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Consistent quality in concrete production is an essential requirement in the ready-mix concrete industry. Insuring batch-to-batch uniformity is, in fact, an ever-present challenge. Moreover, modern concretes mixture designs often have complex behaviors in their fresh states, and yet the focus is on the slump (related to the yield stress) only, presenting an incomplete picture when one wants to assess the properties of various types of concrete. A second parameter known to be highly informative in this situation is the plastic viscosity of the fresh concrete. Few authors have assessed the potential of rheology measurements with a ready-mix truck. After a short introduction on sustainability, the aim of the first part of this conference paper is to offer a review of the different methodologies and results available on rheology measurements in/with a ready-mix truck. The second part is focused on the description of the in-drum sensor system used in this project. Finally, methodology and results about the slump evaluation with the in-drum sensor system are presented.

The second industrial revolution has been accompanied by the massive use of concrete. However, rejected loads or demolitions of new structures due to poor quality indicates that quality control still needs to be improved. Furthermore, with the advent of the third industrial revolution, the industry needs more advanced production systems; for example, high-tech implementation processes like 3D concrete printing will require continuous rheology control systems. In that respect, to answer today’s and tomorrow’s questions, Universite Laval has initiated a research program using an on-board sensor system for ready-mix concrete production to monitor real-time concrete fresh properties.

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.

Tests were carried out to study the influence of set accelerating admixtures on the properties of dry-mix shotcrete, especially its durability. A total of 27 mixtures were shot using two different ...

Le beton projete est un materiau de plus en plus utilise, car il est tres bien adapte a la reparation des degradations d'ordre superficiel, degradations de plus en plus presentes particulierement dans les pays nordiques. Il existe deux procedes pour projeter le beton: le procede par voie seche et le procede par voie humide. Les travaux a l'Universite Laval ont permis de mieux connaitre l'influence de plusieurs parametres (ajout de fumee de silice, de fibres, d'adjuvants, etc.) sur les proprietes du beton projete selon les deux procedes. Tres souvent, les proprietes du beton projete durci ressemblent de tres pres a celles d'un beton conventionnel de meme composition et les resultats des essais ont bien montre qu'il est possible de fabriquer des betons projetes durables, et qui resistent meme au gel en presence de sels fondants. Ils ont aussi montre que la technique de preparation des surfaces a reparer a beaucoup d'influence sur la durabilite de l'adherence des reparations minces en beton projete. L'hydrodemolition semble etre la technique qui donne les meilleurs resultats.

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]

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.

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.