Foam Concrete properties.
This page describes the properties of Foam Concrete (FC) that I found most relevant for understanding the issues that have an influence on the strength and other aspects of FC with a density between 400 and 1800 Kg m3.
See also: Making Foamed Concrete Stronger.
This explains what to do to make a FC that can be used for constructions
All the research, trial and error, and sensational failures add up to understanding how things work. Trying to know what can be done with FC is of great interest to many. I have not been able to count the universities and other organisations that are doing R&D in FC yet, but there are many. However it seems to me that everyone who comes up with an excellent idea to make “strong” FC is keeping it a secret in the hope to make money from it. That seems fair enough, but there is a point where “open source” information is more valuable for mankind. Fast dispersion of knowledge is now possible, so let’s use it. At the same time I want to support people and organisations to create that exceptional “GOOD” FC.
New R&D usually builds on existing knowledge; I hope that the explanations of the FC properties help you to understand the basics before you go into the deeper realms of the FC world.
I have made reference and links to articles relating to the property discussed.
Most of these properties are comparable with “normal” concrete. For more detailed knowledge on “normal” concrete, there is a 3500 page book (sorry, lost the link) on the subject, and many others dealing with a specific subject. Just shows you how much there is to know.
The properties of cured FC are:
- Compression strength
- Tensile strength
- Insulation value / Thermal conductivity / Sound insulation
- Thermal mass
- Fire resistance
Density is relates to the mass, the weight per volume, here we use kg m3.
The density of Foam Concrete depends on how much foam we put in the mortar and the make-up of the mortar.
Lightweight concrete is made with aggregates that have a much lower mass than the usual stones. Some of them are: Pumice, expanded clay, and vermiculite, but there are many more.
Relationship between properties
There is a direct relationship between density, compression strength, tensile strength, permeability and thermal properties. This relationship is by no means linear, but always related to the composition of the FC.
Adding small amounts of admixtures, fibers and what else some people think of can have a drastic influence on the properties.
Compression strength: Mpa
This is the most used indicator of the FC and concrete quality. It indicates the resistance break under pressure. Mpa Is a measure of the compression strength of concrete. One Mpa is equal to one million Pascals (Pa); as a Pascal is one newton of force per square meter, a Mega-pascal is one million Newtons per square meter. The higher the Mpa of concrete, the more resistance to compression the material will be.
The English unit equivalent to Mpa is pounds per square inch (psi). While the U.S., U.K. and a few other countries use psi, the majority of countries rely on Mpa. Many manufacturers, builders and suppliers provide product data showing both Mpa and psi ratings for concrete materials. To convert psi to Mpa, multiply the psi value by 0.0068915. For example, 2,500 psi = 2,500 x 0.0068915, or 18 Mpa.
The Mpa of new concrete is determined using a cylinder-testing process.
For FC purposes a test sample can be poured into a cylinder. Testing can be done as soon as it is set, after 7 days, or 28 days, or much later, depending what you want to know. For how to test FC go to Testing FC page to come soon)
Tensile strength or Tension strength.
I have not found an explanation why two different words are used that seem to mean the same in concrete. Can someone give me a simple explanation?
There is a direct relationship between the Mpa and the tensile strength this is about 10% in most mixes.
This is the opposite force to compression, pull rather than push. FC is normally not designed to resist direct tension, the knowledge of tensile strength is used to estimate the load under which cracking will develop. This is due to its influence on the formation of cracks and its propagation to the tension side of the structure. Shear, torsion and other actions also produce tensile stresses to the particular section of concrete structure. In most cases the structures behavior changes upon cracking. So tension strength of FC is considered in designing the structure. This strength is of interest in designing structures as shear strength and resistance to cracking are very important to sustain loading. The tensile strength of FC is relatively low, about 10 to 15% of the compression, occasionally 20%.
Another way to express the tensile strength is the Modules of rupture. That means the force measured when a rupture was visible.
This is by far the most important property of the FC, and one of the reasons it is such a suitable material to build houses. Thermal conductivity is the “opposite” of Insulation value
The thermal conductivity of FC with 1000 kg/m3 density is one-sixth the value of normal concrete. The density of FC plays an important role in determining its thermal conductivity. A reduction in FC density by 100 kg/m3 results in a lessening of its thermal conductivity by 0.04 W/mK.
It is also possible to reduce the thermal conductivity of FC by using Fly ash. A reduction in thermal conductivity by 12-38% was attained with the introduction of 30% Fly ash in the mix compared to the FC with only Portland cement. This was attributed to the lower density of fly ash particles.
Thermal conductivity is expressed in a couple of ways for building materials, thermal conductivity or transmittance called the U- value and insulation value or also called thermal resistance, expressed in R- value.
The R and U value depends on the thickness of the product.
The R value thus depends on the density, the thickness of the block or wall, and the porous structure. The R value is usually measured of an “oven dry” product, as the water content has also a high influence on the R value.
