A survey of the literature
Ir. C. v.d. Veen
T U Delft
Faculty of Civil Engineering Division of IVIechanics and StructuresSection of Concrete Structures Stevin Laboratory Delft University of Technology
stevin Laboratory Report No. 25-87-2 Faculty of Civil Engineering Research No. 2.1.83.03 Delft University of Technology January 1987
Properties of concrete at very low temperatures; A survey of the literature
by
Ir. C. v.d. Veen
t
u
Mailing address:Delft University of Technology Concrete Structures Group Stevin Laboratory II Stevinweg 4
2628 CN Delft The Netherlands
No part of this report may be published without written permission from the author.
Technische Universiteit Delft Bibliotheek Faculteit der Civiele Techniek
c-r-y (Bezoekadres Stevinweg 1) ^ ' <-. Postbus 5048 olC^dl^^ 2600 GA DELFT
ACKNOWLEDGEMENTS
These investigations have been carried out under supervision of prof.dr.ir. A.S.G. Bruggeling and prof.dr.-ing. H.W. Reinhardt and with the support of the Netherlands Technology Foundation (STW), which is gratefully acknowledged. Furthermore, the author is indebted to Mr H. Spiewakowski, who prepared the drawings, and Mrs D. Pat for typing the report.
CONTENTS
SU^WAHY
1. INTRODUCTION
2. MECHANISMS OF FREEZING AND FROST DAMAGE
2.1 Structure and moisture content of concrete 2.2 Freezing of pore water
2.2.1 Supercooling
2.2.2 Antifreeze effect of a salt solution 2.2.3 Freezing point as a function of pore s 2.3 From water to ice
2.4 Mechanisms of frost damage 2.4.1 Hydraulic pressure 2.4.2 Mils-theory
2.4.3 A model based on thermodynamics
3. THERMAL DEFORMATION
3.1 The components of concrete 3.1.1 Hardened cement paste 3.1.2 Mortar
3.1.3 Water 3.1.4 Aggregates
3.1.5 Comparison of the different components 3.2 Concrete
3.2.1 Moisture content 3.2.2 Aggregate
3.2.3 Coefficient of thermal expansion 3.2.4 Influence of the stress level 3.3 Model according to Wiedemann
4. MECHANICAL PROPERTIES OF CONCRETE
4.1 Compressive strength 43 4.1.1 Initial strength, age, mix proportion 43
4.1.2 Moisture content 47 4.1.3 Influence of the type of aggregate 49
4.1.4 Effect of thermal cycling and cooling rate 51
4.1.5 Cyclic loading 57 4.1.6 The stress-strain diagram 59
4.2 Tensile strength 63 4.2.1 Influence of the curing conditions 64
4.2.2 Effect of the type of aggregate 68 4.2.3 Effect of thermal cycling and shock loading 69
4.2.4 Stress-strain diagram 72 4.3 Young's modulus of elasticity 75
4.3.1 Influence of the curing condition 76 4.3.2 Influence of the type of aggregate 79
4.3.3 Effect of thermal cycling and shock loading QQ
4.4 Conclusions 83
5. BOND STRENGTH
5.1 Influence of the curing conditions 86
5.1.1 Concluding remarks 93 5.2 Influence of some parameters on bond resistance 94
5.2.1 Concrete grade 94 5.2.2 Concrete cover and location of reinforcement 95
5.2.3 Embedment length 96 5.2.4 Bar diameter 97 5.3 Effect of thermal cycling 98
5.4 Crack width and spacing 101 5.4.1 Concrete elements subjected to direct tension 101
5.4.2 Concrete elements subjected to pure flexure 103 5.4.3 Discussion and analysis of the experimental results 106,
SLJWARY
This literature survey is focussed on the behaviour of concrete at very low temperatures down to -196°C. To give the reader some theoretical background, first the mechanisms of freezing and frost damage are discussed. Based on these mechanisms, reasonable explanations can be given for the characteristic behaviour of concrete under cryogenic conditions. The thermal deformation, the compressive and tensile strengths and the elasticity of concrete will be successively treated in separate Sections. Special attention will be paid to the effect of the mix proportions, moisture content, type of aggregate and thermal cycling on the behaviour of concrete.
Finally, the cryogenic bond behaviour will be discussed. Particularly, the local bond stress-slip relationship will be analysed in more detail. Furthermore, the effect of low temperatures upon the crack width and spacings conclude this literature survey.
1. INTRODUCTION
Since the Second World War the world economy has led to a greater demand for energy. Particularly the use of natural gas has undergone exponential growth leading to an increase in its use both for consumptional as well as industrial applications. Because of this development a growing need has arisen for suitable storage and transportation equipment. Natural gas can be transported and stored both in the gaseous phase and in liquefied form. Nowadays gases are in general preferably stored in liquid form at atmospheric pressure. Therefore gases are cooled to their boiling point, which for most gases is below 0°C, Table 1 [Turner 1].
Table 1 Some physical properties of liquefied gases, [1]
liquefied gases boiling point
OC volume reduction factor Butane Ammonia Propane Ethane Methane (LNG) Oxygen Nitrogen 11 33 42 89 162 183 196 1/240 1/950 1/310 1/430 1/620 1/800 1/690
Liquefied Natural Gas (LNG) consists of 80-90SJ (by vol.) of methane. Liquefied Petrolevmi Gas (LPG) consists mostly of propane and/or butane.
Natural gas undergoes a decrease in volume in a ratio of 620:1 as a result of liquefaction at -162°C and is then commonly referred to as L.N.G.
For safety reasons mostly a double-walled tank is used for the bulk storage of large quantities of L.N.G. Such a tank comprises an inner tank ("primary tank")
usually made of nickel steel and an outer tank constructed either of reinforced concrete or prestressed concrete, Bruggeling [2]. Thermal insulation in the gap between the outer and inner tank minimizes the loss of energy. Although prestressed concrete with its high toughness against cracking is superior to steel for constructing the inner tank, prestressed concrete has as yet been used only in a few instants. For this reason and also because it is cheaper to build, prestressed concrete is bound to supersede steel for the inner tank. However, for the containment of L.N.G., the leak tightness of the structure is the single most important design criterion. In an unlined concrete tank this is determined by the inherent permeability of the concrete and the extent to which flaws occur during construction and operation of a tank.
Whilst a well designed concrete can be sufficiently impermeable to contain L.N.G., concern over the possibility of cracks Euid localised areas of high permeability causes the designer and user to demand an impermeable liner to prevent leakage. To develop the potential of vinlined concrete as a primary containment material for cryogenic liquids, more information about the possibility of cracking is needed.
Because bond between the concrete and reinforcement is one of the most essential properties that determine crack spacing and crack width, information is necessary about the characteristic features of bond at very low temperatures. However, experimental information about the bond characteristics at very low temperatures is very scarce. Therefore not only the limited information about the cryogenic bond behaviour will be discussed, but also the properties of concrete which may affect this bond behaviour. It should be possible to analyse the bond behaviour with the aid of these properties such as elasticity, compressive and tensile strength of concrete. Though the bond behaviour is the main subject of this study, much attention will be paid to the other mechanical properties of concrete.
Because mechanisms of freezing and frost damage could be the starting point from which typical experimental observations will be explained. Section 2 is
focussed upon this subject.
