Effect of high temperature exposure on cassava peel ash concrete
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https://www.eduzhai.net American Journal of M aterials Science 2013, 3(5): 142-148 DOI: 10.5923/j.materials.20130305.06 Effect of Elevated Temperature Exposure on Cassava Peel Ash Concrete M. A. Salau1, E. E. Ikponmwosa1,*, K. A. Olonode2 1Department of Civil and Environmental Engineering, University of Lagos, Akoka, Yaba, Lagos, Nigeria 2Department of Civil Engineering, Obafemi Awolowo University, Ile-Ife, Nigeria Abstract Th is paper presents the effects of elevated temperatures on the physical and structural properties of concrete containing diffe rent percentages of cassava peel ash (CPA) as part ial replace ment of ce ment. Blended concrete of mix ratio 1:2:4 was prepared and cast into cubes of sizes 100 mm. The concrete specimens were cured for 120 days for pozzo lanic reaction to take place. Test samples were subjected to elevated temperatures ranging from 200 to 600°C for a period of 2 hours. After exposure, it was allo wed to cool at room temperature (24 ℃) before weight losses, thermal strain were determined as well as residual comp ressive strength. Test results showed that weight of all the specimens significantly reduced with an increase in temperature. Th is reduction was very sharp within the 200℃ exposure and gradual after that. The presence of CPA does not significantly affect the residual weight. The results further indicated that residual strength was high at 200℃ when compared to room temperature strength. Later, the strength dropped gradually till 600℃ but strengths of 5 to 15% CPA concrete were relatively h igher than that of the normal concrete. There was thermal expansion at about 180℃, for all the mixes, with normal concrete on the lead. At above 200℃, thermal shrinkage began and continued till 600℃. But, linear thermal shrinkage of the normal concrete was still h igher. It is concluded that for structural applications involving service temperatures that is not more than 400°C and for limited period of 2 hours, the use of up to 10% cassava peel ash in replacement of cement in concrete can be considered. Keywords Residual Co mpressive Strength, Weight Loss, Thermal Strain, Cassava Peel Ash, Pozzolan ic Reaction 1. Introduction Cassava (Manihoc esculanta) is a t ropical root crop, originally fro m A mazonia, which provides the staple food of an estimated 800 million people worldwide (FAO 2013). In Nigeria, almost all the States of the Federation grow cassava with annual production of 51 million metric tonnes, making it the largest producer of cassava in the world (Adesina, 2012). During p rocessing of cassava, either for do mestic or industrial usage, large quantity of cassava peels are generated. According to Adesanya et al. (2008), about 20 30% by weight of cassava tubers are peels in case of hand peeling. Usually, cassava peels are left to rot or burnt which constitute pollution to the at mosphere, soil surface and underground water. Providing beneficial use for cassava peels is desired. Cassava peel ash has been investigated for use in concrete and mortar as mineral additive with positive results (Salau and Olonade, 2011 and Salau et al. 2012). It was shown that, * Corresponding author: ef email@example.com (E. E. Ikponmwosa) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved it imp roves the mechanical properties of concrete when up to 15% is used as replacement for cement. Nevertheless, to our knowledge, not a single study has been reported on the performance of concrete containing cassava peel ash on exposure to elevated temperatures. Elevated temperature e xposure either due to fire or heating devices around the concrete, like furnace, remains a major threat to concrete structures durability. Concrete is a heterogeneous multiphase material with relatively inert aggregates that is held together by the hydrated Portland cement paste. When concrete is exposed to high temperatures, its durability is under threat. Nonlinearit ies in material properties, variation of mechanical and physical properties with temperature, tensile cracking, and creep