The amount if insulation you need depends on the climate the building is in. For example in a “moderate” climate the R value of 3 to 4 is recommended.
For FC it means that the better you want your house walls, roof, and floor insulated, the thicker the layer of FC needs to be.
The link below is a simple explanation of insulation properties. Written in 1983, but still relevant as physical property facts do not change over time.
Compared to “normal” concrete, FC absorbs more sound energy, this means there is less sound reflecting off the surface, and less sound transmitted trough the material. This is one of the great properties why it is such a good building material. The rate / value of these qualities depends mainly on the density of course.
I am looking for some data that can support and quantify this sound absorption quality.
This is the amount of energy in the form of heat a material can absorb. It is also only a part of the story as the speed of absorption is also important. These two factors influence the “living comfort” of a house.
The thermal mass of normal concrete is high, and that of FC is low depending on its density.
The speed of absorption is important because it determine the comfort of a house when the outside temperature changes. What the best rate is for a house depends on the climate. The most comfortable rate for us humans is usually that it stays stable over a 24 hour period.
This is an indicator how much it can “move” / deform before it breaks. It is also a measure of elasticity. In our DIY world this is a factor that is difficult to measure, because it needs electron microscopy to detect when the damage starts.
Comparing FC to “normal” concrete
A study on an FC beam reinforced with steel and a density of around 1750kg m3 found that the sustained ultimate load was lower, deflects more, and showed less displacement. However the test beam exceeded the design capacity. The total conclusion was that it can withstand large displacements in an earthquake.
This is a very important characteristic as it determines how we can stay dry and comfortable in the house. We will include the water absorption in this chapter as well, but there is a difference. The definition of permeability is a measure of the ability of a material to transmit water (fluids).
Water absorption is the ability of FC to absorb water, often measure how long it stays afloat. This mainly depends on the pore structure connected to the outside, and the amount of micro cracks.
The water permeability of the FC based on the of flow of fluid through a porous material, and measured by the amount of water flows through the material per m2 over time and under a certain pressure.
A study found that the lower the density of the FC the higher the permeability. This is because the air bubbles do not form a barrier for the water to flow through.
The water cement ration has also an influence on the permeability; lower cement ratio has a lower permeability, but also a lower Mpa in some tests.
For us FC users it is good enough to know that FC has good resistance to water flow.
As most surfaces of FC probable will be painted this is not an very important issue in most climates. The durability of FC is also affected by the permeability, in particular in the freeze-thaw climate, and the possible ingress of chemicals that can attack the steel re-enforcing.
There are chemicals that you can apply to the surface that make concrete less permeable. I am intending to test this on FC. If you know about this let me know please.
Energy absorbing qualities
Due to the dense cell structure of FC, the material is compressed during an impact, the resistance of the foam concrete increases absorbing the kinetic energy. FC forms a solid matrix; the material is not so vulnerable to seismic shock waves, thus ideal for building structures in areas with seismic activity.
This also gives reason for another use, as it is highly energy absorbent it also stops flying objects in the material very well. This can be a reason to build with FC in a combat zone to stop bullets etc. It is also used in military training facilities to capture bullets. These mixes usually have also a high content of polymer fibers.
Below is a excerpt from a letter send to my by A. Mandemaker. He has been involved in many Foam Concrete projects.
In 1996 I started a housing project in Cavite, the Philippines. Due to a dispute about the land titles, the project was stopped. From about 100 houses all walls (foam concrete 1,400 density) were up, roofs were installed and some walls were painted. My friend in Manila visited the project last year (2017) and inspected the houses. All walls, also the unpainted walls, were in excellent condition without any visual defects: no cracks, no damage at all. Please note that these houses are about 5 km from the sea and were subjected to earthquakes, torrential rains and typhoons. The walls have been reinforced with light mild steel and polypropylene fibres (0.9kg/m3).
Durability is no issue with properly made foam concrete walls. Check out the website below
More on durability to follow when it comes on hand
High resistance to breakdown in a fire is a desirable feature of any building material. FC is not combustible!
FC is highly fire resistant, a load bearing wall, 15cm thick, had a fire resistance exceeding 7 hours in a test. Unfortunately it did not state the density, but it was a load bearing wall thus we can assume probably somewhere around the 1200 to 1500 Kg m3.
Because FC has also a high thermal insulation rate, depending on the density, the temperature on the other side of the wall will stay below the flash point for longer.
FC does NOT burn, check this out
https://www.airium.com Go to the video clip at the bottom of the pace in the center.
While you on this site look at the claims they make about FC as insulation material.
The link below is a short and funny presentation on the properties of FC.
Testing the properties
If you want to know how concrete is tested the following link is to an article that discribes these. I is also an interesting method to create lightweight concrete. Go to 3rd article dow on the page.