Civil Engineering

Basic Interview questions- Civil structural design

2020-04-21T06:22:00.000-07:00

Basic Interview questions for Civil Design Engineer

01. What is the Minimum reinforcement percentage for a column?

Answer: The minimum reinforcement percentage is 0.8% of the cross-sectional area of a column or A_st= 0.85db / f_y.

02. What is the Maximum reinforcement percentage for a column?

Answer: The minimum reinforcement percentage is 6.0% of the cross-sectional area of a column or 0.04 bD.

03. How do we determine the effective length of a column?

Answer: The effective length of a column is expressed as L_eff = k x L

Where k stands for effective length ratio.

L stands for unsupported length or clear height of the column.

04. What is the Effective length ratio of a column?

Answer: The effective length ratio is basically a constant depending upon the degree of rotational & translational restraints at the ends of the column.

05. Define Slenderness Limits of a column?

Answer: The Slenderness Limits of a column ensures that failure shall only occur due to material failure and not because of buckling. Therefore, the column is checked for slenderness limit, which shall never exceed 60 times the least lateral dimension of the column.

06. How to classify short columns & long or slender columns?

Answer: When the slenderness ratio's L_ex/D & L_ey/b are less than 12, the column is termed as a short column, whereas if the value is greater than or equal to 12 then it is termed as a slender or long column.

Where L_ex stands for effective length in respect of the major axis.

D stands for depth in respect of the major axis.

L_ey stands for effective length in respect of the minor axis.

b stands for the width of the column.

07. What is the minimum eccentricity of the column that needs to be taken into consideration during designing?
Answer: The minimum eccentricity of the column that needs to be taken into consideration during designing is e_min = L/500 + D/30, but not less than 20 mm.

Where L stands for unsupported length or clear height of the column.

D stands for the lateral dimension in the plane of bending.

08. What is the maximum moment that occurs in a column & where does it occur?
Answer: The maximum moment that occurs in a column is expressed as M_max = P (e + Δ_max)

Where P stands for the load on the column.

Δ_max stands for the lateral deflection of the longitudinal axis.

09. What are the non-dimensional parameters used for designing a column with uniaxial eccentric loading?
Answer: The non-dimensional parameters used for designing a column with uniaxial eccentric loading are P_u/b.d.f_ck & M_u/b.d²f_ck .

10. What is the codal method (IS Code) for designing a compression member subjected to biaxial bending?
Answer: The codal methods suggests the use of Bresler's empirical formula which is expressed as follows - (M_ux / M_ux1)^α_n+ (M_uy / M_uy1)^α_n ≤ 1.0.
Where M_ux stands for the moment about X-axis due to design loads on the column.

M_uy stands for the moment about Y-axis due to design loads on the column.

M_ux1 stands for the maximum uniaxial moment capacity with as axial load P_u & bending about X-axis.

M_uy1 stands for the maximum uniaxial moment capacity with as axial load P_u & bending about Y-axis.

αⁿ is an exponent whose value depends upon the ratio (P_u/P_uz)

11. What shall be the overall depth to width ratio of a beam?

Answer: The overall depth to width ratio of a beam shall lie between 1.5 to 2. However, there are other types of beams such as hidden/concealed beams that don't follow this criterion.

12. What are the hidden/concealed beams?

Answer: The hidden/concealed beams are those beam whose depth equals the depth of the slab. When there is a restriction in providing a beam in between slabs due to architectural features, a hidden/concealed beam is used.

13. What is the minimum & maximum number of bars to be used as tension reinforcement in a beam?

Answer: The minimum number of bars to be used as tension reinforcement in a beam is 2 & a maximum of 6 numbers of bars can be placed in a single layer.

14. What is the minimum percentage of tension reinforcement to be provided in a beam?

Answer: The minimum percentage of tension reinforcement to be provided in a beam shall be 0.3% of the crossectional area of the beam.

15. What is the effective width of a flanged beam?

Answer: The effective width of a flanged beam shall not be greater than the breadth of the web of the beam plus half the sum of the clear distances to the adjacent beams on either side. It is computed by the following equations-

For T-beam: b_f = [ (L_o/6) + b_w + 6 D_f ]
For L-beam: b_f = [ (L_o/12) + b_w + 3 D_f ]

Where b_f stands for effective width of the flange

L_ostands for the distance between points of zero moments

b_w stands for the breadth of the web

D_f stands for the thickness of the flange

Civil Site Engineer interview Questions

2020-04-12T04:53:00.000-07:00

Basic Interview questions for Civil Site Engineer

01. What does the M20 grade of concrete mean?

Answer: 'M' stands for mix design & 20 stands for the average compressive strength at 28 days in N/mm².

02. What is the ratio for M10 M15 & M20 grade of concrete?

Answer: The ratio is - M10 - 1:3:6

M15 - 1:2:4

M20 - 1:1.5:3

03. In the above answer what are the components of the ratio?

Answer: The ratio stands for Binder (Cement): Fine aggregate: Coarse aggregates.

04. What does SCC stand for?

Answer: SCC stands for Self-compacting / Self Consolidating Concrete.

05. In general, which component of concrete is responsible for cube / concrete failure?

Answer: The binder material (cement mortar) is mainly responsible for concrete failure as the failure/ crushing strength of the Coarse aggregate is much higher.

06. Why is 10 mm coarse aggregate is used in concrete along with 20 mm Coarse aggregate?

Answer: 10 mm coarse aggregates are used to fill the void in between 20 mm coarse aggregates which gives a higher density of the overall mix, and also it reduces the chances of mortar failure.

07. In construction projects what is the basic test conducted on normal concrete?

Answer: In construction project basic test conducted on normal concrete is

(a) Slump test - To measure the workability of the concrete

08. In construction projects what is the basic test conducted on SCC concrete?

Answer: In construction project basic test conducted on normal concrete is

(a) Flow table test - To measure the workability of the concrete.

(b) L-box test- To measure the workability of concrete.

(d) J-ring test- To measure the passing ability of concrete.

09. During construction of columns/walls why shear keys are provided?

Answer: As it is the joining point between old & new concrete shear key provides the load transfer mechanisms & better joining along with a concrete bonding agent.

10. Why is starter provided before casting a column from the base such as footings/pile cap?

Answer: Starters are provided for maintaining a better alignment of the column & also it would be easier to maintain the alignment of the column during fixation of the shuttering board in case of conventional shuttering.

11. Why is lap length required in TMT bars in any structural member?

Answer: Lap lengths are provided to maintain the continuity of the TMT bars for better distribution of loads throughout the structural member.

12. What is the highest height from which free fall of concrete is allowed?

Answer: The highest height from which free fall of concrete is allowed 1.5 meters as per IS code 456:2000.

How to estimate quantities of brickwork or AAC Blockwork

2020-04-05T07:54:00.001-07:00

Estimation of Brickwork & AAC Blockwork

The file attached can determine the quantities of Cement, Sand required in typical brickwork including plaster on both faces of the structure. This excel can also be utilized to find the estimated quantities of AAC blockwork, where plaster needed is zero (0). It is also very flexible, with easy customization.

Download this excel:

Link:

Estimation_Brickwork.xlxs

Waterproof coating & membrane

2020-04-03T02:04:00.000-07:00

What is waterproof coating & membrane

To prevent the entry of water through seepage or moisture damp inside an RCC structure waterproofing is used. Generally, after completion of construction work of a structure waterproofing work is carried out. The process of waterproofing can be carried out in many ways. However, it can be broadly classified as:

Coating – By the use of Polyurea coating as a waterproofing membrane.
Membrane - By using the waterproofing membrane.

Waterproofing coat (PU):

Surface Preparation – The surface is inspected for any structural defects such as cracks, undulations, and honeycombs if found it shall be rectified before coating. Right angle junctions between slab & perpendicular wall/column shall be treated by coving the junction (75mm x 75mm generally) & filling it with a polymer-modified mortar. The surface shall be free from dust, loose materials, cracks & in saturated surface dry (SSD) condition before the application of a waterproofing chemical.

