fibrereinforced plastic (frp) an innovative approach to...

Topics

DOI: 10.1002/mire.201400027

200 © 2014 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin · Mining Report 150 (2014), No. 4

Fibre-reinforced plastics, known as CFRP or GRP, are used as se-curing elements in the form of rock bolts or concrete reinforce-ment in mining and tunnel engineering, civil engineering and ge-otechnics. Applications according to exposure classes followingEurocode 2 [1] in the ground, in rock and concrete, which are ex-posed to enhanced chemical attack by groundwater and pollu-tant loading, mine water flow, alkalinity of the concrete, frost andthaw conditions and de-icing salt exposure, seawater and othermoist-warm changeover conditions, also place the highest de-mands on permanent chemical resistance, quite apart from thehigh mechanical demands on the composite material.The applications discussed here require the lowest possible elas-tic expansion and long-term resistance, apart from high serviceloads as well as good composite behavior, slight creeping, self-extinguishing and a series of other properties. A somewhat moreprecise consideration of these requirements, the state of the artand other development trends show that several products arenow available that for the most part meet all of these require-ments.In the medium term material and product development will facili-tate quite new application possibilities like pre- and post-stress-ing technology or combinations of primary and secondary sup-port construction, which in technical and commercial respectswill open up new perspectives in mining, but in particular also intunnel engineering and as before in civil engineering.

1 Introduction

Worldwide anchor bolt technology is a key industry inmany fields of application in mining and tunnel engineer-ing as well as geotechnics, with almost 500 mio. units in-stalled annually [2]. It has become indispensable on ac-count of its advantages, e.g. great flexibility, early load-bearing capacity that can be planned and adjusted, lowmaterial and cost expenditure, low education and trainingexpense for planners, monitors and those in charge of exe-cution. On the other hand, the relatively high work expen-diture for installing system anchoring during heading hasto be taken into account in any discussion of anchor bolttechnology.

During chamber construction in underground miningsimple steel rod- and increasingly also pipe anchors areused as supports. In mining such roof bolts combined withcladding mats are also used as the sole support equip-ment. However, in tunnel engineering rod anchors are al-most exclusively installed as temporary rock anchoring

(nails or dowels), where they interact on their own or incombination with shotcrete, mats, lattice girders, piperoofs and other materials. The final secondary supportconstruction, e.g. in the form of tubbing or cast-in-situ con-crete final linings, is only installed once the stress redistri-bution has for the most part subsided.

Rapid stabilization of the rock surrounding the cavityis an indispensable condition in underground mining, inparticular in order to be able to conduct mining work withcommercial success.

In compressive or friable rock, system anchoringmust be combined with other support construction mate-rials and procedures, e.g. with pliable arching and back-filled lean concrete. Here steel anchor bolts as well as an-chor bolts made of fibre-reinforced plastic (FRP) alsoreach their current limits.

Non-metallic systems made of fibre-reinforced plas-tics currently still represent a technical niche in anchorbolt technology. They are named after the fibres used andconsequently often termed using the very common abbre-viation GRP-bolt (Figure 1) on account of the glass fibrecommonly used. As a result, GRP is basically synonymouswith all fibre-reinforced plastics, because apart from thevery wide group of diverse glass fibers, all other fibre ma-terials used quantitatively only represent a marginal phe-nomenon in anchor bolt technology. Plastics with otherreinforcements are named correspondingly after the fibers

Fibre-reinforced plastic (FRP) – an innovativeapproach to further development of rock bolttechnology and the development of new outletsin tunnel and structural engineering

Erich Borgmeier

Fig. 1. From fibre strand to fibre reinforced plastic rod (photo: Firep)

E. Borgmeier · Fibre-reinforced plastic (FRP) – an innovative approach to further development of rock bolt technology and the development of new outlets in tunnel and structural engineering

201Mining Report 150 (2014), No. 4

used, e.g. BRP for composites with basalt fibers or CRP forcomposites with reinforcement made of carbon fibers.

The pultrusion procedure used almost exclusively tomanufacture such composite anchor bolts represents aspeciality in the plastic processing industry. But it possess-es several decisive technical as well as economic advan-tages for long products such as fibre-reinforced plastic an-chor bolts. As an alternative to pultrusion processes, com-posite anchor bolts can also be produced in moldingprocesses with other special properties. In this respect thelength of the bolt depends on the dimensions of the mouldand heating tool. However, this procedure is only appliedby one manufacturer in series production from the fewwho operate internationally.

Composite anchor bolts became established at the lat-est from the mid-1990s in coal mining in Europe withoutspecial R&D work being required for these GRP bolt ap-plications. For the most part they replaced wooden pegs,which had been inserted to secure coalfaces. The GRPconstruction types were simply forced into the “technicalcorset” of the steel anchor bolt versions and their applica-tion procedures (rock dowels). With their material- andproduct-related special features, fibre-reinforced plasticanchor bolts already complement the various establishedsteel anchor bolt systems such as mortar anchor bolts (re-bar anchor bolts), adhesive anchor bolts, injection anchorbolts and increasingly also self-drilling anchor bolts, that isto say almost all temporary systems that are installed un-tensioned.

Current developments now also indicate a (partial)substitution of these conventional steel anchor bolt sys-tems. However, there is still a considerable need for a greatdeal of R&D work for further developments especially intunnel engineering and geotechnics, e.g. for permanent ap-plications as secondary support constructions.

The application advantages of the non-metallic com-posite anchor bolt systems can only undergo noticeableimprovement with several important new features. The ex-tension of the application limits requires for this purposeindependent consideration and planning irrespective ofsteel anchor bolt systems.

2 FRP technology

Composite anchor bolts for mining or for temporary workface securing work in tunnel engineering are produced al-most exclusively on the basis of cheap unsaturated poly-ester resins (UP), which are reinforced exclusively bymeans of glass fibers (Figure 2).

Unsaturated polyester resins are thermosetting matrixresins, which unlike thermoplastic resins such aspolypropylene (PP) cannot be remelted and remolded, butinstead decompose under high temperatures above crack-ing temperature. During curing the molecules undergomore intensive three-dimensional networking to macro-molecules than in thermoplastics. Consequently, as com-posite materials they possess higher mechanical strengths(Figure 3).

The for the most part unidirectionally arranged end-less glass fibers used in these anchor bolt materials fre-quently consist of pure e-glass (e = electric), which is re-garded as the standard fibre with a market share of about

90 %, but which is attacked in alkaline and acidic envi-ronments, or of the more resistant EC- or higher qualityEC-R glass (e-glass corrosion-resistant fibers with en-hanced or especially high corrosion resistance). Other fi-bre materials, such as AR glass (AR = alkaline-resistant en-riched with zirconium (IV)-oxide), aramide (aromaticpolyamides) or carbon fibers have so far not been used inthe anchor technology customary technology on costgrounds alone.

The endless glass filaments are pulled through toolswith extremely fine exit openings (spinnerets, diameter 7to 30 μm) after the melting process of the raw materialsand aggregates and processed to fibre bundles named according to their weight per kilometer (e.g. 1,200 tex =1.2 kg/km). A facing is applied to the still glowing fila-ments directly after leaving the spinnerets for later easierprocessing, a coating based on silane adjusted to the latermatrix resin, which endows the filaments with flexibility,suppleness and better adhesion (attachment) of the matrixresin in the actual manufacturing process of the rod an-chor bolts.

After the filaments have been produced, they undergofurther processing directly or assembled in so-called rov-

Fig. 2. FRP field anchor bolt systems on rod and pipe basis(photo: Firep)

Fig. 3. Molecular structures of thermosetting plastics (left)and thermoplastic resins (right)


202 Mining Report 150 (2014), No. 4

ings. During the actual manufacturing process of the an-chor bolt rod material, they are pulled through a pultru-sion system, which gives the manufacturing process itsname.

On the journey through the pultrusion system the fi-bre bundles pulled off the many roving coils are united ina parallel arranged, unidirectional collective strand,drenched with the thermosetting heat-cured artificial resinsystem (matrix resin), optionally coated or bound withflats (e.g. fabric tape or laying), brought into the shape ofthe desired cross-section profile and then pulled through aheated tool. After leaving the heating tool, the endlessstrand, still about 170°C, undergoes controlled coolingand then runs through the pulling mechanism (drawbead), behind which the endless material is cut to storageor customer length and prepared for dispatch or furtherprocessing (Figure 4).

Alternatively, the drenching and impregnation of theraw strand with resin can occur only upon injection intothe heated tool. Here the resin is thermally stimulated bythe energy input as a result of which the exothermal net-working process of the molecule groups is launched andthe composite material is hardened.

