Floor Space Index/ Floor Area Ratio- Importance, Calculation, Advantages, Disadvantages

The maximum area we can construct in a piece of land or a plot is known as Floor Space Index (FSI) or Floor Area Ratio. It can be further said as it is the ratio of the floor area of the building we are going to construct ie. build area of the building to the total plot area of the land.

The value or the ratio we receive, tells us how much area we can construct on each floor of the building.

Regulation Bodies

The state government is the regulating body under which municipal corporations or the local bodies carry out the work. The norms of FSI/FAR are set by considering the National Building Code (NBC) of India.

It is the regulating bodies, who provide the FSI which varies based on different cities, building structures, and other amenities.

How to calculate FSI/FAR.

Calculating FSI

Consider land area = 10000 sq.ft

As we have seen FSI vary on the different building structure, Location & floors, So firstly we have to consider the type of building i.e

• Residential

• Commercial

Let’s assume the FSI for residential building is 1.45 in that location, So we can easily find out

                                                            = FSI x Land Area

                                                            = 1.45 x 10000

                                                            = 14500 sq.ft

This further can be explained as 14500 sq. ft is the maximum area we can construct in that plot. If we are planning to construct 2 floors,

Then, 14500/2 = 7250 sq.ft each floor.

If we are willing to construct for three floors then

Then, 14500/3 = 4833.33 sq.ft each floor

Premium FSI –

Premium FSI is, the extra money we pay to the government resulting in receiving extra FSI or more plot area for the construction of the building. The Premium FSI can only be availed to the buildings which are near to the road at least 30 ft.

The % of premium FSI availed based on building location to the road can be defined as

Abutting Roadways	Premium FSI
1. 30ft – 40ft	20%
2. 40ft – 60ft	30%
3. More than 60ft	40%

 

Advantages of FSI

·         Planned Growth.

·         Clear view of open area and build-up area in the plot.

·         Equilibrium between open area and build-up area, resulting in reducing congestion.

Easy calculation to find quantity of Cement, Sand, Aggregate in 1m3 Concrete ( M5 - M20)

Introduction - Concrete Mix

For the small scale construction, we does-not have the availability of batching plants, there are two types of methods which are followed for the mixing purpose of concrete, they are

(i) Weight Batching

(ii) Volume Batching

On the above two, the weight batching is done by calculating the weight of the materials required for the concrete mix which is the most appropriate method, which we are going to look at in this article.

On the other side, the volume batching is done by pouring the materials inside the pre-fabricated molds/gauges or farma’s (Hindi). The method is not considered must appropriate since, there are chances of pores, bulging, etc of the aggregate and sand, due to which volume may get increased but the desired ratio of the mix cannot be achieved.

This calculation method is preferred mostly to the Nominal mix of the concrete, which is from grade M5 to M25 mix.

 Important units for calculation - Concrete Mix

Shrinkage Factor / Factor of Safety	1.54 - 1.57
Density of Cement	1440 kg/m3
Density of Sand	1600 kg/m3 - 1800 kg/m3
Density of Aggregate	1500 kg/m3 - 1850 kg/m3
Water to Cement Ratio (w/c)	0.50 (assuming)
Concrete Mix	M20 (assuming)
M20 mix grade	1:1.5.3 ( IS 456)

 

Calculation - Concrete Mix

Finding out the quantity of Cement, Sand, Aggregate in 1m³ Concrete

We are going to calculate the weight of materials from the M20 grade of concrete, where M is referred to as mix and 20 is the compressive strength at N/mm².

The Ratio of M20 grade concrete is = 1:1.5:3

Where,

1 part is cement, 1.5 part is sand and 3 parts is aggregate.

Here, while calculating for 1m³ of concrete, we use to multiply the quantity with the value ranging between 1.54 to 1.57 as a factor of safety or shrinkage factor of the concrete.

This is done since the volume of the dry concrete is different compared with wet concrete. More easily, we can say, while pouring concrete the concrete which we use be wet in nature but once it starts hardening, the concrete start showing shrinkage behavior in it making in the reduction of the volume of concrete.

To overcome the problem we take the factor of safety or the shrinkage factor.

 Note - Lower grades between M5 - M 15, the shrinkage factor tends to be lower than 1.54. This is because of the low workability of the concrete. The less water in the concrete, the lower shrinkage will be seen.

1. Calculating the weight of cement in 1m³ concrete mix

We know the density of cement is 1440 kg/m^3.

