17/07/2017
It is painful when you read about engineers and politicians attributing the cause of cracks at Karuma and Isimba Dams to heat produced in a reaction between water and cement, the heat of hydration. That because concrete casting was done on a hot day, it was necessary to use ice to avoid the current cracks as we now know them. Keeping concrete materials in sheds, cooling aggregates before using them in concrete mixing and using ice to lower the temperature of concrete in hot weather are all good practices for concrete performance but they are not the reason why we have 0.6 meters deep cracks appearing periodic and perpendicular to the river direction across Karuma dam.
Do these engineers and politicians have any clue about the coefficients of thermal expansion of concrete? Do they still remember the heat equation they studied in advanced engineering mathematics? Do they know that large concrete structures in Russia and Europe go through rapid temperature change every year as the weather turns from summer to winter, a drop in temperature associated with an energy quantity that can boil water weight for weight of concrete? I hope they remember the difference between thawing and thermal straining. The problem at Karuma will be understood when a study is done on all Design-and-Build projects in Africa that are implemented by Chinese firms. I have followed arguments advanced for the causes of cracks at Karuma and I also visited the site twice. The cracks at Karuma are purely due to a flaw in steel reinforcement design and detailing. The inexperience of engineers on these projects further complicates the problem. Africa is not yet attractive for experienced Chinese engineers yet our excitement for establishing complex and large scale projects is without equal. African engineers need to rise up to the challenge, stop procrastinating and take on the mantle of leading infrastructure development on the continent. The government needs to stop corruption and other shortcuts and focus on developing local content. The desire for shortcuts will leave us with poor quality infrastructure that will demand heavy budgets for maintenance. Just like doing anything worthwhile, it is not cheap and not easy to build a self-sufficient citizenry and government needs to stop hiding its head in the sand. Shortcuts need to stop.
Below are the reasons why the theories advanced on Karuma are wrong.
1. On heat of hydration and casting concrete on a hot day; Two mechanisms are at play when it comes to heat of hydration, (a) thermal straining and (b) Plastic shrinkage.
a) Thermal straining is significant after concrete has hardened. If the source of heat energy causing thermal straining is internal, i.e heat of hydration, then the point with the highest temperature is at the geometric centre of concrete. For this reason, the point with the highest stress buildup is the core of a concrete mass and, accordingly, cracking is explosive. The cracks are radial since stresses reduce toward the surface of concrete. There was no explosion for the cracks at Karuma and Isimba. The cracks are also not radial but roughly linear and periodic, typical of drying shrinkage cracks.
b) Plastic shrinkage leads to shallow surface cracks at a time when concrete is still plastic (moldable) and is due to a higher rate of water loss from the surface (evaporation). It is true that a hotter concrete will accelerate the loss of water but this mechanism is normally limited to a few millimetres on the surface of the concrete. The cracks are clearly seen 24 hours after casting. Cracks at Karuma are 0.6 meters deep and happened way after casting.
2. On sand being the cause of cracking at Karuma: This is the most clumsy of all reasons. The contractor has a laboratory for the purpose of quality control and quality assurance. How did they use poor quality sand? Could it be the inexperience of the engineers and consultants? Did they use a different type of sand in sections that do not show cracks?
3. On the argument that there was a differential movement between the bed rock and the concrete mass, given that their coefficients of expansion are different; In this case, the point of maximum stress is the contact between the bed rock and concrete. Concrete cracks from the point of highest stress and therefore the cracks would be wider at the bottom of the dam. The cracks start from the surface of the dam and terminate at about 0.6 meters without reaching the bottom of the dam.
4. There was a fourth theory but I just forgot it, maybe because it wasn’t worth remembering.
Advancing my argument that a poorly designed dam structure which is being constructed by inexperienced technical personnel from China is the cause of the shoddy works at the dam, I conclude as follows. The steel profile in the surface of the dam did not cater for uniform distribution of cracks that would inevitably occur due to drying shrinkage. As a consequence, the cracks concentrated in specific locations that are periodically distributed on the surface of the dam. Drying shrinkage is normally minimised by curing. Because curing was done by wetting blankets placed on the surface of the dam, the desired effect was undone. Blankets increase the specific surface for evaporation and lead to higher water loss in exposed surfaces such as in the case of Karuma dam. Blankets also absorb heat and keep the concrete at elevated temperatures leading to faster drying. Moreover, water poured on concrete does not pe*****te into the microstructure of concrete but remains on the surface only to evaporate in a few minutes. This is because hardened concrete has a very low water sorption properties.
I hope my government will renegotiate the contract to shift the maintenance liability to the developer otherwise we shall remain with a dam that will always require large annual expenditures in maintenance.
