The lecture begins with a discussion of the definition of high-strength concrete, followed by a discussion of its design considerations. New design concepts for high performance concrete are discussed in detail, including optimizing particle composition, optimizing paste content to provide adequate compatibility, and optimizing paste composition for optimum flow.
Professor: Manu Santhanam
Indian Institute of Technology Madras Department of Civil Engineering
Lecture – 53 Specialty Concrete – High Strength Concrete
The following is the discussion of the lecture:
Definition, Design and Particle Buildup Concepts Okay, good morning everyone.
Today we will start discussing about special concrete.
We will start with high strength concrete.
First, we will define what high strength is.
What do we mean by high strength concrete?
What are some of the design considerations?
How do you successfully design high strength concrete?
As well as ultra-high strength concrete that reaches over 120 MPa.
We will then look at some of the properties of high strength concrete.
What do you need to keep in mind when designing for these properties?
Of course, the definition of high strength has evolved over time.
In the 1950’s, concrete with 5000 PSI or 34 MPa was considered high strength in the 1950’s according to the ACI 363 committee’s definition.
So, 34 MPa concrete is practically everyday concrete today, but was considered high strength in the 1950s.
Today, we routinely use 55 MPa and above as high strength concrete.
But even 55 MPa is already a low number in today’s concrete strength levels.
Today, at least in high-rise buildings, concrete strengths of at least 80 to 90 MPa are used.
For bridges, 45 to 60 MPa is more typical.
High-rise buildings usually have special requirements for concrete, which we will discuss later.
But in general, at least in India, the accepted definition is that concrete over 55 MPa is high strength.
In fact, there is a new code for high-rise buildings, IS 16700, developed about four years ago.
This was developed specifically for the concrete requirements of high-rise buildings, which are different from those of ordinary buildings.
Considering high-rise buildings, it is obvious that wind and seismic effects are major factors.
Therefore, at least in low-rise buildings, concrete needs to be very stiff.
So you have to design adequately.
It is not enough to rely on the code-recommended creep and shrinkage based on concrete strength; these properties need to be determined every time you design new concrete.
Ultra-high-performance or ultra-high-strength concrete is usually defined as a grade in excess of 120 MPa.
In research, grades up to 800 MPa have even been formulated.
In practice, however, one does not use concrete that exceeds 250 to 300 MPa.
But we will also look at how to achieve this level of strength.
Sometimes High Strength Concrete (HSC) and High Performance Concrete (HPC) are used interchangeably.
But they don’t necessarily mean the same thing.
High performance is not necessarily high strength.
High performance may be other properties, such as fluidity.
So when we have self-compacting concrete, it’s actually a high-performance concrete, but not necessarily a high-strength concrete.
So high strength and high performance are not necessarily the same thing.
But in the literature, you’ll see one referred to as the other because generally speaking, high strength involves a low water-cement ratio.
Typically, low water-cement ratios also lead to better durability.
So high performance can also come from high strength.
Obviously, we need to use a cement mixture with a high cement content.
We want to maximize the use of cementitious materials, not only to benefit from the hydration reaction, but also because of the build-up effect that comes from using this fine material.
When we use only cement for normal concrete proportioning, we are not taking full advantage of the smaller particle sizes.
That’s why when we started using fillers instead of aggregates, we tried to fill in the voids in the smaller particle areas.
When we replace cement with finer mineral additives, this also helps to fill the voids in the smaller particle sizes.
So in a nutshell, for high performance concrete or high strength concrete, we’re looking for blended cementitious materials with high cement content.
Low water-cement ratio, usually less than 0.35 is chosen for high strength concrete.
We have to use high efficiency water reducers because at this water-cement ratio, you can’t get workability without using high efficiency water reducers.
You can also optimize the particle skeleton through the particle stacking process.
The idea is to find a strategy that maximizes the buildup of solid components to minimize voids.
Because if you want to increase strength, you need to minimize voids.
So you have to develop a strategy to minimize the voids.
One of the examples we saw earlier was in the discussion of biomass ash where they were looking at applying compaction during the solidification process.
When you apply compaction during the solidification process, you’re obviously inhibiting the formation of voids, which overcomes the problem of void creation.
