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Durability and Curing of Shotcrete

Durability and Curing of Shotcrete. A.Boniface GIBB & D.R.Morgan AMEC Earth & Environment. Durability & Curing. Shotcrete is increasingly being seen as a permanent structural material and a design life of 120 years is now being stipulated for some projects

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Durability and Curing of Shotcrete

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  1. Durability and Curing of Shotcrete A.Boniface GIBB & D.R.Morgan AMEC Earth & Environment

  2. Durability & Curing Shotcrete is increasingly being seen as a permanent structural material and a design life of 120 years is now being stipulated for some projects It follows that such a material must be durable

  3. Durability & Curing Basson and Ballim (1994) make an interesting analogy between human longevity and the durability of concrete

  4. Durability & Curing

  5. Durability & Curing

  6. Durability & Curing Melby (1994) lists ten factors that affect the durability of shotcrete • Buildable designs • Modern relevant specifications • Capable and trained applicators • Wet or dry method • Hand or mechanised application………..

  7. Durability & Curing ……………. • Correct mix design and accelerator choice • Curing method • Ground and ground water conditions • Site control • Future environment and load conditions

  8. Durability & Curing Many of these are self evident to those familiar with the requirements for producing high quality concrete At the end of the day all comes down to two issues………

  9. Durability & Curing • The appropriate design of the mix to suit the exposure conditions to which the shotcrete will be subjected to • The actual physical (and chemical) properties of the insitu shotcrete

  10. Durability & Curing A great deal of work and effort has been put into the former – designing the shotcrete mix to suit the environment of the completed work (see references in the paper)

  11. Durability & Curing It seems to the authors that not enough has been done in terms of the latter – ie checking the quality of the in place shotcrete

  12. Durability & Curing There are a number of construction parameters that can affect the quality and long term durability of the end product Our paper addresses two of these: • The ‘denseness’ of the in place shotcrete • The role of curing

  13. Durability & Curing ‘Denseness’ of in-situ sprayed concrete There seems to be general agreement that for shotcrete to be durable it needs to be uniformly dense and relatively impermeable

  14. Durability & Curing ‘Denseness’ of in-situ sprayed concrete Some specifications require the permeability to be less than 10-12 m/s But how do you readily monitor the quality of the finished product in terms of ‘denseness’?

  15. Durability & Curing ‘Denseness’ of in-situ sprayed concrete Density testing (alone) has proved unreliable, and classic forms of testing permeability are generally unsuitable for regular day by day testing of finished work

  16. Durability & Curing ‘Denseness’ of in-situ sprayed concrete Some specifications refer to the use of the simplified permeability test as described in DIN 1048-5, which has been incorporated into BS EN 12390-8:2000

  17. Durability & Curing ‘Denseness’ of in-situ sprayed concrete The authors are not familiar with this test and wonder about its suitability for routinely testing sprayed concrete

  18. Durability & Curing ‘Denseness’ of in-situ sprayed concrete For nearly twenty years the shotcrete specifications for most major underground civil engineering projects in Sn Africa have required the routine in-situ testing of in place shotcrete to provide assurance of it potential durability

  19. Durability & Curing ‘Denseness’ of in-situ sprayed concrete The properties of Boiled Absorption (BA) and Volume of Permeable Voids (VPV) are measured for this purpose (see ASTM C 642-06)

  20. Durability & Curing ‘Denseness’ of in-situ sprayed concrete The first published reference to the use of these material properties for assessing the quality of shotcrete was made in a paper by P.Seabrook in Canada, in the 1970s

  21. Durability & Curing ‘Denseness’ of in-situ sprayed concrete Suggested indicators of shotcrete (making reference to these indicators) have been published eg Morgan, 1994

  22. Durability & Curing

  23. Durability & Curing ‘Denseness’ of in-situ sprayed concrete This approach continues to be widely used in N Amerca and Sn Africa However with only two exceptions known to the authors, specifications drawn up in Europe are strangely silent on this topic, eg EFNARC, Norwegian NB & NPRA specifications

  24. Durability & Curing ‘Denseness’ of in-situ sprayed concrete The two European specifications which do mention VPV and BA testing are the BTS/ICE Tunnelling Specification and a similar spec which appears as an Appendix to a BASF publication (Melby, 2006)

  25. Durability & Curing ‘Denseness’ of in-situ sprayed concrete Whilst these two specifications mention VPV & BA testing they leave it to the designer to stipulate the acceptable limits

  26. Durability & Curing ‘Denseness’ of in-situ sprayed concrete A number of authors have rightly pointed out that the question of shotcrete durability is a complex one However it should be possible to agree on methods which can demonstrate the durability of in place sprayed concrete

