Durability of concrete

Durability of concreteSubmited by,Pranam Raval(10CL050)Saumya Shah(10CL055)Yash Khandol(10CL019)

What is Durability!!!Durability of concrete is defined as its ability to resist weathering action ,chemical attack, abrasion or any other process of deterioration.

So that durable concrete will retain its original form, quality and serviceability when exposed to its environment.Factors affecting DurabilityDivided into two groupsExternal factorsInternal factors

1.External factors:

It includesPhysical, chemical or mechanical causesEnvironmental, such as extreme temperatures, abrasion and electrostatic action.Attack by natural or industrial liquids and gases.

2. Internal factors

It includesPermeability of concreteAlkali aggregate reactionVolume changes due to difference in thermal properties of the aggregate and cement paste. Physical causes

Weathering-freezing and thawing effectAs the temperature of a critically saturated concrete is lowered during cold weather, the freezable water held in the capillary pores of the cement paste and aggregates expands upon freezing.This factor is not so important in India. But it is great importance in cold countries.Freezing and thawing causes expansion of concrete. Hence, if concrete mass is subjected to alternate cycles of freezing and thawing, it has detrimental effect on the strength of concrete.Freeze and thaw deterioration generally occurs on horizontal surfaces that are exposed to water.The expansion causes localised tension forces that fracture the surrounding concrete matrix.

Other physical causes..

When concrete is damaged by impact or abrasion, the cause is usually obvious and protection may be necessary as well as repair. Drying shrinkage is a slow process in thick members, so it may lead to a gradual build-up of tensile stress if it is restrained. The use of excessively wet, high shrinkage concrete mixes will aggravate matters, as will the use of shrinkable aggregates that occur in some regions. Restrained thermal contraction is a fairly frequent cause of cracking, and often designers do not make adequate provision for thermal movements. Concrete may be damaged by environmental factors such as fire or frost. Concrete gradually loses strength with increase in temperature above about 300C.Carbonation

The carbon dioxide(CO2) present in the atmosphere reacts in the presence of water with hydrated cement minerals, converting calcium hydroxide [Ca(OH)2] to calcium carbonate [CaCO3].The carbonation penetrant rates beyond the exposed surface of concrete only very slowly.

The import factors affecting rate of carbonation are

Grade of concreteRelative humidityPermeability of concreteCover to reinforcementTime

Chemical causes1.Acid attack

Portland-cement concrete is a highly alkaline material and is not very resistant to attack by acids. The deterioration of concrete by acids is primarily the result of a reaction between the acid and the products of the hydration of cement. Calcium silicate hydrate may be attacked if highly concentrated acid exists in the envir- onment of the concrete structures. In most cases, the chemical reaction results in the formation of water-soluble calcium compounds that are then leached away. If the acid is able to reach the rein- forcing steel through cracks or pores in the concrete, corrosion of the reinforcing steel will result and will cause further deterioration of the concrete. Prevention.

A dense concrete with a low water-cement ratio (w/c) may provide an acceptable degree of protection against a mild acid attack. Portland- cement concrete, because of its composition, is unable to withstand attack by highly acidic solutions for long periods of time. Under such conditions, an appropriate surface coating or treatment may be necessary.2.Alkali silica reaction(ASR)

Some aggregates containing silica that is soluble in highly alkaline solutions may react to form a solid nonexpansive calcium-alkali-silica complex or an alkali-silica complex which can imbibe considerable amounts of water and then expand, disrupting the concrete.Leading to loss of strength, elasticity and durablity of concrete. Prevention.In general, the best prevention is to avoid using aggregates that are known or suspected to be reactive or to use a cement containing less than 0.60 percent alkalies.3.Sulphur attack

Naturally occurring sulfates of sodium, potassium, calcium, or magnesium are sometimes found in soil or in solution in ground water adjacent to concrete structures. The sulphate ions in solution will attack the concrete. There are apparently two chemical reactions involved in sulphate attack on concrete.Sulphates in solution can combine with the tri-calcium aluminate (C3A) in Portland cement, to form a sulphoaluminate hydrate, causing expansion of the matrix which turns white and becomes soft.The form of sulphate present in uncontaminated groundwater is normally calcium sulphate, which has limited solubility. Some other salts, such as magnesium sulphate, are much more readily soluble in water and can form stronger solutions, so they are more dangerous.

Prevention.

By using following methods...Use of sulphate resisting cementAddition of pozzolanaQuality of concreteUse of air-entrainmentHigh-pressure steam curingUse of high-alumina cementLining of polyethylene sheetWhat is corrosion?

The chemical or electrochemical reactionbetween a material and its environments that properties. Corrosion process:Electrochemical:

Physical Process:

Methods of Corrosion Control:Barrier ProtectionProvided by a protective coating that acts as a barrierbetween corrosive elements and the metal substrateCathodic ProtectionEmploys protecting one metal by connecting it toanother metal that is more anodic, according to thegalvanic seriesCorrosion Resistant MaterialsMaterials inherently resistant to corrosion in certainenvironments

Barrier Protection

PaintPowder coatingsGalvanizing

Sulphur Infiltrated concrete (SIC)A relatively new corrosion resistant material that contains aggregates, sand and sulphur polymer cement binder.Sulphur concrete is mixed and placed at a elevated temperature.It rapidly gains high strength over a few hours of cooling and provides an economic long-term performance in many harsh environments.However, sulphur-infiltrated concrete is unstable in alkaline solutions and when submerged in water over long periods.

Sulphur ConcreteCauses of cracks in concrete Anatomy Of Repair

Step 1: Understanding the CrackStep 2: Preparing the surface for RepairStep 3: Proper procedure for Repair

Deteriorated surfaceSurface preparationRepair systemDurable repair materialBonding new to oldRein. Steel protectionConcrete surface conditionRemoval of contaminated concrete and under cutting of exposed reinforcement steelSteel reinforcement cleaningEdge conditioningcrackDe-laminationspallHeavily corroded reinforcement steelContaminated concreteWhat leads to cracking??Cracks are categorized as occurring either in plastic or hardened concretePlastic concrete: 1. plastic shrinkage 2. settlement cracks

Fig: plastic shrinkage cracksFig: settlement cracksWhat leads to cracking??In Hardened concrete: 1. Drying shrinkage 2. Thermal stress 3. Chemical reactions 4. Weathering 5. Corrosion of reinforcement 6. Poor construction practice 7. Construction Overload 8. Design and Detailing Error 9. External loads