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Samadar Salim
• BY BÜŞRA TÜRKERİ
CE-444 PROPERTIES OF FRESH AND HARDENED CONCRETE
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Definition: Concrete having a 28-day compressive strength greater than 17 MPa and an airdried unit weight not greater than 1850 kg/m³.
Composition: Similar to normal concrete except that it is made with lightweight aggregates or combination of lightweight and normal-weight aggregates. All lightweight concretes use both lightweight coarse and lightweight fine aggregates.
Sanded lightweight concretes used natural sand instead of lightweight fine aggregates.
Lightweight Concrete
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Lightweight aggregates that used in structural lightweight concrete are typically expanded shale, clay or slate materials that have been fired in a rotary kiln to develop a porous structure.Other products such as air-cooled blast furnace slag are also used. There are other classes of non-structural LWC with lower density made with other aggregate materials and higher air voids in the cement paste matrix, such as in cellular concrete.
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1. Lightweight Aggregate CONCRETE Porous lightweight aggregate of low specific gravity is used in this concrete.
such as pumice, scoria and most of volcanic origin and the artificial aggregate such as expanded blast-furnace slag, vermiculite and clinker aggregate
Types of Lightweight Concrete
The lightweight aggregate concrete can be divided into two types according to its application :One is partially compacted lightweight aggregate concrete and the other is the structural lightweight aggregate concrete.
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compacted lightweight aggregate concrete is mainly used for two purposes that is for precast concrete blocks or panels and cast in-situ roofs and walls. The main requirement for this type of concrete is that it should have adequate strength and a low density to obtain the best thermalinsulation and a low drying shrinkage to avoid cracking
Structural lightweight aggregate concrete is fully compacted similar to that of the normal reinforced concrete of dense aggregate. It can be used with steel reinforcement as to have a good bond between the steel and the concrete. The concrete should provide adequate protection against the corrosion of the steel
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Foamed Slag – was the first LWA suitable for reinforced concrete.
(that was produced in large quantity in (UK)
Sintered Pulverised – fuel ash aggregate – is being used in the UK for a variety of structural purposes and is being marketed under the trade name Lytag.
Expanded Clays and Shales – capable of achieving sufficiently high strength for prestressed concrete
Pumice – is used for reinforced concrete roof slab, mainly for industrial roofs
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2) AERATED CONCRETE
Aerated concrete is a lightweight, cellular material consisting of cement or lime and sand or other silicious material it does not contain coarse aggregate.
Two methods to prepare the aerated concrete. The first method is to inject the gas into the mixing during its plastic condition by means of a chemical reaction. The second method, air is introduced either by mixing-in stable foam or by whipping-in air, using an air-entraining agent. Concrete of this type has the lowest density, thermal conductivity and strength.Aerated concrete used as a structural material usually of high-pressure steam-cured. It is thus factory-made and available to the user in precast units , for floors, walls and roofs. Blocks for laying in mortar or glue are manufactured without any reinforcement. Larger units are reinforced with steel bars to resist damage through transport, handling and superimposed loads.
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AERATED CONCRETE
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3) NO-FINES CONCRETE
The term no-fines concrete generally means concrete composed of cement and a (9-19mm) coarse aggregate only (at least 95 percent
should pass the 20mm BS sieve), and the product so formed has many uniformly distributed voids throughout its mass.
No-fines concrete usually used for both load bearing and non-load bearing for external walls and partitions.
The structure of NFC makes it ideal for use as a drainage layer under reservoir and basement floors. It can also serve as an insulating layer
and as a damp-proofing material , is NOT suitable for drainage purposes where the water is soft or aggressive to concrete.
