Afonso Pascoal de Araújo Ennes da Lage. May Abstract

Transcription

1 Application of photogrammetry in monitoring of shear tests for the assessment of the bond between aggregate and matrix at the interfacial transition zone (ITZ) Afonso Pascoal de Araújo Ennes da Lage May 17 Abstract With the technological evolution, the acquisition of information from images reveals huge potentialities and advantages over traditional monitoring, namely, automatic and fast procedure, low budget and unlimited measurement points, among others. In this dissertation, a method based on photogrammetry and post-processing of data was applied, aiming at monitoring a campaign of shear tests in concrete specimens. Two main objectives were defined: methods validation and assessment of the aggregate-to-matrix bond behaviour at the interfacial transition zone (ITZ). To achieve these goals, twenty concrete specimens from four concrete strengths ( MPa, MPa, 8 MPa and MPa) and five types of aggregates (steel, glass, granite, limestone, and polypropylene) were tested and monitored. The study of aggregate-to-matrix bond was performed from the results of the displacements computed with the proposed method, which allowed analysing three different sections of the surface. Failure modes from a proposed parameter were defined for each specimen. The method reached an average RMS precision of.5 mm (in displacements). The values obtained were also compared with traditional instrumentation, resulting in a relative accuracy of.22 mm. It was concluded that the method applied is suitable for monitoring aggregate-to-matrix bond tests and guarantees a reliable study of specimens behaviour. In addition, it has shown advantages over traditional instrumentation, mainly due to the discretization achieved. Key-words Photogrammetry, Monitoring, Concrete, Aggregate, Interfacial Transition Zone (ITZ), Bond. 1. Introduction Maintenance operations deal with several problems related to structural durability. For existing structures, it is crucial to guarantee periodic inspections, to evaluate the state of conservation. In the actual economic-financial crisis, it is important to reduce costs related to structural interventions. In this context, the development of new monitoring tools, that allow mitigating expensive interventions, is mandatory. Currently, most of structural assessments are based on periodic visual inspections complemented by traditional instrumentation. For example, displacements are usually computed using: (i) topographic methods in the case of real infrastructures; (ii) LVDT s (near Variable Differential Transformer) and dataloggers for experimental tests. Both procedures are time consuming and only allow collecting information from a limited number of points (Valença, 11). With the technological evolution, the acquisition of information from images has shown huge potentialities and advantages over traditional monitoring, namely automatic and fast procedure, low budget, and unlimited 1

2 measurement points, among others (Chen et al., 16)(Thomas and Cantré, 9). Currently, it is accepted that physical, chemical and thermal aggregates properties affect concrete performance (Coutinho, 2). Bond behaviour between aggregate-to-matrix has been studied, more precisely in the interfacial transition zone (ITZ). The definition of the ITZ is not consensual and there are not enough studies at the scale of the aggregate in order to better understand the bond between aggregates-to-matrix (Scrivener et al, 4). Generally, ITZ are zones with higher porosity and water/cement ratio that provide cracks expansion due to mechanical, physical or chemical actions (Ollivier et al, 1995). For the study of bond behaviour between aggregate and matrix at mesoscale level, a method based on photogrammetry and postprocessing of data was applied. The method allows monitoring concrete specimens considering different aggregates and concrete strengths. Two main objectives were defined: (i) method s validation; (ii) assessment of the aggregate-to-matrix bond behaviour at the ITZ. To achieve these objectives, a campaign of shear specimens, tested up to, performed in the scope of a PhD thesis (Simões, T, ongoing) was monitored Set-up and specimens In Figure 1 and Figure 2 are represented the set-up where is possible to identify a photographic station, a concrete specimen, testing machine and LVDT s. Figure 1 - Test set-up (front view) 2. Photogrammetric procedure The method applied is based on photogrammetry and post-processing of data. The proposed method allowed to calculate displacements along the shear surface and to define a parameter in three different zones of the specimen which helped characterizing different modes. These tests were also monitored using traditional instrumentation (LVDT s). Twenty specimens from four types of concrete ( MPa, MPa, 8 MPa and MPa) and five types of aggregates (steel, glass, granite, limestone and polypropylene) were subjected to shear until. Figure 2 - Test set-up (back view). 2

