Keywords: high strength concrete, beams, helical reinforcement, finite elements.

In this paper finite element modelling (FEM) is used to analyse the behaviour of helically-confined high strength concrete beams. High strength concrete (HSC) has different characteristics than normal strength concrete (NSC). High strength concrete is less ductile than normal strength concrete. However, by using helical reinforcement in HSC; the ductility issue can be improved. Helices confine the concrete and bring it to a three dimensional state of stress due to the Poisson's effect.

A main problem of high strength concrete is ductility. According to the dictionary of civil engineering, ductility or ductile deformation is the ability to undergo cold plastic deformation without cracking and breaking. An increase in concrete strength increases its brittleness. Yet, very high strength concretes are considerably more brittle. The stress-strain relations for high strength concrete are steeper than normal strength concrete. It seems that the ascending branch behaves nearly linear for much higher stress. Moreover, the descending branch in the post-ultimate range appears very sharp and it becomes vertical at higher strength concrete. It means that high strength concrete is brittle and explosive when the ultimate load is reached. For high strength concrete, the value of the strain epsilon₀ at the maximum stress f_cm can be significantly higher than 0.002. Building codes generally apply a maximum compressive strain in high strength concrete designed at 0.003.

Hadi and Schmidt [1] reported that HSC in a reinforced beam has a low ductility, which consequently creates the brittle failure, even though it enhances the strength of concrete. In fact, ductility is very important to determine whether a large deformation and deflection of a structure can occur under overload conditions or it will otherwise experience catastrophic collapse. Therefore, due to the natural behaviour of concrete material, the use and continual improvements of HSC seems to be hampered by a decrease in ductility.

Studies indicate that the ductility and the strength of full potential flexural strength of reinforced high strength concrete can be increased by confining the concrete in the compression zone with helical reinforcement. Several studies found that the helical pitch and diameter significantly influence the ductility and strength of helically confined HSC beam. If the helical pitch equals to the confinement core diameter then the failure becomes brittle. Conversely, the pitch size of helical reinforcement should not be very small so as it can separate the concrete cover from its core. In this paper, the analysis of helical reinforcement in high strength concrete beams with five different pitches 25, 50, 75,100, and 150 mm, are performed by using the finite element software: ANSYS and Strand7. Results of the experimental programme conducted by Elbasha and Hadi [2] are used to compare the results of the finite element analyses. The finite element analyses resulted in load-mid span deflections that are comparable with the experimental results.

1

M.N.S. Hadi and L.C. Schmidt, "Use of helices for enhancing the properties of reinforced concrete beams", Int. J. Mat. And Prod. Tech., 19(3/4), 247-258, 2003. doi:10.1504/IJMPT.2003.002509

2

N. Elbasha and M.N.S. Hadi, "Experimental Testing of Helically Confined High Strength Concrete Beams", Structural Concrete, 6(2), 43-48, 2005. doi:10.1680/stco.6.2.43.66714

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £120 +P&P)