|SCIENCE TRIBUNE||Thursday, March 6, 2003, Chandigarh, India|
waste to wealth
When flyash is used in concrete, it acts like cement due to spherical glassy particles and actually replaces certain percentage of the Portland cement normally used. Till recent past, flyash replacing 15% of cement in the concrete was practised, but today, we can replace 50% or more. Concrete with high percentages of flyash looks and finishes the same as regular cement based concrete. Using high percentages of flyash is not only economical but it also improves the durability of concrete. A potent illustration of the effectiveness of pozzolanic material utilised in construction are the Roman structures that are still standing after 2000 years. The Pantheon and other concrete-roofed structures used volcanic ash similar to flyash for their concrete, and these structures are still strong.
While most concrete incorporates up to 15% flyash, studies have shown that high percentages i.e. 50% or more flyash can also be used to obtain the improved performance i.e., reduced permeability and increased durability and so on. This results in a product known as "high volume flyash" (HVFA) concrete. In the past, builders have been reluctant to use higher percentages of flyash because it reacts slower than Portland cement, providing the potential for delays in construction time resulting in lesser strength. However, the mix can be designed to catalyse and compensate for the different reaction speed by keeping the total amount of water very low.
Flyash particles below 10 microns provide the early strength due to higher reactivity needed in concrete, while particles between 10 and 45 microns react slowly. Most flyash supplied to the concrete industry contains 80 to 85% fines. Larger micron particles are less than 15%. More than 50% of particles are smaller than 10-micron which are excellent water reducers and produce durable concrete and increase workability.
The key to successfully using flyash in concrete is in the mix design. The mix design will vary from one source to the next since it depends on the mineralogy of the materials being used, including the chemical composition and other characteristics of the flyash, cement, sand and aggregates.
There are two ways to successfully use high volume flyash in concrete. The first uses superplasticiser to get the necessary workability in the concrete while keeping the water content very low (0.32 water-cement ratio). A typical mix design includes up to 60% flyash. Because the cement content in this mix is less than 300 pounds, the setting time is somewhat longer than that required for a typical concrete mix. However, it generally produces adequate strength (10 Mpa or 1500 psi) within one day, which is acceptable in the building industry. This mix results in a durable, environmentally friendly concrete, but the superplasticisers are a bit expensive.
The second method, pioneered by Dr P.K. Mehta in San Francisco, CA, uses a slightly higher water-cement ratio (0.45), with 50% flyash and a common water-reducing admixture. It was established, when used in high percentages, the flyash itself acts like a superplasticiser - no other superplasticisers are required. Dr Mehta also discovered that early strengths are determined only by the amount of water in the concrete and the cement quality. As long as the concrete contains at least 300 pounds of high early strength cement and a low enough water content, the HVFA concrete will give reasonably good strengths even at an early stage.
HVFA concrete mixes may gain strength slightly slower than ordinary mixes, but they continue to gain strength for a longer period of time and ultimately end up significantly stronger. Also, curing is very important for concrete and is rarely done adequately. If good quality concrete of any kind is needed, and especially HVFA concrete, curing is essential in a methodical way, preferably wet curing.
The construction industry always seems to resist change and this prevents improvement. It will take some perseverance to get everyone on the team educated about high volume flyash, but it is perfectly feasible. HVFA mixes can be used for most applications.
Once on the job site there are a couple of things to remember. The key to get good strengths is to keep the water content very low. In case a higher slump is needed, use more water-reducing admixture - do not add more water.
Another benefit of using flyash in concrete is that flyash makes beautiful, "architectural" concrete. Flyash of today is light in colour and its extreme workability ensures smoother finishes.
HVFA concrete promises many benefits. Flyash is cheaper than cement, so the cost of ready mix concrete will be reduced by 5 to 10%. Other advantages include:
The cost of HVFA concrete road is comparable to bitumen road. The life of HVFA concrete road has been estimated at least 30 years without major maintenance. Apart from this, there are many plus points over bitumen roads, like:
for better pavements
Asphaltic (Bitumen + Aggregates) pavements are a crucial part of our nation’s transportation network. Asphalt construction is fast and relatively simple, it is economical, and the materials used for it are widely available.
Asphaltic pavements generally fail due to: Stripping of the aggregates (Stripping occurs when the bond between the asphalt cement and the aggregate breaks down due to the presence of moisture, and the binder separates from the aggregate), Due to rutting (Rutting is permanent deformation of the asphalt, caused when elasticity is exceeded), Cracking due to weathering (weathering occur over time to generate a brittle pavement).
Asphaltic concrete can be optimised in many ways to create high performance pavements. Hydrated lime (lime produced by just adding enough water to quicklime to slake it), is one of the modifiers that improves performance of asphaltic pavement to overcome the above shortcomings.
Hydrated lime is the most effective anti-stripping agent available, and is universally used to deal with serious stripping problems. Certain types of aggregates are particularly susceptible to stripping. When lime is added to hot mix, it reacts with aggregates, strengthening the bond between the bitumen and the stone, while it treats the aggregate, lime also reacts with the asphalt itself. Lime reacts with highly polar molecules that can otherwise react in the mix to form water-soluble soaps that promote stripping.
The hydrated lime is able to make an asphaltic mix stiffer and resistant to rutting. Hydrated lime significantly improves the performance of pavement in this respect. Lime is a chemically active filler. It reacts with the bitumen, removing undesirable components at the same time that its tiny particles disperse throughout the mix, making it more resistant to rutting and fatigue cracking. The addition of the lime will not, however, cause the mix to become more brittle at lower temperatures. At low temperatures the hydrated lime becomes less chemically active and behaves like any other inert filler.
Another benefit that results from the addition of hydrated lime to many asphalt cements is a reduction in the rate at which the asphalt oxidises and ages. This is a result of the chemical reactions that occur between the calcium hydroxide and the highly polar molecules in the bitumen. If left undisturbed in the mix, many of those polar molecules will react with the environment, breaking apart and contributing to a brittle pavement over time.
Hydrated lime combines with the polar molecules at the time that it is added to the asphalt and thus, they do not react with the environment. Consequently, the asphalt cement remains flexible and protected from brittle cracking for years longer than it would without the contribution of lime. hydrated lime also reduces asphalt cracking that can result from causes other than aging, such as fatigue and low temperatures.
Synergistic benefits also accrue when lime is used in conjunction with polymer modifiers. In some situations lime and polymers used together can produce improvements greater than each of them used alone.
Generally 1 per cent hydrated lime by weight of the mix is used, and is added to the drum at the same time as the mineral filler. The hydrated lime comes in contact with the aggregate itself, directly improving the bond between the bitumen and the stone, while the balance enters the bitumen. This method is called "dry method" and is the simplest to implement.
Alternatively, lime is applied to damp aggregate in order to insure more complete coverage of the stone than is achieved using the dry method. Lime that does not adhere to the stone is dispersed throughout the mix where it will contribute to the other improvements that have been described and is called "dry on damp" method and is also relatively simple, but driving off the additional water required by the process uses additional fuel and may slow down plant production to some degree.
Lime slurry (mixture of lime and
water) that is also applied at a metered rate to the aggregate,
insuring superior coverage of the stone surfaces. After the slurry is
applied, the aggregate can either be fed directly into the plant or
marinated in stockpile for some period of time, allowing the lime to
react with the aggregate. Because the lime is bound to the stone, it
is also the method that results in the least dispersion of the lime
throughout the rest of the mix.
Why does the sun look
bigger when near the horizon than when it is high in the sky?
This observation applies equally to the moon. The moon also looks much bigger when close to the horizon than when it is high in the sky. There is a consensus that this universal experience has nothing to do with the physics of seeing or the size, or shape, of the image. You can easily check that the actual size of the image is the same, independent of the location in the sky. It is very conveniently done for the full moon using nothing more than a scale held at arm’s length. Do it using a transparent scale with one eye closed. You will convince yourself that the size does not change as the moon climbs up in the sky. It is a bit hazardous for the eyes to try it with the sun, unless you use a certified filter. In any case you can take it from me that physics is not fooled by the proximity of the sun or the moon near the horizon. But we are. This optical illusion has something basic to do with the way our brain interprets images. It uses past experience — some facts, and some prejudices — to give meaning to the visual signals received in the cortex. We are aware of a large number of optical illusions. But the field of psychology is not as certain, or crisp, as mathematics. Take two of the explanations that have been give for this observation. See if they satisfy you.
When the sun, or the moon, is near the horizon we see it in the company of other distant objects — buildings, trees, and hills —whose size our brain knows. These distant objects produce a small image, but the brain is not fooled. It knows how big they are. But the brain is fooled a bit when it gives a similar amplification while assigning a size to the sun — or the moon! Another explanation goes like this:
When we stand out in the open, very far from anything else, we get an impression that the sky is an inverted bowl. This has been so for thousands of years. We do not know the height of this bowl, but get a feeling that it cannot be as much as the distance to the horizon. After all we know that the horizon, where the top of the bowl seems to meet the ground, must be very far because known things look so small when they move close to it. So it seems natural for the intelligent interpreter sitting in our brains to make the sun and the moon appearing near the horizon look bigger than they are! When high up in the sky these objects cannot be as far as the horizon since the bowl is believed to be rather shallow! So the interpreter applies no correction!
I can appreciate the qualitative directions of this explanation, as also the previous one, but the quantitative angle escapes me. Why just so much bigger near the horizon? Perhaps someone knows the answer — or some day in future we will?
How do fish get oxygen in water?
Some oxygen is always dissolved in water.
Fish extracts it out using its gills. You must have noticed that in a home
aquarium we usually keep blowing air into the fish tank. This is to keep the
water "breathable" — that is for creatures with gills.
NEW PRODUCTS & DISCOVERIES
Aerial utility vehicle
In the race to build a so-called personal flying machine, a few developers have got much past the tethered-hop stage, with promises of one in every garage sometime soon. Generations of hopeful flyers have died waiting. But Israel’s Urban Aeronautics at least addresses a key point: A machine like the X-Hawk concept shown here (which the company recently released, saying it’s the design they’ll build) has less chance of serving your average frustrated commuter’s needs than of playing a utility role in commercial and government transport. Projected uses include urban rescue, repair and patrol. The ducted fans mean the machine can safely approach tight areas where helicopters can’t go — for example, as the company’s literature ominously notes, up against "a high-rise building in New York City."
claims it is fast-tracked to actual flight testing because it will
build X-Hawk around already FAA-approved engines and rotors, and say
it has received a patent. If that doesn’t fly, there’s always the
military: The company, with a retired commander of the Israeli Air
Force on its board, says it’s working on a bigger TurboHawk design. —