Durability

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
Factors that determine
human longevity
Factors that determine the
durability of concrete
Types and ratios of genes
(genome) inherited at
inception
Types and ratios of
components used for mixing
the concrete
Care and nutrition after birth Compaction and curing after
placing
Vicissitudes of life maturity Environmental insults after
hardening
Medical and nursing care Maintenance of structure

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
1) The appropriate design of the mix to suit
the exposure conditions to which the
shotcrete will be subjected to
1) 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
VPV % BA % Suggested Quality
Indicator
Less than 14 Less than 6 Excellent
14 to 17 6 to 8 Good
17 to 19 8 to 9 Fair
Greater than 19 Greater than 9 Marginal

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!