Effect of helical pitch and tensile reinforcement ratio on the concrete cover spalling off load and ductility of HSC beams

[extract] In recent years a marked increase in the use of High Strength Concrete (HSC) has been evident in Australian building construction despite the fact that the current Australian design standard, AS3600 provides no design rules. HSC has been used extensively in civil construction projects world wide because it reduces the cross section and the weight for long construction members. High strength concrete and high strength steel are used together to increase the load capacity and reduce the beams\u27 cross section. Using these two materials to design over reinforced beams will lead to huge reduction of cost, which is a desirable issue. However, the problem is the lack of ductility, hence such use is not allowed by the current codes of practice

Behaviour of over reinforced HSC helically confined beams

The technology of high strength concrete and high strength steel have improved over the last decade although high strength concrete is still more brittle than normal strength concrete. As this brittleness increases, particularly with the use of over reinforced sections, they may, suddenly fail without any warning. The research reported in this thesis deals with the installation of helical confinement in the compression zone of over-reinforced high strength concrete beams. This study is divided into three parts as follows: 1) State of the Art & Literature Review This part deals with state of the art and literature review. Helical confinement is more effective than rectangular ties, compression longitudinal reinforcement and steel fibres in increasing the strength and ductility of confined concrete. Helical reinforcement upon loading increases the ductility and compressive strength of axially loaded concrete due to resistance to lateral expansion caused by Poisson’s effect. Based on this concept helical reinforcement could be used in the compression zone of over-reinforced high strength concrete beams. The effectiveness of helical confinement depends on different important variables such as helical pitch and diameter. Thus there is a need for an experimental programme VI to prove that installing helical confinement in the compression zone of an overreinforced concrete beam enhances its strength and ductility and to study the behaviour of over-reinforced high strength concrete beams subjected to different variables. 2) The Experimental Programme & Test Analysis This part deals with an experimental programme and analysis of test results. Extensive experimental work was done because the beams should be full size in order to accurately represent real beams. Twenty reinforced concrete beams, 4 m long &#;&#;200 mm wide &#;&#;300 mm deep were helically confined in the compression zone and then tested in the civil engineering laboratory at the University of Wollongong. In this programme the following areas were studied: the effect of helical pitch, helical diameter, concrete compressive strength and longitudinal reinforcement ratio, on the behaviour of over-reinforced HSC helically confined beams. 3) Analytical Models to Predict the Strength & Ductility This part deals with the analytical models used to predict the strength and ductility of over-reinforced high strength concrete beams based on the findings of this study. A comparison between the experimental and predicted results shows an acceptable agreement. This study concludes that helical reinforcement is an effective method for increasing the strength and ductility of over-reinforced high strength concrete beams

Displacement Ductility of Helically Confined HSC Beams

This paper presents an experimental investigation of the effect of helix pitch and helix diameter on beam behaviour through testing 10 helically confined beams. Two groups of beams had exactly the same geometry and reinforcement; with the only differences being the helices diameter and pitch. 8 mm helix was used in the first group of beams and 12 mm bars in the second group. The helix pitches varied between 25 mm and 160 mm. Beams\u27 cross section was 200 x 300 mm, with a length of 4 m subjected to four point loading. The main results indicate that the helical effectiveness is neglected when the helical pitch is 160 mm (helix diameter) and the displacement ductility index increases as the helical pitch decreases. Finally, there is a considerable release of strain energy responsible for spalling off the cover

The effect of helical pitch on the behaviour of helically confined HSC beams

The strength and ductility of HSC beams are enhanced through the application of helical reinforcement located in the compressionregion of the beams. The pitch of helix is an important parameter controlling the level of strength and ductility enhancement of overreinforcedhigh strength concrete beams. This paper presents an experimental investigation of the effect of helix pitch on the beam behaviourthrough testing five helically confined full-scale beams. The helix pitches were 25, 50, 75, 100 and 160 mm. Beams cross section was200 · 300 mm, and with a length of 4 m and a clear span 3.6 m subjected to four point loading, with emphasis placed on the mid-spandeflection. The main results indicate that the helical effectiveness is negligible when the helical pitch is 160 mm (helix diameter), the concretecover spalling off load increases linearly as the helical pitch increased, and the ultimate load decreases as the helical pitch increases.Finally, there is a considerable release of strain energy responsible for spalling off the concrete cover.\u3

