Investigate Durability Of Concrete Structure

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02 Nov 2017

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The durability of concrete structure will often be determined by result of concrete reaction with chemical. From these reactions, the aggressive substance of concrete, such as the ions and molecules are transport to outside, mostly from the environment.

In generally, the reaction between the aggressive substance and the reactive substance take place as soon as the substance meets. But, as the low rate of the transport of the aggressive substance, these reactions may take many years to show their effect.(Durable 1992)

The chemical reaction will decrease the quality of the concrete, the important parts include:

The reaction of acids

The reaction of sulphates

The reaction of alkalis

2.1.1 Acid attack

Hydrated cement paste is a kind of alkaline material, for this reason attack by other alkaline material will not encountered by normal. High concentration of alkaline materials that may come in contact with concrete in industrial processes cause deterioration by processes other than direct chemical reaction with hydroxide ions.(Fig

The hydrogen ion accelerate the leaching if the calcium hydroxide,

Ca(OH)2+2H+→Ca2++2H2O

If the ion is highly concentrated, CSH may also be attacked,

3CaO.2SiO2.3H2O+6H+→3Ca2++2(SiO2.nH2O)+6H2O

Carbonic acid can also be very corrosive because of formation of soluble calcium bicarbonate

Ca(OH)2+H2CO3→Ca(HCO3)2+H2O

(Concrete 2003)

It should be realized, there is a fundamental difference between acid attack, sulphate attack and alkali attack. In the former case, there is a complete processing of the hardened cement, and destroying the pore system.

With the acid attack, the effect on permeability of the concrete is very low, but with the other attack, the effect on permeability of the concrete is very high.

2.1.2 Sulphate attack

In contrast to acid attack, where the pore system as a whole is destroyed because the acid react with all the component in cement, sulphate attacks only certain components in the cement. Sulphate attack is characterized by the chemical reaction of sulphate ion with the aluminate component and ions of sulphate, calcium and hydroxyl of hardened Portland cement or the cement containing Portland clinker, it forming a lesser extent gypsum.(Durable 1992)(Fig

The cement has a high rate of C3A, It is attack by reaction between sulphate ions and calcium hydroxide

CH+SO42-(aq)←→CSH2+2OH- (aq)

It is important to realize that classification or cements for sulphate resistance only takes sulphate resistance as such into consideration. In case of combined attack, the other factors may influence the choice of the cement.

2.1.3 Alkali attack

The mechanism of alkali attack resembles that of sulphate attack more than acid attack, as the reason of the alkali attack is only on certain substances in the concrete. The difference between sulphate attack and alkali attack is that the reactive substance in the former case in the cement, and later in the aggregate.(Fig

Carbonate materials may also be easily attack by alkaline materials. In dolomite or limestone, the reaction may produce magnesium hydroxide. It may result ultimately in the completely destruction of concrete.(Durable 1992)

2.2 Physical processes in concrete

2.2.1 Cracking

Crack may be caused by many different situations and may range from very small internal micro cracks that occur on the application of modest amount of stress to quite large cracks caused by undesirable interaction with the environment, poor construction practices, and errors in structure design and detailing. (Concrete 2003)

Cracking will occur whenever the tensile strain to which concrete is subjected exceeds the tensile stain capacity of the concrete. The tensile strains capacity of concrete varies with age and with rate of the application strain.(Fig

There are lots of kinds of basic mechanisms by which strains may be generated.

Movement generated by the concrete.

Example: drying shrinkage, expansion or contraction due to temperature change, and plastic settlement or shrinkage.

Expansion of material embedded by the concrete.

Example: corrosion of reinforcement.

Externally imposed conditions.

Example: Loading or deformations imposed by differential settlement or foundation.

(Fig

Cracking developed during the execution process and during the initial period of hardening time may be the main reason for the acceleration or deleterious action, it depends on the water or the aggressive substances enter into to concrete from outside.(Durable 1992)

2.2.2 Erosion

2.2.2.1Erosion by abrasion

Abrasive wear of the concrete surface can be caused, for example, by the grinding action of pedestrian traffic on floors, by the scraping, percussive impact of studded tires on pavements or by impact or sliding of loose bulk material.(Durable 1992)

Abrasive wear also can be caused by the action of heavy particles suspended in water, especially at high water velocities. Especially the places which water occurred. Such as the dams, the hydraulic plants, the constructions for steam regulation, the structure protecting embankment, and the bridge piers.(Fig

2.2.2.2 Erosion by the cavitation

If the water without solid is flowing rapidly parallel to a limiting surface, any change in the geometry of the surface causes a flow detachment and zones of low pressure at the limiting surface. If the static pressure of streaming water becomes lower than the vapor pressure of water, the vapor bubbles will fill in this zone. If the bubbles stream to zones where the static pressure exceeds the vapor pressure of water, vapor condenses in the bubbles and bubbles collapse suddenly. This implosion causes impact and pressure waves to develop, it is similar those caused by explosions. This process is called cavitation, and results in damage similar to pitting and excavations.(Durable 1992)

The cavitation or similar impact and pressure waves occur when water hits limiting surfaces with high velocity. The 90 degree angle surfaces constitute an extreme case of this.(Fig

2.2.3 Frost attack

In the case of water freezing in porous building material, for example the cement paste, the physical processes are the major reason to determine the freezing resistance by their mutual interaction.

Transition of the volume from water to ice is increased 9%. In case of completely water filled pore, this will cause spitting of the concrete. (Fig

The over energy at the pore surface results in a reduction of the potential energy of the pore water, and in a depression of the freezing point, due to the wide range of pore radii of cement paste, only about 33% of the pore water will be frozen at the temperature -30oc and 66% of the pore water will be frozen at the temperature of -60oc. A thin film of water coating the pore surfaces will remain even after the pore water has formed as the ice.

As a result of increasing strength of the concrete and the changing pore structure, the frost resistance will growing stronger as the age of the concrete increasing

In further, it should be noted that even in ambient humilities not likely to cause damage by frost, concrete at very early age shows high moisture content, and it has confined expansion space.

This is due to the fact that the retained water from the processing has not been disposed. (Durable 1992)

CEB COMITE EURO-INTERNATIONAL BU BETON, Durable Concrete structure,1992,PP 1-35

Sidney Mindess. J.Francis Young. David Darwin, CONCRETE, second edition,2003

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