Q Bank for Civil Engineer LDCE Exam- Write Short Notes on (15)

Q.No.21. Write short notes on: 

1. Concrete mix design. 

2. Blanketing materials 

3. Chlorination of water. 

4. Achieve earth pressure and passive earth pressure.

Ans. 21. 

 I) CONCRETE MIX DESIGN:- The concrete mix design is a process of selecting suitable ingredients for concrete and determining their proportions which would produce, as economically as possible, a concrete mix of necessary workability in plastic stage and provide required strength and durability in hardened stage. 

 FACTORS INFLUENCING THE CHOICE OF MIX PROPORTIONS:- 

i. Grade Designation 

ii. Type of Cement 

iii. Maximum nominal size of aggregate 

iv. Grading of combined aggregate 

v. Water cement ratio 

vi. Workability required 

vii. Durability 

viii. Quality control at site

MIX DESIGN BY INDIAN STANDARD RECOMMENDED GUIDELINES:- The procedure adopted for the selection of mix proportions is as follows:- 

i. The Target Mean Strength (ft) is determined by using the relation Where fck is the characteristic compressive strength of concrete at 28 days. ‘S’ is the standard deviation ( the value of ‘s’ varies with the degree of quality control and grade of concrete) 

ii. The water cement ratio for the target mean strength is determined from the standard curves released by IRC (IRC:- 44 -1972). The water cement ratio so choosen is compared with the maximum water-cement ratio specified for durability and the lower of the two values is used. iii. The degree of workability required in terms of slump, compacting factor or Vee- Bee time is selected. iv. Approximate air content is estimated from the available table corresponding to the maximum nominal size of aggregate used.

v. The water content per cubic meter of concrete and percentage of sand in total aggregate ( by absolute volume) are next selected from the available table for the standard reference conditions. 

vi. The water content and percentage of sand in total aggregate are adjusted for any deference in workability, water cement ratio, grading of fine aggregate for the particular case from the reference values. 

vii. Cement content is calculated using the selected water cement ratio and the final water content of the mix obtained after adjustment. The cement content so calculated is compared with minimum cement content from the requirement of durability and the greater of the two values is used. 

viii. The total aggregate content ( saturated surface dry condition) per unit volume of concrete is determined by subtracting the air, cement and water quantities per unit volume of concrete. With the percentage of sand in total aggregate as already determined, the coarse and fine aggregate contents per unit volume of concrete are calculated. 

 Vab = 1.0 – [ W+C/Sc]1/1000 – Ѵ Where 

Vab = Absolute volume of total aggregate per unit volume of concrete 

 W = Mass of water (kg) 

 C = Mass of cement (kg) 

 Ѵ = Air content per cubic meter of concrete 

 Sc = Specific gravity of cement 

 Also absolute volume of fine aggregate Vf(ab) = ƥ. Vab 

 and absolute volume of coarse aggregate  Vc(ab) = (I- ƥ)xVab 

 ix.  Now the mix proportions by mass (kg) can be calculated as under:- 

 Water      : Cement             : fine aggregate                       : coarse aggregate 

 W          :     C                   :[Vf(ab)xSfax1000]                 : [Vc(ab)xScax1000] 

Where Sfa and Sca are the specific gravities of saturated surface dry fine aggregate and coarse aggregate in kg/liter respectively. 

 x. The adjustments for free moisture contents in fine and coarse aggregates are to be done in the calculated quantities and the final mix proportions by mass are calculated.

xi. The actual mix proportions are arrived at by means of tests on number of trial mixes prepared by varying the water-cement ratio by + 10% of the pre selected values.

II) BLANKETING MATERIAL:- A layer of specified coarse, granular material of designed thickness provided over full width of formation between sub-grade and ballast is called BLANKET. 

 The main functions of the Blanket layer are as under:- 

i. It reduces the traffic induced stresses at top of sub-grade to a tolerable limit to avoid track foundation failure. 

 ii. It prevents penetration of ballast into the sub-grade and also prevents the upward migration of fine particles from sub-grade into the ballast. iii. It intercepts downward percolation of water, facilitates run off of water from top surface of blanket layer and provide drainage path for upward movement of water from sub- grade ensuring dissipation of excess pore water pressure.

SPECIFICATIONS OF BLANKET MATERIAL:- 

 The blanket material should generally conform to the specifications prescribed as under:- 

i. It should be coarse, granular and well graded. ii. The co-efficient of uniformity Cu should be > 7 and Cc between 1and 3. 

iii. Fines (passing 75 microns) should be limited 3% to 10%. 

iv. Minimum CBR value 25 of the blanket material compacted at 100% of MDD. 

v. The size gradation should be within the specified range or enveloping curves. 

vi. Los Angeles Abrasion value < 35-40% 

vii. Filter criteria should be satisfied with sub-grade layer, as given below: (OPTIONAL) 

CRITERIA-1 : D15 (Blanket) < 5xD85 (Sub-grade) 

CRITERIA-2 : D15 (Blanket) > 4 to 5 D15 (Sub-grade) 

CRITERIA-1 : D50 (Blanket)< 25xD50 (Sub-grade)

III) CHLORINATION OF WATER:-

The process of disinfection of water with the use of chlorine in its various forms is called the chlorination of water. It is cheap, reliable, easy to handle, easily measurable and above all, it is capable of providing residual disinfecting effects for long period, thus affording complete protection against future recontamination of water in the distribution system. 

 DISINFECTING ACTION OF CHLORINE:-

 Cl2 +H2 O ƥH>5 HOCL + HCL 

 Hypochlorous Acid

In the ƥH range below 5, chlorine does not react and remains as elemental chlorine The hypochlorous acid is unstable and may break into hydrogen ions and hypochlorite ions 

 HOCL ƥH>8 H+ + OCL- 

 ƥH<7   Hypochlorite ions

The above reaction is reversible and depends upon the ƥH value of water. Out of these forms of free available chlorine, the hypochlorous acid is most destructive, being about 80 times more effective than hypochlorite ions. For this reason, the ƥH value of water during chlorination is generally maintained slightly less than 7, thereby keeping more HOCL in solution compared to OCL ions. Moreover, the chlorine will immediately react with ammonia present in water to form various chloramines as:- 

 NH3 + HOCL NH2 CL + H2 O 

 (Mono chloramines)

 NH2 CL + HOCL NHCL2 + H2 O 

 (Di- chloramines) 

 NHCL2 + HOCL NCL3 + H2 O 

 (Nitrogen Tri-chloramines) 

The chloramines so formed are stable and found to possess disinfecting properties. When the added chlorine has consumed all the ammonia available in water, then it would persist as free chlorine. The combined chlorine with ammonia in the form of chloramines is much less effective in causing disinfection compared to the free chlorine, being about 25 times less effective.

DOSES OF CHLORINE:- The amount of chlorine required for water depends upon the inorganic and organic impurities present in it. When chlorine is added to water, it first of all reacts with the inorganic impurities like S- - , Fe++, Mn++, NO2- etc, that convert the chlorine into chloride, which has no residual oxidizing powder. Excess chlorine, after this point is consumed by ammonia to form chloramines. Simultaneously, chlorine will also react with organic impurities present in water. The chlorine consumed in all the above reactions represents the CHLORINE DEMAND OF WATER. When once it gets satisfied, the chlorine will appear as FREE CHLORINE. In general, most of the waters are satisfactorily disinfected if the free residual chlorine is about 0.5 mg/liter, 30 minutes after the chlorine is applied.

VARIOUS FORMS IN WHICH CHLORINE CAN BE APPLIED:- 

1. In the form of liquid chlorine or as chlorine gas (As free chlorine). 

2. In the form of hypochlorite or Bleaching Powder. 

3. In the form of chloramines i.e. a mixture of ammonia & Chlorine. 

4. Use of Chlorine Dioxide gas. 

5. Use of Chlorine tablets. 

vi) ACTIVE EARTH PRESSURE AND PASSIVE EARTH PRESSURE:- 

The magnitude of the lateral earth pressure depends upon the movement of the retaining wall relative to the backfill and upon the nature of soil. Considering a rigid retaining wall with a plane vertical face, backfilled with cohesion less soil. If the wall does not move even after filling the material, the pressure exerted on the wall is termed as Earth Pressure at Rest. The point A represent it is fig. below:-

ACTIVE EARTH PRESSURE:- If suppose the wall gradually moves away from backfill, mobilization of the internal resistance i.e. shearing resistance of soil builds up in direction away from the wall, hence, the earth pressure on the wall decreases and after a particular displacement of the wall, the pressure reaches a constant value and does not decrease beyond this point B with further movement of the wall. This minimum pressure as a result of the movement of the wall away from backfill is called the 

ACTIVE EARTH PRESSURE.

PASSIVE EARTH PRESSURE:- If on the other hand, the wall moves towards the fill, the earth pressure increases, because the shearing resistance builds up in direction towards the wall. The pressure reaches a maximum value (Point C) when the shearing resistance of the soil has been fully mobilized and any further movement of the wall does not increase the pressure. This maximum pressure as a result of movement of wall towards the backfill is called the PASSIVE EARTH PRESSURE.