International Concrete Abstracts Portal

An investigation was conducted to assess the structural integrity of a 17-year-old precast prestressed concrete conveyor bridge used to transport sodium chloride rock salt from a storage building to an outside stockpile area. The stockpile, depending on storage requirements, quite often buried most of the structure and/or subjected it to sodium chloride dust. The investigation revealed that the structure had performed remarkably well, considering the small concrete cover used to protect the reinforcing elements and the inadequate consideration of structural cracking induced by unanticipated loading from stockpiled salt. The concrete strength of the single tee members was estimated to be 7000 psi (48 MPa), with cover to the stirrups varying from virtually 0 to 1 1/2 in. (0 to 38 mm) and cover to the prestressing strands varying from 3/4 to 2 in. (19 to 51 mm). It was observed that aggressive prestressing strand corrosion causing pitting and some brittle wire failures occurred locally at flexural crack locations in single tee column members with little corrosion activity immediately adjacent to the cracks, even after 17 years of aggressive chloride exposure. This observation seems to conflict with the prevailing theory of the role of cracking on corrosion--that cracks perpendicular to steel reinforcement should result in limited early localized corrosion but, with time, chloride ions penetrate even uncracked concrete and initiate widespread corrosion.

Report presents the results of a multi-year laboratory exposure of more than 150 concrete samples to alternate immersion exposure in flowing sea water and flowing fresh water. Other exposure variables included loading, cracking, and electric currents. The validity of the controlled-exposure samples was determined by comparing the results with the results from selected samples removed from concrete structures throughout the United States. The results from a marine seawall are presented in this report and compared with previously reported results from marine masonry structures, highway bridges, and other structures.

The ultimate test of concrete durability to natural weathering is how it performs in the environment in which it is to serve. Laboratory testing yields valuable indications of service life and durability. However, the potential disrupting influences in nature are so numerous and variable that actual field exposures are highly desirable to assess the durability of concrete exposed to natural weathering The U.S. Army Corps of Engineers, through the Waterways Experiment Station, Structures Laboratory, maintains a natural weathering exposure station. It is located on Treat Island in Cobscook Bay near Eastport, Maine. This station has been in use since 1936 and is an ideal location for exposure tests, providing twice-daily tide reversals and severe winters. The average tidal range is about 18 ft (5.4 m) with a maximum of 28 ft (8.5 m) and a minimum of 13 ft (4 m). In the winter, the combined effect of air and water temperatures creates a condition at meantide where specimens are repeatedly thawed and frozen. There have been 23 completed investigations and many of these have been previously reported. There are currently 40 active investigations. Four of these investigations are briefly discussed in this paper.

The relative durability of air-entrained concrete with and without boiled linseed oil coating was evaluated using the rapid freeze-thaw method of ASTM C 666, Procedure A, by measuring the reduction in dynamic modulus and weight loss of the test prisms. Surface scaling was also monitored visually. The richness of the mix was varied by using cement factors of 267 kg/m3 (450 lb/yd3) and 300 kg/m3 (506 lb/yd3). Freeze-thaw durability was reduced in some test prisms of the richer mix through the addition of a chloride-based setting and strength accelerator. Curing prior to freeze-thaw cycling was varied in the lean mix. Prisms were either water-cured for 2 weeks prior to coating with linseed oil before testing, or were coated with linseed oil after removal from molds at 24 hr and then cured under ambient low humidity before freeze-thaw testing at 2 weeks. Uncoated specimens that were water-cured for 2 weeks prior to freeze-thaw cycling served as the control. The use of undiluted linseed oil proved to be superior to using a 50 percent Varsol-50 percent boiled linseed oil mixture, contrary to what was expected. Mixes proved to be quite durable except for the mix in the third series, in which the chloride-based accelerator reduced the freeze-thaw resistance below acceptable limits. The use of a linseed oil coating provided some, though insufficient, improvement in the durability of this mix.

Samples from nine air-entrained concrete mixes made with and without a superplasticizer were examined under a scanning electron microscope to determine the size distribution of the voids in the 0.5 to 50 æm range. Concurrently, samples of the same mixes were examined under a binocular microscope to determine the size distribution of the voids in the 10 to 1000 æm range. The voids observed under the electron microscope were separated into two categories: air voids (spherical in shape or nearly so) and large capillary pores (irregularly shaped). The results show that, in mixes, the amount of capillary pores with diameters ranging from 0.5 to 50 æm is relatively important (the number of these voids generally represents approximately half the total number of entrained air voids). The role of these pores in the frost resistance of concrete is believed to be strongly dependent on their degree of saturation at the time of freezing. The number of air voids smaller than 10 æm in diameter, however, was found to represent less than 10 percent of the total number of entrained air voids. These small air voids are thus expected to have little influence on frost durability. The results also indicate that the distribution of the ir-void diameters is influenced by the nature of the air-entraining agent but not by the use of a superplasticizer. The distribution of air-void diameters was found to be approximately the same for all mixes, irrespective of the value of the spacing factor.