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Recycled glass fiber reinforced polymer additions to Portland cement concrete
Construction and Building Materials 146 (2017) 238–250
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Recycled glass fiber reinforced polymer additions to Portland cement concrete Alireza Dehghan a, Karl Peterson a,⇑, Asia Shvarzman b a b
Univ. of Toronto, Dept. of Civil Eng., 35 St. George St., Toronto, ON M5S 1A4, Canada Antex Western, 1340 Church Ave., Winnipeg, MB R3C 2L4, Canada
h i g h l i g h t s Recycled glass fiber polymers did not cause expansive alkali silica reactions. Recycled glass fiber polymers exhibited pozzolanic behavior. An absorption test for recycled glass fiber polymer additions is presented.
a r t i c l e
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Article history: Received 24 October 2016 Received in revised form 2 April 2017 Accepted 3 April 2017 Available online 20 April 2017 Keywords: Recycled glass fiber reinforced polymer Alkali silica reaction Drying shrinkage Concrete microscopy
a b s t r a c t With recent developments in grinding and sorting technology it is possible to recover glass fibers from waste glass fiber reinforced polymers (GFRPs). The recycled fibers still retain some of the polymer and filler materials, but have the potential to provide some of the same benefits achieved by conventional fiber additions. The research program explored the influence of recycled GFRP on compressive strength, splitting tensile strength, and drying shrinkage in concrete, and alkali silica reaction (ASR) expansion in accelerated mortar beam and concrete prism tests. Compressive strength and drying shrinkage were not improved by recycled GFRP additions at a substitution level of 5 wt% of the coarse aggregate, but splitting tensile strengths were improved in most cases. Negligible expansion was observed from the ASR testing. A scanning electron microscope investigation of the concrete prisms indicated a pozzolanic reaction of the glass fibers. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Fiber reinforced polymer (FRP) is a composite material which is usually made with glass (GFRP), carbon (CFRP), or aramid (AFRP) fibers dispersed in a thermoset polyester resin and has a wide range of applications in the construction industry. Among GFRPs, E-glass is the most common reinforcement, and represents approximately 99% of the commercial market [1]. The ‘‘E” in E-glass is a carryover from its initial application, as used in electrical standoff insulators [2]. The compositional ranges for E-glass used in general applications (such as GFRP) are outlined in ASTM D578 [3] (Table 1). At the end of the life cycle of FRPs, fibers cannot be easily separated from the resin and the resin itself cannot be easily decomposed or recycled. Hence, landfill and incineration are the most common methods for FRP waste management [4,5]. Globally, GFRP
⇑ Corresponding author. E-mail address: [email protected] (K. Peterson). http://dx.doi.org/10.1016/j.conbuildmat.2017.04.011 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
production is estimated at 8 million metric tons annually, with GFRP waste production at 1.5 million metric tons [6]. Given the potential negative environmental impacts of waste FRP, the material has gained the attention of other industries to develop techniques and methods to recycle FRPs. Thankfully, recent advances in grinding and sorting technologies allow for the partial recovery of fibers from FRP and has made their utilization in Portland cement concrete an option. Considering that grinding and sorting equipment are readily available, and that the process produces negligible atmospheric pollution in terms of volatile organic compound emissions, size reduction by mechanical recycling is preferred over other recycling processes [7]. The topic of recycled GFRP in concrete necessarily overlaps with a number of different areas of research, including: glass fiber reinforced concrete (GFRC), waste glass powder concrete, and concrete made using waste plastic fine and coarse aggregate. As such, recycled GFRP combines some of the beneficial aspects of fibers and powdered waste glass, as well as some of the shortcomings of waste plastic aggregates.
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A. Dehghan et al. / Construction and Building Materials 146 (2017) 238–250 Table 1 Certified chemical composition for glass fiber products used in general applications [3]. Chemical
% by Weight
B2O3 CaO Al2O3 SiO2 MgO Na2O + K2O TiO2 Fe2O3 Fluoride
0–10 16–25 12–16 52–62 0–5 0–2 0–1.5 0.05–0.8 0–1
ACI 544.R-96 [8] constitutes a state-of-the-art report on fiber reinforced concrete, with an entire chapter dedicated to GFRC. Fibers in concrete are used to control cracking and to improve tensile and flexural strength. Early formulations of GFRC utilized Eglass fibers, but their use declined after the development of alkali resistant (AR) glass fibers, which performed much better over the long-term in the high-pH environment of concrete pore water. More recently, basalt fibers, made from melting basaltic rock, have been used to produce basalt fiber reinforced concrete (BFRC) [9– 11]. In this research, fibers recovered from recycled GFRP are utilized, but the fibers tend to occur in grouped masses bound by residual resin, as opposed to individual clean fibers. Waste glass is increasingly being utilized in concrete, both as an aggregate and as a powdered pozzolanic addition to concrete. Shi and Zheng [12] provide a thorough review of the topic, and it remains a very active area of research today. However, the majority of the research to date focuses on waste soda lime glass sources from containers or float/plate glass, with relatively few studies on the pozzolanic aspects of recycled GFRP. Xu et al. [13] investigated incinerated waste GFRP from the automotive industry, and used the ash as a pozzolanic additive. Chen et al. [14] investigated the use of ground waste E-glass fibers left over from circuit board manufacture that had never been used in FRP. They documented improvements in compressive strength, chloride and sulfate resistance. Similarly Mastali et al. [15] reported improvements in compressive and flexural strength for recycled glass fibers recovered from woven fiber sheets that had never been used in FRP. However, ground recycled GFRP, a combination of E-glass fibers, resins, and filler materials, is more problematic, as the resins and fillers provide no pozzolanic benefit. Gu and Ozbakkaloglu [16] conducted an extensive review of the utilization of recycled plastics in concrete from the standpoint of plastic fine aggregate, plastic coarse aggregate, and plastic fibers. While recycled plastic fibers tended to improve mechanical properties of concrete, recycled plastic aggregates led to reductions in compressive and tensile strength, and in most cases increased drying shrinkage. A common concern with the usage of recycled GFRP in cementitious binders is the potential for interference with the mechanical performance, particularly reductions in compressive strength. The
feasibility of recycled GFRP concrete has been explored by a number of researchers worldwide, with a comprehensive review recently provided by Yazdanbakhsh and Bank [17]. Asokan et al. [18] reported reduced compressive strengths with increasing recycled GFRP powder substitution when cured in water at 20 °C, but found increased compressive strengths compared to the control when oven cured at 50 °C. Tittarelli and Moriconi [19] and Tittarelli and Shah [20] reported reductions in compressive strength with recycled GFRP powder additions, but some improvements in terms of reduced drying shrinkage, and lower values for capillary water absorption. Correia et al. [21] explored the use of fines produced during the cutting of pultruded GFRP, and found similar reductions on compressive strength. Osmani [22] also reported reductions in compressive strength for concrete produced with recycled GFRP powder. Alam et al. [23] and Yazdanbakhsh et al. [24] both explored the substitution of larger aggregate sized FRP scrap particles, and reported reductions in both compressive and flexural strength. Alternatively, García et al. [25] explored the use of fibers recovered from recycled FRP, termed ‘‘glass fiber fluff,” and reported improvements in compressive and flexural strength when grinding and sieving is optimized. Expansive ASR is still cited as a concern for concrete as E-glass fibers are not stable in the high-alkali environment of the pore water [26,27]. Meanwhile the potential for powdered waste glass to mitigate ASR has been extensively documented, but not necessarily from the standpoint of waste E-glass [28–41]. Chen et al. [14] found negligible expansion due to alkali silica reaction (ASR) for concrete made with powdered recycled E-glass fibers that had never been used in GFRP. García et al. [25] reported expansions of <0.04% for concrete beams produced with glass fiber fluff recovered from recycled GFRP. Tittarelli and Moriconi [19] tested recycled GFRP powder using the recently withdrawn standard ASTM C289 [42], and found it to be innocuous. However, it is widely recognized that ASTM C289 is not a reliable test for predicting the reactivity of carbonate aggregates [43], and calcium carbonate is a common filler material in FRP. 2. Materials and methods Descriptions of the commercially produced GFRPs that were investigated in this study are presented in Table 2, and images of the fibers recovered from recycled GFRP are provided in Fig. 1. The recycled GFRP was produced using a model GM2411-50 ECO GrinderTM [44] single stage hammer mill grinding system with a 19 mm (3/4 in.) screen coupled to a pneumatically fed hopper with a dust collection bag and an enclosed auger feed to a series of 9.5 mm (3/8 in.) and 4.75 mm (3/16 in.) perforated opening trommel (rotary) screens. The GFRP feed consisted of sheets with a maximum thickness of 25.4 mm (1 in.) and nominal dimensions of 150 914 mm (6 36 in.). Table 3 provides a summary of the wt% glass fiber fluff retained by the screens. Materials separated by both screens were recombined for the purposes of this study. The bottom fines were not included. The results of sieve analyses performed on the combined materials are provided in Fig. 2.
