Technical Seminar on The effect of the cover depth on the cracking of concrete prisms with various arrangement and type of reinforcement bars

Date, time & venue
2019-09-18;6:30 pm – 8:30 pm (Talk);Hong Kong Polytechnic University, Room PQ303


Speakers: Dr Viktor Gribniak, Civil Engineering Department, Vilnius Gediminas Technical University (VGTU), Lithuania.



Cracking effects are the intrinsic attribute of serviceability analysis of reinforced concrete elements. These effects are the research object of numerous studies, which have been carried out for more than half a century. Despite extensive research, there is no commonly accepted methodology for cracking analysis. The existing approaches employ empirical models customised using experimental results of laboratory specimens with simplified geometry that are not able to represent common cracking tendencies adequately. Although different setups developed for the analysis of the serviceability properties (cracking and deformations) of reinforced concrete elements, tensile tests are remaining the most often used testing layouts. Recent studies, however, have revealed noticeable limitations of the traditional experiments on concrete prisms reinforced with a centre bar. Inter-correlation of the basic cross-section parameters (i.e. bar diameter, reinforcement ratio, and cover depth) does not enable the adequate interpreting test results. Furthermore, the test equipment has a limited possibility comparing outcomes of the tensile prisms reinforced with bars made of steel and fibre reinforced polymer materials. Special equipment for the anchorage of multiple bars was developed in VGTU to solve these problems.


The seminar consists of three parts. The focus of the first part is to examine the influence of the cross-section parameters on the cracking behaviour of concrete elements subjected to tension. The experimental campaign encompasses 30 concrete prisms reinforced with multiple bars. All specimens had an identical 150×150 mm cross-section. A number of the bars, bar diameter, and cover depth were among the test variables. The bars were distributed in the cross-section ensuring 15 mm, 30 mm, 40 mm and 50 mm depth of concrete cover. The cracking results only partially supported the generally accepted concept that relates crack widths to crack spacing. The maximal crack was located next to the uncracked block of maximum length in 60% of the considered prisms, whereas the maximum crack-opening occurred between the two blocks of whose sum is maximum in only one case. Adequacy of the crack prediction model from Model Code 2010 was examined identifying tendencies that may point to potential improvements in the model. Deformation behaviour of the specimens also modelled with a tailor-designed bond modelling approach for rigorous finite element analysis. The modelling results reveal that the average deformations of the concrete and steel reinforcement are different and are dependent on the reinforcement configurations. Thus, the efficiency of concrete in tension requires a reconsideration for the rational design of structural elements.


The second part of the seminar reports the results of the test of 16 prismatic specimens reinforced with steel and glass fibre reinforced polymer (GFRP) bars provided by different producers. At the same deformation range of reinforcement, almost identical crack distances are characteristic of the prisms reinforced with steel and GFRP bars with similar axial stiffness. This result enables formulating a hypothesis that the crack spacing in tensile elements of equivalent axial stiffness is predominantly dependent on the geometry of the concrete and, mainly, on the cover depth.


The third part of the seminar considers the serviceability properties of concrete prisms reinforced with steel bars and GFRP bars in different combinations. The analysis employes the test results of 11 prismatic elements. The cracking process in terms of crack width and crack spacing is analysed considering the hybrid reinforcement particularities, and a preliminary approach proposed for the prediction of the crack width for this type of reinforced concrete elements.


About the Speaker:

Dr Viktor Gribniak is a Chief Researcher, the Head of Laboratory of Innovative Building Structures. He is also a Professor of Department of Steel and Composite Structures. Research interests of Dr V. Gribniak cover mechanics of composite materials; long-term effects, fracture mechanics, high-temperature effects on material properties of structural components; experimental investigation, constitutive and numerical modelling of engineering structures. He is a co-author of more than 160 scientific publications, 59 articles published in journals with Impact Factor referred in Clarivate Analytics (former Thomson Reuters) Web of Science database. H-index of Dr V. Gribniak is 15. In 2013, V. Gribniak (together with Prof. G. Kaklauskas) received the “Moisseiff Award” by the American Society of Civil Engineers (ASCE). Since 1948, the best article nomination annually awarded for the essential input in structural design and theoretical analysis. In 2010, the PhD thesis of V. Gribniak became a laureate of the “Best Dissertation of the Year” competition organised by the Lithuanian Society of Young Researchers (LJMS).


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