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Ever wonder why some roads fail so often - so fast?
Howard Chin
Saturday, June 25, 2011
CONSIDER some simple stuff first. Do you know how to make a dumpling? Does it stick to your hands? No? Why? Because you dusted your hands with flour.
So the number one principle is to coat things with a dry powder and other things won't stick to them.
What is the most common thing road beds are made with nowadays? Marl - and that's decayed limestone. That whitish or faintly yellowish powdery stuff mixed with various sizes of white rocks mixed in, that you see in piles all over the place. When people drive over this stuff when it's dry, you call the bigger boss and tell him to get the sprinkler truck to wet the dust. Then, it sticks to the soles of your shoes when you walk on it. If it's not kept damp, your whole house will be coated in white dust and you and your children will be coughing all day. That's marl.
Remember, exposed marl will easily wash away when rained on. You're a dummy if you don't know this. In fact, engineers know that it's a really bad thing to let the marl in the road bed become soaked with water.
Next, what is most often put on top of a marl road bed nowadays?
A sprayed-on asphalt dissolved in a solvent is next, also called cut-back. This is supposed to make the next stuff stick on to the road bed. Next they apply what looks like a medium to small mix of sand in asphalt called asphaltic concrete, and it's rolled nice and flat. You can see this kind of construction in photos 1 and 2, and how it has failed. Typical of the many failures we see nowadays, the asphaltic concrete layer develops small openings to the marl road bed below, then lifts and breaks up in slabs, exposing more of the easily washed out marl below. What eventually forms is a massive "front-end-breaking pothole" as seen in photo 2. It can then proceed to develop fully into the "car-swallowing" variety. If it's a patch job, they might just spray on a layer of hot asphalt and drop some gravel on it, roll it, and go away.
Let's back up for a minute. Older folk might remember, and if you are younger, you can ask them what old-time asphalt was like. They'd tell you that you had to heat up a drum over a fire to melt it before the workmen could splash or spray it on.
Old-time tar, if you hit it really quickly, broke like glass, but if you pushed a stick into it, it would slowly make a dent. Also, if you were stupid enough to leave a piece of tar on your driveway, after a few weeks it would be stuck on and might come off with a piece of your driveway if you really tried to pry it off.
Nowadays, asphalt used on roads seems to act a bit peculiar. What used to be called asphaltic concrete was nearly as hard and as impervious to water as concrete, and stuck firmly on to the road bed stones. Roofing tar nowadays is still very much like "old-time tar" used to be.
Look at a slab of today's asphaltic concrete in photo 3, you can see some small stones stuck to the bottom, but there are areas with nothing stuck on.
In photo 4, I was able to bend, and break (!) the slab with one finger. (This is typical as I have seen with a sample taken from Old Hope Road near the gas station at the corner of Mountain View when the area was being paved years ago. It was left at the Bureau of Standards in a labelled cardboard box, but I think it's been lost.) I was able to see that the slab was slightly porous as well, so that water could seep through to the marl road bed.
In the olden days, before modern machinery to lay asphaltic concrete mixtures, melted old-time asphalt was spread all over the rolled two-inch (5-cm) rock road bed, before applying alternating layers of smaller gravel-sized stones and tar and stones, and rolling it. Look at an old road and you may be able to make out that some of the smaller stones in the top layer look like rounded river pebbles.
Here's a magic trick - how to make a new road disappear:
Want to see a road wash away? Just before the rainy season, put down a base of marl. Moisten it. Roll it. Let it dry hard flat and dusty. Then spray on cut-back tar with one or more of the spray nozzles clogged up. Put on a layer of asphaltic concrete and roll it flat, but don't squash it all the way down as you should, so that it's porous like a sponge.
Next, block up the drains that keep the water from covering the road, and don't "camber" the road. That is, make it a little higher in the middle so the water runs quickly off to the side drains (if they're there).