2.1. Water absorption, density, loss on ignition, and fiber cluster length From each recycled GFRP source three representative samples, each with an approximate mass of 100 g, were produced by the ASTM C702 [45] quartering method for a determination of water absorption. Samples were completely immersed in tap water for 72 ± 2 h and stirred at least once every 24 h for one
Table 2 GFRP types, content, and abbreviations used in this study. Type
Resin
Glass content (vol.%) as manufactured
Abbreviation
Structural sheet molding composite Structural sheet molding composite Structural sheet molding composite Light resin transfer mold E-glass fiber
Bisphenol-A epoxy vinyl ester Novolac-based epoxy vinyl ester Flame retardant epoxy vinyl ester Unsaturated polyester None
40 40 40 25 100
EVE1 EVE2 EVE3 UP Virgin
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Fig. 1. Overall appearance of the recycled GFRP materials (a–d), and virgin E-glass fibers (e).
Table 3 Size-separated materials expressed as wt% of initial GFRP feed. Type
EVE1 EVE2 EVE3 UP
wt% 9.5 mm screen
4.75 mm screen
Bottom fines
Loss to dust collection
24 27 27 26
18 7 8 4
55 63 58 71
3 3 7 Negligible
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Fig. 2. Size distribution of recycled GFRP materials.
Table 4 Density, water absorption, glass fiber content, and length of recycled FRPs. Source
Density (g/cm3)
Absorption (%)
E-glass content (wt%)
Max. fiber cluster length (mm)
Avg. fiber cluster length (mm)
UP EVE1 EVE2 EVE3 Virgin
1.63 ± 0.02 1.66 ± 0.02 1.56 ± 0.02 1.60 ± 0.02 2.60 ± 0.04
63.1 ± 3.4 30.1 ± 4.9 25.5 ± 4.5 28.8 ± 6.4 0.4 ± 0.05
54.3 ± 1.2 43.1 ± 1.0 41.1 ± 0.8 42.0 ± 0.5 98.1 ± 0.0
36.4 26.8 33.5 31.3 25.6
21.7 ± 6.3 17.2 ± 5.7 21.2 ± 6.3 19.5 ± 7.5 25.4 ± 0.4
minute to ensure complete submersion. After soaking, the excess water was decanted over a 75 mm sieve to avoid loss of material, and the samples placed in individual open containers. A saturated surface dry (SSD) condition was gradually achieved through exposure over a period of hours in an environmental chamber at 35 ± 2 °C and 15 ± 2% relative humidity (RH) with frequent stirring to ensure homogenous drying. To determine if surface moisture was still present, the containers were inverted onto an absorbent sheet of unbleached paper towel to check whether or not the recycled GFRP material would dampen the paper. If any moisture was observed on the paper, the specimen was returned to the environmental chamber and re-tested in the same manner until it reached a SSD condition. After achieving an SSD condition, the samples were weighed, and then dried in a vented oven at 100 ± 2 °C. Earlier experiments conducted at a drying temperature 105 ± 5 °C resulted in a strong odour and color change of the recycled GFRP, so the alternative drying approach at 100 ± 2 °C was employed. A helium pycnometer was used to measure the densities of the recycled GFRP. Three representative dry samples with an approximate mass of 4–5 g were used and the volume of each sample was measured three times. The averages of nine readings are provided in Table 4. UN1046 compressed helium as recommended by the manufacturer at a pressure of 1.2 kg/cm2 was used in this study. Water absorption values for the recycled GFRPs are also listed in Table 4. Loss on ignition was measured with three representative test specimens, each with an approximate mass of 5 g and conditioned at 23 ± 2 °C and 50 ± 10% RH to meet the requirements of ASTM D2584 [46]. The samples were heated in a muffle furnace at 300 °C for 2 h, then cooled to room temperature in a desiccator, and
weighed. Next, the samples were placed back into the muffle furnace, heated at 560 °C for 2 h, cooled, and reweighed at room temperature. Preliminary investigations found that heating at 560 °C for 2 h was sufficient to remove all organic material. Finally, the samples were heated to 960 °C for 2 h, cooled, and the mass of the residues measured at room temperature. Negligible mass loss was found between 560 °C and 960 °C, and indicated minimal inclusion of carbonate-based fillers. The difference between initial mass and the loss on ignition at 560 °C was used as an approximation of glass content. Mass losses are presented in Fig. 3. The glass contents reported for the crushed and sieved GFRP by ignition were slightly higher than the as-manufactured GFRP. The enrichment may partly be attributed to the loss of resin fines during the crushing and sieving operations. To characterize fiber lengths, representative 5 g samples were obtained, and fiber clusters manually plucked from the fluff, and images recorded with a flatbed scanner (Fig. 4). Digital image processing was used to obtain the fiber cluster lengths reported in Table 4.
2.2. ASR testing 2.2.1. Accelerated mortar bar test Susceptibility to ASR was tested in accordance with CSA A23.2-25A [47] on four 25 25 285 mm mortar bars, but with 5 wt% replacement of the coarse aggregate (an inert crushed dolostone) with recycled GFRP. CSA A23.2-25A was followed,
Fig. 3. Mass loss of recycled GFRPs and virgin E-glass fibers at different temperatures.
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Fig. 4. Fiber clusters manually plucked from glass fiber fluff (a–d), and virgin E-glass (e).
