The effect of antifreeze additives on fresh concrete subjected to freezing and thawing cycles

Cold Regions Science and Technology 127 (2016) 10–17

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The effect of antifreeze additives on fresh concrete subjected to freezing and thawing cycles Rıza Polat ⁎ Department of Civil Engineering, Atatürk University, 25240 Erzurum, Turkey

a r t i c l e

i n f o

Article history: Received 20 August 2015 Received in revised form 23 February 2016 Accepted 13 April 2016 Available online 16 April 2016 Keywords: Urea Calcium nitrate Cold weather concreting Compressive strength SEM Freeze–thaw

a b s t r a c t This study focused on the effect of antifreeze additives on the microstructural changes and physical and mechanical properties of fresh concrete subjected to freezing–thawing cycles produced by cold weather. For this purpose, antifreeze additives, urea and calcium nitrate, were used at the level of 6% by weight of cement dosage and were compared with control samples. After casting, one group of control samples was cured in moist curing conditions for 1 day and then cured in lime-saturated water at 23 ± 1 °C for 28 days. Another group of controls, urea and calcium nitrate mixtures, were subjected to freezing–thawing cycles 1, 3, 5, 7, 10, 15 and 28 times. Scanning electron microscopic (SEM) images, ultrasonic pulse velocity (UPV), water absorption and compressive strength tests were conducted. The results showed that the lowest water absorption value after 28 freezing–thawing cycles was 5.8% for the calcium nitrate mixes. The 28-day compressive strength of the control, calcium nitrate and urea mixes subjected to freezing–thawing 28 times was reduced by 72.0%, 27.8% and 52.9% compared to those of the control samples cured in lime-saturated water at 23 ± 1 °C for 28 days. The SEM images showed that the samples containing calcium nitrate had a more compact and denser micro-structure compared to urea and the control. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Concrete is fundamentally a composite material composed of cement, aggregate and water and gains strength from a reaction between the binder and water to form a gel that hardens and binds the aggregates together. This chemical reaction or curing process depends on favorable temperature and humidity conditions. Generally, humidity above 80% RH and temperatures between 10 (50) and 20 °C (70 °F) are best (Korhonen, 1990). A low concrete temperature reduces the rate of the strength gain and extends the setting time (Korhonen, 1990). ACI 306R-10 defines cold weather “as a period in which, as more than 3 consecutive days, the following conditions exist: 1) the average daily air temperature is less than 5°C and 2) the air temperature is not greater than 10°C for more than one-half of any 24-hr period” (ACI Committee 306R-10, 2010). The average daily air temperature is approximately 1–2 °C during the autumn and spring months for cold provinces, such as Erzurum, in Turkey. During the study, the temperature reached − 10 °C at night and reached + 10 °C during the day (T.C. Ministry of Forestry and Water Works, Meteorological Service, 2015). Cold weather concreting faces two primary problems: 1) a delay of strength gain when the concrete is placed at low temperatures, and 2) subsequent deterioration caused by cycles of freezing and thawing. These problems require special preventions during concrete production. ⁎ Corresponding author. E-mail addresses: [email protected], [email protected].

http://dx.doi.org/10.1016/j.coldregions.2016.04.008 0165-232X/© 2016 Elsevier B.V. All rights reserved.

Cement-based materials should be protected from freezing–thawing until they reach a minimum compressive strength of 3.5 MPa according to ACI 306R-10. Fresh concrete that is saturated with water should not be exposed to cycles of freezing and thawing until it reaches a compressive strength of at least 27.58 MPa (ACI Committee 306R-10, 2010). A cycle of freezing and thawing of fresh concrete can reduce the compressive strength by 20–40% (Ratinov and Rosenberg, 1994). The mechanism of internal frost damage for cement-based composites has been investigated for several decades (Powers, 1949; Beaudoin and Macinnis, 1974; Scherer, 1999; Scherer and Valenza, 2005; Coussy and Monteiro, 2008). Researchers are focused on the source of stress, the dilatation of concrete during freezing, ice lens formation and crack propagation. Freezing and thawing cycles deteriorate concrete because water freezes to ice and expands, and the volume change of water to ice causes high stress if there is no room for expansion. The ice then melts to water and increases the saturation level of the voids. The following cycles of freezing and thawing in cold climates aggravate the resistance of the concrete (Polat et al., 2010). The increase of the volume of a fluid phase at freezing results in the occurrence of some effects: the crystallizing pressure of ice upon the walls of the pores and capillaries, the hydraulic pressure of the porous liquid and the osmotic pressure caused by the freezing of water (Usherov-Marshak et al., 2002). Powers (1949) originally proposed that hydraulic pressure (as water freezes, it expands and generates pressure on the pore wall) was the source of stress that causes damage. The role of hydraulic pressure in freezing–thawing damage led to the introduction of air voids into concrete. Scherer and

