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A STUDY OF MICROCRACKING IN MASONRY CONSTRUCTION:

THE USE OF PULSE VELOCITY MEASUREMENTS

PIETRO BOCCA Associate Professor

Istituto Universitario di Architettura di Venezia Tolentini 191 - 30125 VENEZIA - ITALY

ABST RACT

Cracking development of a compressed historical masonry is controlled in order to deepen some unknown aspects of their failure mechanism.

Experimental results have shown that: - in representa tive specimen of historical masonry in good

conditions of preservation, microcracking sets on at a stress level amounting from 50% to 60% of the failure stress and takes place in the mortar;

- the tests have revealed that brick microcracking occurs at a stress level higher than those observed in mortar, from 80% to 90% of the value of failure stress;

- at a stress level from 30% to 40% the failure load, a mortar compacting effect has been observed, owing to triaxial compression.

INTRODUCTION

Ultrasonic tests [1] show that microcracking in concrete on when the 50-60% of the ultimate strenght is reached. condition lowers to 35-40% if very small specimens are [ 2 ] •

sets Such used

The same problem is now approached for clay brick masonry but to do this we need more sofisticated tools than those used for concrete [3] [4]. We study, in fact, the behavior of both components mortar and bricks and then the behavior of the material as a whole (*).

(*) Tests have Dipartimento di Torino.

been carried out at Ingegneria Strutturale

the del

Laboratorio politecnico

del di

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Masonry specimens have been obtained from 18th century buildings existing in Torino and Venice [5] [6].

Specimens are tested with different loads and the cracking evolution process is controlled to deepen some aspects of the failure of solid masonry.

MATERIALS AND METHODS_

The ultrasonic method is based on the fact that ultrasonic velocity is proportional to the medium density. An increase in the volume of voids of mortar and bricks, due to pore lengthening and to the state of internai microcracking, lowers the ultrasonic velocity. Equipement of the Terratest type, piezoelectric transducer working at 50 kHz and oscilloscope are used for velocity measurements. Direct transmission measures [7] are obtained in the compressive phase (fig.1). Specimens described in tab.1 are used.

Material number of Building site Dimensions Type and code specimens em T Brick

V Brick

T Masonry

V Masonry

the Cores

load compresses rate.

10 TORINO cl>=3 H=6

10 VENICE cl>=3 H=6

2 TORINO L=50 B=25 H=80

2 VENICE L=50 B=25 H=80

Tab.1. Specimens' Features.

of 3 cm diameter were drilled in the direction of to test brick characteristics. A MTS machine the specimen at constant longitudinal deformation

Original T masonry samples were compressed with increasing load cycles, according to fig . 2, at constant load increase rate. Ultrasonic velocity was plotted at every cycle.

While V masonry samp1es were tested with only one cycle to give stress-displacement curves and stress levei at wich microcracking sets.

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(a) (b)

25cm

figo 1.Real dimensions of specimens and transducers position. a) masonry samples. b) brick specimens.

fig.2. Compressive load cycles of T masonry samples function of time.

time

RESULTS AND DISCUSSION

BRICKS Diagrams a-E obtained from representative cores drilled from T and V bricks are plotted in fig.3. Summary of the results of all cores is reported in tab.2. Microcracking begins when load, while increasing, reaches a level such that ultrasonic velocity decreases largely. On the other hand, macrocracking is detected with magnifying glass.

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Material Number of Microcrack. Macrocrack . Failure Type and code specimens set stress set stress stress

[MPa]

T,V bricks 20 0.87 o 0.91 o 11. 5

variation parameter -- 19% 21% 15%

C"

Tab.2. Results from T and V bricks.

From the diagrams and table 2 it can be noted that : - 0im/oR ratios, microcracking stress set/failure stress, are

hlgher for historical bricks respect to those recorded for concrete [1], [2]. They range from 0.8 to 0.9.

- Macrocracking occurs at a stress value quite close to the failure one.

- 0im is, practically, equal to 0iM'

o (MP.)

, .. , .

•. ,ao

4._

0R- ' •. 7 MP.

OiM-o.·.OR

Oimso..,OR

o ~r ______ ~ ________ ~ ______ ~ ____ ~C~

• 10 ,. fig.3. O-€ curve of hystorical bricks.

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MASONRY T masonry samples were loaded cyclically following the scheme of fig . 2. 0max' maximum compression stress, increases at every cycle and the minimum 0min was kept constant. The test proceeded until failure occured (for T1 specimens aR = 8,3 MPa and for T2 specimens aR = 7,8 MPa). Figure 4 plots the variations of the ultrasonic velocity at the end of every cycle .

2700

2500

ã; 8i .,1 Jel :1 =1

1 gol gI :;(' :;;1 u, ::1 !!!I 8, ~I ~,

~I ~, E, EI

• 10 11 12 " M n number or cOltlpression IOad cycles

N .. .. ..

Fig.4. T masonry ultrasonic velocity function of the number of cycles, 0min = 0.6 MPa.

Relatively to specimen T1 there is a noticeable decrease of the velocity during the cycle number 11 whose 0max is 4,2 MPa. The same is true for specimen T2 but during cycle number 10 whose 0max = 3,6 MPa. Microcracking sets at the following:

~~~di '7..tll>

ri ,<J1

.

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v ( m/IIIIC)

., .. / ~. · ~ Li .

LlI I . /I I 1

~ L 111_ , ~

111 I ~C1~ 0.34 1 I · TI LlI ... Lll I --L

/ I 1. · ..e / 11 .... , -1 I . 1 \,....- Jor-$

HO 111~n321 I i I !

V' ... V' cri ]/, V· V·

V· lL" 7

MPa)

elocity function of the compression stress during the loading phase.

Figure 5 is a threedimensional diagram of the velocity variation function both of 0max and of the number of cycles.

Both specimens show a remarkable increase of the propagation velocity from the 7th cycle whose 0max is approximately 0,3 0R' Before microcracking sets, mortar is compactified, probably due to triaxial compression. Such phenomenon as been already noticed by other researchers [9], particularly with bad quality mortars.

In both specimemmacrocracking, detected with optical methods, sets on when 0max is almost equal to 0,8 0R'

V masonry specimens, tested with a uniformely increasing load until failure, gave the fOllowing results (fig.6.):

VI Gim = 0,52 0R V2 Gim 0,50 0R

Microcracking set stress values are equal to macrocracking set stress values.

-

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() (MPa) 9

8

7

6

5 V,: ()R= 4.8 MPa

4

V2 : ()R= 3.2 MP. 3

2

o 0.5 ,.0

Fig.6. 0-8 curves of V masonry samples

CONCLUSIONS

From the tests carried we can draw the following conclusions: 1. Single bricks have microcracking set stress 0im equal to 80-

90% of the failure stress aR' These percentage values are higher than those recorded with concretes and concrete mortars. Bricks behave with no difference regarding microcracking set stress and macrocracking set stress (oim is almost equal to 0iM)'

2. On Hystorical masonry in a good preservation state microcracking occurs when 45-55% of the failure stress is reached and is localized always in mortar .

3 . Macrocracking set stress values on masonry are scattered. Macrocracking set values, in fact, range from a lower bound of 45-55% of the failure stress to a upper bound of 80% of the failure stress value.

4. Stress at wich macrocracking sets in masonry is lower than the one relative to the component brick. This prooves that failure in bricks need not to pass through the generalized microcracking phase but it occurs always due to a concentration of stresses on localized defects. Then, if we consider stresses in compressed masonry [8], brick's failure is caused by opening.

5. At about 30-40% of the failure stress there is a compactification effect somehow similar to settlement

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phenomena [9] caused by triaxial compression.

NO'I'A'I'ION

Gim microcracking set stressi GR = failure stressi GiM = macrocracking set stressi G* mortar compactification stressi v ultrasonic propagation velocity; t ultrasonic propagation time.

REFERENCES

[1] Facaoaru, Jones - "Incercarea Nedistructiva a Betonolui".

[2 ]

Editura Tehnica Bucarest 1971.

Bocca P. - "Sul microcarotaggio - Basi teoriche e prime esperienze" - La Prefabbricazione pp. 651-664 - December 1986.

[3] Bocca P., Zago F. - "11 metodo dell'auscultazione dinamica applicato alle strutture in muratura" - Atti dell'Istituto di Scienza delle Costruzioni - Facoltà di Architettura di Venezia - n° 54 - 1983.

[4] Baronio G., Binda L., Scirocco F., - Controlli non distruttivi per la determinazione delle caratteristiche dei laterizi. Atti del 15° Congresso Nazionale Andil. September 1980.

[5] Pistone G., Roccati R. - Alcuni aspetti del comportamento delle malte in muratura dell' 800. Riunione annuale Gruppo Italiano RILEM - Seminario scientifico sulle malte Torino June 1987.

[6 ] Zago F., Riva G. - Proprietà fisico-meccaniche e comportamento della muratura del centro Venezia. Parte Seconda: La muratura. Atti dell' Scienza delle Costruzioni. IUAV, July 1982.

dei mattoni storico di Istituto di

[7] Reccomandations RILEM -" Les éssais sur béton par la méthode de l'auscultation dynamique". Annales ITBTP

- n° 309-310 September-October 1973.

[8] Mc Nary W.S:, Abrams D.P. -" Mechanics of Masonry in compression" - Journal of Structural Engineering - ASCE Vol. 111 n° 4 April 1985.

[9] Mattone R., Pasero G., Pavano M., Pistone G., Roccati R., "Prove sperimentali su campioni di varie dimensioni volte alla determin.azione delle caratteristiche meccaniche delle vecchie murature" Vlth IBMAC Roma 1982.