Table 5 Mix designs of mortar mixtures for the accelerated mortar bar test. Source
Recycled GFRP (kg/m3)
Crushed agg. (kg/m3)
GU cement (kg/m3)
Water (kg/m3)
Control UP EVE1 EVE2 EVE3
0.0 65.7 65.7 65.6 65.7
1327.1 1247.9 1248.3 1247.1 1247.6
589.8 583.8 583.9 583.4 583.6
294.9 291.9 292.0 291.7 291.8
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A. Dehghan et al. / Construction and Building Materials 146 (2017) 238–250 Table 6 Fresh properties of the mortar mixtures. Source
Temp (°C)
Gravimetric air content (%)
Mix yield (kg/m3)
Flow (%)
Control UP EVE1 EVE2 EVE3
25.7 23.3 23.9 22.8 23.3
2.3 12.3 4.4 9.2 4.4
2243.9 2032.5 2192.0 2108.9 2191.9
83 36 67 51 68
Table 7 Mix designs for concrete prism test. Source
Recycled GFRP, dry mass (kg/m3)
Cement (kg/m3)
Water (kg/m3)
Coarse aggr. dry mass (kg/m3)
Fine aggr. dry mass (kg/m3)
NaOH pellets (kg/m3)
Moisture held by the SSD GFRP (kg/m3)
Control UP EVE1 EVE2 EVE3 Virgin
– 19.5 19.9 18.7 19.2 31.2
415.0 415.0 415.0 415.0 415.0 415.0
186.7 186.7 186.7 186.7 186.7 186.7
1079.5 1060.0 1060.0 1060.0 1060.0 1060.2
714.4 701.4 701.4 701.4 701.4 701.6
1.28 1.28 1.28 1.28 1.28 1.28
– 12.3 6.0 4.7 5.4 0.1
Table 8 Fresh properties of concrete mixtures for concrete prism test. Source
Temp (°C)
Slump (mm)
Pressure meter air content (vol%)
Unit weight (kg/m3)
Control UP EVE1 EVE2 EVE3 Virgin
21.3 21.1 21.3 21.1 20.7 27.1
145 75 140 170 170 30
2.5 4.0 2.9 3.1 2.9 4.6
2411.1 2361.8 2408.3 2397.4 2405.3 2344.3
Table 9 Fresh properties of concrete mixtures. Source
Temp (°C)
Slump (mm)
Pressure meter Air content (%)
Unit weight (kg/m3)
Control UP EVE1 EVE2 EVE3 Virgin
21.5 20.7 22.0 22.0 20.3 27.7
125 80 165 165 175 50
2.6 3.9 2.9 2.7 2.9 4.3
2417.1 2365.7 2417.1 2407.9 2412.9 2372.1
except that the exposure time was extended from 14 d to 28 d. The mix designs of the mortars are presented in Table 5, and fresh properties of the mortar mixtures are presented in Table 6. CSA A23.2-25A specifies that the aggregate used for mixing be in an oven dry state. Since the recycled GFRP was being treated on the basis of a wt% replacement of aggregate, it was also brought to an oven dry state prior to mixing. CSA A23.225A prescribes a fixed w/cm (water to cementitious ratio) of 0.50 for mixtures con-
Fig. 5. 7 and 28 d compressive strengths for non-air entrained concrete cylinders made with recycled GFRP, without any GFRP (control), and with virgin E-glass fibers.
taining crushed aggregate, and a mass ratio of cement to aggregate of 1:2.25, but makes no definitive statement regarding corrections to the water content based on aggregate absorption. As such, no corrections to the water content were made to account for water absorption by the aggregate or recycled GFRP. As shown in Table 5, utilization of recycled GFRP increased the air content. Given the morphology, air is necessarily entrapped within the glass fiber fluff when added in a dry state. From Table 5, recycled GFRP additions also decreased the flowability of the mixtures. Absorption of mix water by the dry glass fiber fluff likely contributed to the reduction in workability. As such, a vibration table was selected as the
Fig. 6. 7 and 28 d tensile strengths for non-air entrained concrete cylinders made with recycled GFRP, without any GFRP (control), and with virgin E-glass fibers.
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Fig. 7. Relationship between water retained in the SSD recycled GFRP materials and virgin fibers and the 7 and 28 d compressive strengths.
Fig. 8. Length changes due to drying shrinkage of concrete prisms made with recycled GFRP, without any FRP (control), and with virgin E-glass fibers.
method of compaction. The reported flow results were obtained after 10 drops of an ASTM C230 [48] flow table, and all of the mortar mixtures were workable enough to be cast into the molds. 2.2.2. Concrete prism test The risk of expansive ASR was assessed in accordance with CSA A23.2-14A [49] on three 75 75 285 mm prisms per recycled GFRP source, but with one significant modification: the recommended temperature of 38 °C was elevated to 50 ± 3 °C. The change in temperature was made to accelerate the rate of ASR, and thereby shorten the duration of testing. As with the accelerated mortar bar test, the inert coarse aggregate was substituted by 5 wt% with recycled GFRP. CSA A23.2-14A specifies that concrete be mixed according to CSA A23.2-2C [50], so aggregate moisture corrections must be accounted for. Prior to mixing, the recycled GFRP materials were prepared in SSD condition using the same procedure used to measure absorption. After achieving a SSD condition, the recycled GFRP materials were sealed in air-tight containers and allowed to equilibrate to room temperature.
Coarse aggregate for mixing was immersed for 24 h prior to use, with excess water decanted prior to mixing. Water clinging to the coarse aggregate was accounted for as a contribution to the mix water. Similarly, fine aggregate of a known moisture condition was used for mixing, and water clinging to the fine aggregate was accounted for as a contribution to the mix water. Mix designs are shown in Table 7. Results for fresh properties of the concrete mixtures are presented in Table 8. 2.3. Mechanical properties A second batch of concrete without the addition of sodium hydroxide was cast for each source of recycled GFRP using the same mixing procedure as described in Section 2.2. Twelve 100 200 mm concrete cylinders and three 75 75 285 mm concrete prisms were cast to measure compressive strength, tensile strength, and length changes due to drying shrinkage at 23 ± 2 °C and 50 ± 4% RH. Samples were cured in lime-saturated water for the periods specified by the corresponding standards. Fresh concrete test results are provided in Table 9.
Fig. 9. Relationship between the water retained in the SSD recycled GFRP materials and virgin fibers and drying shrinkage.
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Fig. 10. Expansions during accelerated mortar bar test for samples made with recycled GFRP, and without GFRP (control).
Fig. 11. Expansions for concrete prisms made with recycled GFRP, without any GFRP (control), and with virgin E-glass fibers.
3. Results and discussion 3.1. Concrete strength CSA A23.2-9C [51] compressive strength results for the concrete samples made with recycled GFRP, virgin E-glass fibers, and without any fiber additions are presented in Fig. 5. Fig. 6 presents the results of CSA A23.2-13C [52] tensile splitting tests, where the sample is failed through indirect tension by applying a compres-
sive load to the diametral plane of a cylinder. Fig. 7 plots the water retention of the SSD recycled GFRP materials versus compressive strength. Compressive strength decreased as recycled GFRP water retention increased, which suggests that the water contained within the SSD recycled GFRP glass fiber fluff may have been released during concrete mixing, locally increasing the water to cementitious ratio (w/cm). Alternatively, in the case of ultra high performance concrete (UHPC) with very low w/cm levels, such sources of internal curing water has proved beneficial in terms of
Fig. 12. Mass changes for concrete prisms made with recycled GFRP, without any GFRP (control), and with virgin E-glass fibers.
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Fig. 13. A porous cluster of glass fibers in thin section from sample UP concrete as observed in fluorescent mode (a), back scattered electron image (b), and close-up (c) with black arrows showing locations of characteristic X-ray spectra from interior of glass fiber (d) and exterior (reacted) glass fiber (e).
strength gain [53,54]. Tensile strength in most cases increased with recycled GFRP as compared to the control, with the exception of the EVE2 mixture. 3.2. Drying shrinkage Length changes in concrete prisms due to drying at 23 ± 2 °C and a relative humidity of 50 ± 4% were tested according to CSA A23.2-21C [55] with the results shown in Fig. 8. During the first week of testing, the mixture containing virgin Eglass fibers experienced the least amount of drying shrinkage. However, in the subsequent weeks and months, the virgin mixture performed similarly to the EVE1 and UP mixtures, with shrinkage
values exceeding the control by 17%. The mixtures with the highest values for water retention (EVE1 and UP) experienced the most drying shrinkage of all the recycled GFRP mixtures (Fig. 9). This result was unexpected, as the purposeful addition of saturated light weight aggregate (LWA) to help reduce autogenous shrinkage is now a widely adopted strategy to counteract shrinkage [53,54,56–58]. However, at the w/cm level of 0.45 explored in this study, autogenous shrinkage is not an issue. Wang et al. [59] reported similar shrinkage issues for concrete made with polypropylene fiber mesh, as well as recycled fibers from tires or waste carpet. They attributed the increase in shrinkage to porosity contributed by the fiber masses. An alternative explanation for the slightly higher levels of drying shrinkage could be that the inher-
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ently lower stiffness of the recycled GFRP provided less resistance to shrinkage [16].
