What is Light weight Concrete?
Lightweight concrete mix is those concrete that have low specific gravity compared to the other concrete due to the presence of minute voids or air entrainment. The void can be created either as in form of microscopic pores in a mortar or through the improper intersectional joint between particles of concrete or within aggregate itself.
Lightweight concrete has distinctive low structural strength compared to the normal or high-density concrete. As we know not all the parts of the structure are responsible for the strength of the structure, such concrete is very useful to those parts construction.
They can decrease dead load up to 20% – 80% depending on their types and components used within them with alteration in strength. Moreover, even they have low structural strength; their strength is still higher than the masonry elements like a brick.
The air is an excellent insulating agent. As the lightweight concrete contains air entrainment within it, this lightweight concrete has extremely high thermal insulation and sound insulation.
How lightweight concrete are made and what are the components of light weight concrete?
Lightweight concrete can be made in different ways, either by creating numerous intersectional joint void or by modifying the cementing agent with numerous pores giving cellular structure or by using extremely lightweight aggregates.
Whatever method is accepted, the principal mechanism of lightweight concrete is to introduce the air voids or entrainment in order to decrease its weight and increase volume.
On the basis of how lightweight concrete are made they are divided into three types:
1) Light weight aggregate concrete:
The lightweight porous aggregate such as pumice, thermally treated shale, diatomite, scoria, volcanic cinder, etc is used for decreasing the density of concrete. The specific gravity of such concrete is generally less than 2. The structural strength difference in concrete form by using normal aggregate and such aggregates keeping all other material as in the same quality and proportion is only 30%.
Thus due to these reasons, the concrete using lightweight are also subjected to structural strength design along with the purpose of decreasing its density. The density of such concrete varies from 1400 kg/m3 – 1800 kg/m3 and the strength of such concrete generally starts with 17MPa and more which depends upon their design mix.
2) Aerated concrete:
The microscopic pores of evenly well-distributed form in concrete is achieved by using the additional foam-forming materials like aluminum in the concrete mix. The aluminum powder reacts with the calcium hydroxide to give hydrogen gas, which forms the bubble of the air entrains gas within the concrete while setting.
Also, some times foam-forming materials (hydrolyzed protein or resin soaps) are used to form the foams. The air entrainment makes the concrete spongy or cellular in structure.
The concrete along with a comparison of its equivalent density with other material, it can be said it has comparative high strength. This concrete can decrease the load up to 80% but are not good for structural strength. Thus this concrete is also precast in the forms of blocks which can easily lift and placed as masonry units. Moreover, these long precast blocks are also used as a slab for the steel structures.
Due to high air entrainment voids, it can facilities the function of internal curing to concrete. They have extremely less density ranges from 550kg/m3 – 990kg /m3, so that can even float on water. The help of using both light aggregate, aluminum, and other admixture can make this. However, they have very little structural strength ranging from 4MPa- 12MPa.
3) No fines use concrete:
In this, the fine aggregate is completely omitted from the mix to create the interstitial voids. This concrete is prone to internal cracks formation and are not well compacted due to which there is a decrease in density.
The gap graded aggregate is preferred in order to prevent the interstitial bonding or compaction. They are useful for damp proof coursing, temporary structures, and external load-bearing walls. The density of this concrete is also 25%-30% less than normal concrete.
There density can ranges from 700kg/m3 (using lightweight aggregate) -1600/1900kg/m3(using normal aggregate). And their strength lies within 4MPa-14MPa. They exhibit low shrinkage on drying and tend to segregate less. But it must be noted that compaction via vibrating machines is prohibited however simple compaction through rod are allowed.
How to make lightweight concrete mix?
As already discussed, there are different types of lightweight concrete depending on their method of preparation. Thus the design of mixes and the proportion ratio of the constitution of the mix may vary or depend upon which type of approach is adopted. However, the generally adopted procedure and mix are explained below
1) For no fine aggregate
- The coarse aggregate passing from the sieve of 20mm and retain in 10 mm is preferred for the preparation of such concrete with absolute no used of fine( no material smaller than 5mm) aggregates. Moreover gap graded aggregate is more preferable in order to create more interstitial voids.
- Aggregate with round edges is preferred as they form a more interstitial gap.
- The cement to the aggregate proportional ratio for such concrete generally ranges from 1:6 to 1:10.
- The water ratio is also adopted of range 0.38 to 0.52 which depends upon the cohesiveness of the mixture.
- This concrete generally doesn’t have significant strength. Thus are not subjected to the design mix approach and general thumb rule or conventional method as explained in the table below are accepted to prepare.
|Aggregate /cement ratio by volume||Water cement ratio||Density In kg/m3||Compressive strength (28 days) In MPa|
2) For the aerated concrete
- The general procedure for the concrete mix preparation can be adopted along with the addition of the aluminum powder as an admixture in the fresh concrete mix.
