International Concrete Abstracts Portal

Retarders are very important during the production of cement-based materials. The delay in setting might be helpful in avoiding negative phenomena related to the long-term transport of the fresh concrete mix, unforeseen breaks in the transport, or laying of concrete. These admixtures prevent the local temperature rise of the concrete, and thus the formation of cracks and also extent the workability. Set-retarders provide a correct development of the microstructure and the undisturbed setting and hardening of cement which lead to higher strengths of cement-based materials. An investigation of the cement mortar with potassium methylsiliconate (MESI) applied as set-retarding admixture was carried out. Siliconates are a highly alkaline water solution of methylsiloxane resin in the potassium or sodium hydroxide. The study involved the cement paste and mortar with three dosages (1%, 2%, and 3% per cement mass) of organosilicon admixture. So far, the siliconates were not applied as admixtures for cement mortar or concrete. The mortar specimens were tested for compressive strength after 1, 2, 7, and 28 days and frost resistance after 25 freeze-thaw cycles. Moreover, the impact of the methylsiliconate admixture on the hydration (by isothermal calorimetry) and setting time of the ordinary Portland cement was also studied.

Probably the most common report in the concrete industry is the concrete test report. Typically, a report is sent when a 7-day break is completed. Later, the same report is updated and resent with the 28-day test results. The concrete test report serves one basic purpose: to assure those involved with a project that the right concrete was delivered to the job site. While the format of test reports can vary from one testing laboratory to the next, each will contain the information needed to determine whether the concrete meets the job-site requirements.

Specifications for our job require casting one cylinder to be tested at 7 days, two cylinders for testing at 28 days, and one backup cylinder to be tested at a later date if the results of the 28-day test don’t meet specification requirements. What action should be taken when 7-day results are unsatisfactory, but concreting beyond the member from which concrete was sampled has already progressed substantially?

Historically, resistance to wave action has been the main design criterion for large concrete armor units. To study the influence of fabrication practices on cubic armor units, six different concrete mixtures were first evaluated using small test cubes. Tensile and compressive strengths and adiabatic temperature development were evaluated for the concrete mixtures during the initial 7 days of curing. To simulate the hardening process at the macro scale, the data were used in finite element analyses, which were used to evaluate temperatures and stresses. Several parameters were varied in the analyses, including the volume of the blocks, the concrete mixture properties, formwork type, stripping time, and ambient conditions. Analyses showed that temperature differentials do have the potential of creating stresses sufficient to crack the concrete. Specifically, the analyses indicate that blocks may lose shape as their corners break off, or they may fracture completely through the thickness and fall apart. Cracking and breaking observed in blocks on an existing breakwater agree very well with the cracking patterns predicted during the analyses.

This paper describes current practices and proposed changes for the seismic analysis and design of bridges at Caltrans. It includes an explanation of the new performance criteria for ordinary and important bridges. It provides a description of Caltrans' new site-specific ground input motions which are based on magnitude, acceleration, and the shear wave velocity of the surrounding soil. A variety of new bridge analysis tools and new bridge models, many of which seek to describe the nonlinear behavior of bridges and bridge elements, are described in the paper. Finally, current and proposed seismic design procedures for columns, joints, and superstructures are briefly illustrated. Caltran's seismic bridge procedures reflect the geological, industrial, and economic conditions of California. California's high seismicity requires analysis and design methods that are state-of-the-art. Often, new procedures are implemented only days after the research is completed. Since most bridges in California are made of cast-in-place reinforced concrete, there are few breaks in the structure for the placement of isolators or dampers. Dissipation of energy must occur in ductile columns rather than in special devices. Because, California's economy is dependent on its highway system, methods of protecting vital transportation arteries during and after disasters are being studied.