Crack repairs: Grooves filled with PMM

Priming - The surface in SSD condition shall be primed, in cases where there are deep undulation & can affect the waterproofing coating shall be repaired by mixing silica floors with primer to give the primer a body by which it can be filled. Not all waterproofing coating requires primer applications.
Waterproof coating – The chemical is generally in liquid form & has to be sprayed mechanically or brushed manually depending upon the chemical & area of application. Polyurea coating has to be done by spraying by special machines that mix the hardener & coating just before spraying as it sets within minutes. Usually, 2 coats of these chemicals are provided & in a perpendicular direction for better tensile effect.

PolyUrea waterproofing coat

Protective Layer – To protect the coating from wear & tear as the coating thickness usually varies between 1.5 – 2 mm, screed concrete is done. A minimum of 15 mm thickness around the drainage point shall be kept for better protection. Integral waterproofing compounds shall be used in the screed to give a better watertight effect.

Waterproofing Membrane:

Surface Preparation – The surface is inspected for any structural defects such as cracks, undulations, and honeycombs if found it shall be rectified before coating. Right angle junctions between slab & perpendicular wall/column shall be treated by coving the junction (75mm x 75mm generally) & filling it with a polymer-modified mortar. The surface shall be free from dust, loose materials, cracks & in saturated surface dry (SSD) condition before application of waterproofing chemicals.
Priming - The surface in SSD condition shall be primed, in this case, the primer has to be applied for better bonding between the waterproofing membrane & concrete surface.
Waterproof membrane – Typically the membrane comes in 250 mm x 25 m x 3 mm (the thickness depends generally on the area of application along with its purpose). The membrane is to be laid on the concrete surface with an overlap of the membrane not less than 100 mm. The overlapped portion is heated using blow torch for better sealing. The end of the membrane is filled with PU sealant or bituminous sealant.

Waterproofing membrane

Protective Layer – To protect the coating from wear & tear the membranes are coated with a bituminous coating to extend its life as these membranes are generally followed by screed concrete as in case of waterproofing coatings.

Slab & types of slabs

2020-03-28T09:34:00.000-07:00

What is a slab & its types?

In any building the most common structural element of construction to create a flat surface such as floor & roof is called slabs. The depth of the slab is very less compared to its effective length. Different loads act on a slab, such as the dead load of the slab, live load on a slab, super-imposed dead load, earthquake load. There are different kinds of slabs based on their support conditions.

Types of slabs:

One-way Slab – Slabs whose longer dimension exceeds two times the shorter dimension is termed as a one-way slab. They are supported by beams on two opposite sides. In one-way slabs the imposed loads are transferred to the two opposite faces of the slab & in one direction, hence the name one-way slabs. In one way slab, the main reinforcements run along the shorter edges while distribution reinforcements run along the transverse direction. The most common example of a one-way slab is a balcony (generally). The load distribution is illustrated in the following figure.

Load Distribution in One-Way Slab

Two-way Slab – Slabs whose longer dimension does not exceed two times the shorter dimension is termed as a two-way slab. They are supported by beams on all four sides. In two-way slabs the imposed loads are transferred to the two opposite faces of the slab & in both the direction, hence it is called two-way slabs. Generally, floor slabs in a multi-storeyed building is a two-way slab. In Two-way slab, the bending moment is maximum at the center of the slab and the larger moment invariably develops along the shorter span. When a slab is simply supported on all four sides and is subjected to transverse loading, the bending effect in two main directions of loading causes the corner at the supports of the slab to curl and lift up. This bending moment effect in two directions gives a torsional effect to the slab, therefore during designing the corners are restrained.

Load Distribution in Two-Way Slab

Cantilever Slab – Slabs that are supported on one side only are termed as cantilever slabs. In cantilever slab, the main reinforcement runs along the shorter span and from the top, as the main moment is generated at the top supports. Horizontal projections of a building are generally cantilever slab.

Load Distribution in Cantilever Slab

Flat Slab – Places where the clear height is an important factor or where the use of beams restricts the usability of the space, to avoid beams flat slabs are used. More suitably it is a multi-spanned slab supported by columns directly without the use of beams. There are different types of flat slabs such as (a) Slabs without drops & column head (b) Slabs without drops & column with column head (c) Slab with drop & column head.

Bending Moment in a slab:

The bending moment in a slab depends upon the following criteria:

Support conditions of the slabs, also known as edge condition. (Like free, fixed continuous)

The short & long span (L_x and L_y)

Type of loading on the slab (Uniformly distributed load, Concentrated load, etc).

Bonding Mechanism in RCC Structure

2020-03-27T08:50:00.000-07:00

Bonding Mechanism in RCC Structure

In an RCC structure is considered safe when the relevant loads on the structure is under the permissible limits for that structure, the impact of load is sustained by the RCC main components like concrete & steel. The composite action of reinforced concrete is mainly due to the bonding or the adhesion between the reinforcement steel’s ribs provide better bonding with the surrounding concrete. This bonding between the reinforcement steel & concrete enables the transfer of axial force from the reinforcement steel to the surrounding concrete & the bond also ensures strain compatibility and composite action of the composite material (Reinforced Concrete). The assumption of plain section remains plain even after bending in the simple bending theory, is valid only when there is effective bond between the steel & surrounding concrete.

In steel reinforcement of an RCC member, the stress varies from point to point along its length mainly due to the bond resistance. If there is no bond, the stress in the steel member will be constant throughout its length as it is found in the case of straight cable used in prestressed concrete section.

Bond Mechanisms:

The Bond between the concrete & steel develops due to the following important factors:

Chemical Adhesion- It is the grip force generated due to the gum like properties of the hydration products of cement in a concrete. It provides initial adhesion properties within RCC structure.

Frictional Resistance- The resistance generated between the steel & surrounding concrete due to their relative movement is termed as frictional resistance. It depends upon the ribs present in the steel & the grip developed after shrinkage of concrete.

Shearing Resistance- It is the mechanical interlock developed due to surface protrusions or ribs provided in deformed bars.

Ribs in steel is an important factor in determining the bond resistance between steel & surrounding concrete, without which the interlocking effect cannot be achieved in an RCC structure. That is why plain bars cannot develop the bond resistance.

Bond Stresses:

A shear stress or tangential stress is developed along the contact surface of the steel & concrete in a RCC structure, which is generally termed as bond stress. Bond stress is expressed by tangential force per unit nominal surface area of the reinforcing bar. If the bond stresses are very high or beyond permissible limits then horizontal cracks develops at the reinforcement level of such locations.

Different types of bond stresses are mentioned below:

Flexure Bond Stress- Stresses that develop in an RCC member under flexure due to variation of bending moment or shear force at a section is termed as Flexure Bond Stress.

Anchorage (Development) bond stress- The stress developed at the end portion of a steel bar in an RCC structure under compression or tension is termed as Anchorage (Development) bond stress. Therefore, Development length is provided in reinforcements at the termination components.

Design Bond Stress in Limit state method for plain bars in Tension

Foundation Structure: Footings

2020-03-23T09:00:00.000-07:00

Foundation Structure: Classification of Footings

Generally, In an RCC structure the applied load, moments & other forces are transmitted through the column to the foundation structure below ground level such as pile via pile cap or footings, without exceeding the safe bearing capacity. The settlement of the structure shall be kept as per tolerable limit and nearly uniform as possible.

In a footing, the plan area is much larger than the cross-sectional area of the column, the load is transferred from the slabs via beam to the columns and the total load is distributed throughout the area of the footing via reinforcements & concrete. The effect of the load is transferred to the soil by bearing pressure, normally the bearing pressure of the soil is much lower than concrete & steel. In generally in India the safe bearing capacity of a particular soil ranges between 100 – 400 kN/Sq.m whereas, the permissible compressive strength of concrete is approximately 5-15 N/mm² & for steel it ranges between 130 – 190 N/mm²in reinforced concrete columns under working loads. These types of supporting structure are termed as foundation structure and sometimes sub structure.

