Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

HPFRC plates for ground anchors

M. di Prisco, D. Dozio, A. Galli & S. LapollaDepartment of Structural Engineering, Politecnico di Milano, Milano, Italy

M. AlbaSurveying Department (DIIAR), Politecnico di Milano, Milano, Italy

ABSTRACT: In order to stabilize a ground slope, special plates of reduced sizes made of High Performanceconcrete reinforced with straight steel fibres were designed and built. The plates are reinforced also with specialhigh bond steel bars with special steel threaded bushes welded at the ends to guarantee their tensile action onoverall the slab size. Their weight is limited in order to assure the transportability by helicopter everywherein mountain regions. After the experimental characterization of the material aimed to identify a constitutiverelationship, a limit design approach was carried out. Ten plates were placed in situ and two of them wereinstrumented in order to follow the real stress state inside of the anchor plates: the main results are here described.

1 INTRODUCTION

In the Pre-Alps region there are wide slopes originatedin the glacial era, made of heterogeneous material char-acterized by a high geological risk. The mitigation ofthe risk associated to this kind of slopes is not aneasy engineering problem for several geotechnical andstructural reasons.

A new precast retaining structure was designed totake advantage of High Performance Fibre ReinforcedConcrete in order to develop a faster and more effectiveprocedure of intervention. The structure geometry wasdesigned to reduce as much as possible the own weightin order to allow the helicopter transport (maximumweight <8 − 10 kN), but at the same time the highperformances of the material guarantee huge surfacehardness and local toughness and a high resistance tolocal pressures, whose distribution on the structure aredifficult to predict due to the roughness and the het-erogeneity of the slope. A plate (0.24 × 0.8 × 0.8 m),reinforced by means of four steel B450C bars alignedin the two directions and located in the intradosface, is anchored to the slope by a prestressed cable(7 strands 0.6” each made of 7 wires; diameter15.2 mm,; Ap = 973 mm2; fptk = 1860 MPa), 14.5 mlong (Fig. 1) passing in a central hole suitably designedto accommodate the cable slope-face end. The barends are welded to threaded bushes able to guaranteea direct anchorage to the concrete at the extremitiesand allowing the reinforcement to be active in thewhole bar length. Steel fibre reinforcement was used,without the introduction of any further conventional

Figure 1. Geometry of anchor plate (measures in [mm]).

reinforcement. Three more plate elements were castin order to carry out experimental tests in the lab-oratory with well known boundary conditions. Thepaper describes the material characterization and thehandling phases of ten plates placed in an environmen-tal laboratory of the Politecnico di Milano located inCaslino d’Erba close to Como (di Prisco et al. 2006).A careful monitoring of three plates (the two heredescribed anchored by means of traditional prestressedsteel cables and the last one anchored by means ofa GFRP anchor) was planned to detect the structuralbehaviour along a time period of at least five years.

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2 MATERIAL CHARACTERIZATION

2.1 Mix design

The composite was selected by comparing differentsolutions starting from the aggregates generally usedby the precast producer and limiting their maximumaggregate size to 2 mm. The material presents typi-cal proportioning of a self compacting concrete. Themix design of the HPFRCC material is specified inTable 1. Steel fibres are high carbon straight fibres13 mm long, with a 0.16 mm diameter; their content isequal to 100 kg/m3.

2.2 Fresh concrete tests

The mix design was used to cast 13 reinforced plateswithout vibration and the specimens, cubes and beams,useful to identify both uniaxial compression and uni-axial tension constitutive relationships (di Prisco et al.,2008).The fresh behaviour was controlled by means of

Table 1. Material mix design.

C 52,5 I Slag Sand 0/2 Fiber Additive Water[kg/m3] [kg/m3] [kg/m3] [kg/m3] [l/m3] [l/m3]

600 500 983 100 33 200

Figure 2. Plate casting.

Figure 3. Fresh behaviour tests: (a) slump flow; (b) V –funnel; (c) L-shape box; (d) J-ring.

several tests (Fig. 3): (a) slump flow; (b) V–funnel; (c)L-shape box and (d) J-ring tests. The results (Table 2)confirmed a SCC consistency (Fig. 2).

