Advances in studies on concrete durability and countermeasures against freezing-thawing effects

WuJian Yan ,FuJun Niu ,XianJun Zhang ,Jing Luo ,GuoAn Yin

1.State Key Laboratory of Frozen Soil Engineering,Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences,Lanzhou,Gansu 730000,China

2.Key Laboratory of Loess Earthquake Engineering,China Earthquake Administration,Lanzhou,Gansu 730000,China

3.Engineering Management Center of China Railway Corporation,Beijing 100038,China

1 Introduction

Concrete building materials play an increasingly important role in the development of the national economy,being used more and more in construction engineering,bridge engineering,and water conservancy engineering because of its advantages of convenience,good integrity,and high compressive strength.However,the durability of concrete is not high,which can cause serious damage to concrete structures and bring huge economic losses.In recent decades,concrete durability research has become a hot topic in civil engineering studies.The durability of concrete in a water-saturated state and subjected to freezing-thawing cycles can be seriously undermined,so freezing-thawing damage has become a major challenge in concrete construction in cold regions.Many buildings,especially hydraulic structures in northeastern and northern China,have incurred freeze-thaw damage,so improving the frost resistance and durability of concrete needs more attention.

2 Mechanisms of freezing-thawing effects on concrete durability

Many scholars have done much research and advanced many theories on the mechanisms of freezingthawing effects on concrete durability,such as the hydrostatic pressure theory of Powers (1945a),the osmotic pressure theory of Powers and Helmuth (1953),the critical water saturation degree theory of Fagerlund(1977),the dual mechanism theory of Cady and Weyers(1983),and the micro-ice-crystal lens model theory of Setzer (2001).Of these,the hydrostatic pressure and osmotic pressure theories are the most widely applied.

2.1 Hydrostatic pressure theory

Powers proposed the hydrostatic pressure theory in 1945 (Powers,1945b).He posited that the volume expansion of concrete results from the partial freezing of the pore solution during the freezing process,which induces the pore solution to remain somewhat liquid,not icy,and to migrate outward from the freezing zone.As it migrates,the pore solution has a certain hydrostatic pressure from overcoming the cement slurry viscosity resistance.With the increase of pore solution flow length,the amount of the hydrostatic pressure increases.Thus,when the flow length of the pore solution is greater than a certain limit length,the hydrostatic pressure produced by pore solution migration will be greater than the tensile strength of the concrete,which causes damage to the concrete.

By the hydrostatic pressure theory,the stomatal interval distance and the critical water saturation are important quantitative indexes of the frost resistance of concrete (Figure 1).Powers (1945b) determined the frost resistance of concrete during the freezing-thawing cycle by measuring the air bubble content and specific surface area under the microscope.He then proposed a calculation method of the coefficient of the average stomatal interval distance (an important parameter in the study of frost resistance of concrete).According to Powers’ recommendations,the American Concrete Institute determined that the coefficient of the average stomatal interval distance is 250 μm.Although some scholars (Mielenzet al.,1958;Yanet al.,2001) questioned whether the limit value is reasonable and whether all kinds of cement concrete can use the same limit value,there has been general acceptance that the coefficient of the average stomatal interval distance is an important parameter of the frost resistance of concrete.

Figure 1 Hydrostatic pressure theory

Subsequently,Powers (1949) quantitatively discussed the relationship between the frost resistance of concrete and the stomatal interval distance,and proposed a definition and measurement methods of the coefficient of the average stomatal interval distance on that basis.Pigeon (1985) considered the coefficient of the average stomatal interval distance to be the most meaningful parameter of the frost resistance of concrete because it is convenient and has a good practical effect,which makes it the most commonly applied basis for the design of frost resistance of concrete to date.

