ReSHEALience Project started with the purpose of increasing service life by at least 30% and decreasing maintenance costs by at least 50% of those structures exposed to aggressive environmental conditions, such as chemical attacks or chloride induced corrosion, throghout a thorough research and innovation 4-years project focused on four main areas: (i) improvement of material durability properties; (ii) improvement of design criteria in service life conditions and prediction of structure lifespan; (iii) full scale monitored prototypes to check technical feasibility; and (iv) evaluation of business opportunities to check economical, social and environmental feasibility. We want longer-lasting structures with lower maintenance costs !! We believe in concrete as the best choice !! And we want them to be sustainable and competitive !! Does it a lot to ask?
Achieving such ambitious goals will require the joint effort, commitment and determination of 14 different partners (companies, universities and research institutions) for the development and realisation of what we have called: Ultra High Durability Concrete (UHDC). It is not just the development of an hiper-ultra-mega-super concrete, but a lot more than that. So, what does this new concept mean?
First time I heard the term Ultra-High-Durability concept in one of the project proposal drafts, I have to admit that first picutre in my mind was one of the Roman bridges still standing. Look at the Alcántara Roman bridge, built in the second century. A beautiful, functional and durable structure, a survivor of nature and human aggressions. Despite it was reconstructed in the sixteenth century due to wars and its old age, could you think in a better definition of whatever may mean the Ultra-High-Durability concept applied to a structure?
Let’s come back to the present. Reproducing this type of structures in our time is something pointless and even impossible. Nowadays engineers have to face tons of imposed restrictions concerning economy, natural and human resources, environment, sustainability, security of both the structure in service and the whole construction process, functionality, timing, … So, how the Ultra High Durability concept can be applied to our days?
The challenge of current civil engineering is to find a proper balance: increasing lifespan of structures as much as resources and technology available allowed and, at the same time, and at the same time, keeping the overall costs (construction, explotation and recycling) both affordable and sustainable to our society.
And we can say that we (mankind) have achieved that in a broad variety of structures, specially those placed in non aggressive atmospheres. However, when it comes to structures subjected to severe environments (sea, open waters or acid environments) it seems that there is still a long way to go. Many are the examples in which structures failed in such aggressive conditions. They do not collapsed (generally) but they loss their functionality or require higher maintenance efforts than initially expected. Yet there are a few others (increasingly more number of them) in which engineering prevails over nature (so far 😉 ). The next two pictures show two different structures made of Ultra High Performance Concrete (UHPC) in Spain designed by RDC. The first one shows a UHPC pedestrian bridge very close to the Mediterranean Sea; the second one a raft for mussel cultivation in the Galician Rías (Atlantic Ocean). Time will tell if they really fits the idea of Ultra High Durability structures.
ReSHEAlience project is focused on those specific structures and applications, relying on concrete as the most suitable material to overcome all detected problems under severe environments. Not a conventional one, but a concrete that improves what has been done so far by including the most recent affordable technologies, new functionalities and adapted design criteria. That is the concept of Ultra-High-Durability Concrete (UHDC). This the overall goal pursued by this project.
It can be said that concrete, as we currently know it, appeared at the end of the 19th century. Combination of concrete with steel rebars (what we call reinforced concrete) rapidly became the construction material par excellence in civil engineering. First investigations on concrete were carried out to ensure the strength of structures. As it was a new material for construction with high expectations for construction industry, emerging cement industry couldn’t afford the risk of messing up with it. It had to be launched as soon as possible. Mistrust of concrete structures was not a choice.
In this context, do you think someone was worried about the state of these first concrete structures fifty years later? Probably not at the beginning. But with time, design concrete codes evolved and started including some clauses to prevent quick degradation of concrete. What was (still is) the main problem of concrete? Cracking. So, first durability clauses were focused on minimising cracks width in concrete under service load conditions.
It was found out in the first half of the 20th century that using rebars with large diameters was the main cause of large cracks in their concrete. But, why is a large crack bad in concrete? It must be always kept in mind that in most concrete structures the steel rebars are the main responsible of their strength. Concrete is just a protrective coating which prevents oxidation of rebars. When oxidation process in steel rebars is triggered due to lack of protection from concrete, steel may loss a significant bearing capacity, and structure becomes weaker. Cracking may leave steel rebars exposed to the environment and potentially favours steel rebars oxidation. That is why around 1950s, design concrete codes started limiting rebars diameters.
Later on, around 1980s minimum distance from rebars to the outer face of the concrete element was also limited according to environment exposure, to avoid induced corrosion of steel rebars due to carbonation process of concrete which is due to its porous nature. When carbonation front in concrete reaches steel rebars, pH value is reduced which favours, again, oxidation of steel rebars.
