification could be considered relatively benign with no contact with either groundwater or atmospheric chlorides. The temperature ranged from 6 to 42 °C seasonally, and averaged 22 °C. The relative humidity averaged 45–50%. No deicing salt was used on the adjacent roadways. According to the Model Code 2010 (2012), this corresponds to an XC3 exposure classification. The fibres used in the trial included both steel and macro-synthetics (Table 1). The Dramix RC65/35 BN fibres were hooked-end and made of cold-drawn steel wire of minimum 1100 MPa tensile strength. These had a diameter of 0.55 mm. The Novotex 0730 steel fibres were flattened-end and made of cold-drawn steel wire of 1400 MPa tensile strength and were 0.70 mm diameter. The Kyodo macro-synthetic fibres were (like all the macro-synthetics) made of polypropylene. They were embossed, 48 mm in length, 1.4 mm equivalent diameter, and 550 MPa tensile strength. The Enduro 600 fibre was a crimped fibre of 50 mm length and 540 MPa tensile strength. The BC54 fibre was embossed, 54 mm in length and 1.4 mm width, and 640 MPa tensile strength. The shotcrete mixtures used in the trials are listed in Table 2. The cement was a General Purpose (GP) type conforming with AS3972 (2010), and the Supplementary Cementitious Materials (Fly Ash and silica fume) conformed with AS3582 (2016).
3. Results The results of the investigation comprised load-deflection curves obtained for each of the C-1550 panel specimens and Unconfined Compressive Strength (UCS) data obtained from the cores and cylinders (Table 1). The results for the pre-cracked panels subject to exposure in the field were divided into a load-deflection curve obtained during the initial cracking test and then a second load-deflection curve obtained following exposure. The post-crack performance of these specimens has been summarized in terms of cumulative energy absorption at 5, 10, 20, and 40 mm total central deflection. Example sets of load-deflection curves for the Dramix RC 65/35 specimens produced using 50 MPa and 25 MPa shotcrete and continuously wet cured in the laboratory are shown in Fig. 1. These curves reveal that a dramatic change in toughness characteristics occurred with age for the higher strength mixture. 3
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Fig. 1. Load-deflection curves for specimens reinforced with Dramix RC65/35 in a) 50 MPa, and b) 25 MPa shotcrete tested at various ages after spraying showing an increase in load resistance at small deflections with aging for the higher strength mix but a fall in resistance at large deformations.
Fig. 2. Load-deflection curves for specimens reinforced with a) Kyodo in 32 MPa shotcrete, and b) Barchip BC54 in 50 MPa shotcrete tested at various ages after spraying showing a steady increase in load resistance both at small and large deflections with aging.
and absence of corrosion products around the fibres exposed within the cracks for the 50 MPa Dramix-reinforced mix suggested that limited corrosion occurred in these specimens, even for an initial crack width of 1.0 mm (Fig. 6). The 40 MPa mix exhibited a steady fall in performance with age, but the inconsistency with crack width and relative absence of corrosion products within the cracks suggested that this was at least partly attributable to the late effects of embrittlement (Fig. 7). For the 25 MPa mix, there was no evidence of embrittlement even after 10 years aging. However, corrosion was evident at the crack surfaces and appeared to have progressed to a greater extent in the 25 MPa mix than in the higher strength mixes despite identical exposure conditions (Fig. 8). Corrosion products were observed around many of the fibres intersecting each crack, particularly near the tensile face of the panels.
be less sensitive to embrittlement than the more slender hooked-end type of steel fibre. This was probably assisted by the higher tensile strength of the wire used to produce these fibres (1400 MPa). The results for the pre-cracked specimens shown in Figs. 6–11 indicate that the residual post-crack performance of steel FRS (normalized relative to uncracked performance at 180 days age) is quite variable but may decrease appreciably with the passage of time. Performance has been normalized with respect to energy absorption at 180 days because embrittlement was judged to have been largely complete by this age and thus any further change in performance was taken to be attributable primarily to corrosion. The magnitude of the decrease in performance after 180 days age is sensitive to both crack width and strength of the concrete matrix. The stability of the results 4
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Fig. 3. Energy absorption at a) 5 mm, b) 10 mm, c) 20 mm, and d) 40 mm for Dramix RC65/35 as a function of age.