3.3. Alkali silica reaction Results of the accelerated mortar bar test are presented in Fig. 10. The expansion values for all of the mortar mixes made with recycled GFRP in this study were below the recommended 0.1% limit. Furthermore, results of the concrete prism test for the five types of the recycled GFRP are provided in Fig. 11, and the expansion values are all below the recommended 0.4% expansion limit. Changes in mass were recorded, and are shown in Fig. 12. The concrete prisms made with recycled GFRP (except EVE1) experienced mass losses between the initial reading (measured after demolding) and up to 14 d of exposure at 50 °C. This unusual reduction in mass could possibly be attributed to water from the SSD mats of the recycled glass fibers migrating to the exterior of the prisms at the elevated temperature of 50 °C. Drying of the prisms due to the escape of water from the sealed pails was ruled out, since the level of water ponded at the base of the pails appeared constant during testing. Furthermore, the absorbent sheets lining the inside walls of the pails remained fully saturated throughout the duration of the test.
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3.4. Microstructure of concrete prisms Figs. 13–16 show images of recycled GFRP and virgin glass fibers in polished thin sections prepared from the concrete prisms after four months of ASR testing. Prior to sectioning, a fluorescent-dyed low viscosity epoxy resin was used to fill the capillary pores and air voids present in the concrete. In the fluorescent images darker regions (less fluorescence) represent areas with low porosity, and brighter regions (more fluorescence) represent areas with high porosity [60]. ASR-related cracking was not observed in any of the samples. Typically, concrete that has undergone expansive ASR will exhibit deposits of alkali silica gel lining air-voids or within cracks. Alkali-silica gel deposits were not directly observed in the samples, although in some cases it appeared that the capillary pores of the hardened cement paste immediately surrounding some recycled GFRP fragments may have been filled/densified through the uptake of alkali-silica gel (Fig. 13a). Concrete containing recycled GFRP from the higher absorption UP material more frequently exhibited porous mats of glass fibers (Fig. 13) as compared to concrete made with the relatively lower-absorption recycled GFRP sources (Figs. 14 and 15). Fig. 16 shows virgin E-glass fibers.
Fig. 14. A fragment of flame retardant (brominated) epoxy vinyl ester resin with embedded glass fibers in thin section from sample EVE1 as observed in fluorescent mode (a), backscattered electron image (b), and close-up (d), showing location (black arrow) of characteristic X-ray spectrum collected from the FRP resin.
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Fig. 15. A fragment of bisphenol-A epoxy vinyl ester in thin section with embedded glass fibers from sample EVE3 as observed in fluorescent mode (a), back scattered electron image (b), and close-up (c) with black arrows showing locations of characteristic X-ray spectra from interior of glass fiber (d) and exterior (reacted) glass fiber (e).
4. Conclusions Reductions in workability have been previously reported for concrete with fibers recovered from recycled GFRP and attributed to increased water demand due to the irregular geometry and the high specific surface of the fibers [25]. In the research presented here, the reduction in workability was overcome by preconditioning the recycled GFRP to a SSD condition. However, the relatively large amounts of water retained in the SSD condition may have had a negative contribution in terms of compressive strength; the water retained by the SSD recycled GFRP in many cases appears to have had a local influence on w/cm, increasing
the capillary porosity of the adjacent cement paste (Figs. 14a and 15a). Furthermore, rather than being dispersed as individual fibers, in most cases the recovered fibers from recycled GFRP and from the virgin E-glass fibers occurred in the concrete as porous clumps, and these would also be expected to have a negative impact on compressive strength. On the other hand, the fibers from recycled GFRP and the virgin E-glass fibers did contribute to improvements in splitting tensile strength. Drying shrinkage was not improved by the addition of recycled GFRP or virgin E-glass fibers as compared to the control concrete, a finding that contrasts with García et al. [25] who reported a modest improvement. But, the overall values for drying shrinkage
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Fig. 16. Virgin E-glass fibers in concrete thin section, backscattered electron image (a), and close-up (b) with black arrows showing locations of characteristic X-ray spectra from interior of glass fiber (c) and exterior (reacted) glass fiber (d).
reported here remain within a narrow range similar to the control. Although a fixed replacement level of 5 wt% of recycled GFRP for aggregate was used throughout this study, the replacement level could be revisited and refined in an effort to minimize shrinkage, and to maximize improvements to tensile strength. Accelerated mortar bar and concrete prism ASR testing showed expansion values well below the recommended expansion limits. Comparisons of X-ray energy dispersive spectra (EDS) collected from points at the perimeters of the glass fibers to points further within the glass fibers show an enrichment and calcium (Figs. 13d & e, 15d & e, 16d & e), which suggests the pozzolanic reaction of the fibers to form hydration products. Given the slender profile of the exposed glass fibers, on the order of 10 mm, the glass fibers perform as a pozzolanic addition similar to expansive ASRmitigating coal combustion fly ash or ground granulated blast furnace slag as opposed to performing as an ASR expansion-inducing aggregate particle.
Acknowledgements This research was funded by the NSERC Engage Grants Program, Canada, EGP #468708-14.
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[41] K. Afshinnia, P.R. Rangaraju, Impact of combined use of ground glass powder and crushed glass aggregate on selected properties of Portland cement concrete, Constr. Build. Mater. 