Project topic

Project description

The verification of the conformity of ultra-high performance concretes (UHPC), which will be regulated in the future in the draft DAfStb guideline "Ultra-high performance concrete", requires form factors that allow a conversion between the compressive strengths obtained on UHPC specimens of different shapes and sizes. Until now, there has been a lack of reliable test results on the basis of which generally valid form factors could have been determined with certainty. Therefore, as part of a research project funded by the German Committee for Reinforced Concrete, the influence of specimen geometry on the results of compressive strength tests on normal, high-strength and ultra-high-strength concretes was investigated experimentally.

The test program comprised a total of 8 series, including one series of normal-strength concrete (NFB), one series of high-strength concrete (HFB) and 6 series of fibre-free UHPFRC. The average cylinder compressive strengths of the individual series were between approx. 30 N/mm² and 200 N/mm². With maximum grain diameters _Dmax of 0.5 mm, 3 mm, 5 mm and 8 mm, the UHPFRC systems largely covered the application range of the future DAfStb guideline.

The experimental work was carried out under the leadership of the Chair of Solid Construction at the University of Siegen together with the University of Kassel and the RheinMain University of Applied Sciences. In addition to cylinders with h/d [mm] = 300/150 and cubes with an edge length d = 150 mm, the characteristic compressive strength of which is used to classify UHPFRC, cylinders with h/d [mm] = 200/100 and cubes with an edge length d = 100 mm were also examined in view of the limited maximum loads of the testing machines used in practice.

The results of preliminary tests on sample preparation led to the decision to also grind the load application surfaces of the cubes for HFB and UHFB in order to minimize the scatter of the test results. For this purpose, a special grinding procedure consisting of four grinding passes and a special flatness test was developed, which all specimens made of HFB and UHFB were subjected to before the compressive strength test. In addition, the UHPFRC specimens were stored in water until the test.

The tests confirmed that results with low scatter can also be achieved for fiber-free UHPFRC if appropriate care is taken during production and sample preparation. For six individual values per series and specimen geometry, the coefficients of variation of the compressive strength were between 0.7 % and 3.0 %. In only four cases was the standard deviation over
4 N/mm². In only one series could the result of a specimen type not be included in the evaluation, as the standard deviation exceeded the self-imposed limit of 6 N/mm².

When evaluating the test results, a distinction must be made between the influence of specimen slenderness and the influence of specimen size. The influence of specimen slenderness decreased significantly with increasing concrete compressive strength. Thus, the ratio between cylindrical and cube compressive strength was approx. 0.79 for concrete with _fcm = 26.8 N/mm², 0.84 for concrete with _fcm = 78.5 N/mm² and - with increasing strength - between 0.94 and 1.04 for UHPFRC. In contrast, the influence of the specimen size on the results of the compressive strength tests was only slight, regardless of the concrete compressive strength. The test specimens with d = 150 mm in the individual series exhibited on average 0.95 to 1.02 times the compressive strength of the test specimens with d = 100 mm.

Based on the test results, a classification of UHPFRC into the strength classes C130/140, C150/155 and C175/180 can be recommended. To convert the test results obtained in conformity tests on a specimen with d = 100 mm to the corresponding specimen with d = 150 mm, a reduction of the measured compressive strength by 5 N/mm² is recommended.

Publications

LEUTBECHER, T.; RIEDEL, P., 2020 Compressive strength classes and performance classes of ultra-high-performance concrete (Part 2) | Compressive strength and performance classes for ultra-high performance concrete (Part 2). Betonwerk und Fertigteil-Technik/Concrete Plant and Precast Technology. 86(10), 46-53. ISSN 0373-4331

LEUTBECHER, T.; RIEDEL, P., 2020. Compressive strength classes and performance classes of ultra-high-performance concrete (Part 1) | Compressive strength and performance classes for ultra-high performance concrete (Part 1). Betonwerk und Fertigteil-Technik/Concrete Plant and Precast Technology. 86(9), 46-54. ISSN 0373-4331

LEUTBECHER, T., 2020 Classification of ultra-high performance concrete: Background and test methods. In: Congress documents 64th BetonTage: Concrete construction of the future. Neu-Ulm, February 18-21, 2020. Betonwerk und Fertigteil-Technik/Concrete Plant and Precast Technology. 86(2), 72. ISSN 0373-4331

RIEDEL, P.; LEUTBECHER, T.; PIOTROWSKI, S.; HEESE, C., 2019. Ratios of Compressive Strengths of Ultra-High-Performance Concrete Specimens of Different Shapes and Sizes. ACI Materials Journal. 116(6), 139-149. ISSN 0889-325X. doi:10.14359/51716983

RIEDEL, P.; LEUTBECHER, T.; PIOTROWSKI, S.; HEESE, C., 2018. Influence of specimen geometry on the results of compressive strength tests on ultra-high strength concrete. Beton- und Stahlbetonbau. 113(8), 598-607. ISSN 0005-9900. doi:10.1002/best.201800027

LEUTBECHER, T.; RIEDEL, P., 2018. Influence of test specimen geometry on the results of compressive strength tests on ultra-high performance concrete (UHPFRC) [PowerPoint presentation]. Epiphany meeting 2018 of the MPA Wiesbaden. RheinMain University of Applied Sciences, Wiesbaden, January 16, 2018.
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RIEDEL, P.; LEUTBECHER, T., 2017. Effect of specimen size on the compressive strength of ultra-high performance concrete. In: TOUTLEMONDE, F.; RESPLENDINO, J., eds. UHPFRC 2017, Design and Building with UHPFRC: New large-scale implementations, recent technical advances, experience and standards. Proceedings of the AFGC-ACI-fib-RILEM International Conference on Ultra-High Performance Fiber-Reinforced Concrete. Montpellier, France, October 2-4, 2017. Paris: RILEM Publications, Volume I, pp. 251-260. RILEM Proceedings PRO 106. ISBN 978-2-35158-166-7
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