PRACTICAL TIPSPRACTICAL TIPS

Presented by Er.T.Rangarjan, B.E,M.Sc(Struc.Engg), F.I.E, CEng

Even though we learn many Theories, Even though we learn many Theories, Studies and Text books it is difficult to Studies and Text books it is difficult to get many practical tips that are known get many practical tips that are known through practical experiencethrough practical experience

INTRODUCTION

The following PRACTICAL TIPS are from my The following PRACTICAL TIPS are from my long experience with the construction long experience with the construction industry both in India and abroad.industry both in India and abroad.

The tips are only on Concrete and its The tips are only on Concrete and its design principle.design principle.

1)1) ABRASIVE RESISTANCE of ABRASIVE RESISTANCE of concrete increases with concrete increases with compressive strength and use compressive strength and use of aggregate having low of aggregate having low abrasion.abrasion.

2)2)SULPHATE RESISTING CEMENT is considered SULPHATE RESISTING CEMENT is considered INEFFECTIVE in an environment where both INEFFECTIVE in an environment where both Sulphates and Chlorides are present.Sulphates and Chlorides are present.

ReasonReason: SRC has alow content of C: SRC has alow content of C33A to reduce A to reduce the influence of Sulphatethe influence of Sulphate attack. But in environment with both sulphates attack. But in environment with both sulphates and chlorides, the Cand chlorides, the C33A inA in

the cement reacts preferentially with the the cement reacts preferentially with the Sulphates and enough CSulphates and enough C33A is left to bind the A is left to bind the chlorides. chlorides.

3)3)The basic mechanical properties for The basic mechanical properties for “Structural design” for steel “Structural design” for steel

reinforcement are:reinforcement are: a)a) The characteristic yield strengthThe characteristic yield strength b)b) Ultimate tensile strength Ultimate tensile strength c)c) ElongationElongation

4) 4) Why Fe500 and above grade of steel Why Fe500 and above grade of steel reinforcing bars are not allowed for reinforcing bars are not allowed for members subject to SEISMIC forces?members subject to SEISMIC forces?

Reason: Reason: The bars having yield strength The bars having yield strength higher than 500N/mmhigher than 500N/mm22 tend to possess tend to possess lower percentage elongation which is not lower percentage elongation which is not acceptable for Seismic prone structures acceptable for Seismic prone structures since plastic hinge formation is not since plastic hinge formation is not possible.possible.

5)5)Do you know that:Do you know that: For steel bars to loose one mm diameter For steel bars to loose one mm diameter

due to corrosion, it takes about 12.5 years. due to corrosion, it takes about 12.5 years. But due to practical reasons the number of But due to practical reasons the number of years reduces due to hostile corrosive years reduces due to hostile corrosive environment.environment.

For 6mm dia. To corrode completely it For 6mm dia. To corrode completely it takes about 75 years.takes about 75 years.

6)6)Cracking levels depend on,Cracking levels depend on, d)d) tensile strength of concrete.tensile strength of concrete. e)e) The cover thickness.The cover thickness. f)f) The diameter of rebar & The diameter of rebar & g)g) Rate of corrosion.Rate of corrosion.

7)7)Corrosion takes place Corrosion takes place only only in the in the presence of presence of MOISTURE & OXYGEN.MOISTURE & OXYGEN.

8)8)The relation between the cube The relation between the cube strength & cylinder strength isstrength & cylinder strength is

ff’’cc =0.8 fck where f =0.8 fck where f’’

cc= cylinder strength,= cylinder strength, fck= cube strength.fck= cube strength.

9)9)The static Modulus Ec(Mpa) in terms of The static Modulus Ec(Mpa) in terms of characteristic cube strength fck(Mpa) ,characteristic cube strength fck(Mpa) ,

Ec=5000√fck N/mmEc=5000√fck N/mm22,(IS code),,(IS code), Ec=0.0427√βEc=0.0427√β33f’c (ACI code),f’c (ACI code), =4500√fck where β =2400Kg/m=4500√fck where β =2400Kg/m33..

10) Poisson’s ratio:10) Poisson’s ratio: A value of about 0.2 is usually A value of about 0.2 is usually

considered for design.considered for design.

11) Direct tensile strength of 11) Direct tensile strength of concrete is equal to about 7 to 15% concrete is equal to about 7 to 15% of the compressive strength. of the compressive strength.

