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22 CHAPTER 5 TESTING OF FRESH AND HARDENED CONCRETE Testing of fresh and hardened EOT concrete is required to monitor the construction process as well as ensure that desir- able concrete properties are achieved. Routine standard tests for fresh concrete include workability, air content, and matu- rity. The most common, and often the only, testing of hard- ened concrete is measuring compressive strength and/or flex- ural strength. Other tests of hardened concrete that could be considered include methods to assess volume change, dura- bility in freeze-thaw environments, absorption/permeability, and microstructural characterization. A number of tests are recommended for evaluation of EOT concrete mixtures. 5.1 TEST METHODS 5.1.1 Testing Fresh Concrete Testing of fresh EOT concrete entails measuring worka- bility, air content, and maturity. Workability is most often assessed through the slump test (AASHTO T 119), with a desired range in slump for EOT concrete often specified between 50 and 150 mm (2 to 6 in.). The time of setting (AASHTO T 131) can also be used to establish the time in which the mixture is workable. This time should be sufficient to provide time for mixing, placing, and finishing the EOT concrete. Air content of the fresh concrete is measured as an indirect indication of the air-void system in the paste. In a freeze-thaw environment subjected to deicer use, the air content for con- crete mixtures containing coarse aggregates with a nominal maxium size of 1 in. (25 mm) or less should range from 6 to 7.5 Â± 1.5 percent depending on coarse aggregate size, as recommended in Table 3. Air contents are commonly mea- sured using the pressure method (AASHTO T 152), although the volumetric method (AASHTO T 196) is also used. The AVA is currently being used for the analysis of paving concrete (Price 1996). The AVA measures changes in buoy- ancy using a special buoyancy recorder that captures bubbles as they rise from the concrete through a viscous medium. Since larger bubbles rise more quickly, monitoring the change in buoyancy as a function of time provides a measure of the air-void size distribution, total air content, and specific sur- face from which a spacing factor can be calculated. Although the use of this device for large paving projects might be jus- tified, it seems unlikely that the device will be used exten- sively to monitor field installations of EOT concrete. The device, however, might be useful in the laboratory during mix design to verify the sufficiency of the air-void system. Another fresh concrete test that will see increasing use in EOT concrete applications is maturity (ASTM C 1074). The maturity concept relates the time-temperature relationship directly to strength gain for a given mixture. It is an excellent way to determine the time to opening. The relatively new use of wireless technology has eliminated the need for wires and continual recording of data common in older maturity meters. As wireless technology continues to develop, it is foreseeable that each repair will have a wireless maturity gauge installed that reports to a hand-held unit, which in turn computes the time to opening based on stored mix design information. 5.1.2 Testing Hardened Concrete Strength Testing Strength testing is an integral part of the mixture design process and construction monitoring for EOT concrete. Com- pressive strength (AASHTO T 22), measured on cylindrical specimens, is often specified to be at least 13.8 MPa (2,000 psi) at time of opening (FHWA 2003). Some agencies prefer the use of a flexural strength (AASHTO T 97) opening criterion. Testing is conducted on beam specimens with common cri- terion of 1.7 MPa (250 psi) for third-point loading and 2.1 MPa (300 psi) for center-point loading. The minimum required compressive or flexural strength is commonly raised for the slower-setting 20- to 24-hour concrete mixtures. For construction monitoring, early strength testing is often not conducted because of the short time available. If logistics preclude strength testing prior to opening, the use of the maturity concept is recommended. Strength-maturity relation- ships established for a given mixture can be used during con- struction to ensure that adequate strength has been achieved prior to loading.
