Development of a High Early-Strength Anchor Mortar for Tunnel Construction

This article describes the development of a high early strength anchor mortar that is to be used in tunnel construction to secure the outer lining of the tunnel. Anchor mortars currently available on the market have a curing time of one to several days, while the anchor may not yet be loaded. Accelerating the curing process could significantly speed up the load-bearing capacity and improve the utilisation of the anchors. In this context, the early creation of a load bearing vault plays an important role, as it would allow to reduce the thickness of the outer lining. The construction progress could be accelerated by an anchor mortar with high early strength, thus contributing to cost savings.

1 Introduction

In tunnel construction, a reduced construction time leads to significant cost savings. Therefore, the demand for faster construction processes is high. Any curing time that exceeds the required working time is an economic and technical disadvantage. Also, due to construction constraints, anchoring may be scheduled for specific time windows (e.g. night shift). This procedure requires a high early-strength anchor mortar. Furthermore, in areas where it is necessary to ensure the full load-bearing capacity of the anchor within a short period of time (e.g. fault zones), so far mainly folded steel tube anchors, which are expanded with water pressure (friction anchors), can be used [1]. The use of anchor mortar with high early-strength would expand the spectrum of alternative anchoring systems. Figure 1 shows the intended strength development (orange) in comparison to the strength developments of products currently on the market, which were determined in preliminary tests in the laboratory.

1 | Target strength development of the high early-strength anchor mortar
Credit/Quelle: FH Münster, IuB

1 | Target strength development of the high early-strength anchor mortar
Credit/Quelle: FH Münster, IuB

The aim of the research project presented here was to develop different anchor mortar mixes that have a much faster strength development than all products currently available on the market, but at the same time fully meet the high current requirements for anchor mortars.

This development work was carried out in cooperation between Münster University of Applied Sciences and AVG Mineralische Baustoffe GmbH within the framework of a research project of the “Zentrales Innovationsprogramm Mittelstand” (ZIM), which is funded by the German Federal Ministry for Economic Affairs and Energy (BMWI).

2 Load-Bearing Effect of Anchors in Tunnel Construction

In tunnelling, single anchors can be used to secure individual rock blocks, wedges or benches, for example. These measures also serve to protect the excavation teams and to maintain the planned excavation geometry. This involves securing coarse blocks of rock to prevent them from sliding. A dowel effect of the anchoring also increases the friction in the rock strata.

With an anchor system, the loosening zone that occurs during the excavation of the tunnel is reduced. The installation of a regular anchor grid supports the formation of a load-bearing ring support. Since the stress redistributions will expand into the interior of the rock over time, an early prevention of the disintegration by the anchor system should be aimed for [2]. In combination with the sprayed concrete lining, the anchoring continues to create a bonding effect with the rock. Figure 2 shows the different areas of application for anchors in tunnel construction.

2 | Use of anchors in tunnel construction
Credit/Quelle: FH Münster, IuB

2 | Use of anchors in tunnel construction
Credit/Quelle: FH Münster, IuB
The faster the full anchor support effect is activated, the lower the loosening pressures can be and the required support resistance (e.g. thickness of sprayed concrete lining) can potentially be reduced.

The excavation of a tunnel changes the stress conditions in the rock and causes a redistribution of the stresses. The three-dimensional stress state changes into a two-dimensional stress state. Figure 3 shows the tangential and radial stress states at the breakout rim. Due to the excavation of the tunnel, the radial stress at the breakout rim is lost. In the crown area, a loosening zone forms, which increases in size as time progresses. The emerging support ring forms around the loosening zone and moves outwards as loosening increases [2].

3 | Load-bearing principle of an anchor system
Credit/Quelle: FH Münster, IuB

3 | Load-bearing principle of an anchor system
Credit/Quelle: FH Münster, IuB

Mohr‘s circle [3] shows that an increase in cohesion (“anchor cohesion”) is associated with the anchoring used at the breakout rim (see Figure 3). The installation of an anchor system in combination with sprayed concrete restores the three-axial stress state in the cavity and counteracts the disaggregation. Therefore, the aim is to create this triaxial stress state as soon as possible after excavation of the tunnel and thus to keep the resulting rock pressure as low as possible by installing a sprayed concrete lining and an anchor system [2].

The compressive stresses acting on the anchor plates must be transferred back via the anchor, the mortar and the rock in the anchor hole (see Fig. 4). In this case, a joint stress is created between the rock bolt and the anchor mortar as well as between the anchor mortar and the rock mass. The forces acting in the anchor mortar can be described in a simplified way using the displayed framework.

