1.8 Appendix
1.8.4 Appendix IV. Changes from an intergenerational per-
Tunnels and other underground construction projects have some of the worst conditions for curing due to the ventilation that blows continuously dry (cold or hot) air into the tunnel. It can be compared with concrete exposed to a windy area. One would think that tunnels have ideal cur-ing conditions with high humidity (water leakage), no wind and no sun exposure. However, this is not the case.
..1 Background
Curing is one of the basic and most important jobs in sprayed concrete because of the large cement and water content of the mix and the con-sequent high shrinkage and cracking potential of the applied concrete.
Another reason is the danger of rapid drying out due to the heavy ven-tilation as is usual in tunnels, the fast hydration of accelerated sprayed concrete and the application in thin layers. Therefore, sprayed concrete should always be cured properly by means of an efficient curing agent.
However, the use of curing agents involves several restrictions: They must be solvent-free (use in closed rooms), they must have no nega-tive influence on the bonding between layers and they must be applied immediately after placing of the sprayed concrete. Most of the in-place sprayed concrete around the world has no bonding and many cracks, due to the fact that no curing is applied.
With the use of sprayed concrete as permanent final lining, long-term quality and performance requirements have built up significantly. These requirements are: good bonding, high final density and compressive strengths to ensure freeze / thaw and chemical resistances, watertight-ness and a high degree of safety.
When curing sprayed concrete with a curing agent, one has to be very careful with the cleaning procedure of the substrate before applying a subsequent layer. Cleaning must be done with high pressure air and a lot of water (use spraying pump and nozzle, adding air at the nozzle).
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Another problem with curing agents is to be able to apply them quickly enough after finishing of spraying. To secure proper curing of sprayed concrete, the curing agents must be applied within 15 to 20 minutes after spraying. Due to the use of set accelerators, the hydration of sprayed concrete takes place a very short time after spraying (5 to 15 minutes).
The hydration and temperature are most lively during the first minutes and hours after the application of the sprayed concrete and it is of great importance to protect the sprayed concrete at this critical stage.
Application of curing agents requires two time consuming working operations: Application of curing agent and cleaning / removal of the curing agent from the sprayed concrete surface between the layers in the case of multiple layers. In many countries with experience in wet-mix sprayed concrete like in Norway and Sweden and in big projects world-wide, there is an obligation to cure the sprayed concrete with a curing agent.
Very good experiences have been made with the use of a special curing agent for sprayed concrete (Masterkure® 112). This product is solvent-free and easy to apply and remove. It is used in many big projects and in different countries, everywhere with very good results. The use of specially designed curing agents for sprayed concrete improves bond-ing by 30–40 % compared to no curbond-ing (air curbond-ing), reduces shrinkage and cracking and also gives a slightly higher density and compressive strength (at 28 days). These results are confirmed by several laboratory tests and field trials. However, in order to achieve these results, proper cleaning is required before subsequent layers of sprayed concrete can be applied. Even with easy-to-apply products, curing of sprayed con-crete remains a time consuming job and is often felt as a hindrance to other tunnelling operations.
.. Concrete improving with MEYCO
®TCC7
UGC International has developed a new system for more efficient and secure curing of wet-mix sprayed concrete, repair mortars as well as concrete.
Concrete improving (internal curing) means that a special admix-ture is added to the concrete / mortar during batching as a normal
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admixture. This admixture produces an internal barrier in the con-crete which secures safer hydration and better resistances than the application of conventional curing agents. The benefits resulting from this new technology are impressive:
< The time consuming application and, in the case of various sprayed
concrete layers, removal of curing agents are no longer necessary.
< Curing is guaranteed from the very beginning of hydration.
< There is no negative influence on bonding between layers.
As a consequence of this optimum curing effect, all other sprayed con-crete characteristics are improved: density, final strengths, freeze / thaw and chemical resistances, watertightness, less cracking and shrinkage.
In addition, MEYCO ® TCC735 has a beneficial effect on pumpability and workability of sprayed concrete, even with low-grade aggregates.
It particularly improves the pumpability of steel fibre reinforced sprayed concrete mixes, and also the fibre orientation, thus reducing rebound and raising toughness values.
.. A proven technology
The concrete improving system with MEYCO® TCC735 has been tested with good results both in laboratories and on big jobsites.
