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Effect of fly ash on freeze-thaw durability of concrete in marine environment.

1. Introduction

Concrete has been extensively used as the basic construction material for various types of offshore structure over for over 50 years. Concrete structures in such locations are always required to withstand physical, chemical and mechanical action of sea water under varying environmental condition throughout their lifespan (Setina, Gabrene, and Juhnevica 2013). Tidal action causing alternate wetting and drying cycles, abrasion and salt water spray due to wave thrust, high ambient pressure due to large hydrostatic head, freeze-thaw cycles due to climatic change, wide variation of ambient temperature and humidity are the major physical and mechanical loadings of marine environment. While, the chemical loadings include the gradual penetration of detrimental sea salt ions into the cement mortar matrix which, after chemical reaction, initiate the decomposition of mortar matrix and also the corrosion of the embedded rebar. Due to the need of practical application, many reinforced concrete structure were (will be) built in cold regions that inevitably subjected to freezing and thawing action (Richardson, Coventry, and Wilkinson 2012). The most important durability problem of concrete under cold climate is freeze-thaw effect (Karakurt and Bayazit 2015). Thus, structural concrete in cold coastal regions are exposed to coupling effects of freeze-thaw cycles and seawater corrosion (Rao et al. 2017). For reinforced concrete structures, other than the degradation of concrete materials, the existence of cracks under service loads is believed to be one of the main causes of deterioration of reinforced concrete structures serviced in cold marine environment or snow melting conditions (Diao et al. 2011). Chloride or other chemical ions would penetrate into concrete through cracks and induce corrosion of reinforcing steel bars. The amount of permeation is determined by the thickness of the concrete cover and the width of the cracks (Djerbi et al. 2008). To simulate the working conditions of reinforced concrete structures in cold coastal regions, the presence of structural loads in addition to the harsh environmental factors were considered in a number of recent studies (Diao et al. 2011).

Furthermore, because of the wide use of deicing salt for melting ice on the roads, salt is usually available near these structures. Besides creating pressure through osmosis and crystallisation, deicing chemicals increase the degree of concrete saturation and keep concrete pores at or near maximum fluid saturation, thus increasing the risk of frost damage (Litvan 1976). When the surface is exposed to 3% salt solution, surface scaling is 1-2 orders of magnitude higher than when exposed to water (Kaufman 2012). The salt in deicing solution also decreases the freezing point of concrete pore solution, leading to significant hydraulic pressure (Setzer 1976).

1.1. Ice formation in concrete

Concrete deterioration for freeze-thaw cyclic action has been a major problem in cooler areas of different countries. Freeze-thaw deterioration begins when water enters into the voids of concrete. Leaching of calcium hydroxide, which is produced during the hydration of portland cement, provides greater voids for water to occupy, thereby aggravating the rate of deterioration. Freezing of water or salt solution in the concrete pores may cause severe deterioration and considerable reduction of service life (Olanike 2014). When water freezes, the volume increases by 9% as water turns to ice, generating high pressure in the adjacent concrete, which may lead to pressure cracking and scaling. Due to the hydraulic and osmotic pressure, the disruption will be further increased (Ramachandran 1996). The temperature at which water freezes in capillary pores is a function of the size of the pores and pore chemistry. As pore size decreases, the temperature required to freeze the water also decreases (Hale, Freyne, and Russell 2009). The resistance of concrete to frost action depends on the strength of the paste, water/cement ratio, type of aggregate used, age of concrete, duration and extent to which the concrete is subject to freezing action. When a concrete structure is exposed to freezing environments, the pore solution in capillary pores changes into ice and expands by volume. Because of its volume expansion, unfrozen water tends to move into any available place nearby and the movement of the pore solution eventually builds up hydraulic pressure (Maria et al. 2017). When the expansive force exceeds the tensile strength of concrete, micro-cracks start to generate and radiate to the surrounding cement paste.

1.2. Freeze-thaw effect on marine concrete

The marine environment is characterised by typical aggressive loading of various soluble salts in sea water. Average total concentrations of salts in Indian Sea are 35.5 gm/L according to Biczok (1972). Typically, sea water contains about 3.5% soluble salts by weight (Shetty 2002). The relative ionic concentrations are 18 gpl [Cl.sup.-], 12 gpl [Na.sup.+], 2.6 gpl [(S[O.sub.4]).sup.2-], 1.4 gpl [Mg.sup.2+] and 0.5 gpl [Ca.sup.2+]. Normally, pH of sea water is about 8. In a marine environment, in addition to its presence in original mix, the chloride ions penetrate into the concrete either from sea water or sea winds carrying sea salts and reacts with the hydrated cement products which produces complex compounds including Friedels salt which are leachable and expansive in nature. The chloride attacks also destroy the passivity of steel and lead to the initiation of rebar corrosion. On the other hand, the penetration of sulphate ions attack the hydrated cement matrix with the formation of gypsum and a complex compound known as calcium sulphoaluminate (ettringite).

In the regions with a cold climate, the freeze-thaw damage is the most important issue among the durability problems in concrete structures, such as dams, hydraulic and offshore structures, and bridges and highway pavements, during their service (Rao et al. 2017). One of the advances in concrete technology is the development of fly ash concrete and its use in it. Fly ash is a by-product from combustion of pulverised coal. As the coal is heated to high temperatures, it liquefies. It is thereafter cooled rapidly, which forms spherical particles. The fly ash consists mainly of silica (Si[O.sub.2]), aluminium oxide ([Al.sub.2][O.sub.3]), iron oxide ([Fe.sub.2][O.sub.3]), and calcium oxide (CaO). Use of fly ash improves the workability of concrete slurry, reduce the heat of hydration of cement and increase the strength of concrete with age; therefore, it can improve the mechanical properties and durability of concrete (Zhang, Johnson, and White 2016).

