concrete
TRANSCRIPT
CONCRETE The Man Made Stone
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
Colloseum, Greece
Pantheon, Rome
Petronas twin Tower Burj Khalifa
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
Constituents..
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
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
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.
Aggregates
• cheap fillers
• hard material
• provide for volume stability
• reduce volume changes
• provide abrasion resistance
Admixtures
• chemical• set retarders
• set accelerators
• water reducing
• air entraining
• mineral• fly ash
• silica fume
• slags
Fresh Concrete
Fresh concrete is that stage of concrete in which concrete can be moulded and it is in plastic state.
Properties of fresh concrete
•Setting•Workability•Bleeding•Segregation•Hydration•Air Entrainment
Setting Of Concrete
The hardening of concrete before its hydration is known as setting of concrete
Factors affecting setting
• Water Cement ratio
• Suitable Temperature
• Cement content
• Type of Cement
• Fineness of Cement
• Relative Humidity
• Admixtures
• Type and amount of Aggregate
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
Workability
• Workability is often referred to as the ease with which a concrete can be transported, placed and consolidated without excessive bleeding or segregation
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
Test for Workability
Slump Test can be used to find out the workability of concrete
Apparatus for Slump test
• Cylinders
• Small scoop
• Bullet-nosed rod (600 mm x 16 mm)
• Steel float
• Steel plate 8”
4”
12”
Slump Test
slump cone
rod
concrete
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.
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
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
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
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
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
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.
Factors affecting slump-paste content
Low paste contentHarsh mix
High paste contentRich mix
ball bearing effect-start
starting height
ball bearing effect-end
Factors affecting slump
• Aggregates• grading the larger the particle size, the higher the slump for
a given paste content
effect of aggregate size
1”
1”
1”
Consider a single aggregate the size of 1”x1”x1”
Break it up further
Compute the surface area
0.5 in
0.25 in
surface area = 0.25*0.25*6*8*8=24
Larger particles, less surface area, thicker coating, easy sliding of particles
Smaller particles, more surface area, thinner coating, interlocking of particles
Angularity and surface texture of aggregates
angular and rough aggregate
smooth aggregateriver gravel
Temperature
fresh concrete
aggregates paste
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.
Bleeding
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.
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
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
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.
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.
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.
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
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.