Precast Inc. - September/October 2008 - (Page 18) TECHNICAL Durable, Watertight Precast Concrete Common thoughts on what makes any concrete structure durable and watertight, with an emphasis on structures subject to moderate exposures. C oncrete is inherently a natural, durable and versatile material. When properly designed and manufactured, precast concrete products are capable of near maintenance-free performance without the need of protective coatings, except in highly corrosive environments. Durable, watertight concrete is the result of low watercement (w/c) ratios and dense concrete mixes, which are produced through superior manufacturing processes. Highquality concrete products are important for many applications, most notably for underground structures exposed to water and wastewater such as storm sewer manholes, culverts and septic tanks. The controlled manufacturing environment in a precast concrete plant is ideal for the production of high-quality, durable concrete products. Watertight requirements are an important and occasionally understated concern in many applications. For example, unwanted infiltration or exfiltration in a tank can compromise both the efficient operation and life of an on-site wastewater system. From a structural perspective, alternative saturations of a concrete structure are a leading cause of deterioration. Cyclic volumetric changes due to variations in water content in a concrete structure can cause enormous strains. Tests have shown that a prolonged exposure to extreme moisture variations can cause volumetric changes in unprotected concrete that are equivalent to a cyclic temperature ranging from ambient up to 1,000 F. However, proper attention to Causes of low durability High water-cement ratios (above 0.40 to 0.45) Inadequate cement content Inadequate air-entraining Poor finishing Inadequate curing Goals for producing high-quality concrete Low water-cement ratios (below 0.40 to 0.45) Minimum cement content More than 564 lbs cement/cy concrete A well-graded, sound, nonporous aggregate Inclusion of sufficient fines Air entraining agent FIGURE 1 – LOW DURABILITY CONCRETE VERSUS HIGH-QUALITY CONCRETE component design readily addresses this concern. The design of durable, watertight products must consider two main factors: the severity of the exposure and the type of construction. Where exposure conditions are not severe (such as pH greater than 5, non-turbid and non-abrasive flow, low sewage strength), waterproofing of most buried structures becomes a matter of three main factors: • Providing a high-quality, dense mix design • Adhering to good manufacturing practices • Paying adequate attention to joints and penetrations Appropriate measures to prevent or minimize the potential of cracking during production, transportation and installation are also important factors to ensure watertightness. In most applications, special protection such as waterproof coatings or liners is normally not warranted. Domestic sewage is usually harmless to concrete. This is in contrast to conditions in a sanitary manhole. In sanitary manholes and similar cases, microbial or anaerobic conditions can produce acids and hydrogen sulfide that may cause corrosion such that additional protective measures are recommended to ensure durability (in liners and coatings, for example). Specifications for buried products should address minimum requirements for mix design and manufacturing processes. Mix design Consider the following comparison of the leading causes of low durability versus high-quality concrete listed in Figure 1. A mix design for durable, watertight concrete should have a maximum water-cement ratio of 0.40 to 0.45 and require a wellgraded mixture of fine and course aggregates. Water-cement ratio The water-cement ratio, also known as Abram’s Law, is the most important factor in concrete design. The water content in a mix controls the moisture’s rate of entry, which may contain aggressive chemicals and the movement of water during the freeze-thaw process. 18 SEPTEMBER/OCTOBER 2008 | PRECAST INC.
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