effect of freezing-thawing cycles on the...

Second International Symposium on Design, Performance and Use of Self-Consolidating Concrete SCC’2009-China, June 5-7 2009, Beijing, China

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EFFECT OF FREEZING-THAWING CYCLES ON THE PHYSICAL AND MECHANICAL CHARACTERISTICS OF CONCRETE

Mohamed A. S. MOHAMED1, Elhem Ghorbel1 and George WARDEH1

1- L2MGC, Cergy-Pontoise University, 5 mail Gay LUSSAC 95031 CERGY-PONTOISE Cedex, Cergy-Pontoise, France Abstract

Concrete is the most used construction material in practically all of civil engineering fields due to its economical and technical advantages. However, its microstructure is porous and may be completely or partially water saturated. In severely cold climates, this water freezes and degradations develop gradually with the freezing-thawing cycle’s number, in forms of internal cracking, chipping and scaling.

Frost behaviour is based on the coupling between the 9% volumetric increase during water transformation into ice, the cryo-suction phenomena, the non frozen water transport within the porous network and the thermo-mechanical behaviour of each component of the frozen media. It is thus obvious that the frost resistance depends on the microstructure (pore size distribution and permeability) and the mechanical characteristics of the material.

In this paper, physical and mechanical characteristics evolution during freezing-thawing cycles was followed. The results show that the reduction in the mechanical resistance and the elastic modulus is accompanied with an increase in the intrinsic permeability. The damage can be characterized by a scalar parameter, Df, due to frost action.

With this parameter, the resistance reduction as well as the permeability evolution may be described, with a good agreement, as a function of cracks development.

Keywords: concrete, frost action, damage, permeability

1. INTRODUCTION

Concrete is the most used construction material. Its durability is a key factor from an economic point of view and depends mainly on its environment and its composition.

The rigorous climatic conditions, in particular the repeated freezing-thawing cycles, are at the origin of important degradations.

Frost damage is classified in two categories, internal cracking and the surface scaling which affects mainly concrete pavements in presence of deicing salt, resulting in a progressive loss of “flakes” of cement paste of elements.

Standardized tests were set up in order to quantify the frost durability of concretes by subjecting them to repeated freezing-thawing cycles. It arises from these tests, carried out in climatic cells, that frost damage is a function of many factors.


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All the studies prove that W/C ratio has a very important influence on the frost durability of concrete. This ratio affects directly the permeability and the amount of freezable water contained in the porous network.

The traditional materials with a strong W/C ration without air entrained are not durables when they are fully water saturated [1]. Mac Innis showed that mortars cooled to -20°C of W/C ratio equal to 0.4, 0.5 and 0.6 respectively are durable if the saturation degree is lower than 90%. Saturated, these mortars are not durable any more [2] .

Gagné [3] observed that a concrete of 0.3 W/C ratio incorporating a high-strength Portland cement is durable but incorporating a normal cement leads to a destruction of material. One can conclude thus that this ratio only does not allow quantifying the frost resistance of concrete.

The influence of compressive and tensile strengths with respect to the frost behavior is not completely established. According to Gagné et al [4] a strong tensile strength allows a better frost resistance. Conversely, other studies [5] do not show any correlation between mechanical resistances (in particular tensile strength) and frost resistance.

A means of protecting materials from frost damage is to introduce a network of fine bubbles during mixing. For an efficient network a maximum air void spacing factor must be about 200 µm [1]. It is important to note that the air-entraining agents increase the frost resistance, but decrease the mechanical resistances in particular in compression. The problem of the frost behavior is not determinable according to the traditional parameters of the concretes and in particular in the case of HPC and VHPC.

Hammer and Sellevold (1990) [6] showed that a VHPC of 110 MPa compressive strength and 0.26 W/C ratio is very severely frost damaged. One can thus think that the very low permeability of these concretes determines their low frost resistance.

In a preceding paper [7], it was shown that the frost behavior of a consolidated porous media is a function of porosity, permeability, elastic modulus, and the imposed boundary conditions.

The objective of the present work is to study the frost behavior of fully saturated ordinary concrete without air entraining as well as the influence of repeated freezing-thawing cycles on the characteristics of material.

