How to build a US road
Roads - an integral part of today's urban landscape. For someone roading - enjoying fast comfortable ride for someone the problem and the many hours of shaking with traffic jams on the narrow roads.
Today we will go for a few construction sites in Texas and look at how to build the highway, why concrete pavement is 30 years without major repairs (and in general, why, in fact, concrete?), A well will understand why trucks from 36- ton "tails" are not forced road.
Traditionally, we start with history. The first concrete road was laid down in the United States in 1930, in Indiana, and then in the state of Illinois in 1940. In Texas, the first concrete road only appeared in 1951 in the city of Fort Worth. Texas Department of Transportation (DOT) is very quick to recognize the correctness of the chosen path and began actively laying the concrete road along and across the entire state of Texas, and today is a leader in the US by the length of concrete road, with 20117 km of highway.
All US road can be divided into several large groups, some of which, in turn, consist of classes. So, starting from the most significant and less significant roads are classified into the following groups: Interstate Highways (denoted by the index I), US Highways (US), State Highways (in this case - TX), FM roads (FM, meaning Farm to Market ), Local roads (this group includes the other roads, such as road (Rd), Avenue (Ave), Junction (Jct) and others).
The government, represented by the US DOT establishes certain requirements for the construction of roads, and these requirements vary from class to class roads. I'll note that in contrast to the small transport issues, such as speed limits or road route laying, design standards can not be changed shtatovskih the DOT, all states are subject to US DOT and construction standards.
Also note that in contrast to the design (which in all states is one, signs, signs, markings), construction techniques slightly different from state to state, it is connected with seismic some states, winter conditions, wetlands, and so on. D. Shtatovskih DOT conduct their research on the technology of laying roads and amend the codes DOT construction. In general, the chain of command codes and shtatovkih DOT to the US DOT is very confusing for the layman.
I do not think that ever come back to DOT, and even more so, I will write a review of road construction, because immediately after graduation at the university, I spun the fate of the building in a completely different jungle. But once upon a time I wrote a thesis on the construction of transport bridges for the DOT TX, as it turned out, the old do not wriggle.
So, before you start construction, DOT assigns a public hearing (usually at the courthouse, or in your office). Upon hearing can come to any resident of the region, where it will be built or repaired the road, and to express their views.
The project can be modified by collecting opinions. For example, people can look at the plan and say that you need to close out here the road to the highway, security, or install anti-noise wall along the road, so it was quiet in the adjacent cafe.
Any reasonable offer will be examined and if the people are right, the amendment will be included in the project. The hearing shall be appointed as a rule in the early stages of the project, when done only a draft, in order then to remodel less in the case of total corrections. Once the project utryasut with locals for DOT is given the green light to work.
All the highway, indexed as the US and I do concrete (with very few exceptions). This is due to the fact that it is on these roads is transit traffic, and the largest flow of both vehicles and cargo. Yes, the road is often covered with asphalt - and so, this is fake, under the asphalt - concrete. Just after the deterioration of concrete is coated with asphalt to repair. This trick is to extend the life of the road and pull cap. repair, t. k. shift concrete problematic and expensive than throw the top layer of asphalt.
The choice of the concrete was made in connection with several features of the last - his strength, durability, ruggedness to the load and the preponderance of trucks. Of the minuses concrete pavement - its problematic laying and repair. If the paver road ready in 8 hours for a stream of cars, it takes several months of work for the opening of a concrete road, but the result justifies itself.
So, having received the green light to work, DOT first thing begins to think about how close the road without creating traffic jams. For this strip made any time somewhere on the roadside (typically asphalt makeshift concoct), or the road is altered to the imposition on the roadside. But in any case, DOT can not close the road for more than 30% of the bandwidth.
The most common DOT cunning, and repairing or building a section of the strip, first make one side of the road, shifting all of the bands to the other side, and then let flow on the road, taking the side and middle of the road, and starts to mess with another old road .
All repairs are always identified areas for signs of drivers, chips, and information boards, where builders write quick messages about the current state of the road and the planned closure of some bands in the future. At a high-speed roads current area fenced off curbs, as the traffic on the road continues to move at a speed of 100 km / h.
