Sarwar Siddiqui

Effect of Internal Water Pressure on the Measured Coefficient of Thermal Expansion of Concrete

A study was conducted to evaluate the effect of coefficient of thermal expansion (CTE) test procedures and length-change measuring devices on the measured CTE values. Twenty different coarse aggregate sources were tested using the Texas Department of Transportation (TxDOT) and the American Association of State Highway and Transportation Officials (AASHTO) suggested CTE methods. Two different types of length-change measuring devices, linear variable differential transformers (LVDTs) and differential variable reluctance transducers (DVRTs), were used. No significant effects of length-change measuring devices were observed on the CTE values measured by the TxDOT method. However, the test methods have shown effects on the measured values. The TxDOT method yields higher CTE values than the AASHTO method. Data obtained in this study confirmed that the internal water pressure development during the heating and cooling cycles is one of the potential reasons. Internal water pressure can significantly affect the CTE of concrete. Further investigation is needed to determine the effect of internal water pressure on the CTE, which affects the design and service life of concrete pavements.

Optimizing the COTE of Concrete by Blending High and Low COTE Aggregates to Meet TxDOT Limit

The coefficient of thermal expansion (COTE) is an important concrete property that characterizes the dimensional change of concrete subjected to temperature change. In jointed concrete pavements (JCP), expansion joints are used to accommodate length changes due to temperature variations. However, for continuously reinforced concrete pavements (CRCP) with high COTE aggregates, there are no expansion joints to accommodate the length changes. In Texas some districts with high volumes of CRCP, such as Houston, Dallas, and Forth Worth, experience a high degree of transverse cracking, horizontal cracking, and punchouts due to the environmental loading in high COTE concrete CRCP. Very recently the Texas Department of Transportation (TxDOT) imposed a limit on COTE as an acceptance criterion for pavement concrete aggregates. Houston and Beaumont do not have low COTE aggregate sources in the vicinity, and to meet the current TxDOT requirement these two districts have to haul aggregates from long distances. This will not only increase the transportation cost but also limit the use of locally available good quality aggregates, which has high COTE. COTE of concrete can be reduced by blending high and low COTE aggregates. This research study used three high COTE aggregate sources from different strategic locations in Texas suggested by TxDOT; they were each blended with low COTE limestone aggregate at different replacement ratios. Results showed that concrete COTE can be reduced by blending low COTE aggregates with high COTE aggregates. The COTE of concrete decreases linearly with an increase in limestone replacement. These findings will help TxDOT use the local high COTE aggregate sources by blending with imported low COTE aggregates. Aggregate producers with high COTE aggregates can also determine the degree of replacement necessary for their aggregates to be accepted in a TxDOT CRCP paving projects.

Effect of Curing Water Availability and Composition on Cement Hydration

Curing can help concrete reach its full strength and durability potential. The effect of sealing the concrete with plastic or formwork, use of a liquid curing compound, wet curing, and internal curing with saturated lightweight aggregates on the cement degree of hydration (DOH) development with time was examined using isothermal calorimetry. Curing water amount, curing water ionic concentration, and sample thickness were varied. Finally, curing application timing was studied by comparing strength development of concrete cylinders sealed, placed in a moist room after 24 hours sealed, and immersed in a water bath immediately after finishing. Increasing the height of curing water decreased the height of heat of hydration rate peaks. Curing water ionic concentration affected the setting time and heat of hydration rate peak heights. Strength results show delayed curing can result in significant strength loss because of the difficulty for water to penetrate the already-hardened concrete.

Effect of Calculation Methods on Cement Paste and Mortar Apparent Activation Energy

Concrete hydration and strength development rates are a function of the concrete temperature, with higher temperatures leading to faster rates of reaction. The equivalent age maturity method is a commonly used method to model concrete property development under varying temperature curing conditions. The equivalent age maturity method requires the use of an activation energy value to account for the temperature sensitivity of the chemical reactions. Several experimental methods and calculation techniques are currently used to quantify the activation energy for concrete. This study compared the activation energy calculated using several different numerical methods from mortar strength, time of set, chemical shrinkage, and isothermal calorimetry paste and mortar experiments. The activation energy calculated from isothermal calorimetry experiments was found to be similar for paste and mortar. This indicates that aggregates have very little effect on the activation energy, which would permit the use of an activation energy calculated from cement paste to be used on concrete.

Experimental Investigation to Improve the Thermomechanical Response of Concrete Samples Subjected to Coefficient of Thermal Expansion Testing

AbstractThermomechanical response of coefficient of thermal expansion (CTE) test samples plays an important role in the consistency of CTE test results. Saturated concrete samples while subjected t...