A quantitative study is thus carried out on the coupled damage-permeability evolution. These results will allow to better understand the processes of frost damage and to identify the key parameters who determine the frost durability of concrete.

2. FROST ACTION MECHANISMS

During freezing, the interstitial solution does not freeze at the same temperature, in particular because of a non homogeneous pore size distribution. The material thus contains non frozen water and ice [8-11].

From a theoretical point of view, ice formation can be described by two different mechanisms, a heterogeneous germination then propagation in the smallest pores or by a homogeneous nucleation only in the pores [8, 9].

Relating to ice formation, there are several microscopic phenomena which create pressures being able to create internal cracking.

Water transformation into ice is accompanied with a volumetric increase of 9 % which


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puts non frozen water under pressure [12]. This pressure is dissipated by a water flow in the porous network. Damage according to Powers is induced when this pressure, called hydraulic pressure, exceeds the tensile strength of the material.

Moreover, the ice being formed exclusively water molecules; zones over salt concentrated are created. Under the effect of the concentration gradient, a water movement of the non frozen pores towards the sites of ice formation must take place in order to restore thermodynamic equilibrium between ice crystals and water. Due to this water movement, an osmotic pressure is built up within the porosity [13].

Water is not the only liquid which can damage a porous material. According to Beaudoin and MacInnis [14] , a cement paste of 0.5 W/C ration, saturated with benzene, dilates if it is cooled at a rate of 2.5 K/h. The dilation starts at +5 °C where the absorbate starts to freeze. Also, an organic liquid induces the expansion of porous glass during a cooling below their freezing point. The organic liquid and benzene contract upon their transformation in a solid state, and the damage is thus due to crystals growth [15, 16].

When the paste is at the temperature of the melting point, the crystal filled exactly the pore space; it is in equilibrium. If the temperature decreases the crystal will try to grow, so the pore walls exert an additional pressure to prevent this growth. This additional (disjunction) pressure is called crystallization pressure.

These microscopic visions are very interesting and improved the physical comprehension of frost action mechanisms. But it remains quite difficult to make the passage at the macroscopic level to explain the behavior of the porous media during freezing.

On the material scale, it was shown that frost action is coupled thermo-hydro-mechanic phenomenon related to [7, 17]: a) - Difference of density between the liquid water and the ice crystal. b) - Thermal contraction effects on the drained medium and the other components. c) - Disjunction pressure representing the interaction between ice crystal and pore walls. d) - A cryo-suction phenomenon which obliges water under the effect of the heat gradients to leave the non frozen zones towards the formed ice zones.

3. MIX DESIGN OF CONCRETE The material in this study is a ready-mixed concrete of S4 class of consistency

(workability) where the slump with the Abrams’s cone is between 160 and 200 mm [18]. The concrete is intended for a building located in an environment where the frost risk is

moderate “definition in the context of the same standard”. The resistance imposed by the note of calculation is 30 MPa at 28 days. The method used for the formulation of concrete is that of Dreux - Gorisse [19]. It allows determining the proportioning of all components of the concrete.

The method specifies, for a given workability, the maximum water quantity, and the allowable W/C ratio is imposed by the class of environmental exposure. Water then is fixed at 164 l/m3 and W/C is chosen to be 0.47 which imposes finally a 350 kg/m3 of cement.

CEM I CALCIA 52.5 Cement, manufactured in France, was used. The mineralogical composition calculated by the method of Bogue is presented in the following table. A sand 0/4 mm, siliceous, rolled with a density of 2.55 and siliceous semi-crushed gravel 4/20 mm with a density of 2.51, were used.


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Table 1: mineralogical composition and physical property of the used cement C3S

% C2S %

C3A %

C4AF%

Density Blaine m²/kg

Compressive strength a 2 days MPa

CEM I 52.5 66.9 10.7 8.4 7.6 3.11 395 31.7 Calculation of granular mixture has shown that the sand to gravel ratio is equal to 1.5

resulting in a mix of gravel of 1064 kg/m3 and 721 kg of sand. The dosage of superplasticizer was found experimentally using the concrete equivalent

mortar method. Rheologically, a linear correlation exists between the workability of the CEM and that of the concrete because the volume of water and the surface of cement and the aggregates are preserved [20].

