Equivalence of mesh- and fibre-reinforced shotcrete at large deflections

Equivalence of mesh- and fibre-reinforced shotcrete at large deflections

H.A.D. KIRSTEN

Steffeiz Robertso~z and Kirsten Inc., P.O. Box 55291, Northlands, 2116 Traizsvanl, South Africa

Received November 1, 199 1

Accepted January 15, 1993

The object of the study was to determine whether extensively deflected mesh- and fibre-reinforced shotcrete as found in mines were equivalent. Full-size panel tests were accordingly carried out, from which it was concluded that uniformly distributed loading was marginally less severe than point loading, mesh reinforcement was superior to fibre reinforcement, Dramix fibre and Melt Extract fibre were similar in reinforcing characteristics, and 30-35 mm was an optimum fibre length. Consistency of fibre content could not be assured in the practical mining situation considered. A maximum content of about 1.5% could be obtained for fibre lengths not exceeding 35 mm. Owing to its greater flexibility, diamond mesh reinforced shotcrete was found to be preferable to fibre-reinforced shotcrete in excavations subject to squeezing conditions. In view of the difficul- ties associated with assuring the consistency of the fibre content in mining applications, it appears that the sophisticated attributes of fibre-reinforced shotcrete can only be effectively exploited in civil engineering applications in which quality assurance is subject to explicit contractual control.

Key words: fibre reinforcement, shotcrete, mesh reinforcement, large deformation.

Le but de cette Ctude Ctait de dCterminer si le bCton soufflC arm6 d'un grillage fortement dCformC et le bCton soufflC arm6 de fibres tels que trouvks dans les mines sont Cquivalents. A cette fin, des essais ont CtC rCalisCs sur des panneaux pleine grandeur; l'on en a conclu que le chargement uniformCment distribuk Ctait ii peine moins sCvkre que le chargement ponctuel, que l'armature de grillage est supCrieure aux fibres, que les fibres Dramix et Melt Extract ont des caracthristiques d'armature similaires, et que la longueur optimale de fibre est de 30-35 mm. La constance de la teneur en fibres ne peut pas &tre assurCe dans les conditions considCrCes d'une mine. Une teneur maximale d'environ 1,5% pourrait Etre obtenue pour des longueurs de fibre n'excCdant pas 35 mm. L'on a trouvC que, par suite de sa plus grande flexibilitC, le bCton soufflC arm6 de grillage en forme de diamant Ctait prCfCrable ii celui arm6 de fibres dans les excavations soumises ii des conditions d'kcrasement. Compte tenu des dif- ficult& d'assurer la constance dans la teneur en fibres dans les applications minikres, il semble que les attributs sophistiquCs du bCton soufflC arm6 de fibres ne peuvent Etre effectivement exploitCs que dans les applications de gCnie civil oh l'assurance de qualit6 est assujettie A un contr6le contractuel explicite.

Mots cle's : armatures de fibres, bCton soufflC, armature de grillage, grande deformation. [Traduit par la rCdaction]

Can. Geotech. J . 30, 418-440 (1993)

Reinforcement of shotcrete Shotcrete may be reinforced with weld or woven wire

mesh or with steel or synthetic fibre. When reinforced with woven wire mesh, it is particularly suitable as a rock support component in excavations subject to squeezing conditions. Steel fibre reinforced shotcrete has significant construction advantages that can only be exploited if it is equivalent in strength and ductility to woven mesh reinforced shotcrete under extensive deflection. The object in the study was to establish such equivalence by testing full-scale panels to destruction under point and uniformly distributed loads. The panels were alternatively provided with diamond mesh and steel fibre reinforcement. Various fibre types, contents, and lengths were examined.

Review of tests for material strength and structural performance

The material strength and structural performance of shotcrete have been tested extensively during the past 15 years in terms of various parameters which bear to a greater or lesser extent on mining applications. The sin- gle most important aspect in terms of which the use of shotcrete in mining excavations differs from that in civil engineering projects is the extensive bending deflections to which such support systems are subjected in mines. Printed in Canada 1 Imprim6 nu Canada

The bending tests that have been reported in the literature are summarized in Table l , from which it can be seen that only Holmgren (1983) and Little (1985) examined the effects of extensive bending deflections. Further observations are as follows.

Panel thickness is important for the variable design conditions in mines; however, none of the authors referred to in Table 1 have examined it in particular. The supports in mines are usually spaced on a 1 x 1 m grid. The spacing quoted in the literature for the full-scale tests (i.e., 2.5 m) does not represent mining applications. The fibre lengths and contents quoted correspond generally to what would be used in mines.

The loading conditions pursued in the tests referred to in Table 1 were not representative of mining applications. It is unlikely in pracfice for the rock behind every section of shotcrete between adjoining rockbolts to present itself without variation to the shotcrete as a sharp object or as a rigid, relatively free block fitting accurately between the supports. The point or punch loads referred to in the tests are not likely to obtain as representative design criteria in practice. It is more likely for the loading to be uniformly distributed for the purposes of optimum design.

