Structural Strengthening of Brick Masonry Walls of Heritage Building with Carbon Fibre Reinforced Polymers

S. K. Savardekar, Director, FYFE (India) Pvt. Ltd., Mumbai, V. Jain, Director, FYFE (India) Pvt. Ltd. Delhi and K. R. Raikar, Director, Structwel Designers & Consultants Pvt. Ltd, Mumbai.

A sprawling palace complex built in 1893 and belonging to the Nizam family, rulers of the erstwhile princely state of Hyderabad in India, is currently being developed as a luxury heritage palace hotel by a leading hotel group from India. Various functional buildings are integrated to form the palace complex of around 940,000 square meters area. The original construction of the palace features mainly a load bearing masonry structure comprising burnt clay brick masonry in lime mortar. Years of disuse and exposure to environmental vagaries, had resulted in various distresses to the structures. The present developers thought is it prudent to conduct a structural audit of the palace complex to assess the present condition of various structures. Subsequent to a structural audit, it was noted that amongst other anomalies, various sections of the load bearing masonry walls of various buildings were non-compliant to the allowable values of slenderness ratios. This necessitated certain intervention measures to the deficient masonry wall panels. An intervention scheme using carbon fibre reinforced polymer (CFRP) bands as localised intermediate stiffeners onto the walls was found appropriate and thus adopted considering various advantages it offered. This paper describes the site case study on the installation of a particular proprietary CFRP system in achieving the desired objective of carrying out structural intervention of masonry walls.

Introduction

Project Description

Structural Strengthening
Figure 1: Front façade view of the main palace building
A leading hotel group in India has recently commenced development of an existing old palace complex to be converted into a luxury heritage hotel. The palace complex belonged to the Nizam royal family, of the erstwhile princely state of Hyderabad in southern India. The construction had commenced in 1884 and is said was completed in various stages over a period of nine years. The palace complex comprises various functional buildings such as the main palace, Gol bungalow, Zanani mahal and various other ancillary buildings all integrated to form a sprawling complex covering around 940,000 square meters. Fig.1.

Need For Structural Intervention

The various buildings of the palace complex are predominantly load bearing type and constructed of burnt clay brick masonry with lime mortar rendering. Certain sections of the walls at the ground level, are noted to be of stone masonry with lime mortar. Over the years, the palace complex was in state of disuse and had been exposed to environmental vagaries.

Certain sections of the palace buildings exhibited distresses in the form of cracks in the masonry and even partial collapses of the masonry walls at few locations were evident. The developers have therefore considered it prudent to conduct a structural audit of all the palace buildings to determine its suitability for their proposed use and the resulting loading conditions. The developers had thus appointed structural engineers for this purpose. Various surveys, visual inspections, non destructive and partially destructive tests and analysis had been conducted by the structural engineers. Based on their evaluation, the structural engineers had identified certain structural anomalies. One amongst the various anomalies having relevance to this paper was that certain masonry walls of various buildings of the palace complex being slender. This necessitated localized structural intervention to address the issue.

Structural Intervention Details

Structural Intervention Approach

To appropriately address this particular anomaly at hand, the use of various remedial options using different conventional techniques such as, section enlargement and introduction of steel sections as substitute framing systems were explored by the structural engineers. However, at certain locations, the age, aesthetic and heritage nature of the existing structures governed the choice of a remedial technique. As such, it was imperative that a retrofit system to be executed, should have essentially been minimal invasive, quick and discreet. Prime deterrents to the use of the above mentioned conventional techniques were that they were invasive, would aesthetically alter the walls appearance and were time consuming. Thus, the use of CFRP for structural intervention measures in order to appropriately address the above issues was recommended by the structural engineers.

Strengthening Basis

Certain wall panels that were identified by the structural engineers to be non-compliant to the specified slenderness limits were recommended for structural intervention. Two criteria of slenderness had been checked for, viz. the ratio of;
  1. wall height / wall thickness
  2. wall length / wall thickness.
Structural Strengthening
Figue 2: A wall panel at site identified for structural intervention
The structural intervention detail was based on the concept of providing CFRP strips in the horizontal and vertical directions and at certain specified spacing and bonded coincident onto either sides of the masonry walls, anchored with fibre anchors. This was designed to act as vertical and horizontal stiffeners by locally plating or sandwiching the masonry walls thereby increasing the stiffness. These CFRP stiffeners would then essentially act as intermediate pilasters/columns so as to reduce the effective wall height / length and thus bring the slenderness ratios within acceptable limits. Based on this concept shop drawings for individual wall panels were prepared taking into account factors such as openings and aesthetic features. Fig.2 shows an actual wall panel at site and Fig.3 shows part print of a shop drawing of the CFRP installation details for that wall panel.