R. Polat / Cold Regions Science and Technology 127 (2016) 10–17

Valenza (2005) explained that the primary source of stresses during freezing for cement-based composites was the crystallization pressure of ice and not hydraulic pressure (Scherer and Valenza, 2005; Liu et al., 2011). Scherer (1999), 2000, 2006) expressed the stress generated in a porous body, such as the cement paste at the interface between the crystal and the pore wall. The interfacial energy plays a central role in the crystallization pressure, particle pushing by crystals and heterogeneous nucleation. The crystallization pressure is the stress exerted on the pore walls by the growing crystal (Flatt, 2002; Steiger, 2005a, 2005b; Scherer, 2006; Liu et al., 2014a, 2014b; Su, 2015; Tang et al., 2015; Wang et al., 2016). There is a thin film of liquid that surrounds the crystal, so the material is always available to permit growth. The reason that growth is inhibited is that there is a disjoining force between the crystal and the pore wall that is exerted across the liquid film (Taber, 1916; Scherer, 2006). Powers (1945) stated that there is a local influence of crystallized ice on the walls of pores; in these places, there are supertensions capable of causing destruction in the weakest places of a construction. There is relatively high permeability in fresh cement pastes, so they are not very susceptible to hydraulic pressure, particularly if properly air-entrained. However, fresh cement pastes are very sensitive to crystallization pressure, which pushes the paste out of the way and, after thawing, leaves voids that are the fossils of the ice dendrites. It is the largest pores created in this way that weaken the concrete. For cold weather concreting, there are several alternative methods, such as heating the water and aggregates, the use of protective insulation, enclosing and heating the area in which the concrete is to be placed, the use of additional cement or high early strength cement and the use of chemical admixtures to accelerate concrete set and increase early age strength development (Nmai, 1998; Demirboğa et al., 2014). In cold climates, concrete construction commonly occurs during the warmer seasons; however, the construction season is short. Most of the cold weather countries use these methods and spend a large amount of money to produce concrete in the cold weather. However, these methods may be either too expensive for long-term protection or do not provide adequate protection in all cases. Cold weather concreting is usually expensive because of the need for equipment to protect the fresh concrete from frost damage. A technologically simple and economically beneficial alternative is to use antifreeze admixtures to lower the freezing point of the fresh concrete (Saaki et al., 1991). There are antifreeze admixtures, such as urea, calcium nitrate, calcium chloride, sodium nitrite, sodium chloride, potash and calcium chloride–nitrite–nitrate (Korhonen, 1990; Ramachandran, 1995; Nmai, 1998). Antifreeze admixtures enablegaining the strength of concrete in cold weather by lowering the freezing point of the liquid phase of the cement paste and by accelerating the hydration of the cement (Brook et al., 1988; Ramachandran, 1995). Lowering the freezing point of water is related to the antifreeze admixture eutectic point, which is the lowest temperature that depresses the freezing point of the antifreeze additives and is dependent on the type of admixture (Nmai, 1998). Antifreeze admixtures affect the pore structure of the cement paste, increase the surface area of the cement paste and increase the strength (Demirboğa et al., 2014). In studies related to the use of antifreeze admixtures for cold-weather concrete, Saaki et al. (1991) reported the effect of polyglycolester derivatives and calcium nitrite–nitrate on the freezing temperature of pore water, heat evolution, strength development and freeze–thaw resistance. When the antifreeze admixture dosage increased from 4 lt per 100 kg cement to 5 lt, the compressive strength at 28 days increased from 22.3 MPa to 30.1 MPa with a 0.45 water cement ratio and a curing temperature of −5 °C. The admixture used in the study lowered the freezing point of the pore water and improved the strength development. Bennett (1994) reported from U.S. Research that the compressive strengths of mixtures cured at 20 °C, − 5 °C, −10 °C and −20 °C and containing 10% urea were 91%, 88%, 42% and 0% of the 28 days compressive strength of the control mixture cured