Foundation structure is broadly classified into two categories:

Shallow foundations – Isolated footing, Combined footing, Eccentric footing, Trapezoidal footing & Strap footing.

Deep foundations – Piles & Caissons.

Different types of footings are:

In general footings are designed to resist the bending moment & shear force developed due to soil reaction.

Isolated footing – In framed RCC structure where the total load is not huge (generally less than 3-4 storied buildings) it is sufficient to provide separate independent footings connected via tie beam. This type of footing is termed as isolated footing. Generally, its shape varies depending upon the shape of the column’s cross-sectional it can be rectangle, square, circle etc. It consists of a thick slab which can be pad, sloped or stepped.

Types of Isolated Footing

Combined footing – When two heavily loaded column are placed close to each other and the isolated footings overlap each other it is much more sensible & effective to merge two different footing, thus creating combined footing. The use of combined footing also improves the integral behavior of the columns & the footing. The combined footing contains a beam between the two columns known as the connecting beam.

Eccentric footing & Trapezoidal footing – When the column is located near the end of the footing due to some restriction of column/ footing placement (such as near the property line of a plot), Special type of footings are designed to overcome these hindrances. Isolated footings with eccentrically loaded column are termed as eccentric footings whereas, in combined footing it is termed as trapezoidal footing, generally in combined footing near the property line the size of the footing is reduced as requirement, thus giving it a trapezoidal shape.

Strap footing – Strap footing are two isolated footing connected with a connecting beam through the footing. The footing’s thick slab acts a cantilever slab flanged on both side of the connecting beam. These footings are also used where there is problem of column placing in the centroid axis of the column.

Types of Footings.

Classification of columns

2020-03-23T04:36:00.000-07:00

Types of columns & their properties

Structural Concrete member in compression are generally termed as columns & strut. The term column is used for structural member transferring loads to the ground. As per Indian Standard Code IS 456:2000 clause 25.1.1 defines the column as a compression member whose effective length exceeds three times the least lateral dimension. Whereas pedestal is termed as vertical compression members whose effective length is less than three times its least lateral dimension.

Axially loaded columns may fail in any one of the following modes:

Pure Compression failure- Where the column generally fails due to the axial load & Buldge at the center of the unsupported length.

Combined Compression & bending failure- Where the column generally fails due to the combined effect of axial load & bending due to the presence of beams.

Failure by elastic instability.

These modes generally depend on the slenderness ratio of the column which in turns depend upon the cross-sectional dimensions, effective length & support conditions of the column.

Classification of Columns

Classification of columns:

Based on type of reinforcement:

Tied Column- In this type of columns the main longitudinal reinforcements are enclosed within closely spaced lateral ties. Tied columns are generally used in square, rectangle, T-shaped, L-shaped, circular columns etc.

Spiral Columns- In this type of columns the main longitudinal reinforcements are also enclosed within closely spaced ties, but the ties are continuously wounded spiral reinforcement. Spiral columns are generally used in Circular columns & sometimes for square or octagonal sections.

Composite Columns- In this type of columns the longitudinal reinforcement is in the form of structural steel section (such as ISMB, ISMC or steel pipes) with or without longitudinal reinforcements. It is generally used where the limit of providing A_st is reached, & no longitudinal bars can be accommodated to fulfil design requirement.

Types of columns

Based on type of loading:

Axially loaded column- Columns which experiences axial load symmetrically on all four sides are termed as axially loaded column. Internal columns of multi-storeyed building with symmetrical loads from slabs from all sides is an example of axially loaded column.

Uniaxial eccentrically loaded column- Columns which experiences eccentric (the point of application of a load is away from the centroidal axis of the column) load on one side of the column is termed as uniaxial eccentrically loaded column. Columns with beam connected at one side is an example of Uniaxial eccentrically loaded column.

Biaxial eccentrically loaded column- Columns which experiences eccentric load on two sides of the column is termed as biaxial eccentrically loaded column. Corner columns with beam connected at two side is an example of Biaxial eccentrically loaded column.

Based on Slenderness Ratio:

Depending upon the slenderness ratio, which is Effective length / Least lateral dimension columns can be termed as Short columns or Slender / Long column.

According to IS 456:2000 code clause 25.1.2, when both the slenderness ratio’s (L_ex / D) & (L_ey / b) are less than 12 it is termed as short column. If the ratio is equal to or more than 12 then it is termed as Slender / Long column.

Where, L_ex = Effective length in respect of major axis.

D = Depth in respect of major axis.

L_ey = Effective length in respect of minor axis.

b = Width of the member.

In case of Non-rectangular & non circular sections where the slenderness ratio is better defined in terms of radius of gyration rather than the lateral dimensions these definition of Short or long column is not applicable.

-You can also view this link for more information-

By Kpstructures

Corona Virus India (COVID-19)

2020-03-17T10:16:00.001-07:00

Corona Virus India (COVID-19)

A corona virus is originally found in animals, however due to mutation of the virus it has been transmitted from animal to human from which it is spreading like wild fire & has become a major global concern. This new version of the virus is officially named as COVID-19 by the World Health Organization (WHO) on 11th February 2020.

Originated in Wuhan market in China where bat, snakes & other wild animals were traded. The symptoms are almost similar to the SARS virus (Severe Acute Respiratory Syndrome) which first occurred in Guangdong province in southern china.

COVID-19 is large in size (400-500 micron in cell diameter) for which any mask prevents its entry. Due to its large size it does not travel by air, however it can float for a few feet during sneezing & coughing of an infected person. It can stay active on a metal surface for about 10-12 hours & about 9 hours on fabrics/clothes. The virus cannot withstand heat, a temperature of about 26-27 Degrees kills the virus.

Impression of COVID-19

Very little is known about the Novel Corona Virus, however few symptoms were found:

Rise in body temperature (I.e. fever).
Coughing.
Difficulty in breathing or shortness of breath.
Person with a lower immunity power or weakened immune system may experience serious respiratory problems such as bronchitis or pneumonia.
Some reports also show that they are experiencing problem with digestion & some with eye diseases like conjunctivitis.

Basic protective measures that should be taken:

Wash your hands often with soap & water or use an alcohol-based (min 60% Alcohol) hand sanitizer, as alcohol-based sanitizers kill the virus.
Avoid touching your eyes, nose & mouth all the time, hands touch many places & can contain virus. The virus can easily enter your body through these openings.
Avoid crowded places & maintain social distance of at least 3 feet, coming near to a sick person can get you contaminated, especially if the person is having the symptoms. Droplets from coughing can enter your body through breathing.
If you develop the symptoms immediately seek medical attention & maintain self-quarantine as much as possible to prevent further spreading of the virus.

This article is written to spread awareness & preventive measures for everyone.

Stay Safe Everyone.

Design of a singly reinforced concrete beam as per IS 456:2000

2020-03-12T09:38:00.001-07:00

Design of a singly reinforced concrete beam

Example: Design of a singly reinforced concrete beam for the following data.

Clear Span of the beam – 3 m
Width of supports – 200 mm
Working live load – 6 KN/m
Grade of concrete – M20
Grade of steel – Fe-415 HYSD bars

Solution:

Given Data – f_ck = 20 N/mm².

f_y = 415 N/mm².

Load factor = 1.5 for Dead load & Live load.

Determination of the dimensions of the beam:

Let us adopt the span/depth ratio of 20 for the given span & loading

Effective depth = d

= (Span/20)

= 3000 / 20 = 150 mm.

Adopt d = 160 mm.

Overall depth (D) = 200 mm.

Width of beam (b) = 200 mm.

