DIFFERENT TYPES OF MODIFIED PORTLAND CEMENTS BASIC
What Are The Different Types Of Modified Portland Cements?
Increasingly, modern concretes contain a blend of Portland cement and other cementitious materials. When other materials are added to Portland cement at the time at which the concrete is batched, they are referred to as mineral admixtures; however, there are also hydraulic cements, which are produced either by forming other compounds during the burning process or by adding other materials to the clinker and then intergrinding them.
The common types of such modified cements are described in the following sections.
Portland Pozzolan Cements
Portland pozzolan cements are blends of Portland cement and a pozzolanic material. The role of the pozzolan is to react slowly with the calcium hydroxide that is liberated during cement hydration.
This tends to reduce the heat of hydration and the early strength but can increase the ultimate strength of the material. These cements tend to be more resistant to sulfate attack and to the alkali–aggregate reaction.
Portland Blast-Furnace Slag Cements
Ground granulated blast-furnace slag (GGBFS), which is a byproduct of the iron and steel industry, is composed largely of lime, silica, and alumina and thus is a potentially cementitious material. To hydrate it, however, it must be activated by the addition of other compounds.
When the GGBFS is to be activated by lime, the lime is most easily supplied by the hydration of the Portland cement itself. Slags may be present in proportions ranging from 25 to 90%. They react slowly to form C–S–H, which is the same product that results from the hydration of the calcium silicates.
In general, because they react more slowly than Portland cement, slag cements have both lower heats of hydration and lower rates of strength gain.
On the other hand, they have an enhanced resistance to sulfate attack. When the GGBFS is to be activated with calcium sulfate (CaSO4), together with a small amount of lime or Portland cement, the material is known as supersulfated cement.
This cement is available mostly in Europe, where it is used for its lower heat of hydration and its resistance to sulfate attack.
Expansive Cements
Expansive cements were developed to try to offset the drying shrinkage that concrete undergoes. This is particularly important when the concrete is restrained against contraction or when it is to be cast against mature concrete in repair situations.
In both cases, severe cracking may occur as a result of the shrinkage. Expansive cements are based on the formation of large quantities of ettringite during the first few days of hydration; however, they are little used today, in large part because it is very difficult to control (or predict) the amount of expansion that will take place for a particular concrete formulation.
Sunday, January 29, 2012
CRANE SUPPORTED (HANGING) LEADERS USED IN PILING BASICS AND TUTORIALS
CRANE SUPPORTED (HANGING) LEADERS USED IN PILING BASIC INFORMATION
What Are Crane Supported Leaders Used In Piling?
Although the complete piling rig with its base frame and leaders supported by a stayed mast provides the best means of ensuring stability and control of the alignment of the pile, there are many conditions which favour the use of leaders suspended from a standard crawler crane.
Rigs of this type have largely supplanted the frame-mounted leaders for driving long piles on land in the UK and USA. The usual practice is to link the leaders by the head of the crane jib and to control their verticality or backward or forward rake by means of adjustable stays near the foot of the leaders.
The latter bear on the ground through an enlarged foot which can be levelled by a screw jack. BSP International Foundations Ltd. TL series leaders (Figure 3.6) have heights of 19.0m and 21.9m and carry hammers of up to 3 tonne mass.
The 610mm and 835mm square section lattice leaders have a height to the cathead of 22.5 and 38m respectively, and can carry combined pile and hammer loads of 13 tonne and 21 tonne respectively.
Backward and forward rakes of up to 1:3 are possible depending on the stability of the crawler crane. There is a practical limit to the length of pile which can be driven by a given type of rig and this can sometimes cause problems when operating the rig in the conventional manner without the assistance of a separate crane to lift and pitch the pile.
The conventional method consists of first dragging the pile in a horizontal position close to the piling rig. The hammer is already attached to the leader and drawn up to the cathead. The pile is then lifted into the leaders using a line from the cathead and secured by toggle bolts.
The helmet, dolly and packing are then placed on the pile head and the assembly is drawn up to the underside of the hammer. The carriage of the piling rig is then slewed round to bring the pile over to the intended position and the stay and angle of the crane jib are adjusted to correct for vertically or to bring the pile to the intended rake. The problem is concerned with the available height beneath the hammer when it is initially drawn up to the cathead.
Taking the example of leaders with a usable height of 20.5m in conjunction with a hammer with an overall length of 6.4m, after allowing a clearance of 1m between the lifting lug on the hammer to the cathead and about 0.4m for the pile helmet, the maximum length of pile which can be lifted into the leaders is about 12.7m.
A somewhat longer pile could be handled if the leaders were of a type which allows vertical adjustment. Occasionally it may be advantageous to use leaders independent of any base machine. Thus if only two or three piles are to be driven, say as test piles before the main contract, the leaders can be guyed to ground anchors and operated in conjunction with a separate petrol or diesel winch.
Guyed leaders are slow to erect and move, and they are thus not used where many piles are to be driven, except perhaps in the confines of a narrow trench bottom where a normal rig could not operate.
What Are Crane Supported Leaders Used In Piling?
Although the complete piling rig with its base frame and leaders supported by a stayed mast provides the best means of ensuring stability and control of the alignment of the pile, there are many conditions which favour the use of leaders suspended from a standard crawler crane.
Rigs of this type have largely supplanted the frame-mounted leaders for driving long piles on land in the UK and USA. The usual practice is to link the leaders by the head of the crane jib and to control their verticality or backward or forward rake by means of adjustable stays near the foot of the leaders.
The latter bear on the ground through an enlarged foot which can be levelled by a screw jack. BSP International Foundations Ltd. TL series leaders (Figure 3.6) have heights of 19.0m and 21.9m and carry hammers of up to 3 tonne mass.
The 610mm and 835mm square section lattice leaders have a height to the cathead of 22.5 and 38m respectively, and can carry combined pile and hammer loads of 13 tonne and 21 tonne respectively.
Backward and forward rakes of up to 1:3 are possible depending on the stability of the crawler crane. There is a practical limit to the length of pile which can be driven by a given type of rig and this can sometimes cause problems when operating the rig in the conventional manner without the assistance of a separate crane to lift and pitch the pile.
The conventional method consists of first dragging the pile in a horizontal position close to the piling rig. The hammer is already attached to the leader and drawn up to the cathead. The pile is then lifted into the leaders using a line from the cathead and secured by toggle bolts.
The helmet, dolly and packing are then placed on the pile head and the assembly is drawn up to the underside of the hammer. The carriage of the piling rig is then slewed round to bring the pile over to the intended position and the stay and angle of the crane jib are adjusted to correct for vertically or to bring the pile to the intended rake. The problem is concerned with the available height beneath the hammer when it is initially drawn up to the cathead.
Taking the example of leaders with a usable height of 20.5m in conjunction with a hammer with an overall length of 6.4m, after allowing a clearance of 1m between the lifting lug on the hammer to the cathead and about 0.4m for the pile helmet, the maximum length of pile which can be lifted into the leaders is about 12.7m.
A somewhat longer pile could be handled if the leaders were of a type which allows vertical adjustment. Occasionally it may be advantageous to use leaders independent of any base machine. Thus if only two or three piles are to be driven, say as test piles before the main contract, the leaders can be guyed to ground anchors and operated in conjunction with a separate petrol or diesel winch.
Guyed leaders are slow to erect and move, and they are thus not used where many piles are to be driven, except perhaps in the confines of a narrow trench bottom where a normal rig could not operate.
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