12) For normal density concrete the 12) For normal density concrete the

splitting strength is about 2/3 of the splitting strength is about 2/3 of the modulus of rupture.modulus of rupture.

13)13) Modulus of rupture: Modulus of rupture: fcr=0.7 √fck --IS CODE fck in fcr=0.7 √fck --IS CODE fck in

N/mm2.N/mm2. Fcr=0.623√f’c—ACI Code .Fcr=0.623√f’c—ACI Code . Use of lower value of fcr will result in Use of lower value of fcr will result in

more conservative (lower) estimate more conservative (lower) estimate of cracking moment.of cracking moment.

14)14) Sherar strength: The strength of Sherar strength: The strength of concrete in PURE SHEAR has been concrete in PURE SHEAR has been reported to be in the range of 10 to reported to be in the range of 10 to 20% of its compressive strength.20% of its compressive strength.

15)15) Temperature and shrinkage Temperature and shrinkage causes tensile forces in concrete.causes tensile forces in concrete.

16)16) CREEP: When concrete is subject CREEP: When concrete is subject to sustained compressive loading , to sustained compressive loading , its deformation keeps increasing with its deformation keeps increasing with time and this time dependent time and this time dependent component-(excluding strains component-(excluding strains introduced by shrinkage and introduced by shrinkage and temperature variations) of the total temperature variations) of the total strain is termed as CREEP.strain is termed as CREEP.

17) Factors influencing creep:17) Factors influencing creep: Creep increases when, Creep increases when,

a) a) cement content is high,cement content is high, b) b) w/c ratio is high,w/c ratio is high, c) c) aggregate content is low,aggregate content is low, d) d) air entertainment is high,air entertainment is high, e) e) relative humidity is low,relative humidity is low, f) f) temperature (causing moisture loss) is high,temperature (causing moisture loss) is high, g) g) size / thickness of the member is small,size / thickness of the member is small, h) h) loading occurs at an early age & loading occurs at an early age & i)i)loading is sustained over a long period.loading is sustained over a long period.

18)18)Effect of creep:Effect of creep: Determental results in RC structures due Determental results in RC structures due

to creep:to creep: aa) ) increased deflection of beams and slabs.increased deflection of beams and slabs.

b) b) Increased deflection of slender Increased deflection of slender columns( possibly leading to buckling)columns( possibly leading to buckling)

c) c) Gradual transfer of load from concrete to Gradual transfer of load from concrete to reinforcing steel in compression members.reinforcing steel in compression members.

d) d) Loss of prestress in prestressed concrete.Loss of prestress in prestressed concrete.

19) In order to reduce the effect of 19) In order to reduce the effect of creep-deflection it is advisable to use creep-deflection it is advisable to use 0.2% of cross sectional area at the 0.2% of cross sectional area at the compression face.compression face.

20) Ultimate creep co-efficient ө,20) Ultimate creep co-efficient ө, ө = 2.2 for age of loading at 7 days,ө = 2.2 for age of loading at 7 days, = 1.6 –do- at 28 days,= 1.6 –do- at 28 days, = 1.1 –do- at one year.= 1.1 –do- at one year.

21)Effective modulus of elasticity for 21)Effective modulus of elasticity for concrete, Ece=Ec/1+ ө.concrete, Ece=Ec/1+ ө.

22) Alternating drying and wet 22) Alternating drying and wet conditions will cause alternating conditions will cause alternating volume changes in concrete. So, volume changes in concrete. So, curing by this method is not curing by this method is not recommended. recommended.

Continuous curing is mandatory to Continuous curing is mandatory to get the best results.get the best results.

23) Symmeterical arrangements of 23) Symmeterical arrangements of reinforcement will aid to avoid the reinforcement will aid to avoid the differential restraint.differential restraint.

24)24)IS code recommends the co-efficient of IS code recommends the co-efficient of

thermal expansion of concrete from thermal expansion of concrete from 6x10^-6 mm/mm per degree c. to 6x10^-6 mm/mm per degree c. to 12x10^-6 mm/mm per degree c. 12x10^-6 mm/mm per degree c.

25)The co-efficient of thermal expansion of 25)The co-efficient of thermal expansion of

steel is 12x10^-6mm/mm per degree csteel is 12x10^-6mm/mm per degree c

26)26)The water content normally ranges The water content normally ranges from 180 to 200 lit per mfrom 180 to 200 lit per m33in concrete in concrete mix.mix.