Therefore, Effective span = (Clear span + Effective depth) = 3 + 0.160 = 3.16 m.

Center to center of supports = 3 + (2 x (.2/2)) = 3.2 m.

Hence Effective span of the beam (L) = 3.16 m.

Load calculation:

Dead load : Self weight of the beam = (0.2 x 0.2 x 25) = 1 KN/m.

Live Load : as given = 6 KN/m.

Total designed load = 1 + 6 = 7 KN/m.

Design Ultimate Load (Factored Load) = (1.5 x 7) = 10.5 KN/m.

Calculation for Ultimate Moment & Shear force:

Moment of UDL on a simply supported beam is given by – wl² / 8.

M_u = (10.5 x (3.16)²) /8 = 13.1 KN-m.

Shear force on a simply supported beam is given by – wl / 2

V_u = (10.5 x 3.16) /2 = 16.59 KN.

Calculation for tension reinforcement:

Calculate the limiting moment of the RCC section, which is given by 0.138 f_ck x b x d².

M_u,lim= (0.138 x 20 x 200 x 160²) x 10^-6 = 14.13 KN-m

As, M_u,lim is greater than M_u the section is under reinforced.

Therefore, M_u = 0.87 x f_y x A_st x d [1- (A_st x f_y)/ (b x d x f_ck)].

From solving equation for A_st we get: A_st = 275 mm².

Check: Minimum A_st,min required = (0.85 x b x d)/ f_y = 65.5 mm².

As, A_st > A_st,min Hence, Ok.

Provide 3 bars of 12 mm diameter which gives 3 x 113 = 339 mm².

Check for Shear stress:

i_v= (V_u / bd) = {(16.59 x 10³)/ (200 x 160)}

= 0.51 N/mm².

P_t = (100 A_st / bd) = {(100 x 339) / (200 x 160)}

= 1.05.

Design Shear Strength of concrete as per IS 456:2000

From above table we get

i_c = 0.62 N/mm^2, which is greater than i_v so nominal shear reinforcement is provided. Using 6 mm diameter two legged stirrups.

S_v = (A_svx 0.87 x f_y) / (0.4 x b)

= (2 x 28 x 0.87 x 415)/ (0.4 x 200)

= 253 mm.

But, spacing for stirrup shall not be greater than 0.75 x d = 0.75 x 160 = 120 mm & 300 mm.

Therefore, adopting spacing of 120 mm.

Pre-stressed Concrete

2020-03-11T09:48:00.000-07:00

The main objective of pre-stressed concrete is that to provide enough pre-compression (Compression that is generated by it’s own self weight, not due to any external/working load) in the concrete by tensioned steel reinforcements or cables / wires so that under working condition the concrete experiences no tensile stresses or low tensile strength so that no deformation / cracks is observed on the concrete surface.

Pre stressed Bridge Construction

Generally, in concrete members the top flange experiences compression & the bottom flange experiences tension, but in pre-stressed concrete the steel wire is subjected to tensile stress & then concrete is poured in the form work. After setting of concrete it is observed that due to the pre-stressing the bottom flange also experiences compression force, which counteracts with the top flange’s compression force. For this advantage the layout of the tendon (Steel wire / cables) is done by Structural Designer in such a way that the predetermined stresses can easily be dealt with the placement of the tendon.

Prestressed Beam

Individual steel wire / cable is stacked & twisted mechanically by machines to form a larger diameter steel wire. It has to be done mechanically as any loose steel wire can have a devastating effect on the structure. The amount of tensile stresses that it can withstand depends upon the external diameter of all the individual steel wire which as a single unit. Therefore, which the design limitation of prestress concrete is less comparatively to normal RCC. However high grade of concrete (greater than M50) shall be used during any pre-stressed concrete as if the concrete is weak then during or after tensioning the concrete might severely crack or deform.

Post tensioned Concrete work at project execution

There are two types of pre-stressing in a concrete member:

· Pre-tensioned Concrete

· Post-tensioned Concrete

Pre-tensioned Concrete-

In this process of casting the steel wire / cable is put under tensile force & locked before the concrete is poured in the formwork. Only after the final setting time of concrete the tendons are released, by which the prestress force of the steel is transferred to the concrete. In pre-tensioned concrete the bonding between the concrete & steel is much greater than post tensioned concrete as the steel is in direct contact with the concrete throughout.

Post-tensioned Concrete-

In this process, the steel wire is placed inside a conduit or tubes without any tensile force & concrete is poured in the formwork. After final setting of the concrete the steel wire /cable is stretched to give the required tensile forces. In post-tensioned concrete the bonding is not as good as pre tensioned concrete as the steel & concrete is separated by the conduits.

Some advantages of prestressed concrete:

Number of design limitations can be resolved in terms of span and loading which is not possible in case of normal RCC. Because of which longer unsupported span can be achieved, with high load bearing capacity.
Few columns are required for the structure as longer beam span can be obtained, this concept is used for auditoriums / roofs where there is a column restrictions and more open space is required & by normal RCC it is quite impossible. Bridges are also constructed by the use of pre stressed system.

Basic Tutorial on STAAD Pro V8i SS6 version

2020-03-10T07:01:00.001-07:00

Just the introduction & basic of Staad Pro V8i SS6 version

Tutorial No 1

Flow Test of SCC

2020-03-09T09:19:00.002-07:00

Flow table test is conducted on Self compacting concrete to test its work-ability, by testing the flow of concrete on a flat table. It also helps to determine the quality of SCC (which is consistency, cohesiveness, Less prone to segregation & bleeding).

Equipment required for Flow table test:

The flow table is made up of metal with a thickness of 1.5 mm and dimensions 750 mm X 750mm
A Metal Cone (Lower Día = 20cm, upper Día = 13 cm, Height of Cone = 20cm).
A tamping rod made of hardwood.
A metal scoop.
A measuring tape.

Flow Table Test

Procedure for Flow table test:

The test shall be conducted when the concrete is fresh & after the retention period (which is at the pouring point) place the flow table on a horizontally leveled surface.
Clean the flow table, it shall be free from dust or other foreign material on Flow table and the table shall be wiped with a wet cloth just before pouring the concrete in the metal cone for testing.
Place the metal cone at the middle portion of the flow table centered with the circular marking on the table and hold it tightly so that it does not shift during concrete pouring.
Pour the design mixed concrete in the metal cone in two layers, each layer shall be tamped with tamping rod for 25 times. After tamping the last layer, the overflowed concrete on the cone is struck off using a trowel. The table is cleaned again with hand / cloth so that it is completely cleaned & moist.
Lift the metal cone vertically up & let concrete fall on its own and take the shape of a circular disc. Stopwatch shall be maintained during this period, as we have to see in how much time the SCC is attending the targeted flow.
Measure the spread of concrete diameter using measuring tape in horizontal direction of the largest diameter and perpendicular to it.
The result is submitted as the flow table test result of that particular design mix of SCC, Flow range is 550 mm – 600 mm in T_{-50 sec} = 3.5 Sec.

Exposed reinforcement in Aluminium shuttering

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Reinforcement that is visible on the concrete surface of any reinforce concrete is termed as exposed reinforcement, it is one of the common types of defect that needs to be repaired immediately after de-shuttering work is completed.

For any concrete repair work sufficient prop supports should be used as per the instruction of engineer in charge, especially if the repair work is to be carried out in structural columns, walls or beams. In case of slabs if the affected area is less, then prop is not required.

Exposed reinforcements

Causes of exposed reinforcement:

Many factors contribute for exposed reinforcement in RCC structure including poor workmanship & quality control, however, some points are listed below

· Most common reason for these defects is due to improper / un-sufficient cover block for any particular segment. Although in some instances it has been noticed that after proper placing of cover blocks, during fixing of shuttering (Aluminium shuttering) some cover block gets damaged, reoriented and even fall off.