3 Tests were divided in four groups, depending on the concrete class: i. Specimens with MPa concrete; ii. Specimens with MPa concrete; iii. Specimens with 8 MPa concrete; iv. Specimens with MPa concrete. Each group was tested considering five different types of aggregate materials (Figure 3): a. eel; b. ass; c. anite; d. mestone; e. lypropylene. Figure 3 - Different aggregates; Left to right: lypropylene, anite, ass, mestone, eel. Concrete specimens were quadrangular prisms (mm mm mm) with a shear surface with a º slope to the horizontal (Figure 4 and Figure 5) and the aggregates were spherical with mm of diameter. The limestone and granite were not perfectly spherical. Figure 5 - General representation of the test (front view). For images acquisition, a Nikon D3 with resolution and a 55 mm focal lens was used. A tripod and a remote shutter ensured the stability of the system. The camera was fixed in the station and with the same luminosity conditions. Traditional monitoring uses three LVDT s CDP, two for horizontal measurements and one for vertical measurement Monitoring method based on photogrammetry Procedure In Figure 6 the procedure followed is summarized: Specimens' preparation age images aqcuisition age i images aqcuisition age i targets detection Homography and orthoretification age targets detection Displac. calculation Failure mode evaluation Figure 4 - Concrete specimen schematic views. Figure 6 - Procedure flowchart. 3

4 Specimens preparation At each surface of the concrete specimens a mask was painted with thirty four red static targets: four reference targets with 7.5 mm diameter in each corner, for image orthorectification, and thirty targets with 5 mm diameter along the shear surface for displacements measurement (Figure 7). Radial shape was used because it is precisely detected, being also independent of scale and image rotation (Thomas and Cantré, 9). Figure 8 - Reference targets detection. Figure 9 - Displacement targets. Figure 7 - Concrete specimen Image acquisition Image acquisition can be divided into two main phases: i. age : ten images were acquired before each test for homography computation and experimental calculation of precision; ii. age i: images were acquired with an approximated step of kn Targets detection The algorithms for targets detection were, previously, developed in-house and implemented in MatLab environment (MatLab R15a) (Valença, 11). The routine used had two steps: i. Detection of reference targets for image orthorectification (Figure 8); ii. Detection of targets along the shear surface for computation of displacements (Figure 9) Homography and image orthorectification Images taken during age and age i were subjected to distortion and perspective effects. The calculation of the homography matrix allowed turning these images into orthorectified images, making targets coordinates detection more accurate. Homography can be defined as an invertible mapping of points and lines on a projected plan from the equation 1 (Valença, 11): (1) where are the coordinates in the real world, are the coordinates in the image plan, is the scale factor and the homograph matrix (3 x 3). Every image is subjected to perspective effects. In order to get a correct measurement of displacements of each target, it is necessary to match the image plan and the specimen s surface plan with the homography matrix. Besides, it is also possible to find a match between pixel and mm, fundamental for displacements evaluation in a metric scale (Valença, 11). 4

5 8_ 8_ 8_ 8_ 8 F (kn) Relative accuracy(%) The error of homography was computed from distances of pairs of targets that are identified in Figure. For that purpose, theoretical results and results from the photogrammetric method were compared. Theoretical results were acquired from the real targets position. This calculation was made for each test with age images. (Figure 12). Afterwards, these values were compared with LVDT s information and relative accuracy was calculated. Figure 12 - Coordinates change of basis. Figure - Pairs of targets age i images selection Although plenty of images were taken during the tests, only the ones corresponding to 3 to 5 key instants were selected to apply the method and proceed to displacements calculation according to load-time curve. Figure 11 shows an example: 7 Photogrammetry Time (sec) 8 Figure 11 - age i images selected Displacements calculation From the photogrammetric method, coordinates variation were calculated in each image in (x,y) axes, corresponding to horizontal and vertical direction, respectively. For the study of aggregate-to-matrix bond behaviour, it was important to obtain tangential and normal displacements of the shear surface Photogrammetric method vs LVDT s The results were compared with the ones obtained with the traditional instrumentation (LVDT s). A relative accuracy less than % was achieved for fourteen tests (Figure 13). In the other cases, displacements observed were smaller than the precision of the method, i.e, the method is not applicable for this kind of displacements values Figure 13 - Relative accuracy in percentage Failure modes Three s modes were considered: (i) cut of the aggregate; (ii) concrete crush; (iii) mixed with concrete crush and aggregate drop. These s can be related to tangential and normal displacements. While more tangential displacements reveal the of the aggregate or the concrete crush, higher values of normal displacements reveal the drop of the aggregate. Relative accuracy 5