3 Properties of pultruded composites

The unidirectionally arranged – i.e. almost exclusively par-allel to the rod axis – reinforcement fibers are tightlypacked in the resin matrix, meaning very high tensilestress and tensile forces can be transferred in the rod axis,ones that are between 50 and 100 % higher than in com-parable steel anchor bolts.

However, shear forces, which can occur later more orless transversely to the longitudinal axis of the anchor boltinstalled, can be absorbed all the less and reach onlyabout 50 % of the magnitude of comparable anchor bolts[3].

This also applies in a similar manner to torsion loads,which for example occur upon tensioning of the tie nutsagainst the anchor plate and act along the free anchor rodlength (span length). The fibers are “twisted” as a result in-to a bundle and despite being embedded in the resin, theybecome “overwound” faster due to their great elasticitythan a homogeneous steel rod. The maximum torsion re-

sistance, depending on the diameter, achieves only about15 to 35 % of that of anchor bolts made of naturally hardsteel.

The e-module of steel is about 210 GPa, whereas GRPanchor bolts of current design based on polyester resin on-ly have an e-module of 40 to 50 GPa in the axial directionof pull. Higher quality epoxy or vinyl-based rods achieve55 to 63 GPa.

The composite materials have no yield point. In thestress-strain diagram the rod material has almost linear-elastic stress-strain behavior in the axle direction withoutplasticisation. The strain at break is about 2 % and as a re-sult has a friable-elastic break behavior (Figure 5).

Consequently, by contrast with homogeneous steel,the anisotropic structure with the properties described al-so lead to the disadvantages of the anchor rod. Additionalreductions in the mechanical properties arise if the fibrecontent is low (< 70 wt %), fibers are not sufficiently even-ly stretched and not evenly parallel embedded, but insteadprocessed with “deserters” and intertwined with differentlengths and with differing “stiffness.”

All fibre filaments made of silicate material (glass,basalt) already have no crystalline structure after the pro-cessing to filaments, but are instead amorphous. Normallythe raw material (e.g. sand) is generated in the oventhrough melting. Considered thermodynamically, glass isto be regarded as a frozen, undercooled liquid, because itis cooled off extremely fast after being removed and afterthe application of the silane. During the solidification ofthe oven melt to glass filaments, crystal nuclei do form,but there is not enough time left for the crystallizationprocess itself.

The solidifying filament becomes solid too fast to per-mit reorganization of the molecular building blocks ascrystals. In this regard the transformation point, the grad-ual transition point between melt and solid is about 600°C

Fig. 4. Pultrusion (pullwinding) (diagram: Ecel)

Fig. 5. Tension-expansion behavior of steel and FRP (diagram: Firep)



in many inorganic glass types The softening point fromwhich glass apparently begins to “flow” due to reduced vis-cosity is noticeable, when glass fibers stretch.

This is similar for organic resin. Here the glass transi-tion temperature TG applies, which likewise does not de-scribe any exact temperature point like the melting pointduring the phase transition of solid crystalline materials.The range beneath TG indicates the friable energy-elasticrange and differs from the “soft” entropy-elastic range(rubber-elastic range) above it. In the case of many UPresins the transition is below 100°C, in the case of VEresins usually above about 100 to 120°C, without ther-mosetting resins softening (irreversibly by contrast withthermoplastics), but instead can be used to just belowtheir cracking temperature (decomposition temperature)from which the macromolecules decompose.

The rovings produced from filaments are impregnat-ed almost exclusively with polyester resins (UP) for costreasons for temporary anchor bolts, whereas permanentanchor bolts for permanent use from 50 to 100 years arevery often manufactured on the basis of special vinyl esterresins (VE).

Such VE resins have to be inert, i.e. chemically stable.After solidification, no further chemical reactions occur.In addition, they have to be quasi diffusion-tight. Even wa-ter vapor molecules do not migrate or scarcely through thehighly networked resin matrix on the fibre surfaces. Theymay not absorb any other liquids or chemicals (imbibingor taking up other chemical substances or molecules),which would lead to an increase in volume and bursting ofthe rod, and also to scaling of the surrounding mortar orconcrete (e.g. superficial bridge damage as a result of thevolume increase of rusting steel).

