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14 CHAPTER 3 PERFORMANCE CONSIDERATIONS RELATED TO THE DURABILITY OF EOT CONCRETE A number of material characteristics impact the behavior concrete cannot be totally eliminated (excluding the use of of EOT concrete. Various test methods can be used to deter- expansive cements), it can be reduced or controlled by the mine these characteristics, including methods to assess the use of an appropriate mix design and proper construction tech- strength, shrinkage, durability, microstructure, and absorbtion/ niques. Controlling shrinkage contributes to crack prevention, permeability characteristics of the mixture. which helps prevent the physical and chemical attack of con- crete. This section discusses three types of shrinkage that can affect EOT concrete mixtures: plastic, drying, and autogenous. 3.1 STRENGTH The mechanical behavior of EOT concrete is commonly 3.2.1 Plastic Shrinkage assessed through measurement of its compressive or flexural strength. Although concrete strength is not directly related to Plastic shrinkage is the result of free, or "bleed," water durability, strength criterion are an important consideration in evaporating from the surface of concrete faster than it appears deciding when a pavement can be opened to traffic. This is during finishing operations (Kosmatka et al. 2002). Gener- especially true where concrete strength is not assessed in days ally, an evaporation rate of 0.5 kg/m2/hour (0.1 lb/ft2/hour) or weeks, but in hours. By definition, early strength is readily or more is considered critical (Mindess et al. 2003). If the attainable in EOT concrete mixtures; however, if any concrete amount of evaporation is significant, small irregular cracks is loaded prematurely, its long-term performance will be com- can form over the entire surface of the concrete. These cracks, promised. Therefore, EOT concrete repair materials must meet although at first isolated to the slab surface, can progress into or exceed criterion set for opening strength. full-depth cracks under the influence of additional shrinkage Concrete strength is essentially a function of the constitu- and/or traffic loading. They also provide pathways for chem- tive materials used, their proportions, temperature, and time ical attack by destroying the water tightness of the concrete. of hydration. Strength development is typically accelerated Generally, the potential for plastic shrinkage is increased in mixtures by using high cement content, low w/c ratios, and by elevated temperatures (both concrete and air), low relative accelerating admixtures. High curing temperatures also pro- humidity, high wind velocity, a low w/c ratio, and high cement mote rapid strength gain. content (Mindess et al. 2003). Many of these factors are The compressive strength test on cylindrical specimens, accentuated with EOT concrete installations. This concern is described in AASHTO T 22, is the most common strength thus particularly important. Any factor that either increases test made on concrete. For pavement applications, the flex- the rate of evaporation or decreases the rate of bleed water ural strength test (AASHTO T 97 [third-point loading test] or rising to the surface makes the concrete more susceptible to AASHTO T 177 [center-point loading test]) is often used. plastic shrinkage cracking. Plastic shrinkage can be virtually The major limitation to the use of flexural strength is that the eliminated by maintaining a wet surface during finishing specimens are more difficult to properly prepare, and thus the operations and initial curing (Neville 1996, Mindess et al. variability within the test is higher. 2003), but this is almost impossible to do under EOT con- As discussed in Chapter 2, the compressive and flexural crete construction limitations. Instead, curing compounds are strengths are both commonly used as strength criterion in used to minimize evaporation. Curing compounds must be SHA specifications for EOT concrete. applied early and uniformly, thoroughly coating the exposed concrete surface. 3.2 SHRINKAGE 3.2.2 Drying Shrinkage The total shrinkage that occurs in a concrete mixture is composed of several types of shrinkage that occur at differ- Drying shrinkage occurs after the paste has hardened and ent ages in the life of the material. Although shrinkage of results from the strain produced by the loss of water from the