performance even at lesser levels of deformation (Fig. 9a). The cracked macro-synthetic FRS specimens showed no sign of deterioration with the passage of time up to 10 years field exposure (Figs. 10 and 11). While some variation in performance was evident, there was no systematic change in performance with age both for the uncracked and cracked specimens. This indicates that macro-synthetic fibres made of polypropylene are highly resistant to in-field degradation, at least for the exposure conditions presently imposed. The mean energy absorption measured at 10 years of each set of cracked steel FRS panels has been divided by the measured performance of the corresponding uncracked panels at 180 days and is plotted as a function of nominal crack width in Fig. 12. The data show that the Dramix-reinforced set made with 50 MPa shotcrete was largely unaffected by corrosion at any crack width. The Dramix-reinforced set made with 40 MPa shotcrete was somewhat more affected by corrosion for wider cracks, particularly at a deformation of 40 mm (Fig. 12a). These results reflect the relatively low level of rust observed on the
This contrast between the 25 and 50 MPa mixtures suggests that the richer cementitious content typical of the higher strength mixes provided some protection for the exposed fibres within the cracks. It should be noted that most of the deterioration in performance due to corrosion occurred between 5 and 10 years age (up to 40% loss of energy absorption at 40 mm for the present benign exposure condition), so the observed adverse trend raises concerns about the level of performance likely to be retained by steel FRS linings at 50–100 years age, especially in aggressive conditions. The performance of the pre-cracked specimens with Novotex steel fibres in 40 MPa shotcrete appeared similar to that of the Dramix fibres in 25 MPa concrete (Fig. 9). Up to 5 years exposure, there was little variation in performance. However, at 10 years the fibres at the crack faces showed clear signs of corrosion and post-crack performance fell substantially compared to performance at 180 days (Fig. 9b). The loss of performance occurred primarily at 20–40 mm central deflection, but there appeared to be a small systematic downward trend in 5
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Fig. 4. Energy absorption at a) 5 mm, and b) 40 mm for synthetics as a function of age.
Fig. 5. Energy absorption at a) 5 mm, and b) 40 mm for mesh, Novotex, and Enduro in 40 MPa concrete, as a function of age.
exposed fibres in the cracks within these specimen sets, especially for the 50 MPa mixture. In contrast, the Dramix-reinforced set made with 25 MPa shotcrete revealed significant degradation due to corrosion that was more pronounced for both wider cracks and larger levels of deformation (Fig. 12b). A similar phenomenon occurred for the specimens reinforced with Novotex fibres in 40 MPa shotcrete. Both these specimen sets exhibited more substantial rust deposits around the fibres at 10 years age. It is notable that the compressive strength of shotcrete cores extracted for these latter two mixtures was somewhat lower than for the first two (Table 1), suggesting that the in-place water/binder ratio was lower for the Novotex set despite the same nominal strength grade for the two 40 MPa mixes. For all the cracked SFRS specimen sets, corrosion did not have a significant effect on 5 mm energy absorption, but for most had a significant negative effect on performance at larger
levels of deformation. Note that none of the FRS mixes exhibited a fall in UCS with age (Table 1). Inspection of the crack surfaces after testing revealed that for the uncracked control specimens stored in water, the loss of toughness that occurred with age was due to a change in the mechanism of postcrack fibre behavior as the concrete matrix became progressively stronger and harder. The loss of post-crack performance was therefore not attributable to degradation of the concrete matrix For the pre-cracked steel FRS specimens exposed in the field, the change in performance with aging and exposure appeared to be due to a combination of corrosion (evidenced by deposits of rust around the base of each fibre) and embrittlement. As the concrete matrix hardened with age the bond to the fibre and corresponding resistance to pull-out increased. This was manifested as an initial increase in energy absorption.
6
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Fig. 6. Energy absorption normalized relative to 180 day performance, at a) 5 and 10 mm, and b) 20 and 40 mm, for Dramix RC65/35 in 50 MPa shotcrete.
If the strength of the anchorage at the ends of the fibre exceeds the tensile strength of the fibre, the fibre will rupture rather than pull-out as the crack widens. Exposure to aggressive agents at the intersection with the cracks can lead to concurrent corrosion that may reduce the crosssectional area of fibre available to resist rupture. Examination with a field microscope indicated that many of the ruptured steel fibres suffered a loss of cross-section by 5–10 years age as a result of corrosion, thereby exacerbating their propensity to rupture at a widening crack rather than pull-out. The areas of corrosion were surrounded by black and brown deposits on the crack surfaces that appeared to be rust. This suggests that the effects of corrosion and embrittlement are cumulative and must be considered concurrently when assessing the long-term inservice performance of steel FRS infra-structure. All the 40 and 50 MPa steel FRS mixes exhibited an increase in energy absorption at small deformations with age, while concurrently
exhibiting a fall in energy absorption at large deformations. This meant that the residual load resistance at large deformations was doubly diminished with age (see Fig. 1 for typical load-deflection curves). In contrast, the macro-synthetic FRS mixes exhibited a steady increase in energy absorption with age at all levels of deformation. These results suggest that steel fibres may perform better under service loads (represented by energy absorption at 5 mm), but macro-synthetic fibres offer superior performance under ultimate strength conditions (represented by energy absorption at 20–40 mm), especially at late ages and in high strength shotcrete, because they exhibit a high-energy pullout mode of failure regardless of the age and strength of the concrete matrix.
Fig. 7. Energy absorption normalized relative to 180 day performance, at a) 5 and 10 mm, and b) 20 and 40 mm, for Dramix RC65/35 in 40 MPa shotcrete. 7
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Fig. 8. Energy absorption normalized relative to 180 day performance, at a) 5 and 10 mm, and b) 20 and 40 mm, for Dramix RC65/35 in 25 MPa shotcrete.