Here's something else to understand: Have you ever walked on the sidewalk while it's raining? Then, had a car, or worse yet, a truck whizz past? Didn't that feel like somebody turned a high pressure garden hose on you? Well, when there's water on the road, and a car or truck drives over it, the water has to move from underneath the tyres. The faster the vehicle is going, the faster the water has to move from underneath, so the higher the water pressure under the tyre - until it can't escape fast enough. Even better, when the tyre is, as we say "bald" so the water can't get out through the tread grooves in the tyre. Then, bingo, the tyre floats on a film of water and the driver says afterwards, "Boss, ah doan know wah 'appen. Ah pick up a skid and di tree lik me car. Ah swear!" Really? We know better.
Now the new road can disappear as if by magic!
Why? Because the underlying marl became soaked by water due to poor drainage softening it. High pressure water trying to escape from under the tyres then digs out some of the soft, weak, and porous asphaltic concrete until the water can directly get in and properly remove large amounts of marl. That was a first-class magic trick, wasn't it?
Of course, if we made roads this way, it would help the economy by keeping the money constantly circulating from us, then to the government, then to the contractor who would be, after all, one of us. Right?
On the other hand, if you are lazy and don't want to come back and do this spot, over and over again (especially if it's in some place that could be described as the back side of nowhere), you might consider using a road base that won't soften up and wash out, a top that won't let water through, and camber or slant it a little so the water runs off before it has the opportunity to soak the road bed and damage the road. Mind you, if you used a really thick slab of rock or concrete as the base for the road, this would be ideal. A bit expensive though.
It's not rocket science, but maybe we forgot that there are some nice old roads built when your grand-daddy was a boy, which, even today, have water running over them regularly and have the entire top asphalt layer worn off, which are still in use. Maybe not for much longer though, if we don't replace the top.
So now you understand all about road repairs, road construction and economics.
Howard Chin is a member of the Jamaica Institution of Engineers.
hmc14@cwjamaica.com
Read more: http://www.jamaicaobserver.com/colum...#ixzz1QIOV76L2
Howard Chin
Saturday, June 25, 2011
CONSIDER some simple stuff first. Do you know how to make a dumpling? Does it stick to your hands? No? Why? Because you dusted your hands with flour.
So the number one principle is to coat things with a dry powder and other things won't stick to them.
What is the most common thing road beds are made with nowadays? Marl - and that's decayed limestone. That whitish or faintly yellowish powdery stuff mixed with various sizes of white rocks mixed in, that you see in piles all over the place. When people drive over this stuff when it's dry, you call the bigger boss and tell him to get the sprinkler truck to wet the dust. Then, it sticks to the soles of your shoes when you walk on it. If it's not kept damp, your whole house will be coated in white dust and you and your children will be coughing all day. That's marl.
Remember, exposed marl will easily wash away when rained on. You're a dummy if you don't know this. In fact, engineers know that it's a really bad thing to let the marl in the road bed become soaked with water.
Next, what is most often put on top of a marl road bed nowadays?
A sprayed-on asphalt dissolved in a solvent is next, also called cut-back. This is supposed to make the next stuff stick on to the road bed. Next they apply what looks like a medium to small mix of sand in asphalt called asphaltic concrete, and it's rolled nice and flat. You can see this kind of construction in photos 1 and 2, and how it has failed. Typical of the many failures we see nowadays, the asphaltic concrete layer develops small openings to the marl road bed below, then lifts and breaks up in slabs, exposing more of the easily washed out marl below. What eventually forms is a massive "front-end-breaking pothole" as seen in photo 2. It can then proceed to develop fully into the "car-swallowing" variety. If it's a patch job, they might just spray on a layer of hot asphalt and drop some gravel on it, roll it, and go away.
Let's back up for a minute. Older folk might remember, and if you are younger, you can ask them what old-time asphalt was like. They'd tell you that you had to heat up a drum over a fire to melt it before the workmen could splash or spray it on.
Old-time tar, if you hit it really quickly, broke like glass, but if you pushed a stick into it, it would slowly make a dent. Also, if you were stupid enough to leave a piece of tar on your driveway, after a few weeks it would be stuck on and might come off with a piece of your driveway if you really tried to pry it off.
