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FAQ

Approximate percentages of streangth of ordinary portland cement concrete at different ages in comparison with the streangth at 28 days

Age 1day 3 days 7 days 28 days 3 months 6 months 12 months
Strength              
(Percent) 16 40 67 100 122 146 155

Strength requirement of concrete

Concrete Equivalent Compressive strength on 15 cm  
  Controlled Cubes, Minimum at  
  Concrete 7 days 28 days
    [kg/sq.cm] [kg/sq.cm]
01:01:02 M250 210 315
1:1/4:1/2   195 290
1:1,1/2:3 M200 175 265
1:1,3/4:3,1/2   160 240
01:02:04 M150 140 210

Allowable Stresses for Steel Conforming

Sl no Form Thickness or Diameter Axial tension Bending Max Shear Av.(in unstiffened web) Bearing Yield guaranteed Equivalent
      Pt Pbc/Pbt Pq Pq Pb Minimum Pe
1 Rolled I-beams and channels All 1500 1650         2285
2 Plates,bars universal beams and columns and Upto and inclg 20mm 1500 1650         2285
  Sections other than above Over 20 mm 1420 1575         2205
3 Compound girders composed of rolled I-beams or channels Upto and inclg 20mm   1650          
  With cover plates of thickness Over 20 mm   1575          
4 Plate girders with single or Upto and inclg 20mm   1575          
  Multiple webs Over 20 mm   1500          
5 Plates, Sections and bars All     1100   1890    
6 Rolled I-beams All       945      
7 Plate girders and single Upto and inclg 20mm       945      
  Channels Over 20 mm       865      
8 (A)St, 44-0 if I.S. 1977-1962                
  Plates,sections and flats 6mm and over           2600  
  Bars(rounds,square and hexagonal) 10mm and over           2600  
  Other than rivet bars 6mm and over           2600  
  (B) IS : 226-1962 Upto and inclg 20mm              
  IS : 2062-1962 Over 20, upto and              
  Plates, sections and flates Incl. 40 mm           2400  
    Over 40 mm           2300  
  Bars(rounds,square and hexxagonal)                
  Other than rivet bars 10 mm & over              
    Upto & inclg.20mm           2600  
    Over 20mm           2400  

Reduction in strength of concrete made with stored cement

Period of storage of cement(month) 0 3 6 12 24
Approx.relative streangth at(28 days percent) 100 80 70 60 50

Removal of forms

a Sides of foundation,Columns,beams and walls 2 days
  Under sides of slabs up to 4.5m span 7 days
c Under sides of slabs above 4.5m span and  
  Under sides of beam and arches up to 6m span 14 days
d Under sides of beam and arches up to 6m span and upto 9m span 21 days
e Under sides of beams and arches over 9m span 28 days

Relation between cube strength and water-cement ratio by weight for fully compacted concrete(portland cement)

Water cement ratio Cube crushing strength  
by weight (kg./sq.cm.)  
  7 days 28 days
0.35 400 530
0.4 350 470
0.45 300 420
0.5 250 370
0.55 220 320
0.6 180 280
0.65 150 250
0.7 130 220
0.75 110 200
0.8 105 180

** Volume of one bag of cement is 0.0347 cum
** Measuring box to be used at site(size 35 x 25 x 40cm)
** Voids to be deducted from stack measurements of
a   Stone metal 40mm guage and above
b   Soling stone/boulders 100mm and above
c   Excavated rocks

Permissible design stress for concrete(Kg/sq. cm.)

Grade of conc Equivalent Concrete Mix Como Strength at 28 days(min) Permissible stress in compression Permissible stress in shear Permissible stress in bond Perssible bearing pressure on full area
    Preliminary test Works test Direct Bending or in Tension in bending Average Local (Plain CC only)
                   
M100 1 : 3 : 6 135 100 25 30 3 4 7 20
M150 1 : 2 : 4 200 150 40 50 5 6 10 30
M200 1 : 1.5 : 3 260 200 50 70 7 8 13 40
M250 1 : 1 : 2 320 250 60 85 8 9 15 50
M300   380 300 80 100 6 10 17 60
M350   440 350 90 115 10 11 18 70
M400   500 400 100 130 11 12 19 80

Permissible design stresses in steelreinforcement( Kg/Sq.Cm)

Type of Stress Mild steel grade 1 Medium tensile seel Cold twisted steel
  (IS : 432-1996) (IS : 432-1966) (IS :1786-1966)
         
Tension other than in helical reinforcement in column or shear reinforcement   Half the guaranteed    
Up to 40mm dia 1400 Yieldstress, sublect to a maximum of 1900 1900
Over 40 mm dia 1300      
         