2.2 Osmotic pressure theory

Powers and Helmuth (1953) found the hydrostatic pressure hypothesis has certain rationality when the porosity of the cement paste is high and it is in the fully water-saturated state,but this hypothesis does not explain the continuous expansion of non-air-entrained slurry at a constant temperature,nor does it explain the shrinkage phenomenon of air-entrained slurry during the freezing process.Therefore,based on the hydrostatic pressure hypothesis,they proposed the osmotic pressure hypothesis,which states that,due to the formation of ice crystals,the concentration of unfrozen pore solution is greater than that of other small-pore,non-freezing pore solution,so that a concentration difference forms.The pore solution migrates from small pores to larger pores,and osmotic pressure generates the concentration difference.Migration of pore solution eventually leads to increased volumes of ice and pore solution,and increased osmotic pressure.The osmotic pressure leads to internal cracks in the cement paste,which results in damaged concrete.

The difference between the osmotic pressure hypothesis and the hydrostatic pressure hypothesis is the direction of migration of the unfrozen pore solution.The hydrostatic hypothesis considers that pore solution of ice crystals will migrate from the large pores to small pores,whereas the osmotic pressure hypothesis considers that the pore solution migrates from small pores to ice crystals.Tang (1984) expressed doubts about the osmotic pressure,and Li (1989),by theoretical calculations and experimental results,concluded that hydrostatic pressure is the main factor of frost damage of concrete.

2.3 Other main theories

The osmotic pressure hypothesis and the hydrostatic pressure hypothesis posit that the frost damage of concrete depends mainly on the cement paste containing water,and the water content determines the scope of the failure stress.Therefore,Fagerlund (1977)proposed the critical water saturation degree theory(Figure 2).He pointed out that the critical saturation of concrete is 80%,but in later studies he found the critical saturation of concrete that has low permeability and high strength may be lower.Setzer (2001)found that internal damage can occur only when the concrete reaches a certain degree of saturation,and that the shrinkage and melting of concrete during the freezing process results in the saturation effect.The shrinkage during the freezing process makes the pore solution migrate irreversibly to the micro-ice lens,ice into water.The swelling process almost stops during the melting process,and external water will enter the internal concrete faster.In effect,the freezing-thawing cycle seems to cause a "pump" of submicroscopic structure and leads to a saturation effect that is stronger than the capillary saturation under isothermal conditions.When the critical degree of saturation is reached,freezing-thawing damage to concrete will occur rapidly.

Figure 2 Critical water saturation degree theory (image courtesy of Penttala,2006)

Under Cady and Weyers’ (1983) dual mechanism theory,concrete cools due to the increased adsorption water volume,which enhances the unfrozen water hydrostatic pressure.Therefore,the adsorption mechanism strengthens the hydrostatic pressure,causing damage to concrete.The China Institute of Water Resources and Hydropower Research (1990) proposed the ice layer separation hypothesis and the water-filling coefficient hypothesis in the 1990s.Mihtaet al.(1992) proposed the temperature stress hypothesis,which considers that freezing-thawing damage to high-strength or high-performance concrete is mainly caused by the large difference of thermal expansion coefficients between aggregates and cementitious materials,and there is a large difference between the deformation of those two materials during temperature changes,so temperature stress fatigue destruction occurs.Pigeon and Lachance (1981),Pigeon (1985)determined the relationship between the stomatal interval coefficientLcrand different cooling rates through experimental studies on the slow freezing method.Chatterji (1999a,b) studied the properties of water and ice in pore structures at low temperatures.Li and Xu (1999) explored the damage mechanism of concrete during the freezing and thawing process by the rapid freezing method,and concluded that if the freezing temperature is lower and freezing rate is faster,the damage to concrete will be more serious,and that the freezing temperature of-10 °C is a critical value.According to Sidebottom and Litvan (1971)and Litvan (1972a,1972b,1973),water in the capillary pores cannot freeze in situ;instead,the freezing takes place in the vicinity of the outer surface of the structure.The supercooled water causes water movements and drying in the porous body,which can cause freezing and thawing deterioration.Penttala (1998,2000),Penttala and Al-Neshawy (2002) set forth a theory based on thermodynamics by which the pressures in pore water can be derived from temperature and relative humidity data measured in concrete during the freezing and thawing cycle.If the amount of pore water is known,the pressures in the concrete structures can be calculated.Based on thermodynamics,the main reason for the pore water movement toward the ice bodies that are first induced is the difference in chemical potential between the ice and the unfrozen pore water in the smaller pores surrounding the ice,and the osmotic pressures play only a minor role.In this theory,the pore size distribution data are not needed.