11

at 20 °C, respectively. Barna et al. (2011) experimentally investigated eight candidate antifreeze formulations and showed that the strength gain when cured at − 4 °C is as good as conventional concrete cured at +5 °C. Dong et al. (2013) studied the workability and strength development in the standard curing of concrete using different admixtures (high efficiency water reducer agent, accelerating agent, nonchloride antifreeze agent and air entraining agent) and cast concretes at different negative temperature (−5 °C, −10 °C, −15 °C and −20 °C). They stated that admixtures can shorten the early setting time of the concrete, increase the early age strength of concrete, form the frozen critical strength of the concrete early, prevent the early freezing of the concrete, simplify the thermal curing of the concrete, shorten the heat curing time of the concrete and reduce the cost of winter construction. Korhonen (1999) investigated concretes with 3%, 6% and 9% urea at 20 °C, − 5 °C, − 10 °C and −20 °C. The compressive strength was increased by increasing the admixture rate, and the compressive strength for all of the ratios at −20 °C was zero. Karagöl et al. (2013) and Demirboğa et al. (2014) investigated the application of urea and calcium nitrate to cold weather concreting, respectively. Urea and calcium nitrate were used at the level of 6% in the mixtures. It was stated that calcium nitrate can be used in cold weather concreting without additional precautions. In the studies on urea and calcium nitrate, the frost effect at a certain time, in other words, a single freeze–thaw, was studied. However, there was no information on the effect of antifreeze admixtures for fresh concrete with different numbers of freezing–thawing cycles. The purpose of this study is to evaluate the effect of antifreeze additives on the microstructural changes and the physical and mechanical properties of 28-day fresh concretes exposed to different numbers of freezing– thawing cycles produced in the transition seasons of cold climates. 2. Materials and methods 2.1. Materials ASTM Type I normal Portland cement was used. Its physical properties and chemical composition are given in Tables 1 and 2, respectively. The chemical and physical properties of the urea (CO (NH2)2) and calcium nitrate (Ca (NO3)2) are given in Table 3. Super plasticizer (SP) was used at a dosage of 0.5% of cement, and it was constant throughout the study. All antifreeze and SP were added into the mixing water. The dosage of the cement varies between 350 and 500 kg/m3 in cold weather concreting practices (Karagol et al., 2015). The cement content and water–cement ratio were 400 kg/m3 and 0.40, respectively, and both remained constant throughout the study. Locally available cement and aggregate were used in the study. The maximum aggregate size was 16 mm. The coarse and fine aggregates were separated to four different size fractions according to TS 802-2009 (TS 802, 2009). The size fractions were 0˗2, 2–4, 4–8 and 8–16 mm, and their densities were 2.56, 2.60, 2.61 and 2.62, respectively. The full details of these concrete mixes are given in Table 4. Urea and calcium nitrate were used at the level of 6% by weight of cement dosage in the mixtures (15% by mass of mix water). The antifreeze additives used in the mixture reduced Table 1 Physical properties of Portland cement. CEM I 42.5 (ordinary Portland cement)

Results

2 days compressive strength, (N/mm2) 28 days compressive strength, (N/mm2) Initial set time, (min) Final set time (min) Volume expansion, (mm) Specific surface, (cm2/g) Specific gravity

25.7 57.9 140 186 1 3812 3.13

TS EN 197/1 standard data (min)

(max)

20.0 42.5 60

– 62.5 –

– –

10 –

12

R. Polat / Cold Regions Science and Technology 127 (2016) 10–17

Table 2 Chemical properties of Portland cement.

Heating loss (%) Insoluble matter (%) Cl− (%) SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) SO3 (%) Na2O(%) K2O (%) Free lime (%)

Results

TS EN 197/1 Standard data (max)

3.24 0.95 0.0098 18.72 4.54 3.43 62.25 3.34 2.98 0.19 0.68 0.78

5.00 5.00 0.10 – – – – – – – –

the total aggregate volume per unit volume of concrete when compared with that of the control samples. Antifreeze admixtures lower the freezing point of the liquid phase of cement paste, the air entrainment agent creates tiny air bubbles in concrete subjected to freeze–thaw cycles, and both decrease or prevent frost damage. Therefore, for a better understanding of the effects of antifreeze additives, an air entraining agent was not used in the mixtures. This study only focused on the effect of antifreeze additives on the properties of fresh concrete subjected to freezing–thawing cycles produced by cold weather. Three different mixtures were prepared (control, calcium nitrate (6%) and urea (6%)) in the laboratory to fabricate the test specimens: 24 specimens for the control, 21 specimens for calcium nitrate and 21 specimens for urea. The mix type and curing conditions are given in Table 5. The mixes were prepared and mixed in a laboratory countercurrent mixer for a total of 5 min. For each mix, specimens of 100-mm diameter and 200 mm height were prepared. The cylinders were consolidated with a needle vibrator according to ASTM C138/C138M– 14 (C138/C138M-14, 2014). After casting, one group of control concrete samples was cured for 1 day of moist curing and was then submerged in lime-saturated water at 23 ± 1 °C until the 28th day. The other samples from the control, urea and calcium nitrate mixtures were subjected to freezing–thawing for 1, 3, 5, 7, 10, 15 and 28 times. All samples were immediately subjected to freezing (− 10) and thawing (+10), except control sample. A cycle of freezing and thawing was performed with 12 h at −10 °C and 12 h at +10 °C (a total of 24 h for each cycle in automatic temperature-controlled deepfreezes). After the samples completed the freeze–thaw process (for 1, 3, 5, 7, 10 and 15), they were held at + 10 °C up to 28 days. Before the compressive strength tests, samples were kept at room temperature (23 ± 1 °C) for 24 h, and the cylinders were capped. 2.2. Methods After production, the concretes in the plastic state together with the molds were immediately transferred to the automatic temperature Table 3 Chemical and physical properties of urea (CO(NH2)2) and calcium nitrate (Ca(NO3)2·4H2O %98 pure). Urea Chemical and physical properties