2.3 Hardened concrete tests

An average cubic compressive strength of 140 MPaand an elastic modulus close to 40 GPa character-ized the material in the preliminary qualification.Cubic compression tests and four point bending testson notched specimens gave first cracking and resid-ual strengths (Fig. 4a, Table 3) significantly smallerthan those obtained in the preliminary tests (di Priscoet al., 2008). The residual strengths in uniaxial tensionwere measured by using a four point bending tests onnotched specimen according to UNI 11039 Italian rec-ommendations.The geometry of the notched specimenand its set-up are described in Figure 4a; the nominalstress σN vs. crack tip opening displacement (CTOD)are shown in Figure 4b. The recommendations sug-gest to compute three strengths in order to classifythe bending behaviour of the material: the first crackstrength (fIF) that is correlated to the tensile strength ofthe material, and the residual strengths respectively inthe range 0.25–0.625 (feq0−0.6) and 0.625 and 3.0 mm(feq0.6−3) corresponding to serviceability and ultimatelimit states. In the table also standard deviations areindicated.

3 STRUCTURAL DESIGN

The design of the retaining structure is mainly orientedto emphasize the SFRC structural behaviour at the ser-viceability limit state, because the prestressed anchors

Table 2. Fresh behaviour characterization. The indicationbetween brackets are referred to the test type: (a) slump flow;(b) V-funnel; (c) L-box and (d) J-ring test.

D(a)m t(a)

m t(b)m h(c)

1 h(c)2 t(c)

200 t(c)400 D(d)

m

[mm] [s] [s] [mm] [mm] [s] [s] [mm]

752 6 27 112 85 2.5 4 750

870

are chosen as critical ring in the construction design.In the EuroCode 7 framework, the retaining structuredesigned belongs to the third category, because it canbe regarded as a new structure due to its conceptualdesign as well as to the material used. The active con-finement action applied to the ground slope by meansof prestressed anchor strands, allows us to considerthe limit state analysis in the case C, that means toregard the slope instability associated to total or par-tial collapse of the tendons as the critical limit state.The safety check was performed by means of pre-dictive calculations and by adopting the observationalmethod. The research here described is aimed to inves-tigate the structural behaviour of the reinforced SFRCplate during placing. It is conceived to react to a pre-stressing action of about 1150 kN originating by thestretching of the cable made of seven 7W strands. Theplate design follows the overestimation rule which isforced by the impossibility to predict the real bound-ary conditions due to the roughness of the morenicslope face and the need to prevent any local crackingdue to concentrated loads. About the computation ofthe ultimate limit state, limit analysis was used associ-ated to a kinematic mechanism of 2 yield lines alignedto the symmetric axes of the plate and justified bythe unilateral constraint of the ground reaction withthe consequent rotation around the diagonal of eachforth square of the plate.The specific bending momentwas computed adding the contribution of each steelbar smeared along the half of each yield line to thecontribution of the residual tensile strength associatedto a crack opening of 2 mm, which corresponds to asteel strain of 1% when the distance of the bar axisfrom the top fibre (equal to 200 mm) is considered ascharacteristic length.

Figure 4. Notched tests according to UNI 11188: (a) test set-up and (b) nominal stress vs. Crack Tip Opening Displacement.

4 ON SITU PLACING PHASE

4.1 Settlement description

The plates were placed in the established locationon the slope as briefly shown in Figure 5. No mor-tar layer was introduced between the plate and theslope face to accelerate the placing procedure. Due tothis choice, the rigid motion of the anchor plates was

Table 3. Hardened behaviour characterization.

Rc28,m fIf ,m feq(0−0,6),m feq(0,6−3),mTest Age [MPa] [MPa] [MPa] [MPa]

n◦ [days] (std) (std) (std) (std)4 28 116.49 13.16 12.06 9.76

(8.78) (1.22) (1.48) (1.78)

Figure 5. Anchor plate location on slope.

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Figure 6. Placing of anchor plates on slope.

Figure 7a. Laser scanning displacements of top plate.

significant as detected by laser scanning technique andshown in Figure 7 for the two instrumented plates.Alsothe displacements of the four corners of these plateswere measured and the relative displacements associ-ated to each load step were computed. According tothese results, the relative displacement of the barycen-tre of each plate, assumed as completely rigid, was alsoidentified in order to depurate this value by the totalsliding measured between the jacket reference and thestretched strands. The results are proposed for the topinstrumented anchor plate in Figure 8.

Figure 7b. Laser scanning displacements of bottom plate.

4.2 Steel anchor stretching

The force activated in the anchor cables was measuredby means of suitable loading cells placed in order tomeasure the force in only one strand for each cable.Thetotal force was measured by the pressure sensor of theloading system. Moreover, the relative displacementbetween the jacket reference and the stretched strandread at the end of each step for all the strands allowedus to deduce the real stiffness of the anchor cables. Infact, the real stiffness of each cable can be computedby subtracting from the total relative displacement therigid displacement of the plate barycentre that takesinto account also the local punching of the plate inthe slope face (Figs. 7a,b). The relative displacementcomputed can be finally compared with the theoreticalprediction based on the different assumptions on thecable length.

