Jung Jun Park

Combined effect of expansive and shrinkage-reducing admixtures on the properties of ultra high performance fiber-reinforced concrete

In order to investigate the combined effect of expansive and shrinkage-reducing admixtures on shrinkage and mechanical properties of ultra high performance fiber-reinforced concrete, a series of experiments including three types of shrinkage tests, compressive and flexural tests, and penetration-resistance test were performed. Test results showed that about 51–53% of free shrinkage and 35–49% of restrained steel strains were reduced by adding 1% shrinkage-reducing admixture along with 7.5% expansive admixture with slight change of strength. Elastic stress, residual interface pressure and degree of restraint from ring specimens were also significantly reduced by using expansive and shrinkage-reducing admixtures. Although the elastic stress of the ring specimen without expansive and shrinkage-reducing admixtures was higher than the tensile strength, no shrinkage crack was observed for all test specimens due to the relaxation effect.

Combined Influence of Expansive and Shrinkage Reducing Admixtures on the Shrinkage Behavior of Ultra-High Performance Concrete

Since ultra-high performance cementitious composite (UHPCC) exhibits remarkable compressive strength and ductility, its application for structural member provides significant reduction of the section. The use of low water-to-binder ratio and admixtures with high fineness induce large shrinkage strain, leading to the possibility of large occurrence of shrinkage cracks due to the restraints of the form and reinforcing bars. Therefore, this study admixes a combination of expansive and shrinkage reducing agents as a solution to reduce the shrinkage of UHPCC. The eventual appropriateness is computed to evaluate the free shrinkage and restrained shrinkage behaviors. From the test results, the admixing of a combination of expansive and shrinkage reducing agents is seen to achieve a reduction of the free shrinkage by 30% to 50% at 28 days and a reduction of the strain of the external steel by about 19%.

Optimized Ultra High Performance Fiber Reinforced Concrete Mixture with Intensified Volumetric Stability

This study performed compressive, tensile and restrained shrinkage tests to find out the optimized mixture including shrinkage reducing admixture (SRA) and expansive admixture (EA) enabling to reduce the shrinkage of Ultra High Performance Fiber Reinforced Concrete (UHPFRC). The test results showed that the mixture using 1% of SRA and 7.5% of EA satisfied the design strength criterion despite of slight loss in the compressive and tensile strengths. In addition, this mixture exhibited the lowest restrained shrinkage strain by offsetting the quick setting and delaying the development of the restrained shrinkage stress.

Effect of Area Ratio of Coarse Aggregate on Erosion Resistance of Offshore Concrete Structure

This study investigates the effects of the compressive strength and area ratio of coarse aggregate of concrete for offshore construction on the erosion depth. To evaluate the erosion resistance of concrete, test was performed according to ASTM C 1138 using a plain mix with design strength of 30 MPa using only Portland cement and five other mixes including different quantities of fly ash and blast furnace slag. The test results showed that the erosion depth is influenced not only by the compressive strength but also by the area ratio of coarse aggregate.

Evaluation of Setting Time in Ultra High Performance Concrete

Ultra high performance concrete (UHPC), characterized by a high strength and high ductility, is also subjected to large shrinkage due to its low water-to-binder ratio and its large content in high fineness materials. The large amount of autogenous shrinkage of UHPC can induce crack on structural member when it was restrained with reinforcement and form. However, shrinkage of UHPC in plastic state is not generating confining stress, which is the main cause of initial crack. Normally, the setting time in concrete is an index to distinguish shrinkage which occur confining stress or not. An estimation of setting time is conducted in compliance with ASTM C 403 till now however, that test standard reveals error of results due to discordance of test condition as following with concrete type. This study therefore evaluated setting time of UHPC through the modified test method which was proposed by KICT. Test results and analyses proved a discrepancy of setting time between ASTM and proposed method. The proposed method put faith in evaluation of setting time in accordance with UHPC.

An Evaluation on the Restrained Shrinkage of Ultra-High Performance Concrete

Since ultra-high performance concrete (UHPC) is subject to large occurrence of shrinkage at early age due to its low water-to-cement ratio, the mixing of large quantities of powdered admixtures and the absence of coarse aggregates, UHPC presents large risks of shrinkage cracking caused by the restraints provided by the form and reinforcing bars. Accordingly, this study intends to evaluate the shrinkage behavior of UHPC under restrained state by performing restrained shrinkage test using ring-test. The test results reveal that increasing thickness of the inner ring increases the tensile creep at early age leading to the reduction of the average strain and residual stress of the inner ring.