The alkalinity of concrete exposed to unlimitedweathering, which reaches a peak of pH 12 to 13 about1,500 h after the concrete was produced [4], also attackssilicate glass and basalt fibers and finally “leaches” them ifthe matrix resin does not provide the fibers with perfectprotection. Alkaline stability of composite in concrete hasbeen a goal of research for many years, which has been es-sentially solved in chemical terms, but still has potentialfor optimization in process-related terms.

Composite material is in general flammable and con-tinues to burn or smolder independently. GRP materialsare not self-extinguishing without inhibitors like alu-minum hydroxide.

The thermal coefficient of the expansion of compos-ite material is much lower than that of steel. In this con-nection the already mentioned melting point of temperedsteel reinforcements or naturally hard anchor bolt steel(micro alloy manganese steel) is approximately compara-ble with the glass transition temperature of resins or com-posites [5]. However, the glass transition temperature ismuch lower than the melting point of steel, which is onlycompensated for by the fact that the composite materialshave a thermal insulating effect and the heat conductioninto the rock support collar or structural concrete is con-sequently much lower in the event of fire. The hardened“glue” of the polyester resin cartridges is also quickly andseverely affected by the high heat conductivity of the steelanchor bolts along the entire anchor bolt length, whereasGRP anchor bolts fail only close to the surface and gradu-

ally. They can maintain their load-bearing capacity andthe bond along the entire shaft length for longer. Thermalfailure of the adhesion with resin systems (adhesive car-tridges, injection resins) along the entire length as in steelanchor bolts therefore has to be measured differently inthe case of composite anchor bolts.

4 Demands on rods made of composite material

The main demand first concerns the tensile strength of thematerial. It depends significantly on the fibre quality andfibre density. FRP rods and in particular reinforcementrods must have a defined surface profile for bonding withconcrete in concrete applications, as defined in DIN 488[6] for reinforcement steel BST500 as projected rib factor,for example. Profiles that are too weak have a pull-out re-sistance that is too low in higher strength building materi-als and can lead to bonding failure. Profiling that is toostrong can, in the event of unfavorable profiling, lead tocrack formation close to the surface and close to rod pairsand also to composite splitting of the concrete. A diame-ter-related projected rib factor of fR ≈ 0.075 to 0.095 ap-pears to be establishing itself for VE-based GRP reinforce-ment profiles.

By contrast, anchor bolt profiles can be designed forhigher composite values. They must survive load introduc-tion from several blocks of anchored, but cracked dilatingrock over a short length without damage (250 to 400°kNto 30 to 60 cm length).

On top of this come shearing loads as well when dis-placements occur in anchored areas on the layer surfaces,which is not uncommon in tunnel- or development head-ing.

The long-term protection of the fibers requiredagainst chemical attack and decomposition in concretehas already been discussed. The position in the case ofrock anchor bolts is more heterogeneous, which, on ac-count of the diverse chemical composition of mine water,undergo very different chemical attacks and have to beperfectly protected against these as permanent anchorbolts.

Constant electric conduction must be guaranteedagainst theoretical spark formation by a short circuit-similar release of static electricity in the composite mate-rial in mining and imperatively in coal mining and areassubject to a risk of explosion. As a result, all of the rodmaterial must be conducting or the surface of the rodmaterial completely finished with a conductive coatingwith a maximum electrical resistance of R < 109 Ohm(Figure 6).

At least one terminal, better would be a continuousthread has to be applied in order to form an anchor head,consisting of an anchor plate and a tie nut. The rod mate-rial usually serves as adhesive profile.

Threaded screw profiles can be produced through re-shaping in the current pultrusion process or retrospective-ly on the hardened rod material by mechanical process-ing, e.g. by means of rotating diamond grinding disks as aresult of which the most outermost, thread-forming edgelaminate is interrupted in its force flux (Figure 7).

It will certainly be possible to regard reshaping in thepullwinding process (special version of pultrusion) in the



ongoing manufacturing process by wire-wrap methods orless wavy reshaping of the smooth, not yet hardened rodsurface as standard if it proves possible to close the fre-quently open-pored systems and permanently protectthem against chemical attack under loading and expan-sion.

As a result of tight constriction or remaining applica-tion of circular thread material, an endless profile can begenerated in a wrapping process before hardening of theresin matrix, which in some anchor bolt systems simulta-neously provides an anchor thread (Figure 8).

