AN INVESTIGATION OF THE SIGNIFICANCE OF FLATJACK FLEXffiILITY IN THE DETERMINION OF IN-SITU STRESSES

Timothy G. Hughes1 and Robyn Pritchard2

1. ABSTRACT

The paper considers the application of the insitu stress relief technique, in a laboratory controlled experiment, where the results of both stiff (stainless steel) and flexible (nitrile rubber) flatjacks are reported. In addition to the overall calibration response, results showing the measured pressure distribution, obtained using small pressure pads, over the roof of the slot are presented. The results demonstrate the significant non uniformity of the pressure distribution produced by the steel jack whereas the flexible jack produced an essentially uniform pressure over the whole area of the slot. The conclusions suggest that some of the problems previously identified with the use of flatjacks may be ameliorated with the use of more flexible jacks.

2 . INTRODUCTION

The increasing volume of work associated with the assessment of the strength of old or damaged buildings has generated increased interest in the determination of both the insitu material properties and the correct current load path in these buildings, with the added requirement of minimal damage to the fabric of the structure inflicted by testing.

The flatjack technique is one of a number of essentially non-destructive techniques used to determine the insitu properties of masonry [1,2,3] . The method was originally applied to determine the insitu stress but has been extended to assist in the determination of both the deformability and shear characteristics of insitu masonry [3 ,4]. The work reported in the present study is relevant to ali the uses to which flatjacks are currently put but is presented in the context of insitu stress evaluation.

Keywords: Masonry; Insitu Stress; Flatjack.

1 Lecturer , School of Engineering , University of Wales College of Cardiff, PO Box 925 , Newport Road , CARDIFF , CF2 lYF, United Kingdom.

2Research Assistant, University of Wales College of Cardiff

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3. FLA TJACK INSITU STRESS TESTING TECHNIQUE

In the determination of insitu stress the following procedure is generally adopted.

1)

2)

3) 4)

5)

Gauge up area to be assessed either with fixed or demountable gauging systems and take a set of initial state readings. Cut a slot in the bed, or sometimes the masonry unit, using one of a number of cutting techniques. Introduce the flatjack into the slot, shimming the slot as necessary. Pressurize the flatjack until the strain readings taken in 1) are returned and record the strain equilibrium pressure. Use a fla~ack calibration curve to read off the insitu stress from the equilibrium pressure.

The literature on the use of the fla~ack for insitu stress measurement contains details of a number of studies where the conclusions drawn on the value of the flatjack have varied, these include field [4], laboratory [4,5,6,7] and numerical studies [7]. Some laboratory studies have indicated that under controlled conditions accurate estimates of initial stress can be determined [4]. Others have suggested that whilst capable of producing good results the method requires a significant element of subjective judgement in determining the equilibrium pressure as the gauged readings can provide conflicting information [6]. A criticaI review [7] identified a number of areas of concern with the technique some of which are addressed by the present paper.

All the methods use the flatjack to provide a known applied load perpendicular to the bed joint which is then translated in to an insitu stress. Regardless of the detail of the type of application there is a fundamental requirement that the externalload, produced by a known internal pressure within the jack, can be evaluated. This requires that a calibration curve between the internal pressure within the jack and the externai applied load can be determined. Additionally it is necessary that the area of the slot is known to enable the applied stress to be evaIuated.

The normal relationship between the externai stress provided by the jack and the internai stress (hydraulic pressure) is

where

Sf = Kj . Ka . Pf (1)

external stress from flatjack jack calibration constant « 1) slot/jack area constant « 1) internal stress in the flatjack.

In reality Kj is a function of the quality of the slot and Ka is difficult to determine.

The application of the method in practise has previously been undertaken by initially caIibrating the jack within a wall of proportions and bond similar to that of the actual site to which it is to be applied. Such a caIibration provides a direct relationship between the internal flatjack pressure and the insitu masonry stress. Such a relationship naturaIly includes effects such as the jacks' inherent stiffness and its effective area. The effect of the variation of jack response with slot thickness is assumed to be overcome by shimming the slot to the thickness at which the caIibration was undertaken.