Fly ash combines with calcium hydroxide to produce additional cementitious materials, thereby reducing the amount of calcium hydroxide that may be leached out of the concrete. Leaching of the calcium hydroxide increases concrete voids which can accelerate freeze-thaw damage. As a result, permeability and porosity are reduced. Fly ash also fills the minute voids creating a denser and less absorptive concrete. It reduces the amount of water required in the mix by approximately 2 to 10%, because the spherical shape of the fly ash particles reduces bleed channels and void spaces. Reduction of bleed channels limits the entrance of water; fewer void spaces mean less space for water to accumulate (Gullu and Fedakar 2016). High-quality fly ash also produces more cohesive concrete which holds entrained air inside the concrete. Fly ash helps to produce higher compressive strengths in the long term that provide a strong concrete which resists the forces generated during the freezing of water in the voids. As a whole fly ash concrete is more stable, uniform, dense, less absorptive and less permeable--all factors which improve freeze-thaw durability. Concrete with about 40% fly ash of cement weight, or less, is durable regarding de-icing salt scaling, whereas the concrete with higher amounts of fly ash is non-durable. Concrete with fly ash perform well when subjected to freezing and thawing, irrespectively of the amount of fly ash in the concrete, considering internal damage (Knutsson 2010). The performance of concrete with additions of fly ash is in many situations improved compared to that of concrete mixed with Portland cement only.

1.3. Use of fly ash in marine concrete

Fly ash can partly replace cement in concrete. Replacing the content of cement, the production cost as well as the environmental impact, e.g. production of C[O.sub.2], is reduced. Use of fly ash improves the workability, reduces the demand of water and reduces the temperature rise in the fresh concrete (Badr and Gadsden 2013). Furthermore, addition of fly ash gives higher long time strength of concrete and reduces the permeability. The fly ash can either be blended in the cement or added separately in the concrete at mixing. Fly ash is not cementitious by itself, but will together with cement, produce cementitious compounds. The primary contributor to the pozzolanic reaction in fly ash is the silica, which combines with calcium hydroxide and water to form the binder in concrete, calcium silicate hydrate (C-S-H). During hydration, the cement reacts with water and form durable binder. Most properties of the hardened cement paste are developed when tricalcium silicates ([C.sub.3]S) and dicalcium silicates ([C.sub.2]S) reacts with water, forming C-S-H ([C.sub.3][S.sub.2][H.sub.3]) and calcium hydroxide (CH) (Illston and Domone 2001). The silica in the fly ash in a finely divided form and in the presence of moisture, at ordinary temperatures, reacts chemically with calcium hydroxide liberated from the hydration of cement, forming cementitious compounds. Hanehara et al. (2001) reported that the onset of the pozzolanic reaction of fly ash at the curing temperature of 20 [degrees]C is at 28 days or longer, and the pozzolanic reaction of fly ash in cement paste highly depends upon the curing temperature. The reaction is secondary, but it is not possible to differentiate the C-S-H produced from pozzolanic reactions, from that produced by cement hydration (Illston and Domone 2001). Since this pozzolanic reaction is secondary, it will occur somewhat later than the hydration of the cement, for some ashes even up to one week after the hydration of the cement has started.

Bortz (2010) studied how the source of fly ash influences the durability regarding scaling under freeze-thaw. The source of the fly ash, which affects the properties of the fly ash, had high impact on the resistance of freeze-thaw scaling. The rate of strength development for concrete with fly ash is lower at the beginning than for concrete with plain Portland cement. However, concrete with fly ash does continue to gain strength, which means that after some weeks or months, the strength of this concrete will be higher than for the concrete containing Ordinary Portland Cement (OPC). The pozzolanic activity does improve the strength of the transition zone, i.e. the interface between the paste and aggregate, in the concrete by secondary effects. Furthermore, better packing of particles in the fresh state when fly ash is included will reduce the porosity, hence also leading to higher strength (Badr 2015).

1.4. Resistance of chloride ingress for concrete with fly ash

Chloride ions from sea water and de-icing salts can penetrate into concrete by transport of chlorides in water, diffusion of the ions in water and by absorption. If the chloride ions reach the reinforcement, corrosion may occur. Not all of the chlorides in the concrete do affect the corrosion of steel; some ions are chemically bound to the hydration products from the cement, whereas others are physically bound being adsorbed on the surface of the gel pores. It is only the free chloride ions that can damage the reinforcement. The penetration of chloride ions is also dependent on the permeability of the concrete; a more permeable concrete will lead to less resistance against penetration (Neville 2003). Concrete with fly ash has shown better resistance against chloride penetration than concrete with OPC. This is partly due to that fly ash creates a denser structure, which reduces the permeation and also concrete with fly ash binds the chloride ions better, thus leaving fewer ions free (Dhir and Jones 1999). The binding capacity was found to be at maximum at a replacement of fly ash of 50% of the cement, but optimum at about 30%. Concrete with fly ash replacing 33% of the cement, the binding capacity was four times larger than for OPC. Furthermore, the binding capacity increased with the concentrations of chloride ions. Replacement of the cement by 30% fly ash was found to improve the resistance against chloride ions with two to four times. The active alumina ([Al.sub.2][O.sub.3]), which exists in larger amounts in fly ash than in Portland cement, is able to bind the chloride ions.

1.5. Water permeability characteristics of concrete

Permeability is the most important aspect of concrete durability. The rate at which water is transmitted through a saturated specimen of concrete under an externally maintained hydraulic gradient is defined as the coefficient of permeability. It is inversely linked to durability in that the lower the permeability, the higher the durability of concrete (Joshi and Lohtia 1997). To be durable, concrete must be relatively impervious (Berry and Malhotra 1980). The single parameter that has the largest influence on permeability/durability is w/c ratio. As the w/c ratio is decreased, the porosity of the paste is decreased and the concrete become more impermeable. Clearly, the permeability of concrete plays an important role in durability because it controls the rate of entry of moisture that may contain aggressive chemicals and the movement of water during heating or freezing. This leads to enhanced durability because aggressive agents without entering inside the concrete cannot attack the concrete nor the reinforcing steel embedded in it (Bremner and Thomas 2004).

The key to prevention of sulphate attack is to tie up the free lime and calcium aluminates to eliminate the possibility of ongoing reactions. Increased sulphate resistance of concrete containing fly ash may be explained by the reaction of silica, alumina and ferric oxide found in fly ash with calcium hydroxide liberated during the hydration of Portland cement to form relatively stable cementitious compounds. Greater impermeability of such type of concrete reduces penetration of sulphate solutions and results in improved resistance to sulphate attack. Fly ash not only reduces the permeability of the concrete, but because of reaction of these materials with available alkalis, it removes that essential component required for Alkali Aggregate reaction (AAR) and reduce the risk of AAR occurring.