The freezing-thawing effect on the mechanisms of concrete damage is very complicated.It can be related to hydrostatic pressure,osmotic pressure,the discontinuity of moisture migration during the freezing and melting process,the critical saturation of the internal concrete,or the growth of micro-ice-crystal lenses;it can also be due to interactions among all of these.In a word,if the interval distance of porosity in the concrete cement paste is small enough and in a dry state,concrete will resist freezing.But if it is in a saturated state,there will also be large hydrostatic pressure even smaller than the critical size which can cause cracks in the cement paste.

3 Influence of freezing and thawing effect on concrete durability

3.1 Surface damage and mass attenuation

Most of the freezing-thawing damage to concrete structures occurs in the northeastern,northern,and northwestern regions of China,especially in the northeast where almost 100% of the concrete structures have been freeze-thaw damaged (Zhao and Wei,2003).The internal gaps in concrete gradually increase,cracks also increase,and the concrete is looser after freezing-thawing cycles,it causes intensity reduce and structure damage (GB/T50082-2009,2009),which got a large number of experiments confirmed.Sellecket al.(1998) considered that freezing-thawing destruction occurs uniformly in concrete;this damage generally starts with small microcracks which can form macrocracks and then more severe damage.Tikalskyet al.(2004) investigated the freeze-thaw durability of cellular concrete and developed a modified freeze-thaw test procedure based on ASTM C666.Mulheron and O’Mahony (1988)reported that the durability of lean concrete,made with recycled aggregates,appeared to be better than or similar to an equivalent control concrete made with natural gravel when subjected to freezing and thawing conditions.Sunet al.(1999a) and Miaoet al.(2002) reported that the deterioration of concrete could be accelerated when subjected to dual-damaging processes,such as being simultaneously subjected to both external loading and freeze-thaw cycles.Jacobsenet al.(1996) investigated the effect of internal cracking on ice formation in high-strength concrete.Marzouk and Jiang (1994) investigated the tension properties of high-strength concrete after freezing and thawing cycles.Buck (1977) found that freezing and thawing resistance of concrete containing recycled concrete,where chert gravel was the original aggregate,increased in freezing and thawing tests.Penttala (2006) considered surface scaling and internal freeze-thaw deterioration by models using test results of 45 concrete samples frozen in a non-saline environment,as well as 12 high-strength concrete samples when saline freezing liquid was used.Statistically,the most important variables were the water-cement ratio,air content,and curing time.Internal crack growth in the concrete paste resulted in crumbling and eventual total deterioration of the concrete.

Shang (2006) studied the surface deterioration of specimens that underwent 200 and 400 freeze-thaw cycles (Figure 3),and provided the relative dynamic modulus of elasticity (RDME) and weight loss of air-entrained concrete in accelerated freeze-thaw cycling tests (Table 1).

Figure 3 Surface of air-entrained concrete after 200 and 400 freeze-thaw cycles (image courtesy of Shang and Yin,2006)

Table 1 The RDME and weight loss of air-entrained concrete after different cycles of freeze-thaw(adapted with permission from Shang,2006)

The mass loss rate of concrete is a reflection of the peeling condition of concrete in the failure process.Therefore,the freezing-thawing cycle effect on the mass loss of concrete is an important factor affecting the concrete durability.

Cheng (2012) did fast freezing-thawing test research on concrete with different water-binder ratios,and determined that the water-cement ratio had a significant influence on concrete quality loss in freezing-thawing cycles:the smaller the ratio,the less loss of concrete quality there will be.Also,the concrete mass loss with different water-binder ratios increased with the number of freeze-thaw cycles;the higher the water-cement ratio,the higher the mass loss rate of the concrete specimen.Xiao (2010) researched changes in concrete quality due to the interaction of freezing-thawing cycles and the carbonization function,and found greater concrete mass loss with higher water-binder ratios.

