Precast Inc. - September/October 2008 - (Page 19) Leakage, 2sf per hr. 2.5 Voids 2.0 1.5 Water Infiltration 1.0 Poorly Graded Mix Design W /C Well-Graded Mix Design Inclusion of fine materials provides fillings for interspaces between large aggregates =0 Porous concrete resulting from absence of fine materials W /C = .80 64 0. 0.5 /C FIGURE 3 – POORLY GRADED VERSUS WELL-GRADED MIXTURE DESIGN W = 0. 50 7 14 Period of moist curing and age at test, days 28 0 1 3 FIGURE 2 – EFFECT OF W/C RATIOS AND CURING DURATION ON PERMEABILITY OF MORTAR Note: The leakage is reduced as the water-cement ratio is decreased. The more water that is used in a mix, the lower the strength, durability and watertightness. Excess mix water will result in excess entrapped air pockets in hardened concrete, which will reduce its resistance to leakage. Too little water can cause placement difficulties and undesirable effects such as honeycombing. The effect the water-cement ratio has on the watertightness of a concrete mix is illustrated in Figure 2. bubbles that improve the watertight performance of hardened concrete. They also improve a concrete’s freeze-thaw performance and overall durability in addition to easing the placement process. The development of air-entraining admixtures has been cited as the single most significant advance in concrete materials technology in the past 80 years. However, it is noted that the benefits of air entrainment are less pronounced in low water-cement ratio mixes, particularly dry mixes (such as zero slump mixes). Adding air-entrainment admixtures to a mix should be strictly regulated, since adding too much air will weaken the mix significantly. Manufacturing process Quality concrete manufacturing processes are critical to the production of durable, watertight concrete products. Proper attention to important pre-pour activities such as maintaining prescribed mix proportions, form cleanliness and reinforcement placement are very important. For concrete products permanently exposed to earth or moisture, increased concrete cover, as specified in ACI 318, is recommended to ensure the corrosion protection. Adequate consolidation of freshly placed concrete is an extremely important factor to produce a high-quality, dense concrete. Added emphasis is required for the desirable low water-cement ratio concrete, since it requires a higher compactive effort. A summary of preferred practices is illustrated in Figure 4. For further information on consolidation, refer to the NPCA TechNote “Proper Vibration Techniques in Precast Plants.” Minimum cement content Rich concrete mixes provide a denser and more impermeable superior finished product. Consequently, specifying a minimum cement content is recommended. In the case of watertight structures, a minimum cement content of 564 pounds per cubic yard is suggested. (The effect of cement content on permeability of a product is illustrated in Figure 6.) Gradation Concrete mixes that are not well-graded can permit water to pass through the finished structure as illustrated in Figure 3. Rounded coarse aggregates are preferred, if available, due to their ability to be more uniformly placed. Friable, non-sound aggregates may fracture in the mixing and placement process, compromising their integrity. Several measures can be taken to ensure a well-graded mixture with sufficient fines and a low water-cement ratio, such as including mineral admixtures like silica fume, fly ash and slag, as well as chemical admixtures such as water-reducing and airentraining agents. Additional fines serve as fillers, reducing the pore structures in the matrix. Water-reducing agents (superplacticizers) provide highly workable mixes that are readily consolidated. The addition of synthetic fibers has also been cited as a way to improve the watertightness of a mix design. Fibers can improve the toughness and minimize the potential for cracking. Air entrainment agents produce near-microscopic independent Correct Incorrect Concrete with lower water-cement ratios requires greater compactive effort. Vibrators should not be used to move concrete laterally. Fresh concrete should be vibrated until all voids and entrapped air pockets are released. FIGURE 4 – VIBRATION TECHNIQUES SEPTEMBER/OCTOBER 2008 | WWW.PRECAST.ORG 19 http://www.precast.org
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