Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement

Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement Advanced concrete technology7 elasticity, shrinkage, creep and thermal movement

Elasticity, sh ri n kage, creep and thermal movement Jeff Brooks The main learning objectives of this chapter are to explain and describe the following features appertaining to concrete: • • • • • • Principal causes and factors affecting elastic, creep, shrinkage and thermal movements Typical ranges of strain occurring in practice Mechanisms of shrinkage and creep Measurement of shrinkage and creep Effects of movements on concrete in service Practical prediction of movements Although the elastic and thermal deformation behaviour of concrete have been known for some time, it is only relatively recently that the importance of creep and drying shrinkage have been recognized It was probably at the beginning of the twentieth century when Hatt (1907) first reported increased, non-elastic deflections of reinforced concrete beams 7/2 Elasticity,shrinkage, creep and thermal movement under a sustained load Since that time, there have been many hundreds of research publications and design documents, dealing with the subject, such as ACI (1973), BS 1881: Part (1985), CEB-FIP (1990) and RILEM (1995) When concrete is subjected to external stress, there is an initial (elastic) strain followed by a slow time-dependent increase in strain (creep) There can be other time-dependent moisture movement strains that are not associated with external stress For example, drying shrinkage occurs in most structural elements stored at usual temperature and relative humidity To calculate the deformation and deflection of structural members in order to check their serviceability, we need to know the relation between stress and strain Too much long-term deflection or cracking due to induced tensile stress should be avoided in order to provide adequate durability Although this chapter concentrates on creep and drying shrinkage, there are other types of movement that contribute to the total deformation or stress induced by restraint to movement Thermal movement can be significant on a daily as well as a seasonal basis It is equal to the product of the coefficient of thermal expansion (approx 10 x 10-6 per °C) and the change in temperature in °C Autogenous shrinkage is small for normal strength concrete but not for high-strength or high-performance concrete Swelling occurs for saturated concrete and can be significant for lightweight concrete The definition of pure elasticity is that strains appear and disappear immediately on application and removal of load Examples of materials behaving in that manner are steel (linear) and timber (non-linear) Other materials behave in a non-elastic manner, e.g glass (linear) and concrete (non-linear) It should be emphasized the concrete only behaves that way when it is young or loaded for the first time; as seen in Figure 7.1, there are possible ways of obtaining a modulus of elasticity The shape of the stress-strain curve depends to some extent on the rate of application of stress, application of load quickly reducing the curvature The deviation from linearity is also due to microcracking at the interface of aggregate and cement paste (transition zone) Because of these effects, the distinction between elasticity and creep is not clearly defined and, for practical purposes, the deformation during application is considered elastic and the subsequent increases are regarded as creep The slope of the stress-strain curve at the stress considered is the secant modulus of elasticity For estimating the total deformation in design calculations, the static modulus of elasticity is often used as an approximation to the secant modulus, its method of determination being specified in BS 1881: Part 121: 1983 Here, the effects of creep are reduced by loading the specimen three times, the static modulus being determined from the slope of the now-linear stress-strain curve Generally, the stronger the concrete the greater the static modulus of elasticity However, it is usual to estimate the modulus from one of the several empirical relationships between static modulus (Ec in GPa) and compressive strength (fcu in MPa), e.g BS 8110: Part 2: 1985: Ec = 9.1fc°u"33 for normal weight concrete of density _=_2400 kg/m 3, and (7.1) Elasticity, shrinkage, creep and thermal movement Ec = 1.7p2fc°u33 x 10 -6 (7.2) for lightweight concrete of density (p) between 1400-2400 kg/m Other countries use different expressions that are based on cylinder strength, which is approximately 0.8 x cube strength for normal strength concrete For estimating the strains at very low stresses, the dynamic modulus of elasticity is used, as determined by the method in BS 1881: Part 5:1970 The dynamic modulus corresponds to the initial tangent modulus in Figure 7.1, and