27 April 2018
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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:
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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.
Fiber performance is influenced by three characteristics; tensile strength, aspect ratio (calculated as the length/diameter) and anchorage (hooked, crimp, emboss, fibrillation, etc.). One characteristic does not outweigh another; all three items have to work together for optimal performance.
Fiber reinforced concrete is a composite material and therefore, all fibers are tested in the concrete to prove their performance.
The fibers then provide ductility and support by bridging cracks and thus providing post crack strength to the concrete. Fibers begin to function in a structural supportive manner when the concrete matrix starts to crack, just like traditional reinforcement. The crack has to occur for the load to switch from the concrete to the reinforcement.
Contact us to discuss your requirements of crimped steel fiber. Our experienced sales team can help you identify the options that best suit your needs.
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