lecture 2 - progress in concrete
TRANSCRIPT
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Progress in Concrete
Technology
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Introduction Conventional Portland-cement concrete
mixtures suffer from certain deficiencies. Attempts to overcome these deficiencies have
resulted in the development of special
concrete types. Ordinary concrete, made with natural
aggregate, has a low strength-weight ratiocompared to steel.
Steel = 60
Concrete = 20
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Introduction
This places concrete at an economic disadvantagewhen designing structural members for tallbuildings, long-span bridges, and floatingstructures.
There are 3 ways to address this problem:-
1. The unit weight of concrete can be reduced bysubstituting lightweight aggregate (LWA) in placeof conventional aggregate. LWA made by calcination of clay or shale is commonly
used to produce structural lightweight concrete thathas about 1/3 less unit weight than conventionalconcrete.
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Introduction
2. The strength of concrete can be raised
substantially.
3. The third approach, which is a relatively
recent development, combines the first two
approaches.
It involves the use of high-strength LWA particles
in superplasticized mixtures to produce high-strength, lightweight concrete.
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Introduction
Restrained shrinkage on drying is frequently thecause of concrete cracking.
This has long been recognized in the design andconstruction practice of relatively thin structural
elements such as floor and pavement slabs.
To counteract this problem, shrinkage-compensating concrete containing expansive
cements or cement additives were developedabout 40 years ago and are being successfullyused.
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Introduction
Poor impact resistance is yet anotherdeficiency from which concrete suffers as abuilding material.
This characteristic has been substantially
improved by using the concept of microlevelreinforcement.
Fiber-reinforced concrete mixtures containing
steel, glass, or polypropylene fibers are beingemployed successfully in the structures whereresistance to impact is important.
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Introduction
Imperviousness is an important materialscharacteristic for durability to strong chemical
solutions.
Concrete mixtures containing polymers havebeen developed which show very low
permeability and excellent chemical resistance.
Overlays composed of such concrete mixtures are
suitable for protection of reinforcing steel from
corrosion in industrial floors and bridge decks.
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Introduction
Heavyweight concrete made with high-density
minerals is about 50% heavier than normal
concrete containing conventional aggregate.
This type of concrete is being used for
radiation shielding in nuclear power plants
when limitations of usable space require a
reduction in the thickness of the shield.
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Introduction Mass concrete for dams and other large
structures has been around for some time, butmethods selected to control the temperature risehave had a considerable influence on theconstruction technology during the last 45 years.
Pre-cooling of concrete materials has virtuallyeliminated the need for expensive post-coolingoperations and has made faster constructionschedules possible.
Dams are also being built now with Roller-Compacted Concrete, using ordinary earth-moving equipment, at speeds and costs that wereunimaginable only 25 years ago.
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Structural Lightweight Concrete
ACI 213R-87, Guide for Structural LWAConcrete defines as those having a 28-day
compressive strength in excess of 17 MPa and
air-dried unit weight not exceeding 1850
kg/m3
Microstructural Properties:-
Dense, strong TZdue to Pozzolanic reaction
Strength more influenced by aggregate
characteristics than by TZ
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Structural Lightweight Concrete
Durability
Freeze-Thaw Resistance
Permeability
Creep/Drying Shrinkage
Abrasion/Erosion Resistance
Applications
Lower overall cost of the structure
Bridge Decks
Floor slabs
Pre-cast concrete elements
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High-Strength Concrete
Definition:
28-dayf c40 MPa
Materials and mix proportions, e.g.
W/C Ratiof c 0.38 40 MPa
0.36 50 MPa
0.34 60 MPa
Problem: Mixing, placing, and consolidation
Use: Water-Reducing Admixture.
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Calculated Mixture Proportions for the First Trial Batch According to the
Mehta and Aitcins Procedure, kg/m3
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HSCApplications
High-rise projects
Construction of RC frames of buildings more than 30
stories and higher upper 1/3 may be conventional
RC reduction size column lower 2/3 of the building.
Bridges Reduce the risk of thermal cracking
Stilling basin of dams
Long term durability to abrasion resistance
Floating concrete container terminal
High durability in Sea water
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SCC
SCC can be proportioned, batched, placed,and cured similarly to normal concrete with
some precautions necessary to assure the
expected expansion.
Additional information can be found in ACI
223, Standard Practice for the Use ofSCC.
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SCC
SCC is an expansive cement concrete which,when properly restrained by reinforcement, willexpand an amount equal or slightly greater thanthe anticipated drying shrinkage.
Because of the restraint, compressive stresseswill be induced in the concrete duringexpansion. Subsequent drying shrinkage willreduce these stresses.
Ideally, a residual compression will remain in theconcrete, eliminating the risk of shrinkagecracking.
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SCC
As the type Kcement hydrates, large amounts ofettringite are formed.
The concrete bonds to the reinforcement, at thesame time start expanding.
Concretes expansion under the restraininginfluence of the steel will induce tension in thesteel while the concrete itself goes into
compression. SCC at the end of moist curing, it will shrink like a
normal portland cement concrete.
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SCCapplications
Applications
Expansive cements have been used since 1960s.
Water and Sewage-handling structures
Water Storage tanks Spillways
Cooling tower basins
Swimming pools Floors without joints
Airport pavement
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Self-Consolidating Concrete (S-CC)
The use of HSC mixtures, with dense steel
reinforcement, has successfully met the need of
the construction industry for stronger and more
ductile concrete structures.