3.2 Change in internal mechanical properties

The mechanical properties of concrete in freezing and thawing circumstances,such as compressive strength,shear strength,splitting tensile strength,elasticity modulus,Poisson’s ratio,and shear modulus,have very important influences on concrete structures.In recent years many scholars have studied concrete mechanical properties after freezing-thawing cycles to establish a foundation for concrete frost-resistance research.

Shi (1997) studied the influence of freezing-thawing cycles on the mechanical properties of concrete (compressive strength,shear strength,splitting tensile strength,elasticity modulus,Poisson’s ratio,and shear modulus),and analyzed the mechanical properties of high-strength concrete and ordinary concrete.He concluded that the more freezing-thawing cycles there were,the greater was the damage to concrete.After 90 freezing-thawing cycles all of the mechanical properties (except shear modulus) of the high-strength concrete specimens declined by 10%;in the ordinary concrete specimens,the declines were more than 10% and the shear modulus loss was more than 20%.Microscopic examination revealed the relationship between changes in the microstructure of concrete and freezing-thawing cycles:the more freezing-thawing cycles there were,the more and larger were cracks in the cement slurry and the aggregate-cement paste interface.Cracks in the ordinary concrete cement paste and the aggregate-and-cement mortar contact surface were wider than in the high-strength concrete,so the mechanical damage to ordinary concrete was more serious than that to high-strength concrete.

Shanget al.(2005),Shang (2006),Shang and Yin(2006) did experimental research on ordinary concrete cubes after freezing-thawing cycles and found that ordinary concrete uniaxial compressive strength and tensile strength decreased significantly after freezing and thawing.Qin (2003) and Qin and Song (2004a,2004b,2005a,2005b) studied the mechanical properties of uniaxial and multiaxial stress after different numbers of freezing-thawing cycles.They analyzed the strength and deformation tests of ordinary concrete after freeze-thaw cycles in uniaxial stress and biaxial compressive performance,and concluded that concrete compressive strain increases significantly with the number of freezing-thawing cycles.The maximum compressive strain value was achieved when the stress ratio was 0.25.This suggests that,in practical engineering,the frost resistance of concrete structures under biaxial compression will be higher than under single-axis stress.

Zhang (2006) analyzed the relationship of stress and strain in ordinary and air-entrained concrete under uniaxial and biaxial stress after freezing-thawing cycles,and established a constitutive nonlinear elastic model of uniaxial and biaxial-equivalent uniaxial strain.Duan (2009) studied compressive stress-strain curves and derived a concrete stress-strain curve equation in freezing-thawing circumstances.Qin(2003) investigated the strength and deformation characteristics of plain concrete under uniaxial and multiaxial compression after different numbers of freeze-thaw cycles,and analyzed the influence of the stress ratio and number of freeze-thaw cycles on the strength and strain.

The fracture properties of polymer concrete are improved by adding reinforcements such as glass or carbon fibers.Tested under normal conditions in the laboratory,the use of glass-fiber reinforcement improves the fracture parameters such as the stress intensity factor by 10%,and the use of carbon reinforcement under the same conditions improves by 24%.These reinforcements improve the fracture toughness by 40% and 225%,respectively (Reis and Ferreira,2002,2003).

In summary,the durability and compressive strength of concrete decreases significantly with more freezing-thawing cycles.

3.3 Influence of coupling effect of freezing-thawing and other factors on concrete durability

3.3.1 Coupling effect of freezing-thawing and salt solution

The coupling effect of freezing-thawing and salt solution on concrete durability has three aspects:First,the salt solution in pore water of concrete will cause internal osmotic pressure generated during the freezing process,thus aggravating the damage.Second,the cooled water produced by salt in an unstable state increases damage when the pores freeze.And finally,because the salt forms a concentration gradient on the concrete surface,ice stratifications appear during freezing,and stress due to the different expansions causes spalling of concrete (Xing,2011).