117 (2016) 263–272. [42] ASTM C289, Standard Test Method for Potential Alkali-Silica Reactivity of Aggregates (Chemical Method) West Conshohocken, Pennsylvania, USA, 2007, p. 7. [43] D. Stark, Alkali-silica reactions in concrete, in: J.F. Lamond, J. Pielert (Eds.), Significance of Tests and Properties of Concrete and Concrete Making Materials, ASTM STP 1679C, ASTM International, Bridgeport, New Jersey, USA, 2006, pp. 401–409. [44] R.J. Wolf, Method of recycling Fiberglass Reinforced Plastics, U.S. Patent 8,361,358, 2013. [45] ASTM C702, Standard Practice for Reducing Samples of Aggregate to Testing Size West Conshohocken, Pennsylvania, USA, 2011, p. 5. [46] ASTM D2584, Standard Test Method for Ignition Loss of Cured Reinforced Resins West Conshohocken, Pennsylvania, USA, 2011, p. 6. [47] CSA A23.2-25A, Test Method for detection of alkali-silica reactive aggregate by accelerated expansion of mortar bars, in: CSA A23.1/A23.2 Concrete Materials and Methods of Concrete Construction/Methods of Test and Standard Practices for Concrete, Canadian Standards Association, Mississauga, Ontario, Canada, 2015, pp. 425–433. [48] ASTM C230, Standard Specification for Flow Table for Use in Tests of Hydraulic Cement West Conshohocken, Pennsylvania, USA, 2014, p. 6. [49] CSA A23.2-14A, Potential expansivity of aggregates (procedure for length change due to alkali-aggregate reaction in concrete prisms at 38 °C), in: CSA A23.1/A23.2 Concrete Materials and Methods of Concrete Construction/ Methods of Test and Standard Practices for Concrete, Canadian Standards Association, Mississauga, Ontario, Canada, 2015, pp. 350–362. [50] CSA A23.2-2C, Making concrete mixes in the laboratory, in: CSA A23.1/A23.2 Concrete Materials and Methods of Concrete Construction/Methods of Test and Standard Practices for Concrete, Canadian Standards Association, Mississauga, Ontario, Canada, 2015, pp. 552–564. [51] CSA A23.2-9C, Compressive strength of cylindrical concrete specimens, in: CSA A23.1/A23.2 Concrete Materials and Methods of Concrete Construction/ Methods of Test and Standard Practices for Concrete, Canadian Standards Association, Mississauga, Ontario, Canada, 2015, pp. 552–564. [52] CSA A23.2-13C, Splitting tensile strength of cylindrical concrete specimens, in: CSA A23.1/A23.2 Concrete Materials and Methods of Concrete Construction/Methods of Test and Standard Practices for Concrete, Canadian Standards Association, Mississauga, Ontario, Canada, 2015, pp. 582–589. [53] R. Henkensiefken, J. Castro, D. Bentz, T. Nantung, J. Weiss, Water absorption in internally cured mortar made with water-filled lightweight aggregate, Cem. Concr. Res. 39 (2009) 883–892. [54] M. Golias, J. Castro, J. Weiss, The influence of the initial moisture content of lightweight aggregate on internal curing, Constr. Build. Mater. 35 (2012) 52– 62. [55] CSA A23.2-21C, Test Method for length change of hardened concrete, in: CSA A23.1/A23.2 Concrete Materials and Methods of Concrete Construction/ Methods of Test and Standard Practices for Concrete, Canadian Standards Association, Mississauga, Ontario, Canada, 2015, pp. 629–635. [56] L.E. Rendon, M. Rendon, N. Ramirez, LWA as Internal Curing Agent to Control Availability of Internal Curing Water in Concrete, MRS Proceedings, vol. 1612, 2013. [57] P. Lura, M. Wyrzykowski, C. Tang, E. Lehmann, Internal curing with lightweight aggregate produced from biomass-derived waste, Cem. Concr. Res. 59 (2014) 24–33. [58] T. Ji, B.B. Zhang, Y.Z. Zhuang, H.C. Wu, Effect of lightweight aggregate on earlyage autogenous shrinkage of concrete, ACI Mater. J. 112 (3) (2015) 355–364. [59] Y. Wang, H.C. Wu, V.C. Li, Concrete reinforcement with recycled fibers, J. Mater. Civ. Eng. 12 (4) (2000) 314–319. [60] H.N. Walker, S.D. Lane, P.E. Stutzman, Petrographic Methods of Examining Hardened Concrete: A Petrographic Manual, Revised 2004 FHWA-HRT-04-150, U.S. Department of Transportation, Federal Highway Administration, Research and Development, Turner-Fairbanks Highway Research Center, McLean, Virginia, USA, 2006, p. 351.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Recycled glass fiber reinforced polymer additions to Portland cement concrete Alireza Dehghan a, Karl Peterson a,⇑, Asia Shvarzman b a b
Univ. of Toronto, Dept. of Civil Eng., 35 St. George St., Toronto, ON M5S 1A4, Canada Antex Western, 1340 Church Ave., Winnipeg, MB R3C 2L4, Canada
h i g h l i g h t s Recycled glass fiber polymers did not cause expansive alkali silica reactions. Recycled glass fiber polymers exhibited pozzolanic behavior. An absorption test for recycled glass fiber polymer additions is presented.
a r t i c l e
i n f o
Article history: Received 24 October 2016 Received in revised form 2 April 2017 Accepted 3 April 2017 Available online 20 April 2017 Keywords: Recycled glass fiber reinforced polymer Alkali silica reaction Drying shrinkage Concrete microscopy
a b s t r a c t With recent developments in grinding and sorting technology it is possible to recover glass fibers from waste glass fiber reinforced polymers (GFRPs). The recycled fibers still retain some of the polymer and filler materials, but have the potential to provide some of the same benefits achieved by conventional fiber additions. The research program explored the influence of recycled GFRP on compressive strength, splitting tensile strength, and drying shrinkage in concrete, and alkali silica reaction (ASR) expansion in accelerated mortar beam and concrete prism tests. Compressive strength and drying shrinkage were not improved by recycled GFRP additions at a substitution level of 5 wt% of the coarse aggregate, but splitting tensile strengths were improved in most cases. Negligible expansion was observed from the ASR testing. A scanning electron microscope investigation of the concrete prisms indicated a pozzolanic reaction of the glass fibers. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Fiber reinforced polymer (FRP) is a composite material which is usually made with glass (GFRP), carbon (CFRP), or aramid (AFRP) fibers dispersed in a thermoset polyester resin and has a wide range of applications in the construction industry. Among GFRPs, E-glass is the most common reinforcement, and represents approximately 99% of the commercial market [1]. The ‘‘E” in E-glass is a carryover from its initial application, as used in electrical standoff insulators [2]. The compositional ranges for E-glass used in general applications (such as GFRP) are outlined in ASTM D578 [3] (Table 1). At the end of the life cycle of FRPs, fibers cannot be easily separated from the resin and the resin itself cannot be easily decomposed or recycled. Hence, landfill and incineration are the most common methods for FRP waste management [4,5]. Globally, GFRP
⇑ Corresponding author. E-mail address: [email protected] (K. Peterson). http://dx.doi.org/10.1016/j.conbuildmat.2017.04.011 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
production is estimated at 8 million metric tons annually, with GFRP waste production at 1.5 million metric tons [6]. Given the potential negative environmental impacts of waste FRP, the material has gained the attention of other industries to develop techniques and methods to recycle FRPs. Thankfully, recent advances in grinding and sorting technologies allow for the partial recovery of fibers from FRP and has made their utilization in Portland cement concrete an option. Considering that grinding and sorting equipment are readily available, and that the process produces negligible atmospheric pollution in terms of volatile organic compound emissions, size reduction by mechanical recycling is preferred over other recycling processes [7]. The topic of recycled GFRP in concrete necessarily overlaps with a number of different areas of research, including: glass fiber reinforced concrete (GFRC), waste glass powder concrete, and concrete made using waste plastic fine and coarse aggregate. As such, recycled GFRP combines some of the beneficial aspects of fibers and powdered waste glass, as well as some of the shortcomings of waste plastic aggregates.
