Subtleties of CTS®/IBO®  Injection Anchors

Requirements for Hollow Bars in geo-technical applications

CTS®/IBO®  hollow bars are marketed in North America exclusively by Con-Tech Systems as CTS®/IBO®  hollow bars. They are the most copied hollow bars in the world. 

The application of hollow bars for micropiles, soil nails and anchors was developed and introduces into North America by Con-Tech Systems  in 1985.

In the meantime, the European Union (EU) has implemented two standards, according to the ‘EU Construction Products Directive’: EN 14199:2005 Micropiles and EN 14490:2010 Soil Nails. In North America, ASTM A615 can be relied upon for the requirements of the thread type deformations and shape.

In Germany, additionally, a National Approval is mandatory for directly drilled and pressure grouted Micropiles. It concerns design, application and installation. It requires permanent independent production control, and identification marks on all components. It simplifies the application of Micropiles for consultants, contractors and owners. Con-Tech Systems  can provide test and results from independent test labs in North America.

The well known GEWI-bar for Micropiles is a solid hot-rolled thread bar with deformations. It was introduced by DSI (Dywidag systems International) and has the German National Technical Approval and is now accepted worldwide. It demonstrates that the National Technical Approval has promoted the application of the GEWI-bar and has given consultants, contractors and owners confidence. Con-Tech Systems  supplies the same kind of bar, hot rolled thread bar (HRTB™) made in the USA.

In North America, only recommendations are available for hollow bars. They are published by the FHWA (Federal Highway Administration): e.g.

-           Hollow-Bar-Soil-Nails, Publication No. FHWA-CFL/TD-09-001, June 2009

-           Hollow-Core-Soil-Nails/State of the Practice-Federal Highway Administration, April 2006

The dominating factor in hollow-bar pricing is steel quality. It is worthwhile to understand the requirements for hollow-bars, as mandated by the above mentioned EU standards.

EN 14199:2005 “Micropiles”, clause 6.2.1 and EN 14490:2010 “Soil Nails”, clause 6.2.2.2 require:

Yield Stress, fyk, between 400 to 600 MPa (60 to 87 ksi)

These are the same requirements as in EU standards for reinforced concrete, EN 1080 “Rebar” and EN 1992-1:2004 (Eurocode EC2) “Design of concrete structures”. In the USA specifications ASTM A615 and ASTM A706 are applicable. A615 only for the thread type deformations, A706 for the maxCE Value.

The yield stress is limited to 600 MPA (87 ksi) for strain compatibility reasons in composite materials like reinforced concrete and composite Micropiles. The largest allowable strain for cement stone is limited to 0.003 (Euro Norms), or 0.0035 (ASTM A706) to avoid bursting (splitting) and longitudinal cracks in composite Micropiles. Consequently EN 14199, clause 7.6.6 and EN 14490, clause 6.2.2.5, allow corrosion protection by cement stone cover (grout cover) alone only for steel yield stresses of less than 600 MPA (87 ksi).

High yield steels are not accepted for micropiles and neither are oil field tubes according to EN 11960, quality N80.

Ductility (Elongation Agt > 5% required by EN 10080, ASTM A615, ASTM A370)

Ductility means that in design calculation, assumptions for plastification and rotation of hinges are accepted. Agt denotes the elongation up to ultimate load without reduction of cross-section. This is not to be confused with total elongation, A, up to rupture, as described in ASTM A370 and stated on mill certificates. Usually elongation, A, has double the value of Agt. The ductility of Agt > 5% is required for the finished hollow-bar, not for the steel tube it is manufactured from and which is given on the mill certificate of the steel mill.

Ductility can be checked on site by a bending test to 180 degrees as described in ASTM 615A section 10.

Charpy impact bending test with hollow bars (absorbed energy); (Charpy impact testing according to ASTM A370).

Drill Operators know that drill tools are forged from high yield, heat treated, ductile Chrome-Nickel alloy steels to withstand roto-percussion drilling and torques by deviation. Hollow-bars from Cr-Ni alloys would be too expensive and would not fulfill the requirements for re-bar. Damaging the hollow bar during installation must be avoided under all circumstances.

In order to avoid overloading of the bars during drilling, Con-Tech Systems  is using, since the beginning, quality fine-grain structural steel S460 NH according to EN10210 “Seamless Tubes”. This material has the highest available Charpy impact bending resistance with more than 100 J (74 ft lbf) at 20°C (68°F) and still more than 40 J (29.5 ft lbf) at -20°C (-4°F). The success of the CTS®/IBO®  hollow-bar is based on this tough and ductile steel since 35 years.

For comparison, other fine grain structural steels according to EN 10210, e.g. S355, have a Charpy impact bending resistance of only 27 J (20 ft lbf) at 20°C (68°F) and no resistance at all at -20°C (-4°F). Oilfield tubes for gas or petroleum don’t have impact bending resistance values comparable to S460 NH steel used by Con-Tech Systems .

SUMMARY

To make sure that a hollow bar product is suitable for the task, technical data should be verified. Important quality parameters as outlined in this article are:

•           Yield and Ultimate load of the finished hollow-bar

•           Ductility of the finished bar (AGT larger than 5%)

•           Material composition, as shown on mill certs from steel mill

•           Charpy impact bending resistance

Other significant parameters include

•           Deformations, ribs, to increase shear bond (ASTM A615)

•           Ultimate load of coupled hollow-bars

Manufacturing quality parameters are

•           Test reports from independent laboratories

•           Permanent production control

•           Quality management system ISO 9001

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