Despite its brittleness and low capacity to bear tensile stress, ordinary concrete is the most frequently used building material in the world. Concrete is almost always reinforced by some kind of steel elements (e.g., bars, stirrups, meshes) when used for construction to counteract its brittleness. Over the last 50 years, steel fiber has become a more and more popular type reinforcement for concrete. Currently, steel fiber reinforced concrete (SFRC) is one of the main building materials. SFRC is characterized by higher tensile, shear and flexural strength, better performance when exposed to elevated temperatures and lower shrinkage than ordinary concrete of similar composition. Usually, steel fiber is arranged randomly but evenly in the SFRC volume. Nevertheless, in some circumstances, irregularity in the fiber distribution may occur. These irregularities in fiber spacing significantly affect its properties. The homogeneous spacing of randomly oriented fiber within a structural element is crucial for guaranteeing proper structural performance with a satisfactory degree of repeatability. Many attempts are being made to study fiber spacing using different destructive and non-destructive methods. The most successful methods are X-ray tomography and cross-section analysis. Both methods give very precise results but are not feasible in common in-situ test scenarios. The simplicity of a conducted test and affordability of used equipment are two main factors influencing the practicability and popularity of a testing method. In many cases, construction companies already have apparatuses dedicated for non-destructive testing (NDT) location of steel bars in existing concrete structures. Harnessing such apparatuses for the assessment of steel fiber volume and spacing would be the most practical and sustainable solution. Two existing NDT techniques, electromagnetic induction and radar-based techniques, have the materials potential for testing SFRC. The inductive technique was proven as a robust and simple non-destructive method to assess the content and the distribution of steel fiber. Nevertheless, there is still a necessity to define its accuracy. Multiple attempts have also been made to use radar-based technique for the similar assessment of SFRC. The fundamental differences between electromagnetic induction and radar technologies are the detectability of objects of different materials and the dependency on the properties of the base material. Using the induction technology, only ferrous materials can be detected.
Radar techniques allow the detection of objects of different materials, including ferrous and non-ferrous metals, water-filled pipes, voids, etc. Moreover, the radar-based technique can be applied not only to detect location of reinforcement, but also to assess moisture in reinforced concrete. To conduct a research programme focused on using both techniques simultaneously to assess fiber spacing in hardened SFRC. Combining two separate NDT methods proved to be very efficient in assessment of mechanical properties of concretes with no fiber reinforcement. The results achieved this way were much closer to real strength characteristics than the results based on only one method. It is feasible to harness devices which are commercially available (and commonly used to detect rebars) to instantly detect fiber and assess their volume and spacing.
9.2.1 Materials and Equipment
The tests were carried out on the industrial floor located inside a depot building in Koszalin, Poland . The tested floor (493 m2) was composed of two layers. A 100 mm-thick concrete undercoat and a 150 mm-thick main SFRC layer. Hooked steel fiber (dimensions of the fiber 50mm 1.0 mm, tensile strength 1115 MPa) at a volume of Vf = 0.3% constituted the reinforcement. The declared strength class of concrete was C20/25. The tests were carried out six years after the concrete was casted.
Both NDT devices used in the research programme were developed for the assessment of rebar location in hardened concrete. The measuring device using electromagnetic induction technique was designed to scan a flat square area of 0.36 m2. The principle of the device’s operation is as follows. When alternating current runs through the probe coil of the device, an electromagnetic field appears around the coil. If there is a ferromagnetic material in the field, it brings about a change in the voltage of the coil, and the voltage change appears according to the diameter around the cover. The tested industrial floor.
Both NDT devices used in the research programme were developed for the assessment of rebar location in hardened concrete. The measuring device using electromagnetic induction technique was designed to scan a flat square area of 0.36 m2. The principle of the device’s operation is as follows. When alternating current runs through the probe coil of the device, an electromagnetic field appears around the coil. If there is a ferromagnetic material in the field, it brings about a change in the voltage of the coil, and the voltage change appears according to the diameter around the cover thickness of the rebar. The method dedicated to the rebar detection was adopted in the tests to localize steel fiber in the area. Using the electromagnetic induction device, 32 randomly chosen square areas of the floor were tested. The total tested area was equal to 11.5 m2, which represented 2.3% of the area of the whole floor. The measuring device allowed to conduct tests in depth up to 150 mm. During the tests, four measuring depths were scanned: 30, 60, 90 and 120 mm. The applied radar apparatus emitted radar pulses spread over the frequency range from 1.0 to 4.3 GHz. The lower the frequency, the deeper the subsurface is penetrated, while the higher the frequency, the smaller objects can be spotted.
