Camilla Colla

GPR of a timber structural element

GPR of timber structural elements is not yet very common in practise but the technique has the potential for condition evaluation of timber material. In this laboratory investigation, a high frequency (2.3 GHz) radar antenna has been employed for detailed investigation of a historic timber beam from a roof structure. The GPR non-destructive diagnostic of the spruce beam was aimed at obtaining material information with a structural relevance, such as presence of knots, their position and depth, cracks, dimensional variations of geometry and extension of areas of decay, The radar scan was carried out along the beam length. The radargram obtained show very detailed information.

Salt Effects in Plastered and Unplastered Outdoors Brick Masonry: Quantitative Laser Monitoring of Surface Decay Evolution

In masonry materials, the superficial decay is a widespread problem. Aggressive environmental agents such as moisture and salts trigger the damage by propagating through the material capillary pores. Although several studies have been carried out on salt crystallization and their damaging effects, additional research effort is required to better investigate this phenomenon on real cases and real weathering conditions. To this purpose, testing and monitoring tools capable of following degradation process since the early beginning are necessary. Repeated visual inspections are commonly used to monitor the superficial decay, but this technique is subjective and thus not capable of providing any quantitative information. In this work, an experimental campaign, carried out in Bologna, Italy, is presented. A two-header brick wall, one main face unplastered and one plastered, was stored outdoors and exposed to weathering over two summers. Before the start of the second aging season, moisture and salt capillary rise was simulated by low-concentrated sodium chloride solution (0.1% -wt). The aim was to favor solution evaporation and salt crystallization and to provoke material damage. The degradation process was monitored based on a contactless, rapid and accurate image diagnostic technique. In particular, high-resolution laser scanning by triangulation technique was adopted. Three-dimensional data acquisition was repeated at the end of both seasons. The proposed procedure successfully extracted quantitative information approximately areas of material spalling and detachment even in the initial phases of decay.

Investigation of Damp and Salt Distribution in Outdoors Full-Scale Masonry Wall via Wireless Monitoring and Radar Testing

Porous construction materials such as masonry constituents are notoriously affected by weathering, pollutants attack and damp transport with manifest consequences on the masonry visible appearance and its mechanical reliability. Water transport mechanisms inside such materials as well as the chemical reactions and phase changes occurring in salt at varying combinations of temperature and humidity content have been previously studied. Nevertheless the investigation of damp and salts degradation processes since their appearance in real structural elements and the monitoring over time of their evolution is a problem never completely investigated...

Damp and Salt Rising in Damaged Masonry Structures: Numerical Modelling and NDT Monitoring

Existing research on structural deterioration of historic structures generally concentrated on direct mechanical causes of deterioration rather than on physical-chemical effects of aggressive agents on the mechanical parameters of masonry and their interaction. The research presented herein studies deterioration of masonry in a unified environmental framework which accounts for both the mechanical and the physical-chemical effects and their coupling. Numerical models and experiments have considered various cases including mechanically damaged and undamaged masonry walls with moisture capillary rise from foundations, with and without salt inclusions at different concentrations. Examples from on-going work are shown.

Diagnostic by imaging: 3D GPR investigation of brick masonry and post-tensioned concrete

In this laboratory study, the comparison of 4 different antenna frequencies (from 1.0 to 2.6 GHz) and radar systems on the same masonry and post-tensioned concrete specimens has served to investigate the precision achievable in imaging cross sections of masonry walls with different brick layouts and of concrete slabs with metal post-tensioning ducts at varying depths. Through high-density 3D data collection and multi-phase post-processing, the work aimed at 2D and 3D data imaging for eased and enhanced diagnosis of structural elements.

Photoelasticity and DIC as optical techniques for monitoring masonry specimens under mechanical loads

To evaluate the complex behaviour of masonry structures under mechanical loads, numerical models are developed and continuously implemented at diverse scales, whilst, from an experimental viewpoint, laboratory standard mechanical tests are usually carried out by instrumenting the specimens via traditional measuring devices. Extracted values collected in the few points where the tools were installed are assumed to represent the behaviour of the whole specimen but this may be quite optimistic or approximate. Optical monitoring techniques may help in overcoming some of these limitations by providing full-field visualization of mechanical parameters. Photoelasticity and the more recent DIC, employed to monitor masonry columns during compression tests are here presented and a lab case study is compared listing procedures, data acquisitions, advantages and limitations. It is shown that the information recorded by traditional measuring tools must be considered limited to the specific instrumented points. Instead, DIC in particular among the optical techniques, is proving both a very precise global and local picture of the masonry performance, opening new horizons towards a deeper knowledge of this complex construction material. The applicability of an innovative DIC procedure to cultural heritage constructions is also discussed.

Mechanical and Mineralogical Characterization of Mortar in Masonry Buildings Damaged by the 20-29th May Earthquake in Emilia

The aim of the work is to characterize mechanical and chemical properties of the mortar, and thus the masonry, of historical buildings damaged by the Emilia May 20-29th 2012 earthquakes. The attention was focused on historical strategic buildings, located in Modena district. The experimental campaign here reported was carried out for each building through four steps: first, a mechanical characterization of masonry was performed in situ using non-destructive and semi-destructive methods (step 1); then, some materials were collected in order to obtain standard specimens of mortar joint and brick (step 2). In laboratory, those samples were properly prepared for testing (step 3) with compression and flexural test setups; finally, a limited number of those mortar samples were tested in order to obtain their chemical properties (step 4). The in-situ and laboratory test results were separately elaborated for mortar and brick, to characterize the single masonry components. The results were then used to estimate the characteristic masonry parameters. Criteria to obtain the masonry behavior were reviewed in order to give the average masonry parameters. The characteristic and average values of compression strength were compared with ranges provided in the codes for the same masonry typology.

