Materials Used in Tall Structures

7/30/2019 Materials Used in Tall Structures

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MATERIALSUSED IN

TALL STRUCTERES


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Materials Used in Tall structures

RCC

HPC

FRC Pre-stressed concrete.


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R .C. C.

Although RCC began around the turn of century, it doesnot appear to have been used for multistory high risebuildings until 1930.

After world war two the construction of High RiseBuildings commenced with radically new structural andArchitectural solutions.

In 1903 the first high rise building was built in Cincinnati.


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Start ofR.C.C. in High rise Building

In 1903 the first high risebuilding was built in CincinnatiCincinnati.

Official name- Ingalls Building. City - Cincinnati.

State - Ohio.

Country - U.S.A.

Height - 65m

Floors - 16

Construction 1903


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R.C.C.

What is RCC?

Why it is used?

Cost of reinforcement .


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Types of Reinforcement

Up to 1960, Indian construction industry using 250 MPa mild steel for concretereinforcement.

Attempts to increase the yield strength by conventional method of increasing

carbon content is result in reducing Ductility, Bend ability of bars.

In 1970, CTD bars introduced as product of cold Twisting technology toovercome strength, Ductility problem where carbon content restricted to lowlevel and proof strength was increased from 250MPa to Guaranteed value of 415MPA by cold twisting, bond strength was increased by ribbing pattern.

TMT bars are recent technological advancement for production of High strengthDeformed steel bars. Process of Thermo -mechanical treatment increasesstrength, ductility.


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Classification and Identification of Bars

HYSD bars have no clearly marked yield point & which exhibit brittle failure(i.e. Failure occurring before reaching an elongation of less than (3-4)%

Ribbed bars are in grades of Fe250, Fe 415, Fe 500 & Fe550

The bar can be distinguished by surface deformation, grade marking bycolour code is difficult.

Bars less than 12mm dia. are produced in two ribbed design while the barsover 12 mm dia. Produced in four ribbed design.

Grade Fe500 bars can be identified by presence of * after every300mm,other bars do not have such marks.


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TMT bar CTD bar


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Properties of steel reinforcement

1. Tensile strength-Fe415- 10% higher than actual 0.2% proof stress or 485 MPaFe500- 8% higher than actual 0.2% proof stress or 545MPa.

2. Bond strength-Bond between steel and concrete depend directly on thedeformations over bars.Slipping chara. On HYSD bars indicate that slip of 0.1mm corresponds tobond strength almost 3 to 4 times of plain round bar.

3. Ductility- Is very important criteria for desired performance of R.C. memberespecially during Earthquake.Elongation at rupture over standard gauge length is accepted to be an indexfor ductility quality, percentage elongation should never less than 14.5% forany steel.

4. Bend ability- This is an important criteria for reinforcing bars in view ofbending of bars, which are frequently required during fabrication. This in turn

directly related to ductility of member.


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Cont

Fatigue strength- Reinforced concrete member subjected to alternatingloads producing minimum and maximum stresses of sufficiently highintensity may fail due to fatigue in steel.

Maximum repetitive stress that can be sustained by the steel section

without failure for 2 million load cycles is commonly accepted as ameasure of Fatigue strength.

Fire resistance- RCC structures subjected to fire get severely affectedwhen the temp of the reinforcing steel exceeds 500-600 deg. Celsius.

around this temp proof strength of HYSD bars reduces to 250MPa


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Tall buildings

1. Name Garden Towers

Structural design- Taisei construction co.

Date of completion- July 1992

No. of storey - 39

Height - 125.3m

Max grade used - 60MPa
http://images.google.co.in/imgres?imgurl=http://www.aviewoncities.com/img/frankfurt/kvege0519b.jpg&imgrefurl=http://www.aviewoncities.com/building/gardentowers.htm&h=300&w=200&sz=26&hl=en&start=1&tbnid=Tw0Fqg6KdDgzFM:&tbnh=116&tbnw=77&prev=/images?q=garden+towers,+&gbv=2&svnum=10&hl=en&sa=G


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Cont

Name- The scene Johoku

Structural design- Shimizu construction co.

No. of storey- 45

Height - 160m

Grade used - M60

Completion - 1993
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Code of practice for concrete reinforcement

IS 432 - mild steel and medium tensile steel bars (part1)and hard drawn steel wire (part 2).

IS 1139- Hot rolled mild steel, medium tensile steel andhigh yield strength steel for concrete reinforce.

IS 1786- High strength deformed steel bars with wires forfor concrete reinforcement.

IS 2502- Bending and fixing of bars for concrete reinforcement. IS 5525- Recommendations for detailing of reinforcement in R.C. work. IS 6461- Glossary of terms relating to cement concrete reinforcement. IS 1566- Hard drawn steel wire fabric for concrete reinforcement.


