Concrete - The Basics

Sat, Jan 19, 2019 7-minute read

Concrete is one of the most commonly used building materials in the world. It is easy to source (material wise), gains strength quickly and can be quick for building purposes. It has exceptionally good properties and from a design point of view, it is very flexible.

On top of that, it is also a composite material that consists of three different parts. Aggregate (coarse to medium grained material). Cement of which acts as a binding material, lime-based is out and out the most common type encountered but others do exist and water. All this together makes a fluid slurry which hardens over time.

The major advantage and without a shadow of a doubt the best property of concrete is its compressive strength. This is also known as f,ck and is the strength of the material at 28 days.

Before jumping into how we can find the grade of concrete, governed by the strength, let’s look at the common notation and how you will see the values in tables.

Concrete class showing the compressive strength of different mixes.

Concrete Grades

So from the table, let’s pick on C40/50.

Concrete Testing Procedure - How Do We Know the Strength of Concrete?

The cube strength is determined in a laboratory. In the United Kingdom and Europe, we take a 150mm x 150mm cube of concrete for the test sample. After 7 days and 28 days we apply an incremental load until it fails. From the load at failure we can determine the maximum stress of the concrete and therefore the stress. The equation is given below.

Fcu = 0.8 x Fc,k

The relationship between the cylinder and cube strength is between 0.8 and 0.83, both are commonly adopted.

Concrete Grades and Their Uses

We know about the strengths and grades of concrete and how they are determined. Why do different grades exist? Where should we be looking to use this grades? Let’s break the different strengths down and see where we may find them used. This only serves as a rule of thumb, the list is not exhaustive and every designer has preferences. All of the below will be stated using the cylinder strength.

C10 and C20 concrete is used for non structural work, for example, patios, walkways and paths.

C20 can be used for domestic floors and lightweight foundations. Lightweight foundations would typically mean a small domestic extension.

C25 is when the strength becomes more useful for structural applications. It is used for general purpose construction such as foundations.

C30 concrete is a more weather resistant and durable grade of concrete.Suitable for heavy road traffic.

C35 is one of the most frequency used mix for structural applications. Floor slabs, composite floors, heavy duty foundations such as piles and load bearing / retaining walls. Check out posts relating to load bearing walls and retaining walls.

C40 and up for the heavy structural applications and also has the added benefit of being able to resist chemical corrosion. Due to the additional benefit it serves well in water / septic tanks and piers. It is also used in precast beams and foundations that have high loads (think large multi storey buildings).

Advantages of Concrete

Going to keep the advantages and disadvantages a short paragraph. Check out the post about [insert url] construction materials for an overview of the types of materials and where concrete is king. So without further ado..

Concrete has excellent compressive strength and material properties. It is also interesting to note that concrete keeps gaining strength over time.

From a design point of view, it is a very flexible material to work with. For example, architects can create various different shapes using formwork and match the specification and desires of the client.

Its economical and environmentally friendly. Cheap to use and also a material that we can recycle. Adding into the economy side of things, concrete requires very little maintenance during its lifespan saving money for the client.

Concrete is one of the better materials to resist environmental factors such as chemical corrosion. Of course for this, higher grades of concrete would need to be used as already discussed.

Disadvantages of Concrete

Concrete has a lot of pros but also some quite big drawbacks.

The tensile strength of concrete is very poor, typically around a tenth of the compressive strength. This is easy to overcome though but we will jump into that soon.

The cost of the formwork and casting can be expensive and demanding on a construction site in terms of cost and more importantly time. The only difficulty on site is quality control, a lot of care has to be taken to ensure the mix done correctly or that the transport vehicles ferrying the concrete arrive in the proper schedule. Random occurrences such as traffic can actually cause a concrete to harden and be unworkable. Good planning needed!

Concrete is a heavy material when looked at in direct comparison to steel, especially in high-rise/multi storey buildings.

One final one, albeit one seldom occurs often. If concrete is heated to extremes, it can explode!

So, I hope I haven’t put you off using concrete. Overall when it used for the right projects it is a great material. Next we are going to look at how to combat some of the disadvantages and some interesting research and development.

What is Concrete Reinforcement?

Joseph Monier, a French gardener in the nineteenth century was the pioneer behind using reinforcement in concrete structures. He starting by using a wire mesh in a mortar shell for his plant pots and this evolved into larger meshes used for columns and girders. He later sold the patent to a German civil engineer, G. A. Wayss.

So history lesson aside, why are we talking about reinforcement in concrete? Reinforcement helps the concrete by taking the tensile force in the element, this therefore lets concrete take all the compression force, something we all know concrete does best.

Types of Concrete Reinforcement

Originally the reinforcement found in the nineteenth century was iron. Due to corrosion problems of iron, steel was adopted and is still the most common type material used in reinforcement.

Steel reinforcement comes in set sizes, they vary in bar diameters up to 40mm and can also come in a mesh arrangement defined by the bar size and bar centres. If you are new to studying reinforcement starting with steel would be advised. A lot of information and guidance can be found from good quality sources such Eurocodes, British Standards and AISC just to name a few.

Steel reinforcement mesh used in reinforced concrete structures.

Steel Mesh

Another type of reinforcement gaining in popular is fibre reinforcement. Steel or micro synthetic (plastic) fibres are added to the concrete. This type of concrete provides advantages such as greater lifespan, improved durability, reduced permeability, impact resistance and enhanced surface finishes.

Fibre reinforcement used in concrete.

Fibre Reinforcement

Prestressed Concrete

Prestressed concrete was invented years before it came a commonly accepted building material. The aftermath of the world wars left a huge storage of steel globally, therefore we needed to optimise the use of the steel used for the reinforcement.

Prestressed concrete is a process complete by pre-tensioning or post-tensioning steel rods/cables. This provides a large load that produces a compressive force in the concrete. Concrete is naturally great in compression, this force balances the tension in the member thus utilising the concretes properties fully.

Opening in a house, load bearing wall is supported by a lintol.

Concrete Lintol For An Opening In a Load Bearing Wall

A section of a prestressed lintol.

Section A-A - Prestressed Concrete Lintol

Applications for Concrete

Concrete is used everywhere, all we have to do is look around. From beams, columns, floors, foundations and retaining structures. It can be used in harsh environments such as piers and water tanks as well as underground.

Prestressed concrete is commonly found in bridges but also in domestic properties for lintols. This allows for smaller section sizes to be used allowing for larger openings.

Interesting Research

Research is being done to make concrete even more useful and combat some of the drawbacks.

A video published from the ‘Techinsider’, ‘Kickstarter’ and ‘Businessinsider’ shows a type of concrete called Topmix Permeable Concrete. Also known as thirsty concrete. This amazing concrete can absorb 880 gallons of water per minute. It is made by using a very fine granite aggregate as opposed to using sand allowing water to flow through it much easier. It is also stated that different levels of infiltration are possible. Pretty cool right. Check out the video below.

Thirsty Concrete

Just a little note, this research originally, as far as I’m aware, came to light in 20015 and again in 2016. I’ve been keeping a close eye out for any further developments but sadly nothing has surfaced. Rest assured though, if I do here anything, I will publish the finders on here! If you know of any developments please get in touch in the comments!