No Fine Concrete:Possibilities of Use in Indian Conditions
No fine concrete, (NFC) an innovative/modified form of cement concrete (also known as pervious concrete) is a environment–friendly and lower cost material which can be used for the construction of several types of structures with great advantages. Low loading intensity parking pavements, footpaths and walkways have been built on a large scale in many developed countries and their performance has been found excellent. Recognized as the best management practice by US Environment Protection agency (EPA), it is capable of controlling first flush pollution and storm/rain water overflow by allowing the received water to percolate down to earth and then seeping down to the ground water table. In parking lots where spillage of oil/coolants/antifreeze etc is a regular affair and the NFC collect and arrest these things during rains/flooding/washing etc. Preventing their washing off into streams, rivers and water ponds/tanks and polluting these valuable water sources. The arrested polluted water percolate into the ground and soil chemistry and biological elements in soil take care of these naturally. It also reduces the runoff from the pavement areas hence reduce load on drainage systems.
India is facing a typical problem of ground water table falling at a fast rate due to reduced recharge of rainwater into subsoil and unplanned water withdrawal for agriculture and industry by pumping. NFC if adopted for construction of pavements, platform/walkways, parking lots designed for lighter load, can help in improving recharging of rainwater. It can be– come a component of rainwater harvesting schemes being propaged by Government of India on a Priority basis.
What is NFC?Cement concrete is a composites material made of
- Fine Aggregate (also known as sand)
- Coarse aggregate (also known as Gitti)
- Water and selected admixtures-containing Mineral admixtures (like supplementary cementitious materials like Flyash, Silica Fume, Rice Husk, Slag, Metakaolin)–Chemical admixture (like plasticizers accelerations, retarders, water proofers, bond improvers, shrinkage compensating and so on).
The process of mixing, placing and vibrating generally remains same but it needs very high degree of controls in mix proportioning, water–cement ratio and during transportation. From aggregate to aggregate bonding is vital and the cement slurry mixed with chemicals is the key to this effect. In case the water added is more than the desired slurry will have a running down effect leaving aggregate particles unbonded resulting poor quality, and poor abrasion resistant concrete which will not set in place. Hence design mixed NFC can best be produced in RMC plants and then transported to site. Efforts to produce site mixed NFC have not been very successful in most of the cases.
NFC is a performing engineered concrete produced using controlled amounts of aggregate, water cementitious materials, and chemical admixture to create a mass of aggregate, particles covered with a thin coating of paste and bonded to each other firmly.
Durability and Strength of NFC
There is very small amount of drying shrinkage in its placement when compared to dense concrete. The drying shrinkage development is much sooner in NFC. There are far less cracking hence crack-preventing control joints can be very few and concrete appearance is much better. Depending upon the mix design and placement technique, NFC can develop strength even above (3000lbs/" Squ.Inch) 210 Kg/cm2 which can support moderate loads. It will require properly designed and executed subgrades. We should always remember NFC with high porosity will have lower strength compared to conventional cement concrete.
How Much VoidsNFS creates a highly permeable mass with inter connected voids that can drain the water very fast. Hardened mass has a void percentage of 15 to 25% which can provide a water flow rate around 0.34 cm/second.
UseIt is the best option for construction of pavements for low intensity/load driving side roads, parking pathways, walking tracks or as a water retention basin for storing/arresting the rain water/storm water flows and allowing its recharge into ground. Residential Roads, allys, driveways, sidewalks, parking areas, low water crossings, sub base for conventional concrete, Patios, artificial reefs, slope stabilization, well lining, tree guards, Z00/green house structures, noise barriers, etc, Figures 2, 3, 4, 5, 6,7,8,and 9 show various stages of construction and application.
Use of Recycled Aggregate in Producing NFC
The construction agencies handling NFC need specialization which can be gained only through hands on experience. The material is generally finished with a steel pipe roller which compacts it without sealing pores. A leading RMC manufacturing in NCR is already producing and marketing NFC (Previous Concrete), and demonstrated it in an exhibition at India Expo in Greater Noida organized by Indian concrete Institute during an International symposium between 14th to 10th December 2008.
Compressive Strength – 3.5 MPa to 28 MPa – Typical value 17 MPaDensity – 1600 kg / m3 to 2000 kg / m3.
ConclusionThis is not a new technology (as was first used in 1852). NFC is receiving attention by the construction industry because of its several advantages, specially high flow rate of water through its body-allowing rain fall to be captured and to percolate into ground and recharge the falling down ground water table, providing a solution to environment–friendly sensitive construction. This is vital for Indian conditions where Govt. is putting lot of efforts to implement ground water recharging techniques. NFC can extremely be a useful material.
Though it has been a dominate application for pavement construction in USA, it has also been used in Europe as a structural construction material for several years, for walls in two story house, load bearing walls for highrise buildings, infill panels for highrise buildings, roads, parking lots, and so on.