6 8_ 8_ 8_ 8_ 8 Error [mm] 8_ 8_ 8_ 8_ 8 Precision RMS (mm) Correlação 3. Results and discussion 3.1. Method validation Method precision Figure 14 shows the precision of the method calculated from the images of age. For the twenty tests, an average RMS precision based on values of.5 mm in both horizontal and vertical direction. The global average RMS value was.5 mm and shows that this method is precise enough for this kind of experiences.,,9,8,7,,,,,,, Figure 14 - Method precision Homography error Figure 15 shows the homography error for each test with an average value of.1 mm what is in line with previously researches (Valença, 6)(Valença, 11),7,,,,,,, Figure 15 - Homography error Relative accuracy RMSx RMSy εt(avarage) εx(máx) εy(máx) Figure 16 shows the correlation between photogrammetry and LVDT s results, where it can be observed, once again, that in totally tests the method cannot be applied. Except for those cases, correlation is above 9%, showing the reliability of the method. 1,9,8,7,6,5,4,3,2,1 Figure 16 - Photogrammetric method LVDT s correlation Specimens behaviour for different concrete strengths In the post-processing of data, a parameter was established. The visual analysis of images of different surfaces allows establishing the following conditions: (a) type (iii); (b) <2 reveals mode >2 reveals mode type (i) or (ii); (c) whenever displacements were lower than.5 mm this parameter has no meaning. It is important to notice that this parameter does not provide all the information required, because there are some phenomena that are unpredictable and unexpected. For this reason, it should be analysed together with complementary information, such as images and displacement field. Specimens with granite aggregates (Figure 18) had deformation for MPa and MPa concrete, where tangential displacements were higher than normal displacements. With parameters higher than two, the mode was due to concrete crush or cut of the aggregate. Failure surfaces images allowed concluding that concrete was crushed for both situations (Figure 17). Correlação 6

7 ass aggregate had a similar behaviour to the granite aggregates (Figure ): deformation for concrete with MPa and MPa strength and totally behaviour for concrete with 8 MPa and MPa strength. Figure 17- Failure surface for specimen with concrete MPa and granite aggregate. =4.4 Concrete crush =3.1 Concrete crush = - = - MPa MPa 8MPa MPa Figure 18 anite aggregates behaviour. Except for concrete with MPa strength, specimens with steel aggregates had deformation. eel aggregates cannot be cut, so for MPa and MPa specimens were due to concrete crush (Figure 19). With the increasing of concrete, normal displacements became higher due to more difficulty to concrete crushing. For 8 MPa concrete specimens, the is mixed, i.e, a combination between concrete crush and aggregate drop. =2.8 Concrete crush =1.7 Mixed =1.1 Mixed = - MPa MPa 8MPa MPa Figure 19 - eel aggregates behaviour. =1.7 Mixed =2. Concrete crush = - = - MPa MPa 8MPa MPa Figure ass aggregates behaviour. lypropylene aggregates (Figure 21) are the ones with lower Young s modulus and the highest deformation. Concrete is not easily crushed by this kind of material. For MPa and MPa concrete, the aggregate provided a with both concrete crush and aggregate drop. For higher concrete classes, polypropylene was deformed until spheres were cut. =1.7 =1.7 = 2.2 = 2.5 Mixed Mixed Cut of Cut of the the aggreg. aggreg. MPa MPa 8MPa MPa Figure 21 lypropylene aggregates behaviour. For limestone aggregates (Figure 22) different modes were observed. For MPa concrete, limestone aggregates crushed concrete, whereas for MPa concrete, concrete cut limestone aggregates. This was concluded, not only by the parameter, but also from images taken after the test. For 8 MPa concrete specimens, limestone 7

8 Force (kn) Force (kn) aggregates were also cut for lower displacements. =4. =4.9 =1.9 = - Concrete crush Cut of the aggreg. Cut of the aggreg. (images request) MPa MPa 8MPa MPa Figure 22 - mestone aggregates behaviour. Failure parameter was defined for three different zones, as shown in Figure 23. In previous analysis, an average value of displacements was calculated, for simplification. Discretization is one of the main advantages of this method when compared with traditional methods. That helped to conclude that for almost every test, shear surface was homogeneous. Figure 23 - Influence areas for results discretization 3.3. Specimens behaviour for different aggregates materials In the analysis considering different material, it was more difficult to define trends and possible justifications for aggregates-to-matrix bond behaviour, because there are other variables involved. This makes it difficult to understand the influence of different aggregates for a given concrete: Young s modulus, shape, porosity, roughness, etc. In Figure 24 the results of each test are plotted. For MPa and MPa concrete specimens, deformation was achieved for every aggregate. For tests with limestone aggregates lower displacements and higher load were registered. lypropylene aggregates exhibited the higher displacements, due to its deformation. For 8 MPa and MPa concrete, glass aggregates obtained the best response in load, while polypropylene aggregates were the only ones that provided deformation. MPa - Tangencial displacement x=mm, 1, 2, 3, 4, 5, 6, dt (mm) 9 8 7, 1, 2, 3, 4, 5, 6, 7, dt (mm) MPa - Tangencial Displacements x=mm, 2, 4, 6, 8,, dt (mm) 8 8 MPa - Tangencial displacements x=mm MPa - Tangencial displacements x=mm,,5 1, 1,5 2, 2,5 3, dt (mm) -1,, 1, 2, 3, 4, 5, 6, dn(mm) Figure 24 - Displacements with aggregates as variables MPa - Normal displacements x=mm -1,, 1, 2, 3, 4, 5, 6, 7, dn(mm) MPa - Normal displacements x=mm, 2, 4, 6, 8,, dn(mm) MPa - Normal displacements x=mm -,5,,5 1, 1,5 2, 2,5 3, dn(mm) MPa - Normal displacements x=mm 8

