Basic Knowledge for a Civil Site Engineer

A site engineer has to perform a lot of activities on the site. His/her role involves a lot of responsibility which includes giving sufficient advice and supervision when there are any technical issues, or for proper management and for the preparation of day to day reports of the construction works. There are some works which are recurring and hence a few tips might be useful for a civil engineer if they are remembered. They help in faster calculations as well as to address problems at the site.

Points to Remember for Civil Site Engineer

Few general points for the civil engineer to remember to make the construction work easy and hence maintaining the quality of the work, done on the site:

• 36mm is the maximum diameter of the bars used for lapping. Bars having more than this diameter are not allowed.
• Maximum chair spacing is 1m and the minimum diameter used for these bars is 12mm.
• Minimum diameter used for dowels bars is 12mm.
• Longitudinal reinforcement should not be less than 0.8% of gross area of cross section and not be more than 6% of the same.
• Minimum number of bars used for square columns is 4 and that of a circle is 6.
• The main bars used in the slabs should not be less than 8mm when HYSD bars are used and 10mm when plain bars are used.
• The distribution bars in slabs should not less than 8mm and not more than 1/8 of slab thickness.
• Minimum thickness of slab is 125mm.
• Maximum free fall of concrete is allowed to 1.50m.
• Lap slices not be employed for bars greater than 36 mm.
• PH value of the water should not be less than 6.
• Compressive strength of Bricks is 3.5 N/mm2
• Water absorption of bricks should not be more than 15 %.
• Binding wire required in steel reinforcement is 8 kg per MT.
• As per IS code, 3 samples should be taken for core cutting test for every 100m2in soil filling.

Site work

Density of Materials

• Bricks                          : 1600-1920kg/m3
• Concrete block          : 1920kg/m3
• Reinforced concrete : 2310-2700kg/m3

 De-Shuttering time of different RCC Members

• For columns                            : 16-24 hrs.
• Soffit formwork to slabs       : 3days
• Soffit to beams props            : 7days
• Beams spanning upto 4.5m : 7days

 Cube samples required for different quantity of concrete

https://civildigital.com/use-of-steel-tmt-bars-as-reinforcement-different-tmt-rebar-grades-manufacturing-testing/

• 1-5m3                 : 1 sample
• 6-15m3             : 2 samples
• 16-30m3           : 3 samples
• 31-50m3          : 4 samples
• Above 50 m3  : 4 samples  plus 1sample(addition of each 50m3)

CONCRETE GRADE: 

Grade= Cement: Sand: Aggregates

 M5 = 1:4:8 

M10 = 1:3:6 (Generally Use on PCC works) 

M15 = 1:2:4 (Generally use on Beams, Slabs) 

M20 = 1:1.5:3 (maximum use on Column, Beam and Bridge) 

M25 = 1:1:2 (use on heavy structures) 

M35 Use on Post tensioning work 

M40 Use on Pre tensioning work 

CLEAR COVER TO MAIN REINFORCEMENT: 

1. Footings : 50 mm
2. Raft foundation Top : 50 mm
3. Raft foundation Bottom/ sides : 75 mm
4. Strap Beam : 50 mm
5. Grade Slab : 20 mm
6. Column : 40 mm (d>12mm) 25 mm (d= 12mm)
7. Shear Wall : 25 mm
8. Beams : 25 mm
9. Slabs : 15 mm or not less than diameter of the bar.
10. Flat Slab: 20 mm
11. Staircase: 15 mm
12. Retaining Wall on Earth: 20/ 25 mm
13. Water retaining structures: 20 / 30 mm
14. Sunshade (Chajja) : 25 mm

WEIGHT OF STEEL ROD (BAR) FOR 1m LENGTH:

Volume (V) =Area x Length = [{(Pi/4) x Dia x Dia} x L]

 Density of Steel=7850 kg/ cub meter 

Weight = Volume x Density 

= {(3.14/4) x D x D x1) x7850} (if D is in mm & for 1 meter length)

 = ((3.14/4) x D x D x1x7850)/ (1000x1000)

