Structural integrity of glass/epoxy composites embedded in cement or geopolymer mortars

The main goal of the present work is to study the mechanical response of glass/epoxy composites when exposed to cement and geopolymer (metakaolin) mortars, as it happens in Civil Engineering applications. For this purpose, specimens were embedded into mortar followed by a period of curing, which was done under the air laboratory environment. For cement, curing was also performed inside a container filled with water. The specimens were removed from the mortars after 30, 60 or 90 days of curing and then tested. Three point bending (3PB) tests, tensile and transverse impact tests were carried out. The degradation was evaluated, both by the residual mechanical properties and by the failure damage mechanisms. In terms of 3PB, and comparatively to the control samples, decreases around 29.5 %, 31.3 % and 37.9 % were found after 90 days of exposure to cement, cement with water and metakaolin mortar, respectively. The same comparison for the impact strength reveals decreases about 39%, 40.1% and 44.3% for impacts in tensile mode, while these values are 7%, 11.6% and 63.1% for impacts in transverse mode, respectively. Therefore, the exposure to both cementitious and geo-polymeric matrices affects significantly the mechanical properties, but their effects are strongly dependent of the exposure time. Finally, it was possible to conclude that the worst results are obtained with the metakaolin mortar.

Accepted Manuscript Structural integrity of glass/epoxy composites embedded in cement or geopolymer mortars A.M Amaro, M.I.M Pinto, P.N.B Reis, M.A Neto, S.M.R Lopes PII: DOI: Reference: S0263-8223(18)31453-3 https://doi.org/10.1016/j.compstruct.2018.08.060 COST 10110 To appear in: Composite Structures Received Date: Revised Date: Accepted Date: 19 April 2018 July 2018 27 August 2018 Please cite this article as: Amaro, A.M., Pinto, M.I.M., Reis, P.N.B., Neto, M.A., Lopes, S.M.R., Structural integrity of glass/epoxy composites embedded in cement or geopolymer mortars, Composite Structures (2018), doi: https:// doi.org/10.1016/j.compstruct.2018.08.060 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain Structural integrity of glass/epoxy composites embedded in cement or geopolymer mortars A.M Amaro1*, M I M Pinto2; P.N.B Reis3, M.A Neto1, S M R Lopes2 CEMMPRE, Department of Mechanical Engineering, University of Coimbra, Coimbra, Portugal *ana.amaro@dem.uc.pt; augusta.neto@dem.uc.pt CEMMPRE, Department of Civil Engineering, University of Coimbra, Coimbra, Portugal isabelmp@dec.uc.pt; sergio@dec.uc.pt C-MAST, Department of Electromechanical Engineering, University of Beira Interior, Covilhã, Portugal preis@ubi.pt Abstract The main goal of the present work is to study the mechanical response of glass/epoxy composites when exposed to cement and geopolymer (metakaolin) mortars, as it happens in Civil Engineering applications For this purpose, specimens were embedded into mortar followed by a period of curing, which was done under the air laboratory environment For cement, curing was also performed inside a container filled with water The specimens were removed from the mortars after 30, 60 or 90 days of curing and then tested Three point bending (3PB) tests, tensile and transverse impact tests were carried out The degradation was evaluated, both by the residual mechanical properties and by the failure damage mechanisms In terms of 3PB, and comparatively to the control samples, decreases around 29.5 %, 31.3 % and 37.9 % were found after 90 days of exposure to cement, cement with water and metakaolin mortar, respectively The same comparison for the impact strength reveals decreases about 39%, 40.1% and 44.3% for impacts in tensile mode, while these values are 7%, 11.6% and 63.1% for impacts in transverse mode, respectively Therefore, the exposure to both cementitious and geo-polymeric matrices affects significantly the mechanical properties, but their effects are strongly dependent of the exposure time Finally, it was possible to conclude that the worst results are obtained with the metakaolin mortar Keywords: Glass/epoxy composites, Environmental degradation, Mechanical tests Introduction Reinforced concrete is widely used in civil engineering for structural purposes The combination of an artificial stone, such as concrete, with embedded reinforcing elements (like steel bars) can be very effective in structural members However, the possible corrosion of the steel bars inside the concrete is one of the main problems In such context, several research works were developed in order to obtain stronger concretes less permeable to aggressive environments and, consequently, more protective relatively to