University of Texas, El Paso
University Transportation Centers Program
Admixtures; Aggregates; Concrete mixers; Feasibility analysis; Recycled materials; Shrinkage; Water cement ratio
The current specifications encourage the use of supplementary cementitious materials (SCMs) and as much as possible recycled materials in portland cement concrete. The use of SCMs, especially along with low water-to-cementitious material ratio, low total cementitious material content and cements with a low C3A content and low alkali, can produce some incompatibility in the paste due to the fact that they are not optimized for sulfate content when used with SCMs, or that have anhydrite or hemi-hydrate as the main sulfate source. This situation can create a concrete mixture that is sensitive to changes in ambient temperature, admixture dosage, cement and SCM properties, creating problems in the curing process, developing some early cracking, and compromising the performance of the structure. The following five factors are considered as the most significant factors contributing to the stability of the mixes: (1) type of cement, (2) type of chemical admixture, (3) dosage of chemical admixture, (4) type of SCMs, and (5) testing temperature. A study is proposed to develop an easy to use, relatively inexpensive laboratory test and equipment to determine potential concrete mixture incompatibilities among the aggregates and the above five parameters. The proposed equipment is a miniaturized free-free resonant column (FFRC, ASTM C215) device that has been used by many agencies with success. We have prototyped a fully-automated computer-controlled FFRC device that at user-defined time intervals collects data without the need for a technician. The computer used for this device, can also be readily used to gather temperature measurements from thermocouples. Therefore, the proposed test setup can collect the shear and compression wave velocities as well as the thermal maturity of the material up to an age of 72 hours. It will also utilize a thermal and/or digital imaging processes to estimate the patterns of heat dissipation and shrinkage of the concrete during curing. The proposed device will be assembled and tested for accuracy and precession and will be tested with a number of mixed with different levels of SCM and cement content, among other factors, to establish the feasibility of the system.