Also note that the builders did not forget about the layout, whether it is running in a temporary "pipok" glued to the road surface, such button clearly visible at night (often in the repair areas there is no light, ie. A. Pillars are not yet installed) and felt wheel at running, letting the driver know that he gape.
By the way, the fines in construction zones are always doubled for all types of violations, especially violently punished speeding, ie. Sometimes builders are working without protective borders to.. The police, in turn. He loves to watch offenders in such areas.
After all fenced and ready for work, it begins the process of building itself.
The first step is to shovel the rocks about a meter, usually breed is not taken away, t. To. It can be used for various embankments and ennoble the territory. If there is a repair, they begin to shoot the old paint.
Then the fit pad for the road, usually gravel, sand and clay, all carefully compacted after laying each layer, for nothing then ground to spread in different directions. As all communication and reservoirs are built.
each laid layer watered, and then calcium chloride - or rather 35% solution or lime mortar, after which a trigger bulldozers "forks", which plow laid layer, after which it is again compacted. What for? The fact is that no matter how tamp pad is not in its natural form, it is water (especially in the sand and clay), and when the water evaporates, the pad prosyadet to reduce the percentage of subsidence, poured lime. Due to chemical reaction of lime holding a water without allowing it to evaporate and leave the pillow, t. E. Inhibits constant percentage of water in the bed, not allowing it to sink. Studies show that health indicators such cushions 80% better compared to the just rammed a pillow.
After the airbag is laid, double layer of asphalt is laid, this relatively thin layer, the thickness of each layer is typically 5-7 cm.
What are placed under the concrete pavement? There are several reasons. First concrete fragile to failures, and in the case of subsidence of the earth cracks will appear immediately, but as I mentioned above, to repair concrete is not so simple. The cause of land subsidence - it is water. Laying a layer of dense asphalt under concrete, preparing a smooth surface and removed any possible subsidence, t. K. Asphalt tightly compacted and takes up any small holes and cracks in the ground, and secondly, it serves as waterproofing for water without allowing it to fall under the concrete by thermal seams concrete joints.
Let me remind you that the percentage of water being laid in a bed, will vary very little throughout the life of the road due to lime. Thus, the road surface is not prone to subsidence, and if subsidence occurs, and (sometimes surrenders ground under cushion), it turns smooth monotonic fossa at which pleasant ride, and which does not cause cracks in the concrete, thanks to a more flexible pavement.
After all these problems, finally begins installation of reinforcement for the concrete canvas. Typically, a standard steel bars of 16 mm diameter (413 MPa). After collecting the fittings on the ground, it is raised on a plastic or metal stand. The ideal position of the valve - between 1/3 and 1/2 the thickness of the tie from the upper edge, in other words - a little above the midpoint of the screed.
are two reasons for this: if the bed is too close to the valve surface - armature can rust from a lack of material on top, if too deep bed - the steel does not protect the concrete from cracks in the top layer of concrete.
Steel reinforcement comes to the construction site on the rods 18 meters, at the junction of armature overlapped, which are calculated by engineers. Throughout the history of construction engineers "brought" overlapping technology, which is to eliminate the overlap on all fittings roadway width in one place. Today, the most commonly used chess move that is perfectly visible in this photo.
reinforcement in concrete is used to distribute the load and prevent the appearance of cracks in the coating. The percentage of steel in the cross section (perpendicular to the direction of travel) is usually 0.60% in the longitudinal (the direction of travel of vehicles) - 0.85%.
The majority of cracks in the coating occurs at a distance of 1-3 meters, the road cross section (perpendicular to the movement of vehicles), t. K. Automobiles forced wheels coating according to the direction of travel. Where the concrete tensile stress exceeds the allowable design load - there is a crack. Concrete is very bad behaved in tension, and steel - very good, so together they compensate for each other's shortcomings.
By the way, for a long time it was found a linear relationship the number and position of cracks in the concrete to the steel volume, located in the screed. Thus, it was proved that with a reduction in contact area with the concrete steel, there is an increase in the distance between the cracks.
Then road paver starts to fill piece. In the process of laying builders actively ram concrete vibrators to expel as much air out of the coating, the air reduces the strength of concrete. For once handler must fill in the section from one to the other thermal weld seam, the whole piece of the road should be monolithic and without concrete joints. To a mixture of used concrete type III, m. K. Particles of concrete is smaller in comparison, for example, type I, but should also not be forgotten that the use of smaller particles that increases concrete heat during solidification, which introduces its own problems in the process concrete aging. The temperature factor of the environment strongly influences the formation of cracks in the coating.