the embedded reinforced bars On the other hand, other types of concrete are being developed with lower greenhouse gas emissions For example, the introduction of metakaolin in conjunction with cement, or even with no cement at all, has already been tried with some success and with significant environmental advantages As known, metakaolin is the pozzolanic material obtained from calcination of high grade kaolinite clay Metakaolin is less environmentally harmful than cement production, because it requires lower temperatures (typically from 500ºC to 800ºC compared to 1500° C for cement) and, consequently, lower carbon dioxide emissions [1, 2] Shi et al [3] defend that “alkaline activated cement can be positioned at the epicentre of a new and necessary transition from today's Portland cement to the new cements of the future” They present a classification of alkali-activated cements with five main classes One of the five classes is the so-called alkali-activated pozzolan cements This class includes: alkali-activated lime–natural pozzolan cements, alkali-activated lime-fly ash cements, alkali-activated limemetakaolin cements, alkali-activated lime-blast furnace slag cements The name of Kaolin is derived from Chinese word “Kao-ling”, a place where this material was extracted centuries ago for ceramics [4] Nowadays, Kaolin is one of the most widely used industrial minerals [4] In terms of reinforcement material embedded in concrete members, steel bars are a very competitive solution although their degradation by corrosion is one of the highest drawbacks In this context, stainless steel can be an alternative solution, but its price is far more costly than that for normal steel Therefore, literature reports attractive advantages for fibre reinforced plastics (FRPs) comparatively to the conventional steels, because they have high specific strength and stiffness as consequence of their low specific weight [5-6] Carbon, Basalt or Glass fibres were initially used mainly on rehabilitation works evidencing, in such context, significant benefits in terms of price, mechanical performance and resistance to degradation Nowadays, FRP bars are used to replace steel bars in reinforced concrete (RC) structures to overcome the corrosion problem of steel bars, especially the basalt fibres as consequence of their low price [7] However, according to Burgoyne and Balafas [8], FRP materials are typically or times the cost of steel Some studies about degradation of Glass Fibre Reinforced Plastics (GFRPs) were already published For example, Kim et al [9] studied the short-term durability of E-glass/vinylester rods exposed to various environmental conditions: just water, alkaline solution, NaCl and NaCl2 solutions, and under wetting/drying scheme They found that the alkaline environmental condition had more influence on the degradation than the other influencing factors The mechanisms of water uptake in composites have been studied by some authors [10, 11] These materials can be able to absorb water, depending on its chemical composition, which causes volume variation Won et al [12] studied the effect of exposure to alkaline solution and water on the strength, and they found that the tensile strength of GFRP decreased markedly with the exposure time However, regarding to water immersion, the degradation appeared to be minimal He et al [13] studied the degradation of GFRP bars embedded in concrete beams with cracks They focused their analysis on the tensile strength of the bars and concluded that the degradation in beams with cracks was generally higher than uncracked counterparts Wang et al [14], for example, developed studies about long-term performance of basalt- and E-glass-fibre reinforced polymer (BFRP/GFRP) bars in seawater and sea sand concrete (SWSSC) environment Accelerated corrosion tests, for different exposure times, were conducted using two types of solutions at different pH and temperatures They concluded that GFRP had much better durability than BFRP, especially at high temperatures, and the Arrhenius relationship theory was adopted to predict the long-term performance of BFRP and GFRP bars in five places from Canada However, the results were very conservatives and more precise degradation models should be developed considering the real temperature ranges, humility ranges and preloading stress Therefore, it is evident that more research studies are necessaries to improve the knowledge in this area On the other hand, for the authors’ knowledge, very few studies on degradation of FRPs when embedded in metakaolin are present in the bibliography, despite the importance of the topic The potential of geopolymers for the civil engineering constructions is well explained by some authors [15-18] with