MATERIAL Complete

8/3/2019 MATERIAL Complete

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Table of Contents

Introduction 11.0 Batching 22.0 Mixing 33.0 Transporting 7

4.0 Placing1

0

5.0 Compacting1

2

6.0 Curing1

3

7.0 References1

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INTRODUCTION

High-rise building is a trend around the globe to the architects and engineers. It is also

becoming a race for all of them. But how are they are made, especially the important

part of any building in the world concrete.

Concrete is a structural material made from natural ingredients. It is extremely versatile

and simple to use but too often, in basic rules are ignored and an inferior product results.

A column is a vertical support structure while beam is a horizontal support structure of

the building.

The complete process of concrete production is followed from the basic materials

employed until curing of the concrete.

Concreting process has 6 processes, which are:

1. Batching

2. Mixing

3. Transporting

4. Placing

5. Compacting

6. Curing

Those processes are needed for any concrete. As for high-rise building, such

as Kuala Lumpur City Centre (KLCC), Kuala Lumpur or Burj Dubai, Dubai,

that we used for as references, is going for something different. In this report

we include the explanation for those. So what are the process that these

types of building going through?

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1.0 BATCHING CONCRETE

Batching is the process of weighing or volumetrically measuring and introducing

into a mixer the ingredients for a batch of concrete. To produce a uniform quality

concrete mix, measure the ingredients accurately for each batch. Most concrete

specifications require that the batching be performed by weight, rather than by volume,

because of inaccuracies in measuring aggregate, especially damp aggregate. Water

and liquid air-entraining admixtures can be measured accurately by either weight or

volume. Batching by using weight provides greater accuracy and avoids problems

created by bulking of damp sand. Volumetric batching is used for concrete mixed in a

continuous mixer, and the mobile concrete mixer (crate mobile) where weighing

facilities are not at hand. Specifications generally require that materials be measured

in individual batches within the following percentages of accuracy: cement 1%,

aggregate 2%, water 1%, and air-entraining admixtures 3%. Equipment within the

plant should be capable of measuring quantities within these tolerances for the smallest

to the largest batch of concrete produced. The accuracy of the batching equipment must

be checked and adjusted when necessary.

Batching that used for high-rise building is grade 80.

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2.0 MIXING

Concrete should be mixed until it is uniform in appearance and all the ingredients are

evenly distributed. Mixers should not be loaded above their rated capacities and should

be operated at approximately the speeds for which they were designed. If the blades of

the mixer become worn or coated with hardened concrete, the mixing action will be less

efficient. Worn blades should be replaced and the hardened concrete removed

periodically, preferably after each production of concrete.

When a transit mixer (TM) is used for mixing concrete, 70 to 100 revolutions of the drum

at the rate of rotation designated by the manufacturer as mixing speedare usually

required to produce the specified uniformity. No more than 100 revolutions at mixing

speed should be used. All revolutions after 100 should be at a rate of rotation designated

by the manufacturer as agitating speed. Agitating speed is usually about 2 to 6

revolutions per minute, and mixing speed is generally about 6 to 18 revolutions per

minute. Mixing for long periods of time at high speeds, about 1 or more hours can result

in concrete strength loss, temperature rise, excessive loss of entrained air, and

accelerated slump loss.

Concrete mixed in a transit mixer should be delivered and discharged within 1

1/2 hours or before the drum has revolved 300 times after the introduction of water to

cement and aggregates or the cement to the aggregates. Mixers and agitators should

always be operated within the limits of the volume and speed of rotation designated by

the equipment manufacturer.

Separate paste mixing has shown that the mixing of cement and water into a paste

before combining these materials with aggregates can increase the compressive

strength of the resulting concrete. The paste is generally mixed in a high-speed, shear-

type mixer at a w/cm (water to cement ratio) of 0.30 to 0.45 by mass. The cement pastepremix may include admixtures, e.g. accelerators or retarders, plasticizers, pigments, or

fumed silica. The latter is added to fill the gaps between the cement particles. This

reduces the particle distance and leads to a higher final compressive strength and a

higher water impermeability. The premixed paste is then blended with aggregates and

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any remaining batch water, and final mixing is completed in conventional concrete

mixing equipment.

