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Methods of Reinforcement Quantity Estimation in Concrete Structure

Estimation of steel reinforcement quantity is a necessary step in calculating cost of RCC structure along with other building materials as per construction drawing.

Accurate calculation of reinforcement in the building plays an important role in the over all costing of the project. The estimation of the reinforcement is made from the drawings and bar bending schedule.in the cases where there is no availability of drawings and schedules, the quantity is normally described in accordance with the requirements of the Standard method of measurement of building works.

Methods of Reinforcement Quantity Estimation:

There are different methods for estimating the quantities of reinforcement; three methods of varying accuracy are:

Method-1 : Reinforcement Estimation (Thumb Rule Method)

This simplest method is based on the type of structure and the volume of the reinforced concrete elements.

Average values for typical concrete frames:

Heavy industrial = 130 kg/m3
Commercial = 100 kg/m3
Institutional = 90 kg/m3

Residential = 85 kg/m3

However, while this simplest method to check on the total estimated quantity if reinforcement, same time it is the least accurate and it requires considerable experience to breakdown the tonnage down to Standard Method of Measurement requirements. Some of the elements breakdown is given below,

Standard Method of Measurement

Method-2 : Reinforcement Estimation (Accurate Method):

This is the most accurate method for quantity estimation of reinforcement. This method requires the drawings and schedules. The drawings used in this estimation are the representative of actual structure. The sketches include the intended form of detailing and distribution of main and secondary reinforcement. An allowance of additional steel for variations and holes may be made by inspection.

Let us take an example and estimate the quantity of reinforcement in method,

Reinforcement Details of a Typical Beam

Cross Section of a Typical Beam

Calculation:

Bar 1:

b = 4000 + (2 x 230) – (2 x 40) = 4380

No bends, hence, no deductions

Cutting Length = 4380  mm

Bar 2:

a = 200

b = 4000 + (2 x 230) – (2 x 40) = 4380

Deduction :

(2 x dia x no. of bends) = 2 x 20 x 2

Cutting Length = (2×200) + (4380) – (2 x 20 x 2) = 4700 mm

Bar 3:

a = 230 – (2×40) = 140

c = 375 – (2×40) = 285

Cutting Length:

(2A + 2C) + 24d = (2x 140 + 2x 285) + 24×8 = 1042 mm

No. of Stirrups: (4000/180) + 1 = 23.22 = 24

Bar Bending Schedule for the Beam

Number of bars:

Suppose the spacing of stirrups is 150 c/c and the length along which they are placed is 6800 mm, we can find the number of bars by the formula below

[ Length / Spacing] + 1 = number of bars

[ 6800 / 150] + 1 =  46.33

In this case, we always round up. Hence, we require 47 stirrups.

Cutting Length:

We must remember than steel is ductile in nature and is subject to elongation. Hence, the length of a bar is increased when bends or hooks are introduced.  Hence, certain deductions are needed to offset this increase in length.

Cutting Length = True Length of a bar – Deductions

For 45 degree
Cutting length  = Total length – 1 x Dia of bar x No. of bends

For 90 degree
Cutting  length  = Total length – 2 x Dia of bar x No. of bends

For stirrups:

90 degree hook:

Length of stirrup = (2A + 2B) + 20 x dia

135 degree hook:

Length of stirrup = (2A + 2B) + 24 x dia

Quantity Estimate for Reinforcement in Kg:

Quantity Estimation of Reinforcement in Kg

**Unit weight in kg/m is calculated using the formula = D2/162

For 8mm bar = 82/162

= 64/162

= 0.395 kg/m

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Types of Soil Excavation Tools and Machines in Construction

There are different types of soil excavation tools and machines used in construction. Excavation of soil is necessary in construction point of view and it should be done by hand tools or machineries based on the area of the land or depth of excavation.

By the process of excavation, the land is cleaned from tree roots, strums, organic impurities etc., which should harm to the foundation.

Types of Soil Excavation Tools and Machines:

Now a days, for the soil excavation there are so many equipment’s are there and these are classified into two types.

1. Hand tools

2. Machineries

Hand tools for Soil Excavation:

These are generally used for smaller depths of excavations in small areas. Man power is required to operate these tools. The tools come under this category are explained below.

Spade

Spade is a tool which consists metal plate having sharp edges, the plate is attached to long handle which is generally made up of wood. Because of its sharp edges the soil can be dig easily. The metal plate having less curvature in the spade so, we cannot lift the soil by spade.

Shovel

Shovel is tool which is used for the purpose of lifting of excavated soil. It is also similar to spade the difference between spade and shovel is the difference in leading edge. The curvature of metal plate of shovel is generally higher when compared to spade so we can hold the soil easily and lifted it. Shovel can also be used for digging purpose in case of soft soils, sand etc.

Hoe

Hoe is an excavating tool which consists a metal plate attached to a long handle with acute angle. The plate having sharp edge is used to excavate the soil. For small work of excavation it is widely preferred tool. Sometimes metal plate is replaced by fork type plate.

Trowel

Trowel is hand sized tool which is generally used to dig the small trenches in soil or to remove the shallow roots in soil.

Rake

Rake is a tools which is having a horizontal rod having metal teeth and is used to remove the small layers of soil.

Pick axe

Pick axe consists hard spike attached perpendicular to handle. They are used for excavating small trenches in soil. Pick axe can cut the soil even if the soil is of hard type. The metal spike is pointed on one side and wide blade is provided on the other side.

Mattock

This looks like pickaxe. But serious digging is not possible with mattock. Generally it is used as lifting tool because of its curve shapes metal at its bottom.

Machinery Tools for Soil Excavation:

These are the tools which are operated by mechanical force and are used for the larger depths of excavations. There are so many types of machine tools with ease of operation are designed in this modern day period.

Tracked Excavator

This is also called as track hoe. It consists of cabinet and long arm. Long arm again consists of 2 parts. The first part which is closure to cabinet is called as Boom and the other part is called as Dipper-stick. Digging bucket is attached to the end of dipper. This entire system can rotate 360 degrees. In this case Vehicle is moved by traction, so we can use this equipment in mines, forestry, pipeline industries etc. the function of excavator is done by hydraulic fluid so, it is also called as hydraulic excavators.

Wheeled Excavator

This whole arrangement is similar to the tracked excavator except that the movement of vehicle is done by wheels. It can move quickly when compared to tracked excavator but it is not suitable for uneven grounds or hilly areas because of slippery nature. So these are generally used for road constructions etc..

Back Hoe Excavator

In case of back hoe excavator, the hoe arrangement is on back side and loader bucket is arranged in front of the vehicle. So two operations digging and loading or lifting is done by hoe and loader. This is widely used equipment nowadays because of its small size and versatility. This is moved with the help of wheels. so, moving from one workstation to another is quickly done.

Bulldozer

Bulldozer consists of hard steel plate with sharp edge at its front. This sharp edge is helps the plate to cut the soil and for excavation. The metal plate can be raised and lowered with the help of hydraulic arms. The bulldozers are available in both tracked and wheeled form. These are widely used for the works of soil excavations, weak rock strata removal, lifting of soil etc.

Dragline Excavators

Dragline excavators consists large length boom. A cable is hanged from the top of the boom and digging bucket is suspended to the cable. Generally dragline excavators are used for larger depth excavations like port construction, under water sediment removal etc. These are heavy equipment with greater economy.

Trenchers

Trenchers are the equipment’s used to dig trenches in the ground. These are also available in many sizes based on our requirement. The trenches dug are generally used for drainage purpose, pipeline laying, cable laying. The trenchers are generally available in two types.

• Chain trenchers
• Wheeled trenchers

Chain trenchers

Chain trencher consists of a fixed arm called boom around which digging chain is arranged. The chain is very hard and consists sharp tooth. Whenever it is driven into ground the chain rotates around boom with rapid force and cuts the ground. By adjusting the angle of the boom we can manage the depth to be excavated. It cuts the soil with hard strata which cannot excavate by bucket type excavator. In case of smaller chain trenchers wheeled vehicles are available and for larger chain trenchers are available in tracked form.

Wheeled trenchers

In case of wheeled trencher, toothed metal wheel is used as trenching tool. This equipment is available in both tracked and wheeled form of vehicles. When compared to chain trencher the wheeled trencher can cut harder soil strata. It is also used to cut pavement surface while road repair works. Economically also wheeled trencher is better than the chain trencher.



