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2015SpringShotcreteEMag

sticky mixture with low slump and high cohe- them as well as the free water. However, siveness is desired to minimize rebound and because the selected dosage rates for the two increase buildup thickness.15 RC agents were within the manufacturer’s Currently, there are no standardized test recommended limits, the impact on the water methods that can measure sprayability and demand was negligible. pumpability. Therefore, it is a common practice It should be noted that slump flow only to assess consistency, viscosity, rebound, and indicates consistency (ease of flow) and does not buildup thickness to evaluate the sprayability evaluate cohesiveness (tendency to bleed and and pumpability of a particular mixture. In this segregate). It is especially important for shotcrete study, slump flow was tested to evaluate the mixtures to have high cohesiveness, as they are flowability as an indicator of pumpability, and less prone to segregation under pressure. Having rebound was tested to evaluate the sprayability a mixture with high cohesiveness and stickiness characteristics of the tested mixtures. is also desired to maximize thickness buildup Figure 1 shows the effect of two RC agents and minimize rebound. Due to the smaller and SF mixtures on slump flow, which was particle size associated with higher specific targeted as 20 ± 1.25 in. (500 ± 30 mm). All the surface area of the RC agents, they work as tested mixtures were within the acceptable nucleation sites for the precipitation of CSH gel, variation limits of the target slump, and they and have stronger Van der Waals and electrostatic performed similar to each other regardless of ionic forces between particles.8 Considering that their mixture constituents. For a given grade of the main source of cohesion in cement paste is RC agent, increasing the dosage rate slightly the CSH gel,16 it is expected for RC agents to reduced slump due to the increased particle increase cohesion due to a) its impact on packing, which decreased the volume between accelerating and forming additional CSH gels; and b) its reactant surface particles exhibiting stronger tendency for adsorption of ions and increasing the surface adhesion between adjacent particles and to other materials. Rebound The mixture was sprayed with a pump rate of 7.85 yd3/h (6 m3/h) with a pump pressure ranging between 800 and 870 psi (55 and 60 bar) on a vertical concrete wall of 6.6 x 6.6 ft (2 x 2 m) with a thickness of about 4 in. (100 mm). TYTRO® SA alkali-free set accelerator was added at an amount of 6% of the total cementitious materials content directly at the nozzle. After finishing the spraying process, the amount of concrete on the concrete slab and the rebound were measured with a balance. Fig. 1: Effect of two RC agents and SF on slump flow Rebound loss is affected by many factors, such as the position of the application, distance and angle of the nozzle from the sprayed location, skill and expertise of the nozzleman, air pressure, impact velocity, thickness of layer, amount of reinforcement, and mixture design (for example, cementitious materials content, water and air content, size and gradation of aggregates, and the presence and dosage rate of admixtures). According to the ACI 506 guide- line,17 for vertical walls, the approximate range of rebound loss is 10 to 30%. In many field applications, it is common to obtain higher than 15% rebound. According to Fig. 2, mixtures containing SF and RC agent were efficient in reducing rebound to as low as 5 to 6%. Based on the aforementioned information, it is a significant improvement to obtain a rebound loss Fig. 2: Effect of RC agent and SF on rebound at such low percentages for vertical walls. The 28 Shotcrete • Spring 2015


2015SpringShotcreteEMag
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