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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
51. Durability & Curing
Thank you!