The aim of this work is to evaluate the freezing and thawing durability of concrete made with locally available Class F fly ash. Among the several durability problems that arise in concrete exposed to a marine environment, those have been studied extensively are the chemical attack of sea water on cement paste, concrete and the corrosion of embedded steel. The freeze-thaw action on structural concrete in the splash/tidal zone has its own characteristics and is dependent on ambient air and sea water temperature. In addition, the chemical attack of the sea water on the cement constituents is found to lead to a more pronounced deterioration in the concrete structure. The results of this investigation would provide data for establishing appropriate mix proportions for concretes subjected to freezing and thawing resistance against sea water and deicer salts.

2. Experimental programme

The experimental programme was planned to study the suitability of fly ash as partial replacement of cement in making structural concrete taking into consideration of the compressive strength, water permeability and rapid chloride penetration test (RCPT) value of hardened concrete exposed to freezing-thawing environment in plain as well as sea water. Plain water used in the experimental programme is tap (portable) water.

2.1. Materials used

(a) Cement: ASTM Type-I OPC was used as binding material. Chemical compositions of OPC are given in Table 1.

(b) Fly ash: Locally available low calcium fly ash compiling with ASTM Class F Fly ash was used as supplementary cementitious material. Chemical analysis of the fly ash conducted using X-ray fluorescence study is shown in Table 1.

(c) Aggregate: Locally available natural sand passing through 4.75 mm sieve and retained on 0.075 mm sieve was used as fine aggregate. The coarse aggregate was crushed stone with a maximum nominal size of 12.5 mm. Grading and physical properties of the aggregates are given in Table 2.

2.2. Variables studied

(a) Curing water: Plain water (PW) as well as artificially made sea water (SW) were used for curing the specimens. SW was made by mixing tap water with exact amount and proportion of principal salts found in natural sea water. The composition of artificial sea water is given in Table 3.

(b) Exposure condition: freezing-thawing arrangement was created in a freeze-thaw chamber. In each freeze-thaw cycle, the temperature was varied from (-17.8 [degrees]C) to (+4.4 [degrees]C) over a total period of 24 h (8 + 4 h for freezing and thawing; 7 + 5 h kept at two terminal temperatures) (Plate 1).

(c) Exposure periods: 30, 90, 180 and 360 freeze-thaw cycles after 28 days of procuring in plain water.

2.3. Mix design and sample preparation

Three different grades of concrete namely M38, M33 and M28 were used in the programme. Four different mix proportions of cement fly ash (80:20, 70:30, 60:40 and 40:60) were used as cementitious material. Cement fly ash mix ratio of 100:0 i.e. plain concrete specimens were also cast as reference concrete for comparing the properties of fly ash concrete. Fly ash concrete means the concrete made using cement and fly ash as cementitious material with sand, stone chips and water. Relevant information of different concrete mixes is given in Table 4.

2.4. Size of specimens

A total of 300 Nos. of cylindrical specimen of size 150 mm diameter and 175 mm high and 450 Nos. cubical specimens of 100 mm size from five different types of fly ash concretes were cast as per requirement for conducting water permeability and strength test. Another 450 Nos. of cylindrical specimen of size 100 mm diameter and 50 mm height were also prepared for RCPT. The whole experimental programme including the various parameters/variables studied, test conducted are summarised in Table 5. The small size of specimen i.e. 100 mm cube was selected in order to accommodate large number of specimens in the limited sized curing tanks. The specimens were demoulded after 24 h of casting and cured in plain water at 27 [+ or -] 2 [degrees]C. Concrete specimens were designated as per grade of concrete and amount of fly ash as a percentage of total cementitious material. Thus, M38FA20 concrete means grade of concrete is M38 and cement fly ash mix ratio is 80:20.

2.5. Test conducted

(a) Compressive strength

The concrete specimens were tested for compressive strength at 30, 90, 180 and 360 freeze-thaw cycles in accordance with the BS EN 12390-3 2009. The reported strength in each case is taken as the average of three tests results.

(b) Freezing-thawing test

In ASTM C 666 (2011), two procedures of freezing-thawing test are defined: Procedure A, rapid freezing and thawing in water, and Procedure B, rapid freezing in air and thawing in water. In this study, the Procedure A was used and according to this procedure, the temperature of the curing water condition concrete specimens was lowered from 4 to -17.8 [degrees]C and raised it from -17.8 to 4 [degrees]C in 4 h.

(c) Water permeability test

Water permeability test was carried out at 30, 90, 180 and 360 freeze-thaw cycles. The cylindrical specimens were dried in the oven at 105 [degrees]C and then coated with epoxy coating in the circular side to prevent water leakage from the side during the test. After placing the specimen in the apparatus, a water pressure of (500 [+ or -] 50) kPa was applied for (72 [+ or -] 2) hours. After the saturation of the specimen, the flow rate reading was taken from connected burette by measuring the changing of volume of water with time. Coefficient of permeability is calculated using the following equation,

k = (QL/AH)

where k = permeability coefficients (m/s), Q = flow rate ([m.sup.3]/s), A = area ([m.sup.2]), L = depth of specimen (m), H = head of water (m). Depth of water penetrated in the test specimen was calculated in accordance with the BS EN 12390-8 2009. The reported strength is taken as the average of two tests results at each test point.

(d) RCPTs

Cylindrical sample of 100 mm diameter and 200 mm height were prepared in accordance with ASTM C39. In this test, chloride ions are forced to migrate out of NaCl solution subjected to a negative charge, through the concrete, into a NaOH solution maintained at a positive potential. The total charge passed, in coulomb, is used as an indicator of the resistance of the concrete to the penetration of chloride ions. At the end of 30, 90, 180 and 360 cycles of freeze-thaw, the cylinder specimens were tested for RCPT as per ASTM C1202. The average result of three test specimens was taken as the representative data. ASTM guidelines concerning the chloride ion penetrability are given in Table 6.