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A. Dehghan et al. / Construction and Building Materials 146 (2017) 238–250 Table 1 Certified chemical composition for glass fiber products used in general applications [3]. Chemical
% by Weight
B2O3 CaO Al2O3 SiO2 MgO Na2O + K2O TiO2 Fe2O3 Fluoride
0–10 16–25 12–16 52–62 0–5 0–2 0–1.5 0.05–0.8 0–1
ACI 544.R-96 [8] constitutes a state-of-the-art report on fiber reinforced concrete, with an entire chapter dedicated to GFRC. Fibers in concrete are used to control cracking and to improve tensile and flexural strength. Early formulations of GFRC utilized Eglass fibers, but their use declined after the development of alkali resistant (AR) glass fibers, which performed much better over the long-term in the high-pH environment of concrete pore water. More recently, basalt fibers, made from melting basaltic rock, have been used to produce basalt fiber reinforced concrete (BFRC) [9– 11]. In this research, fibers recovered from recycled GFRP are utilized, but the fibers tend to occur in grouped masses bound by residual resin, as opposed to individual clean fibers. Waste glass is increasingly being utilized in concrete, both as an aggregate and as a powdered pozzolanic addition to concrete. Shi and Zheng [12] provide a thorough review of the topic, and it remains a very active area of research today. However, the majority of the research to date focuses on waste soda lime glass sources from containers or float/plate glass, with relatively few studies on the pozzolanic aspects of recycled GFRP. Xu et al. [13] investigated incinerated waste GFRP from the automotive industry, and used the ash as a pozzolanic additive. Chen et al. [14] investigated the use of ground waste E-glass fibers left over from circuit board manufacture that had never been used in FRP. They documented improvements in compressive strength, chloride and sulfate resistance. Similarly Mastali et al. [15] reported improvements in compressive and flexural strength for recycled glass fibers recovered from woven fiber sheets that had never been used in FRP. However, ground recycled GFRP, a combination of E-glass fibers, resins, and filler materials, is more problematic, as the resins and fillers provide no pozzolanic benefit. Gu and Ozbakkaloglu [16] conducted an extensive review of the utilization of recycled plastics in concrete from the standpoint of plastic fine aggregate, plastic coarse aggregate, and plastic fibers. While recycled plastic fibers tended to improve mechanical properties of concrete, recycled plastic aggregates led to reductions in compressive and tensile strength, and in most cases increased drying shrinkage. A common concern with the usage of recycled GFRP in cementitious binders is the potential for interference with the mechanical performance, particularly reductions in compressive strength. The
feasibility of recycled GFRP concrete has been explored by a number of researchers worldwide, with a comprehensive review recently provided by Yazdanbakhsh and Bank [17]. Asokan et al. [18] reported reduced compressive strengths with increasing recycled GFRP powder substitution when cured in water at 20 °C, but found increased compressive strengths compared to the control when oven cured at 50 °C. Tittarelli and Moriconi [19] and Tittarelli and Shah [20] reported reductions in compressive strength with recycled GFRP powder additions, but some improvements in terms of reduced drying shrinkage, and lower values for capillary water absorption. Correia et al. [21] explored the use of fines produced during the cutting of pultruded GFRP, and found similar reductions on compressive strength. Osmani [22] also reported reductions in compressive strength for concrete produced with recycled GFRP powder. Alam et al. [23] and Yazdanbakhsh et al. [24] both explored the substitution of larger aggregate sized FRP scrap particles, and reported reductions in both compressive and flexural strength. Alternatively, García et al. [25] explored the use of fibers recovered from recycled FRP, termed ‘‘glass fiber fluff,” and reported improvements in compressive and flexural strength when grinding and sieving is optimized. Expansive ASR is still cited as a concern for concrete as E-glass fibers are not stable in the high-alkali environment of the pore water [26,27]. Meanwhile the potential for powdered waste glass to mitigate ASR has been extensively documented, but not necessarily from the standpoint of waste E-glass [28–41]. Chen et al. [14] found negligible expansion due to alkali silica reaction (ASR) for concrete made with powdered recycled E-glass fibers that had never been used in GFRP. García et al. [25] reported expansions of <0.04% for concrete beams produced with glass fiber fluff recovered from recycled GFRP. Tittarelli and Moriconi [19] tested recycled GFRP powder using the recently withdrawn standard ASTM C289 [42], and found it to be innocuous. However, it is widely recognized that ASTM C289 is not a reliable test for predicting the reactivity of carbonate aggregates [43], and calcium carbonate is a common filler material in FRP. 2. Materials and methods Descriptions of the commercially produced GFRPs that were investigated in this study are presented in Table 2, and images of the fibers recovered from recycled GFRP are provided in Fig. 1. The recycled GFRP was produced using a model GM2411-50 ECO GrinderTM [44] single stage hammer mill grinding system with a 19 mm (3/4 in.) screen coupled to a pneumatically fed hopper with a dust collection bag and an enclosed auger feed to a series of 9.5 mm (3/8 in.) and 4.75 mm (3/16 in.) perforated opening trommel (rotary) screens. The GFRP feed consisted of sheets with a maximum thickness of 25.4 mm (1 in.) and nominal dimensions of 150 914 mm (6 36 in.). Table 3 provides a summary of the wt% glass fiber fluff retained by the screens. Materials separated by both screens were recombined for the purposes of this study. The bottom fines were not included. The results of sieve analyses performed on the combined materials are provided in Fig. 2.
2.1. Water absorption, density, loss on ignition, and fiber cluster length From each recycled GFRP source three representative samples, each with an approximate mass of 100 g, were produced by the ASTM C702 [45] quartering method for a determination of water absorption. Samples were completely immersed in tap water for 72 ± 2 h and stirred at least once every 24 h for one
Table 2 GFRP types, content, and abbreviations used in this study. Type
Resin
Glass content (vol.%) as manufactured
Abbreviation
Structural sheet molding composite Structural sheet molding composite Structural sheet molding composite Light resin transfer mold E-glass fiber
Bisphenol-A epoxy vinyl ester Novolac-based epoxy vinyl ester Flame retardant epoxy vinyl ester Unsaturated polyester None
40 40 40 25 100
EVE1 EVE2 EVE3 UP Virgin
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Fig. 1. Overall appearance of the recycled GFRP materials (a–d), and virgin E-glass fibers (e).
Table 3 Size-separated materials expressed as wt% of initial GFRP feed. Type
EVE1 EVE2 EVE3 UP
wt% 9.5 mm screen
4.75 mm screen
Bottom fines
Loss to dust collection
24 27 27 26
18 7 8 4
55 63 58 71
3 3 7 Negligible
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Fig. 2. Size distribution of recycled GFRP materials.
Table 4 Density, water absorption, glass fiber content, and length of recycled FRPs. Source
Density (g/cm3)
Absorption (%)
E-glass content (wt%)
Max. fiber cluster length (mm)
Avg. fiber cluster length (mm)
UP EVE1 EVE2 EVE3 Virgin
1.63 ± 0.02 1.66 ± 0.02 1.56 ± 0.02 1.60 ± 0.02 2.60 ± 0.04
63.1 ± 3.4 30.1 ± 4.9 25.5 ± 4.5 28.8 ± 6.4 0.4 ± 0.05
54.3 ± 1.2 43.1 ± 1.0 41.1 ± 0.8 42.0 ± 0.5 98.1 ± 0.0
36.4 26.8 33.5 31.3 25.6
21.7 ± 6.3 17.2 ± 5.7 21.2 ± 6.3 19.5 ± 7.5 25.4 ± 0.4
minute to ensure complete submersion. After soaking, the excess water was decanted over a 75 mm sieve to avoid loss of material, and the samples placed in individual open containers. A saturated surface dry (SSD) condition was gradually achieved through exposure over a period of hours in an environmental chamber at 35 ± 2 °C and 15 ± 2% relative humidity (RH) with frequent stirring to ensure homogenous drying. To determine if surface moisture was still present, the containers were inverted onto an absorbent sheet of unbleached paper towel to check whether or not the recycled GFRP material would dampen the paper. If any moisture was observed on the paper, the specimen was returned to the environmental chamber and re-tested in the same manner until it reached a SSD condition. After achieving an SSD condition, the samples were weighed, and then dried in a vented oven at 100 ± 2 °C. Earlier experiments conducted at a drying temperature 105 ± 5 °C resulted in a strong odour and color change of the recycled GFRP, so the alternative drying approach at 100 ± 2 °C was employed. A helium pycnometer was used to measure the densities of the recycled GFRP. Three representative dry samples with an approximate mass of 4–5 g were used and the volume of each sample was measured three times. The averages of nine readings are provided in Table 4. UN1046 compressed helium as recommended by the manufacturer at a pressure of 1.2 kg/cm2 was used in this study. Water absorption values for the recycled GFRPs are also listed in Table 4. Loss on ignition was measured with three representative test specimens, each with an approximate mass of 5 g and conditioned at 23 ± 2 °C and 50 ± 10% RH to meet the requirements of ASTM D2584 [46]. The samples were heated in a muffle furnace at 300 °C for 2 h, then cooled to room temperature in a desiccator, and
weighed. Next, the samples were placed back into the muffle furnace, heated at 560 °C for 2 h, cooled, and reweighed at room temperature. Preliminary investigations found that heating at 560 °C for 2 h was sufficient to remove all organic material. Finally, the samples were heated to 960 °C for 2 h, cooled, and the mass of the residues measured at room temperature. Negligible mass loss was found between 560 °C and 960 °C, and indicated minimal inclusion of carbonate-based fillers. The difference between initial mass and the loss on ignition at 560 °C was used as an approximation of glass content. Mass losses are presented in Fig. 3. The glass contents reported for the crushed and sieved GFRP by ignition were slightly higher than the as-manufactured GFRP. The enrichment may partly be attributed to the loss of resin fines during the crushing and sieving operations. To characterize fiber lengths, representative 5 g samples were obtained, and fiber clusters manually plucked from the fluff, and images recorded with a flatbed scanner (Fig. 4). Digital image processing was used to obtain the fiber cluster lengths reported in Table 4.
