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15 hardened material (Mindess et al. 2003). The factors that has been speculation that autogenous shrinkage might be influence drying shrinkage that are most relevant to EOT con- partly contributing to microcracking. Unlike drying shrinkage, crete materials are the aggregate volume/cement content, the autogenous shrinkage increases as the w/c ratio decreases. w/c ratio, admixtures, aggregate characteristics, and curing. This increase may be particularly relevant for EOT concrete According to Neville (1996), the most important influence mixtures, which in some cases have w/c ratios as low as 0.32. on shrinkage is the restraint provided by the aggregate. The Similar to drying shrinkage, autogenous shrinkage only amount of restraint provided directly relates to the aggregate occurs in the paste fraction of the concrete. Thus, the rela- volume; as the aggregate volume decreases (with a commen- tive volume of aggregate to paste can directly impact the surate increase in cement paste volume), the amount of shrink- magnitude of the measured autogenous shrinkage. Because age increases. concrete made with higher volumes of aggregate have less The w/c ratio also directly and significantly affects drying measured autogenous shrinkage due to increased restraint, shrinkage, with lower w/c ratio mixtures having reduced increased cement contents generally result in increased auto- shrinkage (Neville 1996, Mindess et al. 2003). Thus, EOT genous shrinkage. concrete mixtures will benefit from the low w/c ratio that they commonly employ. For a given aggregate source and vol- ume, the w/c ratio of concrete is one of the most important 3.3 DURABILITY parameters for limiting drying shrinkage. Holding all other factors equal, a lower w/c ratio reduces the amount of evap- The performance of EOT concrete repairs can be adversely orable water available to cause drying shrinkage of concrete affected by the concrete's lack of durability (i.e., ability to mixtures (Neville 1996). Kosmatka et al. (2002) approach maintain its integrity in the environment in which it was this issue a little differently, stating that the most important placed). In general, durability problems can be attributed to factor affecting drying shrinkage is the amount of water added either physical or chemical mechanisms, although the two mechanisms often act together to bring about the develop- per unit volume of concrete and that shrinkage can be mini- ment of distress. Furthermore, problems with completely dif- mized by keeping the amount of added water low. Obviously, ferent causes may develop simultaneously, thereby compli- the aggregate volume, the w/c ratio, and the water added all cating the determination of the exact cause(s) of material relate to each other, but the main objective is to minimize the failure. The information presented in this section is based on amount of evaporable water in the mixture. research conducted for the FHWA (Van Dam et al. 2002a, There is an AASHTO provisional test method for assess- Van Dam et al. 2002b). Only material-related distress that ing the potential for cracking due to drying shrinkage. The test can be directly attributed to the unique properties of EOT is specified in AASHTO PP 34-99, "Standard Practice for concrete are discussed, including freeze-thaw deterioration, Estimating the Crack Tendency of Concrete." In this test, ring deicer scaling/deterioration, and sulfate attack. Certain types specimens are molded and the top and bottom faces of the of material-related distress, such as alkali-aggregate reac- rings are covered to prevent moisture loss other than through tivity and corrosion of embedded steel, can be significantly the outside circumferential area. A steel ring inside the con- affected by some characteristics of EOT concrete mixtures crete specimen restrains the concrete specimen as it shrinks. (e.g., highcement-content and chloride-based accelerators). This restraint results in internal tangential tensile stresses, These topics are not addressed in these guidelines. which will cause the concrete to crack if its tensile strength is exceeded (Kraai 1985). The time to cracking and the width and length of these cracks represent the damaging effect. 3.3.1 Freeze-Thaw Deterioration Freeze-thaw deterioration is caused by the deterioration of 3.2.3 Autogenous Shrinkage saturated cement paste under repeated freeze-thaw cycles. The mechanisms responsible for internal damage resulting from Concrete with a low w/c ratio can undergo a process of self- freeze-thaw actions are not fully understood, but the most desiccation that can lead to autogenous shrinkage (Mindess widely accepted theories stipulate the development of internal et al. 2003). This process is characterized by the removal of stress in the concrete as a result of hydraulic or osmotic pres- water from the capillary pores through the internal use of water sures caused by freezing. A review of the literature related to in the formation of hydration products. Autogenous shrink- these phenomena is provided by Marchand et al. (1994). age is relevant to EOT concrete materials because it seems Deterioration of the cement paste due to freeze-thaw dam- to increase at higher temperatures, in mixtures with higher age manifests itself in the form of scaling, map cracking, or cement contents, and in mixtures made with finer cements severe cracking and deterioration, most commonly occurring (Neville 1996). at joints where moisture is more readily available. The addi- In the past, this type of shrinkage was considered to be tion of an air-entraining agent (an admixture that introduces quite rare and of little consequence because its contribution a system of dispersed, microscopic spherical bubbles in the to total shrinkage was small. But because low w/c ratios are concrete) could effectively prevent this deterioration if a suf- often used in modern concrete, including EOT concrete, there ficient air-void system forms.