Apart from a low e-module, the higher retardationcompared to steel is worthy of brief mention. Put simply,steel is more rigid than FRP. Consequently, given equaltension a higher elastic reshaping (expansion) occurs. Thisis not however static, but instead depends on the load du-ration. The creep modulus (coefficient of creep) in this re-gard describes the time- and temperature-related plasticreshaping under load, in the case of FRP pseudo-plastic.

The creeping of steel is advantageously lower thanthat of concrete. However, the coefficients of creep of con-crete and FRP are more in a joint scatter range, which iswhy creep in long-term considerations of over 50 years’service life also have to be taken into consideration undernormal conditions in measurements. As a great general-ization, one would have to assume, according to older lit-erature and depending on design, a length change ofabout 10 to 50% over 50 years of use, which would have to

be compensated as a result of higher anchor density andgreater tension cross-sections and as a result lower use ten-sions [7]. Here, values need to be determined on more re-cent products, produced according to new manufacturingprocesses.

Self-extinguishing is a major demand on compositeanchor bolts in coal mining, which is subject to a high riskof fire and explosion. Only a few normative guidelines re-quire examination of self-extinguishing properties in thisregard within 10s after removal of the fire source [8].

5 Normative requirements and testing methods

Apart from the technical application limits, there are sev-eral obstacles in the licensing procedure and as a result inmarket access in the absence of general regulatory activi-ties on composite anchor bolts and composite reinforce-ment, which stand in the way of faster development andmarket penetration. Users hardly have the opportunity todistinguish between the more unsuitable and, as a result,in part dangerous from safe products.

In particular, self-monitoring at the manufacturer endor incoming goods examination at the customer end is forthe most part unregulated and scarcely practicable. As aresult of internationalization and the delivery of severalforeign products, especially from Asia, there is an increas-ing safety risk that has to be taken very seriously, sincethese products are optically almost identical with prod-ucts from European production.

Temporary composite anchor bolts are currently onlysubject to fairly comprehensive regulation by the Britishstandard BS 7861 [9]. The outdated DIN 21521 [10] is onlyapplicable to a limited extent to steel anchor bolts andtheir application as a descriptive standard and not at all tocomposite anchor bolts. In the revised US anchor boltstandard ASTM F432 [11], the first provisions on compos-ite anchor bolts have now been included (working groupFirep, Hilti, Orica) in addition to normative references topipe anchor bolts.

Recourse to the English standard for composite an-chor bolts intended for temporary use currently appears tobe the only solution, but would make the use of a range ofcommon composite anchor bolts used in other countriessuch as Germany, Poland and the Czech Republic impos-sible on account of non-fulfillment of the in part extreme-ly high requirements for composite anchor bolts – proba-bly so designed to protect a national manufacturer in theUnited Kingdom.

Fig. 6. GRP anchor bolts with electrically conducting coat-ing (photo: Firep)

Fig. 7. GRP anchor bolt with coating, subsequently mechan-ically processed (photo: Weldgrip)

Fig. 8. Threaded profile through tight winding and constric-tion of edge fibers before final hardening.



The application of several criteria in accordance withISO 10406, but which rather describe minimum require-ments on permanent composite reinforcement, would be atemporary way out for untensioned but permanent anchorbolt applications in tunneling. Here in Germany, all thatremains is the path via approval in individual cases withthe particular federal state authorities.

6 Current and future applications of FRP systems6.1 Current rod and pipe systems

In Europe there are only a few active manufacturers ofFRP products for mining, tunneling and structural engi-neering [2].

The Firep Group (successor organization of RockboltSystems AG / Weidmann AG) was the first internationallyactive manufacturer for rock anchor bolt systems and hasoperated since the mid-1980s. The anchor rods and an-chor pipes, which all have threaded profiles, are producedon the basis of fibre glass in the three resin systems poly-ester resin, epoxy resin and vinyl resin for various applica-tions in different nominal diameters with correspondingaccessories in each case.

Weldgrip Ltd today markets threaded profile GRProd and anchor bolt systems in the United Kingdom,which differ significantly from the first products producedby the English company MAI Systems UK. Weldgrip prod-ucts are today also in versatile use in civil engineering ap-plications. Slope and flank stabilization play an importantrole.

Other renowned manufacturers of composite rods inEurope supply only civil engineering and do not produceany specialized anchor bolt systems. Schöck cooperateswith the Danish manufacturer Fiberline and supplies rein-forcement material, which could also be suitable for per-manent, untensioned anchoring with several adjustments.