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High performance concrete

Initially compressive strength of concrete is considered as the mostimportant parameter to classify concrete.

However it has been realized at a later stage with experience thatthe strength is not the only important parameter but Durability,workability of concrete are also important for structure.

This led to the evolution of concept of H.P.C.


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As per ACI, Concrete which meets special performance and uniformity

requirements that can not be always achieved routinely by using

conventional methods, normal mixing and curing practices.

According to Neville H.P.C. is concrete selected so far to meet for thepurpose for which it is used. No unusual ingredients are needed and nospecial equipment has to used, all it requires is the understanding of thebehavior of concrete and will to produce the concrete mix within closetolerances.

HIGH PERFORMANCE CONCRETE


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What makes the concrete as H.P.C. is the very low w/c ratio.

Always below 0.35, often around 0.25 & occasionally around 0.20.

High performance concrete requires low volume of pores.

Only way to have low volume of pores is for mix to contain particles

grade down to finest size.

This is achieved by using admixtures.


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Quantities of cementitious materials

Determination of optimum quantity of cement and mineraladmixture is extremely important in designing the HPC mix

This is not only required for cost reduction but also forenhancement of the quality of concrete.


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Types of Admixtures used in H.P.C.

Mineral Admixtures used in H.P.C.

1. Silica fume

2. fly ash3. GGBS

4. Metakaoline


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M60 grade H.P.C. Mix with different mineral Admixtures

Sr. No. Mix Descriptions Aggregate Cementitious Water W/Cm SP Colour

Kg/M^3 materials (Kg/M3) (Kg/M3) (% by wt

Coarse Fine Cement Mineral

of

cem

ent)

Admixture

(%by wt of

cement)

1 H.P.C.- Control mix 1199.2 685.7 500 150.9 0.3 1.25 Light grey

2 H.P.C.- HRM 1188.2 679.4 425 10 163.6 0.35 1.5 Off White

3 H.P.C.-SF 1162.5 676.3 425 10 163.6 0.35 1.25 Light grey

4 H.P.C.-PFA 1059.3 653 333.3 50 166.7 0.325 1.25 Off White


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Properties of H.P.C. mixes

Parameters mixes

H.P.C.- CM H.P.C.- HRM H.P.C.-SF H.P.C.-PFA H.P.C.-GGBS

Fresh concrete

Density (Kg/M3) 2579 2545 2531 2499 2534

slump (mm) 150 165 170 200 180

Hardened concrete

Strength( Mpa) 73.5 77 70.5 69.3 73.3

Split Tensile Strength(Mpa) 0 4.5 4.55 3.68 4.36

Note: Properties at 28 days


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Chemical Admixtures used in HPC

Super plasticizers

1. SMF

2. SNF

3. ASTM F-type

Optimum dosage of super plasticizer can be evaluatedby flow consistency test using Marsh cone. The point atwhich the slope changes corresponds to the optimum

dosage of admixture


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Mixing Method

Multistage mixing sequence is suitable for H.P.C.

At Kaiga 1&2, RAPP-3&4 two stage mixing was found

efficient.

Silica fume was mixed dry along with aggregates, in thefirst stage for 5 seconds

Final mixing of 45 seconds


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Placement and compaction

Properly designed H.P.C. mix is more cohesive than NSC and doesnot tend to segregate but it loses strength rapidly compared to NSC.

This makes H.P.C. more sensitive to temperature

At kaiga 1&2 and RAPP-3&4 aggregates were pre cooled and about90% water is replaced by ice flakes.

High slump HPC mix may not seem to need compaction, butexperience of Kaiga-1&2, RAPP-3&4 suggest that good vibrationafter placement of fresh mix is essential for structural elementshaving high congestion with reinforcement.


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Curing

Performance of HPC at hardened state is rather more sensitive toCuring than that of NSC

Loss of moisture from exposed surface of fresh concrete at earlyage cause plastic shrinkage.

Protection against moisture loss from fresh HPC is crucial fordevelopment of strength and durability.

Curing of HPC is carried out in two stages, initial curing and finalcuring.


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Design of H.P.C.

No specific method of mix design of NSC, as presently prevailing inIndia, was found suitable for mix design of H.P.C.

Absolute volume method has been adopted in mix design of HPC forcontainment domes of Kaiga-1&2 and RAPP-3&4 nuclear power plants

In calculating the mix proportions, air content for concrete may beassumed to be 1 % and unit water content may not be less than

150Kg/m^3

Target compressive strength may be taken as about 20% more thandesired characteristic strength.


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Case Study of Kaiga project

Slump 175 mm

Silica fume 7.5% by wt of cement.

Cement 475 Kg/m^3 Superplasticizer ASTM F- type

Retarder 0.1 % of wt of cement

W/Cm is taken as 0.32 for target split tensile strength of

4.37 MPa.


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Use of H.P.C.

