Civil Engineering

Mechanism of Deterioration of Concrete based Structures or Systems and Remedies

In the civil engineering field concrete is a construction materials is extensively used in construction work. Generally it is mixture of cement, fine aggregate (normally termed as sand) coarse aggregate (termed as stone chips or broken stones), water and some times  admixture . It is used  to develop or construct different type of structures or systems. These  structures are generally two types , they are P.C.C and R.C.C. P.C.C stands for Plain Cement Concrete and R.C.C stands for Reinforced Cement Concrete. Now P.C.C structures are developed by mixing cement, sand, stone chips and water where admixtures are optional. In case of R.C.C along with P.C.C  ingredients  steel bars embedded into it. Performance of concrete in P.C.C and R.C.C is quite satisfactory from the strength point of view as well as many other aspects. One of the important performance of concrete in R.C.C is prevention of steel bars from external hazards. Both of the type of systems  at early stage offers considerable resistance but as the time passes  the degree of resistance of systems reduces.  Thus resulting unsatisfactory performance of structural systems. Structural systems those are experiencing such type of adverse situations if not provided selective or necessary treatments. Most of the concrete structures or systems  deteriorate through four different ways. Those are classified as –

  1. Chemical deterioration

This type of deterioration takes place due to chemical reaction at the concrete covers of R.C.C members. This reaction may take place in several ways –

  1. Carbonation: Generally reinforced steel bars are surrounded by a particular grade of concrete. Harden concrete made up of calcium silicate gel and calcium hydroxide (Ca [OH]2). Ca [OH]2 ensure high alkaline nature of concrete to fight against corrosion and save steel from rust by creating a passive film around the steel bars. Due to this film,  O2 and H2O fail to reach the main steel bars. Thus corrosion avoided.  But if environment surrounding  concrete covers, contains carbon dioxide( CO2) in air it may be vulnerable to RC members. CO2 in moisture enter in concrete cover  through concrete pores. It reacts with Ca [OH]2 and produces calcium carbonate (CaCO3) and water (H2O). Thus reduces pH value of concrete and formation of acidic substance i.e, CaCO3. Due to this acidic substance passive film decays and open up the entry  of O2 and H2O to main steel bars. This initiate the corrosion of steel and in tern resulting damage of RC structures.

Remedy – Application of different types of polymer based coating, stabilising primers on concrete surfaces offers protective barrier against concrete carbonation and slow down this adverse process. Adoption of epoxy coating on  steel bars, safe guard the passive layer of steel bars also controls the carbonation process.

  1. Alkali-Aggregate Reaction : If aggregates containing reactive silica are used in concrete making, it is observed that alkaline solution reacts with silica present in aggregates and produces alkali-silica gel. The product volume is quite higher and bring out all the undesirable properties in concrete. Due to higher volume internal stresses generates and cracks develop in concrete structures or systems.

Remedy – Avoid use of  reactive silica based aggregates in concrete making to control the Alkali-Aggregate Reaction.

  1. Sulphate attack : It is also one of the important cause of concrete based structure’s deterioration. Sulphate may exists in nature e.g, in ocean water, ground water and industrial pollution etc in different soluble forms like calcium, sodium, magnesium etc. Sulphate attach may takes place by two reactions. When concrete structures come in contact with water contaminated by sulphates, then Ca [OH]2 , sodium based sulphate salt and water. Therefore produces gypsum thus reduces pH value due to loss of Ca [OH]2 from concrete. In the other way sulphate ion react with tri-calcium aluminate (C3A) and produces calcium sulphoaluminate hydrate (termed as ettringite) as major product. Both gypsum and ettringite posses higher volume expansion resulting internal stresses in concrete body. Finally concrete surfaces spall of from the actual concrete.

Remedy – Use of sulphur resisting cement, fly ash based cement, adequate compaction of concrete etc prevent this type of attack in concrete systems.

  1. Chlorine attack :  Another adverse situation that happens to concrete when RC element’s constituents or environment contaminated by chlorine. Resulting corrosion of steel in RC structures or systems. If concrete come in contact of sea water or sea water used to produce concrete, higher chloride content in aggregate so on and so forth. Chlorides reached to passive layer surrounding steel bars and destroy it and then the bars are exposed fully to the environment. So corrosion initiates and reduces the cross sectional area of bars. Therefore tension carrying capacity of systems or structures reduces. While rusting occurs in bars, its volume immensely increases causing internal thrust in whole system and cracks generates that further accelerates corrosion process. Finally failure of structures or systems takes place.

Remedy – The best possible and easiest way to slow down this attack is to increase concrete cover, epoxy coating on steel bars and surface coating on concrete systems.

  1. Mechanical deterioration

Mechanical decay of concrete systems may occur through several ways like erosion, abrasion and cavitation. Concrete used to construct hydraulic structures are often subjected to the action of abrasion by fluid flow containing solid substances. Wearing of concrete surfaces due to this flow of fluid results erosion of systems termed as erosion. Sometimes movement of solid substances over concrete surfaces develops friction between each other. Some of the examples related to this type of friction are – acceleration and de-acceleration of moving  vehicle’s  wheels, movement of heavy machine over the industrial floors etc made up of concrete.  Therefore considerable amount of material loss from system will occur. In the other way  loss of materials from surface of concrete systems  may occur as a result of irregular flow of fluid/water , change in the direction of flow creating vapour bubbles. This in turn show up as decay of concrete surfaces.

Remedy – Ensuring elimination of solid materials in fluid flow, treatment of  concrete surfaces, use of higher grade of concrete can avoid and prevent this type of deterioration.

  1. Physical deterioration

This is a type of deterioration takes place mainly due to freezing and thawing. Locations where cold and hot weather conditions occurs frequently, in cold situation freshly produced concrete having adequate quantity of water free zed and converted into ice. Therefore volume of water increases and creates internal stresses into the system. While during in hot weather concrete thaws i.e, ice converted to water. Again freezing occurs and followed by thawing in repeated cycles. Thus resulting disruption of system and reduces the strength.

Remedy – Application of air entraining agents during concrete making dissipate internal stresses in concrete systems.

  1. Deterioration due to constructional errors

Constructional errors may occur through different ways those are given below :

  1. During production of concrete if water-cement ratio maintained is higher than specified value and adequate compaction is not achieved in concrete then it will show up in the form of tiny holes, honey combing etc over the concrete surfaces. This will not only interrupt the performance of concrete systems but the appearance of surfaces as well.
    Remedy – To avoid this error adequate care should be ensured during concrete making from the point of view of compaction and proportions of ingredients.
  2. Inappropriate placing of reinforcing steel bars in form work of RC members may also cause errors which finally become the reason of deterioration.
    Remedy – Placing of bars according to the codal guidelines help to avoid this type of error.
  3. Dislocation of form work and insufficient strength of temporary supports to form work during concrete placing resulting variation of dimensions of concrete structural members as well as violate design stipulations. As a result of which stress distribution throughout the members become non uniform and finally causes deterioration of systems.
    Remedy – Continuous supervision throughout the construction  work ensure avoidance of this errors and deterioration.
  4. Also improper finishing and curing of concrete systems whether it is P.C.C or R.C.C, results deterioration of systems. Because if curing is not ensured properly, hydration reaction and production of calcium- silicate-hydrate (C-S-H)  gel will not be adequate. Thus directly affects the performance of structures from the strength as well as durability point of view.
    Remedy – Based on the situation  of construction  sites (i.e, whether member is vertically or horizontally placed, temperature-atmospheric conditions and availability of resources etc.) finishing and curing (periodically) techniques should be adopted.

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