effects affect the build-up of thermal forces, the load-carrying capacity, and the deformation capability (i.e. ductility) of the structural members. The property variat ions result largely because of changes in the moisture condition of the concrete constituents and the progressive deterioration of the cement paste-aggregate bond, which is especially critical where therma l e xpansion values for the cement paste and aggregate differ significantly (Eg linton, 2004). The ult imate consequence of this action is development of cracks. These cracks, like any other cracks propagation, may eventually American Journal of M aterials Science 2013, 3(5): 142-148 143 cause loss of structural integrity and may result in undesirable structural fa ilures (Aydin, 2008). Hert z (2005) studied the mechanisms that occur when concrete is exposed to elevated temperature and concluded that at 110°C and above, the dehydration, including the release of chemically bound water fro m calciu m silicate hydrate, becomes significant. The dehydration of the matrix and the thermal expansion of the aggregate give rise to internal stresses, and beginning at 300°C, micro-cracks begin to pierce through the material [Tanyild izi and Coskun (2008)]. Calciu m hydro xide (Ca(OH)2), the by-product of cement hydration, dissociates at around 530°C, resulting in the shrinkage of concrete [Ariöz, 2007 and Zega and Maio, 2006]. However, this behaviour may not be generalized as many factors such as aggregate sizes, types and water-cement ratio and binder type dictate response of concrete to high temperatures. According De Souza and Moreno (2010), when concrete is subjected to temperatures up to 150°C, its strength is not altered; but for higher temperatures, the strength begins to decrease. This loss in strength can reach 70% for temperatures close to 600ºC due to gel dehydration and the increase of micro -cracking. They also reported that the studies performed by Galleto and Meneguini (2000) have shown that conventional concrete heated to 300°C and slowly cooled had a 24% loss in co mpression strength in relation to its original, unheated strength. A comparative study between the effect of elevated temperature on the normal and high strength concrete was conducted by Zang et al. (2000), where concrete beam specimens were e xposed to temperature between 100°C and 600°C for 12 hrs at 14 days. The weight loss and compressive strength were monitored. They observed that a significance reduction in strength occurred when the temperature was above 400°C for both specimens but more pronounced with high strength concrete. In a bid to improve fire resistance of concrete, different approaches have been suggested. Xiaoa and Falknerb (2006) and Noumowe (2005) p roposed the addition of polypropylene fibers in a concrete mix. However, the use of pozzo lanic materials is being given more attention in the recent pasts [Demirboga(2007), Wang(2008), Kalifa (2001) and Morsy et. al (2008)]. In addition to studies relating to the mineral additives (silica fu me, fly ash, blast furnace slag, pumice and metakaolin) used to increase the mechanical and physical properties of concrete, its durability and its workability, previous studies are also available on the performance of the concrete containing such additives after exposure to elevated temperatures [Al-Owaisy (2004) and Mazloom et al (2004)]. Nevertheless, to our knowledge, not a single study has been reported on the performance of concrete containing cassava peel ash on exposure to elevated temperatures. Cassava peel ash has been investigated for use in concrete and mortar as mineral additive with positive results (Salau and Olonade, 2011, Salau et al. 2012). It was shown that it improves the mechanical properties of concrete when up to 15% is used as replacement for cement. Thus, the focus of this is study is to investigate the performance of cassava peel ash concrete, when exposed to elevated temperature. 