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.