particular emphasis on the environmental advantages For example, they present good mechanical properties, inflammability at high temperatures, chemical resistance, long-term durability, low permeability and considerable ecological advantages, because it is necessary low temperature for the geopolymers production and, consequently, less CO emissions [19] This study intends to analyse the potential use of GFRPs into cement concrete or into geopolymers (with no cement at all) The aggregates of concrete were not included due to small size of the specimens and, therefore, the technical word used further on in this paper for such material is “mortar” However, the chemistry is the same, as the aggregates not interfere with that In this case, the specimens were embedded in two types of mortars (cement mortar and metakaolin mortar) and, after that, exposed to air and water The cement mortar specimens were immersed into water, just after hardening, in order to simulate situations where the material is in contact with water during the lifetime (e.g earth retaining walls, buried foundations, bridge piles and dams) Materials and Experimental Procedure Composite laminates were produced by Glass fibre Prepreg TEXIPREG® ET443 (EE190 ET443 Glass Fabric PREPREG from SEAL, Legnano, Italy) and, according with the manufacturer recommendations, the process involved different steps: make the hermetic bag and apply 0.05 MPa vacuum; heat up to 130º C at a ºC/min rate; apply a pressure of 0.5 MPa when a temperature of 130 ºC is reached; maintaining pressure and temperature for 60 min; cool down to room temperature (25 ºC) maintaining pressure and finally get the part out from the mould The volume fraction of Eglass fibre is 0.45 and the useful size of those plates was 300×300×2 [mm] with the following staking sequence [452, 902, -452, 02]s Finally, the specimens used in the experimental tests were obtained from those plates with the geometry shown in Figure for the tensile impact tests, and rectangular specimens of 100×12×2 [mm] for the flexural and transversal impact tests Figure Nine of those specimens were used just for control purposes, and the remaining 81 were embedded into cement and metakaolin mortars: a total of 54 into the cement mortar and 27 into the metakaolin mortar After hardening for about one day, the 27 metakaolin mortar specimens and 27 cement mortar specimens were left to cure under laboratory air room temperature, and the remaining 27 cement mortar specimens were immersed into water For each environmental condition, at least specimens were tested for more reliable results The cement mortar was produced with sand, cement, water and a plasticizer, as shown in Table 1, and an electrical mixer was used on the mixing process A plasticizer was added, because it is usual to use it in concrete, in order to increase the fluidity of the mixture This is an important property for the molding process Without a plasticizer, the amount of water required for the mixture would be higher, which would contribute to decrease the mechanical properties of the final material The metakaolin mortar is a geopolymer with mechanical properties similar to those of the cement mortar, but has sodium hydroxide, metakaolin powder and sodium silicate in its composition (Table 1) Metakaolin powder is obtained from kaolin, a natural aluminosilicate, resulting from chemical changes of feldspar minerals Table Both the cement and metakaolin mortars need to have high alkalinity after production, normally presenting a pH value in the range 11-12 To make cement mortar, the initial liquid (water) has a pH of 7.0 and this value increases as the solid components are added For metakaolin, the pH value is initially very high, close to 14, and then decreases as the solid components are added [20] The high values of alkalinity might drop with age, a so-called carbonation process, depending on various parameters If pH values are below a certain limit (typically, around 9.0), the risk of corrosion in embedded steel bars could become very high Therefore, the evolution of the pH is very important [21] In order to allow a good embedment quality of the specimens, nine perpex boxes were built (Figure 2a) with grooves on both lateral walls to allow a correct positioning of the specimens and to minimize the risk of damage when later the specimens are removed from the already cured and stiffen mortars After the specimens were positioned, the boxes were filled with the respective mortar, and then wrapped with plastic wrap to minimize water loss, thereby reducing shrinkage caused by dehydration and thus improving the quality of the curing process (Figure 2b) Figure After a period of about 24 hours, which is the necessary time