3. Results and discussion

Concrete specimens exposed in the sea water and plain water is taken out after specific cycles of freeze-thaw, for conducting various tests. After visual inspection, it is seen that concrete specimens in sea water have lost their dimensional stability with substantial erosion and splitting/crumbling on the surface whereas in plain water, concrete surface tend to become uneven (Plate 2). Some changes in colour from the original dark grey to lime grey of the specimens in sea water have also been observed which indicates either the salts deposition on the concrete surfaces or leaching out of portlandite, Ca[(OH).sub.2]. Also, interior surfaces have indicted a higher level of saturation with increasing number of freeze-thaw cycles both in sea and plain water environments (Plate 3).

3.1. Weight change

The change in weight of the specimens of concrete grade M38, M33 and M28 in different exposure conditions and for various freeze-thaw cycles have been illustrated in Figures 1-3, respectively. A close examination reveals that at the end of 30 cycles of freeze-thaw, the specimens in sea water exhibit a higher percentage (nearly 1.1-2.6%) of weight gain as compared to the plain water cured specimens (nearly 0.9-2.2%). This increase in weight may be primarily due to the ingress of sea water or plain water into the concrete. After 90 cycles, a significant difference in the trend of weight change for the concrete specimens exposed to sea water has been found to occur as compared to that for the specimens placed in plain water. In this condition plain water cured specimen exhibit higher percentage of weight gain as compared to sea water cured concrete. After 360 cycles of freeze-thaw, a considerable change (loss) in the weight lying between -0.3 and -1.8% is observed for the concretes exposed to the plain water environment whereas for the similar specimens placed in sea water, this change is found to lie in the range of -3 to -9.3%. The higher loss in weight of the specimens exposed to sea water is primarily due to crumbling of outer surfaces of the specimens caused by crystallisation of sea salts in the voids of concrete and their subsequent expansion during freeze-thaw cycles. It is seen that, fly ash concrete shows better resistance against weight change as compared to plain concrete particularly after long freeze-thaw loading. It may be due to the development of its resistance to water/salt ions penetration inside concrete as the rate of hydration of fly ash concrete in relatively slow.

3.2. Volume change

The change in volume of fly ash concrete of grade M38, M33 and M28 exposed to sea water and plain water for different period of freeze-thaw cycle are illustrated in Figure 4, respectively. It is clear from these figures that the effect of sea water on the volume change of concrete specimens is relatively higher than that of plain water under freeze-thaw action. At the initial stage, i.e. after 30 cycles of freeze-thaw, volume of all the specimens are observed to be increased. It may be attributed due to hydration reaction of cement in presence of ingress of sea or plain water inside the concrete mass. At the end of 90 cycles, sea water cured concrete specimens have exhibited volumetric change of nearly -0.2 to -0.53%, whereas a volume change of -0.19 to 0.1% has been found in the specimens placed in the plain water. The volume change of the concrete specimens placed in sea water and plain water has been found to decrease due to surface erosion and splitting. After 360 cycles of freeze-thaw, this reduction is observed to lie between -0.08 and -0.31% for plain water and -0.62 to -1.11% for sea water cured concrete. The decrease in volume resulting from erosion/crumbling of outer surfaces of concrete may be attributed to the deposition of chemical compounds into the voids of concrete as well as their expansion due to freezing of the entrapped water inside the voids. Fly ash concrete shows much more resistance against volume change as compared to OPC concrete for relatively high cycles of freeze-thaw.

3.3. Compressive strength

Compressive strength of OPC and fly ash concrete of three different grades M38, M33 and M28 has been graphically presented in Figures 7-9. Also for the ease of comparison, the relative compressive strengths at different freeze-thaw cycles and in different curing water are plotted in Figures 10-12. A close examination of these curves indicates that the strength increases during the first 90 cycles of freeze-thaw in sea water as well as in plain water for all grades of concrete and after that it starts to decrease (Canbaz and Armagan 2016). This decrease in the compressive strength has been found to be significant at 360 cycles of freeze-thaw. It is noted that in comparison to the 28 day compressive strength of plain water cured concrete at constant temperature of 27 [degrees]C, the compressive strength of the concrete specimens subjected to 180 cycles of freezing and thawing have been found to lie in the ranges of 78-82% for sea water and 89-95% for plain water; whereas the same value for the concrete specimens subjected to 360 cycles of freeze-thaw in sea water is 70-74% and in plain water is 80-84%.

At early ages of curing, OPC concretes i.e. no fly ash concrete achieves relatively high compressive strength as compared to fly ash concrete. Test result shows that after 30 cycles of freeze-thaw, compressive strength for OPC concrete is around 8, 9, 14 and 43% higher than M38FA20, M38FA30, M38FA40 and M38FA60 concrete, respectively. At initial age of curing, compressive strength is seen to decrease with the increase in fly ash content when compared with OPC concrete. For relatively large freeze-thaw cycles, compressive strength of the fly ash concrete specimens up to 40% cement replacement level are higher than that of OPC concrete. Compressive strength after 180 cycles of freeze-thaw for M38FA20, M38FA30 and M38FA40 concrete is higher by around by 6, 19 and 16%, respectively, than OPC concrete. Cement normally gains its maximum strength within 28 days. During that period, lime produced form cement hydration remains within the hydration product. Generally, this lime reacts with fly ash and imparts more strength and for this reason, concrete made with fly ash will have lower strength than cement concrete at early ages of curing and higher strength at the later ages of curing. Conversely in cement concrete, this lime would remain intact and with time it would be susceptible to the effects of weathering, loss of strength and durability. M28 grade concrete also shows almost similar trend. Test result shows that compressive strength after 30 cycles of freeze-thaw for OPC concrete is around 4, 1, 7 and 38% higher than M28FA20, M28FA30, M28FA40 and M28FA60 concrete, respectively. On the other side at the end of 180 cycles of freeze-thaw, compressive strength for M28FA20, M28FA30, M28FA40 and M28FA60 concrete are, respectively, 3, 8, 4 and 14% higher than no fly ash concrete. After 365 cycles of freeze-thaw, compressive strength for M38FA20, M38FA30 and M38FA40 concrete are, respectively, 17, 27 and 23% higher than the corresponding OPC concrete. The same strength M33FA20, M33FA30 and M33FA40 concretes are, respectively, 11, 21 and 18% higher and for M28FA20, M28FA30 and M28FA40 concrete are, respectively, 6, 15 and 10% higher as compared to OPC concrete of similar grade.

