Ultra-high durability levels are being imparted to conventional concrete mixes with a simple addition of nano-sized materials. This is helping to construct more durable infrastructure and significantly prolong the service life of existing structural assets.
Colloidal silica post-placement pozzolan (P3), “colloidal nano silica” or “nano silica” is a cutting-edge nano technology. It has been used for more than two decades throughout the world, including in South Africa, to make the oldest commodity-based material more durable and, therefore, longer lasting.
This includes municipal service-delivery infrastructure, such as wastewater-treatment plants and sewers.
More recently, colloidal silica P3 is being specified for manhole repairs. The technology is used to neutralise and remove contaminants from existing concrete before the manholes are repaired. When applied to the entire manhole area, corrosion potential can be substantially reduced. This, in turn, potentially increases the service life of the infrastructure by 20 years or more. A chemical-resistant coating is generally applied over colloidal silica P3-treated concrete in wastewater-treatment plants, as well as sewers and manholes.
These amorphous silicon dioxide particles are less than 100 nano metres in size and suspended in water. They are spray-applied to horizontal, vertical or overhead concrete slabs after they have been cast. This single-spray application is permanent.
Colloidal silica P3’s small size provides a tremendous amount of reactivity and pozzolanic potential – even greater than that of un-densified silica fume. This reaction takes place in the capillary voids and pore space that are created in concrete as bleed water exits the construction material. In this way, they are filled with more calcium-silicate hydrate (C-S-H). C-S-H is the same reaction product that provides concrete with its strength and durability traits. Even under hydrostatic pressure, the movement of water through concrete is restricted. This waterproofing action considerably reduces water-borne contaminant ingress, including the transport of chlorides, through concrete.
Chlorides damage the protective hydroxide layer on reinforcing bar. Also referred to as “passivation”, this layer is formed by the high alkalinity of concrete. Once it has been compromised, wet or dry cycles accelerate the continuous corrosion process. This is the lead cause of premature concrete failure.
By extending the period it takes for chlorides to reach reinforcing bar, years can be added to the lifecycle of concrete structures.
Three laboratories have tested colloidal silica P3 technology’s ability to significantly deaccelerate the rate of chloride through concrete. Using chloride diffusion testing, chloride concentrations at various depths were ascertained. These informed the calculation of an average diffusion coefficient that predicts the length of time it will take chlorides to reach reinforcing steel and penetrate the passive layer around it. This modelling was undertaken with sophisticated software, including Life 365. The simple and transparent model is used by many design consultants to estimate the service life and lifecycle costs of alternate protection systems in the design of reinforced concrete structures that are exposed to chlorides.
Notably, colloidal silica P3 achieved a 69% reduction in chloride diffusion coefficient for concrete with a water-to-cement (W/C) ratio of 0,57 and 0,45. For concrete with a W/C ratio of 0,40, a 75% reduction in chloride diffusion coefficient was achieved.
Certainly, colloidal silica can also be applied as an admixture to concrete as it is being produced.
However, many contractors still prefer to treat concrete slabs after they have been cast with colloidal silica P3. This allows the particles to penetrate the concrete deeply after the voids have been formed and close them in the interaction zone.
According to Carl White, Managing Director of Spraylock Africa, 99% of the world’s concrete is placed with excess water because it is needed to achieve the initial chemical reaction, despite the use of effective chemical admixtures.
“The permanent bleed-water channels created in the concrete as a portion of this excess water exits increases the permeability of the construction material. Concrete permeability and durability are intrinsically linked. Concrete that is less permeable is going to be more durable and will, therefore, last longer and fulfil its service life capabilities at a low maintenance cost,” he says.
In addition to serving as a very effective base waterproofing system and densifying concrete, there is another important way that colloidal silica P3 raises the durability of concrete. This is by facilitating optimal curing.