Comprehensive investigation programmes were carried out in Norway (SINTEF), in Switzerland (LPM Institute) and in Austria (University of Innsbruck). Bond strengths were higher than 2.0 MPa with failures dis-covered in the concrete only and not in the bonding area. Density and mechanical strengths at 28 days were more than 10 % higher than in conventionally cured reference sprayed concrete.
Example results from a large jobsite in Middle East:
< Increased bonding compared to no curing: > 100 % (from 0.5–0.7 to
> 2.0 MPa)
< Increased bonding compared to curing with special curing agent:
>30–50 % (from 0.7–1.2 to > 2.0 MPa)
< All cores of sprayed concrete treated with MEYCO® TCC735 show
bond > 2.0 MPa. Failures were discovered only in the concrete and not in the bonding area.
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< Increased density (> 15 %) compared to sprayed concrete treated
with external curing agents
< Increased strength (28 days) compared to air cured sprayed
con-crete or treated with external curing agent (> 10 %)
< No signs of cracking
Figure 21: Water absorption of a drilling core [g / cm2]
(Ref: M. Testor, Master’s degree at the University of Innsbruck, 1997)
Figure 22: Bonding of sprayed concrete drilling cores on sprayed concrete substrate [MPa]
(Ref: LPM test results from internal spraying trials)
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.. Benefits of concrete improving with MEYCO
®TCC7
< No influence on bonding between the layers. Always good bonding,
high security
< No additional work operation for the application of curing agents or
other curing methods
< No need for additional work operation for cleaning and removal of
curing agents
Table 6: Cost comparison per m3 of concrete improver, external curing and water curing from one of the biggest sprayed concrete works ever carried out: > 200 000 m3 of wet HPS sprayed concrete applied in a period of 2.5 years
< Curing from the first second and therefore during the critical time
< Less cracking
< Better chemical resistance
< Improved watertightness (less cracks)
< Improved freeze / thaw resistance
< Improved workability and especially pumpability
< Works independently from aggregate quality, grading and lack of
fineness
< Works particularly well with steel fibre reinforced sprayed concrete;
better fibre orientation, reduced fibre rebound and increased tough-ness values
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< Less time per m3 / m2 due to increased production and less work
operations. Time is money!
< Increased density
< Improved final compressive strengths
.. A safer and cheaper solution
< With MEYCO® TCC735 overall savings of the spraying job are
achieved: the elimination of additional work operations for applica-tion of curing compounds and preparaapplica-tion of substrate, as well as the reduced rebound and fibre rebound more than offset the extra material cost.
< MEYCO® TCC735, whilst guaranteeing safer curing, provides a new
state of the art application procedure of curing agents in the easy-to-apply form of a concrete admixture.
.. Results from some spraying tests
In the tests a great number of parameters have been fixed in order to evaluate the real performance differences of the three mixes and sys-tems.
117 Table 8: Mechanical performances of the three mixes
Reference
(no curing) External
curing Concrete improver (internal curing)
Flexural strength tests on concrete beams (10 x 10 x 40 cm), UNI 5133, MPa:
7 days
Pull-out test Rilem / CEB/FIP RC6, MPa:
7 days
Adhesion on concrete (*), MPa:
7 days
Static modulus of elasticity, UNI 6556, MPa:
7 days
Dynamic modulus of elasticity, MPa:
7 days
(*): The values are the average of two tests.
P: The breakings have occurred in the application, i.e. in the product.
I: The breakings have occurred at the interface between the applica-tion and the concrete slab.
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. Conclusion
Delvo®crete, the alkali-free set accelerators MEYCO® SA160 / SA162 / SA163 / SA167 / SA175 / SA176 / SA180 / SA181 and the concrete improv-ing system are the new generations of advanced sprayed concrete admixtures which set new standards in the world of sprayed concrete.
They contribute to improve quality and increase production, while at the same time lowering costs per cubic metre of in-place sprayed concrete and thus further promoting sprayed concrete as a construction mate-rial.
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. Fibres in sprayed concrete
Fibre concrete is a new material undergoing fast development with new and better fibres, hand in hand with improved concrete technology and application techniques.
Figure 23: Steel fibres as used for reinforcing of sprayed concrete for rock support
The use of steel fibre reinforced sprayed concrete has advanced sub-stantially in the last few years. It has been accepted for rock support by engineers, specifiers, owners and contractors around the world.