Reasons for Concrete Cancer
It provides various reasons for concrete cancer....
REASONS FOR CONCRETE CANCER Concrete is a composite construction material composed primarily of cement, aggregate and water. There are many formulations that provide various properties. The aggregate is generally coarse gravel or crushed rock along with a fine aggregate such as sand. There are several reasons for reinforcing concrete with steel: 1. Concrete has very high compressive strength but very poor flexural strength. Therefore concrete will immediately crack with the slightest flexural pressure exerted by normal stresses such as thermal changes or ground movements (such as earth tremors, foundation settling, or soil volume changes due to seasonal moisture level fluctuations). Steel provides the essential flexural strength required to support a building or a structure through such pressures. 2. Concrete and steel are fully compatible with each other. They expand and contract with changes in temperature at almost exactly the same rates. Using a reinforcing material of differing thermal expansion and contraction rates would readily result in stress fractures. 3. Steel can be quickly fabricated to accommodate a wide range of architectural designs. The initial cause of concrete cancer is usually water penetration. When calcium oxide reacts with water that penetrates the concrete, it forms a solution of calcium hydroxide. When concrete is placed on a sidewalk, garage floor or anywhere else it is mixed with gravel. However gravel is unappealing to the eyes. So the contractors will attempt to make the surface look white and smooth. To do so, they add an extra amount of water to the concrete surface and create a “cream” of pure concrete on the visible structure which is the clean white finish that you see when the concrete has dried. This cosmetic “creamed” surface is more fragile than the rocky concrete below and it is the first layer to chip and flake away with time. Concrete has two layers: an attractive smooth outer surface, and a rough rocky interior. Concrete spalling occurs when the outer surface chips away, revealing the ugly interior material. It is straight forward to assume that, as long as the steel reinforcements are protected from air and water, they cannot corrode, and therefore concrete cancer can be avoided. The porous property of concrete is something which can be affected by the quality of the constituent ingredients in the concrete material such as the aggregate. Another cause is that the chlorides that is present in the salt water can enter the concrete after it is build and cause corrosion. This is an issue with the buildings near oceans, such as beach apartment blocks. Other factors which causes the “cancer” include the concrete not being set properly or not effectively covering the steel environmental factors like movement of earth under the building causes concrete to crack; thereby exposing it to the elements or water soaking up from underground. Weathering and erosion of concrete, as well as exposure to chemicals such as acids or even bacteria can undermine concrete. Temperature and climate are also
factors which can cause concrete to breakdown as water can freeze and thaw on its surface. Following sections will discuss in detail, regarding the types and reasons behind concrete cancer. 1. AGGREGATE EXPANSION Various types of concrete undergo chemical reactions in concrete, leading to damaging expansive phenomena. The most common are those containing reactive silica, that can react (in the presence of water) with the alkalis in concrete (K2O and Na2O coming principally from cement). Among the more reactive mineral components of some aggregates are opal, chalcedony, flint and strained quartz. Following the alkali-silica reactions (ASR), expansive gel forms that create extensive cracks and damage on structural members. On the surface of concrete pavements, the ASR can cause pop-outs, i.e. the expulsion of small cones (upto 3cm in diameter) in correspondence of aggregate particles. When some aggregates containing dolomites are used, a dedolomitization reaction occurs when the magnesium carbonate compound reacts with hydroxyl ions and yields magnesium hydroxide and a carbonate ion. The resulting expansion may cause the destruction of the material. Far less common are pop-outs caused by the presence of pyrite, an iron sulphide that generates expansion by forming iron oxide and ettringite. Other reactions and recrystallization e.g. hydration of clay minerals in some aggregates may lead to destructive expansion as well. 2. CORROSION OF REINFORCEMENT BARS Corrosion of the reinforcement steel however, is by far the most common cause of spalling and splitting in older concrete structures. The expansion of corrosion products (iron oxide) of carbon steel reinforcement structure may induce mechanical stress that can cause the formation of cracks and disrupt the concrete structure. If the rebars have been poorly installed and are located too close to the concrete surface in contact with the air, concrete spalling can easily occur: flat fragments of concrete are detached from the concrete bars by the rebars corrosion and may fail down. One of the main components of this corrosion protection is provided by the amount of concrete cover protecting the steel. As a thumb rule, one inch of the cover is required to protect the rebars. In other words, no rebar should be nearer than one inch of the concrete surface. 3.
