the spraying nozzle. These accelerators are mostly based upon salts of aluminates or silicates, and cause «strength losses» at 28 days, the greater strength losses mostly being experienced with aluminates. In reality, strength losses do not generally occur, but rather a lack of strength gain after 3 days takes place, when compared with the same unacceler-ated mix as a reference.
With the hydration control system, stabilized and activated sprayed con-cretes can also exhibit depressed or marginal strength gains between 3 and 28 days similar to conventionally accelerated sprayed concretes.
However, there are several types of Delvo®crete Activators for rock sup-port. The choice and dosage of a particular type is dependent upon cement quality as well as early strength and setting time requirements.
It is interesting to note that the hydration controlled mix requires a lower dosage of Delvo®crete Activator than the dosage of accelerator a nor-mal mix would require. This is probably due to the fact that because the cement in the stabilized mix was well dispersed and still «fresh» (i.e.
unhydrated), the Delvo®crete Activator is able to act more efficiently than the accelerator used in the normal mix.
..8 Rebound
The rebound of sprayed concrete mixes is a large «add-on» cost in an underground construction project and it is in the interests of the contrac-tor and owner to keep these costs to a minimum. In field tests in Europe and Asia, it has been found hydration controlled sprayed concrete mixes exhibit lower rebound than plain mixes. This is probably due to the fact that in such mixes, no pre-hydration of the cement has taken place and, consequently, «fresh» cement is being sprayed continually, regardless of the time elapsed between batching and spraying.
In the Flurlinger Tunnel project in Switzerland measured rebound tests comparing normal sprayed concrete with a hydration controlled sprayed concrete showed that the rebound of the stabilized mix was up to ten percentage points lower than that of the normal mix.
10
Mix design (per m3) of sprayed concrete, Flurlinger Tunnel, Switzerland:
Cement 425 kg
Aggregate (0–16 mm) 1 730 kg
Superplasticizer 0.8 %
Delvo®crete Stabilizer 0.6 %
Delvo®crete Activator S51 5.0 %
w / c ratio 0.45
Workability (DIN Flow Table):
At batching 600 mm
After 4 hours 580 mm
After 9 hours 560 mm
Compressive strength (28 days):
Required minimum 25 MPa
Average achieved 33.5 MPa
Measured rebound (mechanized spraying): 8 to 10 % (average) Originally, the contractor started to use stabilized sprayed concrete at night because local environmental legislation restricted the operation of the site installed concrete batching plant to between 7:00 a.m. and 10:00 p.m. This meant that the contractor could not obtain concrete mixes after 10:00 p.m. To overcome this problem, several truck loads of stabilized sprayed concrete were batched into truck mixers before 10:00 p.m. and then driven into the tunnel and sprayed, as required, throughout the night. However, due to the positive experiences in rebound reduction and improved quality and efficiency, the contractor eventually opted to use stabilized concrete mixes in the daytime spray-ing operations as well, the cost of the hydration control system chemi-cals being more than offset by the reduction in rebound. This resulted in substantial savings on the project.
..9 Economics
The estimating and costing of the various components relating to the final in-place price of sprayed concrete in a large underground infra-structure project is a difficult and complex task. Many of these costs are overlooked or underestimated by contractors. They may be sum-marised as follows:
< The cost of the sprayed concrete mix per m3
10
< The cost of in-place applied sprayed concrete, allowing for rebound
and overspray. (This may be 100 % higher than the delivered m3 cost.)
< The cost of collecting, loading, carting away and dumping of rebound
from a project to a suitable approved site (environmental legislation)
< The cost of returned sprayed concrete that has to be rejected
because it is too old to use
< The cost of waste sprayed concrete that has to be washed out of
pumps and hoses when breakdowns and delays cause interruption to spraying operations, and at the end of spraying shifts
< The cost of breaking out, removal and replacement of areas of
defec-tive sprayed concrete resulting from the application of material that may have been too old
< The cost and maintenance of sprayed concrete pumps and equipment
< The cost and maintenance (per m3) of a «trixer» unit if being used on
a project
< The cost of setting up a site batching plant on the project if the
near-est ready-mix concrete plant is too far from site, and the 24-hour operation of the plant
< The cost of down time in equipment and personnel when deliveries
of sprayed concrete are delayed, or interrupted
Many of these costs are difficult to quantify and estimate at the tender stage of a project and consequently get overlooked. It may be seen, however, that a reduction in rebound alone has far more cost advan-tages to a contractor than by merely having to purchase less material.