2. Materials and Experimental Procedures 2.1. Materials Ordinary Portland cement was used in this study. Its physical properties and chemical co mposition are presented in Table 1.The o xide compositions showed that the cement meets the standard requirements of BS EN 196-2. Cassava peel ash (CPA) was blended with cement at 5 to 25% replacement levels at an interval of 5%. It was produced by burning cassava peels in furnace for 90 minutes at 700°C and sieved through a sieve size of 150µm. The o xides composition and physical properties of the ash are also presented in Table 1, in co mparable with Ord inary Portland Cement. The blended cement was used as binder; crushed granite of maximu m nominal size of 19 mm was used as coarse aggregate while river sand was used as fine aggregate with maximu m size of 3.18 mm. Mechanical sieve analysis was conducted on the aggregates and particle size distribution curves were plotted, as shown in Figure 1. The specific gravit ies of the coarse and fine aggregates are each 2.72; but the coefficients of curvature for coarse and fine aggregates were found to be 1.36 and 1.0 respectively while those of uniformity were 5.0 and 5.4 respectively. Thus, the aggregates were well graded and suitable for concrete production. Potable water was used for mixing. Table 1. Chemical and Physical Properties of Cement and Cassava Peel Ash Prop ert ies Ordinary Po rt lan d Cement (OP C) Cassava Peel Ash (CPA) BS EN 196-2 Chemical Composition (%) Silicon Dioxide, SiO2 Aluminium Oxide, Al2O3 Iron Oxide, Fe2O3 Calcium Oxide, CaO Magnesium Oxide, MgO Sulphur T rioxide, SO3 Potassium Oxide, K2O Sodium Oxide, Na2O 19.63 2.42 2.33 61.25 2.17 2.09 0.29 0.00 42.21 9.79 4.01 4.77 1.33 0.72 9.04 0.68 Loss on Ignit ion (LOI), % 9.62 4.18 Physical Properties Fineness (Blain Value), m3/kg 355 330 Specific Gravity 3.09 0.32 18-24 2.6 – 8 1.5 – 7 61 – 69 0.5 – 4 0.2 – 4 0.2– 1.0 - - - 144 M . A. Salau et al.: Effect of Elevated Temperature Exposure on Cassava Peel Ash Concrete Percent Passing (%) 100 90 80 70 60 50 40 30 20 10 0 0.001 0.01 0.1 1 Particle Size (mm) Granite Sand 10 100 Figure 1. Particle size distribution of the aggregates 2.2. Concrete Mix and S peci men Producti on The blended concrete mix was prepared using ordinary Portland cement that was partially replaced by CPA in percentages of 5, 10, 15, 20 and 25%. The normal concrete which serves as control contains no ash (0% CPA). Mixtu re was in the ratio of 1:2:4 (binder: sand: granite) with constant water cement ratio of 0.7. Concrete cubes of sizes 100 mm were cast and cured in water for 120 days. The curing period was deliberately chosen to give room for pozzolan ic reaction to take place, according to Salau et al., 2012. 2.3. Testing of S pecimens In order to determine the effect of elevated temperature exposure on the concrete, the concrete cube specimens were heated in an electric furnace (Carbolite GPC 12/65) at 200, 400 and 600 ℃ for a constant period of two hours. The heating rate was set at 2.5℃⁄???????????????????????? according to Bahar and Keleştemur (2010). Durat ion of exposure to elevated temperatures of 2 hours practically guarantees that the samples are heated uniformly within the whole specimen volume. Two points were initially marked on the cubes at a distance of 50 mm. The heated cubes were later allowed to cool for a period of one hour at room te mperature (24±1℃). Thereafter, the d istance between the points was measured and noted before they were weighed and tested for compressive strength in accordance with BS1881. Average of three readings was recorded and compared with control which was not subjected to any treatment. Linear thermal strain was determined fro m the following relationship: ???????????????? = ???????????????? −????????ℎ ???????? (1) where: ???????????????? = linear therma l strain, mm/ mm; ???????????????? = length reading at room temperature, mm; ????????ℎ = length reading at higher temperature, mm; ???????? = Guage length between inserts, mm; The positive value indicates expansion wh ile negative value indicates shrinkage. 