Page 22

2017SpringShotcreteEMag

Fig. 3: Seismic spalling of brittle shotcrete lining (from Stacey, 2001) SCOPE The intent of the following sections is to provide a quick overview of emerging shotcrete technologies/practices offering solutions for these tunneling applications: • Producing a highly durable shotcrete lining; • Efficiently producing high quality shotcrete on-demand; • Reducing rebound and controlling dust/chemical emission; • Speed-up the development cycle and reduce the curing period of shotcrete before re-entry; and • Improving the spalling/impact resistance of the shotcrete lining in challenging ground conditions. INNOVATION IN SHOTCRETE TECHNOLOGIES Highly Durable Shotcrete Lining Several studies have proven that sprayed concrete materials can provide similar or better transport properties and durability than conventional cast-in-place concrete (Zhang, Morgan, and Mindess, 2016). This section briefly describes the key rules to follow produce a highly durable shotcrete lining. MINIMIZING THE WATER-TO-CEMENT RATIO Porosity in cementitious materials such as concrete/ shotcrete is the first order parameter governing most of the material’s performance, especially its durability. Because porosity of concrete is directly related to the water-to-cement ratio (Neville, 2011), limiting the water content of sprayed concrete is the first preventive action to minimize porosity of the in-place material and therefore Fig. 4: Schematic illustration of temporary high initial air content concept (from Jolin and Ginouse, 2012) guarantee its performance in terms of mechanical properties and durability. In the dry-mix process, even if the amount of water added to the dry mixture is controlled in real-time by the nozzleman, the placement mechanisms limit the waterto cement ratio between 0.35 to 0.45 with 0.40 as typical value, which guarantees performance and durability (Zhang, Morgan, and Mindess, 2016). Even for overhead surfaces, the dry-mix process allows for instantly adjusting the plastic consistency to ensure proper build-up simply by reducing the amount of water added to the dry mixture. This straightforward and instant adjustment of the plastic consistency makes the dry mix process a very robust and reliable way to place durable shotcrete materials with low water-to-cement ratios. This is achieved with a limited amount of additives such as plasticizer and set accelerator. In parallel, it is very common with the new generation of superplasticizers to produce good quality wet-mix shotcrete using typical water-to-cement ratios lower or equal to 0.40. However, as previously mentioned, highly pumpable concrete with a low water to cement ratio does not guarantee its shootability. In other words, the material can be very easy to pump but it can sag and not stick to walls especially overhead surfaces. If special care is not given to the mixture design of wet-mix shotcrete, one could incorrectly believe that adding plasticizing admixtures could be the solution to ensuring adequate pumping and shootability. As illustrated in Fig. 4, a highly plasticized mixture would generally exhibit 20 Shotcrete | Spring 2017 www.shotcrete.org


2017SpringShotcreteEMag
To see the actual publication please follow the link above