material complete
Post on 06-Apr-2018
227 Views
Preview:
TRANSCRIPT
-
8/3/2019 MATERIAL Complete
1/16
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
6
-
8/3/2019 MATERIAL Complete
2/16
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?
2
-
8/3/2019 MATERIAL Complete
3/16
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.
3
-
8/3/2019 MATERIAL Complete
4/16
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
4
-
8/3/2019 MATERIAL Complete
5/16
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.
5
-
8/3/2019 MATERIAL Complete
6/16
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.
6
-
8/3/2019 MATERIAL Complete
7/16
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.
7
-
8/3/2019 MATERIAL Complete
8/16
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
8
-
8/3/2019 MATERIAL Complete
9/16
Ready-mix trucks
and pumps
Bucket Lorry-mounted boom pump
9
-
8/3/2019 MATERIAL Complete
10/16
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.
10
-
8/3/2019 MATERIAL Complete
11/16
Concrete placing by bucket
Concrete placing by ready-mix lorry
11
-
8/3/2019 MATERIAL Complete
12/16
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.
12
-
8/3/2019 MATERIAL Complete
13/16
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.
13
-
8/3/2019 MATERIAL Complete
14/16
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.
14
-
8/3/2019 MATERIAL Complete
15/16
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
15
-
8/3/2019 MATERIAL Complete
16/16
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
16
top related