Q Bank for Civil Engineer LDCE Exam- Write Short Notes on (28)

Q.8. Write short note on :-

(i) Water cement Ratio 

(ii) Workability of concrete 

(iii) Fineness Modulus 

(iv) Curing of Concrete 

(v) Static Water Level and Pumping Water Level

Ans. (i) Water Cement ratio :- Water has two functions in a concrete mix. It enables the chemical reactions, which cause setting and hardening of cement. It lubricates the dry concrete in order to facilitate placing. If less quantity of water is added concrete will not be workable whilst excess amount of water if added it may flow by reducing quantity of cement and consequently will reduce the strength of concrete. Dense and strong concrete can be obtained by using specific quantity of water consistent with the degree workability required to give maximum density. Thus a specific ratio of volume of water to the volume of cement is required. At this ratio concrete is found more strong and durable. This ratio is called water cement ratio and denoted as W/C ratio. 

ii) Workability of Concrete :- Workability means ease with which concrete can be placed  in the forms and compacted. Water content, size shape and grading of aggregate, affect the workability of concrete. The degree of workability depends upon the required strength of concrete and its purpose. Hence workability must be considered in relation to the method of compaction and the type of construction, in order to avoid the necessity for an excessive amount of work to obtain maximum density. 

(iii) Fineness modulus:- Fineness modulus of an aggregate in an index number which gives an average size of particle in the aggregate. Fineness modulus is roughly proportional to the average size of particle in the aggregate. Coarser the aggregate higher the fineness modulus and vice versa. It is determined by adding the cumulative percentage of material coarser than each of the following test sieves and dividing the sum by 100. Test sieves 3.35 mm, 2.36mm,1.8 mm,600 micron,300 micron,150 micron and 75 micron 

 (iv) Curing of Concrete:- Curing of concrete is necessary to prevent rapid loss of moisture from the surface of concrete and to keep the temperature of concrete under control. After the concrete has begun to harden. i.e. about one to two hours after its laying, it shall be protected from quick drying with moist gunny bags, sand or any other material approved by the Engineer. After 24 hours of laying of the concrete the surface shall be cured by flooding with water to a minimum depth of 2.5 cm or by covering with wet absorbent materials. The curing is generally to be done for a minimum period of 14 days. Water used for curing shall be clean, land free from sediments of any sort.

(v) Static Water Level and Pumping Water Level :- It is level at which water stands in a well either penetrating an unconfined or confined aquifer is called static water level. It is measured from the ground surface. Pumping water level :- it is the level at which water stands in a well when the water is being pumped from the well. It is also called working water level.