for the mortars to solidify, the boxes where unwrapped and moved to a place with the chosen cure environment: just left at laboratory air room temperature, or placed inside of a container full of tap water The water in the container covers completely the boxes and was renewed every 10 days Mortar characterization on mechanical strength for Civil Engineering applications is undertaken at 28 days as a reference As this study aims to investigate the effect of time on the mechanical strength of the glass/epoxy composite embedded in two different mortars, it was decided to cover approximately 1x, 2x and 3x the reference age (i.e., 30, 60 and 90 days) This decision was because glass/epoxy composites are not yet traditional materials in Civil Engineering and many aspects of its behaviour are still unknown Finally, at the end of each defined exposure period (30, 60 or 90 days), the specimens were carefully removed from the mortars (hardened mortars) through hammering in the space between specimens, with help of a chisel The removal didn´t damage the specimens as they separate always by the interface specimen/mortar without further actions Then the specimens were proper cleaned with water and paper and finally were ready to testing according to the test program Three point bending (3PB) tests were performed according to ASTM D790-2, using a Shimadzu universal testing machine, model AutographAG-X, equipped with a load cell of kN For each environmental condition, specimens were tested at room temperature and at a rate of mm/min The bending strength was calculated as the nominal stress at middle span section obtained using maximum value of the load, and the nominal bending stress was calculated by:  3P L b h2 (1) where P is the load, L the span length, b the width and h the thickness of the specimen Flexural strain was calculated according with the European Standard EN ISO 178:2003 by the equation: ε  f  S h (2) L2 where S is the deflexion (in millimetres), L the span length and h the thickness of the specimen The spam was 40 mm Tensile impact tests were carried out according to ISO 8256 An impact machine Instron-Ceast 9050 and a hammer with 25 J of energy were used for this purpose Finally, the transverse impact tests were performed in the same impact machine according to ISO 179, and a hammer with J of energy was used At least specimens were used for each condition and tested at room temperature Figure shows details of the equipment, and according to ISO 8256, the tests can be described as tensile tests at relatively high strain rates (see Fig 3b), while on the tests performed according to ISO 179 the specimen is supported near its ends as a horizontal beam and it is impacted by a single blow of a striker with the line of impact midway between the supports, and bent at a high (nominally constant) velocity (see Fig 3c) Figure Results and discussion Bending loads According to Banna et al [22] the bending tests is one of the most sensitive to modifications in the environmental degradation In this context, Figure shows typical bending stress versus strain curves for control specimens and specimens exposed to environmental conditions during 60 days These curves are representative of the others ones correspondent to 30 and 90 days of exposure Figure As expected, the highest flexural strength value was obtained for control specimens (around 530 MPa), and lower values were observed successively for the different environments (cement, cement with water and Metakaolin) In fact, both cement and geopolymer mortars are alkaline environments that degrade glass fibres Kim et al [7] studied GFRP rods under different environments and they concluded that the alkaline environmental conditions had more influence on the degradation of the rods than any other environmental conditions they have studied In theory, the degradation can be due to chemical or physical mechanisms There are few works that tried to explain such degradation, but, for instance, Purnell and Beddows [23] studied the durability of glass fibre reinforced concrete and they found that the presence of Ca(OH)2 at the interface seemed to be the key factor for degradation They added that the action of Ca(OH)2, included in the hydrated cement paste, was more chemical than physical The highest variation of the bending stress, in relation to the control specimen, occurred for specimens embedded in metakaolin mortar, and such variation corresponds to a decrease of about 35.9% These results are in accordance to Amaro et al [24], where studies developed by the authors shown that the corrosive environments affect significantly the flexural strength Figure shows a similar analysis for cement mortar alone, where typical curves are shown for the