lecture 2 - progress in concrete

7/28/2019 Lecture 2 - Progress in Concrete

1/45

Progress in Concrete

Technology


2/45

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


3/45

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.


4/45

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.


5/45

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.


6/45

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.


7/45

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.


8/45

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.


9/45

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.


10/45

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


11/45

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


12/45

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.


13/45

Calculated Mixture Proportions for the First Trial Batch According to the

Mehta and Aitcins Procedure, kg/m3


14/45

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


15/45


16/45

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.


17/45

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.


18/45


19/45

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.


20/45

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


21/45

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.


22/45

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.


23/45

Composition and Properties of Typical S-CCMixtures


24/45

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.


25/45

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.


26/45

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.


27/45

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.


28/45

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.


29/45

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.


30/45

Transportation


31/45

Placement

C i


32/45

Compaction


33/45

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.


34/45

Comparison between the Two Methods for

Proportioning RCC Mixtures


35/45


36/45

Composition, Strength, and Elastic Properties of Some RCCMixtures


37/45

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.


38/45

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.


39/45

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


40/45

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.


41/45

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.


42/45

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.


43/45

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


44/45

Sequence ofRCCconstruction


45/45

lecture 2 - progress in concrete

Documents