Approx. values

Chloride (Cl), % Sulphate (SO4), % Heavy metal (Pb), % Ferrite (Fe), % Density, (g/cm3) pH Solubility in water, (g/l) Melting point, (°C)

– – – – 1.34 9 1000 133

Calcium nitrate

b=0.05 b=0.5 b=0.005 b=0.05 1.86 5–7 1470 45

control deepfreezes calibrated before casting at − 10 °C for 1, 3, 5, 7, 15 or 28 days. There was enough space to provide air circulation between samples. Tests were performed to assess the effect of antifreeze additives on the relationship between the number of freezing–thawing cycles and the properties of the concretes. The water absorption, UPV and compressive strength of concrete were determined by calculating the average of three samples from each curing condition and each group according to ASTM C 642–13, ASTM C 39/C39M-14a and ASTM C 597– 09 on standard cylindrical specimens (ASTM C 642, 2013; ASTM C 39/C39M, 2014; ASTM C 597, 2009). The results of the control samples and the samples containing antifreeze additives were compared for the same curing conditions. An FEI, Quanta FEG 250F scanning electron microscope was used for the microstructure analysis. SEM images and the other tests were evaluated on samples subjected to 5, 15 and 28 and 1, 3, 7, 10, 15 and 28 freeze–thaw cycles respectively, and results were compared with the control samples.

3. Results and discussion 3.1. Effect of urea and calcium nitrate on the water absorption of concrete mixes exposed to freezing–thawing Water absorption defines the amount of water absorbed under specified conditions and is used to determine the degree of porosity of a material. The water absorption results of the control mixture and concrete mixes subjected to freezing–thawing are shown in Fig. 1. The water absorption of the K, C and U samples increased after all freezing–thawing cycles. The increase in the water absorption values due to the freeze– thaw cycles for K1, K3, K7, K10, K15 and K28 compared to KK was 11.1%, 15.6%, 28.9%, 33.3%, 40.0% and 44.4%, respectively. The increase in the water absorption values for C1, C3, C7, C10, C15 and C28 and U1, U3, U7, U10, U15 and U28 compared to KK was 4.4%, 8.9%, 22.2%, 24.4%, 26.7% and 28.9% and 6.7%, 11.1%, 24.4%, 26.7%, 31.1% and 33.3%, respectively. With increasing freezing–thawing cycles, the water absorption increased, especially in the control samples, where the water absorption for K28 increased by 44.4%. The maximum value of absorption after freeze–thaw cycles observed for K28 samples was 6.5%. The addition of urea and calcium nitrate to the concrete reduced water absorption by 7.7% and 12.1% compared to K28 after 28 cycles of freezing– thawing, respectively. The lowest water absorption value, 5.8%, was observed for C28 after 28 cycles. The main cause of the increase in water absorption is that the frost susceptibility of mortar is directly related to its moisture content and fresh mortar is water-saturated; thus, it susceptible to frost (Korhonen et al., 1997). However, the frozen water amount in concretes with urea and calcium nitrate was lower, and the concrete was much less damaged due to the expansion of water. The second cause, the shift in the pore size distribution curve of the cement paste in the presence of calcium-based antifreezes toward the zone of micro capillaries and gel pores, improves the interfacial zone of contact between the cement paste and aggregates and increases the impermeability of the concrete (Ramachandran, 1995). The third cause, the absorption characteristics of cement-based materials indirectly represent the porosity, also informs about the permeable pore volume and connectivity between these pores. Antifreeze admixtures increase the strength by accelerating the hydration of the cement (Karagöl et al., 2013) and decrease the porosity, decreasing water absorption. If the mix is then thawed and allowed to continue to hydrate after this initial freezing without reconsolidation, the resulting concrete is very porous and weak (Schroederl and Woodz, 1996). The other cause is that with an increase in temperature, the soluble-water reacts with the cement, which increases the rate of saturation in the solution, increasing the effectiveness of antifreeze additives.