different exposure periods Figure It is possible to observe that the curves present different profiles, which can be explained by the different damage mechanisms occurred Figure presents images of the typical failure modes, where the main damage mechanisms are the fibres fracture in the tensile surface followed by delaminations between the layers Therefore, the posterior load drop is consequence of the propagation of delaminations initiated at the regions with broken fibres However the delaminations’ length increases with the exposure time, as shown in Fig 6, which justifies the different profiles shown in Figures and On the other hand, when the failure modes are compared for the different environments studied, Figure 7, it is possible to conclude that the metakaolin mortar is the most aggressive solution In this case, besides the fibre breakage, higher delaminations occur and, consequently, lower bending stresses occur According to Amaro et al [24, 25], larger delaminations are consequence of the poor interface fibre/matrix, which affect significantly the load carrying capacity of material Figure Figure Table presents the average bending stress, and respective standard deviation, for all conditions analysed Independently of the environmental condition, it is possible to confirm a decreasing of theflexural strength with the exposure time Comparatively to the control samples, the highest decrease occurred for metakaolin mortar, with values around 33.5 %, 35.9 % and 37.9 % for 30, 60 and 90 days, respectively Similar trends were observed for the other two solutions and, for example, a decreasing around 29.5 % and 31.3 % was found after 90 days of exposure to cement and cement with water, respectively Therefore, independently of the exposure time, it is possible to conclude, exposure the decreasing is about 15.6%, 24.3% and 34.1% for samples embedded in cement, in cement immersed into water and in metakaolin mortar, respectively However, this decreasing reaches values about 39%, 40.1% and 44.3%, respectively, after 90 days of exposure These values evidence, again, that the metakaolin mortar solution promotes the worst tensile impact strength, which agrees with the previous results obtained from the bending tests On the other hand, it is also possible to conclude that the exposure time is a determinant parameter on the low performance observed As shown in Figure 9, and independently of the damage to be associated with matrix and matrix-fibre interface cracking, typical of the laminates with off-axis fibres [33-36], the exposure to those solutions promote an acceleration of the damage mechanisms referred previously In this context, if the matrix contributes for the low impact performance [37], the degradation of the fibre/matrix interface observed by Amaro et al [24] is considered by Stamenovic et al [38] as the main cause of the lower load carrying capacity This is evident on the figure, where the highest damages are observed for samples embedded in metakaolin mortar and, consequently, the worst results obtained Figure Figure 10 presents the results obtained from the transverse impact tests A brief comparison between control samples shows that the values obtained in transverse mode are significantly lower relatively to the ones obtained in tensile mode This difference is around 87.5%, and it is justified by the lack of through thickness reinforcement [39] A detailed study about the damage mechanisms performed by Reis et al [37] show that the damage mechanisms are completely different In terms of transverse impact tests, the failure mode observed are very similar to the ones observed in Figure 7, where the main damage occurs by the fibres fracture in the tensile side followed by delaminations between layers, promoting, in this case, practically the total collapse of the sample This is in total accordance to the observed by Reis et al [37] 11 On the other hand, Figure 10 confirms the tendency observed previously, where the metakaolin mortar continues to be the solution that promotes the most severe degradation, i.e., the highest decrease of the transverse impact strength Purnell and Beddows [23] developed a study on durability of glass fibre reinforced concrete, and they found that the degradation of glass fibre takes place inside the cement environments and describes the reasons of degradation, as mentioned before, in this paper However, to the authors’ knowledge, there are no investigations on the reasons of the degradation of glass fibres inside metakaolin mortar environments Figure 10 shows that degradation effect of the mortars, namely for metakaolin, does not show to reach a constant value at the end of the 90 days of exposure For example, after 30 days of exposure