Steel reinforced cement

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26 Νοε 2013 (πριν από 4 χρόνια και 6 μήνες)

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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

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
capacity at first

(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


have been shown to
increase the ultimate moment and ultimate
deflection of conventionally reinforced beams;
the higher the tensile stress due to the
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