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Concrete
Man made stone
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constituentsconstituents mixture of aggregate and paste 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 CementPortland 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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WaterWater 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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AggregatesAggregates cheap fillers hard material provide for volume stability reduce volume changes provide abrasion resistance
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AdmixturesAdmixtures chemical
set retarders set accelerators water reducing air entraining
mineral fly ash silica fume slags
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Properties of fresh concreteProperties of fresh concrete Workability
ease of placement resistance to segregation homogeneous mass
Consistency ability to flow
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Slump TestSlump Test Inverted cone fill it up with three layers
of equal volume rod each layer 25 times scrape off the surface
8”
4”
12”
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Slump TestSlump Testslump cone
rod
concrete
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Slump testSlump test
Slump
Ruler
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Slump test resultsSlump 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 slumpFactors affecting slump water cement ratio
w/c = weight of water / weight of cementexample:
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 ratiowater cement ratioif 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-Factors affecting slump- paste content 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-Factors Affecting Slump-Water ContentWater 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 Factors affecting slump-paste contentcontent
Low paste contentHarsh mix
High paste contentRich mix
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ball bearing effect-startball bearing effect-start
starting height
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ball bearing effect-endball bearing effect-end
slump
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AdmixturesAdmixtures set retarding admixtures set accelerating admixtures water reducing admixtures superplasticizers air entraining admixtures
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Factors affecting slumpFactors 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 effect of aggregate size
1”
1”
1”
Consider a single aggregate the size of 1”x1”x1”
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Compute the surface area as Compute the surface area as you break up the particlesyou break up the particles
volume = 1 cubic insurface area = 6 square inches volume = 1 cubic in
surface area = 1.5*8= 12 square inches
block surface area = 0.5*0.5*6=1.5block surface area = 1*1*6= 6
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Break it up furtherBreak it up further
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Compute the surface areaCompute 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, Larger particles, less surface area, thicker coating, easy sliding of particlesthicker coating, easy sliding of particles
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Smaller particles, more surface area, Smaller particles, more surface area, thinner coating, interlocking of particlesthinner coating, interlocking of particles
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Effect of aggregate sizeEffect of aggregate size
size # of particles volume surface area
1" 1 1 cubic inch 6 square inches
.5" 8 1 cubic inch 12 square inches
0.25 64 1 cubic inch 24 square inches
0.125 512 1 cubic inch 48 square inches
size # of particles volume surface area
1" 1 1 cubic inch 6 square inches
.5" 8 1 cubic inch 12 square inches
0.25 64 1 cubic inch 24 square inches
0.125 512 1 cubic inch 48 square inches
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Angularity and surface texture Angularity and surface texture of aggregatesof aggregates
angular and rough aggregate
smooth aggregateriver gravel
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TemperatureTemperature
fresh concrete
aggregates paste
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BleedingBleeding
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Water accumulation on surfaceWater accumulation on surface
Examine the concrete surface
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Interaction between bleeding and evaporationInteraction between bleeding and evaporation
surface water
Evaporation
Bleed water
Bleed water = evaporation
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Too much evaporation leads to surface crackingToo much evaporation leads to surface cracking
no surface water
Evaporation
Bleed water < Evaporation
drying
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Side diagram of surface contractionSide diagram of surface contraction
Wants to shrink
Does not want to shrink
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Free Shrinkage, Free Shrinkage, causes volume change, but no stressescauses volume change, but no stresses
before shrinkage After Shrinkage
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Restrained Shrinkage- creates Restrained Shrinkage- creates stresses, which may cause crackingstresses, which may cause cracking
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Restrained shrinkage crackingRestrained shrinkage cracking
Parallel cracking perpendicularto the direction of shrinkage
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Bleeding and its controlBleeding and its control
Creates problems: poor pumpability delays in finishing high w/c at the top poor bond between
two layers
causes lack of fines too much water content
Remedies more fines adjust grading entrained air reduce water content
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Causes of Plastic Shrinkage CrackingCauses of Plastic Shrinkage Cracking
water evaporates faster than it can reach the top surface
drying while plastic cracking
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Plastic Shrinkage Cracking-Plastic Shrinkage Cracking-RemediesRemedies
Control the wind velocity reduce the concrete’s temperature
use ice as mixing water increase the humidity at the surface
fogging cover w/polyethylene curing compound
Fiber reinforcement
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CuringCuring The time needed for the chemical reaction
of portland cement with water. Glue is being made. concrete after 14 days of curing has
completed only 40% of its potential. 70 % at 28 days.
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Curing tipsCuring 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 curingTemperature 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 curingTemperature 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.