International Concrete Abstracts Portal

The current industry practice for acceptance testing of post-tensioning grouts includes an accelerated corrosion test (ACT), as recommended in Post-Tensioning Institute (PTI) M55.1-03, “Specification for Grouting of Post-Tensioned Structures.” The most significant downside of the current ACT method is the length of time required to run the test, which typically ranges from 1 to 2 months to complete. The linear polarization resistance (LPR) technique, on the other hand, requires only a few hours to complete; and the research presented in this paper indicates a strong correlation between the time to corrosion, as measured by the ACT method, and the system polarization resistance, as measured by the LPR technique. Based on the test results to date, the authors recommend the use of the LPR method as a prescreen for very high-quality grouts so that lengthy ACT testing is not necessary.

A nonmetallic anchorage for carbon fiber-reinforced polymer (CFRP) tendons, made from a specially developed ultra high-performance concrete (UHPC), was described. The UHPC anchorage system, however, was too large for use on site (120 mm [7.9 in.] diameter x 180 mm [7.1 in.] long). A smaller anchor of tougher material, requiring less wrapping of the barrel, was needed for use on site. A new reactive powder concrete (RPC) anchorage consisting of a CFRP-wrapped barrel and four wedges has therefore been developed. The system is similar in size to standard steel anchorage. Specimens were cast and tested with CFRP tendons to determine if the new system could meet the Post Tensioning Institute (PTI) test requirements.

Most antiwashout admixtures are water-soluble polymers that modify the rheological properties of fresh concrete. Such admixtures have been incor-porated into concrete intended for underwater placements and repairs, and implemented in production of extremely workable and flowing concrete. They have also been used to enhance resistance to sagging of shotcrete and produce bleed-free cement grouts for filling post-tensioning ducts. This paper presents results from a study aimed at better understanding of the effects of antiwashout admixtures on concrete properties. It will highlight benefits and limitations of employing such admixtures in concrete. Fresh properties of low-, medium-, and high-strength concretes made using two types of antiwashout admixtures, a microbial polysaccharide and cellulose derivative, were evaluated. Fresh properties are compared with those of similar concretes made without antiwashout admixtures. Among the parameters evaluated are fluidity, bleeding, water dilution, segregation, setting time, and air content. Test results show that incorporation of an antiwashout admixture can greatly reduce external bleeding and significantly enhance resistance of concrete to water dilution and segregation. However; there is a signijcant increase in water demand, and a high-range water-reducing admixture (HRWRA) is needed to maintain a desired level of fluidity without excess addition of water. The combined additions of an antiwashout admixture and HRWRA delay setting time, especially at high concentrations of HRWRA. Viscous concretes containing antiwashout admixtures show a greater demand for air-entraining admixture. However; once enough air is entrained, proper air-void parameters needed to insure good freeze-thaw resistance can be obtained.

Alkali-aggregate reactivity in massive reinforced concrete structures often tends to develop at a very slow rate, with little if any of its characteristic signs. The visible symptoms, excessive distortions and cracking, are ambiguous and common to other root causes of structural distress. When suspected, possibly because it is perceived primarily as a chemical rather than structural problem, AAR diagnosis is traditionally delegated to laboratory testing. Being largely qualitative and far from infallible, as well as often devoid of a meaningful structural assessment, this approach may result in wrong or delayed diagnosis, with extremely costly consequences where large projects such as power generating facilities are involved. A new approach is proposed, based primarily on a structural in situ rather than laboratory investigation, and focusing on detecting the major structural symptoms of concrete swelling, namely, dimensional changes, build-up of concrete compressive stresses, and post-tensioning effect on reinforcing bars. The key diagnostic tools are overcoring tests for concrete stresses, reinforcing bar in situ test, and regression analysis on structural deformation-monitoring data, when available, all requiring detailed structural analysis simulation. The proposed methodology has been successfully applied to a major hydraulic power generating project, in which previous investigations have failed for 20 years to identify the root cause of continuing operational and structural problems, resulting in costly outages and maintenance.

Currently, there is a serious problem with the deterioration of concrete bridges due to corrosion induced by chloride intrusion into the concrete. Historically, the problem has been associated with conventionally reinforced concrete bridge structures as opposed to prestressed or post-tensioned structures. However, corrosion of steel tendons in prestressed concrete structures is potentially of even greater concern since the structural integrity of the bridge relies on the high tensile strength of the tendons. Corrosion-induced cracking of the tendons could lead to catastrophic failure of the structure. Grout is the final line of defense against corrosion of the steel tendons. Modifiers and additives for grouts were evaluated regarding their effect on relevant engineering properties of grouts in this application, including the ability of the grout to protect embedded tendons from corrosion. The beneficial effect of various additives and modifiers on grout fluidity, open time, bleeding/segregation, chloride ion permeability, and mechanical properties was demonstrated. An accelerated corrosion test (ACT) method was developed which evaluates the corrosion performance of steel tendons embedded in grout.