Increased infrastructure reliability by developing a low cost and high performance stainless steel rebars

As far as the reinforcing steel bars is concerned, the European industrial production of reinforcing stainless steel has developed, in the last decade, products which have good mechanical and corrosion properties. This characteristics, joined to the opportunity to complete or partial avoiding the maintenance and repair actions and to increase the durability and the service life of the buildings, has conferred at the stainless steel a reasonable diffusion especially in critical environment. In spite of the clear advantages provided by this steel, there is actually a huge problem preventing the construction industries from their wide level use: the cost. The HIPER Project has achieved a wide set of technical and economical data comparing conventional and innovative stainless steels. The main result achieved is to set up an innovative stainless steel (HSS3) that maintain the same mechanical characteristics and similar corrosion behaviour of the well known stainless steel, but while the last one are affected by an uncontrolled oscillations of costs, the innovative ones, by searching low cost raw materials, maintain a low cost oscillation. The research has been focused on high tough austenitic stainless steels with a low content of "noble elements" nevertheless the corrosion resistance results are fairly good in respect of the target. From the application point of view, the innovative stainless steels show an interesting alternative in the seismic structures. Also in structures submitted to high fire risk, like the concrete layers in road or rail tunnels, the innovative stainless steels, on account of its high mechanical properties at very high temperature, can offer a remarkable alternative for designers. Within the framework of this result, Polito assessed the mechanical properties as well as weldability of all the materials selected. This was done so as to provide the consortium and, later, all the technicians of construction sector a good knowledge about inherent properties of stainless steel (ss) and, in particular, of HSS steels. It is highly necessary to make aware the construction sector technicians of ss and HSS properties (materials which are very different from typical construction steels), so that they can design exploiting their all potential. For Polito, the main point of interest on the new materials is the microstructural modification induced by strain. In austenitic ss the stabilisation of austenite is usually achieved through nickel additions, but to reduce production costs in HSS composition, nickel has been substituted by N and Mn, cheaper but less effective elements. The transformation of austenite into martensite is particularly pronounced in HSS1 which is less stable. This mechanism has been studied and described with data mainly through tensile test, but in practice, it may occur in any process involving straining. It results in strain hardening, which is in principle positive as it raise tensile properties, but has to be deeply understood to be aware of the behaviour in the preparation phase and in service.

In view of the increasing level of development of high performance civil engineering infrastructures and the limited capacity of older structures (several years) to provide the increasingly high performance demanded by current severe environmental conditions, it becomes necessary to develop highly innovative materials in Europe. The increase in mechanical characteristics and the higher corrosion resistance within corrosive environments exhibited by stainless steel cannot be ignored, but stainless steel rebar are not used because of their initial high cost. Nowadays it is possible to find in the market some innovative low-cost High Strength Stainless Steels as the steel developed by the Hiper project. The importance of these alloys has always been limited due mainly to their low cold working formability. Ferriere di Stabio's activity focussed on this type of steel to design a new manufacturing process by developing and improving a tools design and a forming process fit to generate ribs with the best bond strength. Ferriere di Stabio carried out the study for the development of a suitable technology and the production trial of the innovative stainless steel to obtain rib patterns with an optimised shape to join together the brittle concrete and ductile stainless steel. Two subjects have to be matched: IR (bond strength), as defined by the technical norms, and rib shape fit to grant the best material flow to avoid surface discontinuities that can affect the ultimate tensile strength or reduce the fatigue resistance. Within the framework of this result, Polito assessed the mechanical properties of all the materials selected in the reinforcing bar form and cold rolled state in order to provide to the consortium and, later, to all the technicians of construction sector a reliable set of data. The properties of the cold rolled innovative products are of course highly affected by the microstructural modifications which occurred in hot rolled and annealed state. The classification of the mechanical properties in the reinforcing bar form was particularly necessary to provide a comparison with the classical reinforcing bar steel (FeB44K), which is well known product for all the construction sector. The assessment of the mechanical levels and of the stress-strain pattern is the essential base on which it is possible to design and build with such material. These mechanical levels are currently used by other partners of the Consortium in their own activity within the HIPER project. In the future, the technicians of the construction sector will, hopefully, refer to the guidelines drawn in this project and reported in Deliverable 10 in their own day by day work of design and building.

Current theoretical shear design models for reinforced concrete member design adopted by current design codes in Europe (EC2, Greek Code for Design of Concrete Works) and in the United States (ACI 318) accept the theoretical model of the truss analogue for the estimation of the shear available resistance of RC elements. Recent seismic behaviour of well designed columns in earthquakes as well as laboratory investigations at the RC Laboratory have, however, demonstrated the inability of this method to reliably predict the actual capacity of members in shear, being on both sides of conservatism: either uneconomic or unsafe. An improved design methodology has been put forward by the investigators for improving the theoretical prediction model, which has been proven to provide reliable estimates of the member capacity both for new and for retrofit design. Monotonic and cyclic tests of full size laboratory beam-column specimens tested herein using both conventional strength (ENV-1992 class C30) and high strength (ENV 1992 class C60) concretes, to evaluate the conventional and the proposed retrofit and design methodology. The results of the investigation show that the proposed method is able to predict the failure load of the elements much better than the conventional methods, therefore providing at the same time a safer and more ductile design.