2.2. ASR testing 2.2.1. Accelerated mortar bar test Susceptibility to ASR was tested in accordance with CSA A23.2-25A [47] on four 25 25 285 mm mortar bars, but with 5 wt% replacement of the coarse aggregate (an inert crushed dolostone) with recycled GFRP. CSA A23.2-25A was followed,
Fig. 3. Mass loss of recycled GFRPs and virgin E-glass fibers at different temperatures.
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Fig. 4. Fiber clusters manually plucked from glass fiber fluff (a–d), and virgin E-glass (e).
Table 5 Mix designs of mortar mixtures for the accelerated mortar bar test. Source
Recycled GFRP (kg/m3)
Crushed agg. (kg/m3)
GU cement (kg/m3)
Water (kg/m3)
Control UP EVE1 EVE2 EVE3
0.0 65.7 65.7 65.6 65.7
1327.1 1247.9 1248.3 1247.1 1247.6
589.8 583.8 583.9 583.4 583.6
294.9 291.9 292.0 291.7 291.8
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A. Dehghan et al. / Construction and Building Materials 146 (2017) 238–250 Table 6 Fresh properties of the mortar mixtures. Source
Temp (°C)
Gravimetric air content (%)
Mix yield (kg/m3)
Flow (%)
Control UP EVE1 EVE2 EVE3
25.7 23.3 23.9 22.8 23.3
2.3 12.3 4.4 9.2 4.4
2243.9 2032.5 2192.0 2108.9 2191.9
83 36 67 51 68
Table 7 Mix designs for concrete prism test. Source
Recycled GFRP, dry mass (kg/m3)
Cement (kg/m3)
Water (kg/m3)
Coarse aggr. dry mass (kg/m3)
Fine aggr. dry mass (kg/m3)
NaOH pellets (kg/m3)
Moisture held by the SSD GFRP (kg/m3)
Control UP EVE1 EVE2 EVE3 Virgin
– 19.5 19.9 18.7 19.2 31.2
415.0 415.0 415.0 415.0 415.0 415.0
186.7 186.7 186.7 186.7 186.7 186.7
1079.5 1060.0 1060.0 1060.0 1060.0 1060.2
714.4 701.4 701.4 701.4 701.4 701.6
1.28 1.28 1.28 1.28 1.28 1.28
– 12.3 6.0 4.7 5.4 0.1
Table 8 Fresh properties of concrete mixtures for concrete prism test. Source
Temp (°C)
Slump (mm)
Pressure meter air content (vol%)
Unit weight (kg/m3)
Control UP EVE1 EVE2 EVE3 Virgin
21.3 21.1 21.3 21.1 20.7 27.1
145 75 140 170 170 30
2.5 4.0 2.9 3.1 2.9 4.6
2411.1 2361.8 2408.3 2397.4 2405.3 2344.3
Table 9 Fresh properties of concrete mixtures. Source
Temp (°C)
Slump (mm)
Pressure meter Air content (%)
Unit weight (kg/m3)
Control UP EVE1 EVE2 EVE3 Virgin
21.5 20.7 22.0 22.0 20.3 27.7
125 80 165 165 175 50
2.6 3.9 2.9 2.7 2.9 4.3
2417.1 2365.7 2417.1 2407.9 2412.9 2372.1
except that the exposure time was extended from 14 d to 28 d. The mix designs of the mortars are presented in Table 5, and fresh properties of the mortar mixtures are presented in Table 6. CSA A23.2-25A specifies that the aggregate used for mixing be in an oven dry state. Since the recycled GFRP was being treated on the basis of a wt% replacement of aggregate, it was also brought to an oven dry state prior to mixing. CSA A23.225A prescribes a fixed w/cm (water to cementitious ratio) of 0.50 for mixtures con-
Fig. 5. 7 and 28 d compressive strengths for non-air entrained concrete cylinders made with recycled GFRP, without any GFRP (control), and with virgin E-glass fibers.
taining crushed aggregate, and a mass ratio of cement to aggregate of 1:2.25, but makes no definitive statement regarding corrections to the water content based on aggregate absorption. As such, no corrections to the water content were made to account for water absorption by the aggregate or recycled GFRP. As shown in Table 5, utilization of recycled GFRP increased the air content. Given the morphology, air is necessarily entrapped within the glass fiber fluff when added in a dry state. From Table 5, recycled GFRP additions also decreased the flowability of the mixtures. Absorption of mix water by the dry glass fiber fluff likely contributed to the reduction in workability. As such, a vibration table was selected as the
Fig. 6. 7 and 28 d tensile strengths for non-air entrained concrete cylinders made with recycled GFRP, without any GFRP (control), and with virgin E-glass fibers.
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Fig. 7. Relationship between water retained in the SSD recycled GFRP materials and virgin fibers and the 7 and 28 d compressive strengths.
Fig. 8. Length changes due to drying shrinkage of concrete prisms made with recycled GFRP, without any FRP (control), and with virgin E-glass fibers.
method of compaction. The reported flow results were obtained after 10 drops of an ASTM C230 [48] flow table, and all of the mortar mixtures were workable enough to be cast into the molds. 2.2.2. Concrete prism test The risk of expansive ASR was assessed in accordance with CSA A23.2-14A [49] on three 75 75 285 mm prisms per recycled GFRP source, but with one significant modification: the recommended temperature of 38 °C was elevated to 50 ± 3 °C. The change in temperature was made to accelerate the rate of ASR, and thereby shorten the duration of testing. As with the accelerated mortar bar test, the inert coarse aggregate was substituted by 5 wt% with recycled GFRP. CSA A23.2-14A specifies that concrete be mixed according to CSA A23.2-2C [50], so aggregate moisture corrections must be accounted for. Prior to mixing, the recycled GFRP materials were prepared in SSD condition using the same procedure used to measure absorption. After achieving a SSD condition, the recycled GFRP materials were sealed in air-tight containers and allowed to equilibrate to room temperature.
Coarse aggregate for mixing was immersed for 24 h prior to use, with excess water decanted prior to mixing. Water clinging to the coarse aggregate was accounted for as a contribution to the mix water. Similarly, fine aggregate of a known moisture condition was used for mixing, and water clinging to the fine aggregate was accounted for as a contribution to the mix water. Mix designs are shown in Table 7. Results for fresh properties of the concrete mixtures are presented in Table 8. 2.3. Mechanical properties A second batch of concrete without the addition of sodium hydroxide was cast for each source of recycled GFRP using the same mixing procedure as described in Section 2.2. Twelve 100 200 mm concrete cylinders and three 75 75 285 mm concrete prisms were cast to measure compressive strength, tensile strength, and length changes due to drying shrinkage at 23 ± 2 °C and 50 ± 4% RH. Samples were cured in lime-saturated water for the periods specified by the corresponding standards. Fresh concrete test results are provided in Table 9.
Fig. 9. Relationship between the water retained in the SSD recycled GFRP materials and virgin fibers and drying shrinkage.
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Fig. 10. Expansions during accelerated mortar bar test for samples made with recycled GFRP, and without GFRP (control).
