II th INTERNA TíONAL BRICKlBLOCK MASONRY CONFERENCE

TONGJI UNIVERSITY, SHANGHAI, CHINA, 14 - 16 OCTOBER 1997

CASE STUDY ON ACOUSTIC PERFORMANCES OF BUILDINGS MADE OF BRICK BLOCK MASONRY

Rossano Albatici1 Paolo Baggi02 Antonio Frattari3

I.ABSTRACT

The paper deals with a research performed at the Laboratory of Building Design of the University of Trento about the behaviour of masonry buildings from the acoustic and thermal standpoint. This case study is about two buildings, built in Trento (ltaly) as experimental ones; the fust having a standard reinforced concrete load bearing frame (pillar and beams) with masonry externaI walls ; and the second having a masonry blocks load bearing structure. The masonry blocks used are newly designed hollow c1ay blocks, brand named "Clever Brick". The acoustic behaviour ofthe two buildings has been tested and compared before the laying in place of the inner and exterior finishing.

2. INTRODUCTION

The study presented in this paper has been carried out on two experimental buildings (built in Trento- Italy in 1995), to define the on site behaviour of some hollow c1ay brick block, brand named "Clever Brick" (Fig.l). Such blocks, recently appeared in the market, are made with a medium mass aggregate with higher porosity in order to obtain better thermal and acoustic properties. The buildings have been constructed within a research project related to a wider fçsearch programme promoted by the EEC and called "Brite Euram Programme".

Keywords: Buildings, Acoustic Behaviour, Clay Brick, Masonry.

2

Postgraduate Fellow, of the Department of Civil and Environmental Engineering, University ofTrento, Via Mesiano 77, 38050 Povo (TN), ltaly Researcher, Institute of Technical Physicis , University of Padova, Via Venezia 1, 35 131 Padova, Italy. Professor, Department of Civil and Environrnental Engineering, University of Trento, Via Mesiano 77, 38050 Povo (TN), ltaly.

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This project has been carried out by: the Consorzio Poroton Italia, the I.T.E.A. (the Housing Council ofthe Trento Province), the University ofPadova and the University ofTrento, whichjoined the research group only during the last phase, aimed at the definition ofthe acoustic, thermal and hygrometric behaviour of the buildings.

The two buildings have the same shape and fenestration and they are oriented with the longer side along the North-South direction. They have been constructed with similar building materiais, but with two different building technologies in order to compare their performance. The first building has a load bearing frame made of reinforced concrete (Rbk 350 concrete, FeB 44k steel bars), outer walls made of hollow c1ay blocks called "Clever Brick" (Poroton P700 type) ; andjoists (intermediate floor and root) consisting of concrete and hollow tiles slabs, 24 em thick. The second building, instead, has load bearing masonry walls made ofhollow blocks "Clever Brick" (Poroton P800 type), cement mortar Ml type and reinforced with Rbk 300 concrete and Fe 44k steel bars; the joists ale two concrete and hollow tiles slabs, 30 em thick, linked to the load bearing walls by means of a concrete girder.

Fig. 1 The two experimental buildings in Trento (Italy)

3. TEST EQUIPMENT

The measurements to determine the acoustic behaviour have been carried out with equipmen~ for architectural acoustics manufactured by Brüel & Kjaer, Denrnark. The used instruments were: an architectural acoustic analyser (Fig.2), a condenser microphone with preamplifier, a reference sound source SJ, a secondary sound source S2, and a standardised tapping machine.

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With the building acoustic analyser it has been possible to perform the automatic measurement oftne parameters relevant to building acoustic i.e., to collect data about the sound levei in the source ambient and in the receiving room, to evaluate the reverberation times and to measure the ground noise leveI. The condenser rnicrophone ( ~ inch diameter) hrui been used to perform precision sound levei measurements in the relevant frequency range. The reference sound source SI has been used, in connection with the aforementioned analyser, for the measurement of sound insulation, of reverberation time and of sound adsorption. Such sound source is capable to generate a sound power levei of 40 .;- 100 dB (ref. 1 p W ) with the following operating modes: wide band noise 100.;- 10000 Hz ; octave band noise (7 bands) 125.;- 8000 Hz; white noise 50.;- 10000 Hz and pink noiselOO.;-10000 Hz . The sound source S2, also used for the measure of the reverberation time, sound adsorption and sound insulation has an continuous output power up to 118 dB in the range 100.;- 8000 Hz (150 RMWS - 400W peak). The standardised tapping machine had five 500 g hammers generating 10 impacts per second.