It is evident from these various comments that the competence of fibre-reinforced shotcrete and its equiva-

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lence to mesh-reinforced shotcrete in mining applications have not been investigated comprehensively. The reinforcing characteristics of woven wire mesh under extensive lateral deflections are of particular importance in this regard. The tests that have been carried out on mesh- reinforced shotcrete have generally been based on weld mesh, which owing to its rigidity cannot accommodate large deflections beyond peak load to an extent comparable to diamond mesh.

Test programme The interaction between the shotcrete and the rock in a

real tunnel is complex and cannot be modelled accurately under laboratory conditions. A continuous shotcrete panel of which the central 1 x 1 m section was secured by means of four regularly spaced bolts and subjected alternatively to uniformly distributed and point loads in bending was, however, considered to be sufficiently representative of the actual situation.

A steel frame was manufactured as shown in Figs. 1 and 2. The frame took a 1.6 x 1.6 m square panel and provided for bolt supports at the corners of a 1 x 1 m section. Bearing plates were placed under the bolt heads to simulate actual conditions. The arrangement ensured the continuity of the shotcrete across the lines of support between bolts as applies under actual conditions.

The frame further allowed a uniformly distributed load to be applied to the panel by means of a hydraulically pressurized bag with a lifting height of 520 mm. Hydraulic pressurization of the bag limited the energy in the loading system and ensured that panel deflections could be tracked in a controlled manner beyond peak loading. A point load could also be applied to the centre of the panel by means of a hand-operated hydraulic jack with a stroke length of 150 mm. A 100 x 100 mm bearing plate was placed between the jack and the panel. Although not considered to be likely in the actual situation, point loading was pursued as an additional loading case. The centre of the panel could be deflected to a maximum of 150 mm for any of the loading conditions pursued.

The pressure in the bag was measured by means of a pressure transducer and the load in the jack by means of a load cell, the output signals from both devices being recorded as analogue voltages. The deflections of the panel were recorded at three positions, one at each of two diametrically opposite bolts and one at the centre of the panel. Linear variable differential transducers with a +25 mm travel and an analogue voltage output were used for this purpose. The output signals were fed into a time- based, five-channel multichart recorder that produced continuous load-deflection traces throughout the tests. Two series of tests were carried out as follows.

First test series Eleven panels were prepared in the first series with

dry mixed shotcrete. Four, four and three number panels were prepared for each of three nominal thicknesses, namely 50, 100, and 150 mm, respectively. Eight of the panels were reinforced with diamond mesh and seven with steel fibre. The panels were shot upright into timber formers, spray cured for 3 days, and moist-air cured to 28 days thereafter. Holding down holes, 30 mm in diam-

P: m " "

m m A * - I n . '? "? c4 -

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CAN. GEOTECH. J. VOL. 30, 1993

BASE

FIG. I . Plan of panel testing frame. Dimensions in millimetres H.D. Bolt, holddown bolt.

1 \SHOTCRETE TEST PANEL

JACK FOR CENTRE POINT LOADING

V- EXISTING HOLDING DOWN BOLTS

FIG. 2. Section A-A of panel testing frame. Refer to Fig. 1 . Dimensions in millimetres.

eter, were cast into the panels at the corners of a 1 x 1 m square, and lifting lugs were cast into the panels next to these holes.

Sixteen control beams and 24 control cubes were cut to the dimensions given in Tables 2 and 3 from the prototype panels and from six simultaneously manufactured control panels, 100 mm deep x 500 mm square. The loads were applied in all instances to the shot surfaces of the test specimens and in the direction of shooting. The control beams were simply supported and subjected to point loading at midspan as prescribed in ASTM (1985). The cubes were crushed between rigid steel platens.

The design mix comprised 37% river sand (maximum particle size of 6 mm), 37.'3% crusher sand, 15% ordinary Portland cement, 6% water, 1.7% silica fume, no accelera- tors and, where applicable, 3% Dramix ZP 30 mm long x 0.50 mm diameter steel fibre. Instead of the fibre reinforcement, diamond mesh of 75-mm aperture and 3.1-mm strand diameter was placed in the middle of half the panels. This gave mesh contents of 1.42, 0.71, and 0.47% by weight of the 50- , 100- , and 150-mm-thick panels, respectively. The prototype and control panels were continuously spray cured for 3 days after shooting and subsequently moist-air cured until tested, between 5 and 7 months later.

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TABLE 2. Control beam test results from the first series

Dimensions Properties at first crack (mm) Fibre Residual

content Strength Toughness Deflection strength Description Width Depth Span (% by wt.) (MPa) (N.m) (mm) (MPa)

Control panel 1 99.5 97 270 1.15 10.08 0.36 0.022 10.08 99 84 270 9.49 0.77 0.063 9.49

Control panel 2 99 99 270 1.17 8.46 1.53 0.093 9.15 100 99 270 9.55 0.83 0.047 9.55

Control panel 3 9 8 101 270 1.13 8.15 - - 8.35 100 102 270 8.48 1.74 0.106 8.80

Fibre-reinforced panel, 50 mm thick 99 47 270 1.36 2.85 - - 2.85

100 42 270 1.38 9.73 - - 9.73 Fibre-reinforced panel,

100 mrn thick 101 90 300 1.49 6.73 0.50 0.054 7.68 99 89 300 1.47 7.75 0.64 0.063 8.79

Fibre-reinforced panel 100 mm thick 100 98 300 1.49 4.99 0.49 0.061 5.91

99 98 300 1.47 7.97 0.92 0.074 7.97 Fibre-reinfored panel,

100 mm thick 100 82 300 1.49 9.40 0.78 0.074 10.43 9 8 82 300 1.47 10.2 1 0.85 0.076 10.2 1

Fibre-reinforced panel, 150 mm thick 99 101 300 1.43 5.95 0.65 0.065 6.22

99 100 300 1.43 6.42 0.7 1 0.067 7.03

NOTE: Double entries represent control beams taken at right angles to each other. -, denotes unreliable result or not determined.