The use of the flatjack technique in more general situations where it is not possible to calibrate directly is more problematic but frequently necessary. An example of such a situation would be the determination of the inner ring stress in a multi-ring tunnel. In

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these situations it would be not be feasible or indeed possible to calibrate the jack in the laboratory in a way that would exactly mirror the on site conditions.

If the way in which a stiff flatjack performs in a thin slot is considered then it appears likely that the pressure distribution would be as schematically shown in Fig. la. A1though this is shown as a section through the wall a similar distribution would be obtained for a front elevation of the wall. Such apressure distribution would likely yield some of the identified problems of gauges at different locations along the front face of the slot having different strain equilibrium pressures.

PRESSURE

APPLlED BY

FU"T JAC<

?OS!TIDN ~lONG SLOT

, ST.:t.HlL::::S S S-~[. fL:.r JAC<~

PRESS URE II

APP~ IED BY ,I ,L'IT JAC K I I

I I

(a) Stainless Steel

O[SITIClN "'c.Cl t\G SLOT

(b) Rubber

I S ~C ":: - e :;h -O;:.J

Fig . I Schematic Representation of Flatjack Pressure Distribution for a Stainless Steel (a) and Rubber (b) Flatjacks.

The use of a more flexible jack would produce a more uniform stress distribution and extend the area of application right up to the very edge of the slot, as shown in Fig . lb. Such a system would largely remove the necessity for Ka , would make Kj almost l.0, would remove the requirement to shim the slot and would enable poorer quality slots to be utilised

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3. EXPERIMENTAL PROCEDURE

Flatjack Construction Details

Two types of flatjack were utilised in the present study. The size of the jacks were set to conform with the standard UK brick unit. A rectangular shape was adopted, in preference to the D or sector shaped sometimes used, for two reasons. Firstly the normal use of a flatjack is in an environment where it is known that the stress varies through the thickness of the wall [8]. To present a different area to each stress, as with alI non rectangular jacks, is therefore problematic. The second reason for using rectangular jacks is to reduce the stiffening edge effects along the front face.

The first flatjack was of a traditional stainless steel construction. The details and dimensions are contained in Fig. 2 and follow the design of a flatjack previously tested and successfully used in the UK [1,2].

r Str. S1:eel 304S:5 ([N 58 D

r.\J " . . , 0315

'E~ i ~ f-s oco

Fig. 2 Stainless Steel Flatjack Fabrication Details

The fabrication of a flatjack made of such thin stainless steel plate causes some difficulties, with the plasma welding generating some initial buckling of the plates. This particular jack has been used several times in the lab and on site at pressures up to 6 MPa without obvious difficulties. Of particular note is the edge detail which clearly restricts the transfer of load to the slot in that region. Despite the overall height of the flatjack of approximately 5mm a slot height of some 8mm is required in order to insert the flatjack without obvi0US distress or damage.

I Fig. 3 Rubber Flatjack Fabrication Details

The second flatjack was designed specifically as part of the current programme of work and was fabricated from a 1mm sheet of Nitrile sheet as detailed in Fig. 3. The majority of the bag joints were sealed and the bag was then inverted. The inner end of the coupling was then inserted and the coupling complete before the final simple joint was sealed. A short length of material with a high in-plane stiffness was included within the coupling and along the entire front face to provide a front face restraint. This material is effectively fixed at the top and bottom face by the friction developed between the material and the masonry as the bag expands under pressure.

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FIatjack Calibration in Testing Machine

Each of the flatjacks were calibrated for overalI performance within a steeI sIot. The sIot was constructed with a separate top and bottom steeI pIate onto which emery paper was attached to simulate the masonry surface roughness, this was onIy strictly necessary for the Nitrile flatjack to deveIop sufficient friction to stop the flatjack expanding out the front opening. The flatjack was placed between the pIates and the testing machine was then closed so that the top and bottom pIates were at the specified slot separation. The required separation was achieved using steel shims with a very small residual load being carried by the system without any pressure in the flatjack. The machine was then run with deflection controI and set not to move from this initial position. The flatjack was then pressurised and this pressure, and the Ioad automaticalIy determined by the testing machine to maintain the separation, recorded.

Pressure Distribution Measurement

The pressure distribution was determined using Force Sensor Resistors (FSR). FSR's were chosen as they are thin, small, robust and produce repeatable resuIts. The FSR' s are availabIe as single units or in matrix form suitabIe to cover an area. Single units were utilised in the present study because of the difficulty of properIy calibrating individual matrix eIements. This difficulty arises because each matrix eIement will produce a different Ioad-resistence (+ 15 %) response and because of the technical difficuIties of Ioading individual eIements within a matrix. The prime difficuIty in using a singIe sensor is that it represents a high spot on the sIot surface that will naturalIy attract higher Ioad, this problem was alIeviated as detailed beIow. The secondary probIem of having to repeatedly move the sensor simpIy takes time but is not thought to have introduced any additional inaccuracies.

Because the sensors are somewhat sensitive to the material between which they are sandwiched for the present study a thin rubber sheet was pIaced between the sensor and the slot and another sheet between the sensor and the steeI flatjack, this second sheet was not necessary for the Nitrile flatjack.

The use of these rubber sheets should eventually allow the work to be extended to actual measurements in slots cut in masonry. The use of the rubber sheets has the likeIy effect of flattering the pressure distributions produced for the steel flatjack since it alIows a more uniform stress distribution to be recorded than is IikeIy in real situations where the surface of the slot may have local undulations caused by the cutting processo These undulations may occur in real situations due to variations in the cutting tool height or to variations in the mortar bed thickness.

The sensor was initially calibrated as detailed in the next section. The sensor was then placed at the required position and the electrical resistance recorded for a set range on internal flatjack hydraulic pressures. The sensor was then moved to a new position and the process repeated. Finally the calibration was repeated to ensure no change had occurred.

Pressure Cell Calibration

The pressure sensors were calibrated using a dead load system. To comply with the situation in which the sensors were to be used the sensors were sandwiched between rubber sheets as detailed in Fig. 4.

The diameter of the sheets coincided with the overall diameter of the sensor rather than its effective area and this larger diameter was used in translating the applied 10ad of the system into apressure. The sensor resistance was recorded for pressures

573

between 0.0 and 1.0 MPa. The small drift in the readings, with time, was also recorded to enable a proper procedure for the actual tests to be developed. The calibration tests were repeated to ensure good repeatability.

Fig. 4 Details of FSR Pressure Calibration

Pressure Cell Distribution

The pressure in one quarter of each of the flatjacks was monitored at the 20 positions marked AI to D5 in Fig . 5. The sarne 19mm diameter, 0.3mm thick, circular sensor was used at each location. It is evident that each sensor covers a significant area and that with the expected pressure gradients across the flatjack face there will be sÍtuations where the sensor is not loaded equally across its area. Because of the complex calibration curves it may be that the pressure will not be well represented in these areas. It should be noted that the sensor positions along the front and side are not at the extreme edge and the entire area of the sensor is within the flatjack area.

~ I

11.0 11 30 j' 25.5 --+-- 2 5 5 --t- 25 5 --+-- 25 5-

I i I

'I I \ ~I i \ ('I"j fI'\ J-----,-B' + -"1- - 1- i- - T -( - 1-'-- 0 10.5 ISS , \d:x '><?<'

570 i- (- ~ ~ -.., - - r r ?- ~ -7 .{ - 1- L c 15.5 '><I>! / ~

I +-(-~tm: -=s--r- ffi' \ - ~ -1'· )- ' I ISS (~ '<:s><'

J ' E3 -t- 0 iJ0' Fig . 5 Position of Pressure Cells on Flatjacks

4. RESULTS AND DISCUSSION

Stainless Steel Flatjack Calibration

The calibration curve for the Stainless Steel Flatjack, determined as outlined above, is shown in Fig. 6 as a relationship between the average external pressure, evaluated as

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the load the testing machine required divided by the area of the slot against the internal (hydraulic) pressure. The steel slot was manufactured to the exact size of the flatjack (226mm x 114mm). For this example there was a slot height of 10 mm, some 2mm larger than the minimum usable for this flatjack.