A target spread is thus fixed in this project to 20 cm (allowing obtaining a S4 concrete with a slump of 20 cm). The dosage of superplasticizer was gradually increased until obtaining a wafer of 20 cm in diameter.

Table 2 recapitulates finally the composition and the mix proportion of the used concrete.

Table 2: Mix proportions of the used concrete Constituents Cement

(Kg/m3) Gravel

(Kg/m3)Sand

(Kg/m3)Superplasticizer

(Kg/m3) Water

(Kg/m3) 350 1065 970 1.89 164

Cylindrical 16x32 cm specimens were prepared in order to follow the compressive

strength evolution during the freezing-thawing test. Cylinders of 15 cm diameter were also prepared to follow permeability evolution and finally 10x10x40 cm prismatic beams in order to study dimensional variations.

At 28 days an average compressive strength of 28 MPa and an open water volumetric porosity of 12 % were found. All the specimens were preserved in water until the moment of the freezing- thawing test.

4. FREEZING-THAWING CYCLES At the end of water saturation, the specimens were immediately placed in a climatic

chamber. Cycles of 12 hours duration were imposed according to the Rilem recommendations [21]. The freezing/thawing cycles are carried out without water contribution.

Starting from +10 °C the temperature is lowered in 3 hours with a cooling rate of 10 °C/h up to -20 °C, and it kept constant during 3 hours at this temperature. Then it is increased in 2 hours to +10 °C with a cooling rate of 15 °C/h and kept constant during 4 hours.

These steps were programmed in order to stabilize the temperatures and to allow the liquid transfers to occur.

The basic loop includes two cycles per day. Every 30 cycles some specimens were left in order to carry out the physical and mechanical tests.

Several studies showed that the test conditions are in general more severe than the natural climatic conditions. The observed natural cooling rate is about 2 to 3 °C/h, and thus definitely lower than those imposed during the freezing-thawing cycles [1].


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5. EVALUATION OF VISUAL DEGRADATIONS DURING THE TEST Cracking is the direct consequence of all the phenomena exposed in the second

paragraph. This cracking leads to a loss of the mechanical properties of material. After each 30 freezing-thawing cycle, the surface of the cylindrical specimens designed

for the mechanical tests was studied. All the cylinders had a single model of cracking for which the cracks are in the axial direction of the cylinder and perpendicular to this axe. These typical modes of cracking were also observed by Pentalla and Al-Neshawy [22]. This phenomenon is often accompanied with a surface scaling when the material is saturated. After 60 cycles the cracks become visible and the situation becomes serious as the number of cycles increases.

Figure 1 illustrates the frost damage and one can notice that after 240 cycles the specimens are completely destroyed.

Cracks orientation can be explained by the hydrostatic nature of the porous pressure developed in the porosity during ice formation as well as by the prevention of strains in the middle of the specimens and the freedom of deformation close to the surface.


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60 cycles

180 cycles

210 cycles

240 cycles

Figure 1: damage evolution during freezing-thawing cycles

6. DIMENSIONAL VARIATIONS EVOLUTION Frost damage can be appreciated by the measurement of the dimensional variation after

all N freezing-thawing cycles.


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A follow-up of the samples height variation is carried out. Measurement is simply taken using a gauged stem corresponding to the initial sample length and an extensometer. The material is considered non frost resistant if the average length variation obtained on the series of three prisms is higher than 500 µm/m [23, 24].

For the studied concrete, a visco-elastoplastic model of mechanical behavior is adopted. The retained cracking criterion corresponds to positive residual deformations (expansion) higher than 10-4 m/m, and negative deformations (contraction) higher than 3.10-4 m/m [25]. The average longitudinal dimensional variation is represented on figure 2.

-1500

-1300

-1100

-900

-700

-500

-300

-100

100

300

0 30 60 90 120 150 180 210 240 270 300 330

number of cycles

stra

in (祄

/m)

Figure 2: Dimensional variations of concrete during freezing-thawing cycles

During the first 90 cycles the concrete presents a light lengthening lower than the tensile

criterion so the material resists the frost action. Between 90th and 120th cycle, an important shrinkage occurs and stabilizes until the

240th cycle around a strain of 5.10-4 m/m. In comparison with the cracking criterion in compression it is clear that the concrete is damaged.