  27. Durability & Curing Curing of Sprayed Concrete – ITA WG 12 1993 International Practice Review This review revealed a very wide range of views on this matter – as reported in our paper

  28. Durability & Curing Curing of Sprayed Concrete – ITA WG 12 1993 International Practice Review This review revealed a very wide range of views on this matter – as reported in our paper

  29. Durability & Curing SANCOT Draft Shotcrete spec for the SABS 1200 series of standard specs (1992) This draft spec required, either: • Moist curing for 3 days by means of a mist spray at intervals not exceeding 4 hrs; or • The use of an approved curing compound

  30. Durability & Curing BTS/ICE Tunnelling Spec Clause 306.5 states: • Curing (where necessary) to be by a method approved by the Engineer • Curing compounds only to be used if approved by the Engineer • Membranes only acceptable if no further layers of shotcrete required • Curing shall ensure proper strength gain and minimise cracking

  31. Durability & Curing North American Specifications Moist curing for a minimum of 7 days is a recommended norm

  32. Durability & Curing A current perspective The benefits of curing newly placed structural concrete are well understood in the civil engineering industry Moist conditions are necessary to ensure that complete hydration is achieved which in turn will ensure optimum strength gain and minimise shrinkage

  33. Durability & Curing A current perspective When it comes to sprayed concrete it often appears that the above mentioned benefits are not fully appreciated Perhaps this stems from the fact that when first introduced in tunnelling work shotcrete was used as a temporary expedient

  34. Durability & Curing A current perspective The expected service life in mining is often a lot less than in the civil industry This simply reinforces an attitude that shotcrete is a temporary expedient and that proper QC is not required

  35. Durability & Curing A current perspective The expected service life in mining is often a lot less than in the civil industry This simply reinforces an attitude that shotcrete is a temporary expedient and that proper QC is unecessary

  36. Durability & Curing A current perspective What seems to be overlooked is that poor QC of the whole process of producing and placing shotcrete (includng curing) can lead to mix designs being used that contain excessive quantities of cement – resulting in unnecessary expense ……

  37. Durability & Curing A current perspective ………… and resulting in undesirable effects such as shrinkage

  38. Durability & Curing Methods of curing • Water-spraying at suitable intervals – but there are difficulties in ensuring this is actually done • Maintaining an humidity RH of 80% or more – not always a practical solution • Curing membranes – but NOK where multiple layers are required

  39. Durability & Curing Methods of curing • Internal curing by using special additives, eg Meyco TCC 735 • Internal curing by using sintered lightweight aggregates – available in the US – but maybe not elsewhere

  40. Durability & Curing Methods of curing Water spraying, humidity control and membranes have their limitations Internal curing methods certainly have an appeal of simplicity and a sense of ‘foolproofness’

  41. Durability & Curing Internal curing using special additives These have been used at South Deep and Impala – where the design life of the shotcrete linings was an important consideration

  42. Durability & Curing Internal curing using special additives In Chapter 4 of the book “Sprayed Concrete for Rock Support” produced by BASF (Melby et al, 2006) the authors go to some lengths to demonstrate that the product Meyco TCC 735 is effective and economical to use

  43. Durability & Curing Internal curing using special additives They claim the following benefits: • Increased bonding compared to no curing; 0,6 to more than 2,0 Mpa • Increased density compared with shotcrete cured with external curing agents (greater by more than 15%)

  44. Durability & Curing Internal curing using special additives • Increased strength compared with air cured shotcreteor shotcrete treated with external curing agent (greater by more than 10% at 28 days) • Substantial reduction in water absorption • Substantial cost savings

  45. Durability & Curing Internal curing using special aggregates Recent ACI publications have highlighted the benefits of using special lightweight aggregates in ensuring the adequate curing of High Performance Concretes where low water cement ratios are increasingly being used

  46. Durability & Curing Internal curing using special aggregates Recent ACI publications have highlighted the benefits of using special lightweight aggregates in ensuring the adequate curing of High Performance Concretes where low water cement ratios are increasingly being used

  47. Durability & Curing Internal curing using special aggregates In one such publication it points out the limitations of external methods of water curing and continues: “The solution is to supply water reservoirs on the inside through incorporation of sufficient absorbent agents in place of a part of the normal weight aggregates….

  48. Durability & Curing Internal curing using special aggregates …… These desorb water to the hydrating cement when and where it is needed. This is called internal curing (IC)”

  49. Durability & Curing Internal curing using special aggregates It is interesting to note that some work in the internal curing of concrete is being carried out at the University of Cape Town

  50. Durability & Curing Internal curing using special aggregates This approach to ‘foolproof’ curing surely deserves some R&D work being done which is specifically directed at sprayed concrete applications This could be especially beneficial where shotcrete is used in open air situations

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