Monitoring of stress distribution in damaged small-scale masonry walls by using two innovative sensors

Structural Health Monitoring (SHM) represents a strategic solution for the preservation of cultural heritage buildings. Existing masonry structures often suffer reductions in mechanical performances due to physiological aging of material constituents, external actions, and effect of catastrophic natural events. In many cases, the prompt prediction of damage in masonry elements is difficult and it can cause sudden collapses, compromising the safety of people. The proposed experimental study examines the effectiveness of two low-cost and innovative stress sensors, i.e. piezoelectric and capacitive stress sensors, for SHM of masonry structures. To this scope, the sensors were embedded in the mortar joints of two small-scale clay brick and calcarenite masonry wall specimens consisting of three panels. Experimental tests were carried out by applying a constant vertical compressive load at the top of each specimen and simulating the damage with a progressive reduction of the cross-section of one of the panels. During the tests, the vertical stress distributions (and their variations), were monitored by the sensors. Experimental outcomes from sensor reading were then compared to that numerically provided by a refined finite element simulation of the test. Results will show that vertical stress variations in masonry structures can be effectively accounted by the adopted sensors and potentially interpreted for the early prediction of structural damage

Effect of FRP Wraps on the Compressive Behaviour of Slender Masonry Columns

In the last decade, Fibre Reinforced Polymer (FRP) wrapping technique has become a common method to retrofit masonry piers or columns with poor structural performances. The passive confinement effect induced by the external wrap allows increasing the compressive strength and ductility of the member. Several studies highlighted as the efficacy of this technique is affected by several key parameters, including the shape of the transverse cross section, stress intensification at the strength corner of sharp sections, amount and mechanical properties of adopted composite. Despite this technique has been widely studied from both theoretical and experimental point of view, most of studies focused on short columns and little information is available on the influence of second order effects on its structural efficacy. This paper presents a simplified method able to assess the effect of FRP confinement on slender columns. A preliminary evaluation of the constitutive law in compression of FRP confined masonry is made and the best-fitting model is adopted to model masonry in compression. Sectional analysis is performed by including the tensile strength of masonry and considerations are made on the increase of ultimate moment and curvature. Finally, the effect of column slenderness is considered using a simple numerical procedure, making it possible to calculate the allowable slenderness ratios as a function of the maximum drift, taking into account both strength and stability

Constitutive Numerical Model of FRCM Strips Under Traction

In this paper, the tensile behavior of Fiber Reinforced Cementitious Matrix (FRCM) strips is investigated through Finite Element (FE) models. The most adopted numerical modeling approaches for the simulation of the fiber-matrix interface law are described. Among them, the cohesive model is then used for the generation of FE models which are able to simulate the response under traction of FRCM strips tested in laboratory whose results are available in the technical literature. Tests on basalt, PBO and carbon coated FRCM specimens are taken into account also considering different mechanical ratios of the textile reinforcement. The comparison between FE results and experimental data allows validating the adopted numerical modeling approach. Finally, some considerations are provided on the effects of the fiber fabric mechanical ratio and the strength and stiffness of the interface on the tensile capacity of the FRCM strips

A new FEM approach for FRP-strengthened RC frames

Owing special mechanical properties has made Fiber Reinforced Polymer (FRP) as one of the best strengthening materials for Reinforced Concrete (RC) structures. The effect of externally applied FRP sheets on RC elements has been the topic of many experimental investigations. On the other hand, simulating the behavior of RC structures strengthened by means of externally bonded FRP sheets is yet to be scrutinized. In this work, a new Finite Element (FE) procedure for inelastic analysis of RC structures has been enriched to account the presence of externally bonded FRP sheets on RC sections. The proposed FE procedure works in the framework of lumped plasticity. It is able to identify the exact location of plastic hinges and imposes rotational discontinuities in the position of plastic hinges. The yield domain of the FRP-strengthened sections is constructed based on the hypothesis of strain linearity and taking into account the debonding failure between FRP sheet and concrete substrate. Numerical applications are presented for unstrengthened and strengthened RC frames. Pushover analysis has been carried out on numerical applications and the results are compared to each other. In order to verify the results of the numerical applications, the same models are built and analyzed in OpenSees, which proves the integrity of the FE procedure and its applicability for FRPstrengthened RC frames

Shear strength of High-strength concrete beams: modeling and design recommendations