TABLE 3. Control cube test results from the first series

Description

Control panel 1

Control panel 2

Control panel 3

Fibre-reinforced panel, 100 mm thick



-

Compressive Dimensions Density strength

(mm) (kg/m3) (MPa)

99x99~98 2438 79.6 100x98~99 2423 63.9 100x99~83 2434 7 1.5 100x99~83 2434 72.9 100x98~100 2482 77.7 9 9 x 9 8 ~ 100 2477 69.9 100x97~99 2398 61.4 100x97~99 2448 60.2 100x99~98 2452 68.5 100x99~101 2372 57.0 100x99~100 2396 52.4 100x99~100 2425 -

Secorzd test series the first series. Eight panels were prepared for each of Twenty-four dry mix panels were prepared for the sec- the three nominal thicknesses referred to above. Six panels

ond series according to the s a m e procedure described for were reinforced with diamond mesh, s ix with 30-mm-

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422 CAN. GEOTECH. J. VOL. 30, 1993

TABLE 4. Control beam test results from the second series

Properties at first crack Dimensions (mm) Fibre Residual

content Strength Toughness Deflection strength Description" Width Depth Span (% by wt.) (MPa) (N.m) (mm) (M Pa)

D30 fibre reinforced panel subject to point load

D30 fibre reinforced panel subject to uniformly distributed load

ME25 fibre reinforced panel subject to point load

ME35 fibre reinforced panel subject to uniformly distributed load

"D30, 30-mm Dramix fibre; ME35, 35-mm Melt Extract fibre

long Dramix fibre, and three, three, and six each with 25- , 35- , and 50-mm-long Melt Extract fibre. The panels were also shot upright into timber formers as for the first series.

Twelve control beams and 24 control cubes were cut from the prototype panels to the dimensions given in Tables 4 and 5. The same testing procedures were followed in the second series as described above for the first series.

The design mix was similar to that described above for the first series. The panels were moist-air cured until tested, between 3 and 7 months later.

Control beam and cube test results The results for the control beam tests are summarized

in Tables 2 and 4 for the two test series, respectively. Although not submitted herein, the load-deflection curves were, in general, linear up to the point of failure. The beams cut from the control panels in the first test series gave slightly higher and more consistent results than those cut from the prototype panels. The flexural resistances and compressive strengths of the beams in the second test series were generally not affected by fibre length or content, and were not significantly different for the transverse and parallel beams in the first test series. The fibres were in general aligned perpendicular to the direction of shooting in all the test specimens.

It is evident from Table 2 that the intended design fibre content of 3% bv mass was not achieved in the first series. The control beams failed catastrophically at peak load, mainly due to the low fibre content. As a result, the toughness indices as defined in ASTM (1985), could not be determined.

The objectives in the second test series were to attain a higher fibre content and to observe the effect of greater

fibre length. The first objective was largely achieved for the Dramix fibre at an average content of 2.53%, as evident from Table 7. The contents in the panels reinforced with Melt Extract fibre were generally very low, even for the shorter fibre lengths. This was due to the panels being made at the mine by a relatively untrained crew. The control beams did not fail catastrophically in this series, which allowed the toughness indices to be determined as shown in Table 4.

The results from the control cube tests are summarized in Tables 3 and 5 for the two series, respectively. The densities for the various cubes were consistent and subject to minimal scatter. The compressive strengths were similarly consistent, but the cubes taken from the control panels in the first test series were generally slightly stronger than those taken from the prototype panels.

In both series, the peak load strengths of the beams were on average only 11% of the cube strength of the shotcrete. This was due to the nonuniformity of the stresses in the test cubes and to the anisotropy in strength of the shotcrete. The direction in which the shotcrete was stressed in the surfaces of the beams, the weaker direction, was at right angles to that in which the cubes were crushed.

General results from panel tests and reduction of load-deflection plots

The panels in both series and for both types of load generally developed an orthogonal midspan tension crack at small deflections. A similar crack followed relatively quickly at right angles. At further stages, compression cracks developed diagonally at the bolt heads.

The mesh reinforcement failed across the cracks at large displacements and was generally accompanied by crushing of the shotcrete on the compression side. This

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TABLE 5. Control cube test results from the second series

Description" Density Compressive (kg/m3) strength (MPa)

D30 fibre reinforced panel subject to point load 245 1 49.4

50.4 22 13 49.4

48.5 238 1 49.3

50.6

D30 fibre reinforced panel subject to uniformly distributed load 2337 61.7

ME25 fibre reinforced panel subject to point load

ME35 fibre reinforced panel subject to uniformly distributed load

62.6 2309 62.8 FIG. 3. Illustration of initial typical crack (1) in a 150-mm-

62.9 thick 30-mm Dramtix fibre-reinforced panel (note strain 2197 63.3 gauges attached next to top right anchor).

