A nano-sized silica-dioxide particle, colloidal silica is significantly finer than other Pozzolans, including silica fume or even micro-silica. SCP’s small size, combined with the unique distribution package used, enables the colloidal silica post-placement pozzolan technology to reach all of the interconnected pore spaces in concrete and provide a larger reactive area. Supplying amorphous silica dioxide, colloidal silica is ready to react with calcium hydroxide to form more calcium-silicate hydrate. This is the reaction product that provides concrete with its strength and durability traits and, in this way, essentially filling the bleed water channels with more concrete.
This is according to Brent Rollins, the Vice President of Business Development for Spray-Lock Concrete Protection, who was invited by Cement & Concrete SA to give an insightful presentation on how colloidal silica improves concrete durability. The webinar was well attended by South African contractors, engineers and client bodies, who were keen to learn more about the company’s state-of-the-art colloidal silica post-placement pozzolan technology.
“The primary way in which concrete is attacked is when water containing contaminants, such as sulphates, chlorides and nitrates, enter through the bleed-water channels that are created when the construction material sets. Some of these contaminants attack the concrete directly, and others, like chlorides make their way to the rebar upon which they have a corrosive effect. Finding a way of halting the corrosion of reinforcing steel will, therefore, extend the longevity of concrete. Generally, the lower the permeability of concrete, the greater its durability and vice versa,” Rollins explained.
He said that concrete had changed significantly since the 1970s. Contractors used less admixtures and had cements that required a lower water content to gain the necessary compressive strength. Durability, therefore, was often a given in the concrete mix design.
Since then, contractors have been able to achieve strengths by using less cement content, which has been ground finer than before, in their concrete mix designs. “We are now able to use higher water-to-cement ratios and achieve the strengths that were previously impossible. However, we now have to consider durability as a completely separate variable in the concrete mix design. Just because you have achieved the desired strength, does not necessarily mean that you have reached the durability targets,” Rollins said.
This is a worldwide problem. For example, a study undertaken by the American Society of Civil Engineers in 2012 revealed that one in six concrete bridges in the United States needed major repairs. To provide an indication of the extent of the challenge, the country has more than 600 000 bridges. According to another study in 2015, 40% of the country’s states were using a standard concrete bridge deck mix design shown by computer modelling to have a corrosion-based lifecycle of less than 30 years. While these structures had all achieved their compressive strength target, many were only expected to last for 30 years.
Meanwhile, the Concrete Repair Institute estimates that between USD18-billion and USD20-billion a year is spent on repairing concrete buildings. Generally, concrete buildings in the country have a lifecycle of 50 to 60 years.
Engaging with many municipalities in various countries of the world, Rollins has also identified serious concerns with the durability of concrete pavements. In instances where de-icing salts are being used on concrete pavements, some have experienced only a lifespan of five years.
He said that this was also had a negative impact on sustainability of the built environment. Building longer-lasting structures reduce the use of finite resources and the carbon footprint of cities.
Rollins explained that SCP could be used in various ways. It can be introduced as an additive at the time of mixing to treat an entire load of concrete. As soon as calcium hydroxide becomes available, it will react. However, it will not be as impermeable as there will be some pore space in the concrete. This is considering that more water is used to create the initial reaction that needs to exit the concrete.
Greatly reducing concrete permeability, one-time treatment remains the best way to use the colloidal-silica post-placement pozzolan technology for concrete durability and waterproofing applications.
SCP is applied when the concrete is ready to receive foot traffic without leaving an impression. Spray-applied on the surface of the concrete, it enters the capillary voids that have formed naturally in the concrete as the bleed water exits and penetrates deeply. The minimum recorded penetration of SCP is 30mm on a 100mm-thick concrete slab. “Using SCP in this manner ensures that you are able to derive the most benefit from our product. This is considering that you are closing the void space early and retaining the evaporable water in the concrete. SCP can, therefore, also be used as a curing mechanism when applied to new concrete. The efficacy of SCP for curing applications can be gauged using compressive strength development and drying shrinkage as metrics. It is even more effective than moist curing for 28 days. This is despite most contractors seldom curing concrete for 28 days. Rarely do you see moist curing undertaken for more than seven days,” Rollins said.
Moreover, the technology can also be applied to existing horizontal, vertical and overhead concrete structures to improve concrete durability. SCP penetrates just as deeply in overhead structures as it does when used on vertical concrete structures.
Rollins concluded by providing examples of the application of SCP in projects in many countries of the world. This includes in the Southern African Development Community, where SCP is represented by Spraylock Africa.