Strength in Tension
The theoretical strength of hydrated cement paste has been estimated to be as high as 10.5Gpa, however the actual strength is considerably less. This discrepancy can be explained by flaws that lead to high stress concentrations in very small volumes of the specimen. These flaws vary in size, and it is only the few largest ones that cause failure.
The flaws can be pores, microcracks or voids, though the exact mechanism by which they affect the strength is not known.
Strength in Compression
Partially due to the reasons given above, concrete is significantly stronger in compression than in tension.
The above graph shows that for most samples of concrete, the compressive strength is around ten times higher than the tensile strength.
When designers are creating a structure to be built from concrete, it is designed to utilise the compressive strength of concrete. Any tensile strength requirements are usually met not by the concrete itself, but by other materials within the concrete, for instance steel bars or steel fibre reinforcement.
Diagram X shows a simple rectangular beam loaded at two points along its length equidistant from the ends. Diagram Y shows the cross section A-A, along with two stress diagrams. The dashed areas represent compressive stress, the enclosed white areas represent tensile stress. Assuming that the stress/strain relationship for concrete in compression is linear, (1) shows the stress distribution without reinforcement. The value of compressive and tensile stress is equal. However because the tensile strength of concrete is around ten times less than the compressive strength (as shown by the previous graph) this concrete will fail. (2) shows the same sample with reinforcement. It is clear that all the tensile stress is taken up by the reinforcement, leaving the concrete to cope with only the compressive stress.
Factors influencing Strength Gain
Time
The strength of concrete usually given is the compressive strength after 28 days.
The above graph clearly shows that firstly, the strength of the concrete increases over time, and secondly that the strength gain is high for the first few days, then the rate of strength gain levels off towards 28 days.
Water/Cement Ratio
It has been said that the water/cement ratio is the largest single factor in the strength of fully compacted concrete. The basic principle is the lower the ratio, the stronger the concrete.
This graph shows the mean results of three samples. It shows quite conclusively that not only does the sample with the lowest water/cement ratio give consistently higher results, but also the lower ratio sample has a greater overall gain in strength over the time period. This graph also links with my previous point, i.e. the strength of concrete continues to increase even after a considerable number of years.
As with most principles, there is often a difference between the theoretical ideas and actually putting them into practice. In theory the strongest concrete should have a very low water/cement ratio, however in practice when the ratio drops below about 0.4 then the concrete becomes difficult to compact (i.e. less workable – see ‘Workability’ section) and in practice there is a significant drop in strength as the graph below illustrates.
It is important to note that when concrete, especially that of a low water/cement ratio is compacted mechanically it is stronger than a similar sample compacted by hand.
Curing
The object of curing is to keep concrete saturated, or as nearly saturated as possible until the products of hydration have filled the originally water-filled space in fresh cement paste. In simpler terms it involves limiting the loss of water during the early stages of setting. Again, this is another area where there is a division between theory and practice. In a laboratory with samples of concrete it is easy to keep them in a tank for curing, whereas on a building site it is much more difficult to keep the concrete moist.
T.C. Powers calculated that hydration is greatly reduced when relative humidity falls below 80 per cent. It has been argued that if the humidity of the air is at least that high then no active curing is necessary, however this will only really work if there is no wind and no difference in temperature between the concrete and the air.
This graph shows the relation of compressive strength to the number of days a sample of the same concrete was kept moist. It is clear that the greater number of days the concrete is kept moist, the greater its compressive strength. It is interesting to note that concrete kept continually moist does not increase in strength as rapidly as that kept moist for only 7, 14 or 28 days, however it’s final compressive strength is greater.
The prevention of the loss of water from concrete during curing is important not only because of its adverse affect on the strength of the concrete but also because water loss can lead to shrinkage, increased permeability and reduced resistance to abrasion.
There are two main methods of curing, which can broadly be described as wet curing and membrane curing. Wet curing involves the provision of water that can be imbibed by the concrete. This can be achieved either through constant spraying or flooding or by laying periodically wetted hessian or cotton mats over the concrete. Membrane curing relies on the prevention of water loss from the surface without the possibility of external water seeping in (i.e. a water barrier method). Generally overlapped polythene sheeting or reinforced paper is used.
Another technique, and one used during my own experience on a construction site was that of spray-applied curing compounds which form a membrane.
Other factors
It is important to realise that these are not the only factors influencing strength gain in concrete. Others include aggregate size, aggregate/cement ratios as well as local environmental factors.
Workability
Concrete that is easily compacted is said to be workable. The process of compacting consists of removing air from the concrete, until it has reached the optimum for that mix.
The requirement for compaction becomes clear when you observe the relations between the degree of compaction (a ratio between the actual density of the sample and the density of the same sample fully compacted) and the resulting strength.
The graph above quite clearly shows that the further the concrete is compacted, the greater it’s strength.
This can be linked with Féret’s expression:
fc=K(c / (c+w+a)) where fc is the strength of the concrete, c, w and a are the proportions of cement, water and air respectively, and K is a constant.
Therefore, when concrete is more workable, the more easily it is compacted, more air can be removed, and the concrete is stronger.
However, in order to increase workability more water has to be added, which in turn reduces strength. Therefore there is a very fine balance involved in making the concrete just right.
Many modern concretes now include workability (water reducing) admixtures to increase workability without increasing the water content, with obvious benefits to strength.
How is Concrete Tested?
On construction sites, regular samples of concrete have to be taken and tested in order to comply with British Standards. A representative sample of the concrete is taken and placed into specialist concrete moulds. Each cube is constructed in 3 layers, to ensure that excess air is removed from the sample. These cubes are then left to harden before being placed in a cube tank to cure.