Commonly used concrete in Western Europe posesses more or less the same thermal expansion as the reinforcement in the temperature range from -20 to +60°C. However, when reinforced concrete is used beyond the temperature range stated
above, the thermal expansion may differ considerably. For that reason the thermal behaviour of concrete is also examined in Section 3.
The elasticity, compressive and tensile strength of concrete will be successively discussed in Section 4.
Finally the bond behaviour between reinforcement and concrete will be the subject of Section 5.
2. MECHANISMS OF FREEZING AND FROST DAMAGE
A comprehensive literature survey and study concerning mechanisms of freezing
and frost damage were made by Setzer [20] and Wiedemann [7]. The latter performed a great number of experiments on mortar and concrete, particularly water-saturated mortar or concrete. So, for more detailed information the
reader is referred to the literature mentioned above.
In the following Sections the material properties affecting the mechanisms of
freezing will be reviewed.
2.1 Structure and Moisture content of concrete
Concrete can be conceived as a two phase material consisting of the natural aggregate and the hardened cement paste. Usually the natural aggregate used in the concrete for the construction of L.N.G.-tanks can almost be considered as non-porous and therefore having a very limited capacity for absorbing water. As contrasted with the natural aggregate, the hardened cement paste is a material of high porosity and of great specific surface which may contain a considerable amount of freezable water. The hardened paste consists of hydrates of the various compounds, referred to as gel, of crystals of Ca(0H)2, unhydrated cement and the residue of the water-filled spaces in the fresh state [3]. These voids are called capillary pores, but within the gel itself there exist
interstitial voids, called gel pores. The gel thus comprises the cohesive mass of hydrated cement in its densest paste, including the gel pores.
Full hydration of cement requires an amount of water of about 40% of the cement by weight; 25% will be chemically bound while the remaining 15% is adsorbed physically. The chemically bound water decreases in volume by about 25%, so that the porosity of the hardened paste is about 30%. Fig. 2.1 shows the volumetric portions of a fully hydrated cement paste as a function of the water/cement ratio, Rusch [reported in 4 ] . The starting point for this relationship is that no capillary pores exist at a water/cement ratio of 0.4. If the water/cement ratio is increased the amount of non-bound water will also
yolumetric portion (%)
0 0.2 O.L 0.6 0.8 ID 1.2 IL
W/C
Fig. 2.1 Volumetric portions versus w/c for a fully hydrated cement paste [4].
pores, called capillary pores, are developed. The concrete commonly used consists of approximately 70% aggregate and about 30% cement paste. Hence it follows that the porosity of the concrete itself can be calculated by multiplying the porosity of the hardened cement paste by 0.3. We have to bear
in mind that air voids will also contribute to the porosity of the concrete. The porosity of the hardened cement paste, which is a siumnation of the gel pores and the capillary pores, can be defined [4] as:
w/c-0.19 Hg
Pe = (2.1) w/c+0,32
in which Hg: degree of hydration
For concrete cured under water (saturated), full hydration (Hg=l) of the cement is possible and all the pores in the hardened paste are filled with water. However, in practice for thick walled concrete structures hardly any moisture movement will take place. So the only water available for hydration is the amount which was added at mixing. For the construction of L.N.G.-tanks the water/cement ratio is normally less than 0.5. Even with a water/cement ratio of
0.5, the hydration would not have been completed, as it ceases before the capillaries have become empty [6]. Assuming an average degree of hydration of 0.8, an estimate of the moisture content of in situ concrete can be made:
12(w/c-0,20)
m = X (by mass) (2.2)
w/c+0,32
In practice the water content of thick concrete structures will range from 3.5 to 5.5%, depending of the water/cement ratio.
2.2 Freezing of pore water
The freezing of pure water at atmospheric pressure occurs at 0°C and is accompanied by a 9% increase in volume. However, the freezing of water in capillary cavities of hardened concrete does not behave like pure (bulk) water. There are several reasons why a significant amount of water freezes after the temperature falls below the normal freezing temperature, Wiedemann [7].
2.2.1 Supercooling
If pure water is cooled, the temperature at which crystallization starts spontaneously is always lower than the melting point. This condition (called supercooling) alone is not sufficient for pure water to begin to crystallize. Before crystals can grow there must exist in the solution a number of minute centres of crystallization. Crystallization may occur spontEineously
("homogeneous") or may be induced artificially ("heterogeneous"). In the case of the "homogeneous" mode ice crystals will appear at a supercooling of about -40°C, Michel [8]. However, this mode is more or less of academic interest; more important is the "heterogeneous" mode which will commonly occur in
practice.
An amount of pore water in the hardened cement paste will freeze at higher temperatures than discussed above. Experimental research on water-saturated cement paste specimens by Meier and Harnik [9], Helmuth [10] and Grubl [11] has
revealed supercooling ranging from -11 to -16°C, -7.1°C and -7,2°C respectively. After freezing has been initiated, the water-ice phase transition occurs rapidly, and the temperature of the specimen will then also rapidly be raised by the liberated heat of crystallization. The maximum temperature reached after rapid ice formation will be the equilibrium temperature at which water and ice can coexist. This maximum temperature, ranging from -1 to -3°C and depending on the actual conditions, can be interpreted as the temperature at which ice formation will start when no supercooling occurs.
It is to be noted that in consequence of the rapid ice formation very high hydraulic pressures will be developed.
2,2.2 Antifreeze effect of a salt solution
Because of the salt content in water, the freezing point will be depressed below 0 °C, Assur [12] gives for several salts the minimiam temperature at which the salt solution coexists in equilibrium with the ice, see Table 2.1.
Table 2,1. Minimum temperature in the liquid solution, [12]
salt solution temperature °C
Na2SO4.10H2O - 8.2 NaCl,2H20 -22,9 KCl -36.8 MgCl2.8H20 -43.2 MgCl2.12H2 0 CaCl2,6H20 -54,0
However, in concrete commonly used for structures the concentration of soluble materials in the pore water is very low. As a consequence, the freezing point will only be lowered a few degrees. Experiments by Verbeek and Klieger [13] show for such cases a maximimi lowering of the freezing point of about 5°C. As contrasted with supercooling, the freezing and thawing points coincide,
2.2.3 Freezing point as a function of pore size
The cement paste is hygroscopic owing to the hydrophilic character of cement coupled with the presence of sub-microscopic pores. Water in hydrated cement is held with varying degrees of firmness. At one extreme there is chemically combined water; at the other, free water (bulk water). Between these two categories there is gel water held in a variety of other ways. The water held by the surface forces of the gel particles is called adsorbed water. As contrasted with adsorbed water the free water is beyond the range of the surface forces of the solid phase, Slate and Meyers [14], When the radius of the pore decreases, the surface forces between water molecules and the pore wall increases. Hence it follows that the freezing point is depressed as a function of pore size, as shown in Fig. 2.2. The depression increases when the radius of the pore decreases, Stockhausen [15].
From Fig. 2.2 it appears that adsorbed water held in the cylindrical pores of 3 nm diameter freezes at approximately -50°C.
