Dynamic modulus of elasticity of masonary units

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Experimental determination of the dynamic Modulus of Elasticity ofmasonry units

J.M. Nichols Department of Civil, Surveying and Environmental Engineering, University of Newcastle,

Callaghan, NSW, Australia.

Y.Z. Totoev

Department of Civil, Surveying and Environmental Engineering, University of Newcastle,

Callaghan, NSW, Australia

ABSTRACT: One of the parameters that has been identified as influencing the structural re-

sponse of buildings is the dynamic Modulus of Elasticity of masonry units. The first aim of this

paper is to experimentally investigate the use of high frequency sinusoidal loading to determine

the dynamic Modulus of Elasticity of masonry units. The Longitudinal Vibration and the Ultra-

sonic Pulse methods were originally developed for the dynamic testing of concrete specimens.

These testing procedures are appropriate as each applies only a minimum stress to the masonry

units. This procedure provides results for the masonry units within the elastic range. The second

aim is to compare the dynamic Modulus of Elasticity results to the Modulus of Elasticity obtained

using quasi-static methods from the same population of masonry units. A test rig has been de-

veloped for measuring the elastic properties of masonry units under uniaxial loading.

1 INTRODUCTION

Seismicity within Australian is now recognized as a design issue, as a result of several major

earthquakes within the last 40 years in NSW, SA and WA. One of the common building materi-

als used over the last century in Australia is masonry. There has been a growth in the use of

unreinforced masonry(URM) in the last 50 years, particularly in light commercial, housing and as

infill panels to steel and concrete framed buildings. These types of buildings can pose a signifi-

cant hazard during seismic events. The main reasons for this performance problem are the rela-

tive mass to strength ratio, ductility issues and poor workmanship (Melchers & Page,1992). Un-

reinforced masonry can however under some circumstances withstand an interplate earthquake

and perform well within its design limitations, when constructed to accepted sta ndards.(Tena-

Colunga & Abrams, 1992). A reasonable proportion of the larger masonry buildings and dwell-

ings built within in Australia and elsewhere in intraplate regions would have been designed on the

basis of static loading design rules and assuming zero or low seismic loads. The design of build-ings within Australian must now consider the minimum loading from the Australian Standard(AS)

Earthquake Loading Code (AS 1170.4) using either an equivalent static loading, frequency do-

main or time domain analysis. Material properties are required for these methods of analysis,

irrespective of the numerical method.

The first objective of this paper is to experimentally determine the dynamic Modulus of Elas-

ticity for masonry units using two non-destructive techniques. The second objective is to meas-

ure the Youngs Modulus and Poissons Ratio using quasi-static methods. The third objective is

to compare the quasi-static and dynamic results. The dynamic results are based on the assump-

tion that Poissons Ratio is invariant and can be measured using the quasi-static procedures.


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Three pressed clay bricks designated by colour (red, brown, biscuit), one calcium silicate and one

concrete brick were used in the experimental work.

2 BACKGROUND

Youngs Modulus is an intrinsic property constant for a material. It can be estimated using the

15 to 85 % stress levels in the elastic range from a quasi-static test procedure on a previously

untested population sample (Krajcinovic 1996, LeMaitre 1992). Hookes law is defined as :

= E (1)

where defines the stress (MPa), is defined by the natural (rather than the engineering) strain

formula (Strains) and E is Youngs Modulus (MPa). Youngs Modulus and Poissons Ratio

have been previously measured for a number of Australian brick types. These results were

within the range of 7,000 to 12,000 MPa and 0.12 to 0.29 respectively (Dhanasekar 1985).

Two distinct failure mechanisms can be identified for a macroscopically homogeneous body

of brittle material. The failure mechanism of the first kind is quasi-static or stress wave load-

ing of low intensity when a single failure plane predominates activated by a flaw in the mate-

rial being stressed to a critical condition. The failure mechanism of the second kind for masonry

or dynamic failure mode occurs during impulsive loading where multiple fractures on different

planes can be nucleated and they grow to a significant size without arresting each other. Ce-

ramics and other brittle material have been shown to have a greater fracture resistance to load-

ing that causes the failure mechanism of the second kind (Freund, 1990).

