Review Category : Building Blocks

Mortar is specified by proportions or by properties. The proportion method is simply a mortar recipe or certain volumes of cementitious materials and aggregate combined with water that gives a workable mix. Experience shows that if a specified recipe is followed, mortar with certain performance characteristics is consistently obtained. Sampling, testing, or measurement of properties in the laboratory or in the field is not required of a proportion-specified mortar.

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A Game Changer

Ultra-high-performance concrete (UHPC) was first introduced as reactive powder concrete (RPC) in the early 1990s by employees of the French contractor Bouygues. When introduced, it came in two classes: Class 200 MPa (29 ksi) and 800 MPa (116 ksi). In the U.S., several state departments of transportation have expressed interest in introducing UHPC in their bridge projects, supported by Federal Highway Administration (FHWA) research as well as research done by universities. Most notably, Virginia has produced I-beams with UHPC and Iowa has built two bridges with UHPC beams and one with a UHPC deck. Significant interest has recently been directed at using UHPC in longitudinal joints between precast concrete beams. Use of UHPC in bridges has increased in applications in the U.S. primarily due to leadership by the FHWA.

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The current seismic design philosophy for bridges in high seismic risk areas is prescribed by nationally accepted design standards that have been based on laboratory testing since the 1960s. For ordinary bridges that are expected to experience strong seismic ground motions, this philosophy prescribes structural details that have been shown to provide a low probability of collapse.

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How Do They Impact Structural and Other Key Properties?

Admixtures are used to modify and improve the properties of fresh and hardened concrete. The use of the proper admixtures can result in increased workability, cementitious efficiency (psi per pound of cement), and optimum setting time with mixes, even those with low water/cementitious ratios.

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Specified and Used Regularly for Reinforced Formed Members

Self-consolidating concrete (SCC) is increasingly used for reinforced formed-members, architectural concrete, and floors and slabs. It is often the only solution for structural repairs. SCC is a highly flowable, non-segregating concrete with a slump/flow of 20 to 30 inches. SCC can be easily placed and completely fills forms under its own weight without mechanical vibratory consolidation.

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Masonry has a long performance history as a durable, economical, and attractive building system. It has been used successfully in all climates and can serve as both the structural system and architectural finish. Construction today requires materials that are easy to use and have dependable performance, so manufactured masonry units that have uniform size and properties are much more common than natural (stone) units. Similarly, portland cement-based mortars and grouts are preferred for their consistent setting and strength characteristics.

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Flat plate voided concrete slab systems have been used for many years in Europe and other parts of the world. These systems are becoming increasingly popular in the U.S. This is due to many inherent benefits which include reduced self-weight (resulting in smaller column sizes and foundations as well as smaller seismic forces); larger allowable superimposed loads for given span lengths; economical longer spans; reduced floor-to-floor heights; and accelerated construction schedules.

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Discussions of cold and hot weather concrete procedures do not occur until the five-day forecast calls for extreme weather. At that point, everything becomes a rush.

Special concrete mixes, as required, need to be submitted for approval at the beginning of a project. Time is required to test and practice with different temperatures and dosages and to adjust the cocktail of admixtures that form the basis of high-performance concrete today.

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The option to use structural steel supplied and fabricated in foreign countries for projects constructed in the United States is a realistic one for many projects in today’s market. Typically, the most obvious factor that is considered by the project team is the economic impact of doing so. However, in addition to economics, project teams must also consider factors that may have a significant impact on the project outcome, including project-specific issues, design requirements, material substitutions, procurement, fabrication, and construction concerns.

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Many buildings are partially or wholly clad with stone. There are many aspects of stone anchorage that can, and do, affect the long-term performance of the building envelope and the initial and long-term costs of the envelope and building operation. This article presents critical stone anchorage issues with a focus on anchor and fastener selection and design based on the author’s experience.

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STRUCTURE magazine