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16 Measurements of the total air content of fresh concrete ommended (ACPA 1992). ASTM C 672, "Scaling Resis- are made during construction. Three AASHTO test meth- tance of Concrete Surfaces Exposed to Deicing Chemicals," ods are available for measuring the air content of fresh con- is the most commonly used test to investigate the scaling crete during construction: AASHTO T 152, AASHTO T 196, potential of concrete. and AASHTO T 121. These methods, however, do not deter- mine whether the air is truly entrained or entrapped or whether an adequate air-void system has been developed to protect 3.3.3 External Sulfate Attack the concrete against freeze-thaw damage. A test method that External sulfate attack results when external sulfate ions has been under investigation for a number of years provides (present in groundwater, soil, deicing chemicals, etc.) pen- a means for measuring the air-void system parameters for etrate into the concrete and react with the hydrated cement fresh concrete. The test equipment, known as the Air-Void Analyzer (AVA), has received mixed reviews (Price 1996, paste. Although the mechanism of sulfate attack is complex, Magura 1996). sulfate attack is likely caused by two chemical reactions: The only currently accepted method to characterize the (1) the formation of gypsum through the combination of sul- air-void system in the hardened concrete is through micro- fate and calcium ions and (2) the formation of expansive ettrin- scopic analysis in accordance with ASTM C 457, "Standard gite through the combination of sulfate ions and hydrated cal- Test Method for Microscopical Determination of Parameters cium aluminate (ACI 2003b). In either case, the reaction leads of the Air-Void System in Hardened Concrete." The freeze- to an increase in solid volume that can be very destructive to thaw resistance of hardened concrete is often tested using the hardened paste. AASHTO T 161, "Resistance of Concrete to Rapid Freezing In EOT concrete repairs, deterioration due to external sul- and Thawing," which is used to assess the resistance of con- fate attack would likely first appear as cracking near joints crete specimens to rapidly repeated cycles of freezing and and slab edges, generally within a few years of construction. thawing. Only Procedure A in the standard, in which the spec- Fine longitudinal cracking may also occur parallel to longi- imens are frozen and thawed in water, should be used (TRB tudinal joints. Actions taken to prevent the development of 1999). Many SHA's have modified this procedure to address distress due to external sulfate attack include reducing the tri- their specific needs and experiences. calcium aluminate (C3 A) content in the cement or using poz- zolanic materials to reduce the quantity of calcium hydrox- ide (CH) in the hydrated cement paste. Both these actions are 3.3.2 Deicer Scaling/Deterioration not easily accomplished in EOT concrete mixtures, and if calcium chloride accelerator is used, even greater amounts of Deicer scaling/deterioration is typically characterized by CH are formed. A w/c ratio should be less than 0.45 to help scaling or crazing of the slab surface due to the repeated mitigate external sulfate attack (ACI 2003b). application of deicing chemicals in a freeze-thaw environ- Performance testing using ASTM C 452 and C 1012 should ment. Although the exact causes of deicer scaling are not be considered to examine the sulfate resistance of portland known, this scaling is believed to be primarily a form of phys- cements and combinations of cements and pozzolans/slag, ical attack similar to paste freeze-thaw deterioration. Both respectively. These tests only evaluate the cementitious mate- thermal stress and osmotic pressures are accentuated, mag- rials. There is currently no standard test to evaluate the sul- nifying the conventional freeze-thaw phenomena (Mindess fate resistance of the mixture. et al. 2003, Pigeon and Plateau 1995). It has also been spec- ulated that pressure exerted by salt crystallization in voids is a contributing factor (Hansen 1963). Recent studies suggest 3.3.4 Internal Sulfate Attack that chemical degradation of the cement paste may also be occurring, resulting in dissolution of calcium hydroxide, Internal sulfate attack is similar in many ways to external coarsening of the concrete pore system, and potentially the sulfate attack, except that the source of the sulfate ions is formation of deleterious compounds. internal. Potential internal sources of sulfate are (1) the slowly Deicer scaling/deterioration is more likely to occur if the soluble sulfate contained in the cement, aggregate, or other concrete was over-consolidated or improperly finished-- concrete constituents (such as fly ash) and (2) the decompo- actions that create a weak layer of paste or mortar just below sition of primary ettringite due to high curing temperatures. the finished surface (Mindess et al. 2003). Even adequately Secondary ettringite formation (SEF) and delayed ettrin- air-entrained concrete can be susceptible to the development gite formation (DEF) might both be considered types of of deicer scaling. Recommendations for the prevention of internal sulfate attack that result for different reasons. SEF is deicer scaling include providing a minimum cement content commonly a product of concrete degradation, characterized of 335 kg/m3 (564 lb/yd3) and using a maximum w/c ratio of by the dissolution and subsequent precipitation of ettringite 0.45, both of which are common in EOT concrete mixtures. into available void space and into preexisting microcracks. Providing adequate curing and a minimum of 30 days of con- SEF is possible if the concrete is sufficiently permeable and crete "drying" before applying deicing chemicals is also rec- saturated, allowing for the dissolution and precipitation