Neville (1995) quotes empirical relationships that exist between the static and dynamic moduli of elasticity Ascending curve Permanent strain ' '~"/" curve ~ \ Secant modulus of elasticity Strain Figure 7.1 Magnified stress-strain curve for concrete loaded for the first time 7.4.1 Structure of cement paste Before discussing shrinkage and creep, it is pertinent to outline the structure of the 'seat' of those long-term movements and, in particular the role of water Figure 7.2 shows the components from concrete observed at the engineering level to the C - S - H at the submicroscopic level Concrete is described as a multi-phase composite material consisting of coarse aggregate particles embedded in a matrix of mortar, the mortar consisting of grains of unhydrated cement embedded in a matrix of the products of hydration of cement These products are a cement gel or C-S-H, with a system of water-filled or empty capillary pores At the submicoscopic level, the C - S - H is a mixture of mostly crumpled sheets and foils, which form a continuous matrix with the water-filled interstitial voids (gel pores) The C - S - H sheets have a thickness of about Nm and the gel pores have a diameter between and Nm, which means that only a few molecules of water can be absorbed on a solid surface Gel pores occupy about 28 per cent of the total volume of the cement gel or C-S-H, and are much smaller than capillary pores (10 -3 mm) Water is held in the hydrated cement paste in varying roles At one extreme there is free water, which is beyond the surface forces of the paste while, at the other extreme, 7/3 7/4 Elasticity,shrinkage, creep and thermal movement Concrete Engineering level Coarse aggrega e Mortar Fine aggregate Ceme~nt paste Unhydrated cement Cement hydration products Microscopic level Cement gel, C-S-H Submicroscopic level Crumpled sheets and foils (dia = 0-4 Nm) Water-filled or empty capillary pores (dia.= 10~ mm) Water-filled gel pores (thickness = Nm) Figure 7.2 Structure of hardened cement paste there is chemically combined water forming a definitive part of the hydrated compounds Between these two extremes there is gel water consisting of adsorbed water held by the surface (van Der Waals) forces of the gel particles, interlayer water (zeolitic water), which is held between the C - S - H sheets, and lattice water, which is water of crystallization not chemically combined The different types of water are difficult to determine quantitatively and in practice water is divided into evaporable water, as determined by the loss in weight on heating to 105°C and non-evaporable water The latter is deduced from the original water content but, if unknown, it can be determined by the weight loss on heating to 1000°C The evaporable water includes the free water and some of the more loosely-held adsorbed water 7.4.2 Mechanism of shrinkage In a drying environment where a relative humidity gradient exists between the concrete and surrounding air, moisture (free water) is initially lost from the larger capillaries and little or no change in volume or shrinkage occurs However, this creates an internal humidity gradient so that to maintain hygral equilibrium adsorbed water is transferred from the gel pores and, in turn, interlayer water, may be transferred to the larger capillaries The process results in a reduction in volume of the C - S - H caused by induced balancing compression in the C - S - H solid skeleton by the capillary tension set up by the increasing curvature of the capillary menisci This is known as the capillary tension theory At lower relative humidity, the change in surface energy of the C - S - H as firmly held adsorbed Elasticity, shrinkage, creep and thermal movement water molecules are removed is thought to be responsible for the reduction in volume or shrinkage Another theory is that of disjoining pressure, which occurs in areas of hindered adsorption (interlayer water); removal of this water causes a reduction in pressure and, hence, a reduction in volume The foregoing theories apply to reversible behaviour and shrinkage is not fully reversible, probably because additional bonds are formed during the process of drying Moreover, carbonation shrinkage can occur, which prevents ingress of water on re-wetting This chapter is mainly concerned with drying shrinkage, namely, shrinkage resulting from the loss of water from the concrete to the outside environment It should be mentioned that plastic shrinkage occurs before setting and can be prevented by eliminating evaporation after casting the concrete Like drying shrinkage, autogenous shrinkage occurs after setting It is determined in sealed concrete and is caused by the internal consumption of water by hydration of cement, the products of which occupy less volume than the sum of the original water and unhydrated cement In normal strength concrete, autogenous shrinkage is small (