R. Polat / Cold Regions Science and Technology 127 (2016) 10–17

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Table 4 The mixture proportions for 1 m3 concrete. Materials

w/c ratio

Control mixtures Antifreeze mixtures

0.40 0.40

Cement (kg)

400 400

Antifreeze admixture (urea–calcium nitrate) (kg)

_ 24

160 160

Mwaiuwinga et al. (1997) studied the effect of urea on the concrete properties and found that it can also enhance both flowability (Karagöl et al., 2013) and the overall durability of the concrete. COðNH2 Þ2 þ 2H2 O→H2 CO3 þ 2NH3 H2 CO3 þ CaðOHÞ2 →CaCO3 þ 2H2 O

Water (kg)

ð1Þ

The water released as a result of the above reaction may increase the concrete slump (1). Thus, the release of water increases the amount of water in the paste phase and the water/cement ratio and the porosity increases. 3.2. Effect of urea and calcium nitrate on the compressive strength of concrete mixes exposed to freezing–thawing The relationship between the compressive strength and freeze– thaw resistance for the control, calcium nitrate and urea specimens is illustrated in Fig. 2. There is a general trend of decreasing strength at 28 days for all concretes, especially control samples exposed to the freezing–thawing cycles, which induced a drastic reduction in compressive strength. The compressive strengths of control samples changed in the range of 32.0, 27.1, 15.4, 11.1, 11.3 and 10.9 MPa at 28 days curing duration for 1, 3, 7, 10, 15 and 28 freezing–thawing cycles, respectively. K1, K3, K7, K10, K15 and K28 decreased by 17.7%, 30.3%, 60.4%, 71.5%, 70.8% and 72.0% compared to KK, respectively. Concrete is most susceptible to frost deterioration at early age for two reasons: a) its pore structure is underdeveloped and b) its moisture content is high. As expected,

SP (%)

0.5 0.5

Aggregate (kg) Quartz sand

0–2 mm

2–4 mm

4–8 mm

8–16 mm

45 44

453 441

256 249

352 344

638 622

the reduction in compressive strength of the control samples was much greater than that of the antifreeze admixture samples. Antifreezing admixtures are added to the mixing water of concrete to lower the freezing point of the aqueous solution (Ramachandran, 1995). With these additives, the cement hydrates and the concrete gains strength, even when the temperature is below freezing. The compressive strength of C1, C3, C7, C10, C15 and C28 and U1, U3, U7, U10, U15 and U28 decreased by 4.9%, 10.5%, 17.3%, 19.4%, 14.5%, 27.8% and 19.6%, 23.6%, 31.0%, 35.5%, 32.3%, 52.9% compared to the strength of the KK sample, respectively. Antifreezing admixtures lower the freezing point of the water in fresh concrete (Ratinov and Rosenberg, 1984; Brook et al., 1993). Calcium salts and carbamide (urea) increase the frost and salt resistance of concrete. In cement paste with antifreeze components, the structure forms earlier and is stronger. Because antifreeze increases the surface area of the cement, it promotes concrete strength. Additionally, antifreeze admixtures improve the pore structure of the cement paste and the zone of contact between the paste and the aggregate and prevent ice formation. At low temperature, the freezing of water is prevented by antifreeze additives, and less-defective hydrate phases are formed (Ramachandran, 1995). Fig. 2 shows that the reduction in strength due to urea was higher than that of calcium nitrate. Lowering the freezing point of water is related to the antifreeze admixture eutectic point, which is the lowest temperature that depresses the freezing point of the antifreeze additives and is dependent on the type of admixture (Korhonen, 2003). The eutectic point of urea is between −5.1 and −6.3 °C. In this study,

Table 5 Curing conditions of all specimens. Mix typea

KKa K1 K3 K5 K7 K10 K15 K28 C1 C3 C5 C7 C10 C15 C28 U1 U3 U5 U7 U10 U15 U28

−10 °C (12 h) and + 10 °C (12 h) Times of cycles (= days)

Duration after freeze–thaw cycles kept at +10 °C to complete 28 days (day)

–

–

1 3 5 7 10 15 28 1 3 5 7 10 15 28 1 3 5 7 10 15 28

27 25 23 21 18 13 0 27 25 23 21 18 13 0 27 25 23 21 18 13 0

Water cuing (23 ± 1 °C) 1 day moist curing and then lime-saturated water cured until 28th days – – – – – – – – – – – – – – – – – – – – –

Demolded (total curing duration, day)

Air curing (under laboratory conditions) (day)

–

1

28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28 28

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

a In the text and tables, K, C and U denote control concrete, calcium nitrate and urea samples, respectively; KK denotes control concrete samples cured in lime-saturated water at 23 ± 1 °C up to 28 days. For the other mixtures, the number next to the letter indicates the number of freezing and thawing cycles. Example: K1 denotes control concrete samples subjected to freezing–thawing for 1 cycle; U3 denotes concrete samples containing urea subjected to 3 cycles of freezing–thawing.

