1.4 Empirical results
1.4.2 Factorial decomposition of the change in the content
Strong acids as well as strong bases can be dangerous to personnel because of their aggressive behaviour upon contact with eyes, skin and the respiratory organs. The general terms to be used in this relation are therefore near neutral (pH value 5–9) and aggressive (pH value 0–4 and 10–14).
... Non caustic alkali-free accelerators in liquid form
The increasing demand for accelerators for sprayed concrete termed alkali-free always contains one or more of the following issues:
1) Reduction of risk of alkali-aggregate reaction, by removing the alkali content arising from the use of the common caustic aluminate based accelerators.
2) Improvement of working safety by reduced aggressiveness of the accelerator in order to avoid skin burns, loss of eyesight and respir-atory health problems.
3) Environmental protection by reducing the amount of released aggres-sive and other harmful components to ground water, from sprayed concrete and its rebound.
4) Reduced loss of sprayed concrete final strength, normally in the range of 15 to 50 % with older accelerator products.
The focus within different markets, regarding the above points, is variable. Where most sprayed concrete is used for primary lining (in design considered temporary and not permanent), points 2 and 3 are the most important. When sprayed concrete is used for permanent structures, items 1 and 4 become equally important. This variation in basic reason why to require a new accelerator technology has caused some confusion.
Requirements in different countries, examples
In some countries almost all sprayed concrete in tunnelling is defined as temporary and is disregarded in the design of the permanent lining.
The possible durability problems created by an alkali aggregate reac-tion in the sprayed concrete is therefore not an issue. There is, however, a growing demand for «alkali-free» accelerators, as legislation is being established, to improve working safety. In other words, the real require-ment is for non aggressive accelerators.
7 In other countries the same use of sprayed concrete prevails, but there exist so far no regulations which prohibit aggressive accelerators. Out of concerns for environmental protection (= ground water) the authori-ties are, however, now requiring «alkali-free» accelerators. No additional high pH leaching components other than cement shall be used. The real requirement is in this case for a non aggressive (highly irritant) accelerator.
There are also countries where most sprayed concrete in tunnelling is temporary from a design point of view, but where, however, it is normal to require an «alkali-free» accelerator, i.e. non caustic and containing no alkali cations to avoid aggressive leaching components and, frequently, to obtain a limited allowed final strength reduction. Regarding personnel safety there exist so far no regulations against aggressive products.
Typical Situation I: The practical situation in the above cases is that of application directly at the tunnel face, mostly spraying on or through mesh reinforcement and lattice girders or steel beams, working on small areas at a time and quickly applying layer thicknesses of > 150 mm. A high early strength is often mandatory for safety reasons.
In the London subway tunnelling the normal procedure when using sprayed concrete lining is to first apply a primary, temporary lining. So far there exist no regulations requiring the use of alkali-free accelerators for this. However, because of the very good results with temporary wet-mix sprayed concrete, the use of sprayed concrete also for the permanent lining is getting increased interest, as described by Annett and Varley. To produce a high quality, durable concrete with marginal final strength reduction, an «alkali-free» accelerator was required. In this case (Jubilee Line, Contract 104), the primary purpose of an alkali-free accelerator was that of durability and hence no alkali cations. Also at Heathrow Express Tunnel the first section of permanent sprayed con-crete lining with alkali-free accelerator has been carried out. Especially in Scandinavia, but also more and more in other areas, permanent sprayed concrete linings are being applied in a separate construction phase, well behind the face area.
These examples could be summarized as Typical Situation II: The practical situation is that of a systematic application on relatively large areas, well behind the tunnel face, partly on mesh reinforcement and
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lattice girders, but to a growing extent using steel fibres. Final layer thicknesses may be large, but can be built in passes and a very high early strength is therefore not required.
In Scandinavia (shallow tunnels) the standard support solution can be termed as single-shell sprayed concrete lining or one-pass sprayed concrete lining. This approach is clearly getting increased attention also in other parts of the world. Basically, it means that the application of sprayed concrete at the face is carried out under quality requirements allowing it to be considered as part of the final and permanent sprayed concrete lining. »Single shell» means a single structure, which may well be produced in a number of steps (at the face and behind), see also chap. 9. Road tunnels, railway tunnels, sub sea road and pipe tunnels, hydro power tunnels, the Gjøvik Olympic ice hockey rink etc. are exam-ples of this approach in Norway.