Although the strength of no-fines concrete is considerably lower than that of normal-weight concrete, and increases as the cement content is
increased
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Type Of
LightweightConcrete
Type Of Aggregate Grading of Aggregate (Range
of Particle Size)
Partially compacted
lightweight aggregate concrete
Clinker
Foamed slag
Expanded clay, shale,slate,
vermiculite and perlite
Sintered pulverized-fuelash
and pumice
May be of smaller nominal single
sizes of combined coarse and
fine (5mm and fines) material to
produce a continues but harsh
grading to make a porous
concrete
Structural
lightweight
aggregate concrete
Foamed slag
Expanded clay, shale or slate
and sintered pulverized fuel
ash
Continues grading from either
20mm or 14mm down to dust,
with an increased fines content
(5mm and fines) to produce a
workable and dense concrete
The differences between the types of light weight concrete are very much related to its aggregate grading used in the mixes
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No-fines concrete
Natural Aggregate
Blast-furnace slag
Clinker
Nominal single-sized material
between 20mm and 10mm BS sieve
Aerated concrete
Natural fine aggregate
Fine lightweight aggregate
Raw pulverized-fuel ash
Ground slag and burnt shales
The aggregate are generally
ground down to finer powder,
passing a 75 μm BS sieves, but
sometimes fine aggregate (5mm
and fines) is also incorporated
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LWC can be classifiy :
1.Low density concrete ( 0.69 to 6.89N/mm²) Compressive strength
2.Moderate strength concrete (6.89 to 17.24N/mm²) Compressive
strength
3.Structural concrete (17.24N/mm²) Compressive strength
Classification
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1. LOW DENSITY CONCRETE
These are employing chiefly for insulation purposes. With low unit weight, seldom exceeding 800 kg/m³, heat insulation value are high. Compressive strength are low, regarding from about 0.69 to 6.89 N/mm².
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2. MODERATE STRENGTH CONCRETE
The use of these concrete requires a fair degree of compressive strength, and thus they fall about midway between the structural and low density concrete. These are sometimes designed as ‘fill’ concrete. Compressive strength are approximately 6.89 to 17.24 N/mm² and insulation values are intermediate.
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3. STRUCTURAL CONCRETE
Concrete with full structural efficiency contain aggregates which fall on the other end of the scale and which are generally made with expanded shale, clay, slates, slag, and fly-ash. Minimum compressive strength is 17.24 N/mm².
Most structural LWC are capable of producing concrete with compressive strength in excess of 34.47 N/mm². Since the unit weight of structural LWC are considerably greater than those of low density concrete, insulation efficiency is lower.
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Structural lightweight concrete offers design flexibility and substantial cost savings by providing: less dead load, improved seismic structural response, longer spans, better fire ratings, thinner sections, decreased story height, smaller size structural members, less reinforcing steel, and lower foundation costs
Reduction of dead load indicates faster building rates and lower haulage and handling costs.
Frame structures, considerable savings in cost can be brought by using LWC for the construction floors, partition and external cladding.
Advantages of Using LWC
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Most building materials such as clay bricks the haulage load is limited not by volume but by weight. With suitable design containers much larger volumes of LWC can haul economically.
Important characteristics of LWC is its relatively low thermal conductivity, a property which improves with decreasing density in recent years, The point is illustrated by fact that a 125mm thick solid wall of aerated concrete will give thermal insulation about 4 times greater than that of a 230mm clay brick wall.
Lightweight concrete precast elements offer reduced transportation and placement costs
The bond between the aggregate and the matrix is stronger in the case of LWAC than in normal concrete.
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Very Sensitive with water content in the mixture .
Difficult to place and finish because of porosity and angularity of the aggregate .In some mixes the cement mortar may separate the aggregate and float towards the surface.
Mixing time is longer than conventional concrete to assure proper mixing .
Lightweight Concrete are porous and shows poor resistance to heavy abrasion, replacement of lightweight fines with natural sand improves the abrasion resistance of concrete.
Disadvantages of LWC
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Definition: Concrete in which heavy aggregate such as
magnetic and iron are used to increase the density of the concrete and protection against radiation.
HEAVYWEIGHT CONCRETE
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For producing heavyweight concrete uses heavy natural aggregates such as barites or magnetite or
manufactured aggregates such as iron or lead shot.
The density achieved will depend on the type of aggregate used. Typically using barites the density will be in the region of 3,500kg/m3, which is 45% greater than that of normal concrete, while with magnetite the density will be 3,900kg/m3, or 60% greater than normal concrete. Very heavy concretes can be achieved with iron or lead shot as aggregate, is 5,900kg/m3 and 8,900kg/m3 respectively.