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R. Polat / Cold Regions Science and Technology 127 (2016) 10–17

Fig. 1. Effect of urea and calcium nitrate on water absorption of fresh concrete mixes subjected to freezing–thawing.

Fig. 3. Effect of urea and calcium nitrate on UPV of fresh concrete mixes subjected to freezing–thawing.

the amount of calcium nitrate is 15% of the mixing water, and the eutectic point of calcium nitrate is between − 4.6 and − 7.6 °C (Ramachandran, 1995). However, because the ambient temperature is changed by the reaction of the hydration of cement, the freezing point of the mixture cannot be precisely determined. Karagol et al. (2015) stated that the compressive strength with 9% urea was lower than that of 9% calcium nitrate after freezing and thawing in exterior winter conditions. However, the compressive strengths of K10, K15 and K28 were nearly the same, as shown in Fig 2. When fresh concrete is frozen immediately after mixing, more than 90% of the mix water turns into ice at −3 °C (27 °F). At this point, the concrete has not set, very little liquid water remains, the hydration is severely retarded and will stop once the liquid water is used up, and the overall volume of the concrete has increased. Unless the concrete is reconsolidated before setting occurs, the resulting concrete will be very porous and weak, even if it cures after the ice thaws. After 1 day of room-temperature curing, less water is available for freezing because of the increased hydration of the cement and also because some water has been confined in the pores, where it is more difficult to freeze. Eventually, the quantity of ice that can be produced is reduced to a point where freezing temperatures will not harm or cause less harm to the concrete (Mironov, 1977; Korhonen, 1990). The highest 28-day compressive strength for all freezing–thawing cycles was obtained for the samples containing calcium nitrate, and the lowest compressive strength was obtained for the control sample. The test results showed that antifreeze admixtures alter the pore structure of the cement paste, increase the surface area of the cement paste and support the increase in strength (Demirboğa et al., 2014). On the other hand, after thawing the concrete, ice melts, and the hydration process continues. This process in turn consumes water, and consequently, because the ice needs a larger volume than water, pores or voids are left in the concrete. There is one advantage of low concrete temperatures, as long as the water does not freeze, slow cement

hydration results in higher strength in the long run. The quality of the fibers produced (CSH-fibers) at slower speed is better than those formed rapidly under higher temperatures. Concrete cured at + 5 °C has at least 20% higher compressive strength after 56 days (Vollset, 2010).

The UPV test is the measurement of the electronic wave velocity through materials (Marfisi et al., 2005). This test can be used to detect internal cracks, porosity and microstructure as well as changes, such as deterioration due to freezing–thawing cycles. This method can be used to estimate the strength of concrete test specimens and in-place concrete (Domingo and Hirose, 2009; Karagöl et al., 2013). There is a relationship between the compressive strength and UPV, but these characteristics are not equal. The relationship depends particularly on the mix proportions, cement type and type of aggregate used (Neville, 1990; Trtnik et al., 2009). UPV tests were performed on the control, calcium nitrate and urea concrete specimens 28 days after the freezing–thawing cycles and curing periods. The values of UPV for the control, calcium nitrate and urea specimens are presented in Fig. 3. Each value is the average of three measurements. The UPV value of KK was 4653 m/s, and the UPV values of K1, K3, K5, K7, K10, K15 and K28 (control samples subjected to freezing–thawing cycle/cycles) were 4502, 4350, 3494, 3400, 3200 and 3397 m/s, respectively. The UPV values of C1, C3, C7, C10, C15 and C28 and U1, U3, U7, U10, U15 and U28 were 4550, 4420, 4407, 4459, 4395 and 4274 m/s and 4350, 4300, 4123, 4125, 4162 and 4062 m/s, respectively. The UPV values decreased with increasing number of freeze– thaw cycles for all concrete mixtures. K1, K3, K7, K10, K15 and K28 reduced by 3.2%, 6.5%, 24.9%, 26.9%, 31.2% and 27.0% compared to KK, respectively. Because UPV is a function of the density and ITZ (interfacial

Fig. 2. Effect of urea and calcium nitrate on compressive strength of fresh concrete mixes subjected to freezing–thawing.

Fig. 4. SEM images of KK (curing lime-saturated water at 23 ± 1 °C until 28th days).

