Steel Fibres and Concrete Flexural Strength

Steel Fibres and Concrete Flexural Strength

Written by David W. Harris, PhD, PE

27 April 2018

PHOTO: Concrete Beam Tested in a Third-Point Flexural Apparatus, Courtesy SELlabmanager

In our Concrete Strength series, we discussed the flexural strength of beams; the failure lines are discussed in the book Concrete Strength 1–2–3. A picture of a laboratory demonstration of a nonreinforced beam in failure is shown above illustrating the propagation of cracks from the extreme concrete fibre in the tension zone. The cracks are moving through the tension zone towards the neutral axis of the concrete beam.

Cracks open in shear in the left and right thirds (diagonal lines) and in tension in the middle third (vertical lines). Once cracks initiate they want to open, increase deflection, normal to the crack. This is a tension stress state. We are familiar with reinforced steel design for beams which is designed to carry the tension stress in the bottom of the beam to resist cracking. What are possible alternatives for carrying load perpendicular to a crack?

Imagine if we could design a mini-rebar across each crack.

We can! Steel fibres and some structural synthetic fibres are available for concrete. Hooked or crimped steel fibres are mixed with the concrete anywhere from 10kg/m3 right up to 40kg/m3. The fibre is mixed through the matrix of the concrete so are positioned in random directions which allows tension loads to be carried by the fibres as they develop from various directions.

The American Concrete Institute specifications gives the shear strength for normal weight concrete as:

Experimental research by Parra-Montesinos (Concrete International, 2006, pp. 57–66) showed that the factor of 2.0 could be increased by 3.0 or greater with hooked or crimped end fibres.

Hooked end fibres increase the shear carrying capacity by 50% or more when used in concrete beams and thereby increasing the flexural strength.

Talk to our engineers today to see how you can use steel fibres to increase the strength and durability of your concrete slab or structure.



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