· Another type of exposed reinforcement occurs when the concrete has aged and deterioration of concrete leads to this defect. In this case, the chance is very high that the reinforcement is highly corroded & replacement of reinforcement is only applicable remedial measure.

· Permeability (passing ability of water through it) of concrete is also an important factor contributing to rusting of steel & eventually leading to exposed & highly corroded steel.

Exposed Reinforcement

Repairing of exposed reinforcement:

Chipping to be carried out on extension portion of the exposed reinforcement identify the area of defect till sound concrete & steel is found. Chipping is also required to make space for the reinforcement to be pushed back. If the reinforcement is highly corroded then the steel is to be replaced by new steel with required L_d (Development length).
All loose concrete shall be removed, and exposed steel & concrete is to be cleaned using steel wire brush and water so that no loose debris are present. Using water before the application of chemicals also helps to achieve SSD condition which helps in the bonding of the chemicals.
Hammering might be required to push back the reinforcements so that it gets the required cover.
It the steel is not highly corroded then corrosion resistant chemical compounds such as Sika Rustop shall be applied on the reinforcement surface to prevent it from further rusting. If replacement steel is required then use of chemical is not compulsory, however providing L_d or welding is required to maintain the continuity of the steel.
Before sealing the void with fresh concrete, bonding agents such as Fosroc Nitobond EP to be used for better bonding of new & old / existing concrete.
Application of micro-concrete such as Sika Microcrete 4 or cement based grouting compound such as Sika 122 Top HS can be used to fill up the chipped-out area depending upon the structural important of that particular RCC member. After which finish to be made with a trowel.
Curing is compulsory as per the technical data sheet of the micro concrete used or as per the instruction of engineer in charge. However, it is always a good practice to cure it for at least 7 days.

Durability characteristics of Light weight concrete

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As with normal weight concrete, durability of the structural lightweight concrete is directly affected by its permeability. In general, concrete permeability is affected by many factors such as w/b ratio, cement type, curing, maturity of concrete, etc. The permeability of concrete as a whole is considerably higher than that of its components, namely the mortar matrix and coarse aggregates. According to Mehta (1986), this is mainly the result of micro cracks caused by the elastic mismatch between these components responding differently to temperature changes, service loads and volume changes due to chemical reactions within concrete (as cited in ACI Committee 213, 2003).

Light weight concrete

Due to similar rigidities of mortar matrix and lightweight coarse aggregate (elastic compatibility), there are reduced number of micro cracks observed in contact zone of lightweight concrete compared to that of normal weight concrete, which in turn results in lower permeability. In addition to this, hygrol equilibrium and pozzolanic reaction are two factors also contributing to improvement of the contact zone in lightweight concrete (ACI Committee 213, 2003).

Hygrol equilibrium can be defined as a state at which aggregate surface and mortar matrix have similar water concentration. In normal weight concrete, mixing water accumulates on the surface of the dense aggregate (wall effect) and increases local water-cement ratio, causing porous matrix at the contact zone. In contrast, porous surface of lightweight aggregate allows water transfer and thus avoids accumulation of water on aggregate surface. Therefore, hygrol equilibrium is reached and formation of weak zones caused by differential water concentration are prevented.

The pozzolanic reaction between silica rich surface of lightweight aggregate and calcium hydroxide formed by the hydration of Portland cement increases the density and strength of the interfacial transition zone.

For all these reasons, the contact zone in lightweight concrete is superior to that of normal weight concrete (ACI Committee 213, 2003) and thus less permeable. It should also be remembered that pore system in lightweight aggregates is generally discontinuous, therefore porosity of lightweight aggregates does not influence the permeability of concrete (Neville, 2003).

Since the permeability of the structural lightweight concrete is low, its durability to aggressive chemical solutions is usually quite satisfactory (Mehta & Monteiro, 2006). Sulphate containing groundwater and chlorides in sea water are some examples of the aggressive chemical solutions. Seawater also contains sulphates; however, productions of sulphate attack are soluble in sea water due to presence of chlorides. Therefore, sulphates in seawater do not cause deleterious levels of expansion (Holm & Bremner, 2000).

Another durability concern for concretes is alkali-aggregate reaction. There is no reported case of deleterious alkali-aggregate reaction in lightweight concrete with natural or manufactured lightweight aggregate (Holm & Bremner, 2000). Nevertheless, ACI Committee 213 (2003) recommends testing of natural aggregates against any potential for alkali-aggregate reaction or having a record of satisfactory service history.

Freezing-thawing resistance of structural lightweight concrete is superior to that of normal weight concrete provided that aggregates are unsaturated before mixing (Neville & Brooks, 2010). This performance is generally attributed to the porous structure of lightweight aggregates which act as pressure relief zones for increasing hydraulic pressure as the water freezes (Harrison, Dewar, & Brown, 2001). Air entrainment is especially beneficial when aggregates are close to saturation. Air entrained lightweight concrete shows similar resistance against freezing-thawing action to that of air-entrained normal weight concrete (Mehta & Monteiro, 2006).

Jensen at al. (1995) reported that even relatively low temperatures between 100 and 300oC may cause significant amount of reduction in compressive strength and elastic modulus of high-strength lightweight concretes and added that spalling depends largely on the moisture content. They also suggested that inclusion of 0.1 to 0.2 percent polypropylene fibres in lightweight concrete mixture results in significant reduction of spalling for lightweight concretes (as cited in Holm & Bremner, 2000). Reduction in spalling is attributed to release of steam pressure through the conduits developed by the melting of the polypropylene fibres (ACI Committee 213, 200).

Types of light weight concrete

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Lightweight concrete can be defined as a type of concrete which includes an expanding agent in that it increases the volume of the mixture while giving additional qualities such as nailibility and reduced dead weight. It is lighter than the conventional concrete with a dry density of 300 kg/m3 up to 1840 kg/m3 and about 87 to 23% lighter. It was first introduced by the Romans in the second century where ‘The Pantheon’ has been constructed using pumice, the most common type of aggregate used in that particular year. From there on, the use of lightweight concrete has been widely spread across other countries such as USA, United Kingdom and Sweden.
The main specialties of lightweight concrete are its low density and thermal conductivity. Its advantages are that there is a reduction of dead load, faster building rates in construction and lower haulage and handling costs. The building of ‘The Pantheon’ of lightweight concrete material is still standing eminently in Rome until now from around 18th centuries It shows that the lighter materials can be used in concrete construction and has an economical advantage.

Lightweight concrete can be prepared either by injecting air in its composition or it can be achieved by omitting the finer sizes of the aggregate or even replacing them by a hollow, cellular or porous aggregate. Particularly, lightweight concrete can be categorized into three groups:

No-fines concrete
Lightweight aggregate concrete
Aerated/Foamed concrete

NO-FINES CONCRETE

No-fines concrete can be defined as a lightweight concrete composed of cement and fine aggregate. Uniformly distributed voids are formed throughout its mass. The main characteristics of this type of lightweight concrete is that it maintains its large voids and do not form laitance layers or cement film when placed on the wall. No-fines concrete usually used for both load bearing and non-load bearing for external walls and partitions. The strength of no-fines concrete increases as the cement content is increased. However, it is sensitive to the water composition. Insufficient water can cause lack of cohesion between the particles and therefore, subsequent loss in strength of the concrete. Likewise, too much water can cause cement film to run off the aggregate to form laitance layers, leaving the bulk of the concrete deficient in cement and thus weakens the strength.