Test results also show that compressive strength of both OPC and fly ash concrete is reduced when it is exposed to seawater as compared to plain water curing. At the end of 90 cycles of freeze-thaw, compressive strength for M38FA20, M38FA30 and M38FA40 concrete is around 8, 16 and 15% higher than 28 days compressive strength of OPC concrete when cured in plain water; whereas the same value is around 2, 8, and 6% higher for M38FA20, M38FA30 and M38FA40 concrete, respectively, as compared to OPC concrete when cured in sea water. Also the compressive strength value after 360 cycle of freeze-thaw is deteriorated by 16, 9 and 11% for M33FA20, M33FA30 and M33FA40 concrete when cured in plain water and 27, 24 and 18% when cured in sea water as compared to 28 days strength of plain water cured concrete. The increase in strength up to the first 90 cycles may due to the fact that the specimens do not get saturated fully by sea water during this period. After 90 cycles, the specimens get saturated considerably by sea water and after crystallisation of the salts together with their reaction with cementitious products within the body of concrete results in a significant decrease in the compressive strength.

Effect of seawater on the compressive strength of fly ash concrete has also been explained in terms of relative strength. In case of 360 cycles of freeze-thaw, reduction of strength is 25, 16, 9, 11 and 32% for M33FA0, M33FA20, M33FA30, M33FA40 and M38FA60 concrete exposed to plain water; whereas the same value is 44, 27, 24, 18 and 40% for the same concrete exposed to seawater, respectively, as compared to 28 days strength of OPC concrete in plain water. Fly ash concrete shows better resistance against strength deterioration compared to OPC concrete for larger freeze-thaw cycle. Strength reduction values at the end of 360 cycles of freeze-thaw for M28 grade concrete is relatively 21, 14 and 17% for 20, 30 and 40% cement replaced fly ash concrete and the same value is 25% for OPC concrete exposed to plain water whereas the same value for M38 grade concrete is reduced by 8, 2 and 3% for fly ash concrete and 22% for OPC concrete. The loss in strength of concrete in freezing-thawing condition observed in the current study is in agreement with results reported by Shang, Cao, and Wang (2014).

Rate of strength deterioration for different types of concrete is observed to vary with the grade of concrete and is lower for the higher grade concrete. Among all the concrete studied, after 180 cycles of freeze-thaw in seawater deterioration of compressive strength deterioration is about 26, 11, 6, 3 and 21% for M38FA0, M38FA20, M38FA30, M38FA40 and M38FA60 concrete, respectively, as compared to 28 day strength of OPC concrete; whereas the same strength is seen to decreased by around 29, 17, 13, 7 and 27% for M33FA0, M33FA20, M28FA30, M33FA40 and M33FA60 concrete, respectively, compared to the 28 days strength of no fly ash concrete. Also for higher grade concrete, relative strength gaining was observed to be higher for the similar span of curing age. In case of 360 cycles of freeze-thaw in plain water, compressive strength compared to OPC concrete of similar action, was observed to 6, 15 and 10% higher for M28FA20, M28FA30 and M28FA40 concrete, 11, 21 and 18% higher for M33FA20, M33FA30 and M33FA40 concrete, 17, 27 and 23% higher for M38FA20, M38FA30 and M38FA40 concrete, respectively. At the end of 360 cycles of freeze-thaw cycles, the overall strength gaining for M38 grade concrete is around 6 and 2% higher as compared to M33 grade and M28 grade concrete, respectively, in plain water; whereas this value is 4 and 9% higher as compared to M33 grade and M28 grade concrete in sea water, which indicates that compressive strength gaining is relatively fast for higher grade concrete as compared to lower grade concrete.

3.4. Water permeability

Permeability characteristics of M38, M33 and M28 grade concrete exposed to different environment for various freeze-thaw cycles are graphically presented in Figures 13-15. Fly ash concrete shows relatively high value of permeability coefficient compared to OPC concrete for early age of curing. But at later age of curing reverse trend was observed. Coefficient of permeability value for M28FA20, M28FA30, M28FA40 and M28FA60 concretes are 9, 2, 6 and 31% higher as compared to M28FA0 concrete, 14, 19, 22 and 46% higher for M38FA20, M38FA30, M38FA40 and M38FA60 concrete compared to M38FA0 concrete for 30 cycles of freeze-thaw in plain water. But this value for 20, 30, 40 and 60% cement replaced fly ash concrete for 180 cycles of freeze-thaw are observed, respectively, 12, 19, 21 and 9% lower for M28 grade concrete and 17, 22, 26 and 12% lower for M38 grade concrete. Fly ash has high fineness and can react with the products liberated during hydration. It forms secondary C-S-H gel that fills all the pores inside concrete specimen that makes the concrete dense and compact, as a result coefficient of permeability decreases with the increase of fly ash content up to certain level. Similar trend was also observed for freeze-thaw action in sea water. After 30 cycles of freeze-thaw in sea water, coefficient of permeability value for M33FA20, M33FA30, M33FA40 and M33FA60 concretes are 16, 25, 31 and 47% higher as compared to M33FA0 concrete of similar freeze-thaw condition; whereas the permeability value for M33 grade concrete is decreased by 15, 18, 20 and 43% for 20, 30, 40 and 60% cement replaced fly ash concrete compared to OPC concrete of similar condition in plain water.

It is seen that as the concrete exposed to sea water loses its mass due to surface erosion and splitting, the coefficient of permeability increases significantly; whereas concrete exposed to plain water show relatively low changes in their permeability. After 180 cycles, the permeability of concrete specimens exposed to sea water has been found to lie in the range of 4.76 to 14.52 x [10.sup.-12] m/s, whereas the corresponding value for the concretes placed in plain water lies in the range of 2.61 to 8.35 x [10.sup.-12] m/s. Thus, it is seen that the permeability of concrete at the end of 180 cycles of freeze-thaw exposed to sea water in the freezing-thawing environment is about two times the permeability value of the concrete placed in plain water environments. Also after 360 cycles, the permeability of concrete specimens exposed to sea water has been found to lie between 12.77 and 46.65 x [10.sup.-12] m/s, whereas the corresponding value for the concretes placed in plain water lies in the range of 4.41 to 15.98 x [10.sup.-12] m/s. Thus, it is seen that at the end of 360 cycles of freeze-thaw, the permeability of concrete exposed to sea water in the freezing-thawing environment is at about three times the permeability value of the concrete placed in plain water environments. The larger increase in the permeability of the concrete specimens in the freezing-thawing environment of sea water may be due to the formation of relatively great amounts of total expansive/leachable compounds as compared to plain water environments (Roziere et al. 2009).