In hot climates (TX, FL), usually try to lay concrete at night, ie. K. A mixture loses less moisture. The recommended temperature range laying concrete from 10 to 25C degrees. Otherwise, the builders used concrete watering to avoid the appearance of cracks in the concrete hardening process. But we must not forget that the addition of water reduces the percentage of the strength of concrete, so used for watering some of its technology and small tricks - mostly concrete Kuta cellophane.
Another factor that strongly influences the concrete cover and its durability - it is the type of crushed stone, for example, river rock, gravel or quarry have different indices of thermal expansion, strength and flexibility. Thus, different rubble is one cause of occurrence of cracks, if neglect of its properties. A detailed study of the various types of rubble was carried out in the 1960s.
When filling each section, made of concrete cylinders of 10 (diameter) x 20 (height) cm, or 15h30 cm, which leaves near the flooded portion after 3 days they are taken to a laboratory and broken, then be calculated strength of the poured concrete . This is done in order to ensure that the poured concrete meets all the requirements. This is done precisely at the site that the cylinders are in the same conditions as the concrete. If you fill cylinders at the plant reinforced concrete, where concrete mixers are very often obtained inflated figures. If the concrete does not pass the test - coverage would have to dismantle and perezalit.
A typical thickness of the screed mezhshtatovkisih highway - 30 cm, smaller highway - 20 cm. In practice, this thickness of concrete, laid on an asphalt pad provides excellent resistance to load and does not require a second grid of steel. By increasing the thickness of the concrete - cracks, t to concrete has not become sufficient for load distribution, with decreasing thickness -.. The concrete can not withstand the load of vehicles.
After penetration, paver typically draws a rough surface, which makes the rain water to run as well as increases the coefficient of coupling with the road wheels of automobiles. Sometimes roughness just do not, and cut after using a special machine. Usually this happens in the monolithic bridges.
Also, after pouring in the road do not forget to sign, when the segment has been filled in, then it is useful in order to repair, as well as to know when the road can be opened for the transit of construction machinery in the process of work. Concrete should be based on a minimum of 7 days without any load after seven days on the road, you can drive a light construction equipment, but do not forget that the full strength of the concrete will acquire only after 28 days, and if for some reason on the cover gets heavy machinery, the structure concrete can be completely damaged. Therefore, if you need somewhere to move the road heavy crane - under it stele mats or concrete is poured with a thick layer of sand.
After 28 days, the road is completely ready for operation. The term of service of the road without major repairs - 25 years. In the cities of Houston and Dallas have a stretch of highway, flooded in 1960, and are in perfect condition until now. These areas are used for scientific activities and monitoring. Scientific papers simmer two largest and most prestigious universities in Texas - in Austin (Texas capital) and in Houston.
In the US, there is a relatively large amount of experimental plots of concrete pavement, t. To. The transport department sees the future in concrete roads. These segments are located in all four climatic zones of the country and have a different design. By 2001, several hundred sites, filled in between 1961 and 2001, left 89 sections of the monitored.
On the basis of experiments conducted in 1999 and 2001 on the basis of areas in age from 25 to 40 years, several conclusions were made:
- depending on the design and materials, concrete cover capable of withstanding loads without cracking over 34 years (compared to an average of 8 years, asphaltic concrete);
- After 34 years, over the next 5 years, only 16 pieces fell into disrepair and were subject to a complete replacement;
- for 20 years, motorists have complained about the deterioration of ride comfort, but in some areas, "surrendered" to maintain the soil, forming a small flowing trough without the appearance of cracks in the upper layer coating the roadway;
- By 2012, 89 experimental plots are 34 active (17 are in Texas). Their average age today - for 31 years. The remaining sections were pereulozheny due to the current expansion of roads, and scientific works to date rudder were successfully completed.
The experience of laying concrete roads to actively learn from the United States and Asian countries not only. Laying of concrete highways actively engaged in China and Japan as well as Australia and some European countries, using the accumulated experience of the American road builders. In the United States, if possible, states are trying to build new roads only of concrete, thereby reducing the cost of maintaining roads in the course of their operation.