- Moreover, the use of lightweight aggregate is also used to formless density concrete instead of normal aggregates. And sometimes even coarse aggregates are omitted (generally in AAC blocks)
- However, this concrete is more preferred in form of precast concrete blocks as Autoclave Aerated Concrete Blocks. So the general procedure for AAC block preparation can also be considered as its design method.
- The water-cement ratio for such concrete is 0.5-0.4 with cement sand ratio 1: 2 and if the coarse aggregate is also added then 1:1:2.and aluminum content should be 0.05%- 0.08% of the concrete mass.
3) For light weight aggregate concrete (DESIGN MIX PROCEDURE)
- The above-formed concrete might not have the distinctive strength. Thus by use of the lightweight aggregate the specific concrete of designated grade can be formed. The minimum strength of such concrete must be 17MPa. The lightweight aggregate concrete can be designed by a design mix method if required for the structural purpose.
- Also the adaptation of the material to make such concrete varies the design procedure and quality of concrete.
- Some basic information needed to know before the design is that the aggregate must sufficiently leave for the absorption over 10% for better performance. Also, the well-graded material must be selected to use less volume of fine aggregate.
- The ACI 211.2-98 method of design is popularly adopted to design the lightweight aggregate concrete for higher strength greater than 17MPa limiting density to 1840 kg/m3. The cement-content strength method is used to form fully lightweight aggregate concrete. (However, the normal mass method design used for normal aggregate can also be used to form semi lightweight aggregate concrete)
Lightweight concrete mix design
- Assume the appropriate slump value according to the requirement of concrete whether to use it in the beam, RCC works, column, Slab according to from the table1 given below.
|Types of construction||Slump value (in mm)|
|RCC foundation wall and footings||20 – 80|
|Plain concrete footings, structure wall||20 – 80|
|Beam and RCC wall||20 – 100|
|Building Column||20 – 80|
|Pavements and slab||20 – 80|
|Mas concrete||20 – 80|
|Concrete for canal lining||70 – 80|
- The maximum size of aggregate should be 1/5th of smallest dimension of member (generally cover in RCC), 1/3 of depth of slab and ¾ of reinforcement bar minimum spacing.
- Determine the air entrained required for higher durability requirement. From table 2.
- Determine rough cement content from Table 3.
|Compressive strength in MPa||Cement content (kg/m3 )|
|17||350 – 520|
|21||400 – 550|
|28||500 – 650|
|34||600 – 750|
|41||700 – 850|
- Assume that the total volume of loose and dry aggregate required for compacted 1m3 concrete mass is 1.2m3. assume the amount of fines is about 50 percentage of the coarse aggregate.
- After that with known dry loose densities of aggregate and required strength, the amount of aggregate, cement is determined and the trial mix is made using sufficient required water to produce required or assumed slump value.
- Then, the density of fresh concrete is measured and the yield is estimated to calculate batching quantities.
- The volume of water already presented in aggregate must be accounted during batching of concrete as the sample aggregate use for the trial mix are completely oven dry and loose aggregate. Thus is deducted from the total constant water content required.
- If the strength and other property such as air content, workability ,cohesiveness are satisfactory volume of aggregate and content of cement are just slightly altered. The general thumb rule is increasing dry aggregate by 0.0006 m3 for each 1kg decrease in cement content and vice versa.
- But if the strength and other property are drastically different, (like 20 MPa strength has need to form using trial mix for 17Mpa) then as such situation the huge amount of cement is needed to be altered. The pycnometer specific gravity factor are used to modify the density of material and so to their mass content.
The pycnometer specific gravity factor (S) is given by formula
Where, A = mass of aggregate tested
B = mass of pycnometer with aggregate filled with water (10 min of full immersion in water)
C= mass of pycnometer filled with water,
This formula is generalized in graph by varying different water content of aggregate given by as in fig 1.
And so cement content is increased abruptly , the volume of only fine aggregate is decreased. Thus other are maintain at constant. The new volume of fine aggregate is estimated by deducting sum of volume of cement, coarse aggregate, eater and air entrap obtain after correction. Moreover, with known sp. Gravity correction factor the mass of individual can be determined as well.
Prepare the light weight concrete of average strength 18 MPa at 28 days with air content 7 percent (assume if not given) and slump value of 75mm (assume if not given).
Dry density of coarse aggregate = 720 kg/m3 , moisture content at site = 5%
Dry density of fine aggregate = 900 kg/m3 , moisture content at site = 10%
From table 3, with reference of strength of 17MPa trial
Assume cement content =350 kg/m3 of concrete
In addition, according to design procedure consider the volume of total loose and aggregate required for 1m3 concrete mass be 1.2 m3, where 0.8 m3 is coarse aggregate and 0.4 m3 is fine aggregate (50 % of coarse aggregate)
Then for the trial mix of the volume 0.02m3 is required to prepare.