Relatively low values of coefficient of permeability are found to associate with relatively high grade of concrete. Among all the concrete studied, for 180 cycles of freeze-thaw, coefficient of permeability value as compared to OPC concrete was observed as 12, 19, 21 and 9% lower for M28FA20, M28FA30 M28FA40 and M28FA60 concrete, respectively, 16, 17, 22 and 11% lower for M33FA20, M33FA30, M33FA40 and M33FA60 concrete, respectively, 17, 22, 26 and 12% lower for M38FA20, M38FA30, M38FA40 and M38FA60 concrete, respectively, for plain water condition; whereas the same value for 20, 30, 40, and 60% cement replaced fly ash concrete in sea water was observed as 65, 54, 46 and 66% higher for M28 grade concrete, 54, 49, 43 and 63% higher for M33 grade concrete and 46, 42, 34 and 53% higher for M38 grade concrete as compared to OPC concrete of similar grade in plain water. Overall observation revels that fly ash concrete has better resistance against water permeability. In case of 365 cycles of freeze-thaw in plain water, coefficient of permeability is decreased by about 13, 17, 22 and 7% for M28FA20, M28FA30, M28FA40 and M28FA60 concrete, respectively, as compared to OPC concrete; whereas the same parameter is decreased by around 15, 22, 26 and 13% for M33FA20, M33FA30, M33FA40 and M33FA60 concrete, respectively; 23, 27, 30 and 18% for M38FA20, M38FA30, M38FA40 and M38FA60 concrete, respectively, as compared to the no fly ash concrete of similar age. On the other hand, the same value in sea water curing is decreased by 28, 35, 51 and 17% for concrete M28FA20, M28FA30, M28FA40 and M28FA60, respectively; 27, 38, 43 and 14% for M33FA20, M33FA30, M33FA40 and M33FA60 concrete, respectively; 28, 34, 39 and 19% for M38FA20, M38FA30, M38FA40 and M38FA60 concrete, respectively, as compared to the no fly ash concrete. Permeability decreases very rapidly at the initial ages of curing and the rate depends on grade of concrete. The progressive decrease in permeability may be connected to the micro-voids dispersed in the mortar matrix of the concrete. As the hydration of cement progresses, crystallisation of compounds take place as a result of which the concrete micro voids keep on getting subdivided into capillary micro pores of increasingly smaller sizes. Many of the micro pores lose their connectivity with the passage of time. The reduction in pore sizes coupled with the loss of pore connectivity result in a substantial progressive decrease in the permeability. The use of fly ash in concrete decreases the required water and this combined with the production of additional cementitious compounds leads to a low porosity and discontinuous pore structure which reduces the permeability of the concrete (Estakhri and Saylak 2004). Among all the fly ash concretes studied up to 365 cycles of freeze-thaw in plain water and sea water, 30 and 40% cement replaced fly ash concrete shows better result from water permeability test point of view.

3.5. Rapid chloride penetration data

RCPT value for OPC and fly ash concrete for 30, 90, 180 and 365 cycles of freeze-thaw in plain water and sea water are graphically presented in Figures 16-18. In case of OPC concrete, amount of passing charge is observed as 3896, 5213 and 6198 coulombs for M38, M33 and M28 grade concrete; whereas the similar value for fly ash concretes of cement replacement level of 20, 30, 40 and 60% are 3224, 3025, 3216 and 3949 coulombs for M38 grade concrete, 5134, 4784, 4435 and 5467 coulombs for M33 grade concrete and 6783, 6891, 7105 and 7518 coulombs for M28 grade concrete at 30 cycles of freeze-thaw in plain water. For longer cycles of freeze-thaw, fly ash concrete show better resistance against chloride ion penetration. After 365 cycles of freeze-thaw, rapid chloride penetration values are, respectively, 16, 20, 17, 10% lower for M38FA20, M38FA30, M38FA40, M38FA60 concretes; 14, 18, 15, 12% lower for M33FA20, M33FA30, M33FA40, M33FA60 concretes and 6, 9, 12, 3% lower for M28FA20, M28FA30, M28FA40 and M28FA60 concretes, respectively, as compared to the RCPT value of OPC concrete of similar grade and under same span of freeze-thaw cycle. The incorporation of pozzolanic materials improved the resistance to chloride penetration of concrete as confirmed by other researchers (Janotka and Krajci 2000). A close observation of the data shows that fly ash concrete has relatively better resistance against chloride ion penetration and hence the use of fly ash in structural concrete may inhibits the risk rebar corrosion. Effects of sea water on RCPT values for freeze-thaw cycles are more noticeable as compared to plain water. After 180 cycles of freeze-thaw, RCPT values are 2, 8, 4% lower for M38FA20, M38FA30, M38FA40 concretes; 7, 11, 14% lower for M33FA20, M33FA30, M33FA40 concretes and 6, 9, 12% lower for M28FA20, M28FA30, M28FA40 concretes in plain water; whereas the same value is 5, 10, 11% higher for M38FA20, M38FA30, M38FA40 concretes; 7, 3, 2% higher for M33FA20, M33FA30, M33FA40 concretes and 12, 5, 3% higher for M28FA20, M28FA30, M28FA40 concretes, respectively, in sea water as compared to the RCPT values of OPC concrete of similar grade in plain water.