3. Results and Discussion 3.1. Thermal Effect on Weight of Cassava Peel Ash Concrete Residual weight, as used in this study, is the measured weight of the concrete after it has been heated to elevated temperature. To estimate the p roperties of concrete structures subjected to high temperatures accurately, it is necessary to study moisture migrat ion in mass concrete that is subjected to high temperatures. The results of residual weight of the cassava peel ash concrete that is exposed to elevated temperature and corresponding percentage loses are summarized in Table 2. It is observed that the residual weight reduces as the temperature increases for all the mixes; and this trend does not depend on the content of cassava peel ash (Figure 2). Th is is an indication that water, wh ich also contributes to self-weight of concrete, is being reduced through evaporation fro m the concrete matrix and degradation of constituent materials. Nevertheless, at room temperature of 24℃, the average weight of all the unheated concrete specimens is 2.425 kg with a standard deviation of ± 0.008 kg, showing that there was no appreciable difference in weight of the concrete mixes, irrespective of the content of CPA. Thus, CPA neither contribute to decrease nor increase in weight of unheated blended concrete. The variations in residual we ight recorded after heating could be attributed to the exposure to high temperatures. At 200℃, the residual weights for normal concrete and 5% CPA concrete were each 2.384 kg, representing 1.731% loss in weight while 10 to 20% CPA concrete has 2.373 kg (2.14% weight loss) each. Similar trend is observed when temperature increased to 400℃. But at 600℃, the residual we ight of norma l concrete (2.354 kg) is less than that of concrete containing up to 15% CPA (2.362 kg each). Th is could be attributed to decrease in capillary pore in the concrete and reduction in moisture as a result of pozzolanic react ion between the silica in the ash and calciu m hydroxide fro m hydration of cement. American Journal of M aterials Science 2013, 3(5): 142-148 145 %CP A Content 0 5 10 15 20 25 24 2.426 2.425 2.425 2.425 2.425 2.424 Table 2. Thermal Effect on Weight and Percentage Weight Loss Residual Weight (kg) Percentage Weight Loss (%) Temperature (℃) Temperature (℃) 200 400 600 24 200 400 2.384 2.364 2.354 0 1.731 2.556 2.384 2.374 2.363 0 1.731 2.143 2.373 2.365 2.363 0 2.144 2.474 2.373 2.362 2.362 0 2.144 2.598 2.373 2.344 2.334 0 2.144 3.340 2.364 2.343 2.333 0 2.475 3.342 600 2.968 2.597 2.557 2.598 3.753 3.754 Residual weight (kg) 2.44 2.42 2.4 2.38 2.36 2.34 2.32 0 0% CPA 5% CPA 10% CPA 15% CPA 20% CPA 25% CPA 100 200 300 400 500 600 700 Temperature (°C) Figure 2. Effect of High T em erature on Weight of Cassava Peel Ash p Concrete However, the residual weights of concrete having more than 15% CPA content were each 2.334 and 2.333 kg for 20 and 25% CPA respectively, meaning that the weight loss is a parameter that can help to distinguish three different regimes during heating of concrete (Bingöl, A.F., and Göu l, R. (2009) and Sancak et al. (2008)). When the heating temperature is under 200℃, the weight loss may be co mpletely caused by the quick evaporation of capillary water, and the concrete undergoes a physical process. For a temperature between 200℃ and 400℃, the weight loss may be due to the gradual evaporation of gel water, and the concrete undergoes a mixed physio-chemical process. But a temperature over 400℃, the weight loss is mainly caused by the evaporation of chemically bound water (dehydration) and decomposition, so the concrete undergoes longer weight loss, but a greater curing age slows down the evaporation rate because of further hydration. Generally, the loss in weight was low (maximu m of 3.72%), this is so because the concrete was allo wed to cure for longer period before testing to give room for pozzolan ic react ion to occur. 3.2. Effect of Temperature on Compressive Strength of Cassava Peel Ash Concrete The results of the strength and deformat ion of cassava peel ash concrete, as compared to normal concrete at the age of 120 days, when exposed to temperatures of 200, 400 and 600 ℃ for 2 hours, are summarized in Table 3. It was observed, from Figure 3, that there was relative decrease in the compressive strength of each blended concrete cubes as the percentage (%) of CPA contents increases at room temperature (24 ℃ ) as compared to normal co mpressive strength before heating. These results were comparable to those obtained by Salau et al. (2012). However, when the concrete specimens were exposed to elevated temperatures, the residual co mpressive strength behaves differently. At 200℃, with the exception of concrete with 25% CPA, the residual strengths were h igher than their corresponding strengths at room temperature. For instance, norma l concrete strength increased by 5.30% while 5, 10, 15 and 20% CPA concrete had an increase of 6.14, 4.70, 4.57 and 4.4.1% respectively. An observed increase in compressive strength may be partly attributed to the effect of autoclave curing. In addit ion to this effect, the increase with up to 20% CPA could be mainly due to pozzolan ic reaction which led to the formation of additional a mount of hydration products. According to Ivan (2004), in the presence of finely ground quartz and/or other SiO2 sources and at temperatures between about 150 and 200°C, a pozzo lanic reaction takes place, y ield ing crystalline 1.1 ???????????????? tobermorite, [Ca4(Si5O16H2]Ca.4H2O (C5S5H5), as the main product of reaction. At even higher temperature, xonolite (C6 S6H) may also be formed. Intermed iate products formed under these conditions include C-S-H (I), C-S-H (II) and ????????-C2SH. The ????????????????3+ , wh ich is present, may substitute for ????????????????4+ in the 146 M . A. Salau et al.: Effect of Elevated Temperature Exposure on Cassava Peel Ash Concrete crystalline lattice o f tobermorite with a simu ltaneous increase of ????????????????2 + in the interlayer reg ion. The formation of both 1.1 ???????????????? tobermorite and xonolite is associated with a favourable development of strength of the autoclaved mater ia l. The compressive strength was drastically affected in higher ranges of temperature exposure. At 400℃, there was reduction in strength, when co mpared with the corresponding strength of unheated concrete specimens, by 12.02, 11.36, 10.47 and 10.58% fo r normal concrete and 5, 10 and 15% CPA concrete respectively. This shows that cassava peel ash concrete has higher residual compressive strength than that of the normal concrete when up to 15% CPA concrete except that hair-like cracks were observed with 15% CPA concrete specimen. However, significant reduction in strength was noticed for 20 and 25% CPA strengths with relative low residual strengths of 12.4 and 10.3 ????????⁄????????????????2 respectively. On further heating to 600℃, all the concrete specimens had lost more than 30% of their referenced room temperature strengths. However, the percentage loss in strength of concrete containing up to 15% CPA was relat ively lower than that of the normal concrete. But, concrete with 20 and 25% CPA had deep cracks with very low strengths of 9 and 7.5 ????????⁄????????????????2 respectively (Figure 3). The general trend for the strength loss with increasing temperature reflects the influence of the ce ment paste and the increasing role of the aggregate materials at h igher temperatures. Factors have been identified that may contribute to the general trend for loss of co mpressive strength with increasing temperature (Naus, 2010): aggregate damage; weakening of the cement paste-aggregate bond; and weakening of the cement paste due to an increase in porosity on dehydration, partial breakdown of the C-S-H, chemical transformation on hydrothermal reactions, and development of cracking. Nevertheless, the relative low loss of compressive strength of concrete with up to 15% CPA could be as a result of lo w porosity in the concrete matrix, occasioned by pozzo lanic reaction. The findings of this study are in agreement with the results of Bahar and Oğuzhan (2010), where finely ground pumice and silica fu me concrete were assessed under elevated temperature only that the age of concrete tested was 28 days, wh ich may not be sufficient to allow a pozzo lanic effect of the finely g round pumice and silica fu me. In general, for structural applicat ions involving service temperatures that is not more than 400°C and for limited period of 2 hours, provided many temperature cycles of large magnitude are not present, the use of up to 10% cassava peel ash in replacement of cement in concrete can be recommended; and for this same time, elevated temperatures of the order 600°C, special procedures would have to be considered; such as removal of the evaporable water by moderate heating. Table 3. Thermal Effect on Strength and Expansion/Shrinkage of Cassava Peel Ash Concrete %CP A Content 0 5 10 15 20 25 24 19.05 18.39 18.15 17.95 14.75 12.60 Residual Compressive Strength (????????