3.3. Effect of urea and calcium nitrate on the UPV of concrete mixes exposed to freezing–thawing

R. Polat / Cold Regions Science and Technology 127 (2016) 10–17

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Fig. 5. SEM images of samples subjected to 5 freeze–thaw cycles (a) K5 (b) C5 (c) U5.

transition zone) characteristics of the concrete, it increases with decreasing total porosity of the concrete (Lin et al., 2003), i.e., the reduction of the UPV values of the mixtures is due to the increasing porosity with increasing freeze–thaw cycles. The maximum reduction, 27%, was observed for control samples after 28 freeze–thaw cycles. However, for the calcium nitrate and urea mixtures, the UPV values were relatively slightly decreased with increasing freeze–thaw cycles compared to the control mixtures. The UPV values of C1, C3, C7, C10, C15 and C28 were reduced by 2.2%, 5.0%, 5.3%, 4.2%, 5.5%, and 8.2% compared to KK, respectively. U1, U3, U7, U10, U15 and U28, similarly, decreased by 6.5%, 7.6%, 11.4%, 11.4%, 10.6% and 12.7% compared to KK, respectively. The UPV values of the concrete mixtures containing urea and calcium nitrate were higher than those of the control concrete for all freezing– thawing cycles. When the curing temperatures in the early age concrete drop below 0 °C, the final product can be severely damaged and compromised due to the freezing of the mix water. Through this investigation, it has been established that up to 6% calcium nitrate results in enhanced UPV values and concretes that are more durable. Whitehurst (1951) suggested that concrete with a density of approximately 2400 kg/m3 should be graded as excellent for 4500 m/s and above UPV, and good for 3500–4500 m/s UPV. Jones and Gatfield (1955) and Demirboğa et al. (2004), however, suggested that the lower limit for good quality concrete is between 4100 and 4700 m/s. There is an increase in UPV with the inclusion of calcium nitrate and urea compared to the K samples; therefore, the quality of the concrete, in terms of density, homogeneity and lack of imperfections, is good. Karagöl et al. (2013) found that antifreeze admixtures ensure the formation of a dense microstructure of concrete and increase the frost resistance of the concrete, the impermeability and the zone of contact between the cement paste and the aggregates. Panzera et al. (2008) studied the effect of the UPV of composites based on Portland cement and silica fume and found a significant correlation between the UPV values and porosity. The UPV measurements confirm the compressive strength results. Both the compressive strength and UPV values decreased with freeze– thaw cycles, and the reduction in compressive strength was greater than that of the UPV values. UPV can be used as a tool to investigate the hydration of cement-based materials at the early stages when the paste is in the plastic state.

3.4. Scanning electron microscope (SEM) analysis The microstructural changes in hardened, cement-based systems after freeze–thaw cycles have been studied using a variety of experimental techniques, such as SEM (Skripkiūnas et al., 2013), X-ray diffraction (XRD) (Skripkiūnas et al., 2013) and ultrasonic imaging (Molero et al., 2012). However, the microstructural changes in cement-based systems containing calcium nitrate and urea subjected to freezing– thawing cycles while in the plastic state have not been studied using SEM. Demirboğa et al. (2014) used visual determination of the relationship between the degree of hydration of cement and the compressive strength of hardened concrete containing urea placed into deepfreezes for 28 days at temperatures of −10 °C, −15 °C and −20 °C. The SEM images of the control samples cured in lime-saturated water at 23 ± 1 °C until the 28th day and samples subjected to freezing–thawing cycles are presented in Figs. 4–9. The specimens used for the SEM investigation were obtained from the compressive strength test samples. Anhydrate cement particles, calcium hydroxide crystals, CSH and capillary pores were studied. Fig. 4 shows the SEM images of the control samples cured in limesaturated water for 28 days; the figure shows the formation of profuse, proper and clear C–S–H gel in various stages and voids in the structure of the cement paste. The KK concretes were characterized by a dense microstructure. Aggarwal and Siddique (2014) stated that calcium– silica–hydrate (C–S–H) is the basic phase. The distribution, size, shape, concentration and orientation of the particles, the topology of the mixture, the composition of the dispersed/continuous phases and the pore structure influence the mechanical behavior of the C–S–H phases. Additionally, the experimental compressive strength test results confirmed the SEM observations. Fig. 5 (a,b,c) shows the SEM images of the K, C and U samples that were subjected to 5 freeze–thaw cycles. As shown in Fig. 5a, short columnar crystals can be observed in the pores. As shown in Fig. 5b and c, needle-like crystals can be seen in the pores. Moreover, in Fig. 5a, voids in the structure of K1 are observed to increase according to KK after 5 freeze–thaw cycles. Dry or hardened concrete will withstand freezing–thawing, whereas highly saturated fresh concrete may be severely damaged by a few cycles of freezing and thawing. The higher the w/c ratio of the fresh concrete, the greater the volume fraction of

Fig. 6. SEM images of samples subjected to 15 freeze–thaw cycles (a) K15 (b) C15 (c) U15.