This is one way to increase the strength of the material.
We’ll also see in our discussion of ultra-high strength concrete that applying compaction during setting is often a strategy to get the best performance out of the concrete, even at strengths that are not actually achievable.
So the new design concept we need to apply combines three aspects.
Not just for high strength, but potentially for all high performance concrete, not just high strength.
We optimize the particle composition.
For example, when we have coarse and fine aggregates, we can also have some medium aggregates such as 20 mm, 12 mm and sand in three different sizes.
We choose the proportions or ratios to minimize voids.
That’s not hard to find, is it?
You can randomly choose the proportions of sand, 12 mm and 20 mm aggregates, mix them together, put them into a cylinder of known volume, and then try to measure the mass of the materials and convert the mass to volume because you know the density of these materials.
You subtract the total volume of the ingredients from the volume of the container to get the volume of the void.
You experiment until you reach the minimum void volume.
But all of this must come with a caveat, because obviously you can’t choose a combination of materials that has too many aggregates.
For example, in concrete, you wouldn’t choose a mixture with only 10% fine aggregate.
That’s not a decision you would make.
You wouldn’t choose a mixture with only 20% fine aggregate.
You would choose a mixture that has a lot of fine aggregate to avoid separation problems that can occur in concrete.
So your choices in terms of material mixes would not be unlimited.
When you perform an experimental evaluation of this mixture, you can get the lowest void content.
As a result, you can arrive at a lesser combination of ingredients to get the desired void content based on your understanding of concrete mix design.
Nonetheless, this must be an optimization process.
It is certainly a more complex process in which contours of equal stacking density are plotted.
I’ll discuss what stacking density is in a moment.
Then there is a single point at which you get the maximum stacking or the minimum void content of the mixture.
So what is the advantage of having minimum void content in a particle mix?
A particle mix does not only include aggregates.
It may also include fines from cement, mineral additives, etc.
Therefore, you are optimizing the sizes and proportions to minimize voids.
But what are the advantages of minimizing voids?
Minimizing voids gives better strength, there is no doubt about that.
But you also need to get maneuverability.
How do you get maneuverability?
You have an initial void content estimate, which means you need at least that much slurry or water to provide some workability.
But that may not be enough.
You may need to add water or slurry to the void content to really get the level of workability you need.
We’ll see this application later when we discuss self-compacting concrete, because having determined the void content, we can now calculate exactly how much slurry needs to be added to the system.
The paste content should therefore be just enough to provide the required workability.
Again, this stems from the fact that we want to maximize the aggregate in the concrete.
Why do we do this?
Because aggregates do not provide the strength of the concrete.
Remember this.
Strength is gained from the bonding ability of the cement paste.
It does not come from the aggregate.
What is the main contribution of aggregates to concrete?
Volume.
No, I mean you can fill the volume with anything, but why are aggregates important to concrete?
Can I make concrete with just cement paste?
No, you can’t.
Why?
I don’t get dimensional stability.
Aggregate is absolutely essential for dimensional stability.
You need more aggregate to get a more stable concrete, and in most cases, aggregate is inert relative to paste, so if you have a good mix of aggregates, you’ll get better durability.
But there is another important aspect.
Why do we maximize aggregates?
This is a very practical aspect.
Cost.
We want to minimize the cost of the concrete.
So more aggregate means lower cost.
So you want to maximize aggregate from multiple angles.
Now, when we’re going after higher grade concrete, you don’t have the luxury of having a lot of aggregate in the system because you need paste to get the buildup.
You can’t accomplish this just by using aggregate.
But in any case, the paste content should be just enough to provide adequate workability.
You don’t want to have too much slurry in the system unless you want flowability characteristics, which we’ll discuss later in Self-Compacting Concrete.
Now, the paste itself can be designed for optimum flow, which is why it’s important to understand the rheology of the paste.
That’s what you saw earlier, how we use rheological parameters to control the properties of the paste for certain performance parameters.
We talked about yield stress, viscosity, shear thinning, and shear thickening.
These properties are introduced by properly designing the cement slurry.
