Practical Tips

PRACTICAL TIPS 1)ABRASIVE RESISTANCE of concrete increases with compressive strength and use of aggregate having low abrasion. 2)SULPHATE RESISTING CEMENT is considered INEFFECTIVE in an environment where both Sulphates and Chlorides are present. Reason: SRC has alow content of C3A to reduce the influence of Sulphate attack. But in environment with both sulphates and chlorides, the C3A in the cement reacts preferentially with the Sulphates and enough C3A is left to bind the chlorides. 3)The basic mechanical properties for “Structural design” for steel reinforcement are: a) The characteristic yield strength b) Ultimate tensile strength c) Elongation

4) Why Fe500 and above grade of steel reinforcing bars are not allowed for members subject to SEISMIC forces? Reason: The bars having yield strength higher than 500N/mm2 tend to possess lower percentage elongation which is not acceptable for Seismic prone structures since plastic hinge formation is not possible. 5)Do you know that: For steel bars to loose one mm diameter due to corrosion, it takes about 12.5 years. But due to practical reasons the number of years reduces due to hostile corrosive environment. For 6mm dia. To corrode completely it takes about 75 years. 6)Cracking levels depend on, d)Tensile strength of concrete. e)The cover thickness. f)The diameter of rebar & g)Rate of corrosion. 7)Corrosion takes place only in the presence of MOISTURE & OXYGEN.

8)The relation between the cube strength & cylinder strength is f’c =0.8 fck where f’c= cylinder strength, fck= cube strength. 9)The static Modulus Ec(Mpa) in terms of characteristic cube strength fck(Mpa) , Ec=5000√fck N/mm2,(IS code), Ec=0.0427√β3f’c (ACI code) =4500√fck where β =2400Kg/m3. 10) Poisson’s ratio:A value of about 0.2 is usually considered for design. 11) Direct tensile strength of concrete is equal to about 7 to 15% of the compressive strength. 12) For normal density concrete the splitting strength is about 2/3 of the modulus of rupture. 13) Modulus of rupture: fcr=0.7 √fck --IS CODE fck in N/mm2. Fcr=0.623√f’c—ACI Code . Use of lower value of fcr will result in more conservative (lower) estimate of cracking moment.

14) Shear strength: The strength of concrete in PURE SHEAR has been reported to be in the range of 10 to 20% of its compressive strength. 15) Temperature and shrinkage causes tensile forces in concrete. 16) CREEP: When concrete is subject to sustained compressive loading , its deformation keeps increasing with time and this time dependent component(excluding strains introduced by shrinkage and temperature variations) of the total strain is termed as CREEP. 17) Factors influencing creep: Creep increases when, a) cement content is high, b) w/c ratio is high, c) aggregate content is low, d) air entertainment is high, e) relative humidity is low, f) temperature (causing moisture loss) is high, g) size / thickness of the member is small, h) loading occurs at an early age & i) loading is sustained over a long period.

18)Effect of creep: Determental results in RC structures due to creep: increased deflection of beams and slabs. Increased deflection of slender columns( possibly leading to buckling) Gradual transfer of load from concrete to reinforcing steel in compression members. Loss of prestress in prestressed concrete. 19) In order to reduce the effect of creep-deflection it is advisable to use 0.2% of cross sectional area at the compression face. 20) Ultimate creep co-efficient ө, ө = 2.2 for age of loading at 7 days, = 1.6 –do- at 28 days, = 1.1 –do- at one year. 21)Effective modulus of elasticity for concrete, Ece=Ec/1+ ө.

22) Alternating drying and wet conditions will cause alternating volume changes in concrete. So, curing by this method is not recommended. Continuous curing is mandatory to get the best results. 23) Symmeterical arrangements of reinforcement will aid to avoid the differential restraint. 24)IS code recommends the co-efficient of thermal expansion of concrete from 6x10^-6 mm/mm per degree c. to 12x10^-6 mm/mm per degree c. 25)The co-efficient of thermal expansion of steel is 12x10^-6mm/mm per degree c 26)The water content normally ranges from 180 to 200 lit per m3in concrete mix. 27)The ratio of Fine aggregate to Coarse aggregate is 1:2 or 0.35:65. 28)Es modulus of elasticity of steel Es=2x10^5Mpa(n/mm^2) (200kn/mm^2)

29)Hanger bars of nominal diameter used for the purpose of holding stirrups DO NOT normally qualify as Compression reinforcement – unless the area of such bars is greater than 0.2% of sectional area of the member. 30)Shall we use Fe500 grade of steel for stirrups to resist the shear forces? No. Under clause c1.39.4, the IS code IS 456 limits the value of Fe 415Mpa as high strength reinforcement may be rendered brittle at sharp bends of the WEB reinforcement, also a shear compression failure could procede the yielding of the high strength steel. 31)The shear capacity of concrete shall not be considered effective for members subject to SEISMIC forces and hence whole value shall be resisted by closed vertical stirrups. 32)Shear Modulus G =0.4Ec

33)Torsional reinforcement is provided in the form of closed stirrups and Longitudinal bars which are distributed around the cross section, close to the periphery. 34)To achieve economy and importantly to get the ductility requirements the members always should be designed as “ UNDER REINFORCED “ section by limiting the Pt(the ration of reinforcement steel area ) to 75% of Pt limit(Balalnced section). 35)IS456-2000 allows only 30 %( max) moment redistribution in General for beams and slabs( NOT FOR COLUMNS)-10%(max) for structures subject to vertical gravity loads only. 36)Reduction of moments on account of moment redistribution is generally NOT APPLIED TO COLUMNS. 37)To find the weight of the steel bars per meter, mutiply by 0.006162times dia^2.(Kg/m) Example : to find weight of 25 mm bar/m=0.006162*25^2=3.85Kg/m 38)Is it good to apply epoxies on WET or DAMP surfaces? No. Because the epoxies do not bond on wet or damp surfaces.