High-Energy Mixed Concrete (HEM concrete) is produced by means of high-speed

mixing of cement, water and sand with net specific energy consumption at least 5

kilojoules per kilogram of the mix. It is then added to a plasticizer admixture and mixed

after that with aggregates in conventional concrete mixer. This paste can be used itself

or foamed (expanded) for lightweight concrete. Sand effectively dissipates energy in this

mixing process. HEM concrete fast hardens in ordinary and low temperature conditions,

and possesses increased volume of gel, drastically reducing capillarity in solid and

porous materials. It is recommended for pre-cast concrete in order to reduce quantity of

cement, as well as concrete roof and siding tiles, paving stones and lightweight concrete

block production.

2.1Overmixing Concrete

Overmixing concrete damages the quality of the concrete, tends to grind the aggregate

into smaller pieces, increases the temperature of the mix, lowers the slump, decreases

air entrainment, and decreases the strength of the concrete. Also, overmixing puts

needless wear on the drum and blades of the transit mixer. To select the best mixing

speed for a load of concrete, estimate the travel time to the project (in minutes) and

divide this into the minimum desired number of revolutions at mixing speed-70. The

results will be the best drum speed; for instance, if the haul is 10 minutes, 70 divided by

10 equals 7. With this drum speed, the load will arrive on the jobsite with exactly 70

turns at mixing speed.

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Reversing drum

Batch mixers are of four main types:

1. Tilting drum

2. Non-tilting drum

3. Reversing drum

4. Forced action

For high-rise building, it is suitable to use tilting drum. Its for medium strength concretes

in the capacity range 100 to 200 litres, and for producing mass concrete with large

(150mm) aggregate. Materials are poured into the drum and allow it to fall, encouraging

mixing.

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3.0 TRANSPORTING

All concrete that already has been mixed need to transported from where it mixed to its

final position. It can be simple job of discharging it down a chute from a ready-mix truck

into a trench on the other hand it may have to be conveyed long distances or pumped

to great heights. The precaution to be taken while transporting concrete is that the

homogeneity obtained at the time of mixing should be maintained while being

transported to the final place of deposition.

4.1 Selection of Transportation Method

The best solution for a particular project is usually reached after consulting company

personnel who can provide information and advice on the following:

a) Access available to and on the site;

b) Plant availability;

c) Quantity of concrete;

d) Quality of concrete;

e) Material;

f) Quality and number

4.2 Methods and plant

There are many methods and plant that can be used. There are:

a) Manual handling

The escalating cost of manpower means a hard look must be taken at all

forms of concrete transport involving labour. Nevertheless, there are jobs

where wheelbarrows, prams or motorised prams are most suitable and

have the advantage of low plant hire cost.

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b) Lorry-mounted boom pumps

Its used for concrete pours for everything from slabs and medium high-

rise buildings, to large volume commercial and industrial projects. They

range from single-axle truck mounted pumps used for their high

maneuverability, suitability for confined areas, and cost/performance

value, to huge, six-axle rigs used for their powerful pumps and long reach

on high-rise and other large-scale projects.

c) Lorries

It is used to transport concrete from factory to the site.

d) Ready-mix trucks

It is a very convenient way of transporting concrete and the concrete

should be discharged if possible, directly into its final position.

e) Mobile cranes with buckets

This mobile can be used to transporting a bucket of concrete into the

higher area, when ready-mix concrete is at bottom of the site.

f) Mobile and static pumps

The 150mm static pump with metal quick-release pipes is suitable for

large pours spread over considerable distances.

Those above requirements are important because each selection will effect on the care

of the concrete. The cost that have been given to produce a workable concrete of the

correct strength and durability which leaks from lorries can seriously reduce the qualities

of good concrete.

