Strengthening of Brick Masonry Walls against Earthquake Loading

ISSN 2319-5347, Vol. 01, No. 01, July 2012

Strengthening of Brick Masonry Walls against Earthquake Loading

K

S

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M

J

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B

A

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M

K

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Z

A

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H

K

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S

S

A

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Department of Civil Engineering University of Engineering & Technology, Peshawar, Pakistan

Email: khanshahzada@nwfpuet.edu.pk, shah_civil2003@yahoo.com

Abstract: This paper presents a research on the enhancement of unconfined and unreinforced brick masonry walls

against earthquake loadings in Pakistan. Different unreinforced brick masonry walls have been examined for compressive strength before and after retrofitting. In this research Ferro-cementing has been used for the strength improvement of unreinforced brick masonry. The impact of plaster on the durability of walls has also been regarded. The research of trial outcomes generate, that appropriate retrofitting can reduce the problems occurring due to future earthquakes. Retrofitting improved not only the overall strength of unreinforced brick masonry walls by 40 % and also enhanced its ductility.

Keywords: Unconfined Masonry Structures, Ductility, Compression Strength, Earthquake Disaster, and Retrofitting 1. Introduction:

The easy construction and availability of the burnt bricks make it one of the most widely used construction material in Pakistan. Ordinary structures like small living units; small government buildings, non- profit agencies, health care facilities and other important facilities are constructed from brick masonry. At the same time, Pakistan’s Khyber Pukhtoon Khwa (previously North West Frontier Province) faced disastrous earthquakes in history. One such earthquake hit the northern areas of Disputed Territory of Kashmir and northern Pakistan rendering many homeless and claimed thousands of lives. In the absence of any legitimate building code and lack of knowledge for behavior of the brick masonry caused even greater damage. The reconnaissance survey reports indicated that the unreinforced masonry buildings such as stone masonry, brick masonry and concrete block masonry were either partially or completely damaged (Durrani et al. 2005, Naeem et al. 2005) [1].

Buildings which are properly designed and detailed on the basis of modern seismic building codes are less affected because these buildings dissipate energy through inelastic behavior. Improper design and detailing of buildings can make these buildings vulnerable to earthquakes. The enormous losses inflicted by the October 8, 2005 earthquake in the Northern Areas of Pakistan were mainly due to the fact that in the absence of a seismic building code, buildings were either non-engineered or designed for gravity loads only (Javed et al. 2008, Naseer et al. 2010) [2]. Application of wire mesh increases the lateral strength capacity of unreinforced masonry walls significantly (Shahzada et al. 2008 [4], Shahzada et al. 2009). In this context a study was carried out to strengthen the existing unreinforced brick masonry walls with

Ferro-cement technique with a potential of constructing new structures with Ferro-cement.

2. Methodology:

The study includes experimental evaluation of the strength of brick masonry under certain conditions. For this purpose total of 20 samples of brick wall segments were constructed. The bricks were layered in English course. The masonry walls were constructed under ordinary conditions having mix proportions of 1:6 (cement: sand). Water cement ratio was kept at 0.8. The dimensions of the walls were 16”x20”x9” (Length x Height x Width). A total of 20 walls were constructed out of which 10 were plastered and the remaining 10 were without plaster as shown in Figure 1 and Figure 2. The walls were cured in open air and were tested after 28 days in the Universal Testing Machine (UTM) in the Structural laboratory of Civil Engineering Department, University of Engineering & Technology Peshawar, Pakistan. Half of the non plastered and plastered walls were retrofitted after initial testing as shown in figure 3 and figure 4.

Figure 2: Plastering of Wall

Figure 3: Retrofitting of Wall with Wire Mesh 2.1. Bricks:

The strength of brick plays key role in the construction and stability of the buildings. For this reason, brick samples were collected and were tested in compression to evaluate their compressive strength. The water absorption test was also performed on the brick samples. Sand and cement were also tested according to ASTM standards.

Figure 4: Plastering of Wire Mesh Folded Wall 2.1.1. Compressive Strength:

The compression test for the bricks was carried out on U.T.M. of 200 ton capacity. Vertical alignment was provided so as to get rid of flexural and tensile stresses. A total of 10 bricks were tested as shown in figure 5. The dimensions of these bricks were taken before test. The average compressive strength is 1.89 ksi with standard deviation of 0.87. The details are given in Table 1.

