FAQ

Penetron® should be applied to concrete or block structures exposed to potential water or chemical attack and this requiring permanent waterproofing and protection. It is applicable to either the positive side (side exposed to water) or the negative side (side opposite water) and meets all waterproofing requirements. Here are just a few examples:

• Drinking water reservoirs
• Sewage and water treatment tanks
• Aquariums
• Tunnels
• Foundations
• Elevator shafts
• Underground vaults
• Industrial installations
• Parking decks
• Traffic bearing structures
• Base slabs
• Diaphragm walls
• Basements
• Concrete roofs
• Bathrooms

The size of the molecule depends solely on the inter-molecular chemical bonds between the two (2) hydrogen atoms and the one (1) oxygen atom, and not on the forces that act between the H2O molecules. Therefore, the simple answer is the molecule remains the same size no matter what the state. The more complex answer is that bond lengths are a result of interactions between the atoms inside the molecule.

The measurement of 95.84pm or 104.45 degrees does not alter in the gas or liquid state. A picometer – symbol pm – is a unit of length in the metric system equal to one-trillionth (1/1,000,000,000,000) of a meter.

In the gas phase, each molecule is far from neighbouring molecules and so this interaction is not affected be neighbouring molecules. However, in liquid phase, molecules are much closer to one another, meaning that an atom in one molecule can affect the binding of an atom in another molecule.

One can calculate bond lengths in gas phase, but it is much more difficult to make the quantum mechanical calculation in liquid phase, owing to the interactions between molecules in the liquid phase.

The difference between gas and liquid phase is not significant, so the simple answer is usually better. The molecule itself does not change it is the specific relation to other water molecules that changes.

In the liquid phase, all the molecules are closer together. In the gas phase all the molecules are further apart but the molecule remains the same size regardless of the phase.

Water in ice phase expands owing to the molecules all aligning in a geometric formation and not randomly as in the liquid phase, thus creating a larger mass.

This depends entirely on the following:

• The concrete mix design,
• The type of cement used,
• The amount of water present,
• If Penetron® was applied to the negative or positive side.

Therefore, we cannot give a definitive answer, as there are too many variables.

If the slurry is applied to dry concrete, the rate of growth will be very slow.

In a constant wet environment, growth would be continuous and penetration will be much faster. Application on the positive side will result in faster penetration as there will always be water present to stimulate the growth of the crystals. Water from the negative side will not promote growth inside the concrete once all the existing water has dissipated.

Suffice to say, if the concrete is of a good quality, has been compacted properly and is kept damp from the negative side, crystal growth will be sufficient to dry out the concrete internally within 7 to 21 days.

The depth of the crystal growth can only be determined by EM analysis.

In all but the least critical applications, care needs to be taken to properly cure and protect concrete in order to achieve the best strength and hardness.

This happens immediately after the concrete has been placed. Concrete requires a moist, controlled environment to gain strength and harden fully.

The theory behind protection and curing: Cement paste hardens over time, initially setting, becoming rigid, though initially very weak, and gaining in strength in the weeks following. At around 3 weeks, typically over 90% of the final strength is reached though strengthening will continue for decades.

The conversion of calcium hydroxide (the by product of the cement reaction) in the concrete into calcium carbonate from absorption of CO2 from the atmosphere over several decades further strengthens the concrete and makes it more resilient to damage. However, this reaction, called carbonation, lowers the pH of the cement pore solution and combined with moisture/water will cause the reinforcement bard to corrode, reducing the life span of the concrete structure.

Controlled hydration and hardening of concrete during the first three days is critical.

Abnormally fast drying and shrinkage due to factors such as evaporation from wind and sun during and immediately post placement may lead to increased tensile stresses at a time when it is not yet gained strength to resist these forces, resulting in shrinkage cracks occurring.

The early strength of the concrete can be increased if it is kept damp and covered during the first three days, further curing processes will minimize stresses prior to strength being gained, and this will minimise shrinkage and cracking.

High-early-strength concrete is designed to hydrate faster, often by increased use of cement that increases shrinkage and cracking. Strength of the concrete changes (increases) for up to three years and depends on cross-section dimension of elements and conditions of structure exploitation. The addition of Penetron Admix will provide an early gain in strength as well as an increase in comprehensive strength beyond what is anticipated and designed for in 28 days.

