Monday, 9 October 2017

Dumpy level

Dumpy level is an optical surveying leveling instrument consisting a telescope tube firmly secured in two collars fixed by adjusting screws to the stage by the vertical spindle. The telescope can rotate only in a horizontal plane. Relative elevation of different points of a surveying land is determined with dumpy level.

Use of Dumpy Levels

The dumpy level is mainly used in surveying for the following purposes:

To determine relative height and distance among different locations of a surveyingland.To determine relative distance among different locations of a surveying land.

Advantages of Dumpy Level

Simple construction with fewer movable parts.Fewer adjustments to be made.Due to the rigidity of dumpy levels, it retains its two adjustment for a long time.High optical power.

Disadvantages of Dumpy Level

Civil Engineers may be find it difficult in making accurate measurements.Difficulty in using.

Monday, 2 October 2017

Segregation

Segregation is the separation of the different materials of concrete. A good concrete is one which is homogeneous in nature.If a sample of concrete exhibits a tendency for separation of say, coarse aggregate from the rest of the ingredients, then, that sample is said to be showing the tendency for segregation. Such concrete is not only going to be weak; lack of homogeneity is also going to induce all undesirable properties in the hardened concrete.

There are considerable differences in the sizes and specific gravities of the constituent ingredients of concrete. Therefore, it is natural that the materials show a tendency to fall apart. 

Segregation may be of three types — firstly, the coarse aggregate separating out or settling down from the rest of the matrix, secondly, the paste or matrix separating away from coarse aggregate and thirdly, water separating out from the rest of the material being a material of lowest specific gravity. A well made concrete, taking into consideration various parameters such as grading, size, shape and surface texture of aggregate with optimum quantity of waters makes a cohesive mix. Such concrete will not exhibit any tendency for segregation. The cohesive and fatty characteristics of matrix do not allow the aggregate to fall apart, at the same time, the matrix itself is sufficiently contained by the aggregate. Similarly, water also does not find it easy to move out freely from the rest of the ingredients. 

The conditions favourable for segregation are, as can be seen from the above para, the badly proportioned mix where sufficient matrix is not there to bind and contain the aggregates. Insufficiently mixed concrete with excess water content shows a higher tendency for segregation. Dropping of concrete from heights as in the case of placing concrete in column concreting will result in segregation. When concrete is discharged from a badly designed mixer, or from a mixer with worn out blades, concrete shows a tendency for segregation. Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper, long lift by skip and hoist are the other situations promoting segregation of concrete. 

The most important method of concrete compaction is Vibration. Only comparatively dry mix should be vibrated. When a too wet a mix is excessively vibrated, it is likely to get segregated. Vibration also to be continued just for required time for optimum results. If the vibration is continued for a long time, particularly, in too wet a mix, it is likely to result in segregation of concrete due to settlement of coarse aggregate in matrix. 

Concrete is used with very high slump now a days particularly in RMC. The slump value required at the batching point may be in the order of 150 mm and at the pumping point the slump may be around 100 mm. At both these points cubes are cast. One has to take care to compact the cube mould with these high slump concrete. If sufficient care and understanding of concrete is not exercised, the concrete in the cube mould may get segregated and show low strength. Similarly care must be taken in the compaction of such concrete in actual structures to avoid segregation.


In case of floors or pavement finishing, with a view to achieve a smooth surface, masons work too much with the trowel, float or tamping rule immediately on placing concrete. This immediate working on the concrete on placing, without any time interval, is likely to press the coarse aggregate down, which results in the movement of excess of matrix or paste to the surface. Segragation caused on this account, impairs the homogeneity and serviceability of concrete. The excess mortar at the top causes plastic shrinkage cracks.  


So it can be concluded that the tendency for segregation can be remedied by correctly proportioning the mix, by proper handling, transporting, placing, compacting and finishing. If segregation is observed, it is advisable to remixing for a short time which would make the concrete again homogeneous. As mentioned earlier, a cohesive mix would reduce the tendency for segregation. For this reason, use of certain workability agents and pozzolanic materials greatly help in reducing segregation. The use of air-entraining agent appreciably reduces segregation. 
Segregation is difficult to measure quantitatively, but it can be easily observed at the time of concreting operation. The pattern of subsidence of concrete in slump test or the pattern of spread in the flow test gives a fair idea of the quality of concrete with respect to segregation.