 = D^2/162.27 kg Wt of bar for 1m

= (D^2/162) kg (where D in mm) 

Some Examples:- 

6mm = 0.222Kg/m                  20mm = 2.466 Kg/m 

 8mm = 0.395 Kg /m              25mm = 3.853 Kg/m 

10mm = 0.616 Kg/m              32mm = 6.313 Kg/m 

12mm = 0.888 Kg/m              40mm = 9.865 Kg/m

16mm = 1.578 Kg/m          

STANDARD CONVERSION:

01. 1 RM =3.28 Rft 

02. 1 Inch =2.54 cm 

03. 1 Feet =12.00 Inch 

04. 1 Feet =0.3048m 

05. 1 Meter= 3.2808 ft 

06. 1 Sqm= 10.76 Sft 05. 1 cum =35.32 Cft

 07. 1 sft =0.09Sqm 

08. 1 Cft =0.028 Cum 

09. 1 cum =1000.00 Litre 

10. 1Gallon =4.81 Litres 

11. Link 8" = 200mm 

12. 1 Cent= 435.60 Sft

 13. 1 Acre=100cent = 0.04 Hectare 

14. 1 Hectare= 2.47 Acres= (10000 m2)  

Site Engineers Must Know About Reinforcement and Steel Bars

Hook for stirrups is 9D for one side
No. of stirrups = (clear span/Spanning) + 1
For Cantilever anchorage length for main steel is 69D
“L” for column main rod in footing is minimum of 300mm
Chairs of minimum 12 mm diameter bars should be used.
Minimum diameter of dowel bars should be 12 mm
Lap slices should not be used for bar larger than 36 mm.
In steel reinforcement binding wire required is 8 kg per MT.
Lapping is not allowed for the bars having diameters more than 36 mm.
Minimum number of bars for a square column is 4 and for circular column are 6.
Longitudinal reinforcement should not be less than 0.8% and more than 6% of gross C/S.
Weight of rod per meter length = d2/162 where d is the diameter in mm
All reinforcement shall be free from mill scales, loose rust & coats of paints, oil or any other substances.

Main bars in the slabs shall not be less than 8 mm (HYSD) or 10 mm (Plain bars) and the distributors
not less than 8 mm and not more than 1/8 of slab thickness.

In Case of spacing of Bars

Provided the diameter of the bar, if the diameter of the bar is equal.

Provide the diameter of larger bar, if the diameter is unequal.

5mm more than the nominal maximum size of the coarse aggregate.

NUMBER OF BRICKS IN 1 CUM:

The Standard size of the 1st class brick is 190mmx 90mm x90mm and motor joint should be
10mm thick So size of brick with motor=200 x 100 x 100. 

Volume of 1st class brick = 0.19 x 0.09 X 0.09 =0.001539 cu.m

 Volume of 1st class brick with motor =0.2 x 0.1 x0.1=0.002 cu.m 

No. on bricks per 1cu.m= 1/volume of 1st class brick with motor =1/0.002 = 500 no’s of bricks

UNIT WEIGHT: 

01. Concrete- 25 kN/m3 

02. Brick -19 kN/m3 

03. Steel -7850 Kg/m3 

04. Water -1000 Lt/m3 

05. Cement -1440 Kg/m3 

DEVELOPMENT LENGTH: 

01. Compression 38d

 02. Tension 47 & 60d 

DIFFERENCE BETWEEN MAIN BARS AND DISTRIBUTION BARS

In any reinforcement detailing there are two sizes of bars used in the slab. That is main reinforcement
bars and distribution bars. In this article, we will discuss the difference between main bars and distribution
bars.

To understand properly we need to know the bending moment on slab.

Let’s take an example.

When load is applied to a slab the bottom of the slab will experience positive moment (Sagging)
and the supports will experience negative moments (Hogging). The bottom part, as well as the
top part of the slab, will experience high tension at the same time.

The deflection will be very high in shorter span and low in longer span. High amount of tension
will be acting in the support which is near than the other one.

