Download - CONCRETE
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CONCRETE The Man Made Stone
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Introduction..
• Concrete is a composite material composed mainly of water, aggregate, and cement.
• The word concrete comes from the Latin word "concretus" meaning compact or condensed
• It is believed that romans were the one who introduced concrete and used extensively from 300 BC to 476 AD.
• After the Roman Empire collapsed, use of concrete became rare until the technology was re-pioneered in the mid-18th century
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Colloseum, Greece
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Pantheon, Rome
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Petronas twin Tower Burj Khalifa
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Types of Concrete
• Light weight concrete: used for load bearing external walls for building. They are also used for temporary structures because of low initial cost and can be reused as aggregate
• High density concrete : They are mainly used in the construction of radiation shields (medical or nuclear)
• Mass concrete : used in building dams, foundation etc
• Ready mix Concrete : Ready Mixed Concrete is manufactured under computer-controlled operations
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Constituents..
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Constituents..
•Mixture of aggregate•paste 30 to 40%• portland cement 7% to 15% by Vol.• water 14% to 21% by Vol.
•Aggregates 60% to 70%• coarse aggregates• Fine aggregates
•Admixtures
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Portland Cement
• Dry powder of very fine particles
• forms a paste when mixed with water
• chemical reaction-Hydration
• glue
• paste coats all the aggregates together
• hardens and forms a solid mass
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Water
• needed for two purposes:• chemical reaction with cement• workability
• only 1/3 of the water is needed for chemical reaction
• extra water remains in pores and holes• results in porosity• Good for preventing plastic shrinkage
cracking and workability• Bad for permeability, strength, durability.
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Aggregates
• cheap fillers
• hard material
• provide for volume stability
• reduce volume changes
• provide abrasion resistance
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Admixtures
• chemical• set retarders
• set accelerators
• water reducing
• air entraining
• mineral• fly ash
• silica fume
• slags
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Fresh Concrete
Fresh concrete is that stage of concrete in which concrete can be moulded and it is in plastic state.
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Properties of fresh concrete
•Setting•Workability•Bleeding•Segregation•Hydration•Air Entrainment
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Setting Of Concrete
The hardening of concrete before its hydration is known as setting of concrete
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Factors affecting setting
• Water Cement ratio
• Suitable Temperature
• Cement content
• Type of Cement
• Fineness of Cement
• Relative Humidity
• Admixtures
• Type and amount of Aggregate
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Water-cement Ratio
The water–cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix and has an important influence on the quality of concrete produced
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Workability
• Workability is often referred to as the ease with which a concrete can be transported, placed and consolidated without excessive bleeding or segregation
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Factors affecting concrete workability
• Water-Cement ratio
• Amount and type of Aggregate
• Amount and type of Cement
• Weather conditions
• Temperature
• Wind
• Chemical Admixtures
• Sand to Aggregate ratio
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Test for Workability
Slump Test can be used to find out the workability of concrete
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Apparatus for Slump test
• Cylinders
• Small scoop
• Bullet-nosed rod (600 mm x 16 mm)
• Steel float
• Steel plate 8”
4”
12”
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Slump Test
slump cone
rod
concrete
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Procedure
• Clean the cylinder mould and coat the inside lightly with form oil, then place on a clean, level and firm surface, ie the steel plate. Collect a sample.
• Fill 1/2 the volume of the mould with concrete then compact by rodding 25 times. Cylinders may also be compacted by vibrating using a vibrating table.
• Fill the cone to overflowing and rod 25 times into the top of the first layer, then top up the mould till overflowing.
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Procedure
• Level off the top with the steel float and clean any concrete from around the mould.
• Cap, clearly tag the cylinder and put it in a cool dry place to set for at least 24 hours.
• After the mould is removed the cylinder is sent to the laboratory where it is cured and crushed to test compressive strength
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Slump test results
• stiff 0-2” • massive sections, little reinforcement
• use vibration
• medium 2-5”• columns, beams, retaining walls
• Fluid 5-7”• heavily reinforced section, flowable concrete
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Factors affecting slump
• water cement ratio• w/c = weight of water / weight of cement
example:
weight of water mixed at the plant 292 lbs.
weight of cement 685 lbs./cu. yard
w/c = 292/685 = 0.43
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water cement ratio
if you add 10 gallons of water per cubic yard at job site, then:
extra water
10 gallons/cubic yard * (3.8 liters/gallon) * (2.2 lbs./kg) *( 1kg/liter) = 83.77 lbs.
total water 282 + 83.77 = 365.77
new w/c = 365.77 / 685 = 0.534 >> 0.43
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Factors affecting slump- paste content
• constant water cement ratio• increase paste content• increase slump
• NO GOOD
• constant cement content• increase water content• increase slump
• NO GOOD
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Factors Affecting Slump-Water Content
• Add water at the constant cement content, w/c increases, slump increases.
• Add water at a constant water cement ratio, have to increase cement as well, slump increases.
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Factors affecting slump-paste content
Low paste contentHarsh mix
High paste contentRich mix
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ball bearing effect-start
starting height
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ball bearing effect-end
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Factors affecting slump
• Aggregates• grading the larger the particle size, the higher the slump for
a given paste content
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effect of aggregate size
1”
1”
1”
Consider a single aggregate the size of 1”x1”x1”
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Break it up further
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Compute the surface area
0.5 in
0.25 in
surface area = 0.25*0.25*6*8*8=24
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Larger particles, less surface area, thicker coating, easy sliding of particles
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Smaller particles, more surface area, thinner coating, interlocking of particles
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Angularity and surface texture of aggregates
angular and rough aggregate
smooth aggregateriver gravel
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Temperature
fresh concrete
aggregates paste
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Bleeding
• Bleeding is a form of segregation where some of the water in the concrete tends to rise to the surface of the freshly placed material.
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Bleeding
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Prevention of Bleeding in concrete
• Bleeding can be reduced by proper proportioning and uniform and complete mixing.
• Air-entraining agent is very effective in reducing the bleeding.
• Bleeding can be reduced by the use of finer cement or cement with low alkali content. Rich mixes are less susceptible to bleeding than lean mixes.
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Curing
• The time needed for the chemical reaction of cement with water results in greater bonding and makes stronger.
• concrete after 14 days of curing has completed only 40% of its potential.
70 % at 28 days
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Curing tips
• ample water
• do not let it dry
• dry concrete = dead concrete, all reactions stop
• can not revitalize concrete after it dries
• keep temperature at a moderate level
• concrete with flyash requires longer curing
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Temperature effects on curing
• The higher the temperature the faster the curing
• best temperature is room temperature
• strongest concrete is made at temperature around 40 F.(not practical)
• If concrete freezes during the first 24 hrs., it may never be able to attain its original properties.
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Temperature effects on curing
• real high temperatures above 120 F can cause serious damage since cement may set too fast.
• accelerated curing procedures produce strong concrete, but durability might suffer.
• autoclave curing.
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Advantage Of Concrete
• Concrete is economical when ingredients are readily available.
• Concrete’s long life and relatively low maintenance requirements increase its economic benefits.
• It is not as likely to rot, corrode, or decay as other building materials.
• Concrete has the ability to be molded or cast into almost any desired shape.
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Advantages Of Concrete
• Building of the molds and casting can occur on the work-site which reduces cost.
• Concrete is a non-combustible material which makes it fire-safe and able to withstand high temperatures.
• It is resistant to wind, water, rodents, and insects. Hence, concrete is often used for storm shelters
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Disadvantage
• Concrete has a relatively low tensile strength. In order to over this, steel is reinforced
• low ductility
• low strength-to-weight ratio, and
• Concrete is susceptible to cracking.