As for high-rise building like Kuala Lumpur City Center (KLCC) or Burj Dubai, the

methods they used were:

1. Ready-mix trucks

2. Mobile cranes with buckets

3. Mobile and static pumps

4. Lorry-mounted boom pumps

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Ready-mix trucks

and pumps

Bucket Lorry-mounted boom pump

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4.0 PLACING

The main objective when placing concrete is to deposit the concrete as close as possible

to its final position, quickly and efficiently to minimise segregation.

No water should be added to the concrete once it has left the mixer, otherwise the

properties might be adversely affected. The concrete should not be allowed to fall in

heaps and then moved along the form. It should be spread evenly, to ensure placement

as near as possible to its final position.

The most suitable plant should be provided, for example a crane bucket with a controlled

chute if filling walls with baffle boards to direct the concrete to prevent spillage over the

forms.

The placing of concrete successfully in deep lifts requires the concrete mix to be

designed to minimise the risk of segregation. The concrete should also be discharged

down some form of trunking as this reduces the likelihood of damage to the forms and

misplacement of reinforcement. Trunking will also prevent the loss of grout which, if

allowed to remain on the forms and steel above the concrete level, will result in gap

concrete at the bottom of the pour. The hose of a concrete pump acts in the same

manner as trunking, providing it is place at the bottom of the pour at commencement and

slowly raised to suit the filling process.

Segregation can occur in transporting concrete where the vibration of the lorry, dumper,

etc. will cause the mortar to flow away from the coarse aggregate. With low workability

concrete, the larger aggregate can segregate from the bulk of the concrete.

By considering above factors, the high-rise buildings would probably place the concrete

by the pump and bucket. This is because since it is high-rise building, concrete cannotbe easily to be transported to the required place. Therefore, concrete must be place

directly and quickly but carefully to prevent segregation.

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Concrete placing by bucket

Concrete placing by ready-mix lorry

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5.0 COMPACTING

Compaction is also very important wherever concrete exposed to view. Compaction

also, should be done as soon as possible once water has been added to concrete.

Compacting by machine can only be thoroughly compacted by vibrator, rollers, press or

by centrifugal casting.

For high-rise building, usually they used vibration. This is because it has a chemical

action with the cement which unites the aggregates to form a hardened material.

Second, it allows easy placement of the concrete. External vibrators are use also but

they are restricted to heavy robust forms that are use for repeated manufacture for

example large deep frame and slab having webs which are inaccessibly with a poker

vibrator.

Vibrating tables are used in pre-cast manufacture for repeat items such as kerbs, and

bollards. Moulds are filled with concrete and placed on the vibrating table which vibrates

and effectively compact the concrete.

All vibrators must be treated with care and be properly maintained if breakdowns are to

be avoided site management must have and use the manufacture instruction booklet to

follow the recommendation for both operation and maintenance.

Care must be taken to avoid disturbance to any fixings to the formwork. They should be

firmly fixed with the screws in preference to nails.

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6.0 CURING

Curing is the process of controlling the rate and extent of moisture loss from concrete

during cement hydration. It may be either after it has been placed in position (or during

the manufacture of concrete products), thereby providing time for the hydration of the

cement to occur. Since the hydration of cement does take time. Curing must be

undertaken for a reasonable period of time if the concrete is to achieve its potential

strength and durability.

Curing may also encompass the control of temperature since this affects the rate at

which cement hydrates. Cement requires a moist, controlled environment to gain

strength and harden fully, gaining in strength in the days and weeks following. In around

3 weeks, over 90% of the final strength is typically.

Hydration and hardening of concrete during the first three days is critical. Abnormally

fast drying and shrinkage due to factors such as evaporation from wind during

placement may lead to increased tensile stresses at a time when it has not yet gained

significant strength, resulting in greater shrinkage cracking. Minimizing stress prior to

curing minimizes cracking. High early-strength concrete is designed to hydrate faster,

often by increased use of cement which increases shrinkage and cracking.

