34 FRANCOIS VAN SCHALKWYK received his BEng in Civil Engineering cum laude in 2014 and BEng Hons (Structural Engineering) cum laude in 2015 from the University of Pretoria. This paper is based on research conducted for his Master’s degree in Structural Engineering. Contact details: PO Box 40772 Moreletapark Pretoria 0044 Gauteng South Africa T: +27 82 574 8165 E: frannavschalkwyk@gmail.com PROF ELSABÉ KEARSLEY is a Fellow of the South African Institution of Civil Engineering (SAICE), of which she was also the president in 2009. She holds a PhD from the University of Leeds and she is currently a professor in Civil Engineering at the University of Pretoria. She worked as a structural design engineer in both South Africa and the United Kingdom before becoming a staf member at the University of Pretoria. For the last 23 years she has been involved with cement and concrete materials research. Contact details: Department of Civil Engineering University of Pretoria Pretoria 0002 South Africa T: +27 12 420 2176 E: elsabe.kearsley@up.ac.za Keywords: stress block parameters, rectangular stress block, size efect, SANS 0100-1, EN 1992-1-1 Van Schalkwyk F, Kearsley EP. The infuence of concrete compressive strength and specimen size on the compression stress block parameters of reinforced concrete. J. S. Afr. Inst. Civ. Eng. 2018:60(4), Art. #0013, 11 pages. http://dx.doi.org/10.17159/2309-8775/2018/v60n4a4 TECHNICAL PAPER JOURNAL OF THE SOUTH AFRICAN INSTITUTION OF CIVIL ENGINEERING ISSN 1021-2019 Vol 60 No 4, December 2018, Pages 34–44, Paper 0013 INTRODUCTION Even though high-strength reinforced concrete may be the most cost-effective solution for many structures, most design codes historically did not cover the design of concrete with strengths in excess of about 60 MPa. The South African code of practice, SANS 0100-1 (2000), limits the characteristic cube compressive strength to 60 MPa, while Eurocode 2 (EN 1992- 1-1 (2004)) can be used for the design of reinforced concrete elements with charac- teristic cube compressive strengths up to 105 MPa. Using concrete with strengths in excess of 60 MPa in flexural elements will require less concrete in the compression zone of beams, which could lead to a sig- nificant reduction in own weight of struc- tures as a result of minimised structural element sizes. However, before adopting the European code of practice for the design of high-strength concrete elements, it is neces- sary to ensure that the code design assump- tions for the stress block parameters are applicable to local conditions and materials. In this study, the influence of concrete compressive strength and specimen size on the flexural compression capacity of concrete was assessed. In total, 27 concrete specimens were tested in flexure, with specific reference to analysis of the com- pression stresses (produced by applying two sets of loads in a configuration aimed at controlling the strain distribution), the stress block parameters derived, and the influence of specimen size on the stress block parameters evaluated. Along with the size effect in flexural tests, with specific reference to an analysis of the compressive stresses, the size effect for cubes and cylin- ders was also evaluated, and the size effect for cylinders used to eliminate the size effect of the stress block parameters. Finally, the difference between the measured moment-axial force (M-N) interaction diagram and the M-N interac- tion diagrams obtained using the BS 8110-1 (1997), SANS 0100-1 (2000), ACI-318 (2014) and EN 1992-1-1 (2004) code recommenda- tions was compared. The infuence of concrete compressive strength and specimen size on the compression stress block parameters of reinforced concrete F van Schalkwyk, E P Kearsley This paper describes the influence of concrete compressive strength and specimen size on the fundamental characteristics of the flexural compressive stress-strain distribution. The main variables were specimen size and cylinder compressive strength. A total of 27 concrete specimens were subjected to flexural tests, with specific reference to analysis of the compression stresses, produced by varying two independent loads in a configuration aimed at controlling the strain distribution. These loads generated a condition of zero strain on the one face of the specimen, and a condition of maximum flexural compression strain on the opposite face. From the strain distribution, the stress-strain curves and stress block parameters were determined, and the influence of specimen size on the stress block parameters described using the Modified Size Effect Law (MSLE). Using a modified form of the moment-axial force (M-N) interaction diagram the BS 8110-1 (1997), SANS 0100-1 (2000), ACI-318 (2014) and EN 1992-1-1 (2004) codes of practice were compared for the design of reinforced concrete beams containing South African materials.