39)For cantilever beams: Where the main bars shall be placed? Top or bottom. If bottom, the member will collapse immediately after removing the centering. It should be placed on top as the tension is on the top surface. 40)For cantilever beams: How will be positioned the hooks of the stirrups in a cantilever beam? Is it at the top as usually done for beam or at the bottom? It should be at bottom only since the hooks if placed at the top will open up when it bends while it is tension state. 41) In frame analysis, centre line dimensions of beams and columns are generally used to define the geometry of frame “line diagram”. The BM obtained is on Centre line which has to be reduced by Vb/3 . ie Ms-Vb/3 where Ms is the moment at centre line and V is the shear at the centre line and b is the width of the column or beam. This enables to get lesser steel area which aids in avoiding congestion of reinforcement at the beam column joint to some extent. (vide page 309 –RC DESIGN By S.Unnikrishna Pillai and Devadas Menon.)

42) The shear also should be taken at a distance of d the effective depth from the face of the column or beam . 43) For all buildings which are more than 3 storeys in height, the min. grade of concrete shall be M20. (clause 5.2 of IS 13920:1993) 44) ….It may be clarified that REDISTRIBUTION of MOMENTS permitted in IS 456:2000 will be used only for VERTICAL LOAD MOMENTS AND NOT FOR LATERAL LOAD MOMENTS. (clause 6.2.4 of IS 13920:1993) 45) The contribution of bent up bars & inclined hoops to shear resistance of the section shall not be considered while designing against the SEISMIC FORCES. (clause 6.3.4 of IS 13920:1993) 46) The mix proportion 1:2:4 or 1:1.5:3 is by weight or by volume? 47) 1000 litre of water weighs to 1000Kg. Is it right? 48) One litre is equal 1cu.m. Is it correct?

49) A first class brick should not absorb water more than ….of its own dry weight after 24 hours immersion in cold water. a)10% b)15% c) 20% d) 25%. 50) A first class brick should have a minimum crushing strength of  70Kg/cm^2  105Kg/cm^2  125 kg/cm^2  140 Kg/cm^2. 51) Excess of Alumina and Silica in the clay:  makes the brick brittle and weak,  makes the brick crack and wrap on drying.  Changes colour of the brick from red to yellow,  Improves the impermeability and durability of the brick  Leaves high powder deposit on the brick. 52) The shrinkage of ordinary concrete is 0.3 to 0.6 mm/m.

53) The permissible limit for solids in water used for concrete mix as per IS456:2000 are: 1.Organic 200mg/l 2.Inorganic 3000mg/l 3.Sulphates(as So3) 400mg/l 4.Chlorides(as cl) 2000mg/l for concrete not containing embedded steel and 500mg/l for RCC works. 5.Suspended matter 2000mg/l. 54) Cracks are of live and dead . That is moving and non moving cracks. For moving cracks use always a material that will accommodate the movement. Ex. Joint sealant. For rigid –structural cracks, use a materila that will add strength by bonding with the parent member. Ex. Epoxies or cementitious proprietary material 55) To find the depth of RCC member from the moment for M20 & Fe 415, d= 670.82√M/b where M is Knm,b=breath of the member in mm & d is in mm. This is for the balanced reinforced section

56) To find the steel for a singly reinforced section of M20 and Fe415, Ast =3077.44*M/d where M in Knm,d in mm & Ast in mm^2. When M/bd^2 is less than 1.27 the steel area should be calculated using the lever arm, z= 0.95d. 57) The lap lenth of bars shall be in tension for Seismic forces and it can be safely taken as Ld= 50*dia of bar. 58)Strength of concrete for various period are: Days/months Strength 7 days 2/3 of 28day strength(CP114) 28 days 1.0 2 months 1.1 (Table 5.1-p298-Properties of Concrete by Adam Neville.) 3 months 1.16 6 months 1.2 12 months 1.24

59) The relation between the 28-day strength and 7 day strength which lies between as given in Germany is fc28 =1.4fc7 +1.0 & fc28 =1.7fc7 +5.9 where fc being expressed in Mpa. (page 300 –Properties of concrete by Adam Neville) ACI RECOMMENDS fcm(t) =f28{t/(4+0.85t)} For 7 days the value comes to 0.71% of 28 days strength. For 3 days the value comes to 0.458% of 28 days strength.

60) For rough estimation of reinf. Steel in construction projects following thumb rules may be adopted: Slab 50 TO 80Kg/m^3 of concrete. Sunshade 50 Kg/m^3 of concrete. Lintels 80Kg/m^3 of concrete. Beams 100TO 150 Kg/m^3 of concrete. Columns 150 to 225 Kg/m^3 of concrete. Footing slab 80Kg/m^3 of concrete.