International Concrete Abstracts Portal

The potential of using the stainless steel coatings to protect steel reinforcing bars from corrosion especially in a salt contaminated concrete environment was evaluated. Type 304, 3 16 and 420 stainless steel coated coupons and reinforcing bars were prepared using the Twin-Wire Electric Arc (TWEA) and High Pressure/High Velocity Oxygen-Fuel (HP/HVOF) processes. Metallographic examinations were conducted to determine the coating density and the oxide content. The corrosion resistance of the coatings was evaluated using linear polarization and salt spray techniques. The stainless steel coating prepared using the HP/HVOF process had a much superior corrosion resistance performance than those prepared using the TWEA process. The former process produced a dense, low oxide content coating while the latter produced relatively porous coatings.The oxide content of the coating in the TWEA process can be significantly reduced using argon or nitrogen as primary and arc jet gases. This did not have a significant effect on porosity of the coating.

Corroding reinforcement is the most common form of deterioration in reinforced concrete structures in New Zealand. The first part of the paper describes the causes, extent and severity of rebar corrosion in New Zealand concretes as revealed by the systematic inspection of several hundred structures since the early 1980s. The second part of the paper discusses local approaches to inspection, maintenance and repair of affected structures. The third part describes research currently being carried out by the authors to reduce the risk of future rebar corrosion in new construction and in concrete repairs. These research programs include investigations of the effectiveness of coatings for reinforcement, different methods of cleaning contaminated steel, and different cement/binder combinations in preventing further corrosion. Electrically non-conductive concrete made from high resistivity concrete and fibre reinforced plastic reinforcement is also being investigated.

The basalts of the central Auckland area, classified as nepheline basanites, exhibit a high potential for alkali release. When they are combined with Waikato River sand in concrete, an expansive reaction could occur with the alkali release. Methods developed at the New Zealand Institute for Industrial Research and Development (IRL) were applied to the few failed public structures at Auckland. These showed levels of contribution by the basalt of sodium and potassium in the pore solutions of the concrete exceeding 5kg Na20eq/m3. Bulk chemical and x-ray diffractometric phase analysis of the Auckland nepheline basanites reveal only minor variations. Feldspathoids (nepheline and in the case of Mt. Wellington basalt also leucite) are singled out as the main alkali-releasing phases. Their content can be estimated from the composition of a simple acid leach. This determines only the full alkali-release potential which is probably not frequently developed in Auckland concrete structures. Experimental mortars attain these high alkali-release levels if small aggregate grain size and high W/C are applied. At moderate W/C (eg 0.47) and curing time (eg one year) the rate of alkali release in concrete develops greater differences between and within quarries. These differences are controlled by the interstitial surface area and the phases that can be accessed by the pore solutions. Their alkali release decreases in the following order: feldspathoids (nepheline and leucite) > interstitial glass > feldspar (labradorite). Slow cooling of the lava, and incipient alteration have a delaying effect on alkali release. Screening tests that are faster than mortar curing and address these variables are: a ) analysis of alkaline leachates using 0.3 M LiOH and 0.3M tetramethylammoniumhydroxide (TMA-OH). b ) the measurement of internal surface area and micropore volume of the basalt by absorption-desorption porosimetry or sorption characteristics for heavy alkalies (Rb, Cs). C> electron microprobe investigation of alkali-bearing phases. Although none of these tests on its own provides all the desired information, the analysis of the alkaline leachates is likely to provide a practical measure for the eve1 of alkali that will be released.

Construction of transverse joints with lateral molds in RCD ( Roller Compacted Dam) method has been omitted because concrete layers are casted continuously. However, transverse joints in dam concrete should be required to prevent the occurrence of cracking. In this study, the development of a vibrating machine which can make the joints was done so that the transverse joints could be constructed in the RCD concrete. Many kinds of experiments were conducted using this machine with respect to period and direction of joint, pattern of joint material and its injection, in order to make clear the method for construction of joints. The proposed method is applicable in the actual construction of RCD concrete, and preferable results are obtained.

The development of the roller-compacted concrete (RCC) technology in the 1980s is considered by many as a significant breakthrough in the construction industry. RCC is now commonly used all around the world for the construction of dams and pavements, and for the rehabilitation of existing structures. A critical review of the recent developments in the mixture proportioning of RCC is presented. Discussions include a brief description of each existing method. Special emphasis is placed on recent innovations based on the optimum paste volume and optimum packing density concepts. Each method is evaluated on the basis of its ability to yield cost-effective RCC mixtures with optimum workability, permeability and mechanical properties. The influence of various parameters, such as the use of supplementary cementing materials, mineral fillers and air entrainment is also discussed.