Many domestic and foreign scholars have done much research on the coupling effect of freezing-thawing and other factors.Maclnnis and Whiting(1979) and Marchandet al.(1999) found that freezing-thawing damage is the most serious when the concentration of NaCl solution is around 3%.Chatterji (1984) studied icing salt solution and the effects of freezing and thawing of concrete,and pointed out that,after freezing,the concrete strength with salt solution is less than that with water,and if the salt concentration is higher that strength will be less.Mu and Miu (2001) studied the frost resistance of concrete in a 3.5% sodium sulfate solution and a 5% sodium chloride solution.Their results showed that different salt solutions have different effects on the frost resistance of concrete.After freezing-thawing,concrete in the 3.5% sodium chloride solution hasn’t phenomenon of post injury aggravation just as the surface spalling concrete seriously.Chenet al.(2009) measured the diffusion depth of chloride ions in different freezing-thawing conditions and established a chloride ion diffusion model based on freezing-thawing condition.Zhang and Yu(2011) studied the effect of a 5% MgSO4solution on freezing-thawing concrete and found that the MgSO4solution can ease the freezing-thawing damage in concrete with a high water-cement ratio and will accelerate freezing-thawing damage of concrete with a high water-cement ratio.

3.3.2 Coupling effect of freezing-thawing and load

In practical engineering,the coupling effect of freezing-thawing and load will accelerate the damage in concrete structures subjected to different types and sizes of load.Especially,pre-stressed concrete often exhibits sudden brittle failure under the effect of freezing-thawing cycles (Narayanan and Brooker,2005).The destruction is greater when the stress is higher;the relative dynamic elastic modulus decreases more quickly when there are more freeze-thaw cycles.Under the freezing-thawing effect,load and stress accelerate the damage and deterioration of concrete.

Yu and Sun (2004) studied the service life of concrete under the interaction of bending load,chloride environment,and freezing-thawing factors.They found that load superposition intensifies multiple damages to concrete and shortens the service life of concrete.Sunet al.(1999a,b) conducted a series of concrete durability tests of the interaction of bending load and freeze-thaw cycles,and found that the number of freezing-thawing cycles that concrete can endure decreases with the increase of stress.Rapid expansion of load-induced microcracks causes concrete damage when the stress is high,whereas the pore water pressure in the freezing-thawing cycle causes the formation of microcracks in concrete when the stress is low.Yanet al.(2003) did reliability analysis of concrete and produced a reliability formula based on load and number of freeze-thaw cycles.Zou and Zhao(2008) and Hanet al.(2012) confirmed that the frost resistance of concrete has a direct relationship with the applied loads.Mu (2000) did damage research on the interaction of freezing-thawing and stress,and found that the bending stress lessens the strength of concrete in freezing-thawing cycles,and the damage is greater when the stress ratio is higher.However,the bending stress has almost no effect on the weight of injury during freeze-thaw cycles.

4 Countermeasures against freeze-thaw damage to concrete

4.1 Controlling the water-cement ratio of concrete

The water-cement ratio is an important parameter of concrete mixture ratio design.Its changes more directly affect the porosity and pore structure of concrete than influence the frost resistance of concrete.Normally,when the concrete water-cement ratio is greater,the porosity is higher so that more water could fill the pores and the frost resistance of the concrete is lower.Conversely,when the concrete water-cement ratio is smaller,the porosity is lower so that the concrete is denser and its frost resistance is higher.Zhang (2001) analyzed the influence of the water-cement ratio (with and without an air-entraining agent) on the frost resistance and durability of concrete,and found that the anti-freeze strength of concrete decreases as the water-cement ratio increases,regardless of whether an air-entraining agent is present.The total volume of the pores and apertures increases with increases of the water-cement ratio,so the concrete anti-freeze durability will inevitably decrease.Mu (2000),after damage testing of C40,C60,and C80 concrete subjected to freeze-thaw cycles,found that the frost resistance of concrete increases as the water-cement ratio decreases.