64.3

FIG. 4. Illustration of development of second (2) and third (3) NOTE: In all tests the control cubes measured 100 x 100 x 100 mm. cracks. "D30, 30-mm Dramix fibre; ME35, 35-mm Melt Extract fibre.

was consistent with the underreinforced state of the shot- Crete, resulting from the extensive straining of the woven mesh concomitant with straightening of its profile.

The fibre reinforcement straddled the cracks and tended to gradually debond from the shotcrete rather than fail suddenly. Failure of the fibres was generally not accompanied by crushing of the shotcrete on the reverse face, which represented a condition of relative underrein- forcement in the conventional sense.

The tests were run to total destruction of the panels at midspan deflections generally exceeding 100 mm and crack widths larger than 20 mm. The panels did not col- lapse beyond the peak load like the control beams. The loads were increased continually during the tests to ensure ongoing deflection. This was considered to sig- nify strain hardening or at least ideally plastic straining of the shotcrete.

Typical photographs of the panels at various stages of deflection are shown in Figs. 3-5. Typical photographs of the failure modes of the fibre and mesh reinforcement are given in Figs. 6 and 7.

The greater ability of the panels to sustain load at pro- longed deformation compared with the control beams was due to the two-way spanning nature of the panels,

but also to the constraint provided by the face plates under the bolt heads.

Load-deflection plots for the variously reinforced and loaded panels are given in Figs. 8 and 9 for the first series and in Figs. 10-13 for the second series. The key results for the various panels are summarized in Tables 6 and 7 for the two series, respectively. The results could not be evaluated in this form, because of the dissimilari- ties in average panel thickness.

The postpeak drop in the load-deflection plots did not represent ideally plastic or strain-hardening behaviour of the shotcrete, but was due to the development and con- tinued propagation of tension cracks through the fibre- reinforced panels. In the case of the mesh-reinforced panels, the postpeak drop- was due to the extensive strains associated with the unfurling profile of the mesh and the concomitant secondary crushing of the shotcrete and associated reduction in effective depth.

Best fit curves were subjectively superimposed over the postpeak sections of the load-deflection plots as shown typically in Fig. 14 for one of the tests. The loads and effective panel thicknesses were determined at four points on each of the curves at peak load and at 50- , 100- , and 150-mm deflections, respectively. The postpeak loads were proportional to the squares of the effec-

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TABLE 6. Key panel test results from the first series P h, P

Panel thickness (mm) Fibrelmesh

Avg. Avg. Peak Shotcrete No. of Reinforcement Loading along along load density Length Aspect Content strain

type" t y p e h o m i n a l edge crack (kN) (kg/m3) Manufacturer (mm) ratio (% by wt.) gauges Remarks

Mesh P 150 162.0 180.0 143.4 - - - - - Conventional failure pattern 0.47 100 128.5 147.5 106.2 - - - - 0.7 1 - 2 orthogonal cracks

simultaneously 50 72.5 73.0 26.1 - - - - - Conventional failure pattern 1.42

U 150 153.0 149.0 210.0 - - - - - Conventional failure pattern 0.47 100 119.5 104.0 135.6 - - - - 0.7 1 - Conventional failure pattern 5 0 66.5 73.0 80.0 - - - - - 2 orthogonal cracks 1.42

simultaneously

P 150 134.5 140.5 101.7 2392 Dramix 3 0 5 0 1.43 - Conventional failure pattern 100 102.5 104.0 69.0 2391 Dramix 3 0 5 0 1.49 - Conventional failure pattern 50 45.0 51.5 17.4 2390 Dramix 3 0 5 0 1.36 - Triple parallel crack pattern;

full separation U 100 67.0 78.0 81.0 2380 Dramix 30 5 0 1.47 - Diagonal cracks also present D

50 48.0 51.0 72.0 2377 30 50 1.38 >

Dramix - Complete separation of panel z segments

"D30, 30-mm Dramix fibre. B

"P, point; U, uniformly distributed. d 0 ? -

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TABLE 7. Key panel test results from the second series

Panel thickness ( ~ n m ) Fi brelmes h

Avg. Avg. Peak Shotcrete No. of Reinforcement Loading along along load density Length Aspect Content strain

type" typet' Nominal edge crack (kN) (kg/m3) Manufacturer (mm) ratio (% by wt.) gauges Remarks

Mesh P 150 180.0 187.0 109.5 5 0 70.0 77.0 29.4

Conventional failure pattern Conventional failure pattern

skewed at 45" with additional radial cracks

Conventional failure pattern Conventional failure pattern Conventional failure pattern Skewed conventional failure

pattern

Dramix Dramix Dramix

Conventional failure pattern Conventional failure pattern Conventional failure pattern

with additional radial cracks Conventional failure pattern E Conventional failure pattern g Conventional failure pattern 9 z

Dramix Dramix Drarnix

Melt Extract Melt Extract Melt Extract

Conventional failure pattern Conventional failure pattern Skewed conventional failure

pattern

Melt Extract Skewed conventional failure pattern with additional radial cracks

Conventional failure pattern Slightly skewed conventional

failure pattern

Melt Extract Melt Extract Melt Extract Melt Extract Melt Extract Melt Extract

Conventional failure pattern Conventional failure pattern . Conventional failure pattern Conventional failure pattern Conventional failure pattern ' Conventional failure pattern

"D30, 30-mrn Dralnix fibre; ME25, ME35, and ME50, 25-, 35-, and 50-111111 Melt Extract fibres, respectively. "P, point; U, uniformly distributed.

