Thursday, July 30, 2020

Lightweight Concrete - Classification, types , Properties, Advantages

Lightweight Concrete

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 1200oC. 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.

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