FUNCTION OF MAIN REINFORCEMENT BARS:

Main reinforcement bar is provided at the shorter span direction in order to transfer
the bending moment developed at the bottom of the slab. The purpose of providing
main bar is to transfer the bending load developed at the bottom of the slab to the beams.

1. Main reinforcement bar is provided at the bottom of the slab at the shorter direction.

2. Stronger dimension bar is used as main bar.

In one way slab, the slab is supported by beams on the two opposite sides where
main reinforcement bar is provided.

In two way slab, the slab is supported by beams on all four sides. So there will be
no difference in bar size because each side will have to transfer equal amount of stress evenly.

FUNCTION OF DISTRIBUTION BARS:

1. Distribution bars are provided to resist the shear stress, cracks developed in the longer span.

2. Distribution bars are provided perpendicularly with the top of the main bar.

3. Lesser dimension bars are used.

4. Distribution bars are provided in the longer span direction.

DIFFERENCE BETWEEN MAIN BARS AND DISTRIBUTION  BARS:

1. Main reinforcement bar is normally used at the bottom of the slab.

Distribution bars are placed on the top of the main bar.

2. Main bar is used in shorter direction but distribution bar is used in longer span.

3. Higher dimension bar is used as main reinforcement bar.

Lower dimension bar is used as distribution bars.

4. Main reinforcement bar is used to transfer the bending moment to beams.

Distribution bars are used to resist the shear stress, and cracks developed at the top of the slab.

WHY CRANK BARS ARE PROVIDED IN SLAB:

Different shape of bent up bars and cranks are provided in the slab and other structural
members. Bars are bent near the supports normally at an angle of 45°. The angle bent
may also be 30° in shallow beams where the (effective depth < 1.5 breadth).

In the above image, you can see how the bent up bars are provided in the slab.

The slab is supported at two ends. The maximum tensile stress that is
positive moments (sagging) acting in the middle of the slab and the maximum compressive stress that
is negative moments (hogging) acting at both ends of support. So bottom steel is required at the mid
span and top steel resists negative moments at the supports. A bent-up bar called as crank bar is
provided to make RCC slab safe from compressive stresses.

When bent up bars are provided, the strength and deformation capacity of slabs with bent up
bars compared to slabs without bent up bars is sufficiently increased.

So crank bars are generally provided

1. To resist negative bending moment (hogging).

2. To resist shear force which is greater at supports.

3. To reduce the risk of a brittle failure of slab-column connection.

4. To reduce the amount of steel used.

5. For the economization of materials.

WHY STEEL RODS ARE USED IN REINFORCEMENT?

The fact which makes it possible to combine steel and concrete is that concrete contracts on setting in
the air and if a steel rod is embedded in a mass of wet concrete, it will be found that considerable force
is necessary to pull out when the concrete is set. If the steel section is in the form of a plate, although it
will resist removal when the concrete is set, yet it can be knocked off by sharp blows.

In the first case, the concrete grips the still, while in the second it only adheres. The grip depends upon
the strength of concrete used in the work as well as the perfection with which the concrete has been
mixed, placed in position, compacted and cured.

Besides this, the grip also depends upon the condition of the surface of the rod (whether it is smooth or
rough). Specially shaped bars (ribbed bars etc.) have therefore been introduced from time to time with the
object of increasing the grip.

It is on account of the similar coefficient of expansion of the two materials, the superior bond value of high
tensile strength and less cost of steel that’s why steel rods are used in reinforcement in R.C.C work.

DIFFERENCE BETWEEN WORKING STRESS METHOD AND LIMIT STATE METHOD:

The cardinal difference between Working state method (WSM) and Limit State method (LSM) is: WSM is an elastic design method whereas LSM is a plastic design method.

In elastic design, i.e. WSM, the design strength is calculated such that the stress in material is restrained to its yield limit, under which the material follows Hooke’s law, and hence the term “elastic” is used. This method yields to uneconomical design of simple beam, or other structural elements where the design governing criteria is stress (static).

However, in case of shift of governing criteria to other factors such as fatigue stress, both the methods will give similar design. Also, WSM substantially reduces the calculation efforts.

Now coming to plastic design, i.e. LSM, as the name suggests, the stress in material is allowed to go beyond the yield limit and enter into the plastic zone to reach ultimate strength. Hence the “moment-rotation” capacity of beam, for example, is utilized making the design more economical. However, due to the utilization of the non-linear zone, this method involves arduous calculation.

All other differences are mostly derived from the above stated fundamental difference along with few general differences. Some of these differences are stated below:

1) Serviceability check in case of LSM is required because after the elastic region strain is higher, which results in more deformation, hence a check is necessary.

2) LSM is strain based method whereas WSM is stress based method.

3) LSM is non-deterministic method whereas WSM is deterministic approach.

4) The partial safety factor is used in LSM whereas Safety factor is used in WSM.

5) Characteristic values (derived from probabilistic approach) are used in case of LSM whereas Average or statistic values are used in WSM.

HOW TO CALCULATE CUTTING LENGTH OF BENT UP BARS IN SLAB

CUTTING LENGTH OF BENT UP BARS IN SLAB:

As a site engineer, you need to calculate the cutting length of bars according to the slab dimensions and give instructions to the bar benders.

For small area of construction, you can hand over the reinforcement detailing to the bar benders. They will take care of cutting length. But beware, that must not be accurate. Because they do not give importance to the bends and cranks. They may give some extra inches to the bars for the bends which are totally wrong. So it is always recommended that as a site engineer calculate the cutting length yourself. In this article, we will discuss how to the calculate length for reinforcement bars of slab. Let’s start with an example.

EXAMPLE:

Where,

Diameter of the bar = 12 mm

Clear Cover = 25 mm

Clear Span (L) = 8000

Slab Thickness = 200 mm

Development Length(Ld) = 40d

CALCULATION:

Cutting Length = Clear Span of Slab + (2 x Development Length) + (2 x inclined length) – (45° bend x 4) – (90° bend x 2)

Inclined length = D/(sin 45°) – dD/ (tan 45°) = (D/0.7071) – (D/1)= (1D – 0.7071D)/0.7071= 0.42 D

As you can see there are four 45°bends at the inner side (1,2,3 & 4) and two 90° bends ( a,b ).

45° = 1d ; 90° = 2d

Cutting Length = Clear Span of Slab + (2 X Ld) +(2 x 0.42D) – (1d x 4) – (2d x 2) [BBS Shape Codes]

Where,

d = Diameter of the bar.

Ld = Development length of bar.

D = Height of the bend bar.

In the above formula, all values are known except ‘D’.

So we need to find out the value of “D”.

D = Slab Thickness – (2 x clear cover) – (diameter of bar)

= 200 – (2 × 25) – 12

= 138 mm

Now, putting all values in the formula

Cutting Length = Clear Span of Slab + (2 x Ld) +(2 x 0.42D) – (1d x 4) – (2d x 2)

= 8000 + (2 x 40 x 12) +(2 x 0.42 x 138) – (1 x 12 x 4) – (2 x 12 x 2)

∴ Cutting Length = 8980 mm or 8.98 m.

So for the above dimension, you need to cut the main bars 8.98 m in length.

WHAT IS THE BEST CONCRETE MIX DESIGN?

The cost of concrete determines the characteristic strength, quality control, workability of mix (cost of labor) which includes the high degree of compactions. The best concrete mix design is the one which satisfies all the aspects for which it was designed.

STRENGTH:

The strength of 95% cube cast after 28 days of curing should be greater than the characteristic strength for which concrete has been designed.

WORKABILITY & PLACING:

As working condition changes so the properties desired from concrete also changes, the concrete which can be easily placed without segregation and with least compaction required.

WATER CEMENT RATIO:

Water should be maximum (0.45 – 0.65).

DURABILITY:

The concrete must be durable enough to face harsh conditions of atmosphere for which it has been designed.

These are the main properties considered while designing concrete and the designed concrete satisfying such conditions can be called as best concrete mix design.