A. Carbonation Carbon dioxide in the air can react with the calcium hydroxide in the concrete to form calcium carbonate. This process is called carbonation which is essentially the reversal of chemical process of calcination of lime taking place in the
cement kiln. Carbonation of concrete is a slow and continuous process progressing from the outer surface inward but slows down with increasing diffusion depth. Carbonation has two effects: It increases mechanical strength of concrete but it also decreases alkalinity which is essential for corrosion prevention of the reinforcement steel. Below a pH of 10, the steels thin layer of surface passivation dissolves and corrosion is promoted. For the latter reason, carbonation is an unwanted process in concrete chemistry. Carbonation can be tested by applying Phenolphthalein solution, which is an indicator, over a fresh fracture surface which indicates non-carbonated and thus alkaline areas with a violet colour. Mild steel rapidly oxidises (oxidises) in the presence of moisture, oxygen, and ions (salts). If, however, it is embedded in fresh concrete, the high alkalinity of the concrete passivates the surface of the steel providing an excellent barrier to oxidation. This corrosion protection lasts as long as the concrete maintains its high alkalinity which (with diligent building maintenance) can be virtually indefinite. However concrete is also quite porous (even “high strength” concrete) and rapidly absorbs moisture(H20), carbon dioxide(CO2), carbon monoxide(CO), sulphur dioxide(SO2) and many other airborne chemicals. These chemicals act as acids and neutralises the calcium hydroxide (Ca(OH)2) present in the Portland cement. (In fact “acid rain” is rain water in which theses acids are dissolved). CO2 + Ca(OH)2 CaCO3 + H2O This neutralisation process is known as “carbonation” of the concrete and the point at which alkaline concrete becomes neutral is called the “carbonation front”. The carbonation front begins at the surface of concrete and steadily moves towards the rebar. As soon as the carbonation front reaches the rebar, the alkalinity at the rebar drops, and the rebar loses its only protection against oxidation. When the embedded rebar corrodes, the corrosion products take up more volume than original steel. This expansion within the concrete exerts a far greater force than the concrete’s flexural strength will allow, resulting in cracks in the concrete around the affected steel. The cracks expose the steel to further corrosion and cause more concrete breakdown. By the oxidation-reduction reactions between iron metal, oxygen and water, a complex molecule of iron of greater molecular weight and volume than the original iron material is formed which is called the “rust”. 4Fe + 3O2 + nH2O 2Fe2O3nH2O
Chlorides particularly calcium chloride, have been used to shorten the setting time of concrete. However calcium chloride and (to a lesser extend) sodium chloride have been shown to leach calcium hydroxide and cause chemical changes in Portland cement leading to loss of strength as well as attacking the steel reinforcement present in most concrete. A totally different cause of concrete spalling is often found along the Australian coastline. Coastal air is laden with chloride ion (from seawater salt). As concrete is porous, it readily allows chloride ions to move easily through the concrete matrix and from incipient anodes on the surface of the steel rebars, causing nodules of rust on the steel surface. Even though the concrete may have a high alkalinity, the surface passivation of the steel is disrupted by the chloride ions. The rust nodules create internal stresses in the concrete, which results in crack in the concrete around the affected steel. The cracks expose the rebar to more chloride ion attack and more corrosion exacerbating the spalling problem. C. Sulphates Sulphates in solution in contact with concrete can cause chemical changes to the cement, which can cause many microstructural effects on leading to the weakening of cement binder. Sulphates and sulphites are ubiquitous in the natural environment and are present from many sources, including gypsum (calcium sulphate) often present as an additive in ‘blended’ cement which include fly ask and other sources of sulphate. With the notable exception of barium sulphate, most sulphates are slightly to highly soluble in water. These include acid rain where sulphur dioxide in the air shed is dissolved in rainfall to produce sulphurous acids. In lightning storms, dioxide is oxidised into trioxides making the residual sulphuric acid in the rainfall even more acidic. Local government infrastructure is mostly corroded by sulphate arising from the oxidation of sulphide which occurs when bacteria (for example in sewer mains) reduce the ever present hydrogen sulphide gas to a film of sulphide (S-) or disulphide (HS-) ions. This reaction is reversible, both either readily oxidising on exposure to air or oxygenated storm water, to produce sulphate or sulphite ions and acidic hydrogen ion in the reaction HS- + H2O + O2 2H+ + SO4-. The corrosion often present in the crown (top) of the sewer is directly attributable to the process – known as crown corrosion. D. Leaching When water flows through cracks present in concrete, water may dissolve various minerals present in the hardened cement paste or in the aggregate if the solution is unsaturated with respect to them. Dissolved ions such as calcium (Ca2+) are leached out and transported in solutions some distance. If the physicochemical conditions prevailing in the seeping water evolve with distance along the water path and water becomes supersaturated with respect to certain minerals, they can further precipitate, making deposits or efflorescence inside
the cracks, or at the concrete outer surface. This process can cause the selfhealing of fractures in previous condition. E. Decalcification Distilled water can wash out calcium content in concrete, leaving the concrete in brittle condition. A common source of distilled water can be condensed steam. Distilled water washes out the calcium because normal water will already contain some calcium ions, which will not dissolve them. F. Seawater Concrete exposed to sea water is susceptible to its corrosive effects. The effects are more pronounced above the tidal zone than where the concrete is permanently submerged. In the submerged zone magnesium and hydrogen carbonate ions precipitate a layer of brucite, about 30 micrometres thick, on which a slower deposition of calcium carbonate as aragonite occurs. These layers somewhat protect the concrete from other processes, which include attack by magnesium, chloride and sulphate ions and carbonation. Above the water surface, mechanical damage may occur by erosion by waves themselves or by sand and gravel the carry, and by crystallisation of salts from water soaking into the concrete pores and then drying up. Pozzolanic cement and cements using more than 60% of slag as aggregate are more resistant to seawater than pure Portland cement. Seawater corrosion contains elements of both chlorides and sulphate corrosion. 4. BACTERIAL CORROSION 5. PHYSICAL DAMAGE 6. THERMAL DAMAGE