Concrete under drying

The creep of concrete under drying conditions is increased by the presence of calcium chloride in the mix [30], as shown in Fig. 5.38 for 1.5% calcium chloride by weight of cement. 

The volume deformations of concrete are shrinkage, which occurs under drying conditions, and creep, which is the additional deformation obtained under an applied stress. Creep does occur under saturated conditions (basic creep) but increases considerably under conditions of moisture loss. The picture is rather complicated in that creep is made up of a recoverable and irrecoverable portion on removal of the applied stress. 

Volume deformations under drying conditions It is under conditions where moisture is lost from concrete that volume deformations under loaded or unloaded conditions occur to any magnitude. It is difficult to say what degree of relative humidity structural concrete will be subjected to in actual practice, but certainly for thin sections or near the surface of large sections, considerable interchange of water due to changing climatic conditions will occur. 

Experiments on the effect of different curing conditions on the compressive and flexural strengths of plain and air-entrained concrete [35] showed that air-entrained concrete has less tendency to lose moisture under drying conditions, which means that if concrete curing conditions are not ideal, air-entrained concrete should develop strength more normally than plain concrete [43],... 

Such work as has been published indicates that superplasticizers do not adversely affect either creep or drying shrinkage with the possible exception that with flowing concrete early age shrinkage cracking may be increased under drying conditions, unless adequate curing precautions are taken. 

Figure 14.2 Schematic development of moisture content of concrete under wetting/drying, for coated or hydrophobised and untreated concrete...

Effects of Curii Conditioiis. Favorable curing condition requirements for latex-modified mortar and concrete differ from those for ordirtary cement mortar and concrete, because their binder consists of two phases of latex and hydraulic cement widi different properties. Optimum strength in the cement phase is developed under wet conditions such as water immersion and high humidities, where strength development in the latex phase is attained under dry conditions. Figures 4.24 and 4.25f lf l show the effect of the curing conditions on the strength of the latex-modified mortars and concretes respectively. 

While the puU-olf test is now used extensively for purposes of quality control, only a limited number of studies have been conducted using the approach to assess effects of concrete quality, surface conditions, environment during application of the FRP, and durability. Sen et al. used the test to assess the durability of the bond of two different carbon fabrics and five epoxy systems under four different exposure conditions and concluded that exposure to wet-dry cycles resulted in the greatest deterioration due to moisture absorption in the epoxy. Malvar et al. tested a bond of CFRP to concrete under combinations of temperature and humidity level for a period of 7 days and reported that the maximum number of failures was seen after exposure to an environment consisting of 95% relative humidity (RH) and a temperature of 38° C (Malvar et al. 2003). While these sets of experiments provide valuable data, they do not allow for an assessment of systems with respect to crucial characteristics such as diffusion coefficients of moisture uptake and the actual deterioration of the resin system used to bond the FRP to the concrete as a result of exposure (Karbhari and Ghosh 2009 Zilch and Miihlbauer 2007 RizkaUa et al. 2008 Keller and Zhou 2006 El Damatty et al. 2005 Shayan et al. 2003). 

The plastic plates and terminals are fitted into cast housings inside concrete columns or in built-up areas, in cable junction boxes installed at walls. Belowground test points should be installed in built-up areas only in exceptional circumstances. In this case watertight, flush-mounted test stations are installed under a street-level covering and can be kept dry only by the most careful construction. 

Lead is relatively easily corroded where acetic acid fumes are present and under such conditions it either should not be used or should be efficiently protected. Generally, any contact between lead and organic material containing or developing acids will cause corrosion for instance, unseasoned wood may be detrimental. Trouble from this cause may be prevented by using well-seasoned timber, by maintaining dry conditions, or by separating the lead from the timber by bitumen felt or paint. Lead is also subject to attack by lime and particularly by Portland cement, mortar and concrete, but can be protected by a heavy coat of bitumen. A lead damp-proof course laid without protection in the mortar joint of a brick wall may become severely corroded, especially where the brickwork is in an exposed condition and is excessively damp. 

The dampproofmg admixtures will, therefore, improve the aesthetic qualities of concrete in terms of maintenance of a clean appearance over a prolonged period of time without adverse effects on other properties, and in the areas of freeze-thaw resistance, shrinkage under wet-dry cycling and reinforcement protection, may contribute beneficially. 

The drying shrinkage of concrete containing calcium chloride is increased in comparison to plain concrete, even though the amount of moisture lost is less [22]. This is illustrated in Fig. 5.37 and it is thought that the reduced moisture loss will be due to the more advanced state of hydration in the specimens containing calcium chloride. The increased shrinkage must, therefore, be a characteristic of the type of cement hydration products formed. Under saturated conditions, such as total water immersion, the amount of expansion of the concrete is reduced when calcium chloride is present. 

Rebound Because of marked improvements of adhesion and cohesion, the amount of rebound is significantly reduced. In controlled tests, rebound of a conventional dry-mix shotcrete, applied to a thickness of 50 mm (2 in) to the smooth overhead surface of concrete deck slabs, was measured at 40% by mass (the rebound was caught in tarpaulins and weighed). Rebound under the same circumstances was reduced to 25% by mass in dry-mix silica fume shotcrete. 

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