International Concrete Abstracts Portal

Paper reports the results of part of a program to determine the extent and severity of carbonation in buildings in Canada. About 350 core samples drilled from 28 buildings in Toronto were tested by two procedures to determine the depth of carbonation. Tests were made on cast-in-place balconies and vertical components and on precast cladding. A proportion of the total sample was found to be susceptible to carbonation damage within a reasonable service life.

Twenty-seven high-performance concrete mixes (with 28-day strengths in the 80 to 100 MPa range) were prepared to evaluate the deicer salt scaling resistance of such concretes after various periods of curing. Three water-cement ratios (0:30, 0:26, and 0:23) were used, and for each water-cement ratio a minimum of three mixes were made with different air-void systems: one with a spacing factor of approximately 200 æ, one with a slightly higher value, and one without any air entrainment. Canadian Type 30 cement with an addition of 6 percent silica fume was used for all mixes. The coarse aggregate was a 14 mm minimum size, crushed, very dense, dolomitic limestone. The curing period varied between 1 and 28 days. A total of 54 specimens (2 for each test condition) were submitted to 150 daily cycles in accordance with ASTM Standard C 672, using sodium chloride as a deicer. Weight loss was measured to evaluate the deterioration of the concrete surfaces. The scaling resistance was found to be extremely good in all cases, irrespective of the length of curing, water-cement ratio, or spacing factor value, weight losses after 150 cycles being always lower than 0.50 kg/mý. No correlation was found between the scaling resistance and the spacing factor or the length of curing. Loss of mass was generally concentrated around a few aggregate particles. These results indicate clearly that it is possible to prepare high-performance concretes with very good deicer salt scaling resistance without using any air entrainment.

The air or oxygen permeability of concrete is usually measured by means of techniques that utilize mechanical driving forces. Thus, air or oxygen is forced to pass through a piece of concrete using different mechanical pressures. The flow of gas so measured is used as an indication of concrete permeability and sometimes is also used to predict the durability of concrete reinforcements based on the relationship between anodic corrosion rate and amount of oxygen, which may be reduced in the cathodic areas. However, this extrapolation may lead to erroneous conclusions, because a dry concrete allows a higher amount of oxygen to pass through it than a wet one, although the corrosion rate should be much lower in dry than in wet concrete. In this paper, comparisons between flow of oxygen measured in paste, mortar, and concrete specimens held at different relative humidities using electrochemical driving forces (polarization at about -750 mV SCE), and corrosion rates (measured by means of polarization resistance) are presented to discuss the inherent relationships. The results show that the oxygen permeability is only dependent on the amount of electrolyte inside the pores, but the corrosion rate is also dependent on the concrete resistivity, which is fixed by the amount of pore water content.

A study was made of three commercially available "second-generation" high-range water-reducing admixtures (HRWR) using cement of high and moderate C3 A content and having a cement content of 545k lb/yd3 (323 kg/m3) and a water-cement ratio (w/c) of 0.50. Second-generation HRWR were used to reduce cement and water contents by 15 to 16 percent. Hardened concrete specimens were prepared and tested for freeze-thaw resistance, resistance to deicer scaling, permeability to chloride ions, drying shrinkage, and compressive strength development. In addition, the air-void systems of concretes containing second-generation HRWR and air-entraining admixtures were analyzed by linear transverse. Similar tests were performedon flowing concretes, where cement and water contents were maintained constant and second-generation HRWR were added to increase initial slump levels to 7 to 9 in. (75 to 225 mm). Results indicate that caution must be exercised when using these admixtures to reduce cement contents in concretes subjected to deicing chemicals, as performance may be adversely affected, especially in high-slump "flowing" concretes. Additionally, drying shrinkage may be moderately increased in these concretes.

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.