2.1.2. Water Absorption of Bricks:

The brick samples were submerged in cold and clean water for 24 hours as shown in figure 6. The weight before and after submersion was recorded and the absorption of each specimen was calculated by the following formula;

Absorption (%) = (Ws-Wd)/Wd * 100 Where,

Wd= dry weight of specimen

Ws= saturated weight of the specimen after submersion in water for 24 hours.

Details for water absorption are given in table 2.

Table 1: Compressive Strength of Bricks

S. No Trade Mark Length (in) Width (in) Height (in) Area (in2) Load (tons) Load (kip) Load/Area (ksi) 1 N 8.60 4.04 3.00 34.74 29.40 64.80 1.87 2 N 8.67 3.96 3.10 34.33 13.70 30.19 0.88 3 N 8.46 3.96 2.87 33.50 46.80 103.15 3.08 4 N 8.62 4.08 2.87 35.16 24.10 53.12 1.51 5 N 8.60 4.04 2.95 34.74 18.10 39.89 1.15 6 N 8.58 4.04 2.94 34.60 30.40 67.00 1.94 7 N 8.56 4.08 2.81 34.92 50.50 111.30 3.19 8 N 8.48 4.18 2.77 35.44 30.50 67.22 1.90 9 N 8.73 4.15 2.87 36.22 11.80 26.01 0.72 10 N 8.65 3.69 2.83 31.91 39.10 86.18 2.70

Figure 5: Brick Compressive Strength Test _{Figure 6: Water Absorption Test of Brick} Table 2: Percentage Water Absorption

S. No Trade Mark Dry wt (lbs)

(w1) Wet wt (lbs) (w2) Wt of Water (w3=w2-w1) lbs %age (w3/w1)*100 1 N 6.16 7.57 1.41 22.89 2 N 5.81 6.96 1.15 19.79 3 N 5.79 6.94 1.15 19.86 4 N 5.85 7.30 1.45 24.79 5 N 5.85 7.42 1.57 26.83 6 N 5.74 7.01 1.27 22.12

The average water absorption is 22.71 %, which is high enough and will absorb water from the mortar.

2.2. Sand:

Sand used was clean, hard, strong, well graded and free of organic impurities, deleterious substances, silt and clay. All the sand passed through sieve no. 16.

2.3. Cement:

In this work, locally manufactured ordinary Portland cement (Kohat cement) was used conforming to ASTM spec. C 150 (type-1).

3. Testing Procedure:

The walls were subjected to compressive loading in the UTM. The Half number of the walls were tested up to the collapse level whereas, the remaining walls were subjected to 40% of the walls collapse load which were later on retrofitted with Ferro- cement. The failure mechanism of walls with plaster before retrofitting is shown in figure 7.

The rest of the 10 walls were then subjected to 40 percent of the ultimate load. Cracks were observed in these walls. Five of the walls subjected to 40 percent loading were plastered; this plaster was removed after

cracking. Wire mesh was wrapped around all of the cracked walls.

Figure 7: Spalling of Plaster

These walls were plastered again with a mortar of 1:5 cement sand ratio. After the period of curing, these walls were tested in UTM again to find their crushing strength. The damage mechanism of retrofitted walls is shown in figure 8 and figure 9.

Figure 8: Damage Pattern of Retrofitted wall

Figure 9: Retrofitted Walls after Test

This was done so as to find the comparative strength between normal masonry walls and wire mesh retrofitted walls.

4. Results and Discussion:

For further evaluation, half of the un-reinforced brick masonry samples were coated with cement-sand Plaster. The plastered samples had an average failure load of 42.5 Tons, whereas the non-plastered walls failed at an average load of 40 Tons. This indicated that the plastered walls could take an additional 5.8% of the load as compared with the non-plastered walls. The crack pattern for both cases was same. The cracks started at 70-75% of the average cracking load and grew rapidly once initiated. The strain rate was kept high for the tests which depict the stresses caused by an earthquake. The test data for un-retrofitted walls is provided in the Table 3 below;

After running these tests, the damaged samples were covered with one layer of ferrocement wire mesh and 5mm thick plain mortar of 1:2 (cement: sand). The axial

load was applied at a slow rate by UTM. For plain plastered walls the first crack was observed at 31 Ton. More cracks started appearing at regular increments of 4-5 Tons. The lowest ultimate load was observed at 53.6 Tons while the highest load was 66.5 Tons. The confinement provided by ferrocement increased the failure load by 15-20 Tons which is 40% increase in the strength. The fact that the specimen was intact and did not shatter into pieces was very intriguing.