Fig. 11. Expansions for concrete prisms made with recycled GFRP, without any GFRP (control), and with virgin E-glass fibers.
3. Results and discussion 3.1. Concrete strength CSA A23.2-9C [51] compressive strength results for the concrete samples made with recycled GFRP, virgin E-glass fibers, and without any fiber additions are presented in Fig. 5. Fig. 6 presents the results of CSA A23.2-13C [52] tensile splitting tests, where the sample is failed through indirect tension by applying a compres-
sive load to the diametral plane of a cylinder. Fig. 7 plots the water retention of the SSD recycled GFRP materials versus compressive strength. Compressive strength decreased as recycled GFRP water retention increased, which suggests that the water contained within the SSD recycled GFRP glass fiber fluff may have been released during concrete mixing, locally increasing the water to cementitious ratio (w/cm). Alternatively, in the case of ultra high performance concrete (UHPC) with very low w/cm levels, such sources of internal curing water has proved beneficial in terms of
Fig. 12. Mass changes for concrete prisms made with recycled GFRP, without any GFRP (control), and with virgin E-glass fibers.
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Fig. 13. A porous cluster of glass fibers in thin section from sample UP concrete as observed in fluorescent mode (a), back scattered electron image (b), and close-up (c) with black arrows showing locations of characteristic X-ray spectra from interior of glass fiber (d) and exterior (reacted) glass fiber (e).
strength gain [53,54]. Tensile strength in most cases increased with recycled GFRP as compared to the control, with the exception of the EVE2 mixture. 3.2. Drying shrinkage Length changes in concrete prisms due to drying at 23 ± 2 °C and a relative humidity of 50 ± 4% were tested according to CSA A23.2-21C [55] with the results shown in Fig. 8. During the first week of testing, the mixture containing virgin Eglass fibers experienced the least amount of drying shrinkage. However, in the subsequent weeks and months, the virgin mixture performed similarly to the EVE1 and UP mixtures, with shrinkage
values exceeding the control by 17%. The mixtures with the highest values for water retention (EVE1 and UP) experienced the most drying shrinkage of all the recycled GFRP mixtures (Fig. 9). This result was unexpected, as the purposeful addition of saturated light weight aggregate (LWA) to help reduce autogenous shrinkage is now a widely adopted strategy to counteract shrinkage [53,54,56–58]. However, at the w/cm level of 0.45 explored in this study, autogenous shrinkage is not an issue. Wang et al. [59] reported similar shrinkage issues for concrete made with polypropylene fiber mesh, as well as recycled fibers from tires or waste carpet. They attributed the increase in shrinkage to porosity contributed by the fiber masses. An alternative explanation for the slightly higher levels of drying shrinkage could be that the inher-
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ently lower stiffness of the recycled GFRP provided less resistance to shrinkage [16].
3.3. Alkali silica reaction Results of the accelerated mortar bar test are presented in Fig. 10. The expansion values for all of the mortar mixes made with recycled GFRP in this study were below the recommended 0.1% limit. Furthermore, results of the concrete prism test for the five types of the recycled GFRP are provided in Fig. 11, and the expansion values are all below the recommended 0.4% expansion limit. Changes in mass were recorded, and are shown in Fig. 12. The concrete prisms made with recycled GFRP (except EVE1) experienced mass losses between the initial reading (measured after demolding) and up to 14 d of exposure at 50 °C. This unusual reduction in mass could possibly be attributed to water from the SSD mats of the recycled glass fibers migrating to the exterior of the prisms at the elevated temperature of 50 °C. Drying of the prisms due to the escape of water from the sealed pails was ruled out, since the level of water ponded at the base of the pails appeared constant during testing. Furthermore, the absorbent sheets lining the inside walls of the pails remained fully saturated throughout the duration of the test.
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3.4. Microstructure of concrete prisms Figs. 13–16 show images of recycled GFRP and virgin glass fibers in polished thin sections prepared from the concrete prisms after four months of ASR testing. Prior to sectioning, a fluorescent-dyed low viscosity epoxy resin was used to fill the capillary pores and air voids present in the concrete. In the fluorescent images darker regions (less fluorescence) represent areas with low porosity, and brighter regions (more fluorescence) represent areas with high porosity [60]. ASR-related cracking was not observed in any of the samples. Typically, concrete that has undergone expansive ASR will exhibit deposits of alkali silica gel lining air-voids or within cracks. Alkali-silica gel deposits were not directly observed in the samples, although in some cases it appeared that the capillary pores of the hardened cement paste immediately surrounding some recycled GFRP fragments may have been filled/densified through the uptake of alkali-silica gel (Fig. 13a). Concrete containing recycled GFRP from the higher absorption UP material more frequently exhibited porous mats of glass fibers (Fig. 13) as compared to concrete made with the relatively lower-absorption recycled GFRP sources (Figs. 14 and 15). Fig. 16 shows virgin E-glass fibers.
Fig. 14. A fragment of flame retardant (brominated) epoxy vinyl ester resin with embedded glass fibers in thin section from sample EVE1 as observed in fluorescent mode (a), backscattered electron image (b), and close-up (d), showing location (black arrow) of characteristic X-ray spectrum collected from the FRP resin.
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Fig. 15. A fragment of bisphenol-A epoxy vinyl ester in thin section with embedded glass fibers from sample EVE3 as observed in fluorescent mode (a), back scattered electron image (b), and close-up (c) with black arrows showing locations of characteristic X-ray spectra from interior of glass fiber (d) and exterior (reacted) glass fiber (e).
4. Conclusions Reductions in workability have been previously reported for concrete with fibers recovered from recycled GFRP and attributed to increased water demand due to the irregular geometry and the high specific surface of the fibers [25]. In the research presented here, the reduction in workability was overcome by preconditioning the recycled GFRP to a SSD condition. However, the relatively large amounts of water retained in the SSD condition may have had a negative contribution in terms of compressive strength; the water retained by the SSD recycled GFRP in many cases appears to have had a local influence on w/cm, increasing
the capillary porosity of the adjacent cement paste (Figs. 14a and 15a). Furthermore, rather than being dispersed as individual fibers, in most cases the recovered fibers from recycled GFRP and from the virgin E-glass fibers occurred in the concrete as porous clumps, and these would also be expected to have a negative impact on compressive strength. On the other hand, the fibers from recycled GFRP and the virgin E-glass fibers did contribute to improvements in splitting tensile strength. Drying shrinkage was not improved by the addition of recycled GFRP or virgin E-glass fibers as compared to the control concrete, a finding that contrasts with García et al. [25] who reported a modest improvement. But, the overall values for drying shrinkage
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Fig. 16. Virgin E-glass fibers in concrete thin section, backscattered electron image (a), and close-up (b) with black arrows showing locations of characteristic X-ray spectra from interior of glass fiber (c) and exterior (reacted) glass fiber (d).
reported here remain within a narrow range similar to the control. Although a fixed replacement level of 5 wt% of recycled GFRP for aggregate was used throughout this study, the replacement level could be revisited and refined in an effort to minimize shrinkage, and to maximize improvements to tensile strength. Accelerated mortar bar and concrete prism ASR testing showed expansion values well below the recommended expansion limits. Comparisons of X-ray energy dispersive spectra (EDS) collected from points at the perimeters of the glass fibers to points further within the glass fibers show an enrichment and calcium (Figs. 13d & e, 15d & e, 16d & e), which suggests the pozzolanic reaction of the fibers to form hydration products. Given the slender profile of the exposed glass fibers, on the order of 10 mm, the glass fibers perform as a pozzolanic addition similar to expansive ASRmitigating coal combustion fly ash or ground granulated blast furnace slag as opposed to performing as an ASR expansion-inducing aggregate particle.
Acknowledgements This research was funded by the NSERC Engage Grants Program, Canada, EGP #468708-14.
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