Relative RCPT values of fly ash concrete compared to OPC concrete is observed to vary with the grade of concrete and replacement level of fly ash with cement. After 30 cycles of freeze-thaw, RCPT values are 77, 71, 76, 93% for plain water and 91, 95, 94, 105% for sea water for M38FA20, M38FA30, M38FA40, M38FA60 concretes, respectively; 96, 90, 83, 103% for plain water and 98, 95, 92, 109% for sea water for M33FA20, M33FA30, M33FA40, M33FA60 concretes, respectively, and 108, 109, 113, 119% for plain water and 111, 112, 114, 125% for sea water for M28FA20, M28FA30, M28FA40, M28FA60 concretes, respectively, as compared to RCPT value of 28 days plain water cured of OPC concrete. Also after 365 cycles of freeze-thaw, RCPT values for 20, 30, 40, 60% replacement level concrete are 136, 129, 134, 146% for plain water and 171, 164, 150, 178% for sea water for M38 grade concrete; 130, 125, 122, 133% for plain water and 149, 132, 128, 146% for sea water for M28 grade concrete, respectively, as compared to 28 days plain water cured OPC concrete of similar grade. This is due to high fineness of fly ash. It can react with the products liberated during hydration, forming secondary C-S-H gel that fills all the pores inside concrete. As a result the pore spaces inside the concrete specimen are reduced, that makes the concrete dense and compact and makes it more impermeable (Sarkar et al. 1995). As a result flow of charge through the concrete sample is decreased. It was also observed that at the end of 365 cycles of freeze-thaw, the overall RCPT values for M38 grade concrete is around 3 and 10% lower for plain water and around 7 and 13% lower for sea water as compared to M33 and M28 grade concrete, respectively.

4. Conclusions

The results of the freeze-thaw investigation carried out on three grades of concrete exposed to sea and plain water over 360 cycles have been critically analysed and interpreted. Based on the limited number of tests and variables studied over the specific freeze-thaw cycles, the following conclusions are drawn:

(1) Concrete exposed to freeze-thaw cyclic loading in sea water is much more vulnerable to deterioration including erosion, splitting and crumbling than in plain water.

(2) The loss in weight is found up to 9.2% in sea water and the corresponding loss is around 1.6% for the specimens placed in plain water due to substantial erosion and splitting of concrete specimens in the freeze-thaw environment,

(3) Concrete also shows a significant decrease in volume as much as 1.11% in sea water under freeze-thaw environment; whereas concrete specimens exposed to plain water show a decrease in volume of around 0.31%.

(4) Sea water causes the most detrimental effect on the compressive strength of concrete, the loss being of the order of 18% in plain water and 30% in sea water after 360 cycles of freeze-thaw as compared to the strength of normal plain water cured concrete. The study reveals that 30 and 40% blending of fly ash exhibited the best results with respect to resistance against compressive strength deterioration. Such concrete shows higher compressive strength around 16-20% in plain water and 27-34% in sea water as compared to OPC concrete after 360 cycles of freeze-thaw.

(5) A significant change in permeability (k value) characteristics of concrete in freeze-thaw environment is observed particularly when it is exposed to sea water compared to plain water. Also fly ash concrete shows better resistance against water permeability. After 360 cycles of freeze-thaw, fly ash concrete with 30% and 40% cement replacement level showed around 22-26% in plain water and 38-44% in sea water lower coefficient of permeability as compared to OPC concrete.

(6) Chloride penetration resistance for fly ash concrete is observed relatively higher as compared to OPC concrete. After 360 cycles of freeze-thaw, 30 and 40% fly ash mix concrete showed better resistance of around 15% in plain water and around 20% in sea water against chloride penetration.

(7) Higher grade concrete showed better resistance against strength deterioration, lower coefficient of permeability value and lower rapid chloride penetration as compared to lower grades of concrete.

ARTICLE HISTORY

Received 29 September 2017

Accepted 13 March 2018

https://doi.org/10.1080/13287982.2018.1453332

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported by the University Grand Commission, Bangladesh.

Notes on contributors

Md. Moinul Islam is a professor in the Department of Civil Engineering, Chittagong University of Engineering & Technology, Chittagong, Bangladesh. Now he is acting as head in the Department of Civil Engineering. He received his Bachelor of Science in Civil Engineering, MSc in Civil Engineering and PhD from Chittagong University of Engineering & Technology (CUET), Bangladesh. He is an active fellow (F/10407) of Institution of Engineers (IEB), Bangladesh. His research interest cover the concrete materials, cement composite and the durability of blended cement concrete in aggressive environment. His email address is mislam@cuet.ac.bd

Mohammad Tarequl Alam is an assistant professor in the Department of Civil Engineering, Chittagong University of Engineering & Technology, Chittagong, Bangladesh. He received his Bachelor of Science in Civil Engineering from Chittagong University of Engineering & Technology (CUET), Bangladesh and now pursuing his MSc Degree from CUET. He is a member (M/23164) of Institution of Engineers (IEB), Bangladesh. His research interest is on concrete materials, blended cement and corrosion of steel. His email address is mtalamcuet@gmail.com

Md. Saiful Islam, PhD, is a professor in the Department of Civil Engineering, Chittagong University of Engineering & Technology (CUET), Chittagong, Bangladesh. Now he is acting as dean, Faculty of Architecture & Planning as well as director, Research and Extension of CUET, as additional responsibilities. He received his BSc and MSc in Civil Engineering from Bangladesh University of Engineering & Technology (BUET), Dhaka, Bangladesh and his PhD from I.I.T, Roorkee, India. He is associated with the various consultancy works related to design, repair and retrofitting of Reinforced concrete structure. He is an active fellow (F/4632) of Institution of Engineers (IEB), Bangladesh. His research interest cover the performance of concrete material, concrete composites, corrosion of steel and durability of reinforced concrete in marine and other aggressive environment. His email address is msislam@cuet.ac.bd

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Md. M. Islam, Mohammad Tarequl Alam and Md. S. Islam

Department of civil Engineering, Chittagong university of Engineering and Technology (CUET), Chittagong, Bangladesh

CONTACT Md. S. Islam ([mail]) msislam@cuet.ac.bd

Caption: Plate 1. schematic freeze-thaw cycle.

Caption: Plate 2. Concrete specimens after 360 cycles of freezing and thawing in plain water.

Caption: Plate 3. Concrete specimens after 360 cycles of freezing and thawing in seawater.

Caption: Figure 1. Weight change--freeze-thaw relation for M38 grade concrete.