4. Consequences of embrittlement
shotcrete incorporating high cement and silica fume contents. FRS mixes intended for final linings also commonly incorporate high cementitious contents, supplementary cementitious materials, and low water/binder ratios due to requirements for impermeability related to a 100+ year design life. The problem with this type of shotcrete is that the late age strength can be extremely high (80+ MPa), leading to a change in post-crack failure mechanism from a high-energy frictionbased pull-out mode to a low-energy yielding mode for some types of fibre (such as the 1100 MPa steel fibres presently examined). This can reduce the ability of a lining to redistribute loads should ground movement occur at late ages which may lead to a requirement for rehabilitation at late ages. Late-age ground movement may occur, for example, in the event of hydrologically-induced changes in ground stability, roadway widening, nearby underground excavation (particularly in mines), or a change in groundwater level many years after the
The results of this trial indicate that, as far as ductility retention with aging is concerned, the consequences of embrittlement are comparable to the consequences of corrosion for steel FRS when placed in a benign environment. For ground support applications in which deformations are expected to be substantial, ductility is as important to the maintenance of satisfactory load resistance as the maximum strength of materials, hence the loss of ductility with age exhibited by many of the steel FRS mixtures examined in this investigation is cause for concern. The data shown in Figs. 3–5 is particularly important in the context of recent rapid underground excavation practice. Early re-entry during in-cycle shotcreting usually demands high early-age strength development, and this is most commonly achieved by using a high strength
Fig. 9. Energy absorption normalized relative to 180 day performance, at a) 5 and 10 mm, and b) 20 and 40 mm, for Novotex 0730 in 40 MPa shotcrete. 8
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Fig. 10. Energy absorption normalized relative to 180 day performance, at 5 and 40 mm for a) Kyodo, and b) Enduro in 40 MPa shotcrete.
5. Conclusions An investigation was undertaken into long-term changes in postcrack performance of Fibre Reinforced Shotcrete mixtures reinforced with either steel or macro-synthetic fibres. The investigation involved a field exposure trial in which both uncracked and pre-cracked ASTM C1550 panels were aged for up to 10 years to examine the effects of embrittlement and corrosion on post-crack energy absorption. The results demonstrated that post-crack performance at relatively small crack widths is not affected by embrittlement but in-field exposure in even a benign exterior environment can promote substantial corrosion and performance loss for steel fibre reinforced mixtures. Aging-related effects including high strength development and hardening of the cement matrix can change the mechanism of failure for some types of fibre leading to a loss of post-crack performance at large deformations. The effect of corrosion and embrittlement is negligible at small crack widths (here represented by energy absorption at 5 mm in the ASTM C1550 panel test) pertinent to service conditions, but quite pronounced at wide crack widths (represented by energy absorption at 20–40 mm) relevant to ultimate limit state considerations. The current exposure tests, in an environment free of percolating groundwater or chloride ions, revealed a loss of 25–40% energy absorption at 40 mm deflection after 10 years of exposure even for steel fibre reinforced specimens with cracks 0.1–0.2 mm wide. In contrast, the macro-synthetic fibre reinforced shotcrete specimens showed no change in performance over 10 years of exposure regardless of in-service crack width. The present results demonstrate that when designing a shotcrete lining, it is necessary to utilise the most suitable fibre type to guarantee performance over the intended service life time for the exposure conditions expected. The use of some fibre types in unfavourable conditions may lead to significantly lower performance at late age than required and measured at 28 days.
Fig. 11. Energy absorption normalized relative to 180 day performance, at 5 and 40 mm for BC54 in 50 MPa shotcrete.
FRS lining was initially installed. It may also occur as a result of loosening of rock in flat roof or shallow-arched tunnels (Decker et al., 2012). If high toughness is required throughout the life of a FRS structure, then it is necessary to look beyond the performance achieved at 28 days and consider the effects of excessive strength development and embrittlement on late age performance. The present data indicate that fibres intended for long life structures should be selected on the basis of demonstrated performance at late ages. The assumption that excellent toughness obtained in 28 day Quality Control (QC) specimens will automatically translate to similar levels of performance throughout the design life of a structure could lead to serious over-estimates of late-age structural ductility in long-life infrastructure.
Declaration of Competing Interest The author declares that there is no conflict of interest. Acknowledgements This work was supported by funding and material support from the 9
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Fig. 12. Energy absorption at 10 years relative to uncracked 180 day performance, for a) cracked Dramix reinforced panels in 50 MPa and 40 MPa shotcrete, and b) cracked Dramix reinforced panels in 25 MPa shotcrete, and cracked Novotex reinforced panels in 40 MPa shotcrete.
Roads and Maritime Service of New South Wales, Transurban Roads, Barchip Australia P/L, Propex Concrete Systems, and Holcim Concrete Australia Ltd. The sponsors played no role in design of the study or the interpretation or reporting of the results.
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