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The Effect of Sodium Silicate on the Behaviour of Shotcretes for Tunnel Lining

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The Effect of Sodium Silicate on the Behaviour of Shotcretes for Tunnel Lining Journal of Scientific Research & Reports 14(2): 1-8, 2017; Article no.JSRR.33641 ISSN: 2320-0227 SCIENCEDOMAIN international www.sciencedomain.org The Effect of Sodium Silicate on the Behaviour of Shotcretes for Tunnel Lining Luigi Coppola1, Alessandra Buoso1, Denny Coffetti1, Patricia Kara2*, Sergio Lorenzi1 and Franco D’Alessandro3 1DISA, University of Bergamo, Dalmine (BG), Italy. 2EMIB, Faculty of Applied Engineering, University of Antwerp, Belgium. 3BASF C.C. Underground Construction, Treviso (TV), Italy. Authors’ contributions This work was carried out in collaboration between all authors. Author LC designed the study. Authors AB, DC and FDA performed the statistical analysis and wrote the protocol. Authors PK and SL managed the analyses of the study, the literature searches, wrote the first draft of the manuscript. All authors read and approved the final manuscript. Article Information DOI: 10.9734/JSRR/2017/33641 Editor(s): (1) Luigi dell'Olio, School of Civil Engineering, Channels and Ports, University of Cantabria, Cantabria, Spain. (2) Luigi Rodino, Professor of Mathematical Analysis, Dipartimento di Matematica, Università di Torino, Italy. Reviewers: (2) Halil Görgün, Dicle University, Turkey. (3) Leo Baldenegro, Center of Engineering and Industrial Development, Mexico. Complete Peer review History: http://www.sciencedomain.org/review-history/19001 Received 24th April 2017 th Case Study Accepted 4 May 2017 Published 10th May 2017 ABSTRACT Present case study investigates the rheological, mechanical and in-placing performances of fiber- reinforced shotcrete manufactured with different fibers (steel, glass and polypropylene) and with sodium silicate based set-accelerating admixture for tunnel linings. The study compares the performances of concretes manufactured and fully compacted with those shotcretes which are manufactured directly on the job-site. The influence of sodium silicate accelerator on mechanical and rheological properties of fiber-reinforced shotcretes with respect to reference concrete were evaluated. It was observed that: The addition of fibers does not influence slump and workability retention with respect to reference concrete, independent of type and dosage of fibers; Spraying and set accelerator dosage determined a decrease about of 10-30% in compressive strength compared to that of concrete placed and vibrated without sodium silicate accelerator; The set- accelerating admixture has negative effect on compressive strength of fiber-reinforced shotcrete (15%). _____________________________________________________________________________________________________ *Corresponding author: E-mail: [email protected]; Coppola et al.; JSRR, 14(2): 1-8, 2017; Article no.JSRR.33641 Keywords: Sodium silicate based set-accelerating admixture; mechanical and rheological properties. 1. INTRODUCTION followed by a sharp reduction of mechanical properties of concrete at later ages. Sprayed concrete, or shotcrete, is used world- wide to construct tunnels or underground For many years, fibers of different nature have structures. The poured mix must set quickly in been used to reinforce shotcrete in tunneling order to stabilize the structure [1-2]. The applications. Several research investigations important basic requirements of shotcrete are have ascertained the significant effect of fiber good adhesiveness, a low amount of rebounding, addition on ductility and punching resistance of good shooting, and the quick formation of initial tunnel segments and shotcrete panels [8]. strength [3-4]. To meet these requirements, Currently, concrete is reinforced by using steel, accelerators have become a fundamental glass, polypropylene or acryl-nitrile fibers and additive in shotcrete to achieve appropriate initial carbon nanotubes in order to assess the stress strength, reduce rebound, and suppress early level in reinforced concrete elements [9-13]. To ground relaxation. This affects not only the early improve durability in severe conditions and to strength of the shotcrete but also the favor replacement of temporary solutions development of long-term strength, durability and requiring subsequent cuttings glass fibers and thickness. Traditionally, shotcrete's quick-setting glass fabric solutions are used [8,14-16]. To properties have been achieved by the injection of increase fire resistance and prevent explosive high-alkaline additives at the spraying nozzle. spalling of concrete cover polypropylene fibers Accelerating admixtures are composed of are used [17]. Compared with traditional steel chemicals that influence the rate of cement mesh, fibers arrange themselves in three- hydration, thereby shortening