Chang Seon Shon

Combined Use of Calcium Chloride and Fly Ash in Road Base Stabilization

Chemical stabilization with cementitious and chemical additives such as portland cement, lime, fly ash, and calcium chloride (CaCl2) is now being extensively used in road base or subgrade primarily to enhance mechanical strength and improve resistance to chemical attack, resulting in a more durable roadway. In recent years, there has been renewed interest in the combined use of these additives to induce a synergistic effect on the strength, as well as to improve deficiencies of a single-chemical treatment process (e.g., slow early strength development of fly ash and susceptibility to leaching of CaCl2 in wet environments). The effect of adding CaCl2 to the fly ash–treated roadbed during construction was investigated. The role of CaCl2 in the system was studied, where determination was made as to how the soluble calcium species plays a role in bringing particles closer in the aggregate system and how it has an effect on the setting time, compaction efficiency, and ultimate strength. Laboratory test data revealed that the addition of CaCl2 not only accelerated the setting rate but also increased the initial and long-term strength of the constructed roadbed. On the basis of these laboratory tests, a mathematical model was developed that predicts the strength of the composites. Finally, field performance data showed that fly ashes can be effectively used in road construction if CaCl2 is added to the system.

Application of modified ASTM C1260 test for fly ash-cement mixtures

The ASTM C1260 accelerated mortar-bar test is a commonly used method for rapid identification of potential alkali-aggregate reactivity and may also be used for assessing the effectiveness of supplementary cementitious materials in suppressing alkali–silica reactivity (ASR). A general criticism of this test method is the severity of test conditions. It is not uncommon for aggregates with a good field-performance track record and no history of ASR to test reactive by this method. The purpose of this study was to evaluate the effectiveness of fly ash in controlling expansion due to ASR, using a modified ASTM C1260 test. Three different strengths of NaOH solution were used to test reactive, potentially reactive, and nonreactive aggregates in the presence of Class F and Class C fly ash at 20% and 35% replacement by mass of cement. The other variables included high- and low-alkali cement, extended curing time, and a longer testing period of 28 days. A correlation was drawn between additional evaporable water and expansion due to ASR.

New Performance-Based Approach to Ensure Quality Curing during Construction

Ensuring sufficient water availability in hydrating concrete is a key to achieving quality curing that protects the fresh concrete in the short term and develops the potential properties of the mixture for long-term performance of concrete pavement. Excessive early-age evaporation from the concrete pavement surface can potentially result in plastic shrinkage cracking, high porosity, and low strength at the concrete surface leading to a variety of surface distresses. Application of liquid curing compounds on newly placed concrete pavement surfaces has been widely used to minimize evaporation. However, the availability of a test method to assess the efficacy of a curing compound relative to the actual environmental conditions encountered in the field is still lacking. Therefore, advancements are needed to further facilitate the evaluation of curing effectiveness (CE). This paper presents a laboratory-based procedure, which bridges laboratory measured parameters to performance in the field, as part of a process to qualifying the effectiveness of a curing compound membrane. The curing monitor system, a device capable of accurately recording relative humidity and temperature at three critical locations along with wind speed and solar radiation, is used as part of a test procedure to assess the efficacy of a curing compound. A compound curing evaluation index (EI) is defined in terms of the effective curing thickness concept and moisture loss at 24 h after placement. The EI is useful to rank the curing compound relative to laboratory test conditions. From the limited field application studies, it has been observed that the proposed test procedure has good potential to be an effective tool to assess CE and quality under field conditions.

Alkali-Silica Reactivity Resistance of High-Volume Fly Ash Cementitious Systems

The role of high-volume ASTM Class F fly ash (58% by mass of cement) in reducing expansion caused by alkali-silica reaction was investigated. A series of modified ASTM C1260 tests were performed with three different concentrations of NaOH solution and over an extended test period of 28 days; a reactive siliceous fine aggregate was used. Test results confirm that replacing 58% of portland cement (by mass) with high-volume fly ash in a cementitious system significantly helps control expansion caused by alkali-silica reaction.

Evaluation of Alkali-Silica Reaction Potential of Aggregate Using Dilatometer Method

Existing test methodology for alkali silica reactivity (ASR) are applicable to only a narrow band of accelerated conditions and doubts remain whether these methods have any relevance to concrete performance under field conditions. Aggregate reactivity is a key factor in predicting the concrete ASR and is a function of alkalinity, temperature, size and crystallinity. Recently developed at the Texas Transportation Institute, Texas A&M University, a testing apparatus called a dilatometer has been used to measure aggregate ASR expansion and introducing activation energy as a single parameter to represent ASR reactivity. The expansion-time characteristics as a function of temperature can be expressed by the term activation energy (Ea). The rationality of the dilatometer test procedure is explored by conducting comprehensive laboratory experiments related to the effects of test solution (NaOH) alkalinity, temperature Ca(2+) contents on Ea. Dilatometer measures the volumetric expansion due to ASR and accounts the direct measurement of expansion produced by the reaction products. Based on the test results, it is observed that this test method will be useful to evaluate ASR potential of aggregates based on their Ea within a very short period of time (e.g., within 3 days). The dependency of Ea on alkalinity, Ca(2+) content and aggregate size provides a means to evaluate ASR potential of concrete relative to levels of alkali and temperature that occur under field conditions.