Tensile behaviour of UHPC (III). Into FRC tensile response

UHPC beam complete failure RDC
In the third post of “Tensile behaviour of UHPC” we start talking about UHPC..., finally :) Don't miss the two previous posts dealing with tensile properties of plain concrete and some important properties of it, such as scale-effect and brittleness. In this post we talk about (i) the reason underlying the use of fibres in concrete; (ii) the mechanical response of fibre-reinforced concretes and how it differs from plain concrete; and (iii) we show a typical constitutive behaviour of UHPC. Enjoy it !!
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What we have to know about fibres in UHPC

Fibres in UHPC
Fibres may be considered the most important ingredient in the recipe for UHPC due economical, ductility and durability reasons. ECONOMICAL. An analysis of the raw materials required to produced UHPC shows that approximately 60-80% of the total price is due to fibres in a common UHPC with 2% in volume of steel fibres. This ratio can be much higher if either stainless steel fibres or larger fibre amount is used. DUCTILITY. On the other hand, it is unsafe to design UHPC structures without fibres since a minimum material ductility is required !! Fibres provide that ductility when using properly. DURABILITY. Besides, fibres help control cracking, which also contributes to increase the lifespan of structures. That is why is completely necessary to know some key points about its use. ¿Interested?
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Tensile behaviour of UHPC (II). Softening behaviour and scale-effect in plain concrete

In the first post of "Tensile behaviour of UHPC" we defined the uniaxial tensile behaviour of plain concrete using both a stress-strain relationship up to maximum load and a stress-crack opening relationship from this point onwards up to a maximum crack width. We explained that one of the main consequences of the ability of concrete to transfer a "stitched force" between the two planes of a crack according to the distance between them (crack opening) is the fact that strength depends on specimen size, what is commonly called scale-effect. It could have been a minor issue for science if not for the fact that linear elastic theory couldn't predict that. In this post we are going to explain (i) the concept of equivalent flexural strength; (ii) the nature of the scale-effect phenomenom, (iii) the way we can easily consider the scale-effect by correcting softening stress-strain behaviour according to structure depth and (iv) some notions about brittleness. Warning 1: We strongly recommend to read post (I) before digging into this one !! Warning 2: It may be kind of tedious to read if you don’t have basic knowledge on concrete design !! Let's get started !!
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Multi-microcracking in UHPC: real case application

One of the key features of UHPC is its ability to create multiple microcracks closely spaced to each other under certain conditions. This is the so-called multi-microcracking process that characterises strain-hardening materials, i.e. those materials that are able to hold a constant stress throughout a crack even at high strain values. However, one may consider that multi-microcracking is not a property of UHPC, i.e. a material property, but a property of the structure made with it. It means that even though we have obtained a strain-hardening behaviour in lab conditions using standard specimen and tests methodologies, it is possible that our designed structure behaves as a strain-softening one. Why is that? The answer is found in the fact that development of strain-hardening process in a structure mainly depends on (i) the nature of forces acting on it and (ii) fibre orientation inside the element which may differ a lot depending on element thickness and pouring system. Therefore, even though UHPC shows a strain-hardening behaviour under standard lab conditions, we need to know in which situations we can consider it or not in design. In this post we show a structural application in which consideration of strain-hardening behaviour is not only possible but a also a must to predict its real behaviour. Do you want more info? Keep reading :)
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Tensile behaviour of UHPC (I). Digging into plain concrete tensile response

High performance in tension is probably the most important feature of UHPC, and fibres are responsible for that. They play a major role in ductility, crack control, durability and strength.  As they also represent 60-80% of UHPC cost, it is completely necessary to understand how they work, i.e. tensile response of UHPC. Its knowledge and suitable consideration in design is a must to achieve safe and competitive designs. With this purpose we start a series of posts dealing with the tensile performance of UHPC. I always like to start with a little bit of background. UHPC tensile behaviour can't be fully understood without a previous knowledge of the tensile behaviour of plain concrete. Let's start with it and with some interesting notions you may not know about its response and scale - effect. Warning: It may be kind of tedious to read if you don't have basic knowledge on concrete design !!
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Having your own UHPC

UHPC mixing
One of the main barriers for an increasingly use of UHPC in concrete precast industry is the difficulty of finding a realiable UHPC mixture at an affordable price. If you want to start producing UHPC elements you have two choices: (i) ask specialised companies, commonly cement companies, to provide you with some of their already developed premixes, or (ii) develop one of your own. If you have already asked these specialised companies for a high-quality and reliable UHPC supply and you think what they offer to you is far from making your products competitive (which may be the most likely scenario) and you are afraid you can't do it on your own, in this post you can find some basics you need to follow. Why not give it a try?
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What is UHPC? – Part 2: Requirements

A lot of types of UHPC have already been developed worldwide with different mixture compositions, type and amount of fibres, flowability, compressive strength, tensile behaviour, thermal curing, etc. Several standards and recommendations (France, Switzerland) propose their own definition of UHPC. In this post a definition of UHPC is given considering that the most important feature for its classification is not only the compressive strength but algso its ductility and durability properties. Are you interested?
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What is UHPC? – Part 1: Birth of UHPC

SF particle in UHPC
The term UHPC stands for Ultra-High-Performance Fibre-Reinforced Concrete. It did not appear from nowhere, but from decades of evolution on concrete technology and the hard and thorough work of both international scientific community and private companies. This term encompasses several properties and technologies which are hidden beyond the literal meaning of each word, requiring a further explanation based on concrete technology history. This post tries to offer a broad perspective of the continuous process that ended up in the birth of UHPC and a definition of what this term really stands for. The way and the reasons why conventional concrete evolved into UHPC are the first step we need to take to start understanding UHPC. Are you ready for it?
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