Concrete and its mix

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

Portland CementPortland Cement Dry powder of very fine particles forms a paste when mixed with water chemical reaction-Hydration paste coats all the aggregates together hardens and forms a solid mass

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.

AggregatesAggregates cheap fillers hard material provide for volume stability reduce volume changes provide abrasion resistance

AdmixturesAdmixtures chemical

set retarders set accelerators water reducing air entraining

mineral fly ash silica fume slags

Properties of fresh concreteProperties of fresh concrete Workability

ease of placement resistance to segregation homogeneous mass

Consistency ability to flow

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”

Slump TestSlump Testslump cone

rod

concrete

Slump testSlump test

Slump

Ruler

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

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

water cement ratiowater cement ratioif you add 10 gallons of water per cubic yard at job

site, then:

extra water10 gallons/cubic yard * (3.8 liters/gallon) * (2.2

lbs./kg) *( 1kg/liter) = 83.77 lbs. total water 282 + 83.77 = 365.77new w/c = 365.77 / 685 = 0.534 >> 0.43

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

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.

Factors affecting slump-paste Factors affecting slump-paste contentcontent

Low paste contentHarsh mix

High paste contentRich mix

ball bearing effect-startball bearing effect-startstarting height

ball bearing effect-endball bearing effect-end

slump

AdmixturesAdmixtures set retarding admixtures set accelerating admixtures water reducing admixtures superplasticizers air entraining admixtures

Factors affecting slumpFactors affecting slump Aggregates

grading the larger the particle size, the higher the slump for a given paste content

effect of aggregate size effect of aggregate size

1”

1”1”

Consider a single aggregate the size of 1”x1”x1”

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

Break it up furtherBreak it up further

Compute the surface areaCompute the surface area

0.5 in

0.25 in

surface area = 0.25*0.25*6*8*8=24

Larger particles, less surface area, Larger particles, less surface area, thicker coating, easy sliding of particlesthicker coating, easy sliding of particles

Smaller particles, more surface area, Smaller particles, more surface area, thinner coating, interlocking of particlesthinner coating, interlocking of particles

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

Angularity and surface texture Angularity and surface texture of aggregatesof aggregates

angular and rough aggregate

smooth aggregateriver gravel

TemperatureTemperature

fresh concrete

aggregates paste

BleedingBleeding

Water accumulation on surfaceWater accumulation on surface

Examine the concrete surface

Interaction between bleeding and evaporationInteraction between bleeding and evaporation

surface water

Evaporation

Bleed water

Bleed water = evaporation

Too much evaporation leads to surface crackingToo much evaporation leads to surface cracking

no surface waterEvaporation

Bleed water < Evaporation

drying

Side diagram of surface contractionSide diagram of surface contraction

Wants to shrink

Does not want to shrink

Free Shrinkage, Free Shrinkage, causes volume change, but no stressescauses volume change, but no stresses

before shrinkage After Shrinkage

Restrained Shrinkage- creates Restrained Shrinkage- creates stresses, which may cause crackingstresses, which may cause cracking

Restrained shrinkage crackingRestrained shrinkage crackingParallel cracking perpendicular

to the direction of shrinkage

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

Causes of Plastic Shrinkage CrackingCauses of Plastic Shrinkage Cracking

water evaporates faster than it can reach the top surface

drying while plastic cracking

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

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.

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

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.

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.