From 240th the contraction is higher than 5.10-4 m/m. The concrete is not any more resistant and the test can be stopped according to the criteria of the standard mentioned previously.

The frost shrinkage can be explained by the fact that during freezing, the ice which occupies a part of the porosity contracts simultaneously with the porous skeleton. This contraction is quite lower than the simple thermal contraction of the solid skeleton [10, 11].

A part of this contraction may also be allotted to a similar mechanism of that auto-desiccation shrinkage. By lowering the temperature, the thickness of the layer of water molecules adsorbed on the surface of pores decreases and consequently the surface energy of solid increases and conversely. This increase in surface energy is balanced by a reduction in size of surface inducing the macroscopic deformations of shrinkage. These strains present a priori an irreversible part which accumulates by repeating the freezing-thawing cycles [8].


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7. INFLUENCE OF FREEZING-THAWING CYCLES ON THE MECHANICAL PROPERTIES OF CONCRETE

Frost action induces often irreversible modifications in the microstructure of the cementious matrix. For the material, the effect of repeated freezing-thawing cycles on the mechanical resistance was studied. The results presented are the averages of three measurements on 16x32 cm cylindrical specimens taken every 30 cycles.

Figure 3 shows a compressive strength reduction in a way similar to the dimensional variation.

At the end of 30 cycles, the concrete loses 20 % of its initial strength at 28 days then the loss almost stagnates until the 150th cycle. An important drop is then produced up to the 240th cycle where the final resistance is only about 40 % of the initial compressive strength.

The elastic modulus was deduced from the stress-strain experimental curves obtained at the end of three loading-unloading cycles for a stress varying between 0.5 MPa and 30 % of the compressive strength.

Figure 4 represents the elastic modulus evolution during the freezing-thawing cycles. One can note that this evolution follows the compressive strength evolution.

After 120 cycles this module presents a drop of about 30% of the initial value and at the end of 240th cycle the value is only 25% of the initial value.

Damage, being a consequence of the progressive deterioration of the microstructure of material, is quantified by a factor, Df, associated to the physical and mechanical degradation process related to frost action.

0,0

5,0

10,0

15,0

20,0

25,0

30,0

35,0

0 30 60 90 120 150 180 210 240 270

number of cycles

com

pres

sive

stre

ngth

(MPa

)

Figure 3: compressive strength as a function of number of cycles


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0

5

10

15

20

25

30

35

40

0 30 60 90 120 150 180 210 240 270

Number of cycles

Ela

stic

mod

ulus

(GPa

)

Figure 4: Elastic modulus as a function of number of cycles

This damage factor may be calculated by the relation [25]

EED f

ˆ1−=

Equation 1 With E : elastic modulus after N freezing-thawing cycles and E the initial elastic modulus related to the non damaged material. Figure 5 shows the evolution of this parameter with the number of cycles where one can state a Gaussian evolution given by the following equation

0,0

0,2

0,4

0,6

0,8

1,0

0 30 60 90 120 150 180 210 240 270

Number of cycles

Fros

t dam

age

fact

or D

f ExperienceFitting

Figure 5: Damage factor evolution with freezing-thawing cycles


469

2

.⎥⎦⎤

⎢⎣⎡ −−

= cbN

f eaD Equation 2

a=0.72, b=242.3 and c=135.2 parameters obtained by mathematical fitting with r²=0.97. It appears that at the end of the 120th cycle, the frost damage factor is equal to 0.3 (or 30 %) which means that the resistance of material is of 70 % of its initial resistance which is in very good agreement with the analysis of behaviour under compressive loads. Studies carried out by several authors on the influence of frost action on the mechanical behavior showed that the stress-strain relationship of damaged material can be described by the relation

εσ .)1)(1( EDD f −−= Equation 3

With D: mechanical damage factor, σ stress and ε the strain.

8. GAS PERMEABILITY EVOLUTION DURING FREEZING-THAWING TEST

Permeability was measured with a constant head CEMBUREAU permeameter, the apparatus used allowed recording of the gas flow [26].