H-Bau is the European exclusive partner of Pultrall inCanada whose rods certainly also appear to be suitable,with a different surface design, as permanently applicableanchor bolt material and next to Firep and Weldgrip couldbecome the number three in Europe.

Sireg in Italy also belongs to the small group of man-ufacturers to be mentioned here, which not only produceround, but also rectangular rod material using GRP and al-so CRP material, which is established in geotechnical ap-plications. Other manufacturers and dealers from outsideEurope are slowly penetrating the European market.These definitely include in the medium term the compa-nies Auroa, Pultron and Jennmar.

6.2 Temporary application in mining and tunneling

The use of composite anchor bolts in mining is limited toa few deployment options for temporary stabilization ofthe breasting and working faces. Heading or extractionmachines can cut or machine the FRP material withoutdamaging the mechanical installations or disrupting theoperational processes in heading or extraction. GRP an-chor bolts are used in the place of steel anchor bolts espe-cially in coal mining to secure the coalfaces. This is play-ing an increasing role in the case of major seams and mul-ti-slice mining. A technically feasible, in terms of costs

even more affordable system anchoring of the roof also,can unfortunately not yet be implemented in the opera-tions.

Anchoring with composite systems in heading or dur-ing the timbering or raising of draw point raises is alsosuitable in hard rock or ore mining, which remain stablefor longer during use and gradually increasing expansion.Destroyed GRP anchor bolt segments in debris, e.g. afterdetonations, do not hinder removal and are easy to sepa-rate in treatment facilities.

Composite systems for long anchoring with or with-out prestressing in ore mining will be available in themedium term given corresponding development work.Apart from technological advantages like weight benefitsand corrosion resistance, they also provide cost advan-tages if the total period of use and the overall costs thenincurred in connection with anchor bolt technology areconsidered.

Today, GRP pipe anchors can only be used to a limit-ed extent on account of the fast and massive power in-creases in modern rock drills. As self-drilling anchor bolts,they permit deep prediction and pre-injection of the rockeven before the heading of dislocations. In order to beable to use GRP pipe anchors for this purpose, the me-chanical load-bearing capacity of composite anchor pipesneeds to be increased. In addition, further adjustment ofthe drilling equipment is necessary.

Composite anchor bolts have to date scarcely playedany role in tunneling. There they are occasionally used for

www.flexco.com

MAXIMISINGBELT CONVEYOR

PRODUCTIVITYIncrease UptimeFlexco® Rivet Hinged Belt Fastening System For the most demanding material handling applications.

Increase UptimeFlexco® Bolt Solid Plate Fastening SystemStrong, sift-free splices with superior holding ability.

Reduce Belt SlipFlex-Lag® Ceramic LaggingPrevents belt slip and extends the belt and pulley life.

Reduce CarrybackMineline® MMP Medium-Duty PrecleanerOptimal belt cleaning without an aggressive, heavy-duty belt cleaner.

Flexco Europe GmbH Leidringer Strasse 40-42D-72348 Rosenfeld

Tel. +49/7428-94060

Fax +49/7428-9406260

europe@fl exco.com



organizations like the FIB in Lausanne [7], the researchon permanent applications, in particular also of novelcomposite anchor bolts, appears very worthwhile.

Current simple steel pipe anchor bolts like elastic orplastically moldable friction bolts are very susceptible tocorrosion and in part have to be exchanged or supple-mented after a very short period of use. Apart from thematerial costs, the preservation work involves a high ex-penditure of time and, as a result, personnel costs for re-peated drilling and moving, especially in ore mining.

Especially when it comes to composite anchor boltsystems designed for corrosion-free long-term anchoring,new application fields will open up in future, e.g.:– intake shafts with humid-warm or cold air,– permanent support of underground structures,– suspensions of intermediate ceilings with rear anchor-

ing,– areas with heavy mine water flows,– support of tunnel portals,– support of slopes and cuttings at risk of sliding or slip-

ping,– stabilization of slopes along roads and railway tracks,– permanent back anchoring behind retaining walls and

facework,– areas with frost-thaw alternation (frost / de-icing salt re-

sistance),– use in seawater and tidal water.

In the case of these and similar applications subject to ex-posed conditions and simultaneously enhanced require-ments for fire protection or fire resistance, permanent an-chor bolts made of profiled fibre composite rods with per-manent thread function on a non-metallic basis offer ad-vantages compared to current steel anchor bolt systems.The profiling should be designed following DIN 488 andmeet the composite requirements [12].

6.5 FRP tensioning and re-tensioning technology

Apart from anchor bolts placed untensioned according tothe SN-process, steel anchor bolts can be tensioned as ad-hesive anchor bolts, which are fixed in a column of adhe-sive cartridges that harden at different speeds. This occursafter partial- /final hardening of the adhesive columnsduring a timeframe until the hardening of the completeadhesive column by tightening of the anchor nut, i.e. viathe torque loading. However, a classic tensioning of up to90 % of the yield strength of steel as in anchor bolt sys-tems made of prestressed steel cannot be carried out. Suchprestressed steels undergo stressing by hydraulic presses(Figure 9). Similar approaches are admittedly undergoingtrials for FRP systems, but have not yet been developed formass production readiness [13].

Experiments with wedge systems and longer presseswith longer strokes do indeed show in general that ten-sioning of anchor bolts made of basalt fibre reinforcedplastic with at least 1,860 n/mm2 material tensile strengthcan be carried out in principle in a manner very similar tothe tensioning procedure with prestressed steel.

However, the underlying idea is to apply a serial useof one and the same anchor bolt first as an untensioned,primary support element and thereafter through activa-

the support of pilot tunnels or in the case of partial exca-vations where roofs, sidewalls, benches or intermediatelevels have to be re-widened quickly.

Apart from the standard use of rod anchor bolts, theinnovative use of GRP pipe anchor bolts for temporary orfinal stabilization and improvement of the independentload-bearing capacity of anchored areas through injectionwith cement or two-component systems is of great techni-cal importance, even if this economical method is still ap-plied very hesitantly.

6.3 Temporary and permanently usable FRP anchor boltsystems

The difference between temporary and permanently us-able untensioned composite anchor bolts is initiallyscarcely detectable. Simple anchor bolts, based on fibreglass with a UP matrix for temporary applications can beproduced very economically, today regionally in part al-ready more cheaply than steel. The manufacture of per-manent composite anchor bolts requires other fibers, resinsystems, production processes and certification proce-dures.

However, manufacturers who have developed pro-duction techniques for both systems and keep materialsin stock, can develop and provide a product capable ofcombining the serial use for both applications. For thispurpose the all-rounder requires material modificationand to be pultruded in process terms in a version thatmay make the costs compared to simple composite sys-tems increase significantly, but which still remain belowthe costs for comparable DCP systems. The price per me-ter would be a maximum of 15 euros. The initially highercosts of the permanent anchor bolt are compensated forcompared to composite anchor bolts that can only beused temporarily by the clear cost advantage vis-à-vissteel anchor bolt systems with double corrosion protec-tion.

Such an anchor system is based in the reinforcementmaterial on a fibre material with great load-bearing capac-ity, very high fibre density (in rod core > 80 wt %), which isembedded and sealed in a permanent, preferably VE-ma-trix. Like a steel anchor bolt system made of rust-freestainless steel, it can be deployed universally as an unten-sioned temporary or permanent anchor bolt system. AsFRP threaded anchor pipe, such composite anchor boltspermit applications “where steel reaches its limits.”

6.4 Permanent applications in mining and tunneling

Basalt fibers undergoing trials – some experts are alreadyspeaking of the material of the twenty first century – indi-cate new application potential with greater tensilestrength, higher tensile moduli and greater temperature re-sistance, which will also play a role in the future develop-ment of anchor bolt technology and, in the medium term,could significantly shift the current technical applicationlimits.

Analogously to fundamental research into compositereinforcement of exposed concrete structures by a rangeof research institutes, especially in North America, South-ern Europe, the United Kingdom and East Asia, as well as



tion as a retensioned, secondary support element, i.e. touse the installed primary anchoring made of modern com-posite systems thereafter also for the final secondary sup-port construction.

Composite Rebar Technologies INC. (CRT) fromMadison (WI) has demonstrated one approach where aBMC paste is applied to a smooth pultruded rod or corre-spondingly pultruded pipe with or without metal core (e.g.litz wire), molded and reinforced with C-fibers and hard-ened (website www.hollowbar.com). Good bonding, gooddurability and economical manufacturing hold outpromising prospects for such a reinforcement rod.

It must be possible to “activate” a corresponding com-posite anchor bolt after use as primary anchor bolt (nail) iftensioning is required. The activation means a changefrom the untensioned installation condition to a con-trolled tensioned condition through grouped, cyclical re-tensioning of the metal core. For this purpose the anchorbolt designs, unlike SN anchor bolts made of steel, need tohave anisotropic / hybrid properties in several respects.

Structurally, this can be realized differently withoutmaking allowance for achieving double corrosion protec-tion compared with current DCP systems. Experimentswith an FRP shaft, which should also be termed dualphase composite, show the direction of the developmenttaking place, but also reveal the still high development ef-fort to be made in order to tension an embedded compos-ite anchor bolt.

In deviation from the standards for pre-stressed steelprocedures, a standard procedure for composites needs tobe developed and standardized accordingly.

Easy installation, easy options for retensioning, eco-nomical cost sequences and the durability “built-in” to thematerial will open up a market segment for the new com-posite systems next to the steel solutions for combinedtemporary and final support construction in tunnelingand geotechnics.

The greater expense to be expected in the case of themanual retensioning of composite anchor bolts is com-pensated for by the better building site suitability and sig-nificantly lower expenditure compared to steel systemswith double corrosion protection.

7 Prospects

New procedural techniques during manufacturing forarrangement and orientation even of several different fibregroups combined with fabrics and laying also permit pul-truding and pre-assembly of rod and pipe material withmore pronounced additional and ancillary functions.

Extended properties are a precondition for proce-dures being available or provided today or in the near fu-ture that permit the manufacture of novel products, suchas composite technology has created e.g. in aircraft- orwind turbine construction.

In this connection the permanent applicability ofsuitable “networks” and mats made of composite materialsimilar to construction steel mats but with considerablyexpanded modeling and application options deservesmention.

Here it becomes evident that composite support sys-tems composed of anchor bolts, mats and other compo-nents should not be shoehorned into the system andprocess technology of steel applications, but instead per-ceived as an independent technology.

The developments outlined demand intensive inter-disciplinary cooperation in production, certification, stan-dardization, measuring and application. The applicabilityof the new anchor bolt systems presumes suitable equip-ment, e.g. rock hammers that can be switched to the vari-ous composite anchor bolt systems and rock-related con-ditions.

The permanent properties described will open upnew applications and markets for composite anchoringand composite reinforcement.

Literature

[1] DIN EN 1992: Bemessung und Konstruktion von Stahlbe-ton- und Spannbetontragwerken. Berlin: Beuth, 2011.

[2] Redora: Market study. Unpublished. Troisdorf, 2014.[3] DMT: test reports.[4] Ludwig, H.M.: Lecture DiBt, 2008.[5] Firep: data sheet Firep K60.[6] DIN 488: Betonstahl – Teil 1: Stahlsorten, Eigenschaften,

Kennzeichnung. Berlin: Beuth, 2009.[7] FIB: FRP reinforcement in RC structures. fib Bulletin No.

40. Lausanne, 2007.[8] DIN 22100: Betriebsmittel und Betriebsstoffe aus Kunststof-

fen zur Verwendung in Bergwerken unter Tage – Teil 7:Sicherheitstechnische Anforderungen, Prüfungen, Kennze-ichnung. Berlin: Beuth, 2011.

[9] BS 7861: Strata reinforcement support system componentsused in coal mines. Specification for rockbolting. BritishStandard, 2007.

[10] DIN 21521-2: Gebirgsanker für den Bergbau und den Tun-nelbau; Allgemeine Anforderungen für Gebirgsanker ausStahl; Prüfungen, Prüfverfahren. Berlin: Beuth, 1993.

[11] ASTM F432: Standard Specification for Roof and RockBolts and Accessories. 2013.

[12] IMB: Lehrstuhl und Institut für Massivbau, RWTHAachen, 2007–2009

[13] Galler, R.: personal communication.

AuthorDipl-Ing. Erich BorgmeierRedOra ConsultRubensstraße 1853844 [email protected]

Fig. 9. Retensioning of pre-stressed steel litz wires(photo: Paul)

本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页文献云下载图书馆入口外文数据库大全疑难文献辅助工具

http://www.xuebalib.com/cloud/

http://www.xuebalib.com/

http://www.xuebalib.com/cloud/


http://www.xuebalib.com/vip.html

http://www.xuebalib.com/db.php

http://www.xuebalib.com/zixun/2014-08-15/44.html


fibrereinforced plastic (frp) an innovative approach to...

Documents