-Q5 -5 -10 -20 -50 -100. Freezing point 1 t 1 / Y
LJ
/A
y
/ / / ["CI • / \ ^V
• / - Kelvin f / ?quaticA
>n -elliptical pores r = — M , — ,10 - cylindrical pores . r - 39 . o J in f • 1 10 20 50 100 200 500 1000 2000 Pore radius [A]The moisture content of concrete stored at room temperature depends on the relative humidity of the ambient air. With the curve of equilibrium moisture content (or desorption isotherm) at room temperature and the use of the Kelvin equation it is possible to determine the diameter of cylindrical pores which are filled with water. This has been the point of departure from which Stockhausen [16] assessed the freezing point as a function of the relative humidity, see Fig. 2,3,
freezing point (°C) -10 -/—
-20
-/--30 y
-to
-50
-/-- 6 0 — /
. 7 0 l 1 1 \ \ 50 50 70 80 90 100 relative humidity (%)Fig, 2,3 Freezing point as a function of the relative humidity [16], 2.3 FrcMB water to ice
During the last three decades various investigators have tried to determine the freezing point of the water in concrete using different techniques. From what has been said above appears that we cannot probably speak of one freezing point. A certain transition zone in temperature is to be expected,
In this Section a review of the results, relating to this subject, found by experimental research, will be presented.
Tognon [17]
With the aid of calorimetric tests the variation of enthalpy has been measured as a function of temperature. Samples of pure paste, prepared with high strength cement, w/c=0.45, cured for 300 days in water, were used. The samples were heated in an oven at 110°C. Some samples were tested in this condition
(oven-dry) other samples were stored in water for three days (moist specimens). So moist, in fact, that the rewetted and the oven-dry samples were used for the experiments.
For the moist specimens the enthalpy variation, referred to cooling temperature on hardened cement paste specimens, indicated that evaporable water freezes progressively from -4°C down to -90°C. The researcher thought that the decrease of the freezing point -4°C might be attributed to the presence of dissolved salts. This observed phenomenon is in agreement with the results of Verbeek and Klieger [13] see Section 2.2.2, It is to be noted that 35% of the evaporable water froze at -4°C. For the dried specimens no change of the state from ice to water was observed.
Scurlock and Yusof [18]
These investigators also measured the variation of enthalpy versus temperature. Samples of hardened cement paste prepared with standard Portland cement, with water/cement ratios ranging between 0.3 and 0.4 and cured for 28 days under water at 20°C, were used. Some samples were tested in this condition
(water-saturated); other samples were first heated in an oven at 110°C (oven-dry) and some were, after drying, resoaked in water for 23 days. It is concluded that no change of phase takes place in the water remaining in the oven-dry sample. The water-saturated samples with different water/cement ratios showed a similar behaviour, suggesting that freezing of the evaporable water takes place gradually over the whole temperature range between 0 and -60°C. On the other hand, the third curing condition, the oven dry and rewetted sample, showed a totally different behaviour. It appeared that only 49% of the water taken up showed a latent heat of freezing on cooling from 0 to -60°C. These observations suggested that some of the evaporable water was strongly physically bovmd and did not exhibit any latent heat of freezing. Furthermore
the temperature range from -10 to 0°C. This phenomenon is in accordance with the results of Tognon [17].
The remaining 10 percent of the freezing appeared to take place between -35 and -45°C. The respective differential enthalpy versus temperature curves are shown in Fig. 2,4. (5H/5T)T [J/g.°Cl 25 2D 1.5 1.0 05 00 -60 -50 -«D -30 -20 — 1 1 — - 1 symbo O • D • A
r
W/C 0.30 037 0.«3 1 0.37 ^r^
curing saturated saturated saturated dry rewettedn
i
Ï
l a i ^« 2 ^
\ \ Au
• • 1 5.5 1J
-10 0 10 temperature ( C)Fig. 2,4 The differential enthalpy versus temperature [18],
Since C=(6H/6T)T, the differential enthalpy plots are effectively heat capacity curves. The value of C at each temperature contains among others the latent heat of freezing for water changing phase at that temperature.
Stockhausen, Dorner. Zech and Setzer [16]
These researchers performed experiments on samples of hardened cement paste with the aid of differential thermal analysis (DTA). Results of the experiments
are shown in Fig. 2.5. The influence of the curing condition on the freezing behaviour of the water in the hardened paste seems clear. No changing of phase (from water to ice) was found for paste cured at room temperature and 60% relative humidity.
time (min LO 30 01 o c c B o Q.
Fig. 2.5 DTA-diagram for cement paste cured with different relative humidities [16],
Samples of hardened cement paste cured at a relative humidity ranging from 60 to 90% showed a "peak" in potential difference at -43°C, which is associated with the phase change from water to ice. It can be concluded from the DTA-diagram that even a phase change takes place at -20°C when the paste is cured at 97% relative humidity. This behaviour is in good agreement with the results reported by Scurlock and Yusof [18],
Based on their research, the investigators distinguished the following stages in the freezing of water:
a) Bulk water stored in pores with r > 0,1 mm, which are filled with water at a curing condition > 99% r.h. This water will freeze between 0 and -4°C. b) Condensed water in capillary pores with r > 0,01 mm, which are filled
with water at a relative humidity between 90 and 99%, This water freezes between -20 and -30°C.
c) Structured water: this is the thicker layer of absorbed water in pores with 3nm < r < lOnm, which are filled with water at a relative humidity between 60 and 90%, This water freezes in the range of -30 to -80°C. d) Absorbed water layers on the walls of very small gel pores r < 3nm do not
freeze within the range of 0 to -160°C, as contrasted with the test results reported in [19],
Fadjy and Richards f 19]
Experiments were performed on specimens moulded from a paste with a water/cement ratio of 0.35, made of high-early strength Portland cement and cured in water. The influence of low temperatures on the behaviour of the hardened paste was investigated by measuring the internal friction and dynamic modulus. The temperature range of the experiments was from -20°C down to
-160°C. The dynamic mechanical measurements in the system comprising paste and water indicated the existence of an internal friction peak and a modulus
transition. This transition occurred in the temperature range of -160 to -60°C and is believed to be associated with a phase change of the adsorbed water. The indications are that all of the adsorbate phase is capable of undergoing this phase change. This low-temperature form of the adsorbate phase substantially increases the rigidity of the hardened paste, and therefore it is reasonable to think of it as a form of ice. For lack of a better name the investigators called this unknown phase the "adsorbate ice". Furthermore, the transition was found to be enhanced by increasing the moisture content and the extent of the internal surface,
2.4 Mechanisms of frost damage
Water-saturated and porous construction materials such as concrete exposed to cyclic temperature changes around 0°C are often damaged. The reasons for this phenomenon have been studied extensively. A comprehensive review of the
literature has been given by Setzer [20]. Furthermore the mechanisms of deterioration involved have been reviewed by various authors [7,21-24].
One can distinguish two main effects according to Rosli and Harnik [24] which could lead to frost damage. The first is the generation of hydraulic pressure and the second is the capillary effect. For the latter one, however. Powers and Helmuth [26] used the name Mils-theory ("Microscopic Ice Lens Segregation") which will be used in this report. A thermodynamic model has recently been developed by Setzer [20] which cannot as a whole be assigned to either of the two above-mentioned effects.
2.4.1 Hydraulic pressure
One of the first investigators who tried to explain the frost damage with the
aid of a model was Powers [25]. He assumed the principal reason for destruction of porous materials by frost to be hydraulic pressure. When the temperature falls to the point where freezing should begin, ice crystals appear in the largest capillary cavities. When the water in the larger cavities starts to change to ice, the volume of water plus ice will exceed the original capacity of the cavity, because the change of water from liquid to ice is accompanied by a 9% increase in volume. Therefore, during the time when the water in the capillaries is changing to ice, the cavity must dilate or the excess water must be expelled from it. In consequence of the growing ice body and the water escaping through the paste toward a (partially) empty void, a pressure is generated. This pressure is the so-called hydraulic pressure.