Nowadays, asphalt used on roads seems to act a bit peculiar. What used to be called asphaltic concrete was nearly as hard and as impervious to water as concrete, and stuck firmly on to the road bed stones. Roofing tar nowadays is still very much like "old-time tar" used to be.
Look at a slab of today's asphaltic concrete in photo 3, you can see some small stones stuck to the bottom, but there are areas with nothing stuck on.
In photo 4, I was able to bend, and break (!) the slab with one finger. (This is typical as I have seen with a sample taken from Old Hope Road near the gas station at the corner of Mountain View when the area was being paved years ago. It was left at the Bureau of Standards in a labelled cardboard box, but I think it's been lost.) I was able to see that the slab was slightly porous as well, so that water could seep through to the marl road bed.
In the olden days, before modern machinery to lay asphaltic concrete mixtures, melted old-time asphalt was spread all over the rolled two-inch (5-cm) rock road bed, before applying alternating layers of smaller gravel-sized stones and tar and stones, and rolling it. Look at an old road and you may be able to make out that some of the smaller stones in the top layer look like rounded river pebbles.
Here's a magic trick - how to make a new road disappear:
Want to see a road wash away? Just before the rainy season, put down a base of marl. Moisten it. Roll it. Let it dry hard flat and dusty. Then spray on cut-back tar with one or more of the spray nozzles clogged up. Put on a layer of asphaltic concrete and roll it flat, but don't squash it all the way down as you should, so that it's porous like a sponge.
Next, block up the drains that keep the water from covering the road, and don't "camber" the road. That is, make it a little higher in the middle so the water runs quickly off to the side drains (if they're there).
Here's something else to understand: Have you ever walked on the sidewalk while it's raining? Then, had a car, or worse yet, a truck whizz past? Didn't that feel like somebody turned a high pressure garden hose on you? Well, when there's water on the road, and a car or truck drives over it, the water has to move from underneath the tyres. The faster the vehicle is going, the faster the water has to move from underneath, so the higher the water pressure under the tyre - until it can't escape fast enough. Even better, when the tyre is, as we say "bald" so the water can't get out through the tread grooves in the tyre. Then, bingo, the tyre floats on a film of water and the driver says afterwards, "Boss, ah doan know wah 'appen. Ah pick up a skid and di tree lik me car. Ah swear!" Really? We know better.
Now the new road can disappear as if by magic!
Why? Because the underlying marl became soaked by water due to poor drainage softening it. High pressure water trying to escape from under the tyres then digs out some of the soft, weak, and porous asphaltic concrete until the water can directly get in and properly remove large amounts of marl. That was a first-class magic trick, wasn't it?
Of course, if we made roads this way, it would help the economy by keeping the money constantly circulating from us, then to the government, then to the contractor who would be, after all, one of us. Right?
On the other hand, if you are lazy and don't want to come back and do this spot, over and over again (especially if it's in some place that could be described as the back side of nowhere), you might consider using a road base that won't soften up and wash out, a top that won't let water through, and camber or slant it a little so the water runs off before it has the opportunity to soak the road bed and damage the road. Mind you, if you used a really thick slab of rock or concrete as the base for the road, this would be ideal. A bit expensive though.
It's not rocket science, but maybe we forgot that there are some nice old roads built when your grand-daddy was a boy, which, even today, have water running over them regularly and have the entire top asphalt layer worn off, which are still in use. Maybe not for much longer though, if we don't replace the top.
So now you understand all about road repairs, road construction and economics.
Howard Chin is a member of the Jamaica Institution of Engineers.
hmc14@cwjamaica.com
Read more: http://www.jamaicaobserver.com/colum...#ixzz1QIOV76L2
Paul Marin -19 is one thing, 20 is a whole other matter. It gets even worse if they win the UCL. *groan*. 
05/18/2011.MU fans naah cough, but all a unuh a vomit?-Lazie 1/11/2015
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