The use of alternative materials in Concrete
Pulverised Fuel Ash (PFA)
Along with GGBS (see below), pulverised fuel ash is one of the most widely used alternative materials in concrete. PFA is the ash precipitated either electrostatically or mechanically from the exhaust gases of coal fired power stations.
PFA contains silica in a reactive form, meaning that it is pozzolanic and therefore able to be used as a cementitious material.
A particular advantage of fly ash is the spherical shape of its particles, which is useful from a water requirement point of view as they make the concrete more workable without having to include as much water.
Ground Granulated Blast Furnace Slag (GGBS)
GGBS is a waste product in the manufacture of iron. Chemically, it consists of lime, silica and alumina, the same oxides that make up Portland cement (though not in the same proportions). During concrete production the slag is generally poured into the mixer at the same time as Portland cement.
Cements with a large content of GGBS can be used in structures where the temperature increase resulting from hydration needs to be controlled, as it is low heat cement. This, of course does mean that concrete containing GGBS could be susceptible to frost damage during cold weather.
Another benefit of using GGBS in concrete is that it increases resistance to chemical attack because of the microstructure of the cement paste.
Rubber Tyres
The disposal of waste tyres in the UK is becoming a major problem. It is estimated that over 37 million vehicle tyres are discarded annually. At present, large percentages are placed in landfill sites, or stockpiled. However EU legislation banning this is coming into place in the very near future, therefore there is a definite need to find a way of disposing or recycling these tyres.
It has been recognised that the inherent stability, impact, crack and thermal resistance characteristics of granulated rubber (GR) can be used effectively in concrete in order to improve thermal resistance, freeze-thaw resistance and impact resistance.
GR is produced through a number of mechanical shredding and ripping stages that reduce used tyres to a reusable size. I am going to look in particular at the impact resistance of GR concrete.
In the particular test, carried out by the University of Dundee, two sizes of GR were used, GR1 (1.5-0.5mm) and GR8 (8-2mm). Impact resistance was measured by dropping a 14kg weight onto a sphere resting on a concrete slab.
The number of blows required for an initial crack to reach the slab edge was recorded.
The above graph clearly shows that concrete containing the larger GR8, resists impact better than that containing GR1, and that the greater the GR content, the higher the impact resistance.
This is particularly useful, because in the past modifying concrete from a brittle material that splinters under impact to a tough, resilient material had meant the use of fibre reinforcement. This, although useful was extremely expensive for use on a large scale. However the use of larger granulated tyres, for instance GR20 (25-8mm) could well prove to be a far more cost effective solution.
Crushed recycled Glass
Another innovative idea has been the possibility of using crushed recycled glass as concrete aggregate. Researchers from the Columbia University in New York have had to develop ways of avoiding alkali silica reaction (ASR), a deterioration that normally affects concrete that incorporates glass.
These methods include grinding the glass finely, coating it with a solution to make it resistant to alkalis, using low alkali cement and sealing the concrete.
Conclusions
Through my research, I have discovered that making good, long lasting concrete is an extremely complex subject, more so than most people give it credit for!
In particular, it became clear to me just how much stronger concrete is in compression than in tension, and how engineers have to overcome this problem whilst designing strong, safe structures.
I have discovered that the ratios of water, cement, aggregate and any admixtures have a fundamental effect on the final strength of the concrete and how fine a balance this is. I was also surprised to learn how much strength concrete gains over time.
In our modern society, with legislation and policies constantly fixed on sustainability and recycling, concrete also has a role to play. The alternative materials that I discussed are just a few of the ones on offer, and I’m sure that more and more ideas will be put forward.
Bibliography
Properties of Concrete, A.M. Neville
This book contains everything one would ever need to know about concrete, and more! It contained many graphs and tables which were well laid out and related to the text, however I did find a large proportion of the book was considerably more advanced than I needed, and this made interpreting some of the data quite difficult.
Laing O’Rourke Plc
As a large, multi-million pound construction company, Laing O’Rourke has a plentiful supply of data concerning concrete, as well as employing professionals with knowledge and practical experience of using concrete. This was particularly useful, as I had a very limited knowledge of the subject.
CONCRETE Magazine
The official magazine of The Concrete Society contains many up to date articles concerning concrete. However most tend to be focused on specific projects, however I did find a useful article on the use of granulated tyres. The graphs in the article were quite small, so I had to considerably enlarge the graph I used. The magazine also contained many useful website links.
The Concrete Society
Although this website did not directly assist me in the preparation of this report, it provided good background reading for my project, as well as useful web links.
Dundee University – Concrete Technology Unit
Researchers at Dundee University have carried out several investigations into concrete. The website was well laid out and informative as background reading. I used information from their research into granulated tyres, however this was mostly the same information used in CONCRETE magazine.
New Civil Engineer +
New Civil Engineer magazine is every civil engineer’s choice of reading! The website contains an archive of the articles, which are all dated making it easier to assess the relevance of the information.
‘An Introduction to Reinforced Concrete’ – Department of Civil Engineering, University of Surrey
Although relatively old, this book provided a clear, understandable diagram that I could use in my report.
Properties of Concrete – A. M. Neville
University of Surrey, Civil Engineering Department
University of Surrey, Civil Engineering Department
Properties of Concrete – A. M. Neville
Properties of Concrete – A. M. Neville
Properties of Concrete – A. M. Neville
Properties of Concrete – A. M. Neville
Properties of Concrete – A. M. Neville
University of Dundee, Concrete Technology Unit
New Civil Engineer+ (21/02/2002)