Caption: Figure 2. Weight change--freeze-thaw relation for M33 grade concrete.

Caption: Figure 3. Weight change--freeze-thaw relation for M28 grade concrete.

Caption: Figure 4. Volume change--freeze-thaw relation for M38 grade concrete.

Caption: Figure 5. Volume change--freeze-thaw relation for M33 grade concrete.

Caption: Figure 6. Volume change--freeze-thaw relation for M28 grade concrete.

Caption: Figure 7. Compressive strength--freeze-thaw relation for M38 grade concrete.

Caption: Figure 8. Compressive strength--freeze-thaw relation for M33 grade concrete.

Caption: Figure 9. Compressive strength--freeze-thaw relation for M28 grade concrete.

Caption: Figure 10. Relative compressive strength--freeze-thaw relation for M38 grade concrete.

Caption: Figure 11. Relative compressive strength--freeze-thaw relation for M33 grade concrete.

Caption: Figure 12. Relative compressive strength--freeze-thaw relation for M28 grade concrete.

Caption: Figure 13. Permeability--freeze-thaw relation for M38 grade concrete.

Caption: Figure 14. Permeability--freeze-thaw relation for M33 grade concrete.

Caption: Figure 15. Permeability--freeze-thaw relation for M28 grade concrete.

Caption: Figure 16. Rapid chloride penetration--freeze-thaw relation for M38 grade concrete.

Caption: Figure 17. Rapid chloride penetration--freeze-thaw relation for M33 grade concrete.

Caption: Figure 18. Rapid chloride penetration--freeze-thaw relation for M28 grade concrete.
Table 1. Chemical composition (%) of OPC and fly ash.

Constituents               Composition         OPC     FA

Calcium oxide                  CaO            65.18    0.65
Silicon di-oxide           Si[O.sub.2]        20.80   51.49
Aluminium oxide        [Al.sub.2][O.sub.3]     5.22   31.60
Ferric oxide           [Fe.sub.2][O.sub.3]     3.15    2.80
Magnesium oxide                MgO             1.16    0.28
Sulphur tri-oxide          S[O.sub.3]          2.19    0.19
Sodium oxide               [Na.sub.2]O         --      0.18
Loss on ignition               --              1.70    4.2
Insoluble residue              --              0.6     --

--Not measured items.

Table 2. Grading and physical properties of coarse and fine
aggregate.

Properties                  Coarse aggregate        Fine aggregate

Grading of aggregates

Sieve size (mm)                       Cumulative % passing
25.0                              100                     --
12.5                              100                     --
9.5                                45                     --
4.75                               0                     100.0
2.36                               --                    94.0
1.18                               --                    78.5
0.6                                --                    55.5
0.3                                --                    13.0
0.15                               --                     2.5

Physical properties of aggregates

Specific gravity                  2.67                   2.59
Unit weight                1635 kg/[m.sup.3]       1540 kg/[m.sup.3]
Fineness modulus                  6.45                   2.57
Absorption capacity               0.8%                   1.2%

Table 3. Specified salt contents of artificial SW used in
experimental programme (Mayers, Holm, and Mc Allister
1969).

                              Chemical          Amount
Salt                           formula            gm

Sodium chloride                 NaCl             27.2
Magnesium chloride          Mg[Cl.sub.2]         3.8
Magnesium sulphate          MgS[O.sub.4]         1.7
Calcium sulphate            CaS[O.sub.4]         1.2
Potassium sulphate       [K.sub.2]S[O.sub.4]     0.9
Calcium carbonate           CaC[O.sub.3]         0.1
Magnesium bromide           Mg[Br.sub.2]         0.1
Total                                           35.00

Salt                          Remarks

Sodium chloride            These amounts
Magnesium chloride         of salts were
Magnesium sulphate       dissolved in plain
Calcium sulphate          water to prepare
Potassium sulphate         1000[degrees]
Calcium carbonate             gm of SW
Magnesium bromide
Total

Table 4. Mix proportions and properties of fresh concrete.

                                Grade of concrete
Mixture constituent
and properties                   M38    M33    M28

Cement (kg/[m.sup.3])            500    480    435
Water (kg/[m.sup.3])             218    224    218
Sand (kg/[m.sup.3])              520    530    545
Stone Chips (kg/[m.sup.3])      1120    1130   1150
Water/cement Ratio              0.44    0.47   0.50
Slump (mm)                       60      63     68
Air content (%)                  1.1    1.2    1.3

Table 5. Experimental programme for the investigation.

                                                  Total no. ** of
             Freeze-thaw        Exposure           specimen for
               cycles *         condition            strength

Mix Type         (a)               (b)            (c = a x b x 3)

M38FA0            0          Plain water and            30
M38FA20           30            sea water               30
M38FA30           90                                    30
M38FA40          180                                    30
M38FA60          360                                    30
M33FA0            0          Plain water and            30
M33A20            30            sea water               30
M33FA30           90                                    30
M33FA40          180                                    30
M33FA60          360                                    30
M28FA0            0          Plain water and            30
M28FA20           30            sea water               30
M28FA30           90                                    30
M28FA40          180                                    30
M28FA60          360                                    30
Total                                                   450

               Total no. ***      Total no. ** of
              of specimen for      specimen for
               permeability            RCPT

Mix Type      (d = a x b x 2)     (e = a x b x 3)

M38FA0              20                  30
M38FA20             20                  30
M38FA30             20                  30
M38FA40             20                  30
M38FA60             20                  30
M33FA0              20                  30
M33A20              20                  30
M33FA30             20                  30
M33FA40             20                  30
M33FA60             20                  30
M28FA0              20                  30
M28FA20             20                  30
M28FA30             20                  30
M28FA40             20                  30
M28FA60             20                  30
Total               300                 450

* 28 days precuring; ** Three samples for compressive
strength & RCPT test; *** Two samples for permeability test.

Table 6. Guidelines for chloride ion penetrability
based on charge passed (ASTM C1202 1993).

Charge passed,    Chloride ion
coulombs          penetrability

>4000             High
2000-4000         Moderate
1000-2000         Low
100-1000          Very low
<100              Negligible
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Publication:Australian Journal of Structural Engineering
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Date:Apr 1, 2018
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