There are three type of mixes,

1. Nominal mixes

2. Standard mixes

3. Designed mixes

1. NOMINAL MIXES:

The mixes which have fixed cement aggregate ratio but the nominal mix concrete for a given workability varies widely In strength.

2. STANDARD MIXES:

1. It is designated by code book of IS456:2000

2. The minimum compressive strength have included

3. May result in under and over rich mixes.

3. DESIGNED MIXES:

1. The mix proportion is designed by producer of concrete

2. The concrete is specified by the designers

3. It does not guarantee the concrete mix proportion for the prescribed performance.

As we normally use the standard mixes which are safe and economic, as per standard code book.

TYPES OF JOINTS IN CONCRETE

CONCRETE JOINTS:

Except in small jobs, it is not possible to place concrete in one continuous operation. Joints are also required for functional consideration of the structure.  Concrete joints can be classified under following categories:

1. Construction Joints.

2. Expansion Joints.

3. Contraction Joints.

4. Warping Joints.

1. CONSTRUCTION JOINTS:

These joints are provided where there is a break in construction programme. Concreting operation should be so planned that the work is completed in one operation. If, however, it has to be stopped before completion of entire work, construction joints are provided. Location of construction joints should be such that it interferes minimum with the functional characteristics of the structure. Best locations for construction joints are as following:

i) Beam: Joint may be located at mid-span or over the center of the column in direction at right angles to the length of the beam.

ii) Columns: Joints should be located a few cm below its junction with the beam.

iii) Slab: Joints may be placed at mid span or directly over the center of the beams, at right angles to the slab.

Formwork for construction joint should be placed at the end of each day’s work.

Before new concreting is started, the concrete surface of hardened concrete should be cleaned, roughened, saturated with water, and applied cement grout. This will ensure proper bond between old and new concrete works. New concreting is started before the applied grout on old surface attains initial set.

2. EXPANSION JOINTS:

These joints are provided to allow for expansion of the concrete, due to rise in temperature above the temperature during construction.  Expansion joints also permit the contraction of the element. Expansion joints in India are provided at an interval of 18 to 21 m. A typical expansion joint is shown in Fig 1. The open gap of this joint varies between 2 cm and 2.5 cm. Sometimes, to transfer load from one slab to the adjacent slab, dowel bars are also used at suitable intervals at these joints.

3. CONTRACTION JOINTS:

These joints are provided to permit contraction of the concrete. These joints are spaced closer than expansion joints. These joints do not require any load transfer device as it can be achieved by the interlocking of aggregates. However, some agencies recommend use to dowel bars fully bonded in concrete.

4. WARPING:

Warping joints are provided to relieve stresses induced due to warping effect. These joints are also known as hinged joints.

DIFFERENCE BETWEEN ONE WAY SLAB AND TWO WAY SLAB

ONE WAY SLAB:

One way slab is a slab which is supported by beams on the two opposite sides to carry the load along one direction. In one way slab, the ratio of longer span (l) to shorter span (b) is equal or greater than 2,
i.e Longer span (l)/Shorter span (b) ≥ 2

Verandah slab is a type of one way slab, where the slab is spanning in the shorter direction with main reinforcement and the distribution of reinforcement in the transverse direction.

TWO WAY SLAB:

When a reinforced concrete slab is supported by beams on all the four sides and the loads are carried by the supports along both directions, it is known as two way slab. In two way slab, the ratio of longer span (l) to shorter span (b) is less than 2.

i.e Longer span (l)/Shorter span (b) < 2

This types of slabs are mostly used in the floor of multi-storey buildings.

DIFFERENCE BETWEEN ONE WAY SLABS AND TWO WAY SLABS:

1. In one way slab, the slabs are supported by the beams on the two opposite sides.

In two way slab, the slabs are supported on all the four sides.

2. In one way slab, the loads are carried along one direction.

In two way slab, the loads are carried along both directions.

3. In one way slab, the ratio of Longer span to shorter span is equal or greater than 2. (i.e l/b ≥ 2).

In two way slab, the ratio of l/b is less than 2 (i.e l/b < 2).