27)27)The ratio of Fine aggregate to Coarse The ratio of Fine aggregate to Coarse aggregate is 1:2 or 0.35:65.aggregate is 1:2 or 0.35:65.

28)28)Es modulus of elasticity of steel Es modulus of elasticity of steel Es=2x10^5Mpa(n/mm^2)(200kn/mm^2Es=2x10^5Mpa(n/mm^2)(200kn/mm^2))

29)29)Hanger bars of nominal diameter used for Hanger bars of nominal diameter used for the purpose of holding stirrups DO NOT the purpose of holding stirrups DO NOT normally qualify as Compression reinforcement normally qualify as Compression reinforcement –unless the area of such bars is greater than –unless the area of such bars is greater than 0.2% of sectional area of the member.0.2% of sectional area of the member.

30)30)Shall we use Fe500 grade of steel for stirrups Shall we use Fe500 grade of steel for stirrups to resist the shear forces? to resist the shear forces?

No.No. Under clause c1.39.4, the IS code IS 456 Under clause c1.39.4, the IS code IS 456 limits the value of Fe 415Mpa as high strength limits the value of Fe 415Mpa as high strength reinforcement may be rendered brittle at sharp reinforcement may be rendered brittle at sharp bends of the WEB reinforcement, also a shear bends of the WEB reinforcement, also a shear compression failure could procede the yielding compression failure could procede the yielding of the high strength steel.of the high strength steel.

31)31)The shear capacity of concrete shall The shear capacity of concrete shall not be considered effective for members not be considered effective for members subject to SEISMIC forces and hence subject to SEISMIC forces and hence whole value shall be resisted by whole value shall be resisted by closedclosed vertical stirrups.vertical stirrups.

32)32)Shear Modulus G =0.4EcShear Modulus G =0.4Ec

33)33)Torsional reinforcement is provided in Torsional reinforcement is provided in the form of the form of closed stirrupsclosed stirrups and and Longitudinal bars which are distributed Longitudinal bars which are distributed around the cross section, close to the around the cross section, close to the periphery.periphery.

34)34)To achieve economy and importantly To achieve economy and importantly to get the ductility requirements the to get the ductility requirements the members always should be designed as members always should be designed as “ “ UNDER REINFORCED “UNDER REINFORCED “ section by section by limiting the Pt(the ration of reinforcement limiting the Pt(the ration of reinforcement steel area ) to 75% of Pt limit(Balalnced steel area ) to 75% of Pt limit(Balalnced section).section).

35)35)IS456-2000 allows only 30 %( max) IS456-2000 allows only 30 %( max) moment redistribution in General for moment redistribution in General for beams and slabs( beams and slabs( NOT FOR NOT FOR COLUMNSCOLUMNS)-10%(max) for structures )-10%(max) for structures subject to vertical gravity loads only.subject to vertical gravity loads only.

36)36)Reduction of moments on account of Reduction of moments on account of moment redistribution is generally moment redistribution is generally NOT NOT APPLIED TO COLUMNS.APPLIED TO COLUMNS.

37)37)To find the weight of the steel bars To find the weight of the steel bars per meter, mutiply by 0.006162times per meter, mutiply by 0.006162times dia^2.(Kg/m)dia^2.(Kg/m)

Example : to find weight of 25 mm Example : to find weight of 25 mm bar/m=0.006162*25^2=3.85Kg/mbar/m=0.006162*25^2=3.85Kg/m

38)38)Is it good to apply epoxies on WET or Is it good to apply epoxies on WET or DAMP surfaces? No. Because the DAMP surfaces? No. Because the epoxies do not bond on wet or damp epoxies do not bond on wet or damp surfaces.surfaces.

39)39)For cantilever beamsFor cantilever beams: : Where the main bars shall be Where the main bars shall be placed? Top or bottom. If placed? Top or bottom. If bottom, the member will bottom, the member will collapse immediately after collapse immediately after removing the centering. It removing the centering. It should be placed should be placed on top as on top as the tension is on the top the tension is on the top surface.surface.

40)40)For cantilever beams: How For cantilever beams: How will be positioned the hooks of will be positioned the hooks of the stirrups in a cantilever the stirrups in a cantilever beam? Is it at the top as beam? Is it at the top as usually done for beam or at the usually done for beam or at the bottom? It should be bottom? It should be at bottom at bottom onlyonly since the hooks if placed since the hooks if placed at the top will open up when it at the top will open up when it bends while it is tension state.bends while it is tension state.

