UHPC/UHPFRC Compressive Strength Tests: Cylinder Vs Cube Conversion

After reading my article "Effect of Size and Shape of Test Specimen on Compressive Strength of Concrete"

https://www.dhirubhai.net/pulse/effect-size-shape-test-specimen-compressive-strength-narayanan?trk=pulse-article_more-articles_related-content-card

One engineer from the North-West of Romania, ca 100km from the Hungary border, asked the following question:

What are the correct conversion formulas from?prims 160 x 40 x 40mm prism to 150mm cube and 150/300mm cylinder? How to test UHPC compression strength with a normal concrete hydraulic press? The following is in response to this question:

Ultra-High-Performance Concrete (UHPC) and Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC)

UHPC is a new class of concrete that was developed during the 1990s. When fibers are also used in UHPC, it is called as Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC)- and offers increased strength and durability. The compressive strength of UHPC may have values exceeding 150 MPa (Graybeal and Davis, 2008). It has been found by researchers, especially at the Kassel Universitaet, Germany, that UHPC can be designed to reach a compressive strength of up to 250 MPa. In addition to its high compressive strength, UHPC offers enhancements of High Strength Concrete (HSC) such as very high tensile strength (over 15 MPa) and flexural strength (over 50 MPa), very high ductility, very high durability to freeze-thaw cycles, chloride penetration, abrasion resistance, and carbonation. These enhanced properties will contribute to the overall performance of the structure using this material, thus improving construction safety, providing longer service life, and lower maintenance costs.

However, the very high compression strength of UHPC and UHPFRC will pose issues regarding their strength in laboratory testing. These issues are the limited capacity of compression testing machines and the surface preparation requirement for testing cylinder specimens. The issue of buying high-load capacity testing machines could be solved by using small-size specimens. Similarly, the problem of the surface preparation requirement of cylinders could be solved by using cube specimens.

Generally, cylinder specimens are used in the USA, Canada, France, Australia, and New Zealand. However, cubes are more commonly used in the UK, Europe, India, and Singapore. For normal concrete and HSC, the concrete compressive strength test results from cube specimens are generally higher than cylinders specimens (Subramanian, 2013). Usually, the compressive strength of concrete cylinder specimens is considered to have 0.8-0.85 times the compressive strength of cube specimens.

Cube Vs Cylinder strength of UHPC and UHPFRC Specimen

?The French guidelines propose to use cylinder specimens having a diameter of 70 or 110 mm, to determine the compressive strength of UHPC and UHPFRC, while the Japanese guidelines propose a diameter of 100 mm. In other countries, such as the USA, a diameter of 102 mm or larger diameter of cylinders and larger dimensions of cubes are accepted.

Kusumawardaningsih et al. (2015) tested UHPC and UHPFRC cylinders of diameter 100 mm and height 200 mm?and cube specimens of dimension? 100 mm x 100 mm x 100 mm. UHPC and UHPFRC specimens used had mean compressive strengths of 165.43 MPa (for UHPC, using cube specimens) and 186.03 (for UHPC, using cylinder specimens), 183.03 MPa (for UHPFRC with 1% fibers, using cube specimens) and 188.97 MPa (for UHPFRC with 1% fibers, using cylinder specimens), and 182.27 MPa (for UHPFRC with 2% fibers, using cube specimens) and 186 MPa (for UHPFRC with 2% fibers, using cylinder specimens). It has to be noted that the UHPC specimens which contained no fiber experienced sudden explosive and brittle failures. On the other hand, UHPC specimens containing fibers had rupture failures, which marked the rupture of fibers.

The mean compressive strength conversion ratio between cube and cylinder specimens was found to be: 0.89 (for UHPC), 0.99 (for UHPFRC 1%), and 1.0 (for UHPFRC 2%), respectively. Similarly, the mean compressive strength conversion ratio between cylinder and cube specimens was found to be: 1.12 (for UHPC), 1.01 (for UHPFRC 1%), and 1.0 (for UHPFRC 2%), respectively. These ratios of UHPFRC with 1% fiber and 2% fiber respectively are somewhat close to the conversion ratio proposed by Leutbecher (2011) which is 0.96, and by AMPA (2010) which is 0.95. This finding is interesting as it is opposite to the converting factors of normal concrete and HSC, which says that cube specimens produce higher concrete compressive strength.

Dr. Satish Jain, who is supplying UHPC/UHFRC in India opinions that 50 mm cubes should be sufficient for all internal testing and initial R&D purposes. For final tests, it is suggested to go with 100 mm cubes. The Swiss standard allows it. Additionally, there are sufficient test data to suggest that 100 mm cube results match up exactly with 75x150 cylinders as suggested per the ASTM C1856-17. The rate of loading shall be 1 MPa/Sec.

References:

1. AMPA (2010)(Amtliche Materialprüfanstalt für das Bauwesen), Druckfestigkeitfactor, Kassel Universitaet, Germany.
2. Graybeal, B. and Davis, M. (2008) Strength Testing of 80 to 200 MPa (11.6 to 29 ksi) Ultra-High-Performance Fiber-reinforced Concrete, ACI Materials Journal, Vol. 105, No.6,?Nov.-Dec. 2008, pp. 603-609.
3. Kusumawardaningsih, Y., Fehling, E., and Ismail, M.(2015) "UHPC compressive strength test specimens: Cylinder or cube?",?Procedia Engineering, Vol. 125, Dec., pp. 1076 – 1080. DOI: 10.1016/j.proeng.2015.11.165
4. Leutbecher, T., (2011) Chapter 4: Material Properties of UHPC, Kassel University, Germany, 2011.
5. Subramanian, N., Design of RC Structures, Oxford University Press, New Delhi, 856 pp.