⁄????????????????2 ) Temperature (℃) 200 20.06 19.52 18.95 18.65 15.40 12.30 400 16.76 16.30 16.25 16.30 12.40 10.30 600 13.02 12.75 12.56 12.42 9.00 7.50 Linear Thermal Strain (× 10−4 ) Temperature (℃) 24 200 400 600 0 3 -3.3 -3.5 0 2.3 -3 -3.3 0 2.3 -3 -3.25 0 2 -3 -3.3 0 1.6 -2 -1.5 0 1.5 -2.1 -1.4 Residual Compressive Strength (N/mm2) 25 20 0% CPA 15 5% CPA 10% CPA 10 15% CPA 20% CPA 5 25% CPA 0 0 100 200 300 400 500 600 700 Temperature (°C) Figure 3. Effect of High T emperature on Residual Compressive Strength of Cassava Peel Ash Concrete American Journal of M aterials Science 2013, 3(5): 142-148 147 Thermal Strain (×10-4) 4 3 2 1 0 -1 0 100 200 300 400 500 600 -2 -3 -4 -5 Temperature (°C) 0% CPA 5% CPA 10% CPA Figure 4. Effect of elevated temperature on thermal strain of cassava peel ash concrete 3.3. Ther mal Shrinkage of Cassava Peel Ash Concrete Under elevated-temperature exposure, the Port land cement paste experiences physical and chemical changes that contribute to development of shrinkage, transient creep, and changes in strength. Therma l shrinkage strains computed on exposure of cassava peel ash concrete specimens to higher temperatures are presented in Table 3. The effect of elevated temperature on therma l strain of the concrete specimens can be examined through three regimes as suggested by Bingöl and Göul (2009): between roo m temperature and 200 ℃, between 200 and 300℃ and after 300 up to 600. In the first regime, it is observed that all the concrete specimens e xpand with the peak linear thermal expansion at about 180 ℃ (Figure 4) but normal concrete takes the lead with linear thermal strain of 3× 10−4 follo wed by 5, 10, 15, 20 and 25% CPA concrete in that order (Table 3). The implication of this may be that there was more mo isture in the normal concrete than others as CPA absorbs more water than ordinary Portland cement. When the temperature increases, the water in the mix gains more kinetic energy wh ich increases the volume and hence expansion of the concrete specimen. In the next regime, when the temperature is more than 180 ℃ , the mo isture in the concrete specimens begins to evaporate and thus the concrete specimens start to shrink until they reach zero thermal strain at about 300 ℃ (Figure 4). But the thermal shrinkage beco mes more p ronounced at higher temperatures. Nevertheless, at 600℃, the normal concrete shrinks more than all cassava peel ash concrete specimens indicating that cassava peel ash (CPA) retards the thermal shrinkage in concrete due to further hydration of cement paste, which increases the bond between the aggregate and blended cement paste. This behaviour could be attributed to pozzo lanic property of cassava peel ash. 4. Conclusions strength and thermal strain of cassava peel ash concrete, exposed to elevated temperature, was investigated. The behaviour was compared with corresponding room temperature concrete specimens and the following conclusions were made. 1. The weight of the concrete decreases with increase in temperature for the entire mixes. This finding was due to the release of bound water fro m the cement paste and the occurrence of air voids in the concrete. The highest weight loss occurred in concrete specimen containing mo re than 15% CPA. 2. At 200℃ temperature exposure, there was increase in compressive strength; but significant reduction in strength was observed thereafter. When the temperature was 600℃, about 30% reduction in strength was noted for all the mixes. However, cassava peel ash concrete with up to 15% CPA had relative low strength loss (30.81%) when compared to that of norma l concrete (31.65%). 3. 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