Shrinkage and creep

Although there are some anomalies in the literature, it is generally agreed that both types of volume deformation are a function of the same fundamental mechanism and that the influence of other factors such as admixtures will affect both shrinkage and creep in a similar manner. As outlined earlier, water-reducing admixtures can be used to obtain different effects on the plastic/hardened concrete and it is this factor, together with the admixture type, that is important in determining the effect on the volume deformations of concrete. 

Direct addition of water-reducing admixture This increases the workability of the concrete. The effect of all types of water-reducing admixture under these conditions is invariably to increase the shrinkage and creep of the concrete. Some typical values are shown in Table 1.31. The considerable increases in both shrinkage and creep in the presence of admixtures containing calcium chloride and triethanolamine are clearly illustrated. 

Addition of admixture to obtain higher strengths In this situation the small amount of reported work indicates that where the water-cement ratio is reduced, the shrinkage and creep of the concrete is also reduced. Figure 1.50 shows the effect of a hydroxycarboxylic acid plasticizer on the creep of the concrete where the material has been used to effect a reduction in the water- cement ratio without any other changes in the mix design. Thus it seems that the reduction of the water-cement ratio will compensate for the increases in creep observed in the data above. 

Most of the studies in this area have indicated that superplasticized concrete has shrinkage and creep characteristics similar to plain concrete. Figure 2.21 illustrates the shrinkage results of plain and superplasticized concrete [66], while Table 2.6 shows similar results for creep [67]. 

Creep cannot be observed directly and can only be determined by deducting elastic strain and shrinkage strain from the total deformation. Although shrinkage and creep are not independent phenomena, it can be assumed that superposition of strains is valid, hence ... 

Calcium chloride increases both drying shrinkage and creep. 

Interlayer water. Concrete loses the water retained between the C-S-H layers if the external humidity falls below 11 %. This water does not matter as far as corrosion of reinforced is concerned, since gel pores are too small to allow transport processes at any appreciable rate. It does influence shrinkage and creep. 

Aggregates are natural or artificial materials with particles of size and shape suitable for the production of concrete. Normally, aggregates occupy a high fraction of the volume of concrete (60-85 %). They allow a reduction in the amount of cement paste (and thus a reduction in both the cost of concrete and the consequences of heat of hydration, drying shrinkage and creep) and they contribute to the mechanical properties of concrete (compressive strength, elastic modulus, wear resistance, etc.). 

Mainly used in TPs, mica reinforcement improves the tensile and flexural strength and flexural modulus. Heat distortion temperature is increased and the coefficient of linear thermal expansion is reduced. Shrinkage and creep are significantly reduced, and warpage is virtually eliminated. Chemical resistance is high and permeability is reduced. Mica can also help to produce a Class A surface finish (Table 2.11). 

Until now, much research work has been done on the prediction of composite material coefficient of thermal expansion and elastic modulus by forefathers, and many prediction methods have been developed such as the sparse method (Guanhn Shen, et al. 2006), the Self-Consistent Method (Hill R.A. 1965), the Mori-Tanaka method (Mori T, Tanaka K. 1973) and so on. However, none of these formulas take into account the parameters variation with concrete age, and there is little research on the autogenous shrinkage and creep. In the mesoscopic simulation of concrete, thermal and mechanical parameters of mortar and aggregate (coefficient of thermal expansion, autogenous shrinkage, elasticity modulus, creep, strength) are important input parameters. In fact, there is abundant of test data on concrete, but much less data on mortar while it is one of the important components. Also parameter inversion is an essential method to obtain the data, but there are few studies on this so far. 

Shrinkage and Creep Results of several drying shrinkage tests on concretes with and without water reducing admixtures indicated that these admixtures have little or no detrimental effect [23, 28] although direct addition of water reducing admixtures to increase the workability increases the shrinkage [26]. A similar discussion holds true for... 

It is generally agreed that the shrinkage and creep of admixture-incorporated concretes follow the same mechanism, obey the same rules and are affected by the same parameters as the concretes without admixtures. 

Issues with material shrinkage and creeping Fiber print through Drying of material quality and time critical... 

The gel pores form a part of CSH (calcium silicate hydrate), and may be classified as micro pores or meso pores. The principal difference between gel and capillary pores is that the former are too small to be filled by the hydration products and for capillary effects, it means that no menisci are formed. The gel pores occupy between 40% and 55 % of total pore volume, but they are not active in water permeability through cement paste and they do not influence the composite strength. Water in the gel pores is physically bonded. It is believed that gel pores are directly related to shrinkage and creep properties of the cement paste. 

According to the generally accepted hypothesis, shrinkage and creep are independent and additive. It means that shrinkage is the time-dependent deformation in an unloaded concrete at constant temperature and creep is defined as the change of deformation since the application of load, corrected for shrinkage, and is related to the stress state and its intensity. Consequently,... 

The test results and models adopted for shrinkage and creep were verified again when new kinds of cement-based composites (high performance... 

A prediction of the final values of shrinkage and creep is needed to determine when a state of certain stabilization is achieved. However, perfect stabilization is possible only in artificial conditions created in a laboratory, because in the natural environment the hygrothermal conditions vary and a flow of heat and moisture to and from hardened cement paste is continued indefinitely. Predictions may also concern the development in time and the rate of both processes. 

Various sets of data may be considered as bases for prediction, but all are not always available mixture composition, hygrothermal conditions of cure and during service life, values characterizing shrinkage and creep as measured over a short period of time, etc. Furthermore, most of the data are subject to stochastic distribution and are random variables. 

The reliable formulae and extensive test results for polymer and fibre-reinforced composites are scarce. Simple relations for the prediction for shrinkage and creep of ordinary concretes for application in structural design were given already in CEB-FIP Model Code (1990) with coefficients to allow for actual conditions. Later, so-called Model B3 was recommended by ACI (1999) and extensively discussed by Bazant and Baweja (1995). A sHghtly different approach was proposed in EN 1992 (2004) and detailed comments may be found in Vandewalle (2000). 

In hardened cement-based composites the transportation of liquids and gases through pore and microcrack systems plays a very important role in many processes, such as hydration of Portland cement, pozzolane effects of microfillers, carbonation, corrosion of cement paste and reinforcement due to reaction with external agents, shrinkage and creep, etc. These processes are partly described in respective Sections 4.1, 4.3, 6.5 and 11.5. Only basic information is reiterated below concerning the flow of liquids and gases through concretes and mortars. 

Swamy, R. N., Theodorakopoulos, D. D., Stavrides, H. (1977) Shrinkage and creep characteristics of glass fibre reinforced cement composites, in Proc. Int. Congress on Glass Fibre Reinforced Cement, Brighton pp. 75-96. 

ACI 209.1R-05 Guide to Factors Affecting Shrinkage and Creep of Hardened Concrete. 

The first requirement imposed on high performance concretes (HPC) of any kind is their good workability, and this property should be obtained with minimum water content to avoid all possible negative effects of additional water over that necessary for cement hydration, such as bleeding, segregation, difficulties in correct placing in the moulds, etc. On the other side, excess cement is also unacceptable to control shrinkage and creep. 

The choice and the distribution of the conventional bonded steel reinforcement in the vessel main areas and around the penetrations are extremely important should initial cracking occur due to moisture migration and unpredictable shrinkage and creep in concrete. The bonded steel reinforcement is needed in areas where extreme stresses under serviceability conditions cannot be avoided. Above all, the progressive failure of vessels depends on the amount and the distribution of such reinforcements. 

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