Cement required = 350 * 0.02 = 7 kg
Fine aggregate = 0.4* 900*0.02 = 7.2 kg
Coarse aggregate = 0.8 * 720 * 0.02 = 11.52 kg
Water content for required slump = 4 kg
Total mass = 29.72 kg
If the density of the fresh concrete trial is found to be 1450 kg/m3
Thus actual quantity rate on dry basis is,
Cement: 7/0.02015= 346 (Approx.)
Fine aggregate: 7.2/ 0.02015= 356
Coarse aggregate: 11.52/0.02015= 570
Water content: 4/0.02015 = 198
Total = 1470 kg/m3
The cement content in trial mix is 4 kg less than the specified
If the property including strength is satisfactory,
We decrease the volume of aggregate on dry loose basis by 0.0006m3 for each increase in 1kg cement. Allocating reduction in equally. Then
Fine aggregate rate = 356 – 0.5 *(4 *0.0006*900) = 355 kg/m3 of concrete
Coarse aggregate rate = 570 – 0.5 * (4*0.0006*720) =569kg/m3 of concrete
For small adjustment water content is kept constant
But the water moisture present on site must be considered. So, the final design mix rate of constituent for batching is,
Cement = 346+4 =350 kg/m3 of concrete
Fine aggregate (of site) = 355* 1.1 = 391 kg/m3 of concrete
Coarse aggregate (of site) =569*1.05 = 598 kg/m3 of concrete
Added water = 198 – [391- 355] – [598- 569] = 191 kg/m3 of concrete
If the strength is not achieved sufficient along with improper requirement and quality in concrete then. The cement content must be abruptly increased. Let the cement content is increased by 50 kg/m3 of concrete mass. So that, 400 kg/m3 is taken for next trial.
Also according to design procedure, let us assume specific gravity correction factor from fig 1. Or by formula and pycnometer analysis.
Let the factors is found to be 1.78 and 1.35 on a dry basis and 1.75 and 1.36 on wet basis.
On dry basis, volume of concrete with initially use trial of 350 kg/m3 of concrete is,
Thus the volume of fine aggregate is 1.00- 0.816 = 0.184m3
The mass of fine aggregate on dry basis is,
0.184* 1000*1.78 =328 kg/m3 of concrete mass
To estimate considering moisture (on site)
Fine aggregate = 328* 1.10 = 361 kg/m3 of concrete mass
Coarse aggregate = 569*1.05 = 598 kg/m3 of concrete mass
The added water here is obtain more accurately by,
thus mass of water required = 0.157* 1000 = 157 kg/m3
Hence the quantity required for batching is ;
Cement : 400 kg/m3 of concrete mass
Fine aggregate: 361 kg/m3 of concrete mass
Coarse aggregate: 598 kg/m3 of concrete mass
Water : 157 kg/m3 of concrete mass
Total =1516 kg/m3 of concrete mass
And at last the 2nd trial is made and subjected to test its compressive strength. If the requirements are not achieve then, again the cement content and dine particles are altered or any other parameter with help of general thumb rule.
Advantages of Lightweight concrete mix
- Lightweight concrete can reduce the dead load of the structure. This concrete has ranges of 20% – 80 % less mass than the normal concrete.
- They contain numerous voids and thus these decrease the thermal conductivity and increase the sound insulation.
- Precast Lightweight concrete blocks and slabs are very easy to handle, transport, and place.
- They have a variable range of strength, which can be useful for the nonstructural component of masonry units to the load-supporting structural units.
- They consume less aggregate.
- They don’t show the problems associated with freezing and thawing due to numerous pores in cement material
- They are comparatively more fire-resistant as they have a lesser tendency to spall. Also, concrete strength is less sensitive to the rise in temperature.
- The sound absorption is also more due to the presence of microspores. Thus they are useful for the auditorium, halls for good sound effects.
- The internal curing is possible in such concrete leading lesser cracks due to heat of hydration.
Disadvantages of Lightweight concrete mix
- These concrete are extremely hard to maintain, supervise for the proper placement and workmanship. The smallest negligence can cause great decrease in quality.
- Contains many voids and pore so they don’t give higher density and strength as that of normal concrete for same volume and cement use.
- Due to less density, they are not use for foundation purpose.
- No proper compaction and vibration of the concrete mass leads to the honeycomb in the concrete structure.
- The porous aggregate might absorb more water causing frequent change in the water to be added in mix.
- These concrete have high absorption rate especially, no fines used concrete and aerated concrete.
- No fines used concrete are not usually preferred with reinforcement bars as there is a high probability of rusting and the improper bonding within the concrete.
I hope this article on “Lightweight concrete mix” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha