Precast Inc. - September/October 2008 - (Page 20) Rapid Chloride Permenbility (Cuulumlis) 10000 8000 52 0 6000 lb sc are to be developed. An adequate supply of moisture, either by covering or other means, is important to ensure full hydration and reduce the porosity level such that the desired durability is attained, as shown in Figure 6. Although a period of moist curing significantly reduces permeability, the effects of curing are less pronounced with lower water-cement ratio mixes. em en t /c y 61 0 4000 lbs Joints and penetrations Michael T. Kubal, author of “Waterproofing the Building Envelope,” repeatedly states that 90 percent of water intrusion problems occur within 1 percent of a structure’s exterior surface area. This statement is also applicable to buried concrete structures, which is possibly an area in need of greater attention. A system is only as strong as its weakest link. Close attention to all jointed, connected and sealed areas is absolutely necessary to ensure watertightness. Potential differential settlements and thermal movements must be addressed in the design and manufacture of joints and penetrations. Construction joints need to be adequately prepared and bonded to ensure a complete contact between the two pours, as illustrated in Figure 7. A well-cleaned and sound surface is very important. In watertight applications, waterstops should be used. In the case of connecting surfaces, in a conventional twopiece buried tank, the use of a preformed flexible sealant is recommended. When applying sealant, the concrete surface should be clean and dry. A simple method of determining the cleanliness of a mating surface is to draw a dark or black cloth across the substrate. For the penetrations of service lines, seals should be selected that can accommodate any differential movements or settlements. cem en t/c y 2000 0 85 90 95 100 105 Percent Consolidation FIGURE 5 – CONSOLIDATION VERSUS PERMEABILITY Hydraulic permeability, cm/sec x 10 -10 50 Non-air-entrained concrete Specimens: 4 x 8-in. cylinders Water Pressure: 3000 psi Curing: 1 day moist, 90 days in air 7 days moist, 90 days in air 40 30 20 10 0 0.3 0.4 0.5 0.6 Water – Cement ratio, by weight 0.7 0.8 Conclusion Precast concrete products are particularly suited for durable, watertight applications such as environmental and underground structures. They will not warp or undergo significant dimensional change under service conditions and, when properly designed and manufactured, are nearly impermeable and extremely resistant to corrosion. The following are key elements required to produce a durable, watertight concrete product. • Use of a high-quality dense concrete • A well-graded, low water-cement (w/c) mix design • Superior manufacturing processes • Adequate consolidation • Proper curing practices • Adequate attention to joints and penetration seals • Prevention of cracking during production, transportation and installation The best investment for a waterproof concrete structure is to emphasize close attention to all concrete manufacturing and installation details. The cost of ignoring quality can be enormous. Successful manufacturers have proven that the best way to improve profits is to increase quality. FIGURE 6 – RELATIONSHIP BETWEEN HYDRAULIC PERMEABILITY, W/C RATIO AND INITIAL CURING Water Stop Solid, well-cleaned surface (typical) Joint Joint FIGURE 7 – CONSTRUCTION JOINTS The degree of consolidation can have a marked effect on the watertightness of concrete. As illustrated in Figure 5, a 5 percent reduction in percent of consolidation can result in a 20 percent reduction in watertightness. This figure also shows that higher cement content improves watertightness. Defect-free surfaces produced by using smooth forms and appropriate release agents can considerably improve the impermeability of a precast concrete product. Concrete must be adequately cured if its optimum properties 20 SEPTEMBER/OCTOBER 2008 | PRECAST INC.
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