Presentation of

BY : suphamongkhon manoch tananat phichitsurathaworn

overview  People are being misinformed about steel reinforcement in concrete structures. Over the last five years or more, there has been a considerable amount of advertising in the various concrete industry trade magazines alluding to the placing of concrete without steel reinforcement. Part of this advertising has stated that steel reinforcement can be left out and compensated with admixtures and enhancers? or just use plain (unreinforced) concrete. Much of this advertising is misleading. Architects and engineers are specifying steel reinforcement and the reinforcing is being removed in the field. Some people are unaware that there is no substitute for concrete steel reinforcement. While supplemental admixtures may be included along with the reinforcement, the two materials do different things in the concrete.

It appears that many people are hearing what’s wrong with steel reinforcement. Of course, it is like anything that has been around for a long time—we tend to have more knowledge about that material (and sometimes more bad news than good news). Concrete steel reinforcing has been in the construction marketplace over 100 years, and we have heard a lot of good news. While most people know that all steel reinforcement must be properly positioned in the concrete and be provided with sufficient cover, some steel ends up in the wrong place. In a slab on grade with one layer of reinforcement the bar mats or flat welded wire sheets should be placed on supports 1/3 the depth from the top of the slab or a minimum concrete cover of 2 in. (51 mm). Many experts believe that the steel area should be reduced or terminated with slip steel dowels used at the control and construction joints to allow for free contraction and load transfer at those locations. The steel reinforcement industry believes these are good rules to follow, especially when large concentrated loads are applied to floors.

Fire remains one of the serious potential risks to most buildings and structures. Since concrete is widely used in construction, research on fire resistance of concrete becomes more and more important. Many researchers all over the world have done some researches on this subject. The mechanical properties of all common building materials decrease with elevation of temperature. The behavior of a reinforced concrete structure in fire conditions is governed by the properties of the constituent materials, concrete, and steel, at high temperature. Both concrete and steel undergo considerable change in their strength, physical properties, and stiffness by the effects of heating, and some of these changes are not recoverable after subsequent cooling.It is necessary to have safe, economical, and easily applicable design methods for steel members subjected to fire.

However, without fire protection, steel structures may suffer serious damage or even collapse in a fire catastrophe. This is because the mechanical properties of steel deteriorate by heat during fires, and the yield strength of conventional steel at 600°C is less than 1/3 of the specified yield strength at room temperature. Therefore, conventional steels normally require fire-resistant coating to be applied. The temperature increase in the steel member is governed by the principles of heat transfer. Consequently, it must be recognized that the temperature of the steel member(s) will not usually be the same as the fire temperature in a compartment or in the exterior flame plume. Protected steel will experience a much slower temperature rise during a fire exposure than unprotected steel. Also, fire effect on steel member is influenced with its distance from the center of the fire, and if more ventilation occurs near the steel in a fuel-controlled condition, wherein the ventilation helps to cool the steel by dissipating heat to the surrounding environment.

Especially, temperature increase of steel and concrete in composite steel- concrete elements leads to a decrease of mechanical properties such as yield stress, Young's modulus, and ultimate compressive strength of concrete. Thus, load bearing of steel decreases when steel or composite structure is subjected to a fire action. If the duration and the intensity of the fire are large enough, the load bearing resistance can fall to the level of the applied load resulting in the collapse of the structure. However, the failure of the World Trade Centre on 11th September 2001 and, in particular, of building WTC7 alerted the engineering profession to the possibility of connection failure under fire conditions. In this study, S220 and S420 ribbed concrete steel rebar were subjected to 7 different temperatures to determine the high temperature behavior of reinforcement steels.

As recommended by ACI Committee 544, ‘when used in structural applications, steel fiber reinforced concrete should only be used in a supplementary role to inhibit cracking, to improve resistance to impact or dynamic loading, and to resist material disintegration. In structural members where flexural or tensile loads will occur ….. the reinforcing steel must be capable of supporting the total tensile load’. Thus, while there are a number of techniques for predicting the strength of beams reinforced only with steel fibers, there are no predictive equations for large SFRC beams, since these would be expected to contain conventional reinforcing bars as well. An extensive guide to design considerations for SFRC has recently been published by the American Concrete Institute. In this section, the use of SFRC will be discussed primarily in structural members which also contain conventional reinforcement.

For beams containing both fibres and continuous reinforcing bars, the situation is complex, since the fibres act in two ways:  (1) They permit the tensile strength of the SFRC to be used in design, because the matrix will no longer lose its load-carrying capacity at first crack  (2) They improve the bond between the matrix and the reinforcing bars by inhibiting the growth of cracks emanating form the deformations (lugs) on the bars.

However, it is the improved tensile strength of SFRC that is mostly considered in the beam analysis, since the improvements in bond strength are much more difficult to quantify. Steel fibres have been shown to increase the ultimate moment and ultimate deflection of conventionally reinforced beams; the higher the tensile stress due to the fibres, the higher the ultimate moment.

Here are the benefits of steel reinforced concrete slabs: Steel reinforced cement  Steel reinforcing is simple to place.  Steel reinforcing reduces random cracking.  Steel reinforcing reduces and controls crack width and helps maintain aggregate interlock.  Displacement and curling can be minimized when steel reinforced concrete is provided.  Strength is increased with steel reinforced concrete even the smallest cross sectional area of steel reinforcement will provide reserve strength of l 6 percent and more.  Most importantly, steel reinforcement saves money over the life of the slab.  Finally, admixtures are not an alternative to steel reinforcement; they both do different things in the concrete. Therefore, admixtures cannot be substituted for steel reinforcement. The steel reinforcement industry is dedicated to providing quality steel reinforcement to the construction industry. It is also essential that steel reinforcement be sized, spaced, and placed properly. It is vital to have a well-graded and compacted granular sub base. Of course, total quality can only be achieved when well qualified suppliers and contractors are on the construction sites.

High cost because we must use a lot of steel

The uses of FRC over the past thirty years have been so varied and so widespread, that it is difficult to categorize them. The most common applications are pavements, tunnel linings, pavements and slabs, shotcrete and now shotcrete also containing silica fume, airport pavements, bridge deck slab repairs, and so on. There has also been some recent experimental work on roller-compacted concrete (RCC) reinforced with steel fibres. The list is endless, apparently limited only by the ingenuity of the engineers involved. The fibres themselves are, unfortunately, relatively expensive; a 1% steel fibre addition will approximately double the 115 material costs of the concrete, and this has tended to limit the use of SFRC to special applications.

Thank you for your attention