o: I .. 0.8 ~ ~ 0 .7 1 ª 0.6 I ~ 0.5 í "ii 0.4 -:: I

~ 0.3 I 02 I 0,1 i

I --Ideal

-:t-- 10mm

o 0 .1 0.2 0.3 0.4 0 .5 0 .6 0.7 0.8 0.9

Internai Pressure MPa

Fig. 6 Calibration for Stainless Steel Flatjack

AIso shown in Fig. 6 is the response of an ideal flatjack. It is apparent that a significant element of the pressure goes in deforming the flatjack with, at IMPa, only some 70% of the pressure, on average, being passed on to the slot. The response is seen to be almost linear with pressure. This is somewhat surprising as it would be reasonable to expect a geometric non linear effect as the bag became more swollen at higher pressures. This effect was not apparent at larger pressures of up to 6 MPa (results not presented).

Stainless Steel Flatjack Pressure Distribution

The pressure distribution for the Stainless Steel Flatjack at an internal pressure of 4 bars (0.392 MPa) is shown in Fig. 7. The general arrangement of the pressure contours are largely as anticipated, see Fig. la, with almost no discernible pressure along the front and side coinciding with the edge effect of the joint. A1though, as previously stated , the sensors will not accurately determine the average pressure for an area subject to large normal pressure gradients the absolute lack of detectable pressure at the edge sensors is considered a true effect.

Fig. 7 Pressure Contours for Steel Flatjack (0.396 MPa)

The pressure is seen to rise rapidly from the edge to plateau out at the second sensor in from both the front and side edge. It is noticeable that the external pressures drop

575

off again towards the centre of the stainless steel jack. This phenomena was noticeable at aII tested pressures, with this jack, and is thought to be related to a smaII initiaI buckle introduced into the flatjack during the joint welding as part of the fabrication processo The sensor pressure at position C4 is significantly above the hydraulic pressure in the flatjack and must be attracting load f TOm the adjacent area associated with some initiaI deformation .

The pressure distributions at higher pressures (not presented) show the sensed pressure at the centre rising closer towards the higher vaIues just in f TOm the edges but never quite reaching them. There is no indication even at 1 MPa internai pressure of any externai pressure at any of the edge sensors.

Stainless Steel Flatjack Integrated Pressure CaIibration

The pressure contours shown in Fig. 7 were numericaIly integrated to determine an effective overall internai-externai pressure caIibration response at 4 bar pressure. This exercise was repeated for the other internai pressures in the range 0.0-1.0 MPa .. This was undertaken as a check by pTOviding a comparison to the direct testing machine caIibration aIready undertaken. For the Stainless Steel Flatjack the results are contained in Fig. 8 using the same axes as Fig. 6.

0 .9

.. 0 .8 "-~ 0 .7 C> ã 0.6

~ 0 .5 "-iS 0 .4

] 0 .3 I

l- Ideal

--O-- l O mm

02 I 0 .1 I

O~------~--~--~----r---~--~------~~~

o 0 .1 0 .2 0 .3 0.4 0.5 0 .6 0 .7 0 .8 0.9

Internai Pressure MPa

Fig. 8 Integrated Pressure CaIibration for Steel Flatjack

The caIibration curves produced by direct caIibration, Fig. 6, and from integrating the sensed pressures, Fig . 8, are similar. Both curves show similar vaIues and trends thus providing confidence in the measured pressure distribution .

0.9

0. 8

0 .7

0 .6

0 .5

0 .4

0 .3

0 .2

0 .1

o o ~1 O~ 0 .3 ~4 ~5 0 .6 0 .7 0. 8 0.9

Intemal Pressure MPa

Fig. 9 CaIibration for Rubber Flatjack

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Rubber Flatjack Calibration

The Rubber Flatjack calibration was undertaken as detailed above and the result is shown in Fig. 9. The Flatjack was calibrated at a slot height of lOmm but the calibration is not sensitive to this dimensiono The calibration indicates values of Kj and Ka dose to unity . A linear response with pressure is obtained, as anticipated.