In the 1990s, formulation to determine crack width was developed. Several durability limitations appeared in the new codes accoding also to new and more complete definition of possible environment exposures. Some of those limitations were (i) crack width, (ii) amount of cement, (iii) water to cement ratio, (iv) compressive strength of concrete or (v) maximum stresses reached by steel rebars. These are the codes and limitations we currently use.
Following the evolution path of durability in concrete it can be said that, in terms of durability of concrete structures, engineers and scientists have always been fighting against two specific properties of concrete: (i) its porous nature which allow the entrance of undesirable agents; and (ii) cracking of concrete which actually becomes an open door to them.
Now we can understand why new concrete technologies attempt to:
- Increase compacity of concrete reducing capillary porosity
- Reduce crack spacing so that crack width can be lower under service loads to keep the “door” as close as possible
- Confer the ability to concrete to seal, or even heal, small cracks to “close the door” to undesirable agents
The term Ultra-High-Durability Concrete refers not to a specific type of concrete but to the whole structure made of it. Why is that? As I live in Valencia, allow me the following analogy 🙂 You can have the best ingredients to cook a paella, but making a good one it is not only a matter of ingredients, but following a good recipe thoroughly, having a good control of the fire and the wood use for it, and adding an extra of love to it 😉
Same thing can be applied to Ultra-High-Durability Concrete. Firstly, we need a concrete with the right mechanical and durability properties (ingredients). Then we need to devise the structure according to suitable design rules (recipe). External conditions (fire) as environment, climate and soil must be properly determined and considered. Finally, a good qulality control (love) must be guaranteed throughout the whole construction process . If the result of the whole process leads to an economic and viable structure that greatly increases its lifetime (at least 30%) with a significant reduction of the maintenance cost (at least 50%) if compared to conventional solutions, then we can talk about Ultra High Durability Concrete.
That said, we are going to use the term UHDC to talk about the concrete itself from now on. Having a UHDC is a necessary but not enough condition to have a UHD structure. Let’s focus now on how to get it. There will be time to learn how to use it as we move forward on the project.
The best way to increase compacity in concerte is by means (i) a special selection of materials, with special care to the grading curve and (ii) the use of suitable water reducer chemical agents to keep low water to cement ratios. This may result in a significant increase of compressive strength of concrete. However, there is a very important condition to be accomplished: economic viability. Resulting UHDC must have a competitive raw material cost. Making a expensive UHDC is worthless and has no merit at all, as we could design nothing with it.
Experience gained on these type of materials over the last twenty years shows that a suitable balance between material quality, raw material cost and future improvements on the design phase may be found with concretes with a characteristic compressive strength of around 100-140 MPa. ReSHEAlience project will be focused on these concretes.
Reduce crack spacing
A significant reduction of crack spacing in advanced concretes is directly related to the use of a minimum amount of fibres to ensure a minimum structural strain-hardening behaviour. In simple terms, we can say that a concrete achieves a strain-hardening behaviour in a structure if once tensile strength is reached at most tensioned layer of concrete, it has the ability of forming a closely spaced cracks pattern in which the crack width remains lower than 0.1 mm up to a strain level of 0.25% (upper limit for service limit state design), with also a minimum loss of stiffness, making that most inelastic strain produced during loading is recovered once the structure is unloaded.
This structural behavior depends on several factors such as (i) amount of fibres; (ii) fibre orientation inside concrete and its variability; (iii) type of fibre (slenderness, length, diamater, material, …); (iv) bond between matrix and fibres which may be related to compacity, compressive strength and/or the presence of specific chemical agents.
Among all parameters above described, only fibre orientation and its variability may be considered a structural parameter. A suitable distribution of fibres must be ensured during the design and construction phase and has less to do with material itself. Knowing that, it has been proposed to study the rest of them at material level (using tests at lab scale). Available fibres and chemical agents will be tested to obtain what we have called a strain-hardening material (at material level) in which a minimum average strain at the end of the hardening phase must be reached. Both test setup and minimum hardening values required are yet to be agreed.
Science has proved that concretes with high cement to volume ratios combined (or not) with self-healing chemical agents can heal (or at least seal) those cracks in concrete with an opening lower than 0.1 mm. The influence of ambient humidity and nature of actions responsible for these small cracks (quasi-permanent or frequent) in the healing process needs to be properly evaluated and quantified. Accordingly, we will be able to consider this phenomenon on the design phase.
The result of this optimisation process must lead to a UHDC material which still has two more tests to pass:
- Increase 100% durability properties in uncracked state
- Increase 30% durability properties in service conditions (cracked state)
All of that in comparison to a conventional high strength concrete commonly used in civil engineering of about 60 to 70 MPa.
We will keep you informed of our advances in the project !! Regards from the UHDC team !! And don’t forget to visit our website about Ultra High Durability Concrete !!