11-07-2018 | Posted by Principia
Concrete is a very old material. Concrete-like mixes were already employed thousands of years BC and were extensively used in Roman times. But they were essentially forgotten afterwards, at least until the Renaissance period, and the concrete we know today only started to become popular in the 19th century.
Be as it may, we tend to think of concrete as a solid, strong, enduring material, that undergoes little change with time other than the degradation caused by abrasion, impacts and, possibly, chemical attack in certain environments. In this situation, it may be hard to imagine our stable concrete undergoing progressive swelling, but it does happen in many cases, with dire potential consequences.
There are a number of processes that can cause the long-term chemical expansion of concrete, with the alkali-silica reaction (ASR) being a relatively frequent one. A typical manifestation of the problem is the observation of an arch dam slowly drifting upstream, in the direction opposite to the pressures exerted by the stored waters, over a decades-long process.
The problem is not unusual: many Spanish dams, primarily built in the 50’s and 60’s, are currently experiencing it; and the same occurs internationally, including the Kariba dam on the Zambezi, which houses the largest reservoir in the world. Over the years Principia has studied the problem in many of our dams and has even participated in an international benchmark exercise to predict the evolution of the Kariba dam.
ASR requires a reactive aggregate, an alkali-rich cement and water. It also needs time: forming the hydrophilic gel takes a latency time and its expansion involves a characteristic time. The reactions are accelerated by temperature and the process has a complex interaction with the state of stress. Perhaps the best model to represent it is that proposed by Saouma and Perotti, evolved from an earlier one developed by Ulm and his co-workers.
The analyses of the affected dams are oriented to predicting their future evolution and, particularly, the safety implications of the process. Those analyses must be based on reliable data, which is provided primarily by the past evolution of the dam, as described by its instrumentation, and by the results of accelerated testing conducted on samples taken from the dam concrete.
The problem is often complicated by the fact different parts of the dam may have been built using different aggregates, with correspondingly different characteristics regarding chemical swelling. In those situations, solving the inverse problem to back-figure the values of the governing parameters may require especial skills.
ASR is still an evolving technical field and an active area of research. But from the experience gathered over the years in the study of many dams undergoing the problem, we are now able to provide reasonably accurate predictions about the future of such structures.