Fig. 2 Architectural acoustic analyser

4. TEST ARRANGEMENT AND PROCEDURE

Three different tests have been performed. The first test, P 1, consisted in the field measurement ofthe airbome sound insulation ofthe externaI walls, according to the UNI 8270/4 standard ; the second te:t, P2, consisted in the measurement ofthe sound pressure ofthe floor, according to the UNI 8270/4 standard. .

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Ali tests have been repeated twice (in identical conditions) for both bui1dings. For each test data have been coliected and recorded for the octave bands with centre frequencies between 125 and 8000 Hz. To effective1y test he wal1 behaviour, ali the ho1es in the outer wal1s (i.e., windows) have been temporari1y closed using dry-wal1 masonry bui1t up with brick b10cks Poroton P800 type, 30 cm thick (same material as the rest ofthe wal1). The remaining openings/slits have been sealed with po1yurethane foam and cement mortar, and the interior side has been finished with a 2 cm thick p1aster made of pre-rnixed mortar powder. The entrance openings were closed with fire resistant (REI 60) stee1 doors. The first two tests (test Pl and P2) were as foliows. At frrst, the reverberation time has been measured, in order to be ab1e to adjust the results of the foliowing tests taking in account

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Fig. 3 Position of sound source and microphone

BUILDING2 BUILDING 1

1 1.24 2.59

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Fig. 4 P1an for te measurement of impact sound inso1ation

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~ 1.59

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the reverberation due to the walls of the buildings. For that purpose, the building acoustics analyser has been used which gives the value ofreverberation time (in seconds) evaluated for a standard SOUlld decay equal to 60dB; such value is obtained as an extrapolation ofthe measured 20dB, 30dB e 40 dB sound decay. The microphone has been placed inside the building and in the centre ofthe room space, 3.30 m fram the outer wall with the smallest openings. Sound source SI has been placed dose to the south-east comer (in the building facing north) and in the north-east comer (for the building facing south). Then, the sound pressure levei has been measured inside the building, with an outer sound source (Fig.3). The microphone has been placed inside the building, in the centre ofthe room space, 3.3 m far from the outer wall with the smallest openings. The sound source S2 has been placed on the outer side, 3.7 m far from the same wall. The sounà source has been tilted up 15° to assure an uniform sound pressure on the outer surface ofthe wall. At last the sound pressure levei on the outer surface ofthe wall has been measured. In this case, the microphone has been placed on the outer side, dose to (0.8ã m) the considered wall .

The third test about the impact sl)und insulation of the floor, has been carried out as following. The standardised tapping machine has been placed on the first floor, in the centre ofthe room space, 0.40 m far from the outer wall with the smallest openings (Fig.4). The microphone has been placed at the ground floor, in the centre ofthe room space, 0.40 m from the outer wall with the smallest openings.

frecJJen. Hz test IcB test" cB test UI cB

100 93 92,2 92,6

125 00.7 00,4 oo,as

100 93,1 93,8 93,45

200 00,1 00.9 00

2m 97,9 00.2 00. as

315 93,6 93,4 93,5

400 92,8 92,7 92,75

500 95,2 95,4 95,3

63) 93 92,4 92,7

Im 96,2 95,5 95,85

1<XXl 101,2 101 101,1

1250 97,7 97,8 97,75

100:> 97,1 97,3 97,2

2000 100,2 100,4 100,3

2500 97,7 97,8 97,75

3150 00,2 00,4 00,3

4000 91,6 91,8 91,7

5<XXl 91,2 00,9 91,as

e:m 00,8 00,6 00,7

!UXl 00,8 00,9 00,85

Tab. 1 The outer sound pressure levei Building 1

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frecpn Hz test I cB test" cB test 111 cB