The paste essentially carries the aggregate and the rheological composition or rheological properties of the paste will determine the workability of the concrete.
These are the three properties you need to address for any high performance concrete mix design.
Of course, the basic dependence of strength on water-cement ratio remains.
The higher the water-to-cement ratio, the lower the strength.
Therefore, this must be determined for any concrete based on the ingredients you have in any geographic location.
As we discussed earlier, the water-cement ratio requirement for concrete in Chennai may be different from that of concrete in Delhi.
Aggregates are completely different.
The degree of buildup you can get from these aggregates is also completely different.
All these can lead to different ways of designing concrete in different geographical locations.
It may be the same grade of concrete, but the design may be completely different.
Therefore, the rheological parameters of the paste – yield stress and plastic viscosity – need to be measured.
We talk about the yield stress as the minimum shear we have to overcome in order to produce flow.
We haven’t really revisited the buildup that can be introduced when using different types of aggregates, especially when we started using filler materials in concrete.
Mineral admixtures brought in a whole different dimension.
As a result, people started looking at how to modify this to really get a better estimate of particle buildup.
Particle buildup is basically optimizing the particle skeleton to get the best buildup density.
Please do not confuse this density with the actual physical meaning of the word “density”.
This does not mean that we are producing concrete with a density greater than 2.5 grams per cubic centimeter.
Within the 2.5 range, we stack the aggregates as much as possible in order to obtain the lowest possible void content.
As a result, the density is still around 2.4 to 2.5, but obviously well-stacked material will have a slightly higher density compared to poorly stacked material.
But we have not fundamentally changed that density.
This has been discussed by many different scientists, including Feret, who says that we get the greatest strength when the initial porosity of the matrix is the smallest.
So, not only to optimize strength, but also to minimize cost, you want to increase the percentage of particulate material in the system.
To this end, various methods have been investigated whereby you can begin to fill a volumetric container with progressively smaller material sizes.
Let’s say you start by filling with coarse aggregates that have gaps between them, and you select the next particle size so that you can start filling those gaps.
Then, after selecting that material there are still gaps.
You fill those gaps with smaller particle sizes and so on.
The idea is that you gradually fill the volume with smaller and smaller particles in order to maximize the filling of the particle space.
There are some factors to consider here.
One is called the wall effect.
The wall effect essentially means that stacking close to the boundary of a container will be worse than stacking away from the boundary of the container.
Suppose we take a cylindrical container and try to stack aggregate in the cylindrical container by doing some compaction using a tamping bar.
Stacking near a wall will be worse than stacking away from a wall.
Therefore, no matter where you touch the interface, the buildup will be affected, which is what you see when defining an ITZ.
There is also more porosity in the ITZ due to this wall effect.
You don’t have as efficient a buildup of cement paste or cement mortar around the aggregate as you do slightly away from the aggregate.
Therefore, you need to be aware of the wall effect when designing particle buildup.
The other one is the loosening effect.
Now you have these larger particles to put into the system.
If you choose a pellet size that is slightly larger than the available gap.
Or, if you choose too many small particles, they now don’t have enough room in the gap and they will start pushing out the larger particles.
This is called the loosening effect.
If you didn’t choose the right proportion of small particles, they don’t have enough room to fill the gap.
As a result, they will start pushing out coarser particles to fill the gap.
This can be detrimental to the overall buildup of the system.
Now, imagine that I choose to replace cement with silica fume.
Silica fume has a much smaller particle size.
However, if I use too much of it, it will get into the spaces between the cement particles and may further disperse the cement particles, thus disrupting the buildup of the cement particles.
Therefore, you can’t keep choosing more and more fine ingredients to fill the spaces.
This is the concept of the loosening effect.
You need to have an optimum level in the system in order to fill the spaces as practically as possible.
Obviously, such a system can be defined using several different mathematical models.
There are several models available.
Some of them are based on discrete methods.
The discrete approach means that you fill each step with uniformly sized particles.
For example, only 20 millimeters, only 10 millimeters, only 5 millimeters, and so on.
Alternatively, you can use a continuous range of particle sizes.
This means that each particle system has a continuous gradation, which you can then use in your model.