In the present paper, the flexural and the shear resistance of high strength reinforced concrete (HSC) beams with longitudinal bars, in the presence of transverse stirrups is analyzed both theoretically and experimentally. The experimental researches here presented are parts of previous researches carried out by the author. Researches refer to HSC beams with high percentages of steel bars failing in shear and in flexure. From the analytical point of view, a model based on the evaluation of the resistance contribution due to beam and arch actions including bond splitting and concrete crushing failure modes is developed and presented. The model was verified against available experimental results and those recently obtained by the author. Some of the more recent analytical expressions able to predict the shear and the flexural resistance of concrete beams were mentioned and design considerations are made referring to a ductile design of HSC beams. Finally, design recommendations were derived with the proposed model and compared with expressions given in most common codes

Experimental calibration of flat jacks for in-situ testing of masonry

Flat-jack testing method is one of the most commonly used techniques for the structural assessment of existing masonry structures. Single and double flat jacks are usually adopted to evaluate the acting normal stress, or the compressive behaviour of masonry material. Test procedures are codified by international standards (e.g., A.S.T.M D4729-87; C1196-04; C1197-04, R.I.L.E.M TC 177–MDT D.4; R.I.L.E.M. TC 177–MDT D.5), which provide the preliminary calibration of an experimental coefficient (km) and of the effective area (Aeff), which determination influences significantly the reliability of the test. This article presents the result of an experimental study on the calibration of flat jacks for masonry testing. The problem is investigated by several tests carried out on two types of common flat jacks, which differ for geometry and producer. Two calibration methods are adopted in order to relate the pressure values of the flat jack with those of the hydraulic press and load cycles are performed in two different pressure ranges. Finally, a theoretical interpretation of results is made, which gives good predictions of calibration parameters. Results of this investigation highlight the influence of constructive features and service pressure of the jack adopted, for obtaining reliable results from the tests on masonry structures

Constitutive Models for the Tensile Behaviour of TRM Materials: Literature Review and Experimental Verification

In recent years, the scientific community has focused its interest on innovative inorganic matrix composite materials, namely TRM (Textile Reinforced Mortar). This class of materials satisfies the need of retrofitting existing masonry buildings, by keeping the compatibility with the substrate. Different recent studies were addressed to improve the knowledge on their mechanical behaviour and some theoretical models were proposed for predicting the tensile response of TRM strips. However, this task is complex due to the heterogeneity of the constituent materials and the stress transfer mechanism developed between matrix and fabric through the interface in the cracked stage. This paper presents a state-of-the-art review on the existing constitutive models for the tensile behavior of TRM composites. Literature experimental results of tensile tests on TRM coupons are presented and compared with the most relevant analytical models proposed until now. Finally, a new experimental study is presented and its results are used to further verify the reliability of the literature expressions

Compressive behaviour of eccentrically loaded slender masonry columns confined by FRP

Fibre Reinforced Polymer (FRP) confinement represents an effective tool for retrofitting masonry piers or columns enhancing their structural performance. This technique has been widely studied in the literature mainly with reference to short columns, while no extensive information is available on the influence of second order effects on its efficacy in case of slender members. Within this framework, the presented study concerns a simplified method able to assess the effects of FRP confinement on the compressive behaviour of slender masonry columns. A proper constitutive law of FRP confined masonry in compression is adopted for performing a sectional analysis, in which also considerations are made on the increase of ultimate moment and curvature depending on the role of some key parameters. The effect of column slenderness is evaluated using a simple analytical iterative procedure, making it possible to calculate the allowable slenderness ratios as function of the maximum drift, taking into account both strength and stability. The effect of slenderness on eccentrically loaded columns is finally examined through a finite element model used for validating the results of the proposed analytical procedure, allowing also to draw safety domains useful for design/verification purposes. It is shown as the column's slenderness affects the efficiency of confinement, being the latter negligible for values of normalized length greater than 20

Modelling of FRP and FRCM-confined masonry columns: critical review for design and intervention strategies

This paper presents a critical review of the most established analytical models for the prediction of the compressive strength of FRP and FRCM-confined masonry columns. In particular, two types of fibres are analysed, i.e. glass and basalt. A wide dataset available in the literature is used for the application of the analytical models and for the development of parametric analyses useful for the critical comparison of FRP vs. FRCM confinement technique and glass vs. basalt fibres to be adopted as reinforcement of masonry substrate. The effects of stiffness and strength of the reinforcement, the number of reinforcing layers, the compressive strength of masonry and the cross-section shape are investigate