Factors affecting this pressure include:
a) the coefficient of permeability of the material through which the water is forced;
b) the distance from the capillary to the nearest void boundary; c) the rate at which freezing occurs,
In general, during the process of freezing, hydraulic pressure will exist throughout the paste. This pressure will be higher for points farther from an escape boundary. So it is possible that the pressure will be high enough to stress the surrounding gel beyond its tensile strength, thus producing permanent damage. This model was subsequently improved by Powers and Helmuth [26], Besides the expansion of the water during the process of freezing, the basic principle of the improved model was that the freezing point is depressed as a function of pore size. The smaller the void, the lower the freezing point will be. In consequence, the expansion of the water in the smaller voids will be hindered by the ice formation in the surrounding greater voids, which will lead to the deterioration of the hardened cement paste,
However, it was recognized that the damage is not necessarily connected with the expansion. Similar damage can be produced to stone and consolidated earth by organic liquids which only contract on freezing, Taber reported in [27].
2.4.2 Mils-theory
Ice in the capillaries of frozen cement paste is surrounded by unfrozen water in the gel pores. There is a thermodynamic equilibrium between the gel water and the ice in the capillary at 0°C, assuming that both are under a pressure of one atmosphere. If the temperature drops below the temperature at which the water in the capillary freezes (assumed to be 0°C) the gel water is no longer
in thermodynamic equilibrium with the ice. The free energy of the gel water is higher than that of the ice. The gel water acquires an energy potential enabling it to move into the capillary cavity containing ice. This diffusion of gel water leads to growth of the ice body (ice lenses), Everett [27]. The growth of the ice body in the capillary places the ice and the film around the ice under pressure which results in high stresses in the matrix and leads to micro-cracking. Consequently, the cementstone expands irreversibly.
v.)
2.4.3 A model based on thermodynamics
Setzer [20] explained frost damage with the aid of his model based on thermodynamics. According to this model, freezing begins in larger pores and ice growth propagates into smaller pores filled with non-frozen water. Due to thermodynamic non-equilibrium, water diffuses from small pores towards the ice and freezes there. When the temperature is low enough, two pores with different radius will be filled with ice. However, according to Setzer there still exists an adsorbed water layer between the surface of the ice body and the pore wall. When the temperature is lowered, the thickness of the adsorbed water layer will decrease and the surface force will increase until at approximately -90°C the change of phase from water to ice is completed. The surface forces acting between the adsorbed water and the ice body will give rise to stresses in the cement paste.
A slow cooling rate leads to underpressure in a small pore (Fig. 2.6.a), while a high cooling rate leads to overpressure in the larger pore. Fig. 2.6.b.
Fig. 2.6 Two pores filled with ice and adsorbed water in which a different pressures are acting [20], a=slow cooling rate, b=high cooling rate.
The latter phenomenon has been explained by Setzer in that the water diffusion can only take place by the adsorbed water layer, which becomes thinner at lower temperatures, so that the diffusion is considerably reduced,
High rates of cooling are detrimental because transfer and diffusion of water at the required rate is not possible. The overpressure in the pore results in high stresses in the hardened cement paste and leads to micro-cracking, in consequence of which the hardened paste expands irreversibly.
3. THERMAL DEFORMATION
When reinforced or prestressed concrete made with siliceous aggregates is used, the thermal expansion of the concrete and the reinforcement is almost the same in the temperature range from -20 to +60°C.
Therefore one single coefficient of thermal expansion tt of approximately 10 to 12xl0"^/°C is usually adopted. However, when the application of reinforced concrete is not confined to the temperature range stated above, the ttc of concrete may differ considerably from this value. As a result of the differential thermal deformations high internal restraint stresses would occur and could affect among other features the bond between the reinforcement and the concrete.
Because of the high volumetric proportion of the aggregates (70%) the coefficient of thermal expansion of concrete flCc is dominated by the influence of the aggregates. Based on the results reported in the previous chapter, the moisture content can be expected to be the governing quantity, particularly for water-saturated concrete. Therefore the literature has been perused with a view to finding out which factors will influence the thermal behaviour of the concrete.
3.1 The components of concrete
Concrete itself is a composite material and the different components will influence the overall behaviour. In order to reduce the effects due to the variability of the specific characteristics of materials, various investigators - Wiedemann [7], Rostasy et al. [29], Tognon [17] - decided to perform experiments on the individual components such as: hardened cement paste, mortar, ice and aggregates. In this way it was endeavoured to get a deeper understanding of the observed phenomena.
In the region of low temperatures certain changes may occur which are associated with the anomalous changes of water in small pores and with the phase transition phenomena. These changes can be anticipated when the thermal strains of concrete and its components are studied. The coefficient of thermal strain is physically defined as the differential quotient:
oCph(T) =
d e (T) d T
°C-i (3.1)
thus representing the slope of the tangent at a certain point e(T),
For engineering calculations it is common to define a(T) as a secant modulus
with the origin at +20°C:
a(T) =
e (T) (T-20°C)
OQ-l (3,2)
The latter will be used in this report,
3.1,1 Hardened cement paste
Two types of cements, a rapid-hardening Portland cement (PC35F), and a
slow-hardening blast-furnace slag cement (PSC35L), were included in the German investigations [20,7]. The hydrated cement paste specimens were stored under water till testing. In order to prevent loss of the initial moisture content,
thermal strain (ID''') thermal strain (10'
-200 -150 -100 -50 0 25 -200 -150 -100 -50 0 25 temperature (°C] temperature ( C)
F i g . 3 . 1 Thermal s t r a i n s v e r s u s t e m p e r a t u r e of h y d r a t e d cement p a s t e w i t h P o r t l a n d cement (PC) o r b l a s t - f u r n a c e s l a g cement (PSC) [ 2 9 ] .
the specimens were sealed prior to testing in a sealed wrapping of PE-foil [29]. Fig. 3.1 shows the typical strain-temperature curves of hydrated cement paste. The results indicate three different temperature regions of behaviour. First, at a cooling rate of 2.5°C/min the specimens contract steadily down to a temperature of about -20°C. Secondly, between -20 and -70°C a pronounced expeinsion can be observed, which is defined as transition range. In the third region, below -70°C, the contraction of the specimens is almost proportionate to the temperature. On reversal from cooling to heating, the curves show a shift which does not reflect a material property but is due to the different temperature profiles across the specimen's section.
The expansion in the second temperature range is less pronounced for the Portland cement than for the blast-furnace slag cement. According to Rostasy et al. [29] this is due to the fact that PSC-stone has a higher proportion of small pores, so that its amount of evaporable water exceeds that of the PC-stone. These observations are in agreement with the results of Wiedemann [7], who examined three kinds of PC with different fineness. It was found that the expansion in the transition range increased as the fineness of the cement increased.
3.1.2 Mortar
Fig. 3.2 shows typical strain-temperature curves of mortars [29]. These mortars differ with respect to cement, water/cement ratio and storage condition. Specimens which were oven-dried after the curing time exhibit an almost linear thermal deformation and perfect reversibility. This linear behaviour was also observed for specimens which were stored at 65 and 85% relative htunidity respectively [29,7].
However, when the specimens were stored at a higher relative humidity than 85%, a thermal expansion occurred in the transition range. This thermal expansion increased with increasing water/cement ratio, i.e. evaporable moisture content. Again, the PSC-mortar showed more pronounced expansion.
occured in the second range. Irreversible strains indicate micro-cracking as a result of frost attack, Wiedeman [7,30].
i n t e r n a l
thermal strain (10''^) PC-mc t = 2 . 5 ' >^ ^ dry rtar C/min W/C ^ ^L,
/^ J ^ a t e satura 1 ^ •^water .aturcte ^ ^ t e d \r
1
/i\
1 0 -1 -2 -200 -150 -100 -50 0 25 temperature (°C) thermal strain (10 -200 -150 -100 -50 0 25 temperature ( C)Fig. 3.2 Thermal strains versus temperature of mortar with different cements, water/cement ratios and curing conditions [29],
, thermal strain 110 ) -08 -200 -150 -120 -80 -iO 0 20 /-I temperature (°C) -150 100 -50 0 25 |-v temperature l°C)
Fig. 3.3 The influence of the cooling rate [7] and cooling-heating cycles [29] on the thermal strain.
The thermal deformations are clearly influenced by the rate of cooling, as shown in Fig. 3.3a. The expansion in the transition range increases with the rate of cooling. This phenomenon is due to the fact that the accelerated freezing of water impedes the inner moisture movement. This process of freezing is accompanied by the generation of high hydraulic pressures. It causes an irreversible strain as a sign of internal destruction and increases with the rate of cooling, Wiedemann [7], The behaviour of a water-saturated specimen during several thermal cycles is shown in Fig, 3.3b. The irreversible strain measured at room temperature increases progressively, but the shape of the
thermal strain-curve remains essentially unchanged.
3.1.3 Water
Owing to the fundamental influence of water and ice on the phenomena under consideration it is important to know the thermal contraction of ice. Various investigators, Tognon [17], Wiedemann [7], measured the contraction of ice as a function of temperature, at atmospheric pressure, see Fig. 3.4. When the different thermal contractions of the various components of the concrete are compared, we see that ice has the greatest thermal contraction.
thermal strain (10'^) ^ — ~~"" 1 ice Itap-water] specimen ©iSmm t» Z^C/min ^ / /^' ^~ / / / / / / /
r /
/ / / / ' / / -71 1 1 1 1 LJ 1 -200 -160 -120 -80 -40 0 40 temperature ( CI3.1.4 Aggregates
The coefficient of thermal expansion of concrete oCc is dominated by the
influence of the aggregates, because of their high volumetric proportion. Tests relating to the thermal deformation were performed on several types of aggregate by Tognon [17], The physical properties of four different aggregates are summarized in table 3,1,
Table 3,1 Physical properties of the aggregates [17],
Type of Mineralogical Density Adsorbed water aggregate composition kg/dm^ % by wt granite feldspars amphiboles 3.06 0.75 limestone calcite 2.68 0.16 porphyry fe1dspars, quartz, micas 2.67 0.46 basalt feldspars, augite 2.89 0.30
olivine
thermal strain (10'^)
-180 -160 -1«] -120 -100 -80 -60 -iX) -20 0 20 temperature (°C)
The results of the thermal deformation versus temperature are shown in Fig, 3,5, The curves representing the trend of thermal contraction are parabolic. On account of their very low coefficient of water imbibition, the differences between the linear deformations of specimens cured in a humid and in a dry room are negligible. Miura [31] performed tests on crushed stone (specific gravity 2.86, adsorption 0.78%) and found at -170°C a thermal contraction of approximately 1.25xl0"3. Furthermore, there is perfect reversibility,
3.1.5 C<»iparison of the different components
To compare the actual thermal behaviour of the different components, only the results of the dried specimens have been used. The results are summarized in Table 3.2. It appears that ice possesses the highest coefficient of contraction. According to Tognon [17], this phenomenon is the starting point with regard to the explanation of the marked increase in the compressive strength at low temperatures. The ice within the capillaries forms a mesh of solid veins which completely permeates the concrete. So its formation contraction causes a general compression within the concrete, which increases as the temperature becomes lower. This process has been described as "internal prestressing" [17]; it places the concrete under a multiaxial state of stress, thereby considerably increasing its strength.
Table 3.2 Thermal deformation of various dried components,
Component Thermal deformation at Reference -170°C (10-3)
ice - 5,8 7 hardened paste - 2 . 0 to - 2 , 5 7
mortar (quartz sand) - 1 . 8 to - 2 . 0 7
aggregates: limestone - 0.6 17 porphyry - 1.3 17
Limestone and certain granites which contain hardly any quartz exhibit the absolute lowest value. It is to be expected that siliceous aggregates show the higher absolute values. Porphyry with an average quantity of quartz is situated
in between. It seems that the higher the quantity of quartz in the aggregate, the higher is the coefficient of thermal expansion and thus the thermal deformation. A similar relation between the various aggregates has been found at room temperature [6].
The thermal deformation of the composite material "concrete" will be further dealt with in the next Section,
3.2 Concrete
From the what has been said above it emerges that the thermal expansion of concrete is dominated by the moisture content and the type of aggregate,
Investigations by Rostasy et al, [29] have shown that curves representing the thermal behaviour of concretes qualitatively correspond to the mortar curves, The coarse fraction of the aggregate does not affect the behaviour. So the conclusions with regard to the thermal behaviour of mortar, see Section 3.1.2,
are also valid for concrete in general.
3.2.1 Moisture content
The amount of freezable water stored within the pores depends mainly on the storage condition and the water/cement ratio. The maximum of freezable water is attained if the concrete is saturated with water.
Fig. 3.6a shows the thermal strains of water-saturated concrete specimens with different water/cement ratios according to Wiedemann [7].
The expansion in the transition zone increases with increasing water/cement ratio, see Fig. 3.6b. These results are in good agreement with the observations of Rostasy emd Scheuermann (w/c ratios 0.6,0.8) [32], Planas et al, (w/c ratio 0.45) [33] and Miura [31]. The last-mentioned author examined three water/cement ratios 0.56, 0.46 and 0.36 respectively. It is to be noted that no expansion was observed for the lowest water/cement ratio. Similar behaviour was found by Wiedemann [7], who observed almost negligible expansion
-160 -120 -80 -40 0 20 (a) temperature (°C) 1.0 0.8 0.6 0.4 02 nri expansion 110-^1 PC-concrele W/C 1 water-SQturated age 90 days • ; / / • / / / /
y
. / / //' z:xL
• / temperat ure1
T/ / / hermol strain 1 /.20°C j 03®
0.4 05 06 0.7 08 water/cement ratioFig. 3.6 a) Thermal strains of water-saturated concrete as a function of
temperature and water/cement ratio [7].
b) Thermal expansion versus water/cement ratio [7],
for a concrete with a water/cement ratio of 0.39, see Fig. 3.6b.
As contrasted with the results stated above, research by Kasami et al. [34], who examined the influence of three w/c ratios 0.5, 0.6 and 0,7 respectively, did not reveal any expansion in the transition zone. Only the increase of the thermal deformation becomes steadily smaller in the temperature range from -30 to -70°C,
If concrete is subjected to a number of thermal cycles, an irreversible possitive strain will generally be observed after reheating to room temperature. However, investigations [30] have shown that the irreversible strains vanish if the water/cement ratio is smaller than 0.45 and if hydration has been completed. This statement is valid for concrete in the vicinity of a tank wall liner, so the reduction of the initial water content by drying is entirely prevented. Higher water/cement ratios do not lead to frost damage if the excess water can diffuse by drying beforehand in the construction phase. However, this weight loss by drying is fairly limited and will only take place at the drying face of a thick concrete member. The influence of weight loss by drying prior to cooling on the thermal behaviour was clearly demonstrated by experiments reported by Wiedemann et al. [7,35]. Fig, 3,7 shows the thermal
deformation of concrete specimens with a constant water/cement ratio. These were moist-cured in climates of varying relative humidities until moisture equilibrium was attained. It appears from Fig, 3,7 that, starting with moisture contents corresponding to 86% relative humidity and higher, expansion occurs which is more pronounced for blast-furnace slag cement (PSC) than for Portland
cement (PC), 1.2 08 0.4 0 -0.4 -0.8 -12 -16 -jn thermal strain (10'-') PC-concrete w/c roto Ö5i ^ X / ^
X
r h in Vo J l o o / ^ ^^ • '-A
'Y/
' ' 7 9 _105 C Or ^y\
/;?
<y
<^\ ^-^y //r
A
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-200 -160 -120 -80 -40 0 20 temperature I 01 1.2 0.8 04 -04 -06 -12 -1.6 -20 ttiermal strain (10' 1 PSC-con W/C ro y y / y ^ Crete io ast , /y
/ / ^ r.h in %H
V
\ _ a i ,^ 105°C d /h
•"^1 \H
^A
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/• Vn
A
-200 -160 -120 -80 -40 0 20 temperature ( CIFig, 3,7 Thermal strains of concrete with different types of cement and variable equilibrium moisture content versus temperature [7],
Below 86% relative humidity only contraction occurred. The specimens with a lower content of freezable water are able to accommodate the expansion of ice without rise of pressure. However, it is to be noted that the equilibrium moisture content in a tank wall at the liner corresponds to 97% relative humidity and higher. In order to represent such in-situ conditions, various investigators [32,36,37] examined the thermal behaviour of concrete cured in a sealed condition. This involved storing specimens in moisture-tight bags,
the only water available for hydration being that which was added at mixing. None of the above-mentioned investigators found any expansion in the transition range, only a smaller amount of contraction was observed within this reinge than either above or below it. In general, a curve similar to that in Fig. 3.7 at 90% r.h. was observed, giving a thermal strain of approximately 1.5 to 1.6% at -170°C.
3.2.2 Aggregate
All the results reported so far correspond to concrete with mainly siliceous aggregates. Various investigators have studied the thermal behaviour of concrete, made with different types of aggregates. It appears from the results reported by Tognon [17] that for dried concrete the type of aggregate greatly influences the thermal deformation. Concrete with calcareous aggregate exhibited the lowest, and concrete with siliceous aggregate [8] the highest thermal deformation. The thermal deformation of concrete made with porphyry or with basalt aggregate is situated in between,
Not all the investigators studied the thermal behaviour of oven-dry concrete, Many of them did experiments on sealed concrete in order to simulate in-situ conditions. Concrete made with crushed granite aggregate investigated by Elices
[38] exhibits greater thermal deformation than the oven-dry specimens studied by Tognon [17],
The influence of a lightweight aggregate on the thermal behaviour of sealed concrete was investigated by, among others, Monfore and Lentz [36], Bamforth et al, [37] and Berner et al, [39], Tests were performed on concrete made with a lightweight expanded shale aggregate [36], This concrete showed an expansion as the temperature was reduced from -18 to -59°C,
At -157°C a contraction of 0.11% was measured. The expansion in the transition range was not observed by Bamforth et al. [37], who found a contraction of 0.12% at -160°C. However, no data was reported concerning the used aggregate. Almost the same amount of contraction was observed by Berner et al, [39], who carried out experiments on air-dry (50% r.h.) concrete made with a lightweight expanded shale aggregate. During cooling no expansion was observed. Some results of the thermal strains of concrete are summarized in Table 3.3.
Table 3.3 Thermal strain of concrete made with different aggregates.
Aggregate Curing condition Reference thermal strain
at -170°C(10-3) siliceous siliceous calcareous porphyry expanded shale expanded shale oven-dry sealed oven-dry oven-dry sealed air-dry 50% r.h, 7 32 17 17 36 39 -1,7 to -1,6 to -0,5 to -1,2 to -1.1 to -1.2 to -1.8 -1.7 -0.6 -1.3 -1.2 -1,3
3.2.3 Coefficient of thermal expansion
With the aid of the results from the preceding Section it is possible to
calculate the coefficient of thermal expansion oc(T) as defined in formula 3,2, which is commonly used in engineering practice. In order to quantify ac(T),
concrete is divided into two main classes. The first comprises concrete with moisture contents corresponding to storage at 86% relative humidity and ranging down to oven-dry concrete. It seems that even sealed PC-concrete belongs to this class. The second class comprises concrete stored at above 86% relative humidity to equilibrium and ranging up to water-saturated concrete. In the first class, changes in moisture content have virtually no effect on the thermal deformation. Thus the type of aggregate is the governing quantity. Fig. 3,8 shows the thermal expansion coefficient as a function of aggregate versus temperature. Concrete with siliceous aggregates corresponds to the upper region of the band, and concrete with calcareous aggregate to the bottom region, while basalt, porphyry and some granites are in the middle region.
Furthermore, the moisture content has a minor influence in the fixed range. In general, the very dry concrete has a somewhat higher ac(T).
For the sake of comparison the thermal coefficient of expansion of the reinforcement reported in [40,38] is given in Fig. 3.8. This coefficient shows a small scatter. It appears from the diagram that if concrete is made with siliceous aggregate, such as river gravel commonly used in Western-Europe, only limited stresses will be introduced in a reinforced concrete member.
coefficient of thermal expansion (10 m/m C)
-180 -160 -UD -120 -100 -80 -60 -10 -20 0 20 temperature (°C)
Fig. 3.8 Coefficient of thermal expansion for dry to moderately moist concrete and for reinforcement [7],
.coefficient of linear thermal expansion (10'^/°C)
-180 -160 -KO -120 -100 -80 -60 -LO -20 0 20 temperature ( C)
Fig. 3.9 Coefficient of thermal expansion for concrete of high moisture content [7].
Concrete stored at above 86% relative humidity to equilibrium shows an expansion in the transition range which greatly depends on the moisture content. Concrete with a medium moisture content (w/c ratio < 0.50) always shows a positive value of ac(T), see Fig. 3.9, Wiedemann [7]. However, an increase in moisture content leads to negative values of ac(T). When moist concrete is reheated, <tc(T) shows a hysteresis. These curves were derived from test results [7] for concrete made with siliceous aggregates and Portland cement and blast-furnace slag cement.
3.2.4 Influence of the stress level
It is important to know the thermal deformation of concrete when reinforced or prestressed concrete structures are subjected to large thermal differences. Reinforcing and prestressing steel exhibit an almost linear thermal deformation. Water-saturated concrete shows an expansion in the transition range, however, so a totally different behaviour occurs. Therefore internal stresses will be introduced in the structure. It is to be expected that during cooling high stresses will develop in the reinforcement, while the concrete will only attain a low compressive stress level. Planas et al. [33] performed tests on prestressed concrete elements to measure such (high) internal stresses. The expected high stress levels were not measured, however. So Planas et al. suspected a different thermal behaviour of the concrete when cooled under compressive loads. In order to take account of the effect of the stress level on the thermal behaviour of concrete three different tests were performed. The thermal deformation of water-saturated concrete was measured on unloaded and loaded specimens and on prestressed beams. The compressive stress of 15 N/ram^ in both cases was applied at room temperature euid remained constant until creep and/or initial prestress losses were negligible. Once the load was stabilized, cooling at a rate of 0.25°C/min started, and the thermal measurements were simultaneously recorded. As can be seen in Fig. 3.10, the thermal strain of the unloaded specimen exhibits the well-known behaviour, as has been discussed in the preceding Section. It appears that the behaviour of the loaded specimens differs significantly from that of the unloaded specimens but is similar to that of the prestressed concrete elements.
thermal strain (ID''')
ll60 -UO -120 -100 -80 -60 -^0 -20 0 20 temperature (°C)
Fig. 3.10 Thermal strain of water-saturated concrete vinloaded, loaded and prestressed [33],
First of all, after a temperature cycle an irreversible compressive strain is observed which is in contrast with the irreversible thermal expansion for the unloaded specimens. This is in agreement with the assumption that pore pressure generated during water freezing causes microcracking and that the stiffness is thus reduced. In consequence, the compressive strain is increased after thermal cycling, this increase being called irreversible compressive strain. Secondly, in the transition range no expansion but a small hump was observed in the curve for the loaded specimens. This behaviour was explained qualitatively according to Planas et al, [33] by assuming that during the generation of pore pressure microcracks were developed anisotropically. Initially, propagation of pre-existing microcracks oriented transversely to the load axis is prevented by
the compressive stress. Only, microcracks oriented parallel to the load axis are free to propagate. So the main effect of the increase in water volume during the phase change is a transverse expansion. When the pore pressure reaches a value high enough to overcome the effect of the external load, microcracks transversely to the load axis open first and subsequently
propagate, resulting in a small axial expansion (hump). The starting point for this explanation is the assumption that microcracking is a dominant deformation mechanism in the transition range.
Comparable tests on unloaded and loaded sealed specimens were performed on a lightweight and a normalweight concrete by Bamforth et al. [37]. The specimens were loaded at room temperature to a constant stresslevel of 12.5 N/mm^. The
results of the unloaded and loaded specimens are shown in Fig. 3.11. The contraction was marginally higher under the loaded condition, the difference being most pronounced for the lightweight concrete. According to Bamforth et al. [37], this increased strain was attributable to creep during the 4-day-period of cooldown which was controlled at a rate of 2°C per hour. It should be noted, however, that creep deformation is greatly reduced in such a cold environment [41],
-160 -^L0 -120 -100 -80 -60 -LO -20 0 20 temperature (°C)
Fig. 3.11 Thermal contraction of normalweight and lightweight sealed concrete loaded or unloaded [37].
From the results mentioned-above it appears that water-saturated concrete loaded to a stress level of 15 N/mm^ did not exhibit an expansion in the transition reinge. It is to be expected that there will be an expansion for water-saturated concrete loaded to a lower stress level. This expectation is in good agreement with the observations of Rostasy and Scheuermann [32] who
performed experiments on concrete members 1 m in length and reinforced centrically with a steel bar. Because steel contracts more than concrete on cooling, the steel will be subjected to tension and the concrete to compression. During cooling, restraint stresses in the steel and concrete up to +200 N/mm2 and -2.5 N/mm^ respectively were measured in water-saturated concrete. For sealed concrete the concrete restraint stresses were negligible and the steel stresses attained values of 20 to 60 N/mm^. After the tests the actual thermal deformation of the concrete member was checked. The actual expansion was found to be less pronounced than the observed results of unloaded cylindrical specimens stored under the same conditions. Probably due to variations in moisture content a different behaviour was observed, but it is also possible that the stress level had influenced the thermal behaviour,
The thermal behaviour of water-saturated concrete probably depends on the temperature and the stress level till a critical level is reached. Above this stress level even water-saturated concrete exhibits an almost linear thermal behaviour,
3.3 Model according to Wiedemann
Wiedemann [7] tried to explain the thermal behaviour of water-saturated concrete with the aid of a pore model representing the cement matrix. Because the model gives insight into the thermal behaviour and the compressive stress-strain relation, it is summarized below. During cooling and reheating nine different temperature ranges are distinguished, see Fig, 3.12.
1. temperature range: 20 to 0°C
All the pores are filled with water and only contraction occurs during cooling.
2. temperature range: 0 to -20°C
The water in the larger pores starts to change into ice, and the excess water will escape to pores partially filled or to air cavities. With a fall in temperature, ice which first filled the largest pores completely will contract more than the matrix (ttice > cCmatrix) so that space will become available in the pore.
For thermodynamic reasons non-equilibrium water diffuses from small pores towards the ice in the larger pores and freezes there. In this temperature range mainly contraction occurs, because the ice bodies will not stress the pore walls considerably. The compressive strength and the ultimate strain show a marked increase, because ice almost completely fills the larger pores.
3. temperature range: -20 to -60°C
The larger pores are completely filled with ice and block the expansion of the water in the smaller pores. In consequence, high stresses and cracks are generated. The concrete will expand greatly while the compressive strength will
increase to a lesser extent as a result of the increasing number of internal cracks. The ultimate compressive strain of the stress-strain curves reaches its maximum value. -170 -90 -60 -20 0 20 temperature (°C)
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Fig. 3.12 Thermal strain, compressive strength and the ultimate strain versus temperature [7].
4. temperature range: -60 to -90°C
The ice contracts more than the matrix when the temperature is lowered. Consequently, the inner stresses will be very greatly reduced. Ice which almost completely filled the pores is able to support part of the forces acting on the concrete. Furthermore the water in the very small pores freezes. However, during the phase change there is enough space available in the form of existing cracks, so stresses will hardly develop. Because of the different coefficient of expansion the matrix will shrink round the aggregates and thus prestress the interface between aggregate and matrix. As a result, crack propagation, which starts at the interface between the matrix and the aggregates at room temperature, will be postponed. For these reasons the compressive strength will increase again and the concrete will exhibit contraction only.
5. temperature range: -90 to -170°C
In this temperature range two opposite phenomena are acting. First, the pores are now partially filled with ice and, secondly, the matrix is still prestressing the aggregates. As a result, the compressive strength will increase to a lesser extent, as stated in range 4. Because the ice volume decreases more than the pore volume with a fall in temperature, the ultimate strain will reach lower values. At these very low temperatures concrete behaves in a linear elastic manner and tends to embrittlement,
6. temperature range: -170 to -60°C
During reheating, the concrete expands similarly to oven-dry concrete and the pores are completely filled with ice again, because of the larger thermal deformation of the ice with respect to the deformation of the concrete,
7. temperature range: -60 to -20°C
During the second temperature range water has been diffused to the larger pores, where it freezes. On reheating, the volume of water in the pore exceeds the pore volume. This is probably the main reason why concrete expands more on reheating than it does on cooling.
8. temperature range: -20 to 0°C
All the ice changes into water and the internal stresses are reduced, so that contraction will take place.
9. temperature range: 0 to +20°C Concrete behaves like dried concrete.
3.4 Conclusions
From the what has been said above, the following conclusions can be drawn with
regard to the thermal behaviour:
- The different components of concrete are characterized by thermal deformation at -170°C, which is smallest for ice and which is greater for hardened cement paste, mortar and aggregates, in that order.