Barite magnetite lead shot
COMPOSITION
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Except for the heavyweight aggregates and some hydrous ores as well as boron minerals, the same minerals and proportioning methods are used for producing heavyweight concrete mixtures as are used for conventional normalweight concrete. For details pertaining to concrete-making metarials for biological shielding, the standard specifications should be consulted: ASTM C 637 (specification for aggregates for radiation shielding concrete) and ASTM C 638(nomenclature of constituents of aggregates for radiation shielding concrete).
Because of the high denstiy of aggregate particles, segregation of fresh concrete is one of the principal concerns in mix proportioning. From the standpoints of high unit weight and a lower tendency for segregation, it is desirable that both fine and coarse aggregate be produced from high-denstiy rocks and minerals
MATERIAL AND MIX PROPORTION
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Due to the rough shape and texture of crushed aggregate particles, heavyweight concrete mixtures tend to be harsh. To overcome this problem it is customary to use a finer sand, a greater proportion of sand in aggregate than conventional concrete, and cement contents higher than 360 kg/m³.
It should be noted that to get around the problem of segregation, sometimes other than conventional methods , such as preplaced aggregate concreting, may be employed. İn this method, after filling the forms with compacted aggregate coarser than 6 mm, the voids in the aggregate are filled by pumping in a grout mix containing cement, fine sand, pozzolans, and other pumpability aids.
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Heavyweight concrete can be pumped or placed by chutes over short distances only, because of the tendency of coarse aggregate to segregate. Concretes containing borate ores, such as colemanite and borocalcite, may suffer from slow setting and hardening problems because these minerals are somewhat solube, and borate solutions are strong retarders of cement hydration. Unit weights of concrete containing barite, magnetite,or ilmenite aggregate are in the range of 3450 to 3760 kg/m³ ; when hydrous and boron ores (which are not of high denstiy) are used as partial replacement for heavyweight aggregate, the unit weight of concrete may come down to about 3200 to 3450 kg/m³.
PROPERTIES OF HEAVYWEIGHT CONCRETE
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Massive shielding walls need not be designed for more than 14 MPa compressive strength; for structural concrete, strengths of the order of 20 to 35 MPa are sufficent and not difficult to achieve with the high cement contents normally used. Strenght is, however, of principal concern in the design of heavyweight concrete mixtures suitable for use in prestressed concrete reactor vessels (PCRV).
These are pressure vessels that operate at higher stress levels and tempratures than conventional structures, and concrete is subject to appreciable thermal and moisture gradients. In such cases, inelastic
deformations such as creep and thermal shrinkage should be minimized because they can cause microcracking and loss of prestres. Obviously,
the elastic modulus of aggregate and compatibility of coefficients of thermal expansion between aggregate and cement paste should be
considered to minimize microcracking.
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The reactor vessels are usually designed to operate with concrete temperatures up to 71°C, but higher accidental tempratures and some thermal cycling is expected during the service life. Considerable strength loss can occur when concrete is subjected to wide and frequent fluctuations in temperature; hence PCRV concrete is designed not only for high density but also for high strength. In a study at the Corps of Engineers, Waterways Experiment Station, using 430 to 575 kg/m³ Type I portland cement, 12 mm or 38 mm maximum magnetite or ilmenite aggregate, and an 0.30 to 0.35 water-cement ratio, heavyweight concretes (3680 kg/m³ unit weight) were produced which gave 52 to 65 MPa compressive strength at 7 days, and 62 to 76 MPa at 28 days.
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The ideal property of normal and high density concrete are high modulus of elasticity , low thermal expansion , and creep deformation
Because of high density of concrete there will be tendency for segregation. To avoid this pre placed aggregate method of concreting is adopted.
The high density. Concrete is used in construction of radiation shields. They are effective and economic construction material for permanent shielding purpose.
Most of the aggregate specific gravity is more than 3.5
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They are mainly used in the construction of radiation shields (medical or nuclear power plants). Offshore, heavyweight concrete is used for ballasting for pipelines and similar structures
It is also used for bridge counter-weight and for weighting down underwater pipelines
USES OF HEAVYWEIGHT CONCRET
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