Durability and Curing of Shotcrete

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Durability and Curing of Shotcrete

• A.Boniface GIBB D.R.Morgan AMEC Earth
  Environment

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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

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Durability Curing

• Basson and Ballim (1994) make an interesting
  analogy between human longevity and the
  durability of concrete

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Durability Curing
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Durability Curing
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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..

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Durability Curing

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

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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

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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

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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)

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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

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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

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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

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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?

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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

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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
  12390-82000

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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

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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

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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)

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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

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Durability Curing

• Denseness of in-situ sprayed concrete
• Suggested indicators of shotcrete (making
  reference to these indicators) have been
  published eg Morgan, 1994

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Durability Curing
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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

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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)

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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

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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

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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

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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

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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

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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

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Durability Curing

• North American Specifications
• Moist curing for a minimum of 7 days is a
  recommended norm

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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

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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

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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

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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

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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

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Durability Curing

• A current perspective
• and resulting in undesirable effects such as
  shrinkage

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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

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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

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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

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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

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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

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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)

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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

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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

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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

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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.

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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)

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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

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Durability Curing

• Internal curing using special aggregates
• This approach to foolproof curing surely
  deserves some RD 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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Durability Curing

• Thank you!