In Figure 8 the load corresponds to that measuredby the loading cell and it is referred to only one strand.The real stiffness of the strand is not known because itdepends by the sliding between the strand and the mor-tar and also by the sliding between the mortar and theground. The former usually is small if the injection iswell done, while the latter can be significant due to therelatively high heterogeneity of the ground. In the com-parison shown in Figure 8, a total active length equalto 10 m, that corresponds to the free length (5.5m) plusthe half of the foundation length (4.5m), is first pro-posed to estimate the total displacement.Afterwards, asecond theoretical curve computed with only the freelength is also shown to fit the slope of the measuredcurve after the application of a significant load. Thecomparison between the experimental curve and the

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Figure 8. Load vs displacement of top anchor plate during placing (normal component with respect to slope face).

Figure 9a. Top plate placing: load vs deformation.

theoretical ones justifies the reliability of the anchorsystem. It is worth to note how the important rigiddisplacement of the plate causes some local plasticityphenomena on the slope face, but this occurrence doesnot compromise the anchor system efficiency.

4.3 Plate deformation

The two instrumented plates were equipped by meansof 6 suitable vibrating wire sensors able to measurethe strains on a 200 mm gauge length: four sensorswere located at 30 mm from the intrados (one for eachside; Fig. 9a,b) and two were located at 30 mm fromthe extrados to measure the compressive strains asso-ciated to the bending along the direction at right anglewith respect to the yield line trace assumed.The valuesshown (Figs. 9a,b) highlight as the maximum relativedisplacement can justifies cracking, even if the crackopening remains always admissible according to ser-viceability limit states. It is also evident how large canbe the scattering between the measures performed on

the four sides of the plate, thus justifying the rathercomplex and unpredictable set of boundary conditionsto take into account at the design level (Fig. 10).

5 CONCLUDING REMARKS

The paper is focused on the design of a light retainingstructure able to insert anchor cables in a ground slopeto increase its safety coefficient by preventing its insta-bility. The main assets of this design solution are thelimited weight, the reduced cost and the high speed inits placing. The limited weight allows the designer touse it in quite inaccessible sites like mountain regionsby helicopters transport on situ. The reduced cost isrelated to the optimized volume and to a substantialconvenience when compared to a steel solution: inthis case the cost ratio is larger than five times andthe environmental impact is not comparable. The highspeed of its placing is guaranteed by the lack of anymortar layer between the slope face and the plate. To

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Figure 9b. Bottom plate placing: load vs deformation.

Figure 10. Anchor plate during loading phase.

this aim the use of HPFRC is particularly promisingbecause it allows to dispose of a very high local tough-ness and hardness, which are essential characteristicsin such structures where boundary conditions andapplied constraints cannot be easily a priori known.The lack of complex reinforcement detailing allows acomplete industrialized production and the structureobtained is versatile, because it can be combined withother similar structures to give rise to modular andquite articulate retaining structure. A system of anchorplates can also be designed in combination with con-ventional steel wire net in order to solve at the sametime the surface and the deep landslide. The surveyingof these structures activated via GPRS in the envi-ronmental laboratory located in Caslino d’Erba willgive precious information on durability of this newpromising retaining structure solution.

ACKNOWLEDGEMENTS

The authors thank Prof. Alberto Giussani and Prof.Marco Scaioni for their technical and financial sup-port in the surveying activity and Dr. Fabio Roncoronifor their precious support in the on situ placing phasesand Prof. Claudio di Prisco for the enlightening dis-cussion in the design activity. The authors would liketo thank also Fumagalli Prefabbricati, Basf and HalfenCompanies for their technical and financial support ofthe overall research.

REFERENCES

CsNR-DT 204, 2006. Instructions for design, execution andcontrol of fibre reinforced concrete structures, ItalianStandards.

Naaman, A.E. & Reinhardt, H.W., Eds. 2003. HighPerformance Fiber Reinforced Cement Composites(HPFRCC4), PRO 30, Rilem Publication S.A.R.L.

Reinhardt, H.W. & Naaman, A.E., Eds. 2007. HighPerformance Fibre Reinforced Cement Composites(HPFRCC5), PRO 53, Rilem Publication S.A.R.L.

di Prisco, C., di Prisco, M., Mauri M. & Scola, M. 2006.A New Design for Stabilizing Ground Slopes, Proc. ofthe 2nd fib Congress, Napoli (Italy), June 5–8, ID 4-1 onCD-ROM.

di Prisco, M., Lamperti, M., Lapolla, S. & Khurana, R.S.2008. HPFRCC thin plates for precast roofing, Proc. ofSecond International Symposium on Ultra high Perfor-mance Concrete, Kassel Germany (in printing).

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