41) In frame analysis, centre line dimensions of beams 41) In frame analysis, centre line dimensions of beams and columns are generally used to define the and columns are generally used to define the geometry of frame “line diagram”. The BM geometry of frame “line diagram”. The BM obtained is on Centre line which has to be reduced obtained is on Centre line which has to be reduced by Vb/3 . ie Ms-Vb/3 where Ms is the moment at 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 centre line and V is the shear at the centre line and b is the width of the column or beam. This enables to is the width of the column or beam. This enables to get lesser steel area which aids in avoiding get lesser steel area which aids in avoiding congestion of reinforcement at the beam column congestion of reinforcement at the beam column joint to some extent. (vide page 309 –RC DESIGN joint to some extent. (vide page 309 –RC DESIGN By S.Unnikrishna Pillai and Devadas Menon.)By S.Unnikrishna Pillai and Devadas Menon.)

42) The shear also should be taken at a distance 42) The shear also should be taken at a distance of d the effective depth from the face of the of d the effective depth from the face of the column or beam .column or beam .43) For all buildings which are more than 3 43) For all buildings which are more than 3

storeys in height, the min. grade of concrete storeys in height, the min. grade of concrete shall be M20. (clause 5.2 of IS 13920:1993)shall be M20. (clause 5.2 of IS 13920:1993)

44) ….It may be clarified that 44) ….It may be clarified that REDIRTIBUTION of MOMENTSof MOMENTS permitted in IS 456:2000 will be permitted in IS 456:2000 will be used only for used only for VERTICAL LOADVERTICAL LOAD MOMENTSMOMENTS AND AND NOTNOT FOR LATERAL LOADFOR LATERAL LOAD MOMENTSMOMENTS.. (clause 6.2.4 of IS 13920:1993)(clause 6.2.4 of IS 13920:1993)

45) The contribution of bent up bars & inclined 45) The contribution of bent up bars & inclined hoops to shear resistance of the section shall hoops to shear resistance of the section shall not benot be considered while designing against the considered while designing against the SEISMIC FORCESSEISMIC FORCES. (clause 6.3.4 of IS . (clause 6.3.4 of IS 13920:1993) 13920:1993)

46) The mix proportion 1:2:4 or 1:1.5:3 is by 46) The mix proportion 1:2:4 or 1:1.5:3 is by weight or by volume? weight or by volume?

47) 1000 litre of water weighs to 1000Kg. Is it 47) 1000 litre of water weighs to 1000Kg. Is it right?right?

48) One litre is equal 1cu.m. Is it correct?48) One litre is equal 1cu.m. Is it correct?

49) A first class brick should not absorb water 49) A first class brick should not absorb water more than ….of its own dry weight after 24 more than ….of its own dry weight after 24 hours immersion in cold water.hours immersion in cold water.

10% b)15% 10% b)15% c) 20%c) 20% d) 25%. d) 25%.

50) A first class brick should have a minimum 50) A first class brick should have a minimum crushing strength of crushing strength of

70Kg/cm^2 70Kg/cm^2 105Kg/cm^2105Kg/cm^2 125 kg/cm^2125 kg/cm^2 140 Kg/cm^2.140 Kg/cm^2.

51) Excess of Alumina and Silica in the clay:51) Excess of Alumina and Silica in the clay: makes the brick brittle and weak,makes the brick brittle and weak, makes the brick crack and wrap on drying.makes the brick crack and wrap on drying. Changes colour of the brick from red to yellow,Changes colour of the brick from red to yellow, Improves the impermeability and durability of Improves the impermeability and durability of

the brickthe brick Leaves high powder deposit on the brick.Leaves high powder deposit on the brick.

52) The shrinkage of ordinary concrete is 0.3 52) The shrinkage of ordinary concrete is 0.3 to 0.6 mm/m.to 0.6 mm/m.

