EFFECT OF SEA WATER ON CONCRETE

Khalid ManzoorEnrol no. 17/2011

Introduction Nowadays concrete is increasingly being used in more

hostile environmental conditions

In marine structure the concrete has to withstand the

physical, chemical , mechanical action of sea water

and alternate wetting and drying condition with salted

water

Important factor is permeability.

• The deterioration is mainly because of sulphate and chloride content of sea water

Sea water constitutes Soluble salt – 3.5%by weightSodium and chloride – 11 to 20 gm/ltrMagnesium and sulphate – 1.3 to 2.8 gm/ltrAnd ph of sea water = 7. 5 to 8.4This all are sufficient to deterioration of concrete

Constituents of sea water

Effect of sea water on concreteWetting and drying Temp. variationCorrosion of reinforcement.Battering by waves and tidesSulphate attackFreezing and thawing

Exposure zones

Type of deterioration in various zones Marine atm. Zone 1. corrosion of reinforcement by chloride. 2.frost action.Splash zone 1. corrosion of reinforcement by chloride. 2.abrasion due to wave action.

3.frost action.

contnd.

Tidal zone 1.corrosion of reinforcement by chloride

2.abrasion due to wave action 3.chemical attack

Submerged zone and sea bed1.chemical attack2. biological fouling

Reinforcement corrosion

• The corrosion products forming on the steel have a larger volume than the original steel, and the expansion of these products exerts a pressure that increase gradually and becomes strong enough to crack the concrete cover.

Effects due to humidityChanges in relative humidity can

lead to dimensional changes in material with deformation & cracking.

Prolonged high humidity promote fungal growth and subsequent decay of organic materials.

And corrosion rate increases due to destruction of protective coatings.

Effects due to temperature The corrosion velocity is doubled for

every 10 degree C increase in temperature. sea salt dissolve more easily at the higher

temperature.The temperature changes causes alternate

expansion & contraction of material. It leads to high stresses and gradual deterioration & rupture.

When surface temperature fall sufficiently, moisture may condense on surface, which become thoroughly wetted. This may cause corrosion of material.

Chemical reactions on marine structure

Acid attack Portland cement is not very resistance to acid attack.In case of sulfuric acid attack it deteriorate concrete and

acid is able to reach to reinforcement.Thus loss of cement paste and aggregate from the

matrix and cracking , rust staining, spanning occurrs.

Alkali silica reactionsome aggregate containing silica that soluble in highly

alkaline solution.expand , disrupting the concrete.Sulfate attack

there are two chemical reaction involved in sulfate attack on concrete.

first the sulfate react with free calcium hydroxide to from calcium sulfate.

Next sulfate combines with tricalcium aluminates to form calcium sulphoaluminate.

Both this reaction result in an increase in volume of concrete.

Concrete contains micro cracks

1. Humidity and temperature.2. Impact of floating objects.3. Chemical attack, leaching of

cement paste.4. Freeze attack, overload and other

factor increase permeability of concrete.

Sea water and air

Highly permeable concrete

Corrosion of embedded steelCrack growth

Cracking- corrosion- cracking cycle

Effects on concrete

Colour of concrete change from deep grey to lime grey expose to sea water.

There is continuous increase in permeability in concrete due to sea water and its attribute to sulphate attack on concrete.

Compressive strength over a year is decrease about 15 to 30% with respect to 28 day compressive strength expose to sea water.

• As per IS 456:2000• Min.grade of concrete for RCC is M30.• Min.cement content is 320 Kg/m3.• Max.W/C ratio is 0.45.• Max.chloride content is 0.60 Kg/m3.• Total sulphate content should not exceed 4% .• Use Pozzolana cement or slag as far as possible.• No construction joints within 600mm of the

upper & lower planes of wave action .• Nominal cover is 45mm – 75 mm.

Various Codal Provisions for Marine environment

• As per BS CP 110• Min.grade of concrete for RCC is M40.• As per Australian code • Min.grade of concrete for RCC is M30.• As per IS 456:2000• Min.grade of concrete for RCC is M30.

•w/c kept as low as possible.•Minimum cover should be increased where abrasion may occur.•Proper curing.

Thank you