Concrete And Reinforcement Properties - An Introduction

Episode #2 of the Reinforced Concrete Series

Hello and happy Wednesday, 👋👋

Today, we’ll quickly introduce the main concrete and reinforcement properties we use to design reinforced concrete beams like the compression strength of concrete, partial safety factors and tensile strength of reinforcement.

These material properties are the fundamentals of structural design.

In this article, we’ll explain what properties are important, where we find them and how to calculate them (if needed).

So, let’s get into it.

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The 5 Most Important Concrete Properties

Material properties of concrete and reinforcement are really important for structural design because they define the resistance of the material like compression or tension resistance.

The resistance or strength properties of concrete are defined in EN 1992-1-1 Table 3.1. We’ll summarize it for you below.

In general, concrete has different strength classes, which are categorized by their compression strength. For example, a concrete with the compressive strength of 30 MPa is named a C30 concrete.

Let’s look at the 5 most used concrete properties.

#1 Partial safety factor γ_c

The value of the partial safety factor can be found in EN 1992-1-1 Table 2.1N. For ULS load combinations, this value is defined as:

\(\gamma_c = 1.5\)

And for accidental loads as:

\(\gamma_{c.a} = 1.2\)

Note that these values are usually defined differently in the National Annex. Make sure to check it.

The partial safety factor is used to calculate the design resistance from the characteristic resistance/strength values, which we’ll define in the next sections.

In general (for all materials) we calculate the design resistance with EN 1990 (6.6c):

\(R_d = R_k / \gamma_M\)

With R_k as the characteristic resistance and γ_M the partial safety factor. In our case, that would be γ_c.

#2 Compressive strength f_ck

f_ck is the characteristic compressive cylinder strength of concrete at 28 days. The values are determined with tests and statistical methods.

The values of f_ck are defined in EN 1992-1-1 Table 3.1. As said earlier, the strength class is named after the characteristic compressive strength. A C25 concrete has a compressive strength of 25 MPa.

Here’s an overview of the values of f_ck of the different strength classes.

The compressive strength is used in almost every structural design verification of reinforced concrete. Here are a few examples:

• Compression verification

• Bending verification

• Shear verification

#3 Tensile strength f_ctm

f_ctm is the mean value of the axial tensile strength of concrete.

We’ll find the values of f_ctm in EN 1992-1-1 Table 3.1. Here’s an overview of the different strength classes.

The tensile strength is also used in many structural design verifications. Here are a few examples:

• Minimum reinforcement requirement

• Bending verification

• Shear verification at the interface between 2 concrete casts at different times

#4 E-modulus E_cm

E_cm is the secant modulus of elasticity of concrete.

The E-modulus E_cm is calculated as:

With,

• f_cm as the mean value of the concrete cylinder compressive strength. The value for f_cm can also be found in EN 1992-1-1 Table 3.1.

Or, we’ll find the values of E_cm in EN 1992-1-1 Table 3.1. Here’s an overview of the different strength classes.

The E-modulus is used in the crack width calculation of rc beams and slabs.

#5 Concrete cover

The concrete cover is the closest distance between the edge of the concrete element and the reinforcement.

The cover is used to protect the reinforcement from external influences, which could damage the reinforcement, like water or fire.

Where,

• c_min = minimum cover according to EN 1992-1-1 4.4.1.2

• Δc_dev = allowance in design for deviation according to 4.4.1.3

Now, we won't dive deep into how to calculate c_min and define Δc_dev, because it's an article itself. Most countries have the minimum values defined in textbooks for the different reinforced concrete elements, concrete types and environmental conditions.

These were the 5 most important concrete properties. Of course these were not all, but the main ones we’ll be using in the next weeks.

Now, let’s move on to the reinforcement properties..

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The 3 Most Important Reinforcement Properties

The second material that is part of reinforced concrete is reinforcing steel or reinforcement.

Let’s check out the 3 main properties of reinforcement, where they are defined and in what verifications they are used.

#1 Partial safety factor γ_s

The value of the partial safety factor can be found in EN 1992-1-1 Table 2.1N. For ULS load combinations, this value is defined as:

\(\gamma_s = 1.15\)

And for accidental loads as:

\(\gamma_{s.a} = 1.0\)

#2 Tensile strength f_y (=yield strength)

f_y.k is the characteristic yield strength of reinforcement.

The 2 most common characteristic yield strengths are either 500 MPa or 550 MPa.

The tensile strength is used in the following structural design verifications:

• Minimum reinforcement requirement

• Cracking verification

• Bending verification

• Shear verification

• etc.

#3 E-modulus E_s

E_s is the modulus of elasticity of the reinforcing steel.

The E-modulus E_s of reinforcement is defined as:

The tensile strength is used in the following structural design verifications:

• Calculation of the neutral axis of a cracked rc section

• Column design

• Cracking verification

• etc.

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Final Words

That’s it - Quick article today.

As I said earlier, these 5 concrete and 3 reinforcement properties are not the only ones, but the main ones we use in most design verifications. We don’t want to overcomplicate things in the beginning.

I’ll see you next Wednesday for part 3 of the reinforced concrete series. ✌️✌️

Cheers,

Laurin.

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What is a beam? [Video tutorial if you need to catch up the basics]

Now, before starting to design reinforced concrete elements, we need to be sure we know the basics of structural engineering such as static systems, loads, cross-section properties.

Beams are the most used element in structural engineering.

Here’s a tutorial on what beams are and how they work, in case you need to catch up.

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