23 Testing of Volume Change Two tests that can be used to assess volume change in hardened concrete are the determination of the CTE (AASHTO TP 60-00) and the restrained drying shrinkage test ring (AASHTO PP 34-99). Neither test is adaptable to field applications, and thus they are of little value for moni- toring construction. However, they may be useful for mate- rial selection and establishing mixture design parameters. For example, measuring the CTE of the EOT concrete mix- ture will help assess the mixtureâs thermal compatibility with the existing pavement concrete. In the laboratory phase of this recent study, it was observed that the CTE results were in general highly repeatable, but the results from the restrained shrinkage ring test were highly variable. Also, the relevance of the results of the restrained shrinkage concrete to the performance of EOT concrete has yet to be established. Freeze-Thaw Testing The durability of EOT concrete in a freeze-thaw environ- ment can be assessed by testing the resistance of concrete to freezing and thawing (AASHTO T 161) and the exposure to deicers (ASTM C 672). These tests are long term and can be used only on hardened concrete and therefore are not appli- cable for construction monitoring. Further, because of the rel- atively long period of testing and the complexity of the test procedure, these tests likely will be used only when evaluat- ing EOT concrete mixtures to establish mixture design param- eters (e.g., to investigate failures and qualify new materi- als). Most agencies in freeze-thaw climatic zones conduct some version of AASHTO T 161 and possibly ASTM C 672 as part of the material approval process for paving concrete. It is recommended that these agencies use their test procedures for evaluating the proposed EOT concrete mixture designs. Absorption/Permeability Testing Absorption/permeability can be assessed by testing the spe- cific gravity, absorption, and voids (ASTM C 642) or sorp- tivity (proposed ASTM test). Both tests are conducted on hardened concrete and are thus not suitable for use in con- struction monitoring. Another commonly accepted method to measure permeability is the rapid chloride permeability test (AASHTO T 277), although this test can be difficult to apply to EOT concrete containing certain admixtures. Other rapid chloride permeability tests may be appropriate, but standards need to be established before these techniques can be employed for EOT concrete. Microstructural Characterization The final type of testing is microstructural characterization of the concrete, such as air-void analysis (ASTM C 457) and petrographic examination of hardened concrete (ASTM C 856). These techniques would not be routinely applied to EOT concrete because of the rigor and expense of the test- ing, but they should be considered when a better understand- ing of EOT concrete performance is desired, especially when unexpected deterioration has occurred. It has been found that an adequate air-void system can be difficult to achieve in EOT concrete, even when the air content of the fresh concrete was satisfactory. Thus, if durability problems have been observed in EOT concrete repairs, an agency should consider measur- ing the air-void system parameters of the concrete. The only accepted method to determine the characteristics of the air- void system is ASTM C 457, which requires manual obser- vation with a stereo optical microscope. Automated methods based on digital image analysis currently under development are expected to significantly shorten the time needed to con- duct this test, thus making it more suitable for routine use. In some instances, full petrographic analysis in accordance with ASTM C 856 might be warranted. The unique charac- teristics of EOT concrete make it more susceptible to vari- ous kinds of material-related distress (e.g., alkali-aggregate reactivity and sulfate attack). When durability-related dis- tress in EOT concrete repairs is observed, it would be pru- dent to conduct a thorough investigation of the cause of dete- rioration using ASTM C 856 to avoid similar distress in future applications. 5.2 TESTING RECOMMENDATIONS There are three reasons for testing EOT concrete: to design a mixture for a specific application, to monitor the mixture during construction, and to conduct generalized investiga- tions to improve specifications. The type and extent of test- ing depends on the purpose for which the testing is being done. For each purpose, the suggested testing is divided into rec- ommended and optional. 5.2.1 Mixture Testing Recommendations Table 8 shows the recommended and optional testing for the mixture design process. 5.2.2 Construction Monitoring Table 9 shows the recommended and optional testing for construction monitoring. 5.2.3 Investigations and Research Table 10 shows the recommended and optional testing for investigations and research.
24 TABLE 10 Recommended and optional testing for investigations and research Test Method Property Assessed Performance Characteristic Recommended Testing AASHTO T 119 Slump Workability AASHTO T 152 or 196 Air Content of Fresh Concrete Freeze-Thaw Durability ASTM C 1074 Maturity Strength Gain AASHTO T 22 or AASHTO T 97 Compressive or Flexural Strength Strength Criterion AASHTO TP 60-00 Coefficient of Thermal Expansion Thermal Stress ASTM C 642 Specific Gravity, Absorption, and Voids Absorption ASTM C 457 Air-Void System Characteristics Freeze-Thaw Durability AVA (non-standard) Air-Void System Characteristics Freeze-Thaw Durability AASHTO T 161 Resistance to Freezing and Thawing Damage Due to Cyclic Freezing and Thawing ASTM C 672 Resistance to Deicer Scaling Scaling Resistance Optional Testing AASHTO PP 34-99 Restrained Shrinkage Resistance to Drying Shrinkage Cracking ASTM C 856 Concrete Microstructure General Appearance at Microscopic Level TABLE 8 Recommended and optional testing for the mixture design process TABLE 9 Recommended and optional testing for construction monitoring Test Method Property Assessed Performance Characteristic Recommended Testing AASHTO T 119 Slump Workability AASHTO T 152 or 196 Air Content of Fresh Concrete Freeze-Thaw Durability ASTM C 1074 Maturity Strength Gain AASHTO T 22 or AASHTO T 97 Compressive or Flexural Strength Strength Criterion AASHTO TP 60-00 Coefficient of Thermal Expansion Thermal Stress ASTM C 642 Specific Gravity, Absorption, and Voids Absorption Optional Testing ASTM C 457 Air-Void System Characteristics Freeze-Thaw Durability AVA (non-standard) Air-Void System Characteristics Freeze-Thaw Durability Test Method Property Assessed Performance Characteristic Recommended Testing AASHTO T 119 Slump Workability AASHTO T 152 or 196 Air Content of Fresh Concrete Freeze-Thaw Durability ASTM C 1074 Maturity Strength Gain AASHTO T 22 or AASHTO T 97 Compressive or Flexural Strength Strength Criterion Optional Testing AASHTO T 131 Time of Setting Early Set