4 | Forces acting on the composite anchor
Credit/Quelle: FH Münster, IuB

4 | Forces acting on the composite anchor
Credit/Quelle: FH Münster, IuB
Possible failure types of the composite anchor can be:

1. rock bolt failure due to exceeding of the steel strength

2. bond failure in the anchor/mortar area

3. mortar failure (shear force failure)

4. bond failure in the mortar/rock area

5. rock failure

 

In order to avoid early failure caused by the mortar (type 2–4), it must be designed to endure sufficiently high bond and shear stresses. Thus, the aim of this research was to create a mortar with sufficient resistance, so that failure occurs when the tensile strength of the rock bolt or the rock is exceeded.

3 Laboratory Tests

3.1 Scope of the Examination

Figure 1 shows the strength development of anchor mortars currently available on the market. It should be noted that there are no high early-strength anchor mortars with curing times of 6 to 8 hours available on the market for tunnel construction.

After examining the systems currently in use, a mortar mix was developed that takes the following factors of influence into account:

Different soil and rock conditions

Different curing time requirements (processing time)

Different hydrological influences

Different areas of application (crown area, bench area)

Different injection methods (via anchor head, mortar pump)

 

First, the mix design of the high early-strength mortar was developed in iterative small-scale tests. With the help of the resulting mix design, large-scale laboratory tests were then carried out. The focus was on making the tests as realistic and practical as possible.

3.2 Small-Scale Tests

3.2.1 Scope of the Examination

During the mortar tests, the consistency and processing properties as well as the early and final strengths were determined. The focus was on a high early strength and good processability of the mortar. Furthermore, the determined mortar mix design was analysed regarding hydrological influences and the influence of different temperatures.

3.2.2 Processing Capability

The high early-strength anchor mortar was designed for a processing time of up to 45 minutes in order to ensure the maintenance of the construction site process even in the case of delayed installation. The processing and consistency properties were determined using the flow table according to DIN EN 1015-3 [4] and DIN EN 12350-5 [5]. This procedure enabled the comparability of the iteratively developed mortar mix designs.

Table 1 | Determination of spreading factor in accordance with DIN EN 12350-5 [5]

Table 1 | Determination of spreading factor in accordance with DIN EN 12350-5 [5]
Due to possible deviations caused by the addition of water on site, a range of w/c-values was investigated in which the mortar guarantees practicable application and sufficient strength development. The lower w/c-value must at least be achieved during mortar production in order to enable pumpability. The upper w/c-value is limited by the intended strength development. The result was a range with upper and lower w/c values 10 % apart. Table 1 shows the spreading dimensions of the anchor mortar after mixing and after 30 and 45 minutes.

The anchor mortar shows constant processability in the lower w/c-value range, with consistency ranges of F4 (after mixing and after 30 min) and F3 (after 45 min). By increasing the w/c ratio by 10 %, the anchor mortar achieves consistency ranges in the flowable range. The pumpability of the developed mortar was determined by pumping tests with a screw pump.

3.2.3 Strengths

As previously described, the decisive stress for the anchor mortar is its bond stress, which was investigated within the framework of the large-scale laboratory tests. Since this test is associated with a high level of effort, the compressive strength fck of the anchor mortar was first determined as an indicator of the bond strength. It was assumed that increasing compressive strength is accompanied by a general increase in bond strength. The compressive strength was determined according to DIN EN 196-1 [6] using mortar prisms (160 x 40 x 40 mm).

The strength determinations are based on mortar production with the increased w/c ratio of 10 % and are thus on the safe side. Figure 5 compares the strength development of the newly developed anchor mortar and the anchor mortars available on the market.

5 | Comparison of strength developments
Credit/Quelle: FH Münster, IuB

5 | Comparison of strength developments
Credit/Quelle: FH Münster, IuB

According to the DIN EN 196-1 test [6], the anchor mortar developed achieves an average compressive strength of more than 14 N/mm² after only 6 hours at the specified storage temperature of 20°C and an average compressive strength of more than 33 N/mm² after 8 hours. In contrast, conventional anchor mortars do not reach these strengths at 20°C until 14 to 20 hours later.

In order to reflect the application in practice, the strength development was tested under different temperatures, as the external and/or ambient temperature has a significant influence on the hydration of the mortar and thus also on the strength development [7]. In tunnelling, for example, the effect of heat extraction could be caused by a colder rock mass, whereby the effect would be amplified with increasing difference between fresh mortar temperature and rock temperature. To counteract the cooling of the fresh mortar at low temperatures, the dry mix could be heated or stored warm, or warm mixing water could be added during the mixing process. In addition to the requirements of DIN EN 196-1 [6], compressive strength tests were carried out at storage temperatures of 15, 10 and 5°C as part of the research project.