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A 1 @ 150mm PANEL U D L ~ IOOmm PANEL U O L

150mm PANEL U D L

I / \ 4 --- 150mm PANEL POINT LOAD @ - lOOmm PANEL POINT LOAD 1, $ ' \ 1_ @ - - 5 O y m PANEL PO!NT LOAD

DEFLECTION (rnrn)

FIG. 8. Load-deflection curves for mesh-reinforced panels, first series. UDL, uniformly distributed load.

DEFLECTION (rnrn)

FIG. 9. Load-deflection curves for fibre-reinforced panels, first series. -

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CAN. GEOTECH. J . VOL. 30, 1993

E I 1 LEGEND I IOOmm PANEL UDL ME35

5 0 m m PANEL UDL M E 3 5 8 M E 2 5

a 0----- 1 5 0 m m PANEL UDL M E 3 5

9 250 @- 1 0 0 m m PANEL POINT LOAD M E 2 5

I 1 5 0 m m PANEL POINT LOAD M E 2 5 I

0 25 50 75 100 125 150

DEFLECTION (mm)

FIG. 12. Load-deflection curves for 25- and 35-mm Melt Extract (ME) fibre-reinforced panels, second series.

LEGEND

@) - IOOmm PANEL UDL

@ --- 1 5 0 m m PANEL UDL

@ ----- 5 0 m m PANEL UDL

0 -------- I 0 0 m m PANEL POINT LOAD

0 ------ 5 5 0 m m PANEL POINT LOAD 0 - . - . - . - . 6 1 5 0 m m PANEL POINT LOAD

,-. 5 - 0 4 S

0 2 5 50 75 100 125 150

DEFLECTION (mm)

FIG. 13. Load-deflection curves for 50-mm Melt Extract fibre-reinforced panels, second series.

in Tables 8 and 9 for the two test series, respectively. Evaluation of panel test results for various The comparative proportions of load and effective thick- parameters ness given in Tables 10-16 for the two series were deter- The conclusions from the panel test results are pre- mined accordingly. sented below in terms of comparisons of the standardized

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TABLE 8. Standardized panel test results from the first series

Standard Effective load (kN) thickness Effective thickness (mm) Peak load at given deflections (mm) at peak at given deflections (mm) at standard

Reinforcement Loading load thickness 150 (also type" typeb (mm) 5 0 100 150 (kN) 5 0 100 ultimate load)

Mesh P 150 100 5 0

U 150 100 5 0

"D30, 3 0 - m m Dramix fibre. "P, point; U, uniformly distributed.

average effective thicknesses given in Tables 10-16. The over point loading in terms of effective thickness was corresponding standardized average effective loads are not affected by panel thickness for either type of rein- not directly referred to. Suffice it to observe that the forcement. It was, however, greatly dependent on panel standardized average effective loads are related to the thickness in terms of effective load. squares of the standardized average effective thicknesses. (3) The superiority of the distributed loading over

225

200-

/75-

I50

12 5 - D

4 I00

75 -

50

First series (Table 10) point loading in terms of effective thickness and load increased with deflection, more so for mesh- than for

Coinparison of distributed and poiizt loading fibre-reinforced shotcrete. ( 1 ) Distributed loading was significantly less severe

than point loading for the mesh-reinforced shotcrete, but Coinparisoiz of inesh aizd fibre reinforcemeizt only very slightly so for the fibre-reinforced shotcrete. (1) Mesh reinforcement was superior to fibre rein-

(2) The superiority of uniformly distributed loading forcement for both types of l o a d i ~ g .

:: 0 25 50 DEFLECTION 75 (mm) 100 12 5 150

FIG. 14. Load-deflection curves for 100-mm-thick mesh-reinforced panel subject to uniformly distributed load, first series.

LEGEND LABORATORY OBSERVED RELATIONSHIP - - - FREE HAND DRAWN BEST FIT CURVE

-- -

REFER TO FIGURE I5 FOR LOADS AT 104 mm AVERAGE THICKNESS

I

REFER TO FIGURE 16 FOR EFFECTIVE THICKNESS AT

I

I

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KIRSTEN

FIG. 15. Load vs. initial average thickness for mesh-reinforced panels subject to uniformly distributed load, first series. DEFL, deflection.

fibre content significantly affected the behaviour of the loading could be said to be marginally, but insignifi- shotcrete or not. cantly superior to the point loading.

Coinparison of distributed arzd point loading (1) As evident from Table 13, the uniformly distrib-

uted loading was not different from the point loading for mesh, 30-mm Dramix fibre, and 25-mm Melt Extract fibre reinforcement in the second test series. It was significantly less demanding than the point loading for the 50-mm Melt Extract fibre reinforcement.