Flexural ductility of helically confined HSC beams

The ductility of HSC beams is enhanced through the application of helical reinforcement located in the compression region of the beams. The diameter and pitch of helix are important parameters controlling the level of ductility enhancement of over reinforced high strength concrete beams. This paper presents an experimental investigation of the effect of helix pitch and diameter on the beam behaviour through testing 10 helically confined full scale beams. Two groups of five beams each had exactly the same geometry and reinforcement; with the only differences being the diameter and pitch of the helices. For one group the helix was made of 8 mm diameter bars and the second of 12 mm diameter bars. The helix pitches were 25, 50, 75, 100 and 160 mm. Beams’ cross section was 200x300 mm, with a length of 4 m and a clear span 3.6 m subjected to four point loading, with emphasis placed on the mid-span deflection. The main results indicate that the helical effectiveness is negligible when the helical pitch is 160 mm (helix diameter) and the displacement ductility index increases as the helical pitch decreases. Finally, considerable displacement ductility is revealed for beams confined with 25 mm pitch helix in both the 8 mm and 12 mm helix bar diameter

Establishing model for the displacement ductility index of HSC beams

Incorporating Sustainable Practice in Mechanics of Structures and Materials covers a wide range of topics, from composite structures, via fire engineering and masonry structures, to timber engineering. Valuable reference for academics, researchers and practicing engineers working in structural engineering and structural mechanics. Summary Incorporating Sustainable Practice in Mechanics of Structures and Materials is a collection of peer-reviewed papers presented at the 21st Australasian Conference on the Mechanics of Structures and Materials (ACMSM21, Victoria, University, Melbourne, Australia, 7th – 10th of December 2010). The contributions from academics, researchers and practicing engineers from 17 countries, mainly from Australasia and the Asia-pacific region, cover a wide range of topics, including: • Composite structures • Computational mechanics • Concrete structures • Dynamic analysis of structures • Earthquake and wind engineering • Fibre composites • Fire engineering • Geomechanics and foundation engineering • Masonry structures • Mechanics of materials • Shock and impact loading • Steel and aluminum structures • Structural health monitoring • Structural optimisation • Sustainable materials • Timber engineerin

A model for predicting the displacement ductility index of HSC beams

It has been proven that helices confinement provided in the compression zone of over-reinforced HSC beams improves the ductility. According to the codes of practice, there is a limit to the ratio of longitudinal reinforcement for a particular cross section. However more longitudinal reinforcement can be installed if the flexural strength required is more than the capacity of a particular cross section, where such a section becomes under-reinforced rather than over-reinforced section. It is basic knowledge that over-reinforced sections fail in a brittle mode but installing helical reinforcement with a suitable pitch in the compression zone will reduce this unwanted effect. Formulating the displacement ductility index for an over-reinforced helically confined HSC beam is required to study and focus on non-dimensional factors. The relationship between displacement ductility index and non-dimensional factors involves a large number of variables, most of which are related to helical confinement. The behaviour of over-reinforced helically confined HSC beams is complex and therefore several variables must be investigated to develop an empirical formula. The development of a model to predict displacement ductility index of over-reinforced helically confined HSC beams is presented in this paper. The displacement ductility index is affected by variables such as the volumetric ratio of helical reinforcement, helical pitch and helical yield strength. The results obtained from this model are compared with the experimental results

Experimental testing of helically confined high-strength concrete beams

The strength and ductility of high-strength concrete (HSC) beams are enhanced through the application of helical reinforcement located in the compression region of the beams. The pitch of the helix is an important parameter controlling the level of strength and ductility enhancement of over-reinforced HSC beams. This paper presents an experimental investigation of the effect of helix pitch on the beam behaviour by testing five helically confined, full-scale beams. The helix pitches were 25, 50, 75, 100 and 160 mm. The cross-section of the beams was 200 300 mm, and with a length of 4 m and a clear span of 3.6 m subjected to four-point loading, with emphasis placed on the midspan deflection. The main results indicate that the helix had negligible effect when the helical pitch was 160 mm (helix diameter), the concrete cover spalling-off load increased linearly as the helical pitch increased, and the ultimate load decreased as the helical pitch increased