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  • Writer's pictureSi Shen

CIRIA Guide C766 preview

Updated: Nov 24, 2019

A new CIRIA guide C766 is coming to the market soon. It is to replace the super popular guide C660 – early age crack control for water retaining structures. I feel privileged to be one of its first users and have been able to achieve massive saving in reinforcement, in the magnitude of millions in £ term, by using this guide. C766 is in final draft at the moment and is expected to be published at the end of the year. Let me give you a quick preview of this new guide. Let me define the problem first. Whenever we design silos, deep basement, tube stations or shafts, water tightness is usually a key design requirement, because water either leaves a stain or compromise functionality of the structure. There are largely speaking three popular ways of providing water tightness: 1, Explicit waterproofing. For underground structures, in principle, you provide a double-shell structure and the outer shell is meant to be leaky. You either provide a gap in between to drain away the water, or provide a waterproofing membrane in between. 2, Crack-elimination. Concrete is theoretically water-proof when it is uncracked, made with a perfect mix, and is perfectly compacted. Although in practice you can never achieve this, one way to prevent concrete from cracking would be to put it into compression. Pre-stressing it is usually the way to go. This is normally used for domes or silos above ground and generally not feasible for underground structures. 3, Crack self-healing. This is achieved by using reinforcement to control the crack of the concrete down to a very small level, so that after the crack leaks for a bit, it gets blocked up by fine particles in the water, ‘left-over’ unreacted cement or chemical reactions that create crystals. This is a well-proven capability for cement-based materials, i.e. concrete, mortar, etc. If you adopt the self-healing approach, a small amount of leakage must be permitted, and Eurocode has a set a very stringent requirement for it. We usually use EC2-3 supplemented by CIRIA guide C660. For structures resisting a great amount of hydraulic pressure, cracks need to be controlled to 0.05mm.

Let me put this into perspective for you. Below is a typical crack gauge. You can see that the smallest crack you are able to measure is 0.1mm. This is roughly equivalent to the diameter of your hair. For 0.05mm, you probably can’t even see it with naked eyes. This set of requirements generates a vast amount of reinforcement for crack-control, which makes the option of self-healing comparatively unattractive. Moreover, sometimes, more is less. More reinforcement means high reinforcement congestion, especially around joints, bends and corners. Reinforcement congestion is not only a health and safety hazard, but also makes the concrete compact less well, which means even more risk for it to crack. As a result, EC2-3 forces the industry to resort to waterproof membranes, even when a small amount of leakage is permitted. Waterproof membranes are not necessarily cost-effective when dealing with complex surface geometries, and whether they can stand the test of time of 120 years design life for monumental structures is yet to be strictly proven.

But there is light at the end of the tunnel – a new CIRIA guide is coming – C766 Control of cracking caused by restrained deformation in concrete. This guide will replace some of the requirements specified in EC2-3 and supersede C660.

You can notice the title is different – it is covering a wider subject than C660. I will only list out some of the biggest changes here: 1, It substantially relaxes the allowable crack width for deep water-resisting structures, as long as the water pressure on the structure is stable. The saving is absolutely huge for deep underground structures. In some occasions, the reinforcement needed for crack-control purpose is basically slashed in half. 2, The heat assumed to be released by GGBS concrete has been revised down – this is based on feedback from the industry. 3, There are various adjustment to factors used in crack calculations and all these adjustments accumulate towards saving. These include: the re-definition of cover to reinforcement when conducting crack width calculations; changes to the minimum area of reinforcement by modification of the concrete tensile strength for crack control; introduction of creep under sustained load. This guide is currently due out in December this year. I will be going to its launching event linked below. If you are interested to come along, see you there.

More on the topic of crack control for concrete

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