As of 12th December 2018, CIRIA guide C766 has come on live. As one of its first practical users, I posted a previous blog post had a preview on this awesome new guide (https://www.si-eng.org/blog/ciria-guide-c766-preview). This blog will delve a little bit deeper into this topic, by answering some typical questions.
How much saving can C766 bring to me?
The answer is HUGE - if what you are dealing with is large scale water retaining and containment structures with high hydraulic pressure. Based on my analysis, it could save as much as 75% of thermal crack control reinforcement in certain situations. Note that the benefits may vary significantly across different projects and should be analysed individually.
The huge saving is the result of the compounding effect of savings in various factors combined. These include the following (with the perceived level of impact indicated for each factor) in the list. This is not meant to an exhaustive list, but should cover the majority of the new features in C766.
Typically, a structure has several design scenarios and each of them requires a different amount of reinforcement; the final design is governed by the most critical design scenario. Thermal crack control is usually one of the design scenarios. With C766, it usually means that the thermal crack control case effectively becomes non-critical and the final reinforcement is governed by other loading scenarios.
For structures with lower hydraulic pressure, the saving is reduced. Therefore largely speaking this guidance is most advantageous for deep heavy civil structures.
If the savings are so huge, is there going to be less use of water-proofing products?
There are multiple ways to achieve water-tightness for a structure – please see my previous blog on this topic (https://www.si-eng.org/blog/ciria-guide-c766-preview). Both water-proofing products and crack-control using reinforcement are popular ways for achieving water-tightness. They are effectively ‘interchangeable’ and in competition with each other, just like chips and rice for a meal, or oil and coal for electricity. After C766, crack control is going to be much easier, which means its competitive edge will be improved against waterproofing products. As a result, we may see less use of waterproofing products.
The rationale of the above is correct. However, we have to bear in mind that a very important prerequisite for choosing the crack control option is that it is ok for your structure to leak a little bit before it gets sealed. Strict water-tightness (equivalent to Class 2 and above in EC2-3) is typically a very critical serviceability design criterion for buildings/structures housing people or sensitive equipments. For these cases, waterproofing products or similar equivalent are still the top choice. Crack-controlled concrete only comes into competition when the tightness criterion is more relaxed.
To sum up, waterproofing products have their unique competitive edge, and the huge saving C766 offers is very situational. Therefore my speculation is that the adverse impact on waterproofing products may not be as big as we thought. Perhaps there is no noticeable impact whatsoever.
Apart from the savings and changes, does C766 provide any further clarification and guidance compared to C660?
Yes – I have noticed some below:
1. It offers clarifications on restraint factors calculation method. Further guidance on restraint conditions: thermal performance for ground slabs have been separated from walls. Ground slabs generally release less heat than walls.
2. For members subject to continuous edge restraint, if offers clarifications on the calculation of shrinkage strain – only the differential shrinkage needs to be considered. the differential shrinkage comes from the difference of placing time between the pours. For example, when a new pour is cast on top of a previous pour at its age of 10 days, in your calculations, only a differential shrinkage of 10 days should be considered. After that, the two pours shrink together at the same rate.
3. It provides detailed calculation method for tensile strain capacity of concrete. It is significantly different from what was used in C660. With new equations for calculation, there is substantial improvement in tensile strain capacity of concrete.
4. It clarifies that long term non-structural cracks such as drying shrinkage is additive to the early age thermal and shrinkage cracks. Also restrained thermal and shrinkage cracks are not additive to those arise from structural loadings.
What is the legal standing of CIRIA C766?
C766 is neither a standard nor a code. It has no legal power. Nevertheless, its use is just like the old British Standard – it represents the best practice in the industry. Everything in the Eurocode firstly appeared as guidance; they get merged into the Eurocode once their use achieve certain level of maturity. If C766 stands the test of time, it will be merged into the Eurocode sooner or later. Although it should be noted that the process can take a few decades. Risk-averse clients may choose not to adopt it from legal standing perspective, but bear in mind that the price to pay, or the lost opportunity cost, would be substantial.
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