By utilizing the Delvo®crete hydration control system in large volume wet-mix sprayed concrete projects, cost savings may be made in the following areas:
< Reduction in rebound
< Less rebound to be carted away and dumped
< No sprayed concrete deliveries have to be rejected because they are
too old
< Waste sprayed concrete in pumps and hoses does not have to be
washed out and disposed of when interruptions occur, nor at the end of a spraying shift
< Less defective in-place sprayed concrete due to the fact that all
applied sprayed concrete contains cement that has not undergone any pre-hydration and has not expired its «pot-life»
10
< The necessity for «trixer» units is eliminated
< Site batching plants may be eliminated (dependent upon other
con-crete requirements on the project)
< The cost of overtime and down time in plant and personnel may
be reduced by ensuring adequate sprayed concrete deliveries are always available at the excavation face
Table 5
TOTAL US$ 121.10 US$ 133.86
The costs of using the Delvo®crete hydration control system in sprayed concrete can therefore be extremely economical. The dosage of the Delvo®crete Stabilizer is in the range of 0.4 to 2.0 % (b.w.). This will stabilize the cement for between 3–4 hours and 3 days. The dosage of Delvo®crete Stabilizer will obviously depend upon the requirements of the contractor, but, generally, a dosage of 0.6 % by weight of cement is used in most works.
10
The dosage of Delvo®crete Activators is normally equal to (or slightly less than) the dosage of accelerator that would be required by the same sprayed concrete mix that contains no Delvo®crete Stabilizer.
Table 5 shows the cost comparison of conventional and hydra-tion controlled wet-mix sprayed concretes for rock support with coarse aggregate (0–16 mm). If the conventional mix had 20 % rebound, then the in-place cost of the sprayed concrete would be US$ 121.10 / 0.8 = US $ 151.38 / m3. In order to make the in-place cost of the hydration controlled sprayed concrete of the same order, then the rebound of that mix should be (1–133.86 / 151.38) X 100 = 11 %.
It is not unusual for rebound reductions of this magnitude (40–50 %) to be achieved, as was done by the contractor in the Flurlinger Tunnel project.
In the case of higher cost steel fibre reinforced sprayed concrete (which may cost in excess of US $ 200 per m3), an even smaller reduction in rebound will be required to offset the cost of the hydration control sys-tem chemicals.
Apart, however, from rebound savings, the contractor will also make cost savings by eliminating return and waste sprayed concrete, being able to reduce down time of sprayed concrete pumps and personnel and generally being assured that every cubic metre of sprayed concrete applied will be of a consistent quality.
..10 Summary
< Hydration controlled sprayed concrete mixes are an economical
and efficient development for use in rock support for large under- ground infrastructure projects.
< By suspending the hydration of the cement (and pozzolans) in a
sprayed concrete mix until it is reactivated and accelerated at the spraying nozzle, a consistent quality of applied sprayed concrete may be assured.
< Cost savings in rebound reduction alone can offset the additional
costs of the hydration control system.
< Other cost savings, resulting from reduced rebound and eliminating
waste sprayed concrete may be achieved by contractors.
10
< Hydration controlled sprayed concrete mixes enable greater flexibility
in the scheduling and programming of spraying operations in under-ground construction projects.
..11 Selected case studies
The flexibility and advantages of modern high performance wet-mix SFRS is directly and tightly lined to the correct use of admixtures and proper equipment. Some examples follow, where a selection of features has been made, to demonstrate some of the more important aspects of practical life sprayed concrete application.
A) Wet-mix spraying with Delvo
®crete Athens Metro
The civil construction part of the project comprises 20 stations and 18 km of tunnels. In the tender documents, the dry-mix method was specified. It was, however, possible to convince the contractors that wet-mix spraying is beneficial in all aspects.
From a central mixing plant, the concrete is distributed by truck mixers to a number of sites. The individual sites have a buffer storage for con-crete, holding about 12 m3 maximum. The buffer is also an agitator that can be operated when necessary. The concrete goes from the agitator into a concrete pump, delivering through a pipe system down the shaft (typically 20 m deep) into the tunnel, ending in the MEYCO® Suprema spraying pump. To the spraying nozzle it is normally 100 to 150 m.
A normal work sequence means application of 3 to 4 m3 of sprayed concrete and then a full stop of 3 to 4 hours until the next application.
During this time, the whole system from day-buffer to the sprayed con-crete nozzle is left untouched with concon-crete inside. A full cleaning of the delivery system is carried out about once a week.
This logistics system is only possible due to the Delvo®crete Stabilizer. By adding a maximum of 2 % drawn on the cement weight, it is possible to prevent any hydration for up to 72 hours. When Delvo®crete is used for sprayed concrete, an activator must be used in the nozzle to start the hydration process.
107