- Concrete with a moisture content corresponding to a relative hiamidity of 86% or less exhibits a parabolic thermal contraction curve during cooling. Furthermore, there is perfect reversibility during reheating. The thermal contraction is dominated by the type of aggregate. Concrete made with limestone, siliceous aggregate or porphyry exhibits the least, the most and medium contraction respectively.
- Concrete stored at above 86% relative humidity to equilibrium will, on cooling from about -20 to -60°C, undergo expansion which greatly depends on the moisture content. When moist concrete is reheated to room temperature an irreversible positive strain will generally be observed as a sign of internal destruction. This positive strain increases with the rate of cooling,
- If water-saturated concrete is loaded to a stress level of about 12.5 N/mm^, no expansion is observed during cooling, the thermal behaviour being almost
linear is observed. However, for lower stress levels in the range of 1 to 4 N/mm2 an expansion is still observed during cooling. The thermal expansion observed during cooling probably depends on the moisture content and on the stress level till a critical level is reached. Above this stress level even water-saturated concrete exhibits almost linear thermal behaviour.
4. MECHANICAL PROPERTIES OF CONCRETE
Various authors [1,37,40-48] have surveyed literature concerning, among other features, the mechanical properties of concrete between ambient and cryogenic temperature. Some of the conclusions presented in the above-mentioned literature will be checked and/or discussed in detail.
4.1 Compressive strength
Investigations have shown that in general the compressive strength increases as the temperature is lower. This increase will be greater in concretes with higher moisture contents.
According to Browne and Bamforth [45], the increase in strength associated with a lowering of the temperature is primarily a function of the free moisture content of the concrete, regardless of concrete grade, aggregate type (dense or lightweight), degree of air entrainment, type of cement (ordinary Portland cement, fly-ash, blast-furnace slag), or whether or not admixtures are used.
4.1.1 Initial strength, age, mix proportion
Goto and Miura [49,50] performed experiments on (J100x200mm cylindrical specimens for 11 different curing conditions at ages ranging from 7 to 42 days. The concrete mixes with three w/c ratios (0.45, 0.50 and 0.55) and six different cement contents (325-420 kg/m^) were made with ordinary and with high-early strength Portland cement. For some mixes an admixture or an air-entrainment was used. Basing themselves on the results of the experimental research, the investigators concluded that with a lowering of the temperature the concrete strength increase is hardly affected by its strength at room temperature, i.e. mix proportions, curing method, or age of the concrete. It appears that the strength increase is directly proportional to the free moisture content of the concrete. For this reason the mean compressive strength
at low temperatures may be expressed as:
in which the strength increase /lfcm(T;m) depends on temperature and moisture content only. From a regression analysis of the test results a formula has been found for the compressive strength increase ^llfcm (T;m):
/ T+180 2\ Jfcm(T;m) = ( l - ( ) 12m N/mm2 (4.1A) \ 180 I for T > -120°C and 2lfcin(T;m) = 10.7 m N/nm2 (4. IB) for T < -120°C
in which m = moisture content % (by wt) T = temperature °C
The measured moisture content varied between 2,9 and 7,5%.
Test results reported by Okada and Iguro [51] indicate that the compressive strength increase is independent of the initial strength and the age of the concrete. Experiments were performed on 4>100x200mm cylinders and 6 different mix proportions with a w/c ratio ranging from 0.38 to 0.63 and an initial strength from 26 to 52 N/mm2 at an age of 28 days. Probably because only water-saturated concrete was investigated and the water content was in the
range of 144 to 159 kg/m^, no influence of the free moisture content and even of the w/c ratio on the strength increase was found. To predict the increase in the compressive strength of water-saturated concrete the investigators derived an expression which is a function of the temperature only:
^fcm(T)=5.4-0.86T-0.00276T2 N/mm2 (4.2)
with -10°C > T > -100°C
The influence of the cement type on the compressive strength was investigated by Tognon [17] for two types of pozzolanic cement and a high strength Portland cement. The mixes contained of 300 kg/m^ cement and had a w/c ratio of 0.5. After curing for six months in a fog room the 100 mm^ cubes were tested at
three temperatures: +10, -78 and -196°C respectively. The investigator found that the increase in strength was not influenced by the type of cement. Because the initial strength varied between 32 and 52 N/mm2, it can also be concluded that no influence of the initial strength on the increase in strength was found.
A set of concrete cubes were cured for 3 years in a fog room. These were made with calcareous aggregate and a high early strength Portland cement
(450 kg/m^). Before testing, half the number of the specimens were dried in a stove at 110°C. The results of the moist Specimens are used in Fig. 4.1, in which the results of various investigators regarding the increase of the
compressive strength of mostly water-saturated concrete are compared.
The influence of the mix proportion on the increase in strength of water-saturated concrete was also investigated by Yamane et al. [52]. Four different mix proportions with w/c ratios ranging from 0.70 to 0.50 and cement contents of 236, 227, 280 and 353 kg/m^ respectively were used. The $100x200mm cylinders were cured in water for 19 weeks. The investigated temperature ranged between +20 and -70°C, The increase in compressive strength was found to be greater for the concretes with the higher w/c ratios. Furthermore no influence of the air content ranging from 1.7 to 6.7% was observed. The first conclusion
Table 4.1 Siammary of experiments discussed.
Symbol Investigator(s) fc(20°C) Age Curing Specimen w/c ratio N/mm2 weeks conditions
• • • Wiedeman [7] 21-38 13-26 saturated $80 x 160mm 0,50-0.70
O A D Yamana et al. [52] 24-45 19 saturated (tl00x200mm 0.50-0.70
Okada et al, [51] 26-52 4 saturated (I)100x200mm 0,38-0.63
• Tognon [17] 73 156 fog room cubes lOOram^ 0.50
is in good agreement with the experimental results of Wiedememn [7, page 103]. Concrete made with three different w/c ratios 0.5, 0.6 and 0.7 respectively and with a constant cement content of approximately 350 kg/m^ showed a greater
increase for higher w/c ratios.
Table 4.1 gives a summary of the experiments discussed so far. Some actual results are shown in Fig. 4.1. Tognon's [17] results were beyond the average values; however no size effect of the tested cubes was taken into account. The formulae of Goto/Miura (4.1A,B) and Okada/Iguro (4.2) predict the results fairly well for w/c ratios smaller than 0.6, which are of practical interest.
Afc increase of compressive strength (N/mm^) 120 100 80 60 LO 20 -200 -180 -160 -UO -120 -100 -80 -60 -^0 -20 0 20 temperature ( C)
Fig. 4.1 The increase in compressive strength of water-saturated concrete, • ^ G o t o / Miura m • É = 7 5 % 1 V r J ' ^ Okada/Iguro k water-saturated w/c ratio : 0.5 0.6 Q7 Wiedemann • • • L: Yamane et al O A D iTognon • T 1
l?'^
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i
However, the Okada/Iguro formula is confined to water-saturated concrete only, so the formula of Goto and Miura, which is generally applicable, is to be preferred. In addition to the mix proportions discussed before, Vandewalle [53] and Wiedemann [8] found that concrete made with blast-furnace slag cement shows an increase in compressive strength similar to that of concrete made with Portland cement. So in general the initial strength, age and mix proportions will not influence the increase in strength associated with a lowering of temperature. Only the free moisture content turns out to be the governing quantity. This is the subject of the next Section,
