Cement paste creep

Adsorbed water. Even when water has evaporated from the capillary pores, some water will still remain adsorbed to the iimer surface in the form of a very thin layer of adsorbed water. This water can be removed if the external humidity falls below 30 % it contributes htde to transport phenomena, thus it is insignificant with regard to corrosion of reinforcement. Its removal, however, causes shrinkage of the cement paste and influences creep behaviour. 

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.). 

Fig. 5.59. a Typical creep curve of cement paste, b creep of paste at simultaneous loading and drying free shrinkage, basic creep, creep at drying, total creep stress, total stress (simultaneous loading and drying)... 

Covering of reinforcing bars with resin, apphed as an anti-corrosion protection, can lower their adhesion to cement paste. Worsening of adhesion occurs in the case of vinyl polychloride, but it does not occirr if the epoxy resins are used. However, some epoxy resins reveal the creep effect [56]. 

Creep—slow deformation of a moist hardened cement paste under the influence of an external force. 

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. 

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. 

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. 

Many theories have been proposed over the years to aeeount for creep mechanisms in cement paste, and eaeh is capable of accounting for some of the observed facts. The deseriptions and mechanisms are based on seepage, change of solid stmeture, and interlayer space. 

However, it is felt that the intrinsic property of most water-reducing admixtures is to increase the creep and shrinkage of the concretes into which they are incorporated. This is minimized by utilizing the effect of the admixture to reduce the cement and, therefore, the paste content of the mix. For corresponding mixes with... 

Neither a model of creep, nor the mechanism of this process is known as well. Obviously, the creep relates to the properties of cement gel and the presence of adsorbed water. The creep growth with moisture content in a paste, while the dried samples do not show this effect [127]. However, this latter statement is not quite sure perhaps only a substantial reduction of creep occurs [128]. The hypotheses assume the squeezing of water from the paste under load. 

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