Rubber Flatjack Pressure Distribution

The pressure distribution obtained from sensors in the same position and for the same pressure of 4 bars but for the flexible rubber flatjack is shown in Fig. 10. The pressure distribution shows no noticeable side edge effect with the pressures along column 5 being similar to columns 1-4. There is some reduction in pressure along the front edge but at 0.2-0.3 MPa it still represents a significant pressure when compared to the internal hydraulic pressure of 0.39 MPa. The significant area of high pressure, shown as 0.4-0.5 MPa, is somewhat misleading as the pressure only just creeps into that range.

0 0 .5-0 .6

0 0 .4-0 .5

0 0 .3-0 .4

.0.2-0 .3

.0.1-0 .2

0 0 -0 .1 5 4 3

Front 2

D

C

B

Q)

" üi

Fig. 10 Pressure Contours for Rubber Flatjack (0.396 MPa)

The results obtained at higher pressures (not presented) have the same characteristic with no fali off in pressure along the side and with the front row of sensors continuing to reproduce an appreciable percentage of the internal pressure.

Rubber Flatjack Integrated Pressure Calibration

o ~ + .. 0 .8 t ~ 0 .7 T

~ 0 .6 + ~ 0 .5 T ... I ê 0.4 í 3 0 .3 1

0 .2 -

--- Ideal

0 .1

o~~--------+-------+---+---+-----------~ o 0 .1 0 .2 0 .3 0 .4 0. 5 0.6 0 .7 0.8 0 .9

Internai Pressure MPa

Fig. Ii Integrated Pressure Calibration for Rubber Flatjack

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Finally the pressures contours detailed in Fig. 10, and those obtained at other internal pressures, were integrated to produced the calibration curve shown in Fig . 11. This compares well with the testing machine calibration shown in Fig. 9.

5. CONCLUSIONS

A comparison between the results obtained for the steel and the flexible flatjack pressure distribution shows the steel flatjack as having a highly non uniform pressure distribution whereas the flexible jack only has some small front edge effect and otherwise produces a uniform pressure.

It is considered that the use of more flexible jacks will;

a) allow lower insitu stresses to be determined, b) allow the utilisation of less stringent slot preparation procedures and c) improve the overall performance of the flatjack technique.

6. ACKNOWLEDGEMENT

The work contained in this paper was funded by the United Kingdom Science and Engineering Research Council through grant number GRlH76296 and their support is gratefullyacknowledged.

7. REFERENCES

1. de Vekey, R. C., "Non Destructive Test Methods for Masonry Structures," Proceedings of the 8th International BrickJBlock Masonry Conference, Dublin, Ireland, 1988, pp 1673.

2. de Vekey, R.C., "In-situ Tests for Masonry," Proceedings of the 9th International BrickJBlock Masonry Conference, Berlin, Germany, 1991, pp 620-627.

3. Rossi, P.P., "Non destructive Evaluation of the Mechanical Properties of Masonry Structures", Conference on Non Destructive Evaluation of Civil Structures and Materials, University of Colorado, Boulder, 1990, p17.

4. Rossi, P.P., "Flat-jack test for the Analysis of the Mechanical Behaviour of Brick Masonry Structures", ISMES Bulletin No 205, Bergamo, ltaly, 1982.

5. Noland, J.L., Kingsley, G.R., and Atkinson, R.H. , "Utilisation of Non Destructive Techniques into the Evaluation of Masonry," Proceedings of the 8th International BrickJBlock Masonry Conference, Dublin, Ireland, 1988, pp.

6. Epperson, G.S., Abrams, D.P., " Non Destructive Evaluation of Masonry Buildings", Research Report No 89-26-03 , University of Illinois, 1989.

7. Maydl, P., "A Critical Review of Application of the Flat-jack Method to Brick Masonry", Proceedings of the 9th International BrickJBlock Masonry Conference, Berlin, Germany, 1991, pp 645-652 .

8. Abdunur, C., "Direct Access to Stresses in Concrete and Masonry Bridges", Bridge Management, Thomas Telford, London, 1993, pp 217-226.

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