100 96,7 96 96,35

125 95,2 95,5 95,35

100 85,1 85,2 85,15

200 92,5 92,6 92,56

250 96,7 96,5 96,6

315 00.2 97,9 98,as

400 96,3 96,1 96,2

500 91,2 91,2 91,2

63) 00,1 00,2 00,15

Im 93,4 93,6 93,5

1<XXl 103,1 103,6 103,35

1250 00.5 00.5 98,5

100:> 98,2 00.3 00.25

2000 00,8 00,8 00,8

2500 97,5 97,4 97,45

3150 98,1 98,3 98,2

4000 91,1 91 91,as

5<XXl 88,6 88,5 88,56

e:m 91,2 91,4 91,3

!UXl 00,8 00,8 00,8

Tab. 2 The outer sound pressure levei Building 2

froquen. liz tast I dB tast 11 dB tast 111 dB

100 65,5 64,8 65,15

125 68,1 68 68,05

160 61 ,6 \ 61 ,5 61 ,55

200 62,5 62,5 62,5

250 58,9 58,6 58,75

315 58 58,8 58,4

400 59,1 58,3 58,7

500 56,7 56,1 56,4

630 54,4 5i,5 54,45

800 53,3 53,4 53,35

1000 55,7 55,8 55,75

1250 52,5 51 ,9 52,2

1600 50,3 50,2 50,25

2000 48,4 48,3 48,35

2500 47 47 47

3150 49,2 49,1 49,15

4000 43,2 43,3 43,25

5000 41,3 41 ,2 41 ,25

6300 41 ,6 41,4 41,5

8000 40,1 40,1 40,1

Tab. 3 The inner sound pressure levei Building 1

frequen. Hz lesll dB lesl 11 dB lesllll dB avo dB

100 66,6 68,5 69 68,03

125 12.8 72 71 ,9 72,23

160 68,5 69,2 69,2 68,97

200 73,7 73.9 73,4 73,67

250 73.5 73,6 73,4 73.50

315 78.7 79.1 78.9 78,90

400 78.5 78,8 79.8 79.03

500 75,2 75.4 75,2 75,27

630 74,4 74,9 74,5 74,60

800 75,1 75,3 75,4 75,27

1000 74.2 74,4 74,4 74.33

1250 75,1 75.3 75,2 75,20

1600 75,7 76,3 76.1 76,03

2000 77.3 77,8 78 77.70

2500 77,8 78,2 78,4 78,13

3150 76,4 76,9 77,1 76,80

4000 72.3 73,3 73,S 73,03

5000 72.2 73,3 73.8 73,10

6300 68,2 69.9 70.7 6S.60

8000 60,8 61 ,9 63,9 62,20

Tab. 5 Acoustic insolation ofthe floor Building 1

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froquen. Hz tast I dB tast 11 dB tast 111 dB

100 60,8 62 61,4

r-i25 67,4 67,2 67,3

160 63,5 : 64,2 63,85

200 64,6 64,5 64,55

250 60,4 60,1 60,25

315 60,4 60,1 60,25

400 59,9 60,1 60

500 56,6 56,2 56,4

630 55,9 56 55,95

800 55,1 55,2 55,15

1000 57,5 57,7 57,6

1250 53,6 53,6 53,6

1600 52,1 52,1 52,1

2000 54,1 54,1 54,1

2500 52,2 52,2 52,2

3150 48,7 48,4 48,55

4000 42,6 42,8 42,7

5000 41,6 41 ,8 41,7

6300 42,6 42,5 42,55

8000 40,9 40,6 40,75

Tab. 4 The inner sound pressure levei Building 2

frequen. Hz lesl I dB tesl 11 dB lesl111 dB avo dB

100 67 63,2 64,7 64,97

125 71 ,3 71 ,6 71,S 71,47

160 68,8 69,1 69,6 69,17

200 75,7 75,7 75,2 75,53

250 75,4 75,2 75,5 75,37

315 78.3 78.6 78,3' 78.40

400 79.2 78.9 79,2 79.10

500 74.7 74,8 74,8 74,77

630 75.1 75,4 75 75,17

800 75,8 76 76,2 76,00

1000 76,3 76,3 76,8 76,47

1250 76.7 76,8 77,1 76.87

1600 76,7 76,8 77.1 76,87

2000 79,1 80 80,1 79,73

2500 81 ,5 81 ,6 82 81 ,70

3150 81,8 81,8 82,8 82,1 3

4000 79 80,1 80,S 79,87

5000 77.1 77,7 79,2 78,00

6300 68,5 71 ,5 72 70,67

8000 58,3 61,5 65,2 61 ,67

Tab. 6 Acoustic insolation ofthe floor Building 2

5. COLLECTED DATA ANAL YSIS

The analysed data are the average of the two resulting values for each testo Some diagrams have been included showing the outer sound pressure leveI, the inner smmd pressure leveI, the sound reduction index of the wall, the normalized impact sound pressure level.