Monitoring of masonry compression tests in the lab via optical correlation without surface preparation

Digital image correlation (DIC) is a recently developed optical technique allowing high-definition maps of displacements and deformations of the element under view. Various fields of application are already known where thermal and mechanic distorsions of the object have been explored by comparing false-colour images recorded in different deformation states. The method is becoming more common in civil engineering where first applications were limited to metal structural elements and composite material reinforcements [1-3]. The main advantages of this optical technique lie in contact-free, full-field measurements. In order to enhance the displacement spatial resolution, one of the recognised requirements of the method – in fact, so far, unavoidable – is the preparation of the investigated surface by applying a thin, white mat coating followed by a distribution of speckles (black dots) of appropriate sizes. This preliminary procedure constitutes a disadvantage of DIC due to the necessary preparation time and to inappropriateness of the requirement in case of testing high-value objects such as cultural heritage [4]. In this work, an innovative experience of DIC is presented for a case of mechanical test monitoring of brick masonry, in the lab (specimen dimensions: 0.50x0.5x0.12 m3). In addition, no coating of the surface of interest has taken place. The attempt is to validate the applicability and reliability of the method for this composite material when the masonry materials’ texture is exploited as the reference pattern to monitor geometry variations of the specimen under increased load levels. A further challenge of the experiment was due to the visible degradation state of the masonry specimen, which prior to the destructive compression load test had undergone accelerated ageing through cycles of salt crystallization and consequent discolouring and partial skin loss. The aims were to obtain from DIC high-definition full-field information able to reliably replace traditional measurement instruments such LVDTs, extensometers or TML gauges, determining instead 3D in-plane and out-of-plane displacements, tensions and strains at different load levels up to specimen collapse. The outcome of the experiment provided a better understanding of the masonry behaviour under axial load and of the local and global distribution of the above mentioned characteristic mechanical parameters together proving the reliability of the DIC technique under the specific monitoring constraints.

Static and Dynamic Investigation of the Taro Masonry Bridge in Parma, Italy

The Taro bridge has a very long history since was built by Antonio Cocconcelli in 1820. The bridge is composed of 20 arch spans and is arranged in the shape of three center Perronet arches with flood holes in the piers. The road over the bridge has two lanes and is set for the highest truck loading category. After the second World War has been used intensively for road transportation and due to some intrinsic features of the structure, in recent times many brick and stone detachments occurred. Recently, in order to assess the health of the bridge, static and dynamic testing was carried out, by using a set of 8 trucks filled with sand. A Finite Element Model was worked out for the data interpretation, and the parameters of the model were identified on the basis of the vibration frequencies. In order to test the effectiveness of the model, a comparison is made with the static load testing, finding a very good agreement. The main identified parameter are the compliance of the foundation blocks, and the elastic modulus of the fill. The analysis shows that models with different number of spans are all capable to identify the main frequencies of the bridge, if the correct value of the main parameters is introduced.

Application of low frequency georadar antenna to fractures detection and 3D visualization in a new quarry bench

Recently, laser scanning systems (airborne and terrestrial mobile mapping systems) have been established as a leading technology for collecting high density 3D information from an objects surface. The availability of generated surface models is very important for various industrial, military, environmental, and public applications. The accuracy of the derived point cloud coordinates from a LiDAR system is affected by inherent systematic and random errors. The impact of random errors depends on the precision of the system’s measurements, which comprise position and orientation information from the GPS/INS unit, mirror angles, and ranges. On the other hand, systematic errors are mainly caused by biases in the mounting parameters (i.e., lever arm offset and boresight angles) relating the system components as well as biases in the system measurements (e.g., ranges and mirror angles). In order to ensure the geometric quality of the collected point cloud, the LiDAR systems should undergo a rigorous calibration procedure to estimate the system parameters that minimize the discrepancies between conjugate surface elements in overlapping LiDAR strips. The main objective of this paper is to look into an existing LiDAR system calibration technique, which is based on manual selection of overlapping regions between LiDAR strips and how to increase the efficiency of this technique by automatic selection of appropriate overlapping strip pairs, which should achieve the minimum optimal flight configuration that maximizes the impact of the discrepancies among conjugate surface elements in overlapping strips as well as automatic selection of regions within the appropriate overlapping strip pairs. The methodology of the proposed technique can be summarized as follows: first, the LiDAR strip pairs are grouped based on the flight configuration; second, appropriate overlapping strip pairs from each group is automatically selected; third, regions within the appropriate overlapping strip pairs are automatically selected based on their angles (slopes and aspects) and distribution; finally, the calibration procedure is applied. The experimental results have shown that the quality of the estimated parameters using the automatic selection is quite comparable to the estimated parameters using the manual selection while the proposed method is fully automated, and much faster.