Therefore, to calculate the weight of the cement we have to multiply the volume calculated by the density

(Mass= Volume x density)

Mass of cement = 0.28 x 1440

= 403.2 Kg cement

Since a bag of cement is 50 kg, we require around 8 bags of cement for 1m³ concrete.

2. Calculating the weight of sand in 1m³ concrete mix

From the above calculation, we have received the volume of the sand in the concrete mix, To convert it to the weight again we have to multiply the volume with the density of sand where we assumed the density of sand 1800 kg/m³

Mass of Sand = 0.42 x 1800 kg/m³ 

= 756 Kg’s

So, for 1m³  of concrete, we will require 756 kg of sand at M20 grade.

3. Calculating the weight of aggregate in 1m³ concrete mix

Same procedure as above we will follow, the volume must be multiplied with the density of aggregate to find the weight of aggregate 

Mass of aggregate = 0.84 x 1850 kg/m³ 

                                  =  1554 kg's

So, from this we can say for 1m³ of concrete we will require 1554 kg of aggregate.

4. Calculating the water requirement in 1m³ concrete mix

w/c ratio = 0.5

Therefore, we require 201.6 liters of water for 1 m3 of concrete for M20 mix.

From the above procedure, we can find out the weight of the materials required in concrete.

Related Article

• Grade of concrete
  

Lightweight Concrete - Classification, types , Properties, Advantages

Lightweight Concrete

Introduction

Lightweight concrete is prepared with the help of lightweight aggregates. It is done to make the concrete light in weight will certainly lead to lowering the density and the strength of the concrete.

Lightweight concrete mostly preferred as an insulating type of concrete used in building structures.

Since the lightweight concrete has a lower density as compared to conventional concrete it can be compared as

Conventional Concrete	Lightweight concrete
Density - 2200 kg/m3 to 2600 kg/m3	Density - 300 kg/m3 to 1800 kg/m3

 Lightweight Concrete - Classifications

The lightweight of the concrete and the lower density is achieved due to its production procedure and the material used while the manufacturing, which is distributed into three areas such as,

Material used	Specific Gravity	Function
1. Using porous lightweight aggregate	> 2.6	Aggregates having lower density, flakes, porous in nature are used to make the concrete lighter
2) Introducing large voids to concrete (Cellular, Aerated, Gas concrete)	-	When the alumina reacts with alkaline, hydrogen gas is formed. This Hydrogen gas results in the expansion of concrete, creating large voids inside it. The large voids result in lowering the density and making the concrete lightweight.
3) No fines concrete	-	To make the concrete lightweight no fines are added, resulting in no fillers between the gaps of the aggregate, which creates interstices voids inside concrete making it lighter weight.

In building structures, it is not always required high strength or high density of concrete. There are many areas where the requirement of the concrete is only for filling gaps, insulation, thermal resistance, etc. Therefore, in those areas, this lightweight concrete is used.

Purpose	Code	28 days cylinder compressive strength	Density
1. Structural lightweight concrete	ASTM C 330-05	Should be greater than 17 Mpa	Should be less than 1800 kg/m3
2. Masonry Unit	ASTM C 331 -05	7 - 14 Mpa	500 kg/m3 to 800 kg/m3
3. Insulating Concrete	ASTM C 332 -99	0.7 Mpa to 7 Mpa	Less than 800 kg/m3

 The lightweight concrete reduces the weight of the structure dead load to a greater extent, which is very convenient in designing the structure.

 Lightweight Aggregate - Types

The types of lightweight concrete aggregate are differentiated based on their occurrence, if the aggregates are extracted naturally then they are considered as Natural Aggregates, whereas the aggregates which are manufactured for the requirement of making the density low of concrete are known as Artificial aggregates.

1. Natural Aggregate

Main Natural lightweight aggregates	Origin
1. Diatomite 2. Pumice 3. Scoria 4. Volcanic cinders 5. Tuff	Sedimentary rock Volcanic rock Volcanic rock Volcanic rock Volcanic rock
6. Pumice (widely used)	Volcanic rock

Pumice - The natural lightweight aggregate can be used as a structural reinforcement member, as can be used in concrete for slab casting. They show the density ranging between 500- 900 kg/m³, and show good resistance against insulation, shrinkage, and absorption.

 Scoria - They have the physical appearance of glassy rock, which shows the characteristics of normal aggregate when mixed in concrete.

 2. Artificial Aggregate

As the natural aggregates are classified based on the origin of the rock surface it is extracted, similarly the artificial aggregates are distinguished based on the materials and the process used in the manufacturing process.

