Strengthening of Roof slab

The RCC roof of Tennis court & Badminton court of Malabar Hill club - Strengthening measures

Soffit of roof

Roof top

Abstract:

The roof covering badminton court and tennis court seating arrangement was found in total distressed condition when concrete was exposed.  The engineer decided to repair, strengthen and restore roof at once without any delay.  Requirement from the Client was to allow the local tournaments to proceeds unhindered.  Why repairs have been undertaken?

• Restore or improve structural integrity 
• Improve appearance 
• Improve durability 
• Improve functional performance 
• Provide water-tightness

M/S. Struct-Mast Engineers a leader in the repair industries was selected for the restoration and strengthening works who successfully accomplished the work without any untoward incident in most safest manner and complying to standards and in time completion without hindering the local tournament to the best satisfaction of the Client. 

Sliding moving platform

Struct-Mast Engineers had put in action innovated sliding moving platform which proved a great tool to deliver the durable work in most safest manner with quality as envisaged by the Consulting Engineer for large span roof structures.  

History:

The club was constructed about sixty years back.  The roof of the slab to give shelter to tennis court and badminton court was constructed out of RCC and both the roof needed severe repairs.  Struct-Mast Engineers was awarded the work to repair both the roofs and both the roof are balancing cantilever type roof.  

The carbonation of concrete caused the cracking of concrete due to corrosion of rebars embedded in concrete was the reason to allow ingress of water into the roof structure which further aggravated serious corrosion of cantilever beams forming main load bearing members.  The engineer diagnosed condition of deterioration of steel reinforcing bars and concrete carbonation as severe and advised to repair the roof without any delay.  Expert engineers of Struct-Mast realised that the conventional tubular scaffold if put in action will have lot of limitations due to geometry of seating arrangement and cantilever roof beams.

Compulsions:

• The Club wanted to use the seating arrangements for both the court as internal tournament was being held and could not afford to cancel the tournament for children.
• The Club did not want any incident and wanted to repair the roof structures in safest manner.
• The Consulting engineer had diagnosed the roof structures as critical and needed repairs to begin the repairs without any delay.
• The location is such that the club is surrounded by elites and if any work involving noise will attract wrath from neighboring building. 
• To Curb noise and dust pollution only could help in smoother way to repairs.

Roof Structure:

The roof structure is self balancing cantilever beams with slab supported on beams.  These roof structure is sheltering seating arrangement on either side for badminton court and tennis court.  The repairs of this roof requires a blend of skills from the leading service provider.  The cantilever beams were restored first in sequential orders.

Road map to achieve formidable repairs:

The roof slab and beams repairs were undertaken with following ultimate goal which was divided into various tasks.

• Health and safety of all personnel;    
• Minimum degradation of environment;                    
• Zero accidents project and goal of zero incidents;        
• Achieve in time completion;                      
• Lasting repairs in demanding conditions;                    
• Proactive planning and force majeure. 

The first step was to evaluate current condition of existing concrete.  A review of structural design, details, structural instrumentation data, visual inspection were assessed by the expert engineer.

The second step was to relate data obtained from above to mechanism that causes the damage / deterioration. The competent person related observations to cause of damage. In compliance to the assessment for corrosion initiation phase more proactive maintenance program was worked out and in corrosion propagation phase more detailed repairs strategy was worked out.  

Access system for ceiling repairs

The classified damages were assessed as: 

•   Surface repair - removal and replacement of deteriorated concrete.
•     Strengthening - strengthen or enhance capacity of a structural member.
•   Stabilization - halting unwanted condition like cracking or settlement
•    Water-proofing - stops fluid from entering or exiting concrete structure.
•     Protection- protect concrete from aggressive environment 

The surface damage is superficial damage mainly due to shrinkage crack /cob web crack  seen on surface and are not more than 1.5 mm deep and widths are less than 0.1 mm.   

For ceiling sliding platform was used for an access which proved very beneficial to obtain the quality repairs and progress as very fast.  

Varying degree of damage observed in ceiling

 The entire ceiling was tapped and loose laminated concrete was removed till carbonation.  For deeper degree of damage the micro-concrete was placed in to form works using specially innovated micro-concrete pump which proved very beneficial to achieve good quality works and faster progress.  All anti-corrosive treatment was applied to rebars and on need base the steel was replaced when found corroded more than 15%.   

Micro-concrete pump used to place grout underneath ceiling

•      Minor 

Crack width < 0.1 mm observed on roof slab

crack widths are narrow and are found in the range of ≥ 0.1mm & < 0.3 mm damage defined as the damage restricted within cover concrete portion of the structure.   Isolated cracks are seen and spalling of concrete is not more than 30mm. depth.  No rebar are found corroded.  It is important to restore concrete cover in order to prevent rebar from eventually exposed to corrosion.  The GI weld-mesh of 100 X 100 mm placed with 10 Gz. and connected with 8mm diameter shear connector anchored with epoxy compounds. The area is uniformly leveled with 30 mm thick concrete screed with a acrylic polymer bond coat to restore bond strength between existing parent concrete and screed mortar overlay.

Moderate damage

•     Moderate damage 

consists of concrete cracks and wider de-lamination exposing reinforcing steel.  The reduction in cross section of rebar are not more than 5%. There is no immediate effect on the structural capacity. Although cracks and exposed reinforcement can reduce structure life due to corrosion and freeze thaw action.  The cracks widths are medium and in the range of ≥ 0.3 mm and < 1.0 mm.  The repair work should commence within one to six months or simultaneously with the major repairs works. The restoration of existing concrete surface was proceeded with Polymer modified mortar upto 25 mm thickness.

Shear connectors.

•     Severe damage 

wherein crack widths are >1mm, the exposed rebar are found corroded.  The damage to rebar along with loss of significant cross section and possible lateral misalignment due to distortion calls for urgent attention to primary structure as negligence can cause problem. The reduction in cross section of rebar are not more than 15%.     The crack widths are wider than ≥ 1.0 mm.  The corroded bars have been replaced with 10 mm diameter bars with anchorage derived as lapped bars. The entire surface was restored with high strength micro-concrete.  Shear connectors were introduced to have better compatibility between existing concrete materail and rendered surface.

Shuttering for beams

•    Potentially hazardous damage 

wherein damages are wide spread and cracks are wider than 3mm.  Main bars are found necked / reduced cross section more than 15%.  Excessive deflection of the load bearing member of cantilever beams.  The concrete was removed till found carbonated.  The steel bars were replaced with steel bars as per actual.  Concrete was restored with micro-concrete layer as thick as 50 mm with addition of anchor shear connectors. To enlarge the beam sections shuttering was prepared and micro-concrete was poured which reduced time to strengthen the existing beam and strengthened  the beam.

    The photographs below shows the finished surface of ceiling as well as the roof structures.

Finished repair system of first half

Finished surface of ceiling and columns supporting roof.

Finished surface of cantilever beams over roof.

Conclusion:

• In the entire repair process the sliding platform could help in finding cause and effect of the damages to the stadium structures with ease.
• Application of rendering could be accomplished smooth and quicker there by reducing the cost and imrpove durability.
• The main problem in strengthening anticipated was to achieve the desired ‘compatibility’ and a ‘continuity’ in the structural behavior between the original material structure and the new material/ repaired structure.  
• Visual survey subsequently conducted by competent person could ascertain sound repairs.  No leakage through roof slab or joints was observed during the progress of complete monsoon period.
• NDT tests conducted over the repaired structures like UPV test, half cell techniques showed improved results.