SLUMP TEST OF CONCRETE

CONCRETE SLUMP TEST:

The concrete slump test is an empirical test that measures the workability of fresh concrete. The test is performed to check the consistency of freshly mixed concrete in a specific batch. Consistency refers to the ease and homogeneity with which the concrete can be mixed, placed, compacted and finished. This test is most widely used due to the simplicity of apparatus and simple test procedure.

Slump Test

The slump test gives satisfactory results for the concrete mix of medium to high workability and unfortunately, it does not give the correct indication of low workability, which may give zero slumps. This test is also known as slump cone test.

APPARATUS FOR CONCRETE SLUMP TEST:

1. Mould or slump cone with a height of 300 mm, bottom diameter 200 mm, and top diameter 100 mm.

2. Standard tamping rod.

3. Non-porous base plate.

4. Measuring scale.

PROCEDURE OF TEST:

1. First, clean the inner surface of the empty mould and then apply oil to it.
2. Set the mould on a horizontal non-porous and non-absorbent base plate.
3. Fill the mould fully by pouring freshly mixed concrete in three equal layers.
4. Stroke each layer 25 times with the standard tamping rod over the cross section.
5. After stroking 25 times the top layer is struck off level, now lift the mould slowly in the vertical direction without disturbing the concrete cone.
6. Use the measuring scale to measure the difference level between the height of the mould and the concrete sample.

The subsidence of concrete is known as the slump and the value of slump is measured in mm.

TYPES OF SLUMP:

• True Slump: The concrete mass after the test when slumps evenly all around without disintegration is called the true slump.
• Shear Slump: When one-half of the concrete mass slide down the other is called the shear slump. This type of slump is obtained in a lean concrete mix.
• Collapse Slump: When the sample is collapsed due to adding excessive water, it is known as collapse slump.
• Zero Slumps:  For very stiff or dry mixes it does not show any changes of the slump after removing the slump cone.

ADVANTAGES OF CONCRETE SLUMP TEST:

1. The procedure of slump test is simple and easy than any other workability test.
2. Inexpensive and portable apparatus are required for this test.
3. Slump test can be performed at the construction site as well as in the laboratory

 LIMITATIONS OF CONCRETE SLUMP TEST:

1. The slump test is limited to concretes with the maximum size of aggregate less than 38 mm.
2. The test is suitable only for concretes of medium or high workabilities (i.e having slump values of 25 mm to 125 mm).
3. For very stiff mixes having zero slumps, the slump test does not show any difference in concretes of different workabilities.

Recommended Values Of Slumps For Different Concrete Mixes:

Types Of Concrete	Slump Range In mm
1. Heavy mass construction	25-50
2. Pavements	20-30
3. Bridge deck	25-75
4. Beams and slabs	50-100
5. Columns, retaining walls and thin vertical members etc.	75-150
6. Vibrated concrete	12-25

CONCRETE MIX DESIGN : DESIGN MIX CONCRETE & NOMINAL MIX CONCRETE

CONCRETE MIX DESIGN:

Mix design is a method which determines the proportions of cement, water, fine aggregates and coarse aggregates to produce the concrete of required strength, workability and durability with minimum cost.

Mix design can be divided into following two categories

1. Design mix concrete and 

2. Nominal mix concrete.

1. DESIGN MIX CONCRETE:

When the proportions of concrete ingredients are decided by adopting certain established relationships ( based on assumptions from a lot of experiments) to produce the concrete it is known as design mix concrete.

In design mix concrete, it is assumed that compressive strength of concrete is totally dependent on 

the water-cement ratio.

2. NOMINAL MIX CONCRETE:

When the concrete is produced by taking standard arbitrary proportions of concrete ingredients, it is known as nominal mix concrete. This method is generally used when the quality control requirement for design mixes are difficult to execute. As we have explained for normal work, nominal mix concrete can be designed by taking cement, fine aggregate and coarse aggregate in the ratio of 1 : n : 2n.