Pumpable Concrete

Pumpable concrete is that type of  concrete which can be pushed through a pipeline for construction. It is made in such a manner that its friction at the inner wall of the pipeline does not become very high and that it does not wedge while flowing through the pipeline. It is very important to have a clear understanding of what happens to concrete when it is pumped through pipeline to any study of concrete pumping. Pumpable concrete emerging from a pipeline flows in the form of a plug which is separated from the pipe wall by a thin lubricating layer consisting of cement paste. The water in the paste is hydraulically linked with the interparticle water layer in the plug. 

The pressure generated by the flow resistance must not be greater than the pump pressure rating for maintaining continuous plug movement.  However, if the concrete is too saturated at higher w/c ratio, the concrete at certain pump pressures may be such that water is forced out of the mix, creating an increase in flow resistance and a possible blockage. In other words, a very stiff concrete is not pumpable and also a concrete with high w/c ratio is also not pumpable. It is interesting to note that if a concrete is pumpable, it is implied that it is a good concrete. 


High-Performance concrete

Recently a new term has come in the field of concrete technology “High Performance Concrete” or HPC. The properties of HPC are.
  • ·     High Workability
  • ·     High Strength
  • ·     High Modulus of Elasticity
  • ·     High Density
  • ·     High Dimensional Stability
  • ·     Low Permeability and
  • ·    Resistance to Chemical Attack

There is a little controversy between the terms high-strength and high performance concrete. High-performance concrete is also, a high-strength concrete but it has a few more attributes specifically designed as mentioned above. It is, therefore, logical to describe by the more widely embracing term “High Performance Concrete” (HPC).

In normal concrete, relatively low strength and elastic modulus are the result of high heterogeneous nature of structure of the material, particularly the porous and weak transition zone, which exists at the cement paste-aggregate interface. By densification and strengthening of the transition zone, many desirable properties can be improved many fold. A substantial reduction of quantity of mixing water is the fundamental step for making HPC. With reduction of w/c ratio strength concrete will increase. But reduction in w/c ratio to less than 0.3 will greatly improve the qualities of transition zone to give inherent qualities expected in HPC. 

Use of silica fume is also found to be necessary to improve the qualities of transition zone.Silica fumes becomes a necessary ingredient for strength above to 80 MPa. The best quality fly ash and GGBS may be used for other nominal benefits. Inspite of the fact that these pozzolanicmaterials increase the water demand, their benefits will out weigh the disadvantages. The crux of whole problem lies in using very low w/c ratio, consistant with high workability at the time of placing and compacting. Neville opines that the lowest w/c ratio that could be used is 0.22.


Only with the use of superplasticizer, w/c ratio in the range of 0.25 to 0.3 can be adopted and a high slump is possible to achieve. Therefore, use of appropriate superplasticizer is a key material in making HPC. The associated problem is the selection of superplasticizer and that of cement so that they are compatible and retain the slump and rheological properties for a sufficiently long time till concrete is placed and compacted.

























Cement Mortar

Mortar is a material used in masonry construction to fill the gaps between the bricks and blocks used in construction. Mortar is a mixture of sand, a binder such as cement or lime, and water and is applied as a paste which then sets hard.

Puzzolana Portland cement and sulphate-resisting cement form mortar which are used for constructions exposed to aggressive and waste waters. Cement mortars are used for plastering, rendering smooth finishes and damp proof courses. 

The mix proportions of cement mortar are given in Table below 


Preparation:-Manual mixing is applied for Small quantities of mortar; mechanical mixers are used for large quantities. 

For manual mixing,sand is sieved, cleaned with water to remove dirt and dust and dried. This dry sand is laid uniformly, on a pucca platform, over which cement is uniformly spread. The whole mass is then thoroughly mixed with spades till it becomes uniform in colour. A depression is then made in the middle of the mix and required quantity of water is added. The dry mix from the sides is moved and placed on the edges of the depression formed till the water is completely absorbed by the mix. The wet mix is then worked with spades to give a uniform consistency to the mortar.

For mechanical mixing the calculated quantity of cement, sand and water are fed into the cylindrical container of the mixer. A rotar with blades, inside the container, rotates and thoroughly mixes the ingredients. 
  
Precautions:-Basic property of Cement mortar should be uniformity and workability. It should be consumed within 30 minutes from the instant of adding water to the mix. The bricks, stones and blocks should be fully saturated in water before laying. The masonry and plastered or pointed surface should be kept completely wet by sprinkling water for at least 7 days. 