In order to obtain representative values of a material, the test was conducted on a minimum of three samples.

Basing on Darcy’s law the following relation gives the effective permeability of the material tested when the flow is assumed to be laminar and unidirectional

).(

...222se

ssa

PPA

PQLk

−=

μ Equation 4

Where L [m] is the sample thickness, A [m2] is the section subjected to flow, Pe [Pa] is the upstream absolute pressure and Ps is the downstream absolute pressure. µ [Pa.s] dynamic viscosity of fluid, Qs [m3/s] volumetric gas flow.

The apparent measured permeability by the preceding relation depends on the gradient of applied pressure, the temperature, the water saturation degree and the microstructure of material.

The permeability given by Equation 4 assumes a laminar flow but in fact a nonviscous contribution due to the pore fineness in concrete is frequently observed. This contribution and the dependence of the pressure are well identified by Klinkenberg [26].

For a given degree of saturation, the apparent permeability is generally a linear function of 1/Pg from where Pg is the total pressure of the gas phase.

Consequently, the intrinsic permeability of concrete subjected to a laminar gas flow can be given by an extrapolation (linear regression) of the apparent permeabilities to an infinite average pressure. For the concrete before frost test a permeability of 1x10-16 m² is retained.

The application of this method on a concrete being undergone 30 cycles of freezing gives the results represented in figure 6. The results confirm very well the linearity between K and 1/Pg.

During freezing-thawing cycles the mechanical and hydraulic characteristics evolve because of consecutive degradations related to the microscopic cracks formation.

Picandet and Al [27] showed that the residual gas permeability (measured after unloading) of concrete increases exponentially with the damage induced by the uniaxial


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loading in the elastic phase.

1,5E-16

2,0E-16

2,5E-16

3,0E-16

3,5E-16

4,0E-16

4,5E-16

5,0E-16

0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9

1/Pm=1/(Pe+Ps) [1/bar]

Perm

eabi

lity

[m²]

Sample 1

Sample 2

intrinsic permeability

Figure 6: gas permeability of concrete subjected to 30 freezing-thawing cycles.

The evolution of gas permeability was followed during the first 120 cycles then was stopped because of a material problem.

The permeability after 60th cycle is 300 times higher than the initial permeability and at the end of 90th cycles is 4000 times higher and at 120th cycles it becomes 20000 times higher than the initial permeability.

One can state that gas permeability is sensitive to frost action. This sensitivity is explained by the fact the small pores have a great influence on the permeability and that these are the pores where the porous pressure related to ice formation is significant [28].

The evolution in relation to the frost damage factor Df is represented on figure 7 where one notes that the permeability increases exponentially with the frost damage factor.

Finally, based on the experimental results one can qualitatively find an exponentially correlation between the frost damage parameter and the permeability obtained by the relation

fADeKK 0= Equation 5

With A=38.3. Consequently, this damage parameter is the best adapted variable to the permeability evaluation following a mechanical degradation of material.


471

y = 1E-16e38,296x

R2 = 0,938

1,0E-16

1,0E-15

1,0E-14

1,0E-13

1,0E-12

1,0E-11

1,0E-10

0,00 0,05 0,10 0,15 0,20 0,25 0,30

Damage factore Df

Per

mea

bilit

y (m

²)

Figure 7: gas permeability as a function of frost damage factor.

9. CONCLUSION

Frost behavior of a given concrete is evaluated by subjecting it to repeated artificial freezing-thawing cycles in a climatic chambers.

Physical and mechanical characteristics evolution was followed. In spite of the respect brought to the minimal quantity of cement and to the maximum W/C ratio imposed by the standard, the concrete employed in this study did not resist 300 cycles. These two criteria are not thus sufficient to have a material frost-resistant.

From 120th cycle the concrete presented a contraction higher than 3.10-4 criterion of cracking selected to compression. This cycle is also marked by a drop of compressive strength and elastic modulus about 30 % of the initial values.

The evolution of the permeability during freezing-thawing cycles was also followed. The results show that the reduction in the mechanical resistance and the elastic modulus are accompanied by an increase in the intrinsic permeability of material.

The scalar frost damage factor Df allowed describing this permeability increase.

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