 Type - 1 - Heating in Kiln

In this type, the raw materials such as clay, shales, perlite are heated inside the kiln at 1200^oC. Because of the heating, certain gases are released which later are entrapped inside the raw material, later on creating an artificial lightweight porous aggregate.

These aggregates show a density range between 300 kg/m³ to 900 kg/m³ and when mixed with the concrete, the concrete possesses a density of 1400 - 1800 kg/m³.

 Type - 2 - Molten blast furnace slag 

It is a modern method, which produces lightweight pellets by blast furnace slag. The process is carried out such that, the blast furnace slag is heated to a high temperature, turning the slag into a molten state. The molten state of the slag contains heat bubbles and later water is sprayed to the slag which results in forming small lightweight pellets.

 Types -3 - Clinker aggregate

They are the burned extract of the industrial wastes which are converted to lumps due to the high temperature. In the United States, it is known as cinders. When these are mixed to concrete, the concrete shows a density ranging between 1100 - 1400 kg/m³ . When used with natural sand, the density of the concrete is increased to 1850 kg/m³ .

Lightweight Aggregate Concrete - Properties

 1. Using lightweight aggregates

Density Range	Strength Range	Cement used compared to normal concrete
300 - 1850kg/m3	0.3 - 40Mpa	Same to up to 70% more

 Using the lightweight aggregate, we can achieve a large variety of concrete having different properties, the strength in this is determined by the parameters of the use to water to cement ratio in it, the degree of the compaction, and the use of cement amount.

The lightweight aggregate concrete is considered sustainable due to its characteristics of being lighter in weight, density, and strength. The only disadvantage of this is, as it shows low thermal conductivity, there is a large amount of heat generated while using this type of concrete, due to which for a good period of time curing is required else cracks are observed on the surface.

 2. Aerated Concrete

The concrete consists of a large number of voids inside the concrete. These large air voids are introduced to the concrete by two methods.

(i) Introducing hydrogen Gas

The reaction of alumina with the alkaline present in the cement results in the formation of hydrogen gas. This reaction is done in the fresh concrete, which results in expanding the concrete and generating large voids.

The alumina used is 0.2% of the cement used in concrete.

 (ii) Introducing Foam agent to concrete.

During the perpetration of concrete, these foaming agents such as resin soaps are mixed to the concrete at a very high speed. Forming voids in the concrete.

Aerated Concrete	Density	Purpose
(i) Without Sand	200 -300 kg/m3	Non - Structural members, ex- Only for insulation purpose.
(ii) With Sand	500 -1100 kg/m3	Used in structural members.

 The codes used to check the different properties of aerated concrete

Codes	Properties
BE EN 678:1994	Compressive strength
BE EN 1351:1997	Flexural strength
BE EN 679:2005	Dry density
BE EN 680:2005	Shrinkage

 Mostly the aerated concrete is preferred for high insulation properties.

 3. No Fines Concrete

In the perpetration of no-fines concrete, we done used the fine aggregates in the concrete, which are sand particles. While mixing the concrete, there is no material available to fill up the gaps between the aggregate which results in creating voids in the concrete. The cement paste makes a coat of 1.3mm above the surface of the aggregate in the process. A large number of voids makes the concrete lightweight, reducing its strength.

The workability of no-fines concrete is very low and the compaction of the concrete with the help of a vibrator is done for a very short period of time. If done for a longer period, there are chances of the cement slurry to leave the adhesive bond with the aggregate there low compaction is observed in this type of concrete.

Range of W/C ratio	0.38 - 0.45
Density range between	1870 - 2020 kg/m3
Compressive strength range b/w	7 - 14 Mpa

 Lightweight Concrete - Area’s used

1. Since they are lightweight and shows good heat insulation properties, they are mostly preferred in roofs.

2. Non-Structural area’s where strength is not always required

(i) Wall partitions

(ii) Closing of cut-off, holes

(iii) Used for floor construction replacing the hollow tiles.

(iv) For light insulation

 Lightweight Concrete - Advantages

(i) Due to its lightweight, they reduce the dead load to an extent.

(ii) The transportation of lightweight concrete is easy compared to normal conventional concrete.

(iii) It is easier to work with lightweight concrete, thus it is efficient in reducing the transportation cost.

(iv) It does not exert much pressure to the surface of the formwork, resulting in reducing supports, materials, and labour charges making it cost-efficient.

(v) They have more resistance to freeze and thaw.