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Fig. 7. SEM images of samples subjected to 28 freeze–thaw cycles (a) K28 (b) C28 (c) U28 (for mag: 30,000).

the larger pores. When large pores are saturated with water, a large amount of water can freeze during cooling. Figs. 6–8 show the SEM images of the K, C and U samples that were subjected to 15 and 28 freeze–thaw cycles. The SEM images related to the microstructure show that the deterioration rate of the concrete increases as the number of freeze–thaw cycles increases. After the introduction of the above antifreeze components, Ramachandran (1995) found that the frost resistance of concrete under alternating freezing in water and thawing increases on an average by 1.5–2 times. Figs. 5b–8b and 5c–8c show when urea and calcium nitrate are added, the concrete internal structure is improved. In the interface and microcracks, a large amount of needle-like ettringite crystals and CSH gel (Figs. 5b and c, 6b and c, 7b and c, 8b and c) were formed and filled. Therefore, the density of the concrete with calcium nitrate and urea was higher than that of the concrete without the admixture. Figs. 7–8 indicate that the hydration of the urea and calcium nitrate concretes continues even during the process of freeze–thaw cycles. Consequently, the specimen would have good compressive strength and UPV. Figs. 7 and 8 indicate that the control samples have more porous internal structures. Figs. 5–8 (a–b–c) show that samples containing calcium nitrate have a more compact and dense microstructure compared to the urea and control samples (subjected to freeze–thaw cycles). Ramachandran (1995) explained that the silicate phases of cement are chemically unreactive to the strong electrolytes used as antifreeze admixtures. Electrolytes in contact with the silicate phases modify the ionic strength of the solution, whereas the hydrolyzing salts modify the ionic strength of the solution and change the pH. When the admixtures do not contain the same ions as in the cement phases (i.e., Ca, Si or Al ions), they accelerate the hydration process principally by increasing the solubility of C3S and β-C2S. The admixtures, such as calcium chloride, calcium nitrite and calcium nitrate, that contain the same cations as alite and belite accelerate hydration by the nucleating action of such ions, which results in an intensification of the processes of crystallization of the hydrate. Karagöl et al. (2013) found that antifreeze admixtures ensure the formation of a dense structure in the microstructure of concrete and the pore solution. For samples (KK) unexposed to freeze–thaw cycles and with calcium nitrate and urea, calcium silicate hydrate plates and calcium hydroxide crystals were clearly identifiable. In Figs. 5–8 (a–b–c), ettringite needles are observed in the fresh concrete samples subjected to freeze– thaw actions, but the needles are shortened compared to the ettringite not subjected to freeze–thaw actions. Such shortening of the ettringite is responsible for the strength and microstructural deterioration.

Fig. 9 (a-b-c) shows that the defects are mostly seen as cracks. However, in most cases, the degradation by freezing and thawing is characterized by the gradual formation of microcracks in the concrete. If freezing is slow, water may redistribute itself throughout the mix by moving toward the colder areas before freezing and forming ice lenses. This process is similar to the way water moves in freezing soils. Voids left by the thawing of the ice lenses weaken the concrete. If freezing is rapid, water has little chance to move toward the colder areas. The water freezes in place, creating a nearly uniform distribution of ice crystals. However, these crystals can still disrupt the immature cement paste and weaken the bonds between the cement paste and the aggregate (Suprenant, 1992). The number of cracks seen in the frostdeteriorated control concretes was much higher than in the calcium nitrate and urea concretes. At 50 μm, several large cracks were seen in K28. At 50 μm for C28 and U28 (the effect of antifreeze admixtures), with continuous hydration, the capillary pores were filled with hydration products, and the density improved continuously. 4. Conclusions The effects of freeze–thaw cycles on fresh concrete were investigated. Based on the experimental work performed in this study and the discussion of the experimental results, the conclusions can be summarized as follows: 1- The addition of urea and calcium nitrate to concrete reduced water absorption by 7.7% and 12.1% compared to K28. 2- The mixtures containing urea and calcium nitrate demonstrated decreased water absorption for all freezing and thawing cycles. However, whereas the reductions for C1 and U1 compared to K1 were 6% and 4% for 1 freezing–thawing cycle, the reductions for C28 and U28 compared to K28 were 10.8% and 7.7% for 28 cycles of freezing–thawing. When the number of freeze–thaw cycles increased, the effect of the antifreeze agents on water absorption increased. 3- The highest compressive strength and UPV values were obtained for the calcium nitrate sample, and the lowest values were obtained for the control sample at 28 days for all freezing–thawing cycles. 4- The reduction in the compressive strength due to urea was higher than that of calcium nitrate. 5- The SEM results corroborate the compressive and UPV test results.

Fig. 8. SEM images of samples subjected to 28 freeze–thaw cycles (a) K28 (b) C28 (c) U28 (for mag: 10,000).

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Fig. 9. SEM images of samples subjected to 28 freeze–thaw cycles (a) K28 (b) C28 (c) U28 (for mag: 2500).

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