Question

Question 6: Define two concrete mixture performance problems caused by aggregate properties.

Please TYPE your answer, no hand writing. Thank you

Answer 1

There are still a number of challenges facing the Concrete Industry. From a political and social viewpoint, improving the sustainability of concrete construction has to be the first challenge, not because concrete solutions are poor when compared with alternatives, but due to the volume needed for the modern economy.

Uncontrolled Addition of Water on Site

Adding water on site in a controlled manner under the full responsibility of the concrete producer is not bad practice and it is a solution to situations where travel time to site is highly variable or where the exact consistence at discharge is critical, e.g. diaphragm walling. In this situation the procedure is agreed between all the parties beforehand and typically some of the mix water is held back and added on site, followed by mixing at high speed for the agreed time. The volume of water added is recorded and samples for testing the consistence and strength are taken from the re-mixed concrete. The producer remains fully responsible for the quality of the supplied concrete.

This section is not about this practice, but about the uncontrolled addition of water to concrete on site. From a supplier's viewpoint, the specification has been changed by the client instructing the producer to add more water, but it is questionable whether the person ordering this change has the authority to do so. If the producer does not have this instruction and the volume added written on the delivery ticket and signed, they have little proof that they were following instructions. What is worse is if the sample for testing is taken prior to adding water. While the producer will claim that this sample represents the quality of concrete supplied to the site, it does not reflect the quality of concrete in the structure and this is the key concern of owners of structures.

This issue is one that continues to blight the industry and resolving it remains one of the challenges facing the Concrete Industry. The solution may appear simple, ban the practice in the concrete specification, but this is ineffective if it is not enforced on site. The one certainty is that nothing will be recorded on the delivery ticket and if this practice is spotted on site, the producer will not want to lose a customer and, therefore, they will work together to claim this was an exception!!!

Achieving the Minimum Cover

Another issue that blights our industry is the lack of the achievement of the specified minimum cover to reinforcement. One of the mantras of the former Cement & Concrete Association, Fulmer Grange, UK was 'halving the cover quarters the life' and this simple message puts the importance of cover into context.

In general, there is little evidence that if a concrete structure achieves the specified minimum cover given in the design code, the concrete conforms to the current recommendations in national standards and it is well compacted and cured, it will not achieve its intended design life. Nevertheless, modern structures are still showing signs of reinforcement corrosion, nearly always due to the lack of the specified minimum cover.

Euro code 2 [15] has addressed one of the issues related to the lack of the minimum cover by clearly distinguishing between nominal cover and minimum cover. The nominal cover is the minimum cover plus a fixing tolerance, typically 10mm but it can be more for concrete cast against the ground and less for certain precast products where the cover is tightly controlled.

Conclusions

1. There are still a number of challenges facing the Concrete Industry.
2. Improving the sustainability of concrete construction has to be the first challenge, not because concrete solutions are poor when compared with alternative solutions, but due to the importance and volume of concrete being used.
3. To get a fair comparison of alternatives, sustainability has to be at the level of the structure and include all three pillars of sustainability.
4. Finding a system for combining the different indicators is essential if sustainability is to be assessed in a holistic way.
5. Another challenge is developing a practical, robust, reliable and cost effective method of specifying durability by performance.
6. There are issues that are known about and solutions are available, but they still blight the Concrete Industry. These are the uncontrolled addition of water on site and the failure to achieve the specified minimum cover in the structure.