These distinct mechanisms have been observed in masonry testing (Tercelj et.al,1969, Klopp

1996). The first or quasi-static mechanism is the basis for the development of key static masonry

design rules (Page, 1979). The results form part of the dynamic test program on masonry panelsbeing undertaken at Newcastle. This research on the two failure mechanisms would suggest

that at an equivalent time the following inequality holds, (where ~ denotes dynamic and - denotes

static Modulus of Elasticity):

~E Et t (2)

This qualitative observation can be attributed of Tercelj et.al.,(1969). The dynamic Modulus of

Elasticity can be determined from Equation (3) for the Longitudinal Dynamic Test Method

(LDTM) and from Equation (4) for the Ultrasonic Pulse Method (UPM). The LDTM uses a

small audio striker to provide pulses along the longitudonal axis of the brick. The UPM uses the

measurement of the travel time of ultrasonic pulses in the transverse and longitudonal axes.

fE

L=

~

42

(3)

VE

=

+

~( )

( )( )

1

1 1 2 (4)

where f(Hz) is the fundamental natural frequency, L (m) is the length of the specimen,

(kg/m3) is the density, V(m/s) is the pulse velocity and is Poissons Ratio.

The ultrasonic pulse method can be used to determine if the masonry unit samples show any

pattern of anisotrophy, such as may be exhibited as a result of the process of manufacture.


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3 EXPERIMENTAL METHOD

The LVTM uses a dynamic test rig that is a modified version of the Electrodynamics Stan-

dard Material Tester EMFCO SCT/5 (EMFCO, n.d.) This test rig is noted in the specification

as to complying with British Standard (BS) 1881: 52 ( Longitudinal Vibration ). Specimens were

saw-cut from a standard range of bricks supplied by local manufacturers. Each specimen was

microwave dried and then tested in the rig. Each was then allowed to stabilize at room tempera-

ture before being retested. A Tektronix Function Generator FG501 with controlled frequency

was used to generate the applied sinusoidal loading function. This signal was amplified using a

Peavey Electronics Corp. XR400 Amplifier to feed the 3 Ohm coil on the test rig. Each speci-

men was clamped on the test rig using an 11 mm rad. jaw clamp at the midpoint of the cut brick.

A piezoelectric crystal pick-up detects the signal which was monitored on an Tektronix Oscillo-

scope 7603 for peak amplitude. The frequencies used were in the range from 5 to 9 kHz. A

schematic arrangement of the equipment is shown in Figure 1.

Figure 1 Layout of the dynamic test rig for the Longitudonal Vibration Test Procedure.

The second dynamic method UPM uses a standard measurement system. This method uses

the CNS Portable Ultrasonic non destructive tester (CNS 1978). A calibrating specimen is pro-

vided with the rig. Testing was at 50 kHz about the longitudonal and transverse axes of the

specimen. A schematic arrangement of the Ultrasonic Puls Method is shown in Figure 2.

Figure 2 Layout of the dynamic test rig for the Ultrasonic Puls Test Procedure


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The quasi-static Test Method uses uniaxial compression applied to the smaller end faces of the

brick samples with a Tinius Olsen 1800 kN Universal Testing Machine(UTM). A pressure cell

in the UTM generates an analog output signal. Initially this signal was converted to a digital sig-

nal using a Gedge Systems(Aus) GS1650P Peak Indicator . The analog to digital signal genera-

tor proved to be too coarse (2.5kN) for this testing and caused a substantial component of the

error in the measurement. Final testing had the analog signal being fed directly into the data log-

ger. Calibration of the signal was undertaken using the UTM dial scale.

Each brick was tested for Youngs Modulus about the longest axis to provide a reasonable

gauge length for the measurements, to protect the Linear Velocity Displacement Transducers

(LVDTs) and to partially negate end effects. The test method was based on the relevant Aus-

tralian Standard for testing masonry units. Bricks were immersed for a minimum of two hours

before being tested, except for the few that were used in both the dynamic and quasi-static test-

ing. Plywood capping was used between the solid platens and the brick.