LIGHTWEIGHT AGGREGATE CONCRETE

Porous lightweight aggregate of low specific gravity is used in this lightweight concrete instead of ordinary concrete. The lightweight aggregate can be natural aggregate such as pumice, scoria and all of those of volcanic origin and the artificial aggregate such as expanded blast-furnace slag, vermiculite and clinker aggregate. The main characteristic of this lightweight aggregate is its high porosity which results in a low specific gravity the lightweight aggregate concrete can be divided into two types according to its application. One is partially compacted lightweight aggregate concrete and the other is the structural lightweight aggregate concrete. The partially compacted lightweight aggregate concrete is mainly used for two purposes that is for precast concrete blocks or panels and cast in-situ roofs and walls. The main requirement for this type of concrete is that it should have adequate strength and a low density to obtain the best thermal insulation and a low drying shrinkage to avoid cracking. Structurally lightweight aggregate concrete is fully compacted similar to that of the normal reinforced concrete of dense aggregate. It can also be used with steel reinforcement so as to have a good bond between the steel and the concrete. The concrete should provide adequate protection against the corrosion of the steel. The shape and the texture of the aggregate particles and the coarse nature of the fine aggregate tend to produce harsh concrete mixes. Only the denser varieties of lightweight aggregate are suitable for use in structural concrete.

AERATED CONCRETE

Aerated concrete does not contain coarse aggregate, and can be regarded as an aerated mortar. Typically, aerated concrete is made by introducing air or other gas into a cement slurry and fine sand. In commercial practice, the sand is replaced by pulverized fuel ash or other siliceous material, and lime maybe used instead of cement. There are two methods to prepare the aerated concrete. The first method is to inject the gas into the mixing during its plastic condition by means of a chemical reaction. The second method, air is introduced either by mixing-in stable foam or by whipping-in air, using an air-entraining agent. The first method is usually used in precast concrete factories where the precast units are subsequently autoclaved in order to produce concrete with a reasonable high strength and low drying shrinkage. The second method is mainly used for in-situ concrete, suitable for insulation roof screeds or pipe lagging.

History of light weight concrete

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Ancient Applications

Known history of the use of lightweight concrete starts more than two thousand years ago in the Roman Empire. The most significant examples of that time were the Port of Cosa, the Pantheon Dome and the Colosseum (ACI Committee 213, 2003).

The Port of Cosa was built on the west coast of Italy, in 273 B.C. The structural designers of the port were aware of the fact that lightweight aggregates were more convenient to use in marine structures. Instead of using locally available aggregates (beach sand and gravel) for the construction, the builders have brought natural lightweight aggregates (pumice and scoria) from the volcanic resources which were located around 40 km away from the Port of Cosa. The harbour consists of four piers, which had resisted the forces of nature except the surface abrasion for almost 2 thousand years and it is now abandoned only due to siltation (ACI Committee 213, 2003).

The construction of the Pantheon was completed in 27 B.C. It has a dome with a diameter of 43.3 m which was the highest record for almost two thousand years. The builders used the lightweight aggregates of varying densities in descending order from the base to the top of the doom. In other words, higher density aggregates were used near the base where the stresses are higher; and lower density aggregates were used near the top where the stresses are lower. When the dome was first constructed, it had a metal cover which was soon removed to be used for another structure. Until it was covered with a lead roof around 15 years back, it had been exposed to the forces of nature for hundreds of years (Holm & Bremner, 2000). Even today, the Pantheon is still in use for spiritual purposes (ACI Committee 213, 2003).

The Colosseum, which is an ancient amphitheatre of massive size with 50 thousand seating capacity, was constructed in 75 to 80 A.D. The foundation of the Coliseum was made of a light weight concrete utilizing crushed volcanic lava as aggregate. Similarly, the aggregates used in its walls were made of porous, crushed bricks. The spaces and the vaults between the walls were made of porous-tufa cut stone (ACI Committee 213, 2003).

Modern Applications

The use of lightweight aggregates after the Romans was limited. This was changed when manufactured lightweight aggregates became commercially available in 20th century (ACI Committee 213, 2003). In 1918, Stephen J. Hyde, a ceramic engineer, patented the process of producing lightweight aggregates through heating and expanding shale, clay or slate in a rotary kiln. At first, the expanded aggregates were used in the construction of concrete ships for U.S. fleet. Later, expanded aggregates started to be used for civilian construction sector.

The first commercial plant for expanded aggregate production was founded in Kansas, in 1920. In 1923, Dan Servey initiated the first production of lightweight concrete masonry units. In 1929, the use of lightweight concrete jumped to high rise construction. 14 additional stories were added to the existing 14-storey building of South Western Bell Telephone Office by using lightweight concrete (Holm & Bremner, 2000).

Starting from the second half of the 20th century, many multi-storeyed buildings were constructed by using structural lightweight concrete. Examples are 42-storey Prudential Life Building (Chicago) with lightweight concrete floors and 18-story Statler Hilton Hotel (Dallas) with all lightweight concrete frame and flat plate floors (ACI Committee 213, 2003).

Today, the applications of structural lightweight concrete extended not only to Highrise buildings but bridges and marine structures also. Stolmen Bridge and Heidrun Tension Leg Platform are significant examples of recent applications.

Structural light weight concrete (SLWC)

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Structural light weight concrete (SLWC) has extreme importance to the construction industry. Most of the current concrete research focuses on the high-performance concrete, by which is meant a cost-effective material that satisfied demanding performance requirement including durability. Light weight concrete consist of structural light weight aggregate has been defined in ASTM C30.

The concrete has a minimum 28 days’ compressive strength of 25000 psi (17 MPa), an equilibrium density between 70 to 120 lb/ft3 (1120 to 1929 kg/m³) and consist entirely of light weight aggregate or combination of light weight & normal density aggregate. But that need to be understood that it’s a definition not a specification, hence project specific or raw materials specific and it may vary.

In India light weight concrete is presently being used as filling materials. It has been observed that the density of such type of concrete is around 7-9 kN/m³ but the compressive strength is very poor (around 3-5 MPa). It is mainly used where dead load reduction is the main focus but structural strength is not a concern. The use of LWC is usually predicted on reduction of project cost, improved functionality or combination of both. Using artificial light weight coarse aggregate production of LWSC is a very sound option but may not be cost effective one. Our aim is to produce a type of concrete which will be lighter in density, satisfy the structural requirement & should be made of using locally available resources.

Aggregates with an oven-dry particle density less 2000 kg/m3 or an oven-dry loose bulk density less than 1200 kg/m3 are called as lightweight aggregates according to EN206-1:2000. ASTM C330 also defines a maximum limit for the bulk density, which is 1120 kg/m3 and 880 kg/m3 for fine and coarse lightweight aggregate, respectively.

Lightweight aggregates are divided into two categories according to their sources:

Natural Lightweight Aggregates.
Manufactured (Synthetic) Lightweight Aggregates.

Natural lightweight aggregates - They are obtained by processing volcanic rocks. Pumice, scoria, tuff and perlite are some of the examples which fall in this category. Pumice is a light-colored porous glass with elongated voids. Scoria is a dark colored porous glass with spherical voids. Tuff is a porous glass formation of consolidated volcanic ash. Perlite is a porous glass with a high silica content. It generally contains 2-5% water (Mehta & Monteiro, 2006).

Synthetic lightweight aggregates - They are expanded forms of materials such as clay, shale, slate, perlite and vermiculite, produced by heat treatment, generally around 1000 degree Celsius. The materials are either reduced to desired size before calcination or crushed after the calcination process. The expansion results from the entrapment of gases, which are generated during heat treatment, inside the processed material. The use of initially pelletized materials in heat treatment process produces spherical aggregate particles with a semi-impervious coating having 12-30% lower water absorption capacity than the particles produced of un-pelletized material. Therefore, coated particles are preferable from workability point of view, however they are more expensive than uncoated ones (Neville & Brooks, 2010).

At Construction Project

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In a single construction project, there are separate departments that work together for better outcome of the project. They also ensure that the project runs smoothly, on schedule, working with quality standards & safety regulations as per Indian Standard Code of practice. A series of tasks performed in correct order creates a project where every departments contribute to particular tasks. Here we are going to talk about construction site departments.

