DURABILITY OF THE CONCRETEThe two main structural engineering materials that are now used for civil engineeringapplications are reinforced concrete (using carbon steel reinforcement) and structuralsteel. These materials have been shown to be very flexible in the construction industryand have proven, or at least known, performance in a wide range of serviceenvironments. However, in some conditions neither material are regarded as inherentlydurable and it is common, in these instances, to modify or provide additionalprotection to improve the durability and achieve the design life. This is particularly soforcivil engineering structures where long (>100 year) is often a basic design requirement.THREATS TO DURABILITY:-•So2 attack•Carbonation•Steel Corrosion•Acid attack•Chloride attack•Alkali aggregatePROTECTION AGAINST DUEABILITY:- SO2 RESISTING CONCRETE:-•Sulfate attack can be external or internal. is the more common type and typicallyoccurs where water containing dissolved sulfate penetrates the concrete.Due to whichthe composition and microstructure of the concrete will change .These changes mayvary in type or severity but commonly include: •Extensive cracking •Expansion •Loss of bond between the cement paste and aggregate •Alteration of paste composition, with monosulfate phase converting to ettringite and, in later stages, gypsum formation. The necessary additional calcium is provided by the calcium hydroxide and calcium silicate hydrate in the cement pasteOTHER SOURCES OF SULFATE:-
Other sources of sulfate which can cause sulfate attack include: •Seawater •Oxidation of sulfide minerals in clay adjacent to the concrete - this can produce sulfuric acid which reacts with the concrete •Bacterial action in sewers - anaerobic bacterial produce sulfur dioxide which dissolves in water and then oxidizes to form sulfuric acid •In masonry, sulfates present in bricks and can be gradually released over a long period of time, causing sulfate attack of mortar, especially where sulfates are concentrated due to moisture movement •Internal sulfate attackOccurs where a source of sulfate is incorporated into the concrete when mixed.Examples include the use of sulfate-rich aggregate, excess of added gypsum in thecement or contamination.Protection against internal sulfate attackProper screening and testing procedures should generally avoid internal sulfate attack. CORROSION:- CARBONATION:-Carbon dioxide, CO2 is a gas form in the atmosphere, it penetrates in the concretepores. This penetration is more fast when concrete is more porous. But, this gas canbe dissolved in the water which is in some pores. It can then react with cement andform carbonates. This reaction decreases the concrete pH, down to a value close to9,5.So, carbonation starts on concrete surface, and concerns some thickness (calledcarbonation depth) of this material. STEEL CORROSION:- CAUSES:-•Before being placed in a framework, a reinforcement is rusted, because it was initiallyexposed to atmosphere.•When a freshly-mixed concrete is placed around this steel,.MECHANISM:-•When a freshly-mixed concrete is placed around this steel, the mixing water penetratesthrough the rust pores, where it gradually forms hydrated calcium ferrite (4.CaO. Fe2
O3 13H2 O). Moreover, this water reacts with steel and forms on it a thin layer of ironand calcium hydroxides, respectively [Fe(OH)2] and [Ca(OH)2].•All these products in the vicinity of steel raise the pH of concrete pore solution, up toabout 13. It should be noted that in contact with an initial rust, cement hydration of isdisturbed : a transition zone is locally formed, and concrete is more homogeneous, farfrom this zone. PROTECTION:-So, the concrete mixing water makes it possible to form around steel, some products,which protect it by passivation. More precisely, under atmospherically induced rust,reinforcement is covered with a thin protective layer of white products, containingferrite and of hydroxide of calcium.Such a protection vanishes, if the pore solution disappeared (e.g. when large cracksreach reinforcements) or does not correspond any more to sound concrete. CHLORIDE ATTACK:-Corrosion caused by chloride attack can dramatically affect mild. steel and even lowergrades stainless enclosures. To help select ... misery for all. Typically the corrosion iscaused by chloride attack from salt waterIN Reinforced Concrete :In the case of reinforced concrete the realisation that the material may not always bedurable in some conditions has occurred over the last 25 years or so as the impact ofchlorides (from whatever source)on the long-term performance of structures hasbecome apparent. The problems can be particular lyacute on highway structures, suchas bridges, and has led to the development of a range of repair strategies to avoiddemolition and reconstruction of these structures. The problems of corrosion arisingfrom the use of de-icing salts can often occur within a relatively small fraction of theintended design life (15 to 20 years out of an intended 120 years). The resultantmaintenance, repairs and monitoring of such structures is:• Difficult• Expensive• Increasing unacceptableCHLORIDE PROTCTION:
For the most part the approach is to attempt to improve the resistance of the concrete todetrimental affects of chlorides by restricting the transport of chlorides through theconcrete to the steel reinforcement. In broad terms this is achieved by one or more ofthe following:• Increasing the thickness of cover to the reinforcement, thereby increasing the timefor chloride transport to the steel.• Altering the specification of the mix to include cement alternatives that resultin a concrete that is less permeable to chlorides; thereby increasing the time forchloride transport to the steel.• Improving the overall quality of the concrete to ensure a low permeability tochlorides.• Treatment of the surface (using coatings or impregnation materials) to preventthe ingress of water and chlorides into the concrete; such methods require futuremaintenance. Alkali-Aggregate Reaction :-In most concrete, aggregates are more or less chemically inert. However, someaggregates react with the alkali hydroxides in concrete, causing expansion and crackingover a period of many years. This alkali-aggregate reaction has two forms—alkali-silicareaction (ASR) and alkali-carbonate reaction (ACR).Alkali-silica reaction (ASR) :In ASR, aggregates containing certain forms of silica will react with alkalihydroxide in concrete to form a gel that swells as it adsorbs water from thesurrounding cement paste or the environment. These gels can swell and induceenough expansive pressure to damage concrete .Alkali-carbonate reactions (ACR) :Dedolomitization, the breaking down of dolomite, is normally associated withexpansion. This reaction and subsequent crystallization of brucite may causeconsiderable expansion. The deterioration caused by ACR is similar to thatcaused by ASR; however, ACR is relatively rare because aggregates susceptibleto this phenomenon are less common and are usually unsuitable for use inconcrete for other reasons.ACID ATTACK:-
Concrete is susceptible to acid attack because of its alkaline nature. Thecomponents of the cement paste break down during contact with acids.Most pronounced is the dissolution of calcium hydroxide which occursaccording to the following reaction: 2 HX + Ca(OH)2 -> CaX2 + 2 H2O (X is the negative ion of the acid)Factors Affecting Acid Attack:-The decomposition of the concrete depends on•The porosity of the cement paste,•On the concentration of the acid, Acids such as nitric acid, hydrochloric acid andacetic acid are very aggressive as their calcium salts are readily soluble and removedfrom the attack front.•SULPHURIC ACIDSulphuric acid is very damaging to concrete as it combines an acid attack and a sulfateattack.• The solubility of the acid calcium salts (CaX2) and on the fluid transport through theconcrete.•Insoluble calcium salts may precipitate in the voids and can slow down the attack.calcium salt, due to their low solubility, inhibit the attack by blocking the pathwayswithin the concrete such as interconnected cracks, voids and porosity CONCLUSION:-The best means of maximizing the probability that concrete will be durable is toproduce concrete that will provide the desired service for the desired service life in theenvironment in which it will be placed and used. Every concrete mixture should beproportioned in accordance with exposure conditions, construction considerations, andstructural criteria. Exposure to freezing and thawing, sulfates, deicing chemicals,acids,varying moisture conditions, and abrasive loadings should all be considered whenselecting materials and proportions.PREPARED BY:-SANA ADNAN.ROLL # CE-107.