(2) The comparisons between the uniformly distributed and point loadings in terms of effective thickness for similar reinforcement types were not affected by panel thickness as in the first series. It was, however, significantly dependent on panel thickness in terms of effective load, also as for the first series.

(3) The comparisons between the uniformly distributed and point loadings in terms of effective thickness and load varied randomly with deflection, which was unlike the first test series.

(4) Over both test series the uniformly distributed

Comparison of Drarnix and Melt Extract fibre ( I ) The 30-mm Dramix fibre appeared from the results

given in Table 14 for the second test series to have been significantly more effective under point loading than either 25- or 50-mm Melt Extract fibre. The average contents for the 30-mm Dramix fibre and for the 25- and 50-mm Melt Extract fibre were, however, 2.43, 1.20, and 1.22%, respectively. The greater Dramix fibre content therefore could explain its apparent superiority.

(2) The 30-mm Dramix fibre also appeared to be significantly more effective than the 25-mm Melt Extract fibre for uniform loading, as is evident from the results given in Table 15 for the second test series. The comparative fibre contents were, however, 2.63 and 1.57%, which as concluded above meant that the difference could have been due to the difference in fibre content.

(3) The 30-mm Dramix fibre and the 35-mm Melt Extract fibre were not significantly different for uniform

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LEGEND: 1 - LOADS AT 5 0 m m DEFL

LOADS AT IOOrnm DEFL

RESIDUAL LOADS

AMRAGE THICKNESS (mm)

FIG. 16. Effective thickness vs. initial average thickness for mesh-reinforced panels subject to uniformly distributed load, first series. DEFL, deflection.

loading according to the results given in Table 15 for the second test series. In terms of the average fibre contents, which were 2.63 and 1.25%, respectively, it could alternatively be concluded that fibre content did not significantly affect the behaviour of the shotcrete.

(4) The results from the second test series for the 30-mm Dramix fibre and the 50-mm Melt Extract fibre were very similar for uniform loading, as given in Table 15. The average fibre contents were, however, 2.63 and 0.64%, which was inconsistent with the finding in the first paragraph above with regard to the 50-mm Melt Extract fibre.

(5) Overall, the Dramix and Melt Extract fibre reinforcement did not appear to be significantly different.

(6) Although it would appear that fibre content did affect the behaviour of the shotcrete, the indications were that such an effect was not conclusively demonstrated.

Compurison of vurio~is lengths of Melt Extract fibre (1) The 25-mm Melt Extract fibre was slightly superior

to the 50-mm Melt Extract fibre for point load from the

results given in Table 14 for the second test series. The corresponding average fibre contents were 1.20 and 1.22%. The 50-mm Melt Extract fibre, at an average content of 0.64%, however, appeared for uniform loading from Table 16 to have been somewhat superior to the 25-mm Melt Extract fibre, at an average fibre content of 1.57%. These results were inconsistent and could not be explained.

(2) The apparent superiority of the 35-mm Melt Extract fibre over the 25-mm Melt Extract fibre for uniform loading as given for the second-test series in Table 16 seemed real in view of the similarity in the average fibre contents of 1.25 and 1.57%, respectively.

(3) The apparent superiority of the 35-mm Melt Extract fibre compared with the 50-mm Melt Extract fibre for uniform loading in Table 16 could be explained in terms of the difference in average fibre content, namely 1.25 and 0.64%, respectively.

(4) On the whole, the results indicated, although not very strongly, that 30-35 mm was an optimum fibre length. Below this length the r~inforc ing effect of the

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TABLE 10. Comparative prototype panel test results from first series

Standard Ratio" (%) of effective thickness at given Ratio" (%) of effective load at given Reinforcement Load thickness deflections (mm) deflections (mm)

condition condition at or or peak load = Peak E Peak

comparison comparison" (mm) load 50 100 150 Average load 50 100 150 Average'

Mesh

Average Standardized to peak load Standardized to deflection

Fibre




"P, point; U, uniformly distributed. " ~ a t i o s were deterniined froin figures corresponding to U and P loads and to mesh and fibre reinforcement conditions given in Table 8. "Averages in parentheses are expressed as percentages of the peak load ratio.

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TABLE 11. Comparison of results from the first and second series of panel tests

Standard Ratio" (%) of effective thickness at given Ratioc (%) of effective load at given Reinforcement Load thickness deflections (mm) deflections (mm)

condition condition at or or peak load = Peak = Peak

comparison" comparisonb (mm) load 50 100 150 Average load 50 100 150 ~ v e r a g e "

1 st:2nd series mesh


lst:2nd series mesh


lst:2nd series D30


lst:2nd series D30


"D30, 30-mm Dramix fibre. "P, point; U, uniformly distributed. 'Ratios were determined from figures corresponding to 1st and 2nd series mesh and D30 reinforcement conditions given in Tables 8 and 9. " ~ v e r a g e s in parentheses are expressed as percentages of the peak load ratio.