A rectangular test rig capable of measuring the relative displacements about two axes was

designed to provide a repeatable measurement protocol. The rig is similar to the standard cylin-

drical concrete test rig, except that it is modified to measure Poissons Ratio. Vertical or longi-

tudinal displacement was measured using two LVDTs Type RDP Electronics D2-200A. These

have a total movement of 11 mm. Horizontal displacement was measured using two LVDTs

Type Solartron DFG 5.0. These have a total movement of 12 mm. Each of the LVDTs was

calibrated using a Mitutoyo gauge with a range of 0 - 25 0.005 mm and the sensitivity tested

using a Mitutoyo gauge with range of 0 - 1 0.0002 mm. . The vertical gauge length was 100

mm and the horizontal gauge length was 90 mm. The final test protocol measured both dis-

placements at the same time.

Signals were fed into a Data Electronics Datataker 600. The signals were logged and con-

verted to an ASCII format using DASYLab 3. The results were analysed using a regression

macro written for MINITAB 10.2.

4 RESULTS

These series of experiments were undertaken to determine the Youngs Modulus, Poissons

Ratio and dynamic Modulus of Elasticity of masonry units. Five masonry units were tested,

three pressed clay bricks, a concrete and a calcium silicate bricks. All bricks were of local State

manufacture and the three pressed bricks were tagged by colour red, brown and biscuit.

Youngs Modulus and Poissons Ratio were measured using the quas i-static test method. The

test results are based on the gross area of the cross section of each unit. No allowance has

been made for the frog. The quasi-static test results for each set of the five brick types are

shown in Table 1.

Table 1. Youngs Modulus, Poissons Ratio and typical dimensions for the five brick types.

Br ick Type Number Tested Y ou ng s

Modulus

MPa

Poissons Ra-

tio

Length : Width

:Depth

mm

Pressed Clay Red 6 14,000 0.22 226: 111: 75

Pressed Clay Biscuit 4 10,000 0.29 230: 110: 76

Pressed Clay Brown 5 7,000 0.21 227: 108: 74

Calcium Silicate 3 6,000 0.17 229: 108: 78

Concrete 4 14,000 0.33 232: 109: 77

The range of Youngs Modulus was from 1 GPa for a pressed clay red brick to 56 GPa for a

concrete brick. The stress-strain curves in compression were generally linearly elastic over the


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range used to measure Youngs Modulus(15 to 85 % of the peak stress), with a regression coef-

ficient of R2

~0.95 being a typical value.

A few of the pressed bricks exhibited stress-strain curves that would suggest that there is a

variation of density within the brick. This variation can probably be attributed to the compaction

associated with the creation of the frog. Initial results would point to a greater material density

on the frog side of the brick. The non-symmetric shape of the pressed brick requires the use of

averaged results between the two sets of displacement for the measurement of Youngs

Modulus for some bricks. Poissons Ratio results ranged from 0.1 to 0.4.

The specimens used for the Longitudinal Vibration Test Method were cut from full bricks.

Three different specimen sizes were used in the experiments . Typical dimensions for each of

the specimen sizes, presented as Length (mm),:Cross Sectional area (mm2) and the density

(kg/m3) are presented in Table 2.

Table 2. Typical Brick Properties for the specimens for the Longitudinal Vibration Test Method.

Brick Type Description Specimen Type 1 Specimen Type 2 Specimen Type 3

Typical Length mm 180 200 230

Typical Cross

section area mm 2

1100 1800-2020 2250-2500

Pressed Clay Red Density kg/m3 2170 2070 2320

Pressed Clay Bis-

cuit

Density 2220 2230 2270

Pressed Clay

Brown

Density 2130 2180 2130

Calcium Silicate Density 1810 1740 1760

Concrete Density 2010 2100 2190

Three full size brick specimens for each of the five brick types were tested using the UltrasonicPulse Method. The Longitudinal Vibration Test Method uses Equation (4) and the results of the

frequency measurements, the dimensions and the density results. The Ultrasonic Pulse Method

uses Equation (5), Poissons ratio from the quasi-static testing, the transit time results, the dimen-

sions and the density results. The results for the quasi-static and dynamic testing are shown in

Figure 3. The calcium silicate and the concrete bricks exhibit isotropic behaviour in the Longitu-

donal Vibration Test as can be seen in Figure 3. The remaining bricks exhibit slight anistrophy

probably attributable to the method of manufacture.


