Issue 29

L. Contrafatto et alii, Frattura ed Integrità Strutturale, 29 (2014) 196-208; DOI: 10.3221/IGF-ESIS.29.17 197 In the paper the application to chemical anchors in natural stone of theoretical models developed for anchoring systems in concrete is commented and the comparison between the theoretical results and the experimental data reported in [7] is performed. The anchor system under investigation consists in threaded rods glued to basalt, sandstone and limestone supports by means of epoxy resin. A set of experimental tests has been numerically simulated by using some models classically implemented in engineering structural software and the limit of these predictions is highlighted. Specifically, both axis-symmetric and 3D Finite Element discretisation of the anchor system have been performed. Drucker-Prager, Mohr-Coulomb or Concrete models were used to rule the stone constitutive behavior. A hardening plasticity model was assigned to the steel element. In 2D axis-symmetric simulations the resin was modeled linearly elastic, while in 3D simulations a bond slip model was introduced at the stone-rod interface. Moreover, the simulation of the pull-out test has been performed by the applications of an advanced software, belonging to the scientific research field, showing that only through a refined Finite Element modeling, based on enhanced formulation, a correct prediction both of the limit strength of the anchor and of the entire failure process can be obtained. E XPERIMENTAL BEHAVIOUR OF THE ANCHORING SYSTEM n [7] the minimum embedment depth for chemical anchoring of post-installed threaded rods in basalt, sandstone and limestone support has been experimentally determined. All the experiments took place at the Laboratory of Structural and Material Testing of Catania University. Three types of natural stone samples were used in the tests, typical of eastern Sicily: sandstone squared blocks from the southeast Sicily (Palagonia sandstone) all of size 15x25x45 cm 3 , basalt irregular blocks coming from a local quarry in the region of volcano Etna (Etna basalt), with dimensions ranging between 20x20x40 cm 3 to 30x40x60 cm 3 , limestone blocks of quite irregular shape from the region of the mountain chain of Nebrodi and dimensions ranging between 20x30x40 cm 3 to 40x40x60 cm 3 . The material data obtained by the mechanical characterization are reported in Table 1. Basalt Limestone Sandstone Uniaxial compressive strength [N/mm 2 ] 500 220 20 Young Modulus E [N/mm 2 ] 50000 19616 12309 Density [kN/mm 3 ] 30.0 26.0 14.3 Table 1 : Mechanical properties of the rocks Threaded steel rods were unmarked, 4.6 class (tensile strength 400 N/mm 2 and yield strength 240 N/mm 2 , verified by performing uniaxial tensile tests. The HIT-RE 500 epoxy resin of the company Hilti was used as bonding agent, specifically designed for fastening into solid materials such as concrete, grout, stone or solid masonry. It is a high strength, two part epoxy adhesive composed by Component A (Epoxy resin, filler material) and Component B (hardener amine base, filler material). The drilling of each rock sample was performed by column drill, with a hole diameter always 4 mm larger than the rod diameter, considering an efficient thickness of resin of 2 mm all around the rod, as usually recommended in these applications. Hole cleaning was carried out using compressed air and specific brushes to allow the optimal adhesion between the resin and the stone material. The epoxy resin was injected into the hole accompanied by the immediate insertion of the rod by screwing. The effect of the embedment length on the pull-out strength of the steel rods chemically bonded into the three types of rock blocks was investigated for three embedment depths (3, 5 and 10 times the bar diameter) and three nominal diameter (10 mm, 14 mm, 20 mm). The experimental tests consisted of applying a static pull-out force to the bar chemically bonded in the stone specimen by means of a hydraulic hollow cylinder FPT CRM-30/100 and measuring the intensity of the force by means a HBM MGC Plus data acquisition system. The triangular reaction frame with height-adjustable feet in Fig. 1(b), bearing on the stone block, was used as a base for the hollow cylinder, because stone blocks had irregular faces. Moreover the device had the role of maintaining the zone possibly interested by crisis mechanism free from confinement. Eighty-one tests were performed. Each test was labelled by using a 4-fields alphanumeric code. The first alphabetic field identifies the stone type on which the experiment was done. A: Palagonia sandstone. B: Etna basalt. C: Floresta I