the setting time dimensional directions inside the cement matrix and, in some cases, increasing the rate of early and they are able to absorb the tensile stress strength development [3]. Set accelerators affect induced by shrinkage and thermal gradients. Therefore, fibers could limit crack width and both the C3A hydration (by interfering with the increase the energy absorption capacity C3A – gypsum reaction) and the C3S hydration (aiding the dissolution of lime) [5-6]. However, (toughness) of the material [18]. In particular, this method resulted in concrete with high fibers determine a considerable improvement in porosity and density, low strength and durability, the post-cracking behavior of concrete [19]. and environmental concerns. Reference concrete fails suddenly once the deflection corresponding to the ultimate flexural Nowadays, shotcrete accelerators are classified strength is exceeded; On the other hand, fiber- as silicate-, aluminate-, or alkali-free, cement- reinforced concrete continues to sustain based minerals, depending on the main material considerable loads even at deflections that is present. The sodium silicate accelerator considerably in excess of that of the reference gives a rapid initial set and a slow final set, while concrete. So, compared to reference concrete, the aluminate accelerator provides a slow initial fiber-reinforced concrete is tougher and more set and a fast final set. Both types suffer the resistant to impact [18], permitting control of the same deficiency of loss in strength and durability local detachment of tunnel linings [7,20-21]. over the long term. Furthermore, their strong alkalinity may endanger workers and lead to The paper presents research results on environmental contamination. Increasing the rheological and mechanical properties of accelerator amount adds to the cost and also reference and fiber - reinforced shotcretes increases the rebound ratio [1,7]. To mitigate manufactured with sodium silicate accelerator these issues, alkali-free and cement-based and glass, steel and polypropylene fibers. mineral accelerators (CMs) that are environmentally friendly and provide good long- 2. EXPERIMENTAL INVESTIGATION term strength are now frequently used in construction sites. Sodium silicate based 2.1 Materials admixtures are more effective – in terms of set acceleration - and cheaper than alkali-free Limestone Portland cement (CEM II/A-LL 42.5R) products. For these reasons, sodium silicate according to EN 197-1 was used. The chemical accelerators are widely used in tunnel linings. composition of cement is shown in Table 1. However, a very fast increase of early strength Coarse-grained sand (40% of the total mass of development in sodium-accelerated shotcretes is the aggregates) and crushed sand (25%) were 2 Coppola et al.; JSRR, 14(2): 1-8, 2017; Article no.JSRR.33641 used as fine aggregates. In addition, crushed (GF) and polypropylene fibers (PF) were added stone (max size 8 mm) was used as coarse to the mix. The main properties of fibers are aggregate (35%). A sieve analysis of fine and listed in Table 4. The amount of fibers added to coarse aggregates is shown in Table 2. the concrete mixes is shown in Table 5. The dosage by volume of steel and glass fibers is Table 1. Chemical composition (% by mass) very similar; Consequently, rheological and of the cement (CEM II/A-LL 42.5R) mechanical properties are directly comparable establishing the efficiency of the two types of CEM II/A-LL 42.5R fibers. The dosage of polypropylene fibers is Loss of ignition, % 0.88 lower than that used for the steel and glass fibers (about 37% and 39% lower, respectively). During SiO2, % 19.31 Al O , % 4.31 casting (at 1/5 and 4/5 of the placing), before the 2 3 addition of set-accelerating admixture, the CaO, % 61.08 workability and the density were evaluated in MgO, % 2.38 order to investigate the fiber influence on SO3, % 2.96 rheological properties of the concrete. The fiber Na2O, % 0.29 content and the shotcrete rebound were K2O, % 0.86 measured according to EN 14488-7. TiO2, % 0.14 Fe2O3, % 2.28 Combined aggregate grading is presented in Fig. 1. A polycarboxylate-based superplasticizer with dosage 1.4% vs. cement mass was used [6,22]. Sodium silicate accelerator was used as a set- accelerating admixture. The dosage of sodium silicate accelerator was in the range of 13% - 17% vs. cement mass and it was added in the nozzle during spraying. The w/c was equal to 0.44. Workability of reference concrete was measured by slump test according to EN 12350- 2 and the consistency class required for the mix was S5 (slump ≥ 220 mm). The composition of Fig. 1. Aggregates grading reference concrete without fibers and set- accelerating admixture is shown in Table 3. 2.2 SPECIMENS In order to compare the influence of different Test panels with and without the addition of set- types and dosages of
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