Curing is an essential component of quality assurance and control during construction. This is considering the significant role that it plays in ensuring high-strength gain; minimising thermal, plastic and drying-shrinkage cracks and making concrete more water-tight; and improving abrasion resistance. Moreover, proper curing advances the microstructure of concrete by assisting the cement-hydration reaction to progress steadily. The calcium-silicate hydrate gel that is created during this process binds aggregates to form a rock-solid mass. This makes concrete denser; decreases porosity; and enhances the physical and mechanical properties of concrete.
Colloidal silica P3 retains water that would normally evaporate. The technology’s ability to facilitate continued hydration is the improved compressive strength and significant drying shrinkage reduction of concrete treated with the technology. Colloidal silica P3-treated concrete even outperforms moist-cured concrete. Moist curing is considered the “gold standard” of curing practices.
Notably, more than 100 research papers authored by over 15 research teams have demonstrated the improved properties of concrete containing colloidal silica that has either been applied as an admixture during mixing or after slabs have been cast. Moreover, two ASTM working groups have been assigned to just work on colloidal silica specifications.
Formerly known as the American Society for Testing and Materials, ASTM develops and publishes voluntary consensus technical international standards for a wide range of materials, products, systems and services.
Notably, a representative of United States-based SprayLock Concrete Protection (SCP) serves on one of these working groups. SCP is the foremost manufacturer and supplier of colloidal silica P3, which conforms to the EN1504-2 durability standard. The company’s spray-applied products have earned a solid reputation the world over for their ability to protect concrete from contaminants.
The United States has been at the cutting-edge of colloidal silica research, not least of which are the significant efforts made by the US Army Corps in the field since the early 2000s. This stimulated worldwide interest in the technology.
Even in a highly developed country such as the United States, concrete structures have not been performing as they should. The leading cause of the premature failure of concrete structures in the country is as a result of alkali–silica reaction. This can be attributed to higher alkali cements, more reactive aggregates and ineffective pozzolans.
About USD48-billion is invested in new concrete structures in the United States every year. Yet, USD8,3-billion is spent annually maintaining them as a result of physical and chemical attack.
According to the American Society of Civil Engineers’ Infrastructure Report Card of 2012, one in six of the more than 600 000 concrete bridges in the country were in severe need of replacement or repair at the time. Meanwhile, the findings of a study undertaken by the American Society of Military Engineers in 2015 revealed that the mix designs for 40% of the country’s concrete bridges had a life expectancy of less than 30 years. At the same time, the International Concrete Research Institute expects building repair costs in the United States to be about USD20-billion a year, with most structures only expected to last between 50 to 60 years. In areas of the country where de-icing salts are used on sidewalks and pavements, this infrastructure only lasts for five years. Where the salt melts, snow build-up creates saltwater slush that enters concrete. Once inside the concrete, it refreezes. This additional pressure starts breaking up concrete. In addition, salt water itself contains magnesium chloride, as well as sulphate and hydrogen carbonation ions. This corrodes the concrete and rebar.
SCP technology has been very effective in safeguarding new and existing concrete that is exposed to fresh and salt water. This includes bridges, dams, piers and other concrete structures that are located within 0,8km or more of a body of water. The company’s top-of-shelf formulation improves concrete durability of structures exposed to harsh marine conditions by 90% or more. This is according to the EN 12390-8 testing standard. It specifies the method for determining the rebound number of an area of hardened concrete using a spring-driven hammer. SCP also improves chloride diffusion by 50% and reduces forced chloride ingress by 25% or greater, according to the ASTM C1556 and NT Build 492 testing standards, respectively. ASTM C1556 is the standard test method for determining the apparent chloride diffusion coefficient of cementitious mixtures by bulk diffusion. An alternative to ASTM C1202, the NT Build 492 laboratory test is used to determine the chloride migration coefficient in concrete and mortar.
“South Africa is also grappling with a growing infrastructure backlog, while existing service delivery assets are deteriorating because of insufficient maintenance. Therefore, the focus is increasingly on extending the life of existing and developing new infrastructure that continues to add value over its entire lifecycle while reducing maintenance costs. SCP products enable this,” White concludes.