The upgraded product, the piezo-electric actuation servo-valve, will facilitate essential high strain rate testing and particularly testing in the intermediate strain rate, which is often the strain rate of seismic action. The available servo-hydraulic valves cannot generate and control high frequency loading, particularly when high loading frequency and large stroke are combined. However, if the driving unit of the servo valve, which is electromagnetic, is suitably substituted by an equivalent piezo-electric unit, the performance of the valve is enhanced. Although the principle seems simple and the relevant dynamic model is well defined, the implementation is partially effective due to the limited expansion of the piezo-actuator. IMMG had already proceeded in the implementation of this unit in the project but the expansion limitation of P?? powerful actuators (< 150 µm) did not allow the use of the improved valve in high strain rate testing, when high ductility was foreseen. During the first part of this project a prototype mechanism was developed which multiplies (by four) the small range stroke of the piezo-electric actuator. Numerical investigation supported the experimental work and optimised the dimensions of the elements of the multiplier. Also, preliminary tests on dummy specimens indicate that the valve performance should be adequate during the main tests, which simulates earthquake motion. The response time of the new valve is 1,5 msec and this reaction speed allows the interruption of a fast developing phenomenon like concrete fracture.

In order to assess the structural response of reinforced concrete in fire, a major research programme was initiated under the umbrella of the Hiper project which involves the testing of the steel, concrete and the composite by means of pull-out tests. Such a comprehensive programme is unique. The tests are partly funded by Hiper and partly by Professor Gabriel Khoury of FSD and Imperial College to provide a comprehensive panoramic view of the effect of fire on reinforced concrete structures and to examine the influence of three types of steel - carbon, AISI 316 Stainless and especially the innovative HSS3 Conge stainless steel. Although the casting was made in December 2004 and January 2005, the steel tests and over half of the concrete and pull-out tests have already been completed. While not part of the Hiper programme, tests on plain concrete are necessary in order to ascertain the thermal stabilities of the two concretes examined: limestone and granite, the latter being the more stable. For the pull-out tests, the pull-out failure induced by shearing off (corresponding to well-confined bars) and pull-out failure induced by splitting cracks (corresponding to less confined bars) are investigated. Furthermore, all tests are conducted at Imperial College, London, in the traditional "steady-state" temperature methodology as in the codes and standards, and by an innovative "transient" thermal test methodology more representative of fire conditions.

The research team contacted the technical offices of the Pavia province (a public administration body) in order to collect all data necessary to develop a LCCA on a new bridge the province is planning to build over the Ticino river. The aim of LCCA was to demonstrate the capability of the new product to meet the normal requirements of designers for a civil structure. In particular, attention focused on the design and construction of the reinforced concrete bridge deck. Building works was scheduled to start at the end of 2004. One hundred tons of HSS3 could be used to reinforce the concrete deck of the new bridge. Two different types of concrete and three different steels have been taken into account. A total amount of 100 tons of inox steel has been considered. Different scenarios were proposed and evaluated in order to compute different LCCA for the new bridge's reinforced concrete deck. The analysis estimated the cost per unit area of the deck for construction, maintenance, consolidation and demolition of the bridge. This information is required for the evaluation of NPD and AV for the direct costs. Traffic data concerning the bridge has been evaluated in order to estimate the user costs.

Concrete mixes have been optimised in compliance to the EN 206-1 standard for the aggressive environments (Seismic Resistant Concrete, Corrosion Resistant Concrete, Fire Resistant Concrete) considered in the project. In each case, a normal strength concrete (C25/30) and a high strength concrete (C60/75) were designed, using the most appropriate local materials. The selection of the best concrete mixes was based on the results of characterisation tests relevant to environment. A complete characterisation of the selected mixes was performed in fresh and hardened state, together with specific tests on some concrete mixes. The performances achieved for high strength concrete were in some cases significantly above the current level of performance for this type of concrete in area considered. The same concrete (components and mixture proportions) was used for tests on plain or stainless steel reinforced concrete, on small specimens in laboratory or on large specimens during field tests. This provides direct information about the impact of concrete mixture proportions on the behaviour of stainless reinforced concrete exposed to aggressive environments, fire or seismic loading. These data will provide potential references for the users of these high performance reinforced concrete not yet commonly used locally.

This result covers proposed amendments to European standards when the new low-nickel stainless steel HSS3 is used as reinforcement in concrete structures instead of normal carbon steel. The current topics for possible amendments were identified, and the following topics are covered: -Durability (EN 1992-1-1, Eurocode 2-1-1). - Structural design of concrete structures (EN 1992-1-1, Eurocode 2-1-1). -Design of concrete structures under seismic action (prEN 1998-Parts 1 and 2, Eurocodes 8-1 and 8-2). -Design of concrete structures under fire action (EN 1991-1-2, EN 1992-1-2 and prEN 1993-1-2, Eurocodes 1-1-2, 2-1-2 and 3-1-2). -Repair and strengthening of concrete structures (prEN 1504-series). For durability, some relaxations are proposed. For the other topics it is concluded that Design rules in the European standards are applicable for the HSS3 steel. However, the properties of the steel are such that it is not beneficial to employ HSS3 steel compared to carbon steel. The main users of the result are standard making organisations, especially CEN, consultants and other users of the new reinforcing steel.

Deliverables

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