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TABLE 12. Comparison of results from first and second series of panel tests

Standard Ratio" (%) of effective thickness at given Ratio' (%) of effective load at given Reinforcement Load thickness deflections (mrn) deflections (mm)

condition condition at or or peak load = Peak T Peak

comparison" comparisonb (mm) load 5 0 100 150 Average load 50 100 150 ~ v e r a ~ e "

1st series D30 : 2nd series ME25 P


1st series D30 : 2nd series ME25








"D30, 30-rnm Dramix fibre; ME25, ME35, and ME50, 25- , 35- , and 50-mm Melt Extract fibres, respectively. "P, point; U, uniformly distributed. 'Ratios were determined from figures corresponding to 1st series D30 and 2nd series ME25 reinforcement condition given in Tables 8 and 9. " ~ v e r a g e s in parentheses are expressed as percentages of the peak load ratio.

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TABLE 13. Comparison of uniformly distributed and point loading for various reinforcement types

Standard Ratio" (%) of effective thickness at given Ratio" (%) of effective load at given Reinforcement Load thickness deflections (mm) deflections (mm)

condition condition at or or peak load 5 Peak Peak

comparison comparison (mm) load 50 100 150 Average load 50 100 150 ~ v e r a ~ e "

2nd series mesh U:P 150 100 105 100 94 99 21 1 234 211 186 (1 00) 100 100 104 103 108 104 248 283 272 295 (110) 50 100 105 105 114 106 294 328 368 383 (117)

Average - 100 105 103 105 - 25 1 282 284 288 276 Standardized to peak load - 100 105 103 106 103 100 122 113 114 (1 10) Standardized to deflection - - - - - - 100 103 106 103 103

2nd series D30 U:P 150 100 - - - - - - - - -

100 100 100 94 77 93 250 252 219 147 (87) 50 100 100 92 69 90 339 342 278 154 (82)

Average - 100 100 93 73 - 295 297 249 151 248 Standardized to peak load - 100 107 99 7 8 96 100 115 96 59 (93) Standardized to deflection - - - - - - 100 100 98 97 99

2nd series ME25


2nd series ME50 150 100 141 162 - 134 240 495 682 -

100 100 109 156 - 122 202 263 511 -

50 100 106 135 - 114 157 173 300 -

Average - 100 119 151 - - 200 310 498 -

Standardized to peak load - 100 118 150 - 123 100 142 239 -

Standardized to deflection - - - - - - 100 103 106 -

NOTE: Abbreviations as in Table 12. "Ratios were determined from figures corresponding to U and P loads given in Table 9 for the 2nd series mesh, D30, ME25, and ME50 reinforcement conditions. " ~ v e r a ~ e s in parentheses are expressed as percentages of the peak load ratio.

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TABLE 14. Comparison of 30-mm-long Dramix fibre with various lengths of Melt Extract fibre for panels subjected to point load

Reinforcement condition

or comparison

Standard Load thickness

condition at or peak load

comparison (mm)

2nd series D30:ME25


2nd series D30:ME50


Ratio" (%) of effective thickness at given deflections (mm)

Ratio" (5%) of effective load at given deflections (mm)

= Peak load 50 100 150 Average

100 102 107 115 106 100 164 164 162 148 100 167 163 160 147 100 144 144 145 -

100 140 141 144 131 - - - - -

= Peak load 50 100 150 ~ve rage"

2nd series ME25:ME50 P 150 100 131 149 - 127 112 232 303 -

100 100 83 112 9 8 85 64 122 - (193)

-

5 0 (106)

100 84 106 97 8 8 59 114 - -

Average - 100 99 122 - - (99)

95 118 180 - 131 Standardized to peak load - 100 98 122 - 107 100 106 175 - (127) Standardized to deflection - - - - - - 100 110 118 - 109

NOTE: Abbreviations as in Table 12. "Ratios were determined from figures corresponding to 2nd series D30 and ME25 reinforcement conditions given in Table 9. " ~ v e r a g e s in parentheses are expressed as percentage of the peak load ratio.

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TABLE 15. Comparison of 30-mm-long Dramix fibre with various lengths of Melt Extract fibre for panels subjected to uniformly distributed load



comparison (mm)


Ratio" (%) of effective load at given deflections (mm) Reinforcement

condition or

comparison = Peak

load 50 100 150 Average = Peak

load 50 100 150 ~ v e r a g e "

2nd series D30:ME25


2nd series D30:ME35


2nd series D30:ME50


NOTE: Abbreviations as in Table 12. "Ratios were determined from figures corresponding to 2nd series D30, ME25, ME35, and ME50 reinforcement conditions given in Table 9. " ~ v e r a g e s in parentheses are expressed as percentages of the peak load ratio.

