DURABILITY OF CONCRETE

Presented byMr. T. Vairamuni.,B.E.,

Lecturer/CivilA.M.K.Tachnological Polytechnic College,

Chennai – 21.

WHAT IS DURABILITY OF CONCRETE?

The ability of concrete to resist weathering action, chemical

attack, and abrasion while maintaining its desired engineering

properties.

DAMAGE OF CONCRETE DUE TO LACK OF DURABILITY

• Fine to wide cracks developed in concrete.

• Scaling (localized small patches) of concrete can take place.

• Spalling of concrete can happen. (It is a result of water entering

brick, concrete or natural stone and forcing the surface to peel,

pop out or flake off)

• Disintegration of concrete takes place.

• Deposits of salts can take place, which is called efflorescence of

concrete.

• Complete structure failure can occur.

FACTORS AFFECTING DURABILITY

• Type and quality of constituent materials.

• Cement content and water-cement ratio.

• Workmanship to obtain full compaction and efficient curing, and

• Shape and size of member.

• Abrasion

• Biological Factors.

• Temperature Effect

• Environmental Related Physical Problems

• Freezing And Thawing

• Chemical Attacks

ABRATION

• Concrete is resistant to the abrasive affects of

ordinary weather

• Abrasion resistance is directly related to the

strength of the concrete

ABRATION

• Examples of severe abrasion and erosion are

particles in rapidly moving water, floating ice,

or areas where steel studs are allowed on tires

• For areas with severe abrasion, studies show

that concrete of grade M80 and above work

well.

BIOLOGICAL FACTORS

Mosses and lichens

these plants of a higher order, cause significant

damage to concrete. These plants produce weak acids in

the fine hair roots.

The acids that are produced will attack the cement

paste and cause the concrete to disintegrate and scale

BIOLOGICAL FACTORS

FIRE

FREEZING AND THAWING

• The most potentially destructive weathering

factor is freezing and thawing while the

concrete is wet

• Deterioration is caused by the freezing of water

and subsequent expansion in the paste, the

aggregate particles, or both.

FREEZING AND THAWING

• Air-entrained concrete with a low water-

cement ratio and an air content of 5 to 8% will

withstand a great number of cycles of freezing

and thawing without distress.

FREEZING AND THAWING

CHEMICAL ATTACKS

• Carbonation

• Chloride Attack

• Acid Attack

• Sulphate Attack

CARBONATION OF CONCRETE

It is a process by which CO2 from the air

penetrates into concrete and reacts with calcium

hydroxide to form calcium carbonates in

presence of water.

CH + CO2-------------------------- CACO3 + WATER

CARBONATION OF CONCRETE

CHLORIDE ATTACK

•  Chloride attack is particularly important

because it primarily causes corrosion of

reinforcement.

• Statistics have indicated that over 40 per cent

of failure of structures is due to corrosion of

reinforcement.

CHLORIDE ATTACK

Prevention measures:

• Use supplementary cementitious materials to reduce

permeability

• Increasing the concrete cover over the steel

• use of corrosion inhibiting admixtures

•  epoxy-coated reinforcing steel, surface treatments,

concrete overlays, and cathodic protection

ACID ATTACK

Concrete is susceptible to acid attack because of

its alkaline nature. The components of the cement

paste break down during contact with acids.

SULPHATE ATTACK

•  Sulphates can attack concrete by reacting

with hydrated compounds in the hardened

cement paste

• Result in disintegration of the concret

SULPHATE ATTACK

External Sources:

• Soil - gypsum - harmless (0.01-0.05)

• Groundwater-high-manganese and alkali

sulphates

• Agricultural soil and water-Ammonium sulphate

• Furnaces - high sulphur fuel

• Furnaces-Chemical industry-sulphuric acid.

SULPHATE ATTACK

Internal source:

• Portland cement might be over-sulphated.

• presence of natural gypsum in the aggregate.

• Admixtures also can contain small amounts of

sulphates.

SULPHATE ATTACK

Control of sulphate attack:The quality of concrete, specifically a low permeability, is the

best protection against sulphate attack.

• Adequate concrete thickness• High cement content• Low w/c ratio• Proper compaction and curing

SULPHATE ATTACK

• The addition of a pozzolanic admixture such

as flyash

• Use of chloride ions:the solubility of sulfate ettringite in sodium and calcium

chloride solutions is about 3 times more, than in water

• Use of low C3A content cement

ALKALI-SILICA REACTION“The Cancer of Concrete”

Alkali-Silica Reaction

The alkali–silica reaction (ASR) is a

reaction which occurs over time in concrete

between the highly alkaline cement paste and

reactive non-crystalline (amorphous) silica,

which is found in many common aggregates.

Alkali Silica Reaction (ASR)Alkalis

+Reactive

Silica+

Moisture

ASR Gel

which expands

Concrete expansion

andcracking

What is ASR?

• Concrete quality• Loss of strength, stiffness, impermeability• Premature failure of concrete structures

• Economic/Environmental impacts• ASR decreases concrete service life • Reconstruction has both environmental and economic

impacts. ex. cement production produces 7% of the world’s CO2 emissions (a greenhouse gas)

Why is it important to study ASR?

When cracks reach the surface of a structure, “map cracking” results.

How to prevent ASR damage

• Avoid high alkali content:– use low alkali Portland cement: Na20eq < 0.69– replace cement with low alkali mineral admixtures

• Avoid reactive aggregate (amorphous silica)• Control access to water: use low water to cement ratio,

monitor curing conditions, use admixtures to minimize water contact.

• Use lithium additives prior to placement of concrete or as a treatment in already existing concrete

Alkalis + Reactive Silica + Moisture ASR Gel

Alkali-Silica Reaction

ASR Damage Examples

Built in 1965, this deteriorated bridge is located 9.7 miles west of LeeVining at 9400 feet elevation on the eastern slope of the Sierra Nevada.

CRACKS IN CONCRETE• Plastic Shrinkage Cracks.

• Settlement cracks.

• Bleeding.

• Delayed Curing.

• Constructional effects.

• Early Frost Damage.

• Unsound Materials.

• Shrinkage.

• Drying Shrinkage.

• Thermal Shrinkage.

PLASTIC SHRINKAGE CRACKS

• When the loss of water from surface of concrete is

faster than the migration of water from interior to the

surface, the surface dries up.

• It depends upon the rate of evaporation of water from

the surface of concrete

PLASTIC SHRINKAGE CRACKS

Prevention measures:

• Moisten the formwork.

• Erect temporary wind breakers to reduce the wind velocity

over concrete.

• Erect temporary roof to protect green concrete from hot sun.

• Reduce the time between placing and finishing. if there is

delay cover the concrete with polythene sheets

PLASTIC SETTLEMENT CRACKS

• Plastic concrete when vibrated or otherwise settles. If

the concrete is not free to settle uniformly, then

cracks are formed.

• Non Uniform settlement caused due to large piece of

aggregates or reinforcement.

PLASTIC SETTLEMENT CRACKS

THERMAL EXPANSION AND SHRINKAGE

• Expansion and contraction of concrete subjected to

ambient increase or decrease in temperature results

concrete cracking.

• Ex: roof slabs, road or airfield pavements , bridge

decks etc.

CONCLUSIONS

Factors affecting durability of concrete

• Environmental factors

• Quality of constituent materials

• Quantity of constituent materials

• Quality of workmanship

• Cover to the reinforcement

• Inadequate design

• Improper use of structure