Bourke place Meilbourne, Australia

Year of completion 1991

No of storeys - 52

Height of building - 223m No of levels below ground- 3

Building use - office

Type of structure - R.C.C.

Concrete strength - 60MPa
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Fibre reinforced concrete

Fibres are used through ancient times3000 BCEgyptians Used mud mixed with straw to bind dried bricks.They also used gypsum mortars and mortars of lime in the

pyramids.

Portland cement association investigated FRC in 1950 with thesurge in Fibre reinforcing, new materials other than steel wereinvestigated

Recently organic & synthetic fibres such as acrylic,aramid,carbon, nylon, polyster, polypropylene

also been used.


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Cont

Useful Improvements in the mechanical behavior of tensionweak concrete (or mortar) can be achieved by incorporation ofshort discrete fibres

Resulting composite generally termed as Fibre ReinforcedConcrete


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FIBRE REINFORCED CONCRETE

Concrete containing a hydraulic cement, water, fine or coarse aggregate

and Discontinuous fibers is called F.R.C.

Unlike plane concrete, a F.R.C. specimen does not break immediately

after initiation of the first crack.

At the cracking section matrix does not resist any tension and fibres carry

entire load of the composite.

With increasing load fibers will tend to transfer the additional stress to

matrix through bending. If these bond stresses do not exceed thebond strength, then there may be additional cracking in matrix.

The process of multiple cracking will continue until either fibres fail or

there may be fiber pullout.


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Constituent materials

Commercially used mixes for matrix in FRC are often not very muchdifferent from what is used for conventional RCC.

In applications when thin sections are to be cast Maximumaggregate size limited to 9.5 to 19mm

It is necessary to use super plasticizers with mixes containing largerfibre volume content (1% or more).

Air entraining agents can also be used with FRC to improve itsability to resist freeze thaw cycling under load( especially inpavement)


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Fibre Aspect Ratio

Mechanical properties of FRC are largely influenced by Fibre AspectRatio, fibre type, fibre orientation.

F.A.R.- Is defined as the ratio of the length to equivalent fibre dia.

To avoid the fibre balling in conventional and to provide uniformdistribution of fibres in mix FAR taken as 100

Practically used fibre volumes in conventional FRC range from (0.1to2)%


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Types of fibres

1. Low modulus, high elongation fibres

such as acrylic, aramid, nylon, polyster, polypropylene

these are approx. 0.25mm in dia. & 12mm to 50mm length, with

FAR=50-100

2. High modulus, high strength fibres such as steel, carbon, glass


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Properties of FRC

1. Tensile behavior


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Cont

3. Flexural behavior

In most practical applications FRC is likely to be subjected toflexural loads.

There are two commonly reported strengths values associated

with flexure, First crack strength & Ultimate flexural strength

First crack strength- Is defined as the flexural tensile strength ofthe composite at a point where loaddeflection curve deviatesfrom linearity which is not influenced by incorporation of fibres.

Ultimate flexural strength is evaluated as the flexural tensilestrength at the peak load carrying capacity, which is greatlyinfluenced by the fibre type, fibre aspect ratio and fibre volumecontent


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Toughness

Toughness isgenerallyaccepted as theenergy absorbing

capacity of thematerial.

The energyabsorbed by thespecimen iscomputed fromthe area underthe loaddeflection curve


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Fatigue resistance

Experimentalstudies showsthat for agiven type offibre there is asignificantincrease inFatiguestrength with

increase in

fibre content.


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Applications

It can be used in both cast-in-place and precast applications where

Durability and crack control are major considerations.

In Tall buildings F.R.C. can be used for exterior panels, shear walls,

floors that carry vehicle traffic (Parking level in tall buildings)

Columns (Avoid congestion of reinforcement)

Beams (Shear strength enhancement).

Foundations and Footings (Shear resistance and dynamic loads).

Beam column joints (Improve ductility and avoid congestion of

steel)


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PRE STRESSED CONCRETE

The development of the early cracks in Reinforced concrete due to

incompatibility in the strains of steel and concrete is the starting point of

the new material known as Pre stressed Concrete.

Pre-stressing means intentional application of a predetermined

force on a system for resisting the internal stresses, developed in

system.


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System of Pre stressing

Pre tensioned Pre stressing.

Post Tensioned pre stressing


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Advantages of Pre stressing

Utilize the full section of concrete.

To obtain the crack free concrete.

Greater resistance to shearing forces due to effect of compressive

pre stress, which reduces the principle tensile stress.

Size of member reduces which results in decreasing the dead load

of structure and minimize the total height of the structure.

For long span structures it is Economical


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High rise building

Paramount building, Sanfrancisco

Post tensioned pre stressing

No. of storey

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Concrete grade used M55


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Materials Used in Tall Structures

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