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Riprap is an erosion-resistant ground cover made up of large, angular and loose stones (rock, concrete or other material) with geotextile or granular layer underneath. Riprap is commonly used in drainage channel to provide a stable lining to resist erosion by channel water. It is also used in channels where infiltration is intended but the velocity of flow is too large for vegetation.
In a typical box culvert, at the interface between rockfill layer and sub-grade, a layer of geotextile filter is usually added to perform separation function. With the addition of geotextile filter layer, it avoids the intermixing of widely different soil granulations so as to reduce long-term settlement.
Flow velocity seldom causes abrasion problem for concrete pipes. Instead, the particles carried by effluent in high velocity may create abrasion problem of concrete. The abrasive effect is dependent on the size of particles and velocity. In most circumstances, the problem of abrasion shall be avoided for flow velocities less than 8m/s.
Concrete pipes are designed to crack in tensile zone so that steel reinforcement could take up the tensile stress. Flexural stresses are developed at the top and bottom inside surfaces and on the outside longitudinal cracks.
Stoplog consists of several sets of horizontal beams/logs stacked vertically. For narrow openings, the logs span between support slots at the ends of the openings. For wide openings, intermediate removable support posts may be required. They are prevalent in the past because of its low establishment cost, simple erection and easy operation.
The freeboard is defined as the vertical distance from water surface to the top of channel bank. The selection of freeboard is dependent on the consequence should overflow out of channel bank occurs. Other than that,consideration should also be given to prevent waves, superelevation and fluctuations in water surface from overflowing the channel banks.
A penstock is commonly used to control the flow and water level and for isolation of fluid. It mainly consists of a sliding door which is controlled by mechanical spindle moving through a hole in a frame built onto a structure. Penstock is the term used in UK while sluice gate is more commonly adopted outside UK. In the design of penstock, it is important to identify if it would take on-seating head or off-seating head.
During the construction of new pavement, vibratory roller and heavy equipment are needed to compact the filling material and bituminous material. These heavy equipment could generate very high impact load with short duration on the concrete pipes.
Concrete pipes are designed to be uniformly supported along the length to carry vertical loads on its top. They are normally not intended to serve as a beam to carry loads in longitudinal direction under poor ground supports (i.e. high and low spots in bedding).
By
Er. Kaushal Kishore ,
Materials Engineer, Roorkee
PORTLAND CEMENT:
Joseph Aspdin, a mason at Leeds prepared a cement in 1824 by heating a mixture of finely-divided clay and hard limestone in a furnace until CO2 had been driven off; this temperature was much lower than that necessary for clinkering. The prototype of modern cement was made in 1845 by Isaac Johnson, who burnt a mixture of clay and chalk until clinkering, so that the reaction necessary for the formation of strongly cementitious compound took place. The name ‘Portland Cement’ was given due to the resemblance of the colour and quality of the hardened cement to Portland stone- a limestone quarried in Doset.
The process of manufacturing of cement consists essentially of grinding the raw materials ( calcareous materials such as limestone or chalk and argillaceous materials such as shale or clay), mixing them intimately in certain proportion and burning in a large rotary kiln at a temperature of upto about 14500C when the material sinters and partially fuses into balls known as clinker. The clinker is cooled and ground to a fine powder, with some gypsum added, and the resulting product is the commercial Portland Cement so widely used throughout the world.
MAKING CONCRETE:
Just mix cement, aggregates and water, cast this mix in a mould, open the mould next day. A uniform hard mass will be found, which is known as concrete, any body can make it. The simplecity in making concrete make this material to be look like very simple in its production, yet it as not so simple. Due to ignorance about concrete no other building materials ever mis-used as concrete in the construction. In India concrete is being used in the construction since the last 70 years. Yet 80% of the builders have no proper understanding of this materials. Go to any construction site (except big construction sites) you will find that sand and aggregates are being taken in iron tasla or cane baskets to charge the mixer without the consideration of site aggregates actual grindings, moisture content and bulking of sand. The water is poured in the mixer without any measured quantity. It could be well imagine what sort of concrete structure will be made with the concrete being produced in this crude method.
Most of the contractors, builders, masons etc. still follow 1:2:4 or 1:1.5:3 mixes they are not aware of Design Mixes and Concrete Admixtures. This paper described how Design Mixes can be converted into volume with 1 Bag Cement, 2 Boxes of sand and 4 Boxes of Aggregate. The site practical problem is the dispersion of water and liquid admixtures into the mixer. For this the site should fabricate a plastic circular graduated measuring container of 30 lit capacity with a tap fitted at its bottom. This container is to be fitted on top of the mixer. From this container water and liquid admixtures can conveniently poured direct into the mixer in a measured quantity.
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