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TABLE 16. Comparison of various lengths of Melt Extract fibre for panels subjected to uniformly distributed load

Reinforcement condition

or comparison



comparison (mm)

2nd series ME25:ME35



Ratio" (%) of effective load at given deflections (mm)

E Peak load 50 100 150 Average

100 79 67 5 0 74 100 81 74 5 4 77 100 87 79 65 82 100 82 73 56 -

100 82 73 5 6 7 8 - - - - -

= Peak load 50 100 150 ~ v e r a ~ e "

2nd series ME25:ME50 U 150 100 86 81 64 83 7 9 54 52 35 (70) 100 100 9 1 79 60 8 3 106 8 1 64 41 (69) 50 100 94 85 74 8 8 138 126 101 80 (80)

Average - 100 90 82 66 - 108 87 72 52 80 Standardized to peak load - 100 90 82 66 84 100 78 66 46 (72) Standardized to deflection - - - - - - 100 96 96 106 100

2nd series ME35:ME50 U 150 100 109 121 129 115 181 280 354 385 (166) 100 100 113 108 111 108 152 192 168 171 (112)

100 100 108 115 108 166 223 214 228 (125) Average - 100 110 112 118 - 166 231 245 261 226 Standardized to peak load - 100 110 112 118 110 100 139 145 154 (135) Standardized to deflection - - - - - - 100 115 116 111 110

NOTE: Abbreviations as in Table 12. "Ratios were determined from figures corresponding to 2nd series ME25, ME35, and ME50 reinforce~nent conditions given in Table 9. !

" ~ v e r a g e s in parentheses are expressed as percentages of the peak load ratio.

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440 CAN. GEOTECH. I. VOL. 30, 1993

fibre may not have been fully developed, and above this length the required fibre content could not be practically achieved.

(5) It again appeared from these comparisons, but still not conclusively, that fibre content did influence the behaviour of the shotcrete.

Summary of findings The overall findings from the two test series may be

summarized as follows. The uniformly distributed loading was marginally less severe than the point loading, the effect of which on support design is negligible.

Mesh reinforcement was found to be superior to fibre reinforcement for uniformly distributed and point loading. This was due to the more effective location of the mesh within the section of the shotcrete with regard to bending and catenary action.

The load-carrying capacities of the panels from the two test series were in general not significantly different. Also, no difference was found between the Dramix and Melt Extract fibre with regard to reinforcing action. Fibre content did affect the performance of the shotcrete, but not conclusively so at the relatively small contents investigated. Indications were found, albeit not very strong, that 30-35 mm represented an optimum fibre length. Below this length the reinforcing effect of the fibre did not develop fully, and above this length the required fibre content could not be practically achieved.

The average contents for the 30-mm Dramix fibre in the two test series were 1.43 and 2.53%, respectively. The average content for the 25- and 35-mm Melt Extract fibre in the second test series was 1.29% and for the 50-mm Melt Extract fibre 0.83%. Despite close control during the manufacturing process, neither the consistency nor target content of the fibre, (3% by weight) could be obtained. It appeared from both test series that an average content of about 1.5% by weight was the maximum which could be obtained in practical mining applications for fibre lengths not exceeding 35 mm. At a greater length, the maximum fibre content can be expected to be less.

Shotcrete is much stiffer than rock reinforcement in the context of an application to the surface of an underground excavation. Consequently, it attracts the greater proportion of the load in heavily converged ground and destructs itself in the process. Under these circumstances, fibre-reinforced shotcrete would be less competent than

mesh-reinforced shotcrete because of the latter's ability to sustain larger deformations., Notwithstanding, fibre- reinforced shotcrete may be an attractive means to ensure the early safety of the workplace.

The strictly controlled civil engineering construction site, in which context underground chambers and tunnels are not subject to deformations of significant magnitude, seems the only application in which the sophisticated attributes of fibre-reinforced shotcrete can be effectively exploited.

Acknowledgements The work described was undertaken on behalf of

De Beers Consolidated Limited, Premier Mine Division, for which the opportunity is sincerely appreciated and permission to publish this paper gratefully acknowl- edged. Sincere thanks are due to Mr Peter Labrum, a fel- low member of staff, for overseeing all the testing and for reducing the results.

ASTM. 1985. Standard test method for flexural toughness and first-crack strength of fibre-reinforced concrete (using beams with three point loading) (C1018-85). In 1985 Annual Book of ASTM Standards. ASTM, Philadelphia. Sect. 9. Vol. 03.04. pp. 637-644.

Fernandez-Delgado, G., Mahar, J.W., and Parker, H.W. 1976. Structural behaviour of thin shotcrete liners obtained from large scale tests. In Shotcrete for Ground Support. Pro- ceedings of the Engineering Foundation Conference, Easton, Md. American Concrete Institute, Detroit, Mich. Publication SP-54. pp. 399-442.

Hanssen, R. 1988. Steel fibre reinforced shotcrete for ground support. Proceedings of Conference on Applied Rock Engineering. Institute of Mining and Metallurgy, Newcastle upon Tyne, England. pp. 83-87.

Holmgren, B.J. 1983. Tunnel linings of steel fibre reinforced shotcrete. Proceedings, 5th Congress of the International Society for Rock Mechanics, Melbourne. pp. D3 11-D3 14.

Little, T.E. 1985. An evaluation of steel fibre reinforced shot- Crete for underground support. Canadian Geotechnical Journal, 22: 501-507.

Morgan, D.R., and Mowat, D.N. 1984. A comparative evaluation of plain, mesh and steel fibre reinforced shotcrete. Proceedings of the International Symposium on Fibre Reinforced Concrete. American Concrete Institute, Detroit, Mich. Publication SP-81. pp. 307-324.

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Documents

Equivalence of mesh- and fibre-reinforced shotcrete at large deflections