Structural Strengthening
Figure 3: CFRP installation detail for the identified wall panel

CFRP System Properties

The CFRP system selected for installation was a proprietary system from a reputed FRP system manufac- turer. This system was selected on the criteria of mat- erial characterization, system performance and environmental durability as speci- fied by structural engineer. The CFRP system comprised a unidirectional carbon fibre fabric as the reinforcement and a compatible two component epoxy resin as the matrix. The key properties of the CFRP system used for carrying out the structural intervention are as shown in Table 1.

Table 1 Properties of CFRP system used for structural intervention
Property Test Method Typical Test Value*
Ultimate tensile strength in main fibre direction ASTM
D-3039
986.0 MPa.
Elongation at break ASTM
D-3039
1.0%
Tensile Modulus ASTM
D-3039
95.8 GPa
Laminate thickness 1.0 mm
* Values reported by CFRP system manufacturer based on gross laminate properties.

Structural Intervention Process

Preparatory Works

The preparatory works commenced with identifying on site, wall panels marked for structural intervention on the drawings. Locations of the vertical and horizontal CFRP strips were then marked onto the walls ensuring that the CFRP strip locations are coincident on the internal and external faces of the walls. The lime plaster rendering was carefully chipped off at these marked locations to expose the brick masonry. Fig.4 shows the CFRP strip locations and removal of lime plaster. A polymer modified cementatious skim coat was then applied onto the exposed masonry to level off all concavities and depressions, so as to provide for a level substrate to receive the CFRP application. After adequate water curing, the leveled surface was then sanded using sand paper to smoothen out any protrusions and make the surface ready to receive CFRP application. Holes for fibre anchors were drilled at locations as indicated on the drawings. Fig.5 shows a wall panel with the prepared surface ready to receive CFRP strips.

Structural Strengthening Structural Strengthening
Figure 4: Marking of CFRP strip locations and removal of lime mortar rendering Figure 5: View of a wall panel with prepared surface ready to receive CFRP strips
Structural Strengthening Structural Strengthening
Figure 6: Installing vertical CFRP strip onto a wall panel Figure 7: Closeup view of fibre anchor installed over CFRP strip

CFRP Installation

The installation of the CFRP was wet wrapping type process as recommended by the system manufacturer. The installation process commenced with sizing and cutting of the carbon fibre fabric to the required dimensions that were obtained from site measurements. The two components of the saturant resin matrix were mixed together in the ratio as per manufacturer’s specifications. The sized fabric was then manually saturated from both sides with the mixed resin matrix. The saturation was carried out manually using fabric rollers. The saturated fabric was then rolled onto spools and taken to site for installation. Prior to installation of the CFRP strips, the prepared surface was first wet primed with a coat of saturant resin. The saturated fabric was then installed onto the primed surface by evenly rolling out the spool along the primed surface. The CFRP strips were adhered onto the surface using uniform hand pressure along the main fibres. This also ensured to remove any entrapped air voids behind the CFRP strip. At a given wall panel, all the vertical CFRP strips were installed followed by the horizontal CFRP strips. Fig.6 shows the installation of vertical CFRP strip.

The fibre anchors were then installed by first saturating the anchors into the saturant resin matrix and inserting one end of the anchor into the pre-drilled holes. The outer end of the fibre anchor was then splayed over the CFRP strips as shown in Fig.7.

Upon the CFRP strip achieving a tacky stage of the curing process, quartz sand was sprinkled over it. This acted as a key for further lime plaster render finish.

Conclusion

The use of CFRP for structural intervention of the masonry walls of the palace buildings was found to be an appropriate technique to achieve the desired objective and at the same time being minimal invasive, quick and discreet as compared to conventional strengthening techniques.

References

  • ACI Committee 440, Guidelines for the design and construction of externally bonded FRP system for strengthening concrete structures. American Concrete Institute, Detroit, 2000.
  • ICC AC125, Interim criteria for concrete and reinforced and unreinforced masonry streng- thening using fibre reinforced polymer (FRP) composite sytems. ICC Evaluation Service Inc. Whittier, California, 2003.
  • Technical Datasheet, Tyfo® SCH41 composite using Tyfo® S epoxy. Fyfe Co. LLC, San Diego, CA. 2006.

Acknowledgment

This article has been reproduced from the proceeding of 'National Conference on Repair & Rehabilitation of Concrete Structures' organized by ICI western U.P Gaziabad, IA Sructural Engg, and Association of Structural Rehabilitation, with the kind permission of the organisers.

NBMCW June 2011