53) The permissible limit for solids in water used for 53) The permissible limit for solids in water used for concrete mix as per IS456:2000 are:concrete mix as per IS456:2000 are:

1.Organic 200mg/l1.Organic 200mg/l 2.Inorganic 3000mg/l2.Inorganic 3000mg/l 3.Sulphates(as So3) 400mg/l3.Sulphates(as So3) 400mg/l 4.Chlorides(as cl) 2000mg/l for concrete not 4.Chlorides(as cl) 2000mg/l for concrete not containing embedded steel containing embedded steel and 500mg/l for RCC and 500mg/l for RCC works.works. 5.Suspended matter 2000mg/l.5.Suspended matter 2000mg/l.

54) Cracks are of live and dead . That is 54) Cracks are of live and dead . That is moving and non moving cracks. moving and non moving cracks.

For moving cracks use always a material that For moving cracks use always a material that will accommodate the movement. Ex. Joint will accommodate the movement. Ex. Joint sealant.sealant.

For rigid –structural cracks, use a materila that For rigid –structural cracks, use a materila that will add strength by bonding with the parent will add strength by bonding with the parent member. Ex. Epoxies or cementitious member. Ex. Epoxies or cementitious proprietary materialproprietary material

55) To find the depth of RCC member from 55) To find the depth of RCC member from the moment for M20 & Fe 415,the moment for M20 & Fe 415,

d= 670.82d= 670.82√√M/b where M is Knm,b=breath of M/b where M is Knm,b=breath of the member in mm & d is in mm.the member in mm & d is in mm.

This is for the balanced reinforced section.This is for the balanced reinforced section.

56) to find the steel for a singly reinforced 56) to find the steel for a singly reinforced section of M20 and Fe415,section of M20 and Fe415,

Ast =3077.44*M/d where M in Knm,Ast =3077.44*M/d where M in Knm, d in mm & Ast in mm^2.d in mm & Ast in mm^2. When M/bd^2 is less than 1.27 the steel area When M/bd^2 is less than 1.27 the steel area

should be calculated using the lever arm should be calculated using the lever arm z= 0.95d.z= 0.95d.

57) The lap lenth of bars shall be in tension for 57) The lap lenth of bars shall be in tension for Seismic forces and it can be safely taken as Seismic forces and it can be safely taken as

Ld= 50*dia of bar.Ld= 50*dia of bar.

58)Strength of concrete for various period are:58)Strength of concrete for various period are: Days/months strengthDays/months strength 7 days 2/3 of 28day strength(CP114) 7 days 2/3 of 28day strength(CP114) 28 days 1.028 days 1.0 2 months 1.1 (Table 5.1-p298-Properties of Concrete 2 months 1.1 (Table 5.1-p298-Properties of Concrete by Adam Neville.)by Adam Neville.) 3 months 1.163 months 1.16 6 months 1.26 months 1.2 12 months 1.2412 months 1.24

59) The relation between the 28-day strength and 59) The relation between the 28-day strength and 7 day strength which lies between as given in 7 day strength which lies between as given in Germany isGermany is

fcfc28 =1.428 =1.4fcfc7 +1.0 &7 +1.0 &

fcfc28 =1.728 =1.7fcfc7 +5.9 7 +5.9 where fc being expressed in Mpa.where fc being expressed in Mpa. (page 300 –Properties of concrete by Adam Neville)(page 300 –Properties of concrete by Adam Neville) ACI RECOMMENDSACI RECOMMENDS fcm(t) =ffcm(t) =f28{28{t/(4+0.85t)}t/(4+0.85t)} For 7 days the value comes to 0.71% of 28 days strength.For 7 days the value comes to 0.71% of 28 days strength. For 3 days the value comes to 0.458% of 28 dyas strength.For 3 days the value comes to 0.458% of 28 dyas strength.

60) For rough estimation of reinf. Steel in construction projects 60) For rough estimation of reinf. Steel in construction projects following thumb rules may be adopted:following thumb rules may be adopted:

SLAB 50 TO 80Kg/m^3 of concrete.SLAB 50 TO 80Kg/m^3 of concrete. Sunshade 50 Kg/m^3 of concrete.Sunshade 50 Kg/m^3 of concrete. Lintels 80Kg/m^3 of concrete.Lintels 80Kg/m^3 of concrete. Beams 100TO 150 Kg/m^3 of concrete.Beams 100TO 150 Kg/m^3 of concrete. Columns 150 to 225 Kg/m^3 of concrete.Columns 150 to 225 Kg/m^3 of concrete. Footing slab 80Kg/m^3 of concrete.Footing slab 80Kg/m^3 of concrete.