When the radar device is moved over the surface, a measurement is taken every 5 mm. At one scanner position, a high number of pulses are emitted and recorded to determine the full reflection pattern of the objects under the surface. Multiple acquisitions are used to reduce the noise in the data, which leads to a clean image.
The signal acquired by the radar front-end is further conditioned by the following steps:
_ Correction of antenna sizes and positions;
_ Background removal with automatic foreground/background detection to mask uniform structures such as the surface and possible stratifications.
_ Automatic gaining to compensate the damping of the radar waves in the base material;
_ Time-zero estimation (automatic recognition of the surface position);
_ Temperature compensation to allow immediate and accurate measurements directly after start-up.
Test Results and Analysis
The exemplary images of the industrial floor tested using an electromagnetic induction technique . The obvious disadvantage of this scanning method is the “shadow” cast by the fiber present in the top layers. The created “shadow” increased blackening of the images in deeper layers. Therefore, in subsequent layers, the shaded areas should not be taken into account to assess fiber presence.
The advantage of the electromagnetic induction method is a possibility of instant assessment of fiber volume and spacing in the scanned area. Images of tested areas located at the depth of 30 mm, are examples of such possibility.
Images of detected fiber (using the electromagnetic induction technique) in different tested areas at the same depth of 30 mm
The relative percentage of detected fiber in the layers located every 30 mm is presented. In subsequent layers, the shaded areas were not taken into account to assess relative percentage of fiber content. The electromagnetic induction technique detected almost 50% of steel fiber in the second layer located at the depth of 30–60 mm. The other layers contained from 15.5% to 20.2% of steel fiber. The low fiber content in the top layer can be explained by the so-called “wall effect”. The results obtained for the 0–30mm and 30–60mm depths closely corresponded to the actual fiber content. The smaller amount of fiber detected at depth of 60–120mm did not harmonize with genuine fiber content. The analysis of specimens was obtained by coring confirmed high uniformity of fiber spacing across the thickness of the floor (apart from the top layer). The explanation for this phenomenon might be a low measure of sensitivity of the apparatus. The depth and “shadow” cast by the fiber present in the top layers play a key role in this phenomenon.
The percentage deviation of fiber volume for 32 tested areas is presented in Figure 9.6. The results were grouped by depth. The top layer was characterized by the highest percentage deviation of fiber volume. The proximity of the surface influenced the homogeneity of fiber distribution. The second layer, placed at the depth of 30–60 mm, was characterized by the lowest value of the percentage deviation of fiber volume. Along with the depth, the value increased. The second layer, due to the best reading parameters and lowest percentage deviation of fiber volume, was the most suitable for assessing volume and spacing of steel fiber volume. The proximity of the surface influenced the homogeneity of fiber distribution. The second layer, placed at the depth of 30–60 mm, was characterized by the lowest value of the percentage deviation of fiber volume.
Two-dimensional visualisation of data obtained by the radar-based technique.
Conclusions
The results obtained during the research programme allowed us to form the following conclusions:
The method based on the electromagnetic induction technique can be applied to estimate the approximate volume of steel fiber in a hardened SFRC. However, the method requires calibration to obtain good quality of results in deeper layers due to the “shadow” cast by the fiber present in the top layers. The method can be applied to detect steel fiber up to the 120 mm thickness of the tested element.
-The method based on the radar technique is suitable for instant detection of the areas with a clearly spaced fiber volume (too low or too high local fiber concentration). Theoretically, the method can be applied to detect steel fiber presence up to the 200 mm thickness of the tested element, yet only fiber present in upper layers is correctly detected. The testing equipment based on the radar technique used for fiber detection is able to recognize fiber concentration fields but not a single fiber.
-Both methods together can detect fiber concentration in SFRC volume but cannot detect a single fiber.
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