Table 3: Test Results of Unreinforced Brick Masonry

Walls Specimen Designation Max. failure load (Tons) Average Failure Load (Tons) 40% load (Tons) Plastered 45.6 42.5 17.0 47.4 29.3 59.9 40.6 Un-plastered 36.3 40.0 16.0 27.9 44.0 39.7 42.3

The non-plastered walls were also retrofitted with the ferrocement and plastered afterwards. The cracks were observed at relatively lower (29 Ton) compressive strength. The lowest ultimate load was 47.1 Ton while the highest load was 55.7 Ton. This shows that the non-plastered walls suffered greater damage during the initial testing. Table 4 presents a summary of the testing results after retrofitting. A small comparison between table 3 and table 4 is presented in the Figure 11. This indicates substantial increase in the strength capacity of the masonry walls after retrofitting.

Table 4: Maximum Failure Load after Retrofitting

Specimen Designation Max. failure load (Tons) Average Failure Load (Tons) 40% load (Tons) Plastered 61.4 59.18 40.00 66.50 57.30 57.10 53.60 Un-plastered 47.10 52.28 30.00 52.30 54.00 55.70

Detailed Analysis of Walls 0 10 20 30 40 50 60 70 plastered retrofitted plastere retofitted

non plastered non plastered retrofitted Columns Lo a ds ( To n) Series1

Figure 11: Comparison of the Results 5. Conclusions:

Experiments were conducted to find out the compressive strength of un-retrofitted and retrofitted brick masonry walls. The brick walls were coated with single layer Ferrocement wire mesh. It was noted that the ferrocement coating on masonry walls increases the compressive strength. Ferrocement specimens having one layer of wire mesh wrapped around showed an increase in failure load of up to 40% as compared to controlled specimen. The excessive mortar thickness applied to cover wire mesh leads to premature cracking. Also the premature cracking can occur if the ferrocement is not properly cured. The biggest advantage of the ferrocement is the fact that it does not disintegrate after failure unlike normal masonry walls, hence reducing the falling hazard. The ordinary brick masonry walls fail suddenly leading to brittle failure,

however the ferrocement walls crack at slightly lower loads but the subsequent widening and growth leading to failure happens at greater loads.

6. References:

[1] Durrani, A. J., Elnashai, A. S., Hashash, Y. M. A., Kim, S. J., Masud, A., [2005]” The Kashmir Earthquake of October 08, 2005: A Quick Look

Report,” Mid-America, Earthquake Center,

University of Illinois at Urbana-Champaign.

[2] Javed M., Khan A.N. and Magenes G.

[2008]“Performance of masonry structures during earthquake -2005 in Kashmir”. Mehran University Research Journal of Engineering & Technology, Vol. 27, No. 3, pp. 271-282

[3] Khan. A.N. et al. [2005] “Reconnaissance Report on the 8th October, 2005 Earthquake, Pakistan”, EERI Website, report, 20 pp.

[4] Khan Shahzada et. al, “Improvement of Masonry Structures Against Seismic Force” Published in Bulletin of the International Institute of Seismology and Earthquake Engineering Tsukuba, Japan September, 2008.

[5] Khan Shahzada et. al, “Sustainable Buildings in Dera Ismail Khan and Adjoining Areas: An Experimental Study” International Conference on “Integrating Disaster Management and Climate Change Adaptation Into Policy Making Baragali, Pakistan October 15- 17,2009.

[6] Naseer, A., Khan, A, N., Ali, Q., Hussain, Z. [2010] “Observed Seismic Behavior of Buildings in Northern Pakistan during Kashmir Earthquake”, Earthquake Spectra, Vol. 26, No. 2, pp 425-449.