the decreasing observed relatively to the control samples is around 21.4%, but after 90 days it reaches about 63.1% However, the samples embedded in cement and cement immersed into water shows a slight decrease relatively to the control samples While no effect is observed until 30 days of exposure, after 90 days a decreasing around 7% for samples embedded in cement and 11.6% for samples embedded in cement immersed into water can be found Figure 10 Conclusions This work studies the mechanical properties, namely bending and impact performance of composite laminates when embedded in cement or in geopolymer mortars It can be concluded that these mortars have a significant effect on the structural integrity of the composites, promoting a significant decrease of their mechanical properties The main damage mechanisms observed is the matrix cracking and fibre failure but, for longer exposure periods (60 and 90 days), the delamination size increases The exposure time was determinant on the degradation degree of the composites, increasing damage and therefore reducing stress The metakaolin mortar shows to be more aggressive than the other two 12 environments (cement and cement in water) Cement mortar cured in water, shows a more degradation effect than the one cured at air laboratory environment for 3PB but that is not so clear for both the impact tests performed The elastic energy also decreases with the exposure time, corresponding to the most severe damage degree As the degradation effect of the mortars, namely for metakaolin, does not show to reach a constant value at the end of the 90 days of exposure, longer exposure periods are recommended on further studies on these materials Acknowledgements This research is sponsored by the project UID/EMS/00285/2013 The authors acknowledge the support of the student André Leonardo, for having collaborated in this work 13 References [1] Duxson P, Fernández-Jiménez A, Provis J, Lukey GC, Palomo A, van Deventer JSJ Geopolymer technology : the current state of the art Adv Geopolymer Sci Technol 2007; 4: 2917-33 [2] Elimbi A, Tchakoute HK, Njopwouo D Effects of calcination temperature of kaolinite clays on the properties of geopolymer cements Constr Build Mater 2011; 25 (6): 2805-12 [3] Shi C, 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with metakaolin mortar 17 a) b) c) Figure - Impact machine Instron-Ceast 9050: a) Global view of the machine; b) Details of the apparatus used in the tensile impact tests; c) Details of the apparatus used in the transverse impact tests Figure – Typical stress-strain curves for control specimens and specimens embedded during 60 days 18 Figure – Typical stress-strain curves for control specimens and specimens embedded in cement mortar 19 Figure - Failure damage morphology for: a) Control samples; b) Samples embedded in cement mortar during 30 days; c) Samples embedded in cement mortar during 60 days; d) Samples embedded in cement mortar during 90 days 20 Figure - Failure damage morphology for: a) Control samples; b) Samples embedded in cement mortar embedment during 60 days; c) Samples embedded in cement mortar and immersed into water during 60 days; d) Samples embedded in metakaolin mortar during 60 days 21 Figure – Effect of the solution and exposure time on the tensile impact strength 1 m m a) m m b) Figure Damage mechanisms observed from tensile impact tests for: a) Control samples; b) Samples embedded in metakaolin mortar during 90 days 22 Figure 10 – Effect of the solution and exposure time on the transverse impact strength 23 Tables Table - Composition for the cement mortar and metakaolin mortar Cement mortar Constituintes Metakaolin mortar Constituintes Concentration Concentration Sand 62.72 Sand 54,34 Cement Portland 25.09 Sodium hydroxide 8,70 Water 12.05 Metakaolin powder 19,57 Plasticizer 0.14 Sodium silicate 17,39 24 Table – Effect of environmental conditions on bending stress Control Cement mortar Cement mortar in water Metakaolin mortar Standard Deviation Exposure days Stress [MPa] - 535 62 30 475 14 60 405 13 90 377 30 424 22 60 401 24 90 367 18 30 355 14 60 343 15 90 332 16 25 [MPa] ... impact performance of composite laminates when embedded in cement or in geopolymer mortars It can be concluded that these mortars have a significant effect on the structural integrity of the composites, ... Samples embedded in cement mortar during 30 days; c) Samples embedded in cement mortar during 60 days; d) Samples embedded in cement mortar during 90 days 20 Figure - Failure damage morphology for:.. .Structural integrity of glass/epoxy composites embedded in cement or geopolymer mortars A.M Amaro1*, M I M Pinto2; P.N.B Reis3, M.A Neto1, S M R Lopes2 CEMMPRE, Department of Mechanical Engineering,