In the case of single-shell permanent sprayed concrete linings both of the two typical situations described above apply. The basic difference is that the quality requirements are the same throughout the construction.
The different practical requirements may call for different accelerators at the face and at later stages, depending on cement type and other local requirements. It has therefore become quite clear from tests with a range of different accelerators that it is not possible to cover all applica-tion situaapplica-tions with a single product.
Properties
UGC International has made an important technology breakthrough with the launch of a range of liquid alkali-free and non caustic accelera-tors for both wet-mix and dry-mix sprayed concrete. With this step UGC International puts itself ahead of the state of the art: Products in powder form are very difficult to use in practice.
Unlike most traditional accelerators, the alkali-free and non caustic products from UGC International cause no or only minimal decrease in final strength. Requirements as demanded for HPS (durable sprayed concrete), e.g. for single shell permanent sprayed concrete linings, can be met more easily. But the most important innovation presented by the alkali-free accelerators launched by UGC International is the safe work-ing environment: Less danger of skin burns for the operators.
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Physical form (1) liquid liquid liquid liquid
Alkali cations (2) <0.5% <0.8% <0.5% <0.5% pH value at +20°C (3) 2.5–3.5 2.4–3.4 2.5–3.5 3.0–4.0 Layer thickness (4) 300 mm 300–700 mm 300–700 mm 300–500 mm
Dosage (5) 4–10% 3–7% 3–10% 3–10%
Early strength devel. (6) good extremely good
extremely good
very good
Corrosiveness (7) high moderate moderate moderate
Equipment (8) stainless stainless stainless stainless
Effect on skin (9) Xi Xi Xi Xi
Handling (10) simple simple simple simple
Cement sensitive (11) no no no no
Comments on Table 3:
(3) MEYCO® SA160 / SA162 / SA167 / SA180 have a pH value of 2.5 to 4.0 and are therefore acidic. MEYCO® SA160 / SA162 / SA167 / SA180 are aggressive to steel and this requires acid resistant steel quality for equipment in direct contact with these products (before spraying).
Once the alkali-free accelerators have been added at the nozzle, they are instantly neutralised by the alkali-rich cements. There is no corrosion risk to steel reinforcement.
(4) Minimum layer thickness that can be applied in the roof in one pass, provided a reasonably compatible cement quality has been chosen.
With well suited cement types, these thicknesses can be substan- tially increased, although it is sound application technique not to push the limit. MEYCO® SA160 / SA162 / SA167 / SA180 have proven to produce extremely large thicknesses, sometimes of up to 700 mm in one pass.
(5) The dosage is given as percentage by weight of binder (cement plus any pozzolanic additives). The lowest values indicated are achieved with good cements (high compatibility), while the maxi- mum dosage may be used with poorer cements or if high early strength is required. It may be mentioned that dosages above the
0
given maximum will produce no added quality, but can cause trick- ling and fall down and will reduce the final strength. Within the indi- cated dosage range there will be no or only marginal final strength loss.
In some cases the final strength is substantially higher than for the same concrete without accelerator.
(6) All products show excellent early strength development from about 1 hour upwards. The rating relates to the first few minutes and can be seen as a parallel to the possible layer thicknesses applied in the roof. MEYCO® SA160 / SA162 / SA167 / SA180 are «very good»
with almost all cements and are partly excellent (meaning as good as, or better, than best results of caustic aluminates).
(11) MEYCO® SA160 / SA162 / SA167 / SA180 work with most cements.
Even with blended cement types causing problems with all other accelerator types, they often work very well.
... Alkali-free accelerators in powder form
The approach with powder products involves numerous practical limita-tions and constraints:
< Costs for an additional dosing unit
< One additional man for the dispensing of the accelerator into the dos-ing/dry spraying machine
< Higher dosages: approx. 7–10 % by b.w.
< More air to feed into the dosing unit (4–5 m3 / min)
< Higher rebound. Experience from practical tests has shown that the rebound is 10–15 % higher as compared to liquid alkali-free accel- erators.
< The approach with powder products is not practical in modern rapid tunnelling where wet-mix high performance steel fibre rein- forced sprayed concrete plays an important role: The set up of the equipment between each spraying cycle is too complicated and too time consuming.
< In addition to all these practical and economical limitations and
con-straints, powder products also raise environmental concerns: They create a lot of dust and therefore cause a very bad working environ-ment.
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