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Figure 3 Modulus of Elasticity Results for the three different test procedures.

5 CONCLUSION

This study was designed to compare the quasi-statically measured Youngs Modulus to the dy-

namically measured Modulus of Elasticity. The work forms part of a research program into the

response of masonry panels subjected to seismic frequency and intensity of loading. The quanti-

fication of the Youngs Modulus and Poissons Ratio was made with three test methods. Thefirst method was quasi-statically in a Universal testing Machine and the second two methods

were variations on standard techniques used extensively in concrete research. A set of proto-

cols and a test rig were developed to quasi-statically measure the Youngs Modulus of masonry

units. This test rig is based on the rig used to test concrete cylindrical specimens.

Two dynamic methods are used to measure the Modulus of Elasticity of concrete cylindrical

specimens. These two methods are the Longitudonal Vibration and Ultrasonic Pulse Velocity

Test Methods. The limitation for these procedures is the frequency dependence of the results.

The results for the quasi-static measurement of the Youngs Modulus and Poissons ratio for

the masonry units is presented in Table 1. The results for the dynamic measurement of the

Modulus of Elasticity for the two dynamic methods are shown on Figure 3. These results are

within the range of Youngs Modulus normally expected for these types of masonry units.

Dynamic measurement of the Modulus of Elasticity is a practicable alternative to quasi-static

destructive testing for clay masonry units. There is no evidence of frequency dependence for

the clay masonry units within the range of frequencies available with these two methods. There

appears to be a strong frequency dependence for the sand based masonry units.Testing is required using many specimens to quantify the relationship. The frequency depend-

ence of the stiffness of masonry walls was noted by others at lower frequencies than used with

these two test methods. Further research is suggested on masonry units in the non elastic range.

REFERENCES

CNS Electronics Ltd. 1978. Pundit Manual for use with Portable Ultrasonic Non-

destructive digital indicating tester. London: CNS.

Dhanasekar, M. 1985.The performance of brick masonry subjected to in plane loading.

Dissertation No. 990. Newcastle: University of Newcastle, Australia.

EMFCO. n.d. Specification for the SCT/5 Test Rig. London: EMFCO.

Freund, L.B. 1990.Dynamic Fracture Mechanics, Cambridge :Cambridge University Press.

Klopp, G.M. 1996. Seismic design of unreinforced masonry structure., Dissertation Adelaide

: The University of Adelaide.

Krajcinovic, D. 1996.Damage Mechanics. New York: Elsevier.LeMaitre, J. 1992.A course on Damage Mechanics. Berlin : Springer-Verlag.

Melchers, R.E. & Page, A.W. 1992. The Newcastle Earthquake. Building and Structures.

94:143-56.

Page, A.W. 1979. The inplane deformation and failure of brickwork. Dissertation No 636.

Newcastle: University of Newcastle, Australia.

Paulson, T.J. & Abrams D.P. 1990. Correlation between static and dynamic response of model

masonry structures.Earthquake Spectra. 6:573 - 91.


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Tena-Colunga, A. & Abrams, D.P. 1992. Response of unreinforced masonry building during the

Loma Prieta earthquake. Proceedings of the Tenth World Conference on Earthquake En-

gineering.3:2292-9.

Tercelj, S. Sheppard, P. & Turnsek, V. 1969. The influence of frequency on the shear strength

and ductility of masonry walls in dynamic loading tests. Proceedings of the Fifth International

Conference on Earthquake Engineering.3:2292 - 9.

Tomazevic, M. Lutman, M. & Petkovic, L. 1996. Seismic behaviour of masonry walls : experi-

mental simulation. ASCE Journal of Structural Engineering,122:1040-7

Dynamic modulus of elasticity of masonary units

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