Aloysius Hettiarachchi

At this very moment the concrete used for the construction of large number of schools in the United Kingdom have been found to be of poor quality. It seems some have collapsed and others are crumbling. According to their BBC World Service around 150 schools are affected and many have been closed. It appears that this is just the tip of the iceberg and many government buildings also may be affected. According to this source the culprit is a certain type of lightweight concrete known as RAAC (Reinforced Autoclaved Aerated Concrete).

When I saw a youtube video recently (link given below) by a young enthusiast with statistics on the Victoria Dam, I thought of highlighting my concern in this forum as a Professional Civil and Structural Engineer:

 Actually, I have raised this question of ‘durability’ and use of concrete on important structures from some experts who were giving a presentation at the institution couple of years ago as well. They were some engineers (of Sri Lankan origin) attached to an Australian University doing research work on design and construction techniques using reinforced concrete. With some hesitance the reply was that the useful life of reinforced concrete structures is about 75 years. In practice, I have known that concrete buildings and bridges become unusable after about 75 years, many a time without giving notice with disastrous consequences. But my concern was lack of such information given to decision makers who plan for such buildings or structures like bridges.

What is Concrete:

Concrete is made with cement by adding it in controlled proportions to fine aggregate (sand or quarry dust) to coarse aggregate (crushed stone or crushed river pebbles) and mixing with water. After placing the concrete, it needs to be cured (by pouring water over it) for a certain period of time (several days) to prevent cracking and for it to gain the required amount of strength. The general consensus is that after about 28 days concrete gets the full specified strength according to the proportions used and stays that way for ever. Most of us know this process but what we do not know is what exactly is cement and its behaviour.

The patent for cement was first obtained by an English mason by the name Joseph Aspdin in 1824. The product was first known as Portland Cement as it resembled a building stone in the Isle of Portland off British coast. We used to refer to this cement as OPC (Ordinary Portland Cement). Since then, the use of concrete has become widespread and the first high-rice concrete building (skyscraper) was constructed in Chicago in the latter part of the 19^th century. It had a useful lifespan of about 40 years according to my search in the internet.

The biggest constituent of cement is lime (about 66%) which is obtained from sedimentary rock formed in river beds and lakes. Perhaps this material containing calcium carbonate is a product of millions of years of biological action. The rest are alumina, silica, compounds of iron, etc. The powdered mixture is capable of absorbing a huge amount of energy that gets released when water is added in the concrete making process.

Various codes of practice for the use of concrete in reinforced concrete have evolved since then, but it seems to me they all follow the pioneer (UK’s codes), perhaps blindly.

In my practice I have used three codes for reinforced concrete. The first was British Standard (BS) 110 in the 70s. Then came BS 8110, in mid 80s. Finally, when UK joined EU, we started designing for Eurocode known as EN 1991. Currently we use EC2 which Sri Lanka also follows. However, what struct me most is that none of these codes use time as a parameter. They all deal with ‘durability’ of a completed or partially completed structure under loads, wind and other conditions without any reference to energy transfer that goes on inside the constituents continuously over a long period. The useful time span is vaguely mentioned as between 50-100 or taken for granted as 100 yrs in some obscure document but not in any of the codes mentioned, to my knowledge. BS5400 for bridges define concrete cover to reinforcements with a view to control the exposure of steel in concrete to elements but not with a view to control the reactions that may be going on in cement.

The codes do not explain how the bond between steel and cement paste occurs but assumes that it stays firmly attached together for ever. This is the same story for bonding between cement paste (calcium silicates) and aggregates.

I have gone through a research paper produced at the time of EN1991 came into force (ie 1990) by a team in University of California, Berkely (Report No. UCB/SEMM-90/14) giving the details of a Finite Element Analysis of Reinforced Concrete Structures around five years ago. I found to my surprise that they too overlooked this very important aspect of ‘time’. And also, I found that the model they used for the bond between steel and concrete unsatisfactory as they have used a method called ‘smeared crack model of concrete’. I have also gone through Reynolds Handbooks used by engineers, world over and handbooks of an expensive software packages used for design of high-rise buildings and found that they all follow the codes mentioned above without regard for time factor.

Coming back to the subject matter about Victoria dam, I believe the statistics given by the narrator is true. I have not visited this dam but witnessed its opening by late Her Majesty the Queen when I was working in a West African county. The job is impressive, but will it serve another 40 years or at least another 15?. There are no structural steel rods as it has been designed as an arch , but what about the bond between calcium silicates and aggregates that gives it the compressive strength. With time they may become weak and the smallest earthquake can cause a huge damage. To be pre-warned is to be pre-armed. This writer also has designed and constructed an earth-dam in a foreign country after studying their structures in Sri Lanka and elsewhere from several books on the subject. It was part of a food prevention scheme and has become a picnic spot.

According to the second law of thermodynamics in every system the entropy (disorder) tends to increase with time to come to a more stable state. We take materials from the ground which are in stable state, heat them up and give energy. We then use them in our projects and think they will stay the same way. How wrong we are.

It’s time to think in terms of quantum physics that explain the way matter exist. In any material the electron spin in some atoms align in a certain direction when we give energy to it. But with time by combining with nearby other atoms that it may come to contact with the direction of spin of electrons gets rearranged to previous state so that the combined effect of magnetic fields become zero. This was beautifully explained in a video by a scientist. Unfortunately, I cannot lay my hands on it now to produce here.  However, it is the same thing that was explained by one engineer using an instrument known as ‘Vector Network Analyzer’ (VNA) where he was doing a Fast Fourier Transform (FFT, which is an interesting subject as it is in all our mobiles) on emissions (or release of energy) coming from some materials to find their wave form. This video appeared in one of my previous write ups in this forum. Here it is:

Understanding Holy Grail in Western Medicines, MRI Machines etc.