CD Designs Blog
It is widely recognised that salt has a corrosive nature to most things in life, concrete is not without exception. Salt alone would pose little grave danger to concrete, but the presence of salt on concrete at various times of year, can age the concrete dramatically.
For ease of understanding we will look at the two extremes of temperature when concrete is most susceptible to damage by salt. In the U.K. we have summers which can reach temperatures in excess of 25 degrees C, and in recent years winters which have reached in excess of -10 degrees C. There are a number of elements which need to be present in order for these processes to take place and affect the concrete.
- Water (Precipitation)
- Freeze Thaw Cycles
- Alkali-Aggregate Reaction
At the onset of our concrete hardening Mother Nature starts immediately by unpicking our work. Processes known as physical and chemical weathering are contributing factors which lead to the deterioration of our concrete. Salt has the ability to accelerate these processes. It is important for us (Users) to understand why this is, in order for us not to contribute to the effects of these processes.
Freeze Thaw Cycles (Physical)
This process is a physical phenomenon which puts the concrete in tension. Concrete is weak in tension (when it is pulled) yet very strong in compression (pushed together). In winter when the temperature drops below 0 degrees water freezes, as a result it forms ice crystals which expand by about nine percent its volume.
Concrete, on close inspection is like a sponge, moisture is able to penetrate the surface. If this happens before it freezes, then it does drop below freezing the water in the concrete will expand, creating tension and can blow the surface. Under normal conditions this process does not pose any great danger to our concrete. (see Air-entertainer an add mixture to prevent this from happening)
By applying salt to your drive in winter it will melt the snow and ice and make a salt water mush. The melting action of the salt allows water to enter the concrete. Salt is hygroscopic. It attracts water. It can cause concrete to become more saturated than it would otherwise. The presence of extra water in freezing conditions within the concrete can spell trouble. As the additional water freezes as it expands, it creates pressure greater than can be with stood by the concrete. This usually leads to the concrete sprawling.
If we have a frost which last for five days and temperatures remain below 0 degrees for the duration, then we would only see one freeze thaw cycle. That is why we only notice pipes which have not been lagged properly in our home start leaking, until after the frost has gone, even though the damage may have been done on day one.
Salt on the other hand lowers the Freezing point, depending on the content of salt in the water. The Freezing point can be any where between 0 degrees and -19 degrees. At 0 degrees there would be very little salt in the water and -19 the salt would of saturated the water.
This is where the real damage can happen, because the temperatures fluctuate and melting point fluctuate, there could be several freeze thaw cycles in one night. Each time the saturated saltwater freezes, expands and sheers the concrete. The melt water penetrates deeper into the concrete. This is probably one of the most important reasons not to put salt on your drive. After all, this type of physical weathering has brought mountains down for thousands of years.
With our pattern imprinted concrete we do apply our acrylic sealant, this provides a barrier which helps protect the surface.
Alkali-Aggregate Reactions (Chemical)
For ease of understanding a simple explanation will be offered in order to aid the end user in understanding the process and its effects on concrete. It is by no means exhaustive and some generalisations have been made.
The Chemical Equation (West, 1996)
A simplistic explanation of the chemical equation is:-
- When salt solution is mixed with the cement a reaction occurs.
- Cement produces an alkali solution.
- The aggregates used in the concrete may contain a reactive salt or silica.
- As a result a chemical reaction occurs, which is known as Alkali Silica Reaction.
- This reaction forms an Alkali Silica gel.
- Rain water is the next contributing factor.
- Silica gel is essentially porous sand.
- The silica gel absorbs water, expands and in turn can crack the concrete.
- Silica gel can absorb about 40 percent of its weight in moisture.
- Once these cracks reach the surface they provide channels for more water to penetrate deep into the concrete.
- De-icing salts can further augment the alkali already present in the concrete.
The pressure created by the expanding silica gel is greater than both the aggregate and cement paste which form the concrete, therefore blast it apart. While this chemical reaction naturally occurs in our concrete, we have little control over inherent raw materials which make up concrete. It is though, essential that us, as users are aware of the negative effects additional salt has on our concrete and is something we can control.
In summer it is more likely that compounds such as fertilisers, weed killer which may contain salts, exaggerate the above effect (Chemically), by producing more Alkali Silica Gel. Creation of the silica gel may initiate the concrete to crack but the greatest damage is caused by hydration of the silica/salt crystals expanding with in the concrete.
If salts are spilled on our concrete it can and will accelerate the deterioration of your driveway. In wet periods salts will be absorbed by rain water which will in turn be absorbed by the concrete. Once the concrete dries, salt crystals will be left within the concrete. It is not until the salt crystals in the concrete are rehydrated by the rain that they grow. It is this expansion which creates forces which busts the concrete. Repeated periods of evaporation and hydration increase the deterioration of our driveways.
Silica gels are commonly used in products we buy, it holds and absorbs water. You would find little packets in any thing that would be affected by excess moisture or condensation. E.g. leather products, electronics.
What can we do about it?
We use a clear acrylic seal on our driveways. This creates a barrier between the concrete and the world.
Protection is the primary reason for sealing our driveways. A by product of the seal is that it enhances the colour, this is commonly mistaken as the only reason for sealing the driveway and therefore is the most criticised due to poor maintenance.
The acrylic seal is only a couple of millimetres thick and is the wearing course. It is tough and designed for tyres of cars and pedestrian traffic. It will scratch, chip and wear revealing the concrete underneath. The deterioration of the seal is not a failure of the product but is proof that it has served its purpose (protecting the concrete). These scratches, chips and wears will highlight the need for rejuvenating.
This is the theory behind the chemical reaction which ultimately leads to the mechanical destruction of our concrete. It is important for us as end users to understand this, so we are prepared to avoid spillage of any contaminating materials and maintain the protective coating we put on our pattern imprinted drives.
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21st January 2011