During this period, concrete needs to be kept under controlled temperature and in humid atmosphere.

In practice, this is achieved by covering with plastic, spraying or ponding the concrete surface with water, thereby protecting the concrete mass from the ill effects of ambient conditions.

Properly protected and cured concrete leads to increased strength and lower permeability and avoids cracking where the surface dries out prematurely.

Care must also be taken to avoid freezing, or overheating due to the exothermic reaction during hydration (setting) of cement.

Improper curing can cause scaling, reduce strength, poor Abrasion resistance and cracking, as well as shorted with life of the concrete structure.

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.

Concrete placing contractors generally place, compact and strike off the concrete and wait for the bleed water to dissipate before they attempt to power float the surface of the concrete. In normal conditions, this often results in the power floating commencing as soon as the concrete is capable of supporting the machine.

In hot, dry or windy weather when evaporation rates are above 1.0 litre per m2 per hour it is often difficult to see the bleed water as it evaporates as quickly as it arrives and it is easy to misread the correct time to finish the surface. The situation can be deceptive, as the rapidly drying surface is normally firm enough to support the power-floating machine well before all of the bleed water has reached the surface.

When concreting in hot weather, it is extremely important to start finishing the concrete at the correct time. This is because starting the power floating too early seals the surface of the concrete before all of the bleed water has risen to the surface. This situation often results in small blisters of concrete around 20 to 30mm in size and 1mm deep (sometimes referred to as laitance) peeling off during the power floating operation. In some cases, you will see larger de-laminations but the depth is always between 1 to 3mm. Another problem can be the discolouration of the surface of coloured concrete as the bleed water brings deposits of calcium carbonate and other salts to the surface, often turning the concrete white.

In circumstances where evaporation rates are high and pouring must proceed we recommend the use of plastic sheeting to control the evaporation of bleed water. The plastic should be applied immediately after the strike off operation. Bleed water will be seen to build up under the plastic sheeting. The primary reason for its use is to prevent plastic or settlement shrinkage cracks from occurring but it can also help with the finishing of the concrete by preventing the surface becoming too dry.

After the bleed water appears to have dissipated, the surface of the concrete should be probed to see if there is more bleed water just below the surface. This water may be as far as 5mm or more below the surface. If there is no bleed water beneath the surface, it is generally a sign that power floating may commence.

Other options for controlling evaporation such as fog sprays, windbreaks and shading should also be considered where they could be used cost effectively.

If the concreter has started the power floating operation too early, it is sometimes possible to flatten the angle of the blades to help release the bleed water and to steepen the angle on subsequent passes where a burnished surface is required in the specification. It is far preferable of course not to start power floating too early.

Mix Design:

High amount of cement paste, fly ash, oxide colouring, and fine sand or even entrained air in the mix can contribute to the rapid sealing of the surface during the power floating operation and it may be possible to reduce one or more of these by adjusting the mix design. The use of a dry shake to colour or harden the concrete can be particularly troublesome, as it will tend to seal the surface and soak up bleed water too rapidly at the same time.

Super-plasticisers in concrete:

If the mix is using a super-plasticiser, the problem can often be severe as the bleed water has a tendency to arrive late and fast. Whenever a super-plasticiser is used, the concreter needs to be involved in the decision and informed as to the likely behaviour of the mix as he may need to wait longer and have more power float machines and operators on hand.

The weather:

When forecasts indicate evaporation losses higher than about 1.0 litre per m2 per hour it is generally preferable to postpone the pour until the weather is more conducive to attaining a high quality finish. Below is a chart that you can use to estimate likely evaporation rates for any given relative humidity, air temperature concrete temperature and wind speed.

Wet down the surrounding area where dust is a problem as dust landing on the slab can rapidly soak up any bleed water and contribute to the problem.

Also, ensure that the sub-grade beneath the slab (if pouring slab on ground) has been soaked but has no free water on the surface. This will provide more bleed water to the surface and help with the finishing.

The above is a brief summary of what is a very complex topic.