However, the constructability of highly congestedreinforced concrete elements requires the fresh
concrete mixtures to be very fluid.
The advent of superplasticizers made it possibleto achieve slump values on the order of 200 to
250 mm, without the use of too much water.
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S-CC
In late 1970s and early 1980s pioneering work
by German, Italian, and Japanese researchers
led to the development of high workability
concrete mixtures that are commerciallyknown by various names such as self-
consolidating concrete, self-compacting
concrete, self-leveling concrete, orrheoplastic concrete.
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Composition and Properties of Typical S-CCMixtures
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Applications ofS-CC
In Europe and Japan, S-CC has been used for
underwater concreting and for the
construction of heavily reinforced structures.
Most ready-mixed concrete plants are
reluctant to produce S-CCdue to the high cost
and additional requirements for quality
control that are necessary when usingviscosity-controlling admixtures.
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Roller Compacted Concrete (RCC)
RCC is based on the concept that no slump
concrete mixture transported, placed, and
compacted with the same construction
equipment that is used for earth and rockfilldams.
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Roller Compacted Concrete (RCC)
The development of RCCcaused a major shift inthe construction practice of mass concrete damsand locks.
The traditional method of placing, compacting,and consolidating mass concrete is typically aslow process.
Improvements in earth-moving equipment has
made the construction of earth and rock-filleddams speedier and, therefore, more cost-effective.
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Materials and Properties ofRCC
For effective consolidation, the concrete mustbe dry to prevent sinking of the vibratoryroller equipment but wet enough to permit
adequate distribution of the binder mortarthroughout the material during the mixingand vibratory compaction operation.
The conventional concept of minimizingwater/cement ratio to maximize strengthdoes not hold.
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Materials and Properties ofRCC
The best compaction gives the best strength,
and the best compaction occurs at the wettest
mix that will support an operating vibratory
roller.
From the standpoint of workability, fly ash is
commonly included in RCCmixtures.
In Willow Creek Dam, the adiabatic
temperature rise was only 11C in 4 weeks.
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Advantages ofRCC
Cement consumption is lower because much leaner concrete
can be used.
Formwork costs are lower because of the layer placement
method.
Cooling is unnecessary because of the low temperature rise.
Cost of transporting concrete is lower than with cable crane
method because concrete can be hauled by end dump trucks;
it is spread by bulldozers and compacted by vibratory rollers.
Rates of equipment and labor utilization are high because of
the higher speed of concrete placement.
The construction period can be shortened considerably.
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Transportation
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Placement
C i
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Compaction
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Roller Compacted Concrete Pavements
(RCCP)
RCCP can be constructed with the sameequipments as asphalt pavements, laid by thesame pavers and compacted by rollers.
The strength grows fast enough to permit
opening for traffic in a short time. Since the drying shrinkage is small, the interval
between joints can be maximized.
Applications: Ordinary roads, Roads in factories,Temporary roads for construction works, Parkingareas, Service areas, Container yards, andMaterial handling yards.
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Comparison between the Two Methods for
Proportioning RCC Mixtures
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Composition, Strength, and Elastic Properties of Some RCCMixtures
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Construction practice ofRCC
The overall planning of a RCC dam isconceptually different from a gravity dam.
To minimize thermal stresses, traditional mass
concrete is built in separate, monolith blocks. This process is slow but allows great flexibility;
if a problem develops in one of the blocks, the
construction front moves to another block.RCCdams do not have such luxury.
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Construction practice ofRCC
The operation is continuous, building onehorizontal lift at a time.
There are no special requirements for
batching and mixing of RCC, which can beproduced using the same equipment as for
conventional mass concrete
Ready-mixed concrete trucks cannot be usedto transport RCC because the zero-slump
concrete is too dry and cannot be discharged.
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Transporting ofRCC
Conveyor systems can be an efficient method oftransporting RCC.
Creek Dam used conveyor belts to deliver
concrete from the mixer to the job site, and fromthe discharge point end-dump trucks were used.
Transporting and placing with end-dump trucksfollowed by dozers that remix and spread is a fast
and economical method. Alternatively, scrapers and bottom-dump trucks
can be used
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Lift thickness ofRCC
The success of a RCCdam is often contingenton the correct selection of lift thickness, which
depends on the mixture proportions and on
the equipment available.
Normally, the thickness of the lifts ranges from
0.15 to 0.90 m; in the United States a lift
thickness of 0.3 m is often used.
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Selection of the roller
Compaction of the lift is achieved by using avibrating steel-wheel roller.
The selection of the roller depends on the
desired compaction force, drum size,frequency, and operating speed.
Compaction of the lift should be performed assoon as possible, typically within 10 min afterspreading and no more than 40 min aftermixing.
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Surface finishing of each lift
Once adequate compaction is achieved, goodcuring conditions for the finished surface areessential; the surface should be kept in a
moistened condition until the next lift isplaced.
Investigations have shown that using specialhigh-consistency bedding mixtures for starting
the new concrete placement is helpful inreducing the cold joints.
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Surface finishing of each lift
Typically, bedding mixtures contain 360 to 460
kg/m3 of cement, 170 to 220 kg/m3 of fly ash,
and 4.75-mm maximum size aggregate
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Sequence ofRCCconstruction
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