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Backfilling of Sewer Sanitary Trench -Compaction and Equipments

Backfilling of sewer sanitary trench is required after installation of sewer pipes. Method of compaction and degree of compaction, equipments and sequence of sewer trench backfilling is discussed. Backfilling process requires substantial cautions and should be carried out properly.

Fig.1: Backfilling of Sewer Sanitary Trench

Backfilling of Sewer Sanitary Trench

Following points regarding backfilling of sewer sanitary trench is discussed:

• Backfilling consideration
• Backfilling sequence
• Compaction degree
• Selection of compaction equipment

Considerations for Backfilling of Sewer Trench

Sewer sanitary pipe disposition and deterioration need to be prevented while backfilling is carried out. This may be obtained by carefully selecting methods and machines for the task.

The determination of backfilling technique is dependent on trench width, properties of excavated materials, degree of compaction and excavation approach.

Sequence of Sewer Sanitary Trench Backfilling

Generally, backfilling begins after concrete structures like arches and cradles have achieved adequate strength and withstand imposed backfill material loads without suffering damages.

Backfilling of sewer sanitary trench is composed three main layers including embedment, final backfill and the third layer is finish grade material.

It should be noted that each layer requires certain considerations that need to be considered. These zones of sewer sanitary trench backfill is discussed in the following sections:

Embedment Zone of Backfilling

It is the first layer and consist of haunching and initial backfill. Backfilling process starts from the selection and placement of embedment materials. The lower backfill layer is extended from the foundation up to 30cm above the pipe.

This zone should be placed by hand or suitable machines and properly compacted to avoid dislocation and deterioration of installed pipes.

It is recommended to use granular material since the application of considerable compaction efforts to gain specified density is not required.

The placement of haunching that extends up to the spring line should be conducted with utmost caution while flexible pipes are used.

In the case of high water table, it is required to decrease voids as minimum as possible to prevent the movement of soils.

Intermediate Backfill Zone

The selection of intermediate backfill material is commonly influenced by the required compaction degree. However, the use of excavated material is recommended especially in the regions prone to frost heave.

The degree of compaction is based on the location of the project for instance 95% modified proctor compaction should be achieved in traffic area, 90% compaction needed in urban areas and small compaction degree will be required in rural area.

Upper Backfill Material Zone

The depth and compaction degree of upper zone is controlled by the type of the required finish grade, for instance, the area might be used for agriculture purposes or paved.

In the latter case, the elevation of the upper zone need to be established properly to support different pavement layers. In the former case, top soil layer with depth of 100mm shall be provided.

Sanitary sewer pipe might be subject to floatation before and during backfilling due to water accumulation in the trench. So, necessary measures need to be considered to prevent such detrimental situation.

Degree of Compaction for Sewer Trench Backfilling

The required degree of compaction for sanitary sewer trench backfill is associated with the location of the project. For instance, high compaction degree shall be provided if the area is designated to be paved, but if the region is not heavily populated and do not subjected to heavy traffics less compaction would be satisfactory. Natural settlement of backfill material may be adequate in certain conditions like along outfall sewers in open country.

The degree of compaction is computed in accordance with specification provided by American Association of State Highway and Transportation Officials. Modified proctor test is used to evaluate optimum moisture content in the laboratory, and various tests are available for the determination of compaction value in the field.

Selection of Compaction Equipment for Backfilling

In this part, the selection of the suitable compaction equipment based on the type of backfill material and the availability of the required space is explored:

Compaction Equipment for Cohesive Materials

Generally, small particles and low internal friction are properties by which cohesive materials are characterized. The strength of adhesive force between cohesive material particles make it difficult to reshape the particles and compact it adequately. So, the application of equipment that impose great impact is advised to overcome adhesive forces and relocate particles and eventually obtain desired compaction.

For restricted areas, the utilization of pneumatic tamper, engine driven rammer and self-propelled rammer is recommended to use whereas sheep foot roller can be considered for wide area and dozers and loaders may be used if high compaction degree is not needed.

Backfill material should be close to optimum moisture content and multiple lifts should be used otherwise the compaction result would be poor and unacceptable.

If the moisture content is excessively high, the backfill material should be dried to reach compactible moisture content. Such moisture content may be realized by drying, mixing the material with granular material and pulverizing.

