1. Aggregate expansion due to alkali silica reactions:
Various types of aggregate undergo chemical reactions in concrete, leading to a damaging expansive phenomenon. 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, chert, flint and strained quartz. Following the ASR, and expansive gel forms, that creates extensive cracks and damage on structural members. On the surface of concrete structures, the ASR can cause pop-outs (i.e. the expulsion of small cones, up to about 3cm in diameter), ASR’s require water for the reaction and expansion of the gel, therefore Penetron prevents ASR’s.
2. Corrosion of reinforcement bars:
The expansion of the corrosion products (iron oxide) of carbon steel within reinforced structures will 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 air and moisture, or if water can penetrate into the concrete via the interconnected pores and capillaries that exist in all concrete matrixes, spalling can easily occur. The result is flat fragments of concrete become detached from the concrete mass by reinforced corrosion.
For steel to rust the must be three elements present: Oxygen, steel and water. Remove the water and steel will not rust even in the presence of oxygen.
Penetron prevents water ingress and hence prevents corrosion of the reinforcement bars.
3a. Chemical damage – carbonation (weathering):
Carbon dioxide from the air reacts with the calcium hydroxide in concrete to form calcium carbonate. This process is called carbonation.
This is essentially the reversal of the chemical process of calcinations of lime taking place in a cement kiln. Carbonation of concrete is a slow and continuous process progressing from the outer surface of a concrete structure inwards, but slows down with increasing diffusion depth. Carbonation has two effects: it increases mechanical strength of concrete, but it also decreases alkalinity of the concrete, which is essential for corrosion prevention of the embedded 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. Both Penetron and Peneseal FH will prevent the carbonation of concrete.
[Passivation: To make (a metal or other structure) un-reactive by altering the surface layer or coating the surface with a thin inert layer.]
3b. Chemical damage – chlorides:
Chlorides, particularly Calcium Chloride, have been used to shorten the setting time of concrete. However, Calcium Chloride and (to a lesser extent) 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 reinforcing present in most concrete. Chloride penetration testing is one of the most common tests done on concrete to ensure durable concrete. Penetron significantly reduces Chloride penetration in concrete, and increases the durability of concrete.
3c. Chemical damage – sulphates:
Sulphates in solution in contact with concrete can cause chemical changes to the cement binder, which can in turn cause significant negative micro-structural effects leading to the weakening of the cement binder and the general wakening of the concrete structure. Typically, Sulphates are delivered to the concrete at depth by water penetration into the concrete. Penetron will prevent Sulphates entering the concrete and causing damage.
3d. Chemical damage – leaching:
When water flows through cracks pores and capillaries present in concrete, water may dissolve various minerals present in the hardened cement paste and/or in the aggregates, if the water solution flowing through the cracks, pores and capillaries is unsaturated with respect to these minerals. Dissolved Ions, such as Calcium (Ca2+), are leached out and transported in solution out of the concrete along this water path. If the physical chemical conditions prevailing in the seeping water evolve with distance along the water path and water becomes supersaturated with respect to certain minerals, this can manifest as deposits or efflorescence inside the cracks, or at the concrete outer surface. This process can cause the self-healing of fractures in particular conditions over a very long period (many years for self-healing to take place). Penetron will accelerate this process into several days or weeks.
3e. Chemical damage – de-calcification:
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 leaches out the Calcium better than un-distilled water as un-distilled water will already contain some Calcium Ions, and does not dissolve them out of the concrete as readily.
3f. Chemical damage – seawater:
Concrete exposed to salt 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 micrometers thick, on which a slower deposit of Calcium Carbonate such as Argonite – a mineral consisting of Calcium Carbonate, typically occurring in white seashells and as colourless prisms in deposits in hot springs – occur. These layers somewhat protect the concrete from other processes, which include attack by Magnesium, Chloride, Sulphate Ions and Carbonation. Above the water surface, mechanical damage may occur by erosion from waves themselves and/or sand and gravel they carry and by crystallization of salts (NaCl) from the seawater soaking into the concrete pores and then drying up inside the concrete matrix.
Pozzolanic extenders like fly ash or slagment in concrete and cements using more than 60% slag as aggregate are more resistant to sea water than pure Portland cement. Seawater corrosion contains elements of both Chloride and Sulphate corrosion.
3g. Chemical damage – bacterial corrosion:
Bacteria themselves do not have noticeable effect on concrete. However, anaerobic bacteria in untreated sewage tend to produce Hydrogen Sulphide, which is then oxidized by an aerobic bacteria present in the bio film on the concrete surface above the water level to Sulphuric Acid. This Sulphuric Acid dissolves the carbonates in the cured cement and causes strength loss, as well as producing sulphates that are harmful to concrete.
Concrete floors lying on ground that contains Pyrite (Iron (II) Sulphide) are also at risk. Using limestone as the aggregate makes the concrete more resistant to acids, and the sewage may be pre-treated by increasing the pH, oxidizing, or precipitating the sulphides in order to inhibit the activity of sulphide utilizing bacteria.
Penetron products prevent this.
4. Freeze thaw cycle:
When concrete is exposed to temperatures below ‘0’ , the moisture inside the concrete freezes. When water freezes it expands and this creates a micro crack. When the ice melts, it penetrates deeper into this crack. This freeze/thaw cycle repeats and the cracks get wider and deeper with each repetition. This water carries Salts, Chlorides and other harmful chemicals with it deeper and deeper into the concrete matrix causing damage to the cement binder as well as to the embedded reinforcing.
Penetron can prevent this damage to concrete caused by freeze/thaw cycles.