Departments that are related to the technical aspect of the project are termed below:

Engineering Department : These consists of General Manager – Technical, General Manager- Commercial, Project Engineers who is the engineer-in-charge of a project, under whose supervision there is Senior Engineer, Engineer & Junior Engineer. This department takes charge of all the activity regarding construction of any structural members including steel structure. Some of the duties of Quality department are stated below:

They are responsible for all the reinforcement works carried out as per the GFC drawing, as well as liaising with the contractors so as to keep the project on track.
There are responsible for reconciliation of materials used in the project & measurement of the work done.
They are also responsible for reconciliation of TMT bars that are issued to the contractor for any particular RCC work.
Contractor are responsible for carring out the RCC work under the direct supervision of Engineering team, consultants check the final reinforcement detailing before concreting of any particular structural member.

Quality Department : These consists of Senior Quality Engineer who is the senior most quality head of a project, under whose supervision there is Quality Engineer & Junior Quality Engineer. This department takes charge of all the activity regarding quality of any structural work including steel structure. Some of the duties of Quality department are stated below:

They are responsible for all the reinforcement that is being used in work are in good condition, free from rust & does comply with the companies working standard.
They are also responsible for Concreting work if the project has it's own concrete batching plant.
Any material related to project work shall be certified & tested if required before being used at work.
All test are conducted by the quality departments which are verified by the respective consultants for approval of work.

Other departments of the project are termed below:

Planning Department : All work schedules with co-ordination between the different departments are fixed & executed by the planning department.

Safety Department : These consists of Senior Safety Engineer followed by safety engineer & junior safety engineer, they are responsible of all the activities running in project as per the safety guidelines, including scaffolding works, rope harness tied to person doing a work at a high height. Wearing of safety equipment such as gloves, goggles etc.

Purchase Department : Thousands of materials & items are required for a smooth progress of work, they are responsible for getting the items purchased and delivery to project within certain time frame. All the transactions, salary & managing funds of the project are coordinated by this department.

Thermo Mechanically Treated Bars (TMT)

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What is TMT bar?

Thermo Mechanically Treated bars is generally termed as TMT bars. These are reinforcement bars with ribs all-around the surface for better bonding between the bars & concrete, it is also known for their high strength. The main characteristic feature of these bars is a soft inner core wrapped in a hardened outer surface. They are manufactured in a way where the hot steel billets are passed through cold water, this hardens the outer surface, enhancing its tensile strength, while the core remains at a higher temperature and increases in ductility. This alteration in micro structure across the bar’s cross-section imparts enormous strength to it. As TMT bars don’t go through any twisting processes, hence their surface is free from cracks and other defects. In general, the TMT bars is denoted by the name of Fe500 or Fe415 and many more, this means that Fe stands for Iron & 500 stands for minimum yeild stress is 500 N/mm².

Thermo Mechanically Treated bars is used for reinforcements in RCC work all over the globe due to the fact it is more corrosion resistance than HYSD or TOR rebars, which is also used as reinforcement.

Test to be carried out before using a consignment of Thermo Mechanically Treated bars:

Generally, the Thermo Mechanically Treated bar samples are taken from different bundles of the consignment so as to get more accurate average result of the consignment. These samples are sent to 3^rd party laboratory for detailed analysis of mechanical & chemical properties of the said consignment.

Different client requires different kinds of test & the acceptance criteria values of these test results are sometimes modified based on the discussion of their structural consultant or by any consultancy firm.

The basic requirements for a Thermo Mechanically Treated bar to be accepted & used for work is clearly stated in IS code 1786:2008.

For Mechanical properties the code states the following:

Whereas, for Chemical properties the code states the following:

Fine aggregates in SCC / Normal concrete

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Fine aggregates in SCC / Normal concrete

Fine aggregates is a key component in SCC as well as normal concrete, it gives volume to the cement mortar without which the concrete will segregate very easily.

Fine aggregates used for concrete can be of different types as well as grades, some broad classification of fine aggregate is stated below:

Natural / River sand – These are produced naturally by weather disintegration of rocks & which has been deposited by stream/river or glacier agencies.
Crushed Stone sand – As the name suggests, it is a man made product, produced by crushing hard stones.
Crushed Gravel Sand - It is also a man made product, produced by crushing natural gravels.

The use of fine aggregate is dependent upon the availability of a particular type of sand at that area. If river sand in available it is generally used, if not any other alternative that is easily available may be used. Based on the aggregates available a concrete mix design shall be determined by trial before starting concreting work.

As per the attached grading chart Fine Aggregates of all zones can be used to produce concrete, but Zone II is the most commonly used, Zone I, III & IV shall be used cautiously as coarser / finer grains would give a different impact on the workability of the concrete. Trials to be finalized, with satisfactory results of V-box, U-Box, J-ring tests as well as flow test in case of SCC concrete. During an ongoing project, the fine aggregate shall be tested per truck load to ensure that only nominal variation between the truck loads is accepted or as to store it in different slot for different application of sand in that project.

Another test that should be carried out at every intake of sand is to check the moisture content of that particular lot, it provides the value we need for moisture correction during creation of batch sheets used at batching plant. The process is to take a finite weight of the sample (say W₁) heat that in an oven & then weight again (say W₂). Now the difference between the weight’s is the amount of moisture present in that given sample (X = W₁– W₂). Percentage moisture content is calculated by (X / W₁ x 100).

Self-Compacting Concrete (SCC)

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SCC stands for Self-Compacting Concrete or Self Consolidating Concrete. It has a high flowability for which it can spread in the formwork with ease and without any mechanical vibrator. This type of concrete has good flowability, stability & for the use of superplasticizer admixtures such as FOSROC Structuro 202 or BASF MasterEase 3602 gives the concrete a flow of around 550 mm - 650 mm in Flow table test without adding more water.

Some advantage of SCC over normal concrete is stated below:

The mix design for any grade of concrete is done with superplasticizer admixture (such as FOSROC Structuro 202 or BASF MasterEase 3602) which gives the concrete the ability for higher flow, greater compacting ability under its own self-weight & without the use of any mechanical vibrator.
Due to its flowability, SCC can occupy every corner of its formwork with ease and gives a better finish of the RCC surface than most of the normal RCC casting.
As for its property of self-compaction, it is generally preferred over normal concrete where there is a congestion of TMT work and use of mechanical vibrators is restricted. The bonding between the TMT bars & concrete also gets enhanced due to high flowability.
The superplasticizer increases the viscosity of concrete which enhances the pump-ability of the concrete.
It is more economic than normal concreting due to the fact it is more easily place-able via pump or Transit mixture & less manpower required for compaction, leveling as well as surface finish. It reduces the cost of labours & also increases the casting schedule of the RCC member.
Due to the addition of superplasticizer it is less prone to segregation than normal concrete.

Some drawbacks for SSC is:

When working with SCC especially when it is going to travel in transit mixture, we have to be very careful and what type of flow range we need at concrete pouring point, cause the flow deceases as time passes, it is generally termed as retention period of concrete. For example: if the flow at batching plant (concrete production) is 600 mm in 3 sec (T₅₀Sec) and had to travel or wait for an hour then the flow will get low around 500 mm in 4.5 sec (T₅₀Sec).
If the SCC is to be traveled by a transit mixer then fully loading the mixture is not a good idea, as the flowability is higher there is a chance of spillage.
The formwork has to be strong enough to withstand the fluid pressure of the concrete, generally for embedded / reusable wall tie (used for holding the aluminium shuttering) has to have a minimum load capacity of 25 N/mm².
Concreting with SCC is easier than normal concrete but making an SCC is harder, and care to be taken at batching plant constantly for a steady standard flow of concrete. Temperature can vastly change the standard of SCC for which minor adjustment of admixture dosage is required during a long casting.
Some case of casting requires additional water at pouring point if the retention period has exceeded beyond allowable point to get the required flow, however for SCC it cannot be done as it will de-stabilize the concrete and can lead to segregation or bleeding of concrete.
In case of underground structure such as rain water harvesting tank, SCC concrete is not suitable as due to evaporation of water from mix can lead to shrinkage crack, which affects the permeability of the structure.