During this period concrete needs to be in conditions with a controlled temperature and

humid atmosphere, in practice this is achieved by spraying or ponding the concrete

surface with water. The pictures to the right show two of many ways to achieve this,

ponding submerging setting concrete in water, and wrapping in plastic to contain the

water in the mix.

Properly curing concrete leads to increased strength and lower permeability, and avoids

cracking where the surface dries out prematurely. Care must also be taken to avoidfreezing, or overheating due to the exothermic setting of cement. Improper curing can

cause scaling, reduced strength, poor abrasion resistance and cracking.

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6.1 Curing Method

Impermeable-membrane Curing

Formwork leaving formwork in place is often an efficient and cost-effective method of

curing concrete, particularly during its early stages. In very hot dry weather, it may be

desirable to moisten timber formwork, to prevent it drying out during the curing period,

thereby increasing the length of time for which it remains effective. It is desirable that

any exposed surfaces of the concrete (e.g. the tops of beams) be covered with plastic

sheeting or kept moist by other means. It should be noted that, when vertical formwork is

eased from a surface (e.g. from a wall surface) its effectiveness as a curing system is

significantly reduced.

Plastic sheeting

Plastic sheets, or other similar material, form an effective barrier against water loss,

provided they are kept securely in place and are protected from damage. Their

effectiveness is very much reduced if they are not kept securely in place. The movement

of forced draughts under the sheeting must be prevented. They should be placed over

the exposed surfaces of the concrete as soon as it is possible to do so without marring

the finish. On flat surfaces, such as pavements, they should extend beyond the edges of

the slab for some distance, eg or at least twice the thickness of the slab, or be turned

down over the edge of the slab and sealed.

For horizontal work, sheeting should be placed on the surface of the concrete and, as far

as practical, all wrinkles smoothed out to minimise the mottling effects (hydration

staining), due to uneven curing, which might otherwise occur. Flooding the surface of the

slab under the sheet can be a useful way to prevent mottling. Strips of wood, or

windrows of sand or earth, should be placed across all edges and joints in the sheetingto prevent wind from lifting it, and also to seal in moisture and minimise drying. For

decorative finishes or where colour uniformity of the surface is required sheeting may

need to be supported clear of the surface if hydration staining is of concern. This can be

achieved with wooden battens or even scaffolding components, provided that a

complete seal can be achieved and maintained.

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For vertical work, the member should be wrapped with sheeting and taped to limit

moisture loss. As with flatwork, where colour of the finished surface is a consideration,

the plastic sheeting should be kept clear of the surface to avoid hydration staining. Care

must also be taken to prevent the sheeting being torn or otherwise damaged during use.

A minimum thickness is required to ensure adequate strength in the sheet. Plastic

sheeting may be clear or coloured.

Care must be taken that the colour is appropriate for the ambient conditions. For

example, white or lightly coloured sheets reflect the rays of the sun and, hence, help to

keep concrete relatively cool during hot weather. Black plastic, on the other hand,

absorbs heat to a marked extent and may cause unacceptably high concrete

temperatures. Its use should be avoided in hot weather, although in cold weather its use

may be beneficial in accelerating the rate at which the concrete gains strength.

Curing

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7.0 REFERENCES

Baker, E. M. (1985). Making and Placing Concrete. Construction Press.

Davis, E. Harmer (1956). Composition and Properties of Concrete. McGraw-Hill.

Wikipedia (11.30 AM, 8 February 2009). Concrete. Retrieved from

http://en.wikipedia.org/wiki/Concrete

Wikipedia (11.36 AM, 8 February 2009). Skycraper. Retrieved from

http://en.wikipedia.org/wiki/Skyscraper

Superseal (11.42 AM, 8 February 2009). Concrete pumping. Retrieved from

http://www.concretefoundation.net/page2.html

Concrete Network (12.00 PM, 8 February 2009). Pumps. Retrieved from

http://www.concretenetwork.com/concrete/concrete_pumping

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