Compaction Equipment for Cohesionless Material

Cohesionless soil is characterized by low adhesion force and large internal friction. The degree of non-cohesive soil compaction is improved though the decrease of surface friction between particle rather than moisture content. Therefore, vibrator compaction is the most suitable machine that can be used to compact cohesionless backfill material and provide acceptable compaction degree.

It is specified to use vibratory roller if the trench is wide. Vibratory plate would provide satisfactory compaction degree in narrow trench. Vibration impact of dozer or truck loader are suitable to consider if high compaction degree is not required.

Compaction Equipment for Borrow Material

Borrow material is used when the excavated soil does not possess necessary requirements to be used as a backfill material. So, in this case material with good property whether it is cohesionless or cohesive may be decided to utilize.

One should be aware that the application of borrow materials would cause variations in the ground water, so this likelihood shall be considered while the application of borrow material is considered.

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Excavation and Backfilling of Soil – Work Sequence and Procedure

Earthwork in excavation and backfilling of soil upto required depth is required for construction of foundation and trenches. The proper sequence of excavation and backfilling is required to optimise the process and prevent safety issues.

Excavation and backfilling of soil is a very important part of construction process, and care must be taken while excavation in safety perspective.

Different soil layers may be encountered while excavation, dewatering may be needed sometimes. These points must be kept in mind to take necessary action during excavation and backfilling.

Correct measurement of excavation and backfilling is required because excavation cost is major part of the foundation construction.

Excavation and Backfilling of Soil

Following are the sequence of works for excavation and backfilling of soil:

Materials and Tools Used for Excavation

The following are the equipments used for the earthwork for foundation:

• Hydraulic excavator
• Tractor / trucks
• Spade
• Kassi
• Pickaxe
• Crowbar
• Rammer
• Wedge
• Boning Rod
• Sledge Hammer
• Basket
• Iron Pan
• Line and Pins
• hydraulic compactor

Drawings required for excavation

1. Centerline Drawing
2. Layout Plan

Scope of Work for Excavation and Backfilling of Soil

• Setting out of corner benchmarks.
• Survey for ground levels.
• Survey for top levels
• Excavation to approved depth.
• Dressing of loose soil.
• Making up to cut off level
• Constructing dewatering wells and interconnecting trenches.
• Marking boundaries of the building.
• Constructing protection bunds and drains

Working Procedure for Soil Excavation

The extent of soil and rock strata is found by making trial pits in the construction site. The excavation and depth is decided according to the following guidelines in the site.

• For Isolated footing the depth to be one and half times the width of the foundation.
• For adjacent footings with clear spacing less than twice the width (i.e.) one and half times the length.
• 1.5m in general and 3.5 m in black cotton soils.

Fig: Earthwork in Excavation by using hydraulic Excavator

Setting out or ground tracing is the process of laying down the excavation lines and center lines etc. on the ground before the excavation is started.

The centerline of the longest outer wall of the building is marked on the ground by stretching a string between wooden or mild steel pegs.

Each peg may be projected about 25 to 50 mm from the ground level and 2m from the edge of the excavation. The boundary is marked with the lime powder.

The center lines of other walls are marked perpendicular to the longer walls. A right angle can be formed by forming 3, 4 and 5 triangles. Similarly, outer lines of the foundation trench of each cross walls and are set out.

Backfilling and Removal of Excess Soil

Estimate the excavated stuff to be re-utilized in filling, gardening, preparing roads, etc. As far as possible try to carry excavation and filling simultaneously to avoid double handling.

Select and stack the required material in such a place that it should not obstruct other construction activities.

The excess or unwanted material should immediately be carried away and disposed off by employing any of the following methods.

• Labour
• Tractor
• Trucks

Quality Checks for Excavation

• Recording initial ground level and check size of bottom.
• Disposal of unsuitable material for filling.
• Stacking suitable material for backfilling to avoid double handling.
• Strata classification approval by competent authority.
• Dressing bottom and sides of pits as per drawing with respect to centerline.
• Necessary safety measures observed.

Quality Checks for Backfilling

• Recording initial ground level
• Sample is approved for back filling.
• Necessary marking/ reference points are established for final level of backfilling.
• Back filling is being carried out in layers (15cm to 20cm).
• Required watering, compaction is done.
• Required density is achieved.