4.2 Adding an air-entraining agent to the concrete

Micropores in concrete are very important to improve the concrete frost resistance;they can prevent or inhibit the tiny ice-slurry bodies generated in the concrete freezing process (Zhanget al.,2008).Many uniform,stable,and closed microbubbles will be introduced after mixing an air-entraining agent into the concrete.These micropores can not only reduce the hydrostatic pressure in the frozen concrete,but can also improve the fluidity of the concrete and indirectly improve its frost resistance.Wang and Qian (2004)studied the effect of gas content on the performance,frost resistance,and impermeability of concrete.Their results showed that an increase of gas content in concrete can greatly increase its frost resistance but its compressive strength will decrease.Fang (2003)found that the freeze-thaw damage processes of ordinary concrete and concrete mixed with an air-entraining agent are very different.The elastic modulus of concrete mixed with an air-entraining agent first decreased rapidly and then slowly,and peeling damage from the outside to the inside slowly occurred.The elastic modulus continued to decline and the overall damages rapidly escalated.Fan (1991,1993) and other scholars (Song,1999;Tan,2006)studied whether an air-entraining agent improved the frost resistance of concrete and reached the same conclusion:ordinary concrete can obtain good frost resistance durability when it is mixed with an air-entraining agent.

Cheng and Zhang (1999) studied the frost resistance of air-entrained fly-ash concrete and concluded that the air content of concrete is the most decisive factor affecting frost resistance of ordinary and fly-ash concrete.Whether an air-entraining agent can improve the frost resistance of concrete depends on the corresponding grade of concrete:the higher the concrete strength,the less gas content is required to improve the frost resistance.For ordinary concrete and fly-ash concrete,the minimum air content required to improve frost resistance is shown in table 2.Gokceet al.(2004) introduced some information about freezing and thawing resistance when air-entrained or non-air-entrained concrete was used as recycled coarse aggregate into air-entrained concrete.

Table 2 Minimum air content required to improve the frost resistance of concrete (Durability coefficient = 90)

4.3 Mineral admixtures in concrete

Some mineral admixtures,such as fly ash and silica fume,can improve the pore structure of the concrete and the thin aperture.The advantage of these mineral admixtures is that they bubble (distribute more evenly) in the concrete in order to improve its frost resistance.Different mineral admixtures have different effects on the frost resistance of concrete.Pan (2003),Xu and Xie (2006) studied the damage effects of different dosages of fly ash,different freeze-thaw methods,and different concrete materials in freezing-thawing circulation.Cheng and Yan (2008)did a study on concrete frost resistance of fly ash replacing cement at 30%,40%,50%,and 60%,and determined that the frost resistance of concrete decreased as the amount of fly ash increased.When the fly-ash content was large it could either increase or not influence the frost resistance of concrete.Ding and Sun (1991) found the anti-freezing capacity of non-air-entrained concrete was high in the same total cementitious materials and slumping constant conditions.In addition,the filling effect of silica fume decreases the internal aperture of concrete and the water molecule channel thins so that water transfer is difficult at negative temperatures;thus,the hydrostatic pressure is small (Litvan,1974).Fan (1990) also demonstrated that silica fume can reduce the frost resistance of concrete.Cohenet al.(1992) believed that the frost resistance effect of only non-air-entrained concrete mixed with silica fume was obvious,but silica fume did not improve the frost resistance of air-entrained concrete.Wanget al.(2005)found that the frost resistance of lightweight aggregate concrete was enhanced as the silica fume quantity increased,but only to a certain value;the frost resistance decreased when the silica fume quantity exceeded this value.Thus,the increase of silica fume quantity does not always improve the frost resistance of lightweight aggregate concrete.

Pigeonet al.(1996) found that the spalling resistance of concrete can be improved when the silica fume quantity is less than 10%.Douglaset al.(1992)studied the performance of high-GGBS concrete(50%-75%) and found that a large amount of GGBS has very high anti-freeze performance.Several scholars (e.g.,ACI Committee 226,1987;Fan,1989;Gangneet al.,1996) have discussed effect of silica fume on the frost resistance of concrete and demonstrated that the pore structure of concrete can be improved and the pore size and spacing coefficient becomes small after the incorporation of silica fume.Thus,the hydration of pores in concrete can be reduced so that the frost resistance is obviously improved.However,the silica fume content should not exceed 15% because the frost resistance of concrete decreases if the silica fume content is too high.

4.4 Mixing fibers into the concrete

Mixing fibers into cement is an effective way to increase the strength and improve the crack resistance of concrete.Fiber-reinforced concrete is widely applied in engineering,and the fibers most often used are steel,polypropylene,carbon,ethylene glass,and plant fibers.

