More Than 15 Years Of Successful Result In Producing Durable Concrete Lining
History Of Sprayed Concrete
Dr Akeley wanted to make an equipment which can spray mortar mixture on concrete surface to make parent concrete durable. He made a special chamber working as a pressure vessel which can spray the mortar mixture on the R.C.C. wall surface. The seed concept of today’s sprayed concrete was born more than one hundred years and was known as “CEMENT GUN”. This unique method became popular within a short period that was patented in 1911, which was owned by Cement Gun Company.
Based on this concept different technical modification enriched the system and worked better than the past.
In this case study we will focus on an innovative idea of sprayed concrete in Graphite Storage Silo, which could prove its outstanding durability for a longer period. The technical analysis will be presented accordingly.
How SYLOCON worked to enhance durability of the lining?
In ancient time it was believed that a complex material containing clay with nanopores into which indigo dye was combined chemically to create an environmentally stable pigment. Based on this concept a corrosion resistant azure pigment was produced, known as Maya Blue in AD 800. It was discovered in the pre-Columbian Mayan city of Chichen Itza.
Today’s Nano technology can meet several requirements in different industry. Nano tubes in SYLOCON® has opened a new window to solve the major problems of debonding between sprayed mortar and parent R.C.C. structure in tunnel lining. SYLOCON® is a complex of different Nano tubes. The bonding of Carbon nanotubes are due to the carbon atoms linked in hexagonal shapes, with each carbon atom covalently bonded to three other carbon atoms. The tensile strength of carbon nanotubes is approximately 100 times greater than that of steel of the same diameter. There are two things that account for this strength. The first is the strength provided by the interlocking carbon-to-carbon covalent bonds. After mixing of SYLOCON in concrete the Nanotubes are widely distributed in the cement mortar.Moreover Carbon Nanotubes have capacity to hold maximum weight. It has a low density for a solid of 1.3 to 1.4 g/cm3, its specific strength of up to 48,000 kN·m·kg−1 is the best of known materials, compared to high-carbon steel›s 154 kN·m·kg−1.Its reaction with cement is more intense than that of the larger fibers. Nanotubes in SYLOCON possess a high modulus of elasticity, tensile strength, and yield strain. Those characteristics make SYLOCON an effective reinforcing material for cement mortar that makes concrete durable. The basic technology of SYLOCON is presented here to understand the dispersion and bond linkage of Nano tubes in SYLOCON.
Similarly the ductility, flexural strength and permeability of concrete is of great importance in view of durability. The change in volume of concrete has also a great impact on durability and other utilities of concrete. This is termed as autogenous shrinkage. Different cracks are observed in concrete leads to rejection in quality due to the autogenous shrinkage. SYLOCON contains the nano particles to overcome this problem. The following graphical presentation shows the result by SYLOCON’S control mechanism to reduce this adverse effect of concrete. Different quantity of nano tubes are used to examine the autogenous shrinkage behaviour of concrete. Temperature of concrete due to heat of hydration is recorded with time. This tool helps to design most effective formulation of SYLOCON. SYLOCON containsCarbon nanotubes which are seamless, carbon cylinders which have unique mechanical and electronic properties. The length is varied with single wall or more than one wall. Young’s modulus of those are five times and eight times (theoretically 100 times) the strength of steel, with 1/6th the density. The major advantages are mechanical durability as well as prevention of cracks in concrete. This is well established 15 years back that SYLOCON can produce an impermeable, crack free and durable concrete. Use of SYLOCON in Graphite India Limited storage silo is one of the authentic examples to know its effect. So, this is clearly understood that SYLOCON is the perfect additive for sprayed concrete in tunnel lining.
Detailed morphology of SYLOCON and cement compound strand was studied using TEM. it was verified from this test that there was a physical bonding between cement matrix and CNT. The diameter of the cement compound strand was almost the same as that of CNT.
Water and soluble contaminants penetrate through the pores of the structure and causes gradual degradation of the concrete.
The degradation appears in the form of efflorescence, laitance, spalling concrete and cracks. These facilitate active ingression of water with various ion contaminants transported from higher concentration to lower concentration within the inner pores of the concrete. This ingression can be avoided blocking the pores of concrete. SYLOCON® produces the crystal on hydration reacting with un-hydrated free calcium oxide present in the cement, which blocks the pores and makes concrete stronger, impermeable and durable.
Reaction And Its Result
Four phase cement reaction-
- Cement before hydration.
- Surface products like C-S-H grows outward with connected gel pores.
- Pore products as calcium hydroxide is polycrystalline and fully dense with no connected pores.
- Capillary pores- the water filled space between the solid phases.
As soon as water is added to cement, rapid heat evolution takes place. Within few minutes, this is decreased and remains constant in plastic stage up to 3-5 hours until the concentration of the ions in the solutions before starting the precipitation of products of hydration.
When the maximum heat is evolved, CH crystallizes from the solution and a slow coating process of C-S-H product is started, which covers unreacted C3S grains. As the hydration goes on, this coating becomes thicker and does not allow water to contact the un- hydrated core particles.
Nano particles of SYLOCON® contacts all the particles of cementing material and produce the crystal products under controlled hydration, which fill up pores and capillary pores The capillary pores are known to be responsible for the impermeability of concrete. The size and number of pores are greatly reduced. As a result, the strength of concrete is also increased.
Scanning electron microscope shows the pores in concrete, development of different sizes of crystal in cement mortar after hydration. Nano particles are observed in the scanned photograph.
Scanning Electron Microscopic Views Of The Mortar With And Without SYLOCON®
The sample is prepared without any additive.
X-ray Diffraction Test Of SYLOCON
After the product is added to the cement mortar, a different pattern is observed. This indicates the growth of the new hydration products in the concrete.
The crystals developed as hydration products fill up the capillaries. The following diffractograph shows the new properties developed after hydration.
Microanalysis Study On The Products Of Hydration With And Without SYLOCON®
The microanalysis report of the mortar specimens show no development of any new product after hydration. A foreign carbon particle from any external source is present in the concrete without SYLOCON®. But, the chemical composition of the concrete is unchanged after the reaction of cement and SYLOCON®. So, the waterproofing properties developed in the treated mortar are due to the presence of the same nature of crystal formed in the capillaries and the pores of the concrete. These crystals of different shapes and sizes plug the water ingression path without changing the basic properties of concrete. In the microanalysis report this is very clear, only the percentage of the elements are changed.
Heat evolution during hydration
Heat evolution during hydration is plotted here.
The rate of heat evolution curve in the presence of SYLOCON® is slightly lower than the curve of a normal mix. Total heat evolved during the hydration is plotted here. This indicates the retardation of hydration.
From experimental results, it appears that in the presence of, mix some nano particles are formed, which the products of hydration are. The extremely nano size particles have some favourable characteristics. Those fill up the pores and increase the strength of concrete. The microstructures of interface of cement and aggregate are greatly improved. This hinders the water percolation through the concrete.
Uses Of SYLOCON In Structure
The practical examples and experiments has established that SYLOCON is the perfect combination for lining with cement mortar or concrete to enhance bond strength and durability of concrete tunnels and silos.
To be continued……………..