Particle image velocimetry (PIV) to evaluate fresh and hardened state properties of self compacting fiber-reinforced cementitious composites (SC-FRCCs)
Introduction
Industrial use of fiber-reinforced self-compacting composites (SC-FRCCs) is increasing with an increasing know how of material properties, processes and applications. SC-FRCCs have changed and developed significantly from the times that fiber reinforcement was first used.
From a structural point of view, the main reason for incorporating fibers is to improve the fracture characteristics and structural behavior through the fibers’ ability to bridge cracks. According to Clarke and his colleagues, the main benefit of fibers is their ability to transfer stresses across a crack, and consequently enhance the toughness and ductility of concrete as well as the energy absorption capacity under impact [1].
Previous studies have shown that existence of cracks significantly affects strength and durability of SC-FRCCs [2], [3]. Therefore, reliable and precise crack monitoring for the members cast using SC-FRCCs is very important since it is a comprehensive study which yields a wide range of information such as first crack load, cracking patterns and crack width. Overall mechanical performance can be evaluated by using the information obtained from the experimental study.
There are numerous experimental methods available for crack monitoring under different types of loads [4]. Measuring related displacements/crack widths is especially important. Some of the conventional measurement equipments are electrical resistance strain gauges, extensometers and linear variable differential transformers (LVDTs). All of these equipments are effective and trustworthy if they are accordingly used with a proper loading set-up and system. However, there are typical drawbacks faced with the use of these conventional measurement devices, such as; (i) they provide only one measurement at a time, either strain or displacement [5], (ii) problems due to proper bonding of the strain gauges, (iii) de-bonding during the loading of the specimen, (iv) alignment of the strain gauge with the fibers when using strain gauges on composite materials [5], (v) tedious and time-consuming sample surface preparation and the strain gauge adhesion [6], (vi) complexity and impracticity of using too many strain gauges when the region of interests is a large one [7], [8], (vii) the points measured by these conventional measurement devices cannot be dense and well distributed [9], deformations can only be measured at points where devices are mounted and in many cases would not cover the entire surface [10], [11], (viii) possible damages to the measuring devices during the experiment, (ix) problems related to removal of the devices prior to specimen failure to avoid damage to the instrument, (x) high cost of measuring devices, and extra charges for data acquisition [12]. As a result, these drawbacks of conventional measurements have led researchers to develop non-contact full field deformation measurement techniques.
Detection of cracks with non-contact measurement techniques, such as analyzing images of concrete surfaces has been explored by many researchers [13], [14], [15], [16] using a variety of methods. Some of these image based methods are, Digital Image Correlation (DIC), photogrammetry and video extensometers.
One of the methods that have been used for non-contact deformation measurement as well as crack monitoring is particle image velocimetry (PIV). PIV is an image-based technique which was originally developed in the field of experimental fluid and gas mechanics [17], [18] and has also been frequently used in granular materials [19], [20], [21], [22], [23], [24], [25]. This technique is used to determine instantaneous fields of the vector velocity by measuring the displacements of numerous fine particles that accurately follow the motion of the fluid [17]. Nowadays, PIV has been used in the field of concrete/composites also, in order to measure displacement and strain fields [26], [27], [28], [29] and monitoring fracture process zones (FPZs) and cracks [12], [30]. Pioneering research done on the subject reveals the potential of this technique for precise and easy measuring of deformation/crack width of concrete specimens [30]. However, the number of studies on the application of this technique for crack monitoring of cement-based materials is still limited. Therefore, the objective of this study is to further investigate the ability of PIV for measuring concrete performance in the fresh and hardened states.
An experimental study was carried out to evaluate both fresh and hardened state properties of SC-FRCC’s with conventional equipments and all the tests are monitored by using a high resolution camera. Images captured during tests are analyzed by using PIV and the ability of PIV for measuring different parameters in both the fresh and hardened state is compared with the results obtained from conventional measurement techniques.
Section snippets
Materials and mix design
The SCC mix used was adapted from the previous studies of the authors. In the previous studies of the authors, an SCC mortar mix was optimized to obtain high flowability, high segregation resistance and high performance for thin section precast elements [31], [32]. Cement, slag, sand, water, superplasticizer and fibers were used. No coarse aggregate was used in the mixture. Mixture had excellent flow ability with good static and dynamic segregation resistance as previously discussed in [32],
Particle image velocimetry (PIV)
PIV technique was originally implemented using double-flash photography of a seeded flow and the resulting photographs were divided into a grid of subsets, called patches. For PIV analysis, the displacement vector of each patch during the interval between the flashes is found by locating the peak of the auto correlation function of each patch. The peak in the auto correlation function indicates that the two images of each seeding particle overlying each other, so the correlation offset is equal
Mini-slump flow
Conventional measurement of mini-slump flow gives only final spread diameter of the material and final spread diameters alone do not give adequate information to distinguish between different materials. For instance, 3 different materials were compared in the scope of this study (plain, 6 mm fiber-reinforced and 13 mm fiber-reinforced) and final spread diameters of 34.1, 30.5 and 27.8 mm were measured, respectively. This data gives limited information about the fresh state behavior of material.
Conclusions
A detailed experimental study was carried out to compare measurements from conventional techniques and an image based non-contact method, i.e. PIV, both for fresh and hardened state properties of SC-FRCCs. Following results are obtained:
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Detailed comparison can be made between the flow behaviors of various materials by using PIV. Information such as slump flow evolution, velocity–time diagrams and strain fields can be used to evaluate fresh state behavior of materials. This information is
Acknowledgements
The authors would like to acknowledge the financial support of Boğaziçi University Research Fund (Project Code 09HA401P). The support of AKÇANSA Cement, BASF-YKS Construction Chemicals and LIMAK Construction is also acknowledged. First author would also like to acknowledge Bogazici University Construction Materials staff Ümit Melep and Yener Aydın for their help and support during experiments.
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