Gao and Zhu (2005),Jiang and Wang (2006),Xie and Gao (2006),Yu and Sun(2006) studied the frost resistance of concrete mixed with steel fibers.Their research showed that the freeze-thaw resistance of concrete mixed with steel fibers significantly increased and further increased when an air-entraining agent was added to the concrete.After freeze-thaw cycles,the compressive strength of concrete mixed with steel fibers was little changed,but the concrete splitting tensile strength and flexural strength improvement was obvious.However,when the number of freeze-thaw cycles was high and the steel fiber volume ratio (ρ＝2%) was large,the effect of steel-fiber reinforcement was poor.

Yao (2005) studied the effect of polypropylene fiber on trends of mechanical properties of concrete in low-temperature environments (temperatures of-15 °C to 5 °C) and discussed the frost-resistance durability of polypropylene-fiber concrete under fast freeze-thaw test conditions (temperatures of-17.5 °C to 7.5 °C).His results showed that polypropylene fiber increased the entrained air content and improved the microstructure and the crack resistance of concrete,and was thus advantageous to strength growth and improved durability of concrete under low temperatures.

Parviz and Mohamad (1992) assessed the frost resistance of concrete mixed with carbon fiber.They found that mixing an appropriate amount of carbon fiber is beneficial to the frost resistance of concrete,but the addition of excessive carbon fiber will weaken the frost resistance.Xu and Huang (2005) studied the effects of different volume fractions of carbon fiber,forming pressures,and curing methods on freezing-thawing properties of concrete.They made carbon-fiber-reinforced composite materials by the pressure molding method;ordinary silicate cement was the base material and carbon fiber was the auxiliary component.Their results showed that there was good frost resistance when the carbon-fiber content was 0.2% and the strength (10 MPa) after the concrete pressure molding was high before freezing-thawing,but the strength decreased greatly later in the freezing-thawing period.

5 Conclusions

We have drawn the following conclusions from the basic research results of many scholars in China and abroad:

1) The main theories put forth regarding the mechanisms of freezing-thawing effects on concrete durability are the hydrostatic pressure theory,the osmotic pressure theory,the critical water saturation degree theory,the dual mechanism theory,and the micro-ice-crystal lens model theory.

2) The concrete mass loss rate and the water-binder ratio have positive correlations in the single freezing-thawing effect.The water-cement ratio has a significant influence on concrete quality loss in freezing-thawing cycle:a smaller ratio will lead to less concrete quality loss.The concrete mass loss increases with an increasing number of freeze-thaw cycles,and the mass loss rate of concrete specimens is higher with higher water-cement ratios.

3) The coupling effect of freezing-thawing and salt solution influences concrete durability in three aspects:First,the salt solution in pore water of concrete will cause the internal osmotic pressure generated during the freezing process,thus aggravating the damage.Second,the cooled water produced by salt in an unstable state increases damage when the pores freeze.And finally,because the salt forms a concentration gradient on the concrete surface,ice stratifications appear during freezing,and stress due to the different expansions cause spalling of concrete.In practical engineering,the coupling effect of freezing-thawing and load will accelerate the damage in concrete structures subjected to different types and sizes of load.Especially,prestressed concrete often exhibits sudden brittle failure under the effect of freezing-thawing cycles.The destruction is greater when the stress is higher;the relative dynamic elastic modulus decreases more quickly when there are more freeze-thaw cycles.Under the freezing-thawing effect,load and stress accelerate the damage and deterioration of concrete.

4) The most efficient measures to improve the durability of concrete in freezing-thawing environments are controlling the water-cement ratio of the concrete,mixing an air-entraining agent into the concrete,incorporating mineral admixtures into the concrete,and introducing fibers such as steel,polypropylene,carbon,ethylene glass,and plant fibers into the concrete.

This research was supported by the National Key Technology Support Program (No.2014BAG05B05),and the Basic Scientific Research Business from Institute of Earthquake Science,CEA (No.2014IESLZ01).

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