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A Case Study –Blast Damaged Industrial Building – Insurance claim settlement

Managing Director – Construction Diagnostic Centre Pvt. Ltd., India
E-mail – ranaderavi@gmail.com

Abstract : –

In one of the chemical manufacturing Industry in Maharashtra, the chemical vessel exploded damaging the RCC structure of the building. Insurance company wanted to assess the extent of damage caused by this blast. Hence a detail investigation was carried out by CDCPL. The owner company had claimed for a total damage of the building; but the investigation revealed only a localised damage.

Blast Damage Assessment Audit : –

This case study reveals the facts about blast damage assessment. The assessment work involved Structural Condition Survey (Structural Audit), collection of data / information from site and from client, conducting various Non-destructive tests, and finding out the extent of damage in RCC building

The detail activities carried out under all above scope of work were as below –
Structural Condition Survey (Structural Audit)

• Detail inspection of the entire structure
• Photographic survey
• Noting various observations such as load transfer system, Structural framing system, Structural deficiencies, settlement if any, Cracks in RCC members, Cracks in masonry / plaster, Leakages, Loads on structure, corrosion, defