W. Micah Hale

Measured Transfer Lengths of 0.7 in. (17.8 mm) Strands for Pretensioned Beams

The use of 0.7 in. (17.8 mm), Grade 270 (1860) prestressing strands has advantages over 0.5 and 0.6 in. (12.7 and 15.22 mm) strands. This study provides design guidelines for estimating transfer length of 0.7 in. (17.8 mm) strands. Sixteen pretensioned concrete beams using a single prestressing strand were fabricated with conventional concrete and self-consolidating concrete. The concrete strengths at 1 day ranged from 5.9 to 9.2 ksi (40.7 to 63.4 MPa). Transfer lengths were determined using concrete surface strains along with the Average Maximum Strain Method. Initial strand end slips were also measured for predicting transfer length at release using an empirical formula. Experimental results indicated ACI 318 and American Association of State Highway and Transportation Officials (AASHTO) specifications are applicable for estimating transfer length of 0.7 in. (17.8 mm) strands at release and at 28 days. The results also showed that a coefficient of 2.32 was the most appropriate value for estimating transfer lengths at release from initial strand end slips.

Effect of High Temperatures and Heating Rates on High Strength Concrete for Use as Thermal Energy Storage

In recent years due to rising energy costs as well as an increased interest in the reduction of greenhouse gas emissions, there is great interest in developing alternative sources of energy. One of the most viable alternative energy resources is solar energy. Concentrating solar power (CSP) technologies have been identified as an option for meeting utility needs in the U.S. Southwest. Areas where CSP technologies can be improved are improved heat transfer fluid (HTF) and improved methods of thermal energy storage (TES). One viable option for TES storage media is concrete. The material costs of concrete can be very inexpensive and the costs/ kWhthermal , which is based on the operating temperature, are reported to be approximately

Measured Development Lengths of 0.7 in. (17.8 mm) Strands for Pretensioned Beams

1. Researchers using concrete as a TES storage media have achieved maximum operating temperatures of 400°C. However, there are concerns for using concrete as the TES medium, and these concerns center on the effects and the limitations that the high temperatures may have on the concrete. As the concrete temperature increases, decomposition of the calcium hydroxide (CH) occurs at 500°C, and there is significant strength loss due to degeneration of the calcium silicate hydrates (C-S-H). Additionally concrete exposed to high temperatures has a propensity to spall explosively. This proposed paper examines the effect of heating rates on high performance concrete mixtures. Concrete mixtures with water to cementitious material ratios (w/cm) of 0.15 to 0.30 and compressive strengths of up to 180 MPa (26 ksi) were cast and subjected to heating rates of 3, 5, 7, and 9° C/min. These concrete mixtures are to be used in tests modules where molten salt is used as the heat transfer fluid. Molten salt becomes liquid at temperatures exceeding 220°C and therefore the concrete will be exposed to high initial temperatures and subsequently at controlled heating rates up to desired operating temperatures. Preliminary results consistently show that concrete mixtures without polypropylene fibres (PP) cannot resist temperatures beyond 500° C, regardless of the heating rate employed. These mixtures spall at higher temperatures when heated at a faster rate (7° C/min). Additionally, mixtures which incorporate PP fibres can withstand temperatures up to 600° C without spalling irrespective of the heating rate.Copyright

COMPARING DIFFERENT CEMENTS IN HIGH-PERFORMANCE CONCRETE

The use of 0.7 in. (17.8 mm), Grade 270 (1860) prestressing strands in construction is slow regardless of the engineering advantages of these types of strands. The limited research data and unavailable design guidelines partially account for the slow use. This study measured development length for 16 pretensioned concrete beams, in which the prestressing strand was tensioned to 75% of its ultimate strength. The beams were fabricated with conventional concrete or self-consolidating concrete (SCC). The concrete compressive strengths at 28 days of age varied from 9.2 to 13.4 ksi (63.5 to 92.5 MPa). The development length was determined by conducting bending tests at different embedment lengths. The experimental results indicated that the measured development lengths did not show a good correlation with concrete compressive strength. The ACI 318 equation significantly overpredicts the measured development lengths. A simple equation was proposed to predict development length of 0.7 in. (17.8 mm) prestressing strands.

Development of Geopolymer Mortar for Field Applications

Eight cements encompassing different types, manufacturers, and plant locations were each examined in two classes of high-performance concrete (HPC) mixtures. Class I and Class 2 mixtures were designed to achieve compressive strengths of approximately 60 and 75 MPa (8700 and 10,880 psi) at 28 days, respectively. Criteria for comparing mixtures included workability, compressive strength, splitting tensile strength, and modulus of elasticity. Mixtures containing a Type III cement achieved the highest compressive strength at all ages tested, most significantly at early ages. At 28 days, cement characteristics influenced splitting tensile strength more significantly than compressive strength and compressive strength more significantly than modulus of elasticity. The ACI 209 equations underestimated the rate of compressive strength development at early ages. The ACI 363R equation was mostly accurate within ′ 10% in describing splitting tensile strength. The ACI 363R equation underestimated modulus of elasticity by more than 10%, while the ACI 318 equation, extended beyond its valid range, underestimated most of the modulus of elasticity results. The applicability of these empirical relationships must be confirmed for different cements used in HPC.

A New Smoothing Technique for Transfer-Length Determination

In recent years there has been an increased demand for environmentally conscious and sustainable construction materials. One such material is “geopolymer” or “alkali-activated” binder. Current industry practice uses ordinary portland cement (OPC) in combination with supplementary cementitious materials as binder in concrete and mortar. Cement production is very energy intensive and accounts for approximately 10 percent of the total carbon dioxide emission in the world. In geopolymer materials, OPC is replaced with waste materials such as fly ash or slag cement along with a chemical activator. When supplied with additional chemical constituents, the aluminate and silicate present in fly ash or slag cement arrange into a polymeric structure with similar properties to hydrated cement. Proper application of this material can reduce, or even replace, the use of OPC in concretes and mortars. In addition to the cutback in OPC and the use of waste products, geopolymer materials require less water for curing and are shown to have increased resistance to chemical attack. While this product offers a “green” alternative to OPC, it is a new technological concept. Fly ash variability creates inconsistencies in quality control and chemical content. The chemical activators used to facilitate polymerization are often quite expensive and dangerous to use. Additionally, heat curing can be necessary for a geopolymer material to achieve specified compressive strength. Present limitations warrant extensive research and development to increase practicality of geopolymer materials for field implementation. In order to aid in the design and use of geopolymer materials, it is important that laboratory studies fully address the use of less than ideal materials and conditions in the creation of geopolymer. By using low cost materials and avoiding heat curing, laboratory research on geopolymer mortar can function as a means of developing a material that can be readily adopted into practice.

Properties of concrete mixtures containing slag cement and fly ash for use in transportation structures

The authors acknowledge the financial support from the Mack-Blackwell Rural Transportation Center (MBTC) and Ton Duc Thang University. The authors would like to thank Insteel Industries Inc. and Sumiden Wire Products Corporation (SWPC) for providing strands for this research. The authors are also thankful to C. Murray, W. Philips, J. Daniels III, D. Davis, and R. Hagedorn for help in experimental procedure at the Engineering Research Center at the University of Arkansas.