Denis Beaupre

Tests to characterise properties of fresh dry-mix shotcrete

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.

USE OF AIR-ENTRAINING ADMIXTURES IN DRY-MIX SHOTCRETE

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.

DEICER SALT SCALING RESISTANCE OF DRY- AND WET-PROCESS SHOTCRETE

American Society for Testing and Materials (ASTM) C 672 deicer salt scaling tests were carried out on both dry- and wet-mix shotcretes. Twenty-five dry-mix shotcretes and eleven wet-mix shotcretes were used to fabricate different test panels. The mix variables included cement type, silica fume, latex, polypropylene and steel fibers, and set-accelerating and air-entraining admixtures. All test panels were wet-cured for 7 days except two additional panels, one of which was not cured and the other of which was cured with a curing compound. Water containing 2.5 or 3 percent salt solutions was used for the scaling tests. The scaling residues were collected and weighed to evaluate deterioration. The mass of scaling residues was found to vary between 0.1 and 24.0 kg/m. These tests indicate that the scaling resistance of both dry- and wet-mix shotcrete improves with an increase in the air content, and that the use of silica fume generally reduces the mass of scaling residues. These tests also indicate that the use of a set-accelerating admixture can significantly reduce the scaling resistance of shotcrete. The use of Type 30 cement and of an air-entraining admixture was found to markedly improve the scaling resistance of dry-mix shotcretes.

Effects of Particle-Size Distribution in Dry Process Shotcrete

Dry-mix shotcrete has been successfully used for many years in construction and repair. Its unique pneumatic application yields an in-place material with low workability and relatively high stiffness well adapted to vertical and overhead applications without formwork. For some time, reduction of rebound--particles not adhering to the surface during shooting--has received attention due to its important effects on costs and in-place properties. The aim of this project is to study the effects of the particle-size distribution of the granular phase in dry-mix shotcrete. The results presented confirm the validity of what is known as ACI Gradation No. 2 for shotcrete recommended by ACI Committee 506, Shotcreting. Moreover, a fundamental approach involving optimal packing density offers a simple and interesting explanation of the observed behavior and a basis for future work.