Water in Concrete

Water may be present in the hydrated cement paste in many forms that may be classified on the basis of the degree of difficulty with which it can be removed [16]. 

Capillary water. The water contained in capillary pores accounts for the greatest part of water in concrete (and the most important part with regard to corrosion). 

Cement Chloride added (by mass of cement) Water/ binder Age (days) Sample Source [OH-] [Na+[ [K+] [Ca++] [C1-] [S04=] 

Interval of values from pastes exposed to environment with different CO2 concentration- 

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None (18) Stable out of contact with oxygen None stored under water in concrete tanks Smoke in sir phos-pborio acid (HaPOd dissolved in water Like matchee Solid pardcie burns flesh vapors very poisonous, cause none, decay smoke relatively harmless... 

In this chapter the mechanisms of transport operating in concrete and the parameters that define them are discussed. Since the liquid present in the pores has an important influence both on the transport of the various aggressive species and in the degradation phenomena that can take place in concrete, it is worthwhile looking first at the composition of the pore solution and the physical forms of water in concrete as a function of environmental conditions. 

Carbonation The process by which carbon dioxide (CO,) in the atmosphere reacts with water in concrete pores to form carbonic acid and then reacts with the alkalis (q.v.) in the pores, neutralizing them. This can then lead to the corrosion of the reinforcing steel. 

If concrete is exposed continuously to water, it exhibits insignificant expansion over a period of several years. Concrete exposed to dry and wet conditions would, however, be subjected to loss of water causing it to contract and leading to its cracking due to development of internal stresses. The shrinkage caused by drying of water in concrete is called shrinkage. ... 

During hardening of concrete, calcium hydroxide Ca(OH)2 is precipitated and alkali hydroxides are formed within the mix water. The pore water in concrete is thus strongly alkaline with pH 13.5. This high pH value of the pore water counteracts corrosion... 

Lithium hydroxide can be used for preparation of numerous lithium salts. The dominant use is the preparation of lithium stearate [4485-12-5], which is added to lubricating greases in amounts up to about 10% by weight. This salt has very low water solubiHty and extends the acceptable viscosity for the grease to both low and high temperatures (see Lubrication and lubricants). Lithium hydroxide is also used in production of dyes (62) and has been proposed as a source of lithium ion for inhibition of alkaH-aggregate expansive reactivity in concrete (63). 

Fig. 4. Integrated vault technology for low level waste disposal where A represents waste containers that are placed in concrete overpacks and sealed with grout B, closed modules covered with a multiple-layer earthen cover, to direct water away from modules, and short rooted vegetation for erosion control and C, overpacks placed in reinforced concrete modules which are closed with a reinforced concrete roof Courtesy of Chem-Nuclear Systems, Inc.

Water resistance is an important factor in concrete and masonry constmetion for the safety, health, and comfort of buUding occupants (see Cement). Several texts on concrete constmetion describe the methods for obtaining water resistance (72—76). The term waterproof describes concrete and masonry that is completely impervious to water and its vapor, whether or not the water is under pressure. Waterproof constmetion involves the use of some type of barrier that covers aU surface pores or capUlaries. Water repeUent describes concrete or masonry that repels water without significantly reduced permeabUity to water vapor. In this discussion, concrete and masonry are used synonymously. 

Type V (High Sulfate Resistance). Type V Pordand cement is used in concrete exposed to severe sulfate attack of 1,500 to 10,000 ppm. Low concentrations of tricalcium aluminate [12042-78-3] give Type V its sulfate resistance. The sulfate resistance is improved with air entrainment and low water to cement ratios in the wet concrete. U.S. production of Type V Pordand cement in 1989 was 0.9% of the total Pordand cement production. 

Concrete, Mortar, and Plaster. Citric acid and citrate salts are used as admixtures in concrete, mortar, and plaster formulations to retard setting times and reduce the amount of water requited to make a workable mixture (172—180). The citrate ion slows the hydration of Portland cement and acts as a dispersant, reducing the viscosity of the system (181). At levels below 0.1%, citrates accelerate the setting rate while at 0.2—0.4% the set rate is retarded. High early strength and improved frost resistance have been reported when adding citrate to concrete, mortar, and plaster. 

For the analysis heat and mass transfer in concrete samples at high temperatures, the numerical model has been developed. It describes concrete, as a porous multiphase system which at local level is in thermodynamic balance with body interstice, filled by liquid water and gas phase. The model allows researching the dynamic characteristics of diffusion in view of concrete matrix phase transitions, which was usually described by means of experiments. 

Cathodic protection can be used to protect steel in concrete (see Chapter 19). There is no fear of damage by H2 evolution due to porosity of the mortar. Local corrosion attack can be observed under extreme conditions due to porosity (water/ cement ratio = 1) and polarization (f/jq = -0.98 V) with portland cement but not with blast furnace cement, corresponding to field IV in Fig. 2-2 [53]. However, such conditions do not occur in practice. 

A similar danger of corrosion lies in cell formation in steel-concrete foundations (see Section 4.3). Such steel-concrete cells are today the most frequent cause of the increasing amount of premature damage at defects in the coating of new steel pipelines. The incidence of this type of cell formation is increased by the connection of potential-equalizing conductors in internal gas pipelines and domestic water pipelines [25], as well as by the increased use of reinforcing steel in concrete foundations for grounding electrical installations [26]. 

Factors that affect cell formation are the type of cement, the water/cement ratio and the aeration of the concrete [6]. Figure 12-1 shows schematically the cell action and the variation of the pipe/soil potential where there is contact with a steel-concrete structure. The cell current density is determined by the large area of the cathode [see Fig. 2-6 and Eq. (2-44)]. In industrial installations the area of steel surface in concrete is usually greater than lO m ... 

Another option is to use open filters, which are generally constructed in concrete. They are normally rectangular in configuration. The filter mass is posed on a filter bottom, provided with its own drainage system, including bores that are needed for the flow of filtered water as well as for countercurrent washing with water or air. 

It may also be replaced with new carbon and disposal of the exhausted carbon Most adsorbers are pressure vessels constructed in carbon steel, stainless steel or plastic. Large systems for drinking water are often eonstructed in concrete. In some cases, a moving or pulsed bed adsorber is employed to optimixe the use of the granular activated carbon. 

Pressure-tubes allow the separate, low-pressure, heavy-water moderator to act as a backup hesit sink even if there is no water in the fuel channels. Should this fail, the calandria shell ilsdf can contain the debris, with the decay heat being transferred to the water-filled shield tank around the core. Should the severe core damage sequence progress further, the shield tank and the concrete reactor vault significantly delay the challenge to containment. Furthermore, should core melt lead to containment overpressure, the concrete containment wall will leak and reduce the possibility of catastrophic structural failure (Snell, 1990). 

The sound speed c, m s , is the velocity of propagation of the pressure variations. This depends on the physical properties of the medium and increases with the density of the medium. In air, for example, it is. 344 m s, while in water, 1410 m s and in concrete, 3000 m s . The elapsed time between successive compressions is called the period time T. 

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