The inner sound pressure leveI was adjusted according to the formula: Lin\, corr = Lint - 10*lg(S/ A)

where S = area ofthe tested surface, A = absorption factor, calculated as A = 0.1 63*VIT

where v = room volume, T = reverberation time.

The sound reduction index ofthe wall has been calculated according the formula:

(1)

(2)

Rs = LexcLint+lO*lg(8*S*cosS/A) (3) whereS = inclination angle of the sound source, and according the formula:

Rtr = Lext-Lint+lO*lg(S/A) (4)

The normalized impact sound pressure leveI has been calculated according the formula: Ln = Lint+lO*lg(AlAo) (5)

where Ao = reference area of 10 sqm.

6. CONCLUSIONS

Tests Pl and P2 show that the reverberation time is lower, in the low frequency range, for the northem building, i.e. about 1.5 second less than the southem building .

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Fig. 5 Outer and inner sound pressure - Building 1

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Fig.5 Outer and inner sound pressure - Building 1

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Fig. 6 Outer and inner sound pressure - Building 2

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Such difference gradually decreases increasing the frequency, and totally disappears for higher frequencies .

The measured sound pressure leveIs suggest that the two walls behave in the same way when exposed to a similar exterior sound source. The sound reduction index can be

summarised as shown in the foUowing table:

low frequencies high frequencies high frequencies

Bulding 1 40 dB \ 45 dB 50 dB

Bulding 2 35 - 40 dB 45 dB 50 dB

Fig. 7 Noise reduction index - Building 1 and 2

Both the buildings required an adjustment ofthe inner sound pressure leveI ranging from 5dB (for low frequencies) to -2.5 dB (for high frequencies) .

The comparison of the sound reduction index clearly indicates that the two waUs have similar acoustic properties. As can be seen in Fig. 7, no meaningful differences have been found in the sound insulation behaviour ofthe considered walls. In fact, the plotted values follow the same trend and cross each other several times showing a substantial homogeneity of results. The difference between values at low frequencies is probably d~e to measurement errors that are higher in this range because ofthe background noise particularly high (and unavoidable) during in field tests.

As can be seen in Fig 5, the real behaviour ofboth the walls is very close to the theoretical one of a fixed wall with a mass per unit area M, where the noise reduction index is predicted by the formula:

R = 18*lg(1O*M*f)-44 where M = mass of the wall ner unit area (kg/m2

) and f = frequency (Hz). (6)

In fact, at lower frequencies the response is unstable (because of the background noise and of resonance effects) in the middle range the measured trend is very similar to the theoretical one and in the high frequency range the noise reduction index tends to become constant.

The results about normalized impact sound pressure leveI for the floors have shown no mea..iingful difference between the two buildings. It must be stressed that the test was carried out in this phase of the research measuring the leveI of the sound pressure in the receiving rooms with the standardised tapping machine placed on the unfinished floor of the above emitting room. These data will become meaningful only when compared with similar data obtained performing again the test when the above floors wiU be finished (and after laying in place an acoustic insulation layer). These measurements wiII be performed in a second phase ofthis research project, aimed at the evaluation ofthe onsite performance of some insulating materiaIs.

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6. ACKNOWLEDGEMENT

The research was financed from the Consorzio Poroton Italia with the partenership ofthe ITEA (by the Housing Council ofthe Trento Province -Italy).

7. BIBLIOGRAPHY

1. Gclrni, A. "Industrial development of reinforced masonry buildings", Murature oggi n. 43, March 94 p. 2 - 12. 2. Frattari, A. "Caratterizzazione acustica di un edificio", Murature oggi n. 53, December 96 p. 13 - 22. 3. Acoustics, vibration and shock - ISO Standards Handbook 4- Geneve,1985 .

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