Tension in helical reinforcement in columns   1000 1300 1600
Tension in shear reinforcement in beams   1400 1400 1400
         
Compression in column bars   1400 1400 1400
Comparision in bar in a beam or slab when the comp   The calculated comp.stress in surrounding concrete    
Resistance of the concrete is taken in to account   Multiplied by the modular ratio    
Comparision in bar in a beam or slab when the comp   Half the gurantee    
Resistance of the conc. Is not accounted up to 40mm dia 1400 Yield stress, subject to   1900
Resistance of the conc. Is not accounted over 40mm dia 1300 A maximum of 1900   1900

Field test for Adulteration of cement

Burn a sample of cement for 20 minutes on a steel plate heated by a stove. Adulterated sample changes colour while unadultered sample remains unchanges
Small blocks(say)2"x2"x1/2" made out of adulterated cement can be broken easily with pressure of fingures

Water assessment for building works

Brickwork, Masonary, Concrete 2250 liters/cu.m
Plastering work 85 liter/sq.m
Pointing works 60 liter/sq.m
White washing per coat 3 liter/sq.m
Road work, metalling or gravelling(6" thick) 130 liters/cum.m

Recommended mixes for various types of construction

Nature of Work Mixture Recom-mended vol. Proport Maximum Size of aggregate to use Water in gallons per bag of C Bast Consistency
         
Long Span R. C. Arches High Load R. C. Columns 1 : 1 : 2 1/2" to 3/4" 3.5 to 4.0 Medium
         
Heavily stressed members od structurers,        
Small precast work such as posts and poles        
: for Fencing, Telegraphs, Signals, Garden, 1 : 2 : 2 1/2" to 3/4" 4.5 to 5.0 Medium or soft
Furniture and decoratives and other        
Work of very thin secion, watertight        
Constructions, for high heads, long piles.        
         
R.C. Columns and members subjected to 1 : 3 : 2      
Medium loads, wall and floors of reservoir to 3/4" 5.5 Medium
and tanks, cisterns, sewers, well kerbs and 1 .2 : 2 : 4      
Platforms and other watertight constructions,        
for moderate heads, non-surfaced roof slabs,        
Concrete deposited under water        
         
General R.C building work subjected to ordinary stresses 1 : 2 : 4 1/2" to 1 1/2 6.0 to 6 1/2" Stiff for
Such as beams, Slabs, Columns, Panel walls basement and retaining   as   roads Medium
Walls, Stairs, Lintels, Sills, Roads, Pavements driveways, Side walls,   required   for others
Floors steps, Bunkers and silos, Bridge construstion dams and piers etc        
Exposed to action of water and frost, Machine foundations        
Subjected to vibrations, R.Cfootings, R.C piles.        
         
Mass concrete work in culverts, Retaining walls, Compound walls 1 : 3 : 5 1" to 2" 7.5 Stiff
and ordinary machine bases, Foundation       or
Walls which need not be water-tight       Medium
         
Mass concrete for heavy walls foundations under column        
Footings and under heavy duty floors, Concrete blocks, 1 : 4 : 8 1 1/2" to 2 1/2" 1.00 to 10.5 Medium
Hollow block construction(1/2" AGG).        

Usefull notes on cement

1 Ton of portland cement = 20 Bags;
1 Cubic foot of portland cement weights 90 o 94lbs, when loodely filed. Average weight of 1 : 2 : 4 concrete; Coke breeze aggregate
  100 lbs. per cubic foot; Clinker aggregate 110 lbs. per cubic foot; brick aggregate 125 lbs. per cubic foot. Ballast aggregate 145 lbs
  per cubic foot
   
  Average weight 1 : 2 : 4 reinforced concrete, 150 lbs cubic foot
  4.1 cu. Ft. of concrete mixed 1 : 2 : 4 5.8 cu ft. of concrete mixed 1 : 3 : 6
  5.1 cu. Ft. of concrete mixed 1.2 1/2 : 5 7.5 cu ft. of concrete mixed 1 : 4 : 8
   
1 Cubic foot of loose portland cement neat as cemet paste will cover about 9.5 sq. feet,
  Inch thick
1 Cubic foot of loose portland cement to 2 of sand will cover 16.8 sq. feet, 1 inch thick
1 Cubic foot of loose portland cement to 2 of sand will cover about 29 sq. feet, 1 inch thick
1 Cubic foot of loose portland cement to 3 of sand will cover about 35.7 sq. feet, 1 inch thick