Sunday, 1 October 2017

ORDINARY PORTLAND CEMENT

Cement is one of the most important base products of construction industry and Ordinary Portland cement or OPC is by far the most important type of cement. Prior to 1987, there was only one grade of OPC [which was governed by IS 269-1976]. After 1987 higher grade cements were introduced. The OPC was classified into three grades, namely..

1.   OPC-33 Grade
2.   OPC-43 Grade
3.   OPC-53 Grade


These classifications are based on the strength of the cement at 28 days when tested as per IS 4031- 1988. If the 28 days strength is not less than 33N/mm2, it is called 33 grade cement, if the strength is not less than 43N/mm2, it is called 43 grade cement, and if the strength is not less then 53 N/mm2, it is called 53 grade cement. But the actual strength obtained by these cements at the factory are much higher than the BIS specifications.

The physical and chemical properties of different types of  OPC are shown in Table below.


It has been possible to upgrade the qualities of cement by using high quality limestone, modern equipments, closer on line control of constituents, maintaining better particle size distribution, finer grinding and better packing. Generally use of high grade cements offer many advantages for making stronger concrete. Although they are little costlier than low grade cement, they offer 10-20% savings in cement consumption and also they offer many other hidden benefits. One of the most important benefits is the faster rate of development of strength. In the modern construction activities, higher grade cements have become so popular that 33 grade cement is almost out of the market. Table shows the grades of cement manufactured in various countries of the world.

The manufacture of OPC is decreasing all over the world in view of the popularity of blended cement on account of lower energy consumption, environmental pollution, economic and other technical reasons. In advanced western countries the use of OPC has come down to about 40 per cent of the total cement production. In India for the year 1998-99 out of the total cement production i.e., 79 million tons, the production of OPC in 57.00 million tons i.e., 70%. The production of PPC is 16 million tone i.e., 19% and slag cement is 8 million tons i.e., 10%. In the years to come the use of OPC may still come down, but all the same the OPC will remain as an important type for general construction.


RHEOLOGY

Originated from a Greek word “rheo” which means “flow” and “logia” which means “study” Rheology is the study of flow of matter in liquid, soft solid or solid state under conditions in which they respond to plastic flow other deforming elastically when a external force is applied. 

Rheological properties of a cement mix means deformation of hardened concrete and placing and mixing of freshly mixed concrete.The mechanical behaviour of hardened cement paste,which exhibits both elastic and inelastic deformations, can be expressed in rheological terms.

Factors Affecting Rheological Properties:- 

1. If the amount of coarse aggregate in a concrete mix is more the desirable amount the voids cannot be filled with the available mortar, which will lead to loss of cohesion and mobility. Such a mix is termed harsh and requires a great amount of effort to place and compact. On the other hand, an excessive amount of fine aggregate or entrained air in a concrete mixture will greatly increase the cohesion and render the concrete difficult to move.

2. Slump test is the measure of consistency of concrete mix or it is an indicator of relative water content of the mix. An increase in the water content or slump above that required to achieve a workable mix produces greater fluidity and decreased internal friction. Thus, a water content more than that needed will not improve the rheological properties of concrete. But very low slump with decrease the workability of a concrete mix making it impossible to place is some areas.

3. Elevated temperature, use of rapid hardening cement, cement deficient in gypsum and use of accelerating admixtures, increase the rate of hardening which reduce the mobility of concrete.

4. The rough and highly angular aggregate particles will result in higher percentage of voids being filled by mortar, requiring higher fine aggregate contents and correspondingly higher water content. Similarly an angular fine aggregate will increase internal friction in the concrete mixture and require higher water contents than well rounded natural sands.

5. A well graded aggregate gives good workability. Gap graded aggregate affects void system and workability. These effects are greater in fine aggregate.

6. An increase in the maximum size of aggregate will reduce the fine aggregate requirement to maintain a given workability and will thereby reduce the surface area to be wetted and hence the cement content necessary for a constant water/cement ratio .

7. The admixtures which have significant effect on the rheology of concrete are plasticizers and super-plasticizers, air-entraining agents, accelerators and retarders.Lignosulphate salt based plasticizers (0.15%) reduce the water content by 10% without any detrimental effect. Super-plasticizers and plasticizers prevent the formation of flocculated structure by changing the inter-particle attraction/repulsion.
With proper attention to the rheological properties can increase work efficiency and reduce the material cost as well as the cost of construction

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