For the series of tests at 10 and 5°C, warm mixing water with a temperature of 25°C was used. To achieve results that are as close to practical application as possible, two different types of formwork were used in addition to the different temperatures: steel formwork (steel grade: C45 and C60, see Fig. 6 min. value) with a thermal conductivity coefficient λ of approx. 48 W/(m*K) and plastic formwork (PE-UHM, see Fig. 6 max. value) with a thermal conductivity coefficient λ of approx. 0.42 W/(m*K). The thermal conductivity of the rock will be between these two limit values. Figure 6 shows the strength development at the storage temperatures 20, 15, 10 and 5°C.

6 | Strength developments at different storage temperatures
Credit/Quelle: FH Münster, IuB

6 | Strength developments at different storage temperatures
Credit/Quelle: FH Münster, IuB

For the required initial support and stabilisation of the rock, the first objective in the development work was to achieve a compressive strength after 6 to 8 hours that is equivalent to the compressive strengths of the anchor mortars available on the market after 24 hours (see Figure 1). The target value of ~15 to 20 N/mm² (red marked area in Fig. 6) is reached after ~ 6 hours (20°C), ~ 9 hours (15°C), ~ 12 hours (10°C) and ~ 20 hours (5°C) depending on the ambient and subsoil temperature.

3.2.4 Hydrological Influence

Since in practice hydration of the mortar can also take place under existing groundwater or accumulating mountain water, investigations were carried out into the hydrological influence. These included adding fresh mortar into formwork stored in water and displacing the water. It was shown that the curing time of the mortar is longer under continuous hydrological influence. The mortar was able to displace standing water without segregating. However, 6- or 8-hour strengths could not be achieved. It should therefore be noted that the early strength of the mortar under hydrological influence is significantly dependent on several parameters: Water temperature, pressing water, drop height, water displacement and segregation of the mortar during placement.

3.3 Large-Scale Laboratory Tests

3.3.1 Approach

After the investigations on small test specimens, the mortar was tested for its suitability in a realistic setting. For this purpose, a rock borehole was simulated in the laboratory using a concrete body (40 x 40 x 100 cm) with a central borehole (diameter 55 mm). The SN anchor technique was tested, in which the anchor is pressed into the borehole previously filled with mortar. The borehole is backfilled with the help of a screw pump. The processability of the anchor mortar was tested on vertically, horizontally and overhead placed concrete bodies. The bond strength of the mortar was then tested using the test set-up of the pull-out test shown in Figure 7.

7 | Test set-up for pull-out test
Credit/Quelle: FH Münster, IuB
7 | Test set-up for pull-out test
Credit/Quelle: FH Münster, IuB
3.3.2 Results of Large-Scale Lab Tests

Processability: The newly designed anchor mortar proved to be pumpable in all three positions and the anchor mortar was successfully placed. When the mortar was placed vertically overhead, the borehole could not be completely filled with the targeted w/c ratio, as the mortar partly seeped back before the borehole was closed.

Failure cases and strengths: During the pull-out tests, different failure cases occurred depending on the strengths, which were determined separately by means of mortar prisms. It was determined that the failure load of the rock bolt (d = 28 mm, B500) of approx. 307 kN is reached when the compressive strength of the mortar exceeds ≥ 10 N/mm². For mortar strengths fck < 10 N/mm², on the other hand, the failure case was always the bond failure between rock bolt and mortar, which occurred abruptly. An overview of the entire test results is shown in Table 2. The mentioned strengths refer to a mortar bond length of 1m.

Table 2 | Results of pull-out tests

Table 2 | Results of pull-out tests

Based on the results of the pull-out tests, the previously assumed minimum compressive strength of approx. 15 to 20 N/mm² can be reduced to 10 N/mm². This assumption is based on the pull-out tests on the 1m long concrete bodies with SN anchors. The required minimum compressive strength can vary depending on the used anchor system, borehole diameter, bedrock and bond length. The anchoring length in tunnel construction is several metres, which means that a reduced minimum compressive strength can generally be assumed.

Assuming a minimum compressive strength of 10 N/mm² results in lower curing times for the mortar, depending on the outside temperature (Fig. 8). The target value of approx. 10 N/mm² (red marked area in Fig. 8) is reached after ~ 5.5 hours (20 °C), ~ 8 hours (15 °C), ~ 10 hours (10 °C) and ~ 18  hours
(5 °C), depending on the outside and ground temperature.

8 | Strength developments at different storage temperatures
Credit/Quelle: FH Münster, IuB

8 | Strength developments at different storage temperatures
Credit/Quelle: FH Münster, IuB

4 Practical Construction Testing

After development and testing in the laboratory, the mortar was submitted to practical construction tests for a realistic assessment. For an initial practical test, the mortar was inserted into additional boreholes with a borehole diameter of 100 mm at a descending inclination of 20° for slope stabilisation. Self-drilling anchors d = 32 mm and SN anchors d = 28 mm were inserted into these boreholes over a length of 4 m and backfilled with a screw pump commonly used on construction sites. In the anchor test after 24 hours at 5°C outside temperature, the anchors proved to be load-bearing. If the w/c-value is maintained and depending on the ambient temperature, the results from Fig. 8 can be estimated. It was thus possible to produce a high early-strength composite anchor on a cementitious basis.

5 Conclusion and Outlook

The anchor mortar developed fulfils the technical requirements of a high early-strength anchor mortar. The individual parameters of the anchor mortar were examined, evaluated and optimised during the tests at the Institute for Underground Construction at Münster University of Applied Sciences. Large-scale experimental tests proved that the anchor mortar can be used for practical applications under laboratory conditions. Since the focus of this work was on laboratory tests, more practical tests with different boundary conditions are necessary to make well-founded statements about the practical application. For a general application, the boundary conditions must be taken into account under which the anchor mortar is to achieve high early-strength results.

The authors would like to thank F. Basler M.Sc. and P. Voitenko M.Sc. for their cooperation and support.
References/Literatur
[1] Müller, L.: Der Felsbau, Dritter Band: Tunnelbau, 1978, Enke Verlag, Stuttgart.
[2] Maidl, B.: Handbuch des Tunnel- und Stollenbaus, Band I, 3. Auflage 2004, Verlag Glückauf, Essen.
[3] Schnell, W., Gross, D., Hauger, W.: Technische Mechanik 2, 1992, Springer-Lehrbuch, Berlin.
[4] DIN EN 1015 Prüfverfahren für Mörtel für Mauerwerk – Teil 3: Bestimmung der Konsistenz von Frischmörtel (mit Ausbreittisch);
Deutsche Fassung EN 1015-3: 1999 + A1 2004 + A2, 2006, Beuth Verlag, Berlin.
[5] DIN EN 12350-5: Prüfung von Frischbeton – Teil 5: Ausbreitmaß, Deutsche Fassung EN 12350-5, September 2019, Beuth Verlag, Berlin.
[6] DIN EN 196-1: Prüfverfahren für Zement – Teil 1: Bestimmung der Festigkeit; Deutsche Fassung EN 196-1, November 2016, Beuth Verlag, Berlin.
[7] Bendix, R.: Bauchemie - Einführung in die Chemie für Bauingenieure und Architekten, 7. Auflage 2020, Springer Vieweg Verlag, Wiesbaden.
x

Related articles:

Issue 2014-06 Bolting Technology

Mineral Dry Mortar

Sika Deutschland GmbH offers SikaRock Anchor Mortar 1 and SikaRock Anchor Mortar HS – two hydraulically hardened and pure mineral dry mortars – for anchoring rock nails and filling anchor drill...

more
Issue 2012-05 Fire Protection Mortar

Adler Tunnel – Renovation with Fire Protection

Requirements The reinforced concrete bars and the rock anchor heads had to be protected for 60 minutes in accordance with the ISO 834 fire curve in the event of fire so that the tunnel inner shell...

more
Issue 2015-05 Hagerbach Test Gallery

Shotcrete: Early Strength and Strength Development

Shotcrete has developed from an auxiliary building agent to an efficient construction material. This is due to intensive development of the starting materials and processing equipment as well training...

more
Issue 2009-06 Construction Equipment

Australia’s longest Road Tunnel in Brisbane

The city of Brisbane on the East Coast of Australia is Queensland’s industrial, economic and financial center. Many service companies are located in the city center. Brisbane Central Business...

more
Issue 2013-03 Anchoring

„Semmering Base Tunnel New” – Securing the Precut with ­Self?Drilling Anchors

In April 2012, work started on one of the most ambitious construction projects of Austria’s ÖBB-Infrastruktur AG: the New Semmering Base Tunnel. Scheduled for completion in 2024, the 27.3 km long...

more