The Actions of Axials Loads on Masonries - Ultrasonic Verifications and Statistical Analysis of Obtained Data

Among the different non-destructive tests used to carry out an exhaustive survey about the mechanical and durability characteristics of a masonry made of solid tile-bricks, the ultrasonic method has experienced a wide diffusion thanks to its low costs and to its easiness of use; the method essentially consists in detecting the propagation velocity of ultrasonic pulses propagating through a solid to be acoustically characterized. The solid considered in this theoretical and experimental survey has been obtained from a meaningful statistical sample of masonry units made of solid tile-bricks blocks bound together by cement mortar. The experimentation made use of one set of masonry panels, prepared for this purpose, consisting of solid bricks of UNI type (size of block cm 5.5 x 12 x 25 produced in Italy), arranged according to the thickness or according to a “one-head” pattern scheme . The total height of the samples of the set is 48 cm inclusive of cement mortar layers, the mortar having been prepared according to the Italian regulations. The issues which came out during the statistical analysis of the experimentation results are basically of inductive nature: the purpose is to move from a limited and incomplete knowledge of the values of the propagation velocity quantity measured on the masonry, to get the most suitable definition of the possible distribution of all the values that can be reached by velocity under stress conditions imposed by the strength applied on the masonry at the moment of measuring the ultrasonic wave. The strength is generated by a press used to simulate the conditions imposed on the axial strength of the specimens under investigation. The purpose of this work is to present a procedure for the interpretation of the data in order to assess the compressive stress state by using the ultrasonic velocity variable obtained from masonry samples expressly prepared for this experiment. Introduction: Among existing non-destructive tests suitable to carry out an exhaustive survey about the mechanical characteristics of wall facings, the ultrasonic method has been fairly well disseminated thanks to his low costs and its user-friendly operation; the procedure mainly consists in detecting the velocity of propagation of ultrasound impulses spreading inside a solid to be acoustically characterized. For the purpose of this work, the solid was made by a fixed number of panels of solid brick masonry connected to each other by means of cement mortar. The ultrasonic investigation is presented and proposed as a predicting tool to extend to the measurements of the simple compressive stress state in the panels assessed during failure tests, the corresponding values of the velocities of longitudinal propagation measured simultaneously through the definition of a functional link. The paper intends to propose an interpretation procedure able to assess the compressive stress state of given masonry elements by using the pulse velocity variable obtained on a series of masonry samples specifically prepared of this kind of experimentation. Results: Six masonry panels have been expressly prepared for the experimentation. Each panel has been prepared according to the scheme (figure 1) foreseeing the arrangement of seven courses of solid bricks UNI-type placed according to the brick thickness, therefore resulting in a one-head positioning after the interposition between courses of a layer of equivalent gauged mortar of type M3 prepared according to what provided for by the Italian standards on masonries (D.M. of 20/11/1987) for this type of mortar.[1] Wall panels with geometrical dimensions BxLxH of cm 12.0 x 52,0 x 48,0, levelled on their upper and lower parts through the interposition of a layer of distribution and levelling mortar have individually undergone to simple compressive test through the application of a centered load generated by a 2000 KN press in order to determine the ultimate compressive load. Figure 1. Preparation of the masonry panel for breaking test after having obtained the ultrasonic velocity data Along with the preparation of the panels, also other specimens have been prepared to determine the most meaningful mechanical characteristics of each individual constituent material of the wall facing by proceeding in accordance to what indicated at paragraph 2.1.1. of the Annex to the above mentioned D.M. Before breaking the masonry samples, the available masonry panels where subjected to ultrasonic characterization. To this end, the panels were subjected to three different loading steps: no load, 200 and 400 KN; at each loading step, the ultrasounds propagation velocity was measured through indirect technique (probes placed on the same wall face) applied with horizontal trajectories on a base of cm 34,7 + D, D being the 5 cm diameter of the probes. For each loading condition, the results expressed in terms of propagation velocity of the ultrasound impulses, are listed in the table below (1). In particular, for the statistical variable “pulse velocity” it is possible to read the number of measurements carried out, the value of the mean of velocities, the amplitude of the sample in terms of minimum and maximum value, the standard deviation and, finally, the coefficient of variatiance, considered as the ratio between standard deviation and the mean. Load Number of individual values Min velocity