9 4. Conclusion A method based on photogrammetry and postprocessing of data was proposed for monitoring shear tests. The method allowed evaluating the bond behaviour between different types of aggregates and concretes in the Interfacial Transition zone from displacements analysis. st-processing of data allowed defining a parameter in three different zones of the specimens, with the purpose of characterizing different types of modes. The method was applied with an experimentally measured RMS precision of.5 mm. This shows that this method is applicable for this kind of studies. Relative accuracy, comparing with traditional instrumentation leads to differences under %. The exception is related to specimens with totally s, with displacements bellow the method s precision. The method allows to understand the aggregate-to-matrix bond behaviour, especially when concrete strength is the variable. It was possible to conclude, as expected, that concretes with higher strength are more suitable to have totally s and that there are three main types of modes: concrete crush, cut of the aggregate and mixed mode ( drop of the concrete combined with concrete crush). The definition of a parameter allows quickly predicting the possible mode. However, the parameter should be analysed together with additional information, such as images and/or displacement field. The definition of this parameter in three different zones is one of the main advantages of this method when compared with LVDT s measurements because it gives information about the behaviour along the shear surface. Generally, in the cases analysed, the surfaces had a homogeneous behaviour. Finally, it can be stated that the method applied presented several advantages for monitoring the bond behaviour between aggregate-to-matrix in the ITZ: i. Definition of a parameter in three different zones, that allowed characterizing the behaviour along the shear surface; ii. The possibility to acquire 2-D information from an almost unlimited number of points, only conditioned by the number of targets used; iii. Contactless method, therefore not susceptible to any associated errors with the placing of LVDT s; iv. Low budget method when compared with other equipments required to traditional instrumentation; v. Precise and accurate information record, from a single image per stage; vi. Large amount of information registered allowing the indirect calculation of several parameters for structural analysis, which can be customized on a case-by-case basis. References Chen, J. et al. ISPRS Journal of Photogrammetry and Remote Sensing Information from imagery : ISPRS scientific vision and research agenda, ISPRS Journal of Photogrammetry and Remote Sensing. International Society for Photogrammetry and Remote Sensing, Inc. (ISPRS), 115, p.: 3 21, 16. Coutinho, J. Agregados Para Argamassas E Betões, in Ciência de Materiais - 1 a Parte, p.: 5 125, 2. Ollivier, J. P., Maso, J. C. and Bourdette, B. Interfacial transition zone in concrete, Advanced Cement Based Materials, 2, p.: 38, Scrivener, K. L., Crumbie, A. K. and Laugesen, P. The interfacial transition zone (ITZ) between cement paste and aggregate in concrete, Interface Science, 12, p.: , 4. 9

10 Thomas, H. and Cantré, S. Applications of low-budget photogrammetry in the geotechnical laboratory, Photogrammetric Record, 24, p.: 332 3, 9. Valença, J. Aplicações de Fotogrametria à Engenharia de Estruturas. MsC Thesis. Faculdade de Ciências e Tecnologia da Universidade de Coimbra, 6. Valença, J. Monitorização do estado de conservação de estruturas de betão por processamento de imagem e análise multiespectral. PhD Thesis. Faculdade de Ciências e Tecnologia da Universidade de Coimbra, 11. Simões, T. Caracterização Mecânica das Interfaces Fibra/Pasta e Agregado/Pasta em Betões Reforçadas com Fibras- PhD Thesis. DECivil. Instituto Superior Técnico. Universidade de sboa (under development).