Volumetric conversion of SCC Mix Design

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To convert a concrete mix design which is generally given in terms of cubic meter into practical application when the mixing quantity required is small such as 1 bag of cement the weight of ingredients is converted into volume. These conversions are generally termed as volumetric conversions of concrete mix design.

For example, let us consider the following mix design at SSD condition for M25 SCC concrete.

Cement: 330.42 Kg

FlyAsh: 116.58 Kg

Fine Aggregate- River sand: 922.35 Kg

Coarse Aggregate- 10 mm: 542.31 Kg

Coarse Aggregate- 20 mm: 450.21 Kg

Water: 158.00 Kg

Admixture: 3.13 Kg

now, we need to get the volumes of ingredients as per 1 bag of cement,

Weight of cement per Bag : 50.00 Kg

Ratio between total cement quantity & per bag: 330.42/50.00 = 6.61

Therefore reduced quantity as per 1 bag of cement comes to -

Fly Ash: 116.58 / 6.61 = 17.64 Kg

Fine Aggregate- River sand: 922.35 / 6.61 = 139.57 Kg

Coarse Aggregate- 10 mm: 542.31 / 6.61 = 82.06 Kg

Coarse Aggregate- 20 mm: 450.21 / 6.61 = 68.13 Kg

Water: 158.00 / 6.61 = 23.91 Kg

Admixture: 3.13 / 6.61 = 0.47 Kg

With these values we can now convert it into volumes by simply multiplying the weight of ingredient with specific gravity of that particular ingredient again multiply by 1000 and it will give the volume of that ingredient in cubic meter.

Fly Ash: 17.64 x 2.23 x 1000 = 0.039 Cubic Meter

Fine Aggregate- River sand: 139.57 x 2.76 x 1000 = 0.385 Cubic Meter

Coarse Aggregate- 10 mm: 82.06 x 2.84 x 1000 = 0.233 Cubic Meter

Coarse Aggregate- 20 mm: 68.13 x 2.87 x 1000 = 0.196 Cubic Meter

Water: 23.91 x 1.00 x 1000 = 0.024 Cubic Meter

Admixture: 0.47 x 1.10 x 1000 = 0.001 Cubic Meter

After getting these data, any combination of sizes may be assumed as long as it maintains the actual volume of that particular ingredients. Generally, when casting is carried out manually (I.e. without a batching plant) these volumetric conversions is required, so as the mix design can be maintained without a strict supervision of any engineer. These data are used to make forma or boxes to keep the volume of ingredients easy to maintain.

How to repair non-structural hair crack

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Hair crack are non-structural cracks that occurs mainly on the concrete surface, not through & through the concrete member.

Shrinkage crack are also non-structural cracks that occurs due to lack of curing for sufficient amount of time after casting of the concrete members.

Repairing methodology for Non-structural hair crack:

· First, using chisel we have to cut a V grove throughout the length of the crack with a width around 25 mm & depth as required depending upon the extent of crack.

· The chiseled-out part shall fill with crack filling mortar such as Sika Crack seal acrylic paste.

However, any crack repair work should be executed as per the engineer in-charge Approval from the structural engineer / consultant shall also be taken before commencing.

Fly ash layer in concrete

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Now a days fly ash is being used with cement as a binding material & as well as for other benefits where economic is one of them, some times due to bad concreting a flyash layer is observed after de-shuttering of the casted member.

Fly ash layer is generally found when bleeding occurs in a concrete mix. The slurry along with the carbon layer rise to the top surface. During successive castings, a layer is formed in between good concrete where weak layer of fly ash is found.

Repairing methodology for Fly ash layer:

· First, we have to determine the extent of fly ash layer in a particular structural member or portion.

· Secondly, using chisel/steel wire brush we have to get rid of the weak fly ash layer till we find good concrete.

· If the depth of the chipped out part is less than 5 mm then the void shall be filled with finish materials such as gypsum/putty as per client's instruction.

· If the depth is more than 5 mm but less than 10 mm, then the void shall be cleaned properly followed by application of concrete bonding agent such as Fosroc Nitobond EP. After which modified cement mortar such as Sika Microcrete 4 or Sika 122 top HS shall be used to fill up the void.

· If the depth is more than 10 mm but less than the cover provided, then the void shall be cleaned properly followed by application of concrete bonding agent such as Fosroc Nitobond EP. After which modified cement mortar such as Sika Microcrete 4 mixed with 10 mm coarse aggregate in the ratio (1:4) shall be used to fill up the void.

However, in the last case & cases where the depth is more than cover the engineer in-charge shall be notified and suggestion shall be taken for carrying out repairing work. Approval from the structural engineer / consultant shall also be taken before commencing.

Civil Engineering

Basic Interview questions- Civil structural design

Basic Interview questions for Civil Design Engineer

Civil Site Engineer interview Questions

Basic Interview questions for Civil Site Engineer

How to estimate quantities of brickwork or AAC Blockwork

Estimation of Brickwork & AAC Blockwork

Download this excel:

Waterproof coating & membrane

What is waterproof coating & membrane

Waterproofing coat (PU):

Waterproofing Membrane:

Slab & types of slabs

What is a slab & its types?

Types of slabs:

Bending Moment in a slab:

Bonding Mechanism in RCC Structure

Bonding Mechanism in RCC Structure

Bond Mechanisms:

The Bond between the concrete & steel develops due to the following important factors:

Bond Stresses:

Different types of bond stresses are mentioned below:

Foundation Structure: Footings

Foundation Structure: Classification of Footings

Foundation structure is broadly classified into two categories:

Different types of footings are:

Classification of columns

Types of columns & their properties

Axially loaded columns may fail in any one of the following modes:

Classification of columns:

Based on type of reinforcement:

Based on type of loading:

Based on Slenderness Ratio:

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Corona Virus India (COVID-19)

Corona Virus India (COVID-19)

Very little is known about the Novel Corona Virus, however few symptoms were found:

Basic protective measures that should be taken:

Design of a singly reinforced concrete beam as per IS 456:2000

Design of a singly reinforced concrete beam

Example: Design of a singly reinforced concrete beam for the following data.

Solution:

Determination of the dimensions of the beam:

Load calculation:

Calculation for Ultimate Moment & Shear force:

Calculation for tension reinforcement:

Check for Shear stress:

Pre-stressed Concrete

There are two types of pre-stressing in a concrete member:

Pre-tensioned Concrete-

Post-tensioned Concrete-

Some advantages of prestressed concrete:

Basic Tutorial on STAAD Pro V8i SS6 version

Just the introduction & basic of Staad Pro V8i SS6 version Tutorial No 1

Flow Test of SCC

Equipment required for Flow table test:

Exposed reinforcement in Aluminium shuttering

Causes of exposed reinforcement:

Repairing of exposed reinforcement:

Durability characteristics of Light weight concrete

Types of light weight concrete

LIGHTWEIGHT AGGREGATE CONCRETE

AERATED CONCRETE

History of light weight concrete

Ancient Applications

Modern Applications

Structural light weight concrete (SLWC)

Lightweight aggregates are divided into two categories according to their sources:

At Construction Project

Departments that are related to the technical aspect of the project are termed below:

Other departments of the project are termed below:

Thermo Mechanically Treated Bars (TMT)

Fine aggregates in SCC / Normal concrete

Fine aggregates in SCC / Normal concrete

Self-Compacting Concrete (SCC)

Some drawbacks for SSC is:

Volumetric conversion of SCC Mix Design

How to repair non-structural hair crack

Fly ash layer in concrete

Just the introduction & basic of Staad Pro V8i SS6 version

Tutorial No 1