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This paper presents the first part of a study under-taken to measure the potential of self-leveling concrete to perform overhead repairs. The performances of self-leveling concrete in terms of its filling ability and bonding characteristics was compared to dry-mixture shotcrete and to normal cast-in-place concrete. Self-leveling mixture characteristics included the presence of superplasticizer, viscosity-modifying admixture (welan gum), air-entraining admixture (AEA) and silica fume.

The intent of this project is to study the fundamental characteristic properties of dry-mix shotcrete. Although several factors are known empirically to influence the shooting properties of the dry process shotcrete, it is generally not clear how and why these variables are important, particularly with regard to the maximum buildup thickness. With this in mind, different dry-mix shotcrete mixtures were shot, and rebound and maximum buildup thickness were evaluated for each. In particular each mixture was shot at a different consistency, which was evaluated from the force required to push a needle into the fresh shotcrete. Relationships were found between the penetration resistance of a fresh mixture and its fresh tensile strength or maximum buildup thickness (cohesiveness). However, distinct relationships were obtained for the different mixtures tested, which implies that consistency (penetration stress) is not sufficient by itself to characterise fresh dry-mix shotcrete. The addition of silica fume or air-entraining admixture allowed one to shoot better quality shotcretes. However, these additions had an opposite effect on the fresh tensile strength.

As part of a larger investigation in steel‐fiber‐reinforced wet‐mix shotcrete, companion specimens were cast using the shotcrete mixes for a direct comparison with shotcrete after hardening. Deformed steel fibers with five different geometries were investigated. Mixes were first shot on wooden forms and then cast using external vibration. Comparisons were made between shotcrete and cast concrete on the basis of compressive and flexural strengths and the flexural toughness. For a given mix, the average compressive strength of cast concrete was found to be significantly higher than that of the shotcrete. Flexural strengths, however, witnessed a reversal in the trend. In the case of flexural toughness, although the results for cast concrete were decisively different from that of shotcrete, no clear trend became apparent. The paper concludes that care should be exercised in predicting the behavior of shotcrete on the basis of the performance of cast concretes of similar proportions.

Five commercially available steel fibers with different geometries were investigated both in dry-mix and in wet-mix shotcretes. In the dry process, bulk-bin bags with dry premixes were used. In the wet process, ingredients were wet-mixed centrally in a batching plant and then transported to the shooting site in ready-mix trucks. Mixes were shot on wooden forms and the rebound characteristics of the fibers and other ingredients were determined. Once hardened, shotcrete panels were sawed and cored to obtain beam specimens for flexural toughness tests and cylinders for compression tests. Hardened shotcrete specimens were also tested for boiled absorption and permeable voids. The boiled absorption and permeable voids data indicated that an adequately dense in-place shotcrete was obtained for both processes. The rebound of fibers in the dry process was found to be much higher than that in the wet process. Both compressive and flexural strengths were found to be improved because of the addition of fibers. Fibers also led to a significant enhancement in the toughness of shotcrete. Key words: shotcrete, wet mix, dry mix, steel fibers, rebound, toughness.

This paper outlines the findings of laboratory tests to check the influence of curing and type of mould on scaling resistance of three types of concrete exposed to frost and deicing chemicals: a plain concrete and concretes with 5 and 10% silica fume (relative to total binding mass). Sodium chloride was the deicer used for the tests, which were performed according to standard ASTM C672 (Standard test method for scaling resistance of concrete surfaces exposed to deicing chemicals). The tests showed that type of mould (steel or wood) and curing (12 or 48 h) have relatively little influence on scaling resistance of concrete but that silica fume in the concrete can cause a significant drop in scaling resistance. Key words: concrete, silica fume, demolding age, formwork type, scaling resistance, freezing, deicing salt, sodium chloride. [Journal translation]

Tests were performed to study the influence of the type of cement and the type and dosage of air-entraining admixtures on the properties of dry-mix shotcrete, particularly on resistance to salt scaling. A total of 21 mixes were made using three types of cement (normal portland cement, high-early-strength portland cement, and silica fume cement), three air-entraining admixtures (a synthetic detergent, a sulfonated salt of hydrocarbon, and a vinsol resin), and five dosages of air-entraining admixtures (0, 10, 20, 30, and 40 ml per liter of water). Test results reveal that the use of an air-entraining admixture can lead to a significant decrease in the spacing factor and a significant increase in scaling resistance. The results further show that the properties of air-entrained shotcretes made with normal portland cement and high-early-strength portland cement were similar, but that drying shrinkage and chloride ion permeability were lower in those containing silica fume.

Bonding tests were performed to evaluate the capacity of different shotcrete mixes to provide a good and durable joint with concrete surfaces prepared for repair in various ways. A total of 21 slabs made of ordinary or conventional concrete were covered with a 75- to 100-mm layer of shotcrete. The concrete surfaces to be covered with shotcrete were prepared in different ways: sandblasting, chipping with jackhammers, grinding, or hydrodemolition. Both dry-mix and wet-mix shotcretes were used. Pullout tests, performed after 2 and 6 months, were used to evaluate the strength and durability of the bonding. The results indicate that the type of surface preparation has a strong influence on the strength and durability of the bonding, and that hydrodemolition is probably the best type of surface preparation. The shotcrete mix composition, however, was found to have relatively little influence on bonding durability.

This paper will discuss ASTM C 672 scaling tests that were carried out on normal concretes and concretes containing 5 percent silica fume, with air-void spacing factors in the 100 to 200 µm range. Five curing methods were compared: a 24 hr heat cycle with a maximum temperature of 70° C, 2 and 14 days moist curing, and two different curing compounds. Results indicate that the use of silica fume does not improve the scaling resistance of concrete. Concretes cured with one particular curing compound were found to have a scaling resistance similar to that of those cured in water for 14 days, weight losses after 50 cycles being lower than 1 kg/m(2) for all mixes. Concretes cured with the other curing compound had a lower and more variable scaling resistance. As expected, specimens cured for only two days in water also had a lower scaling resistance. All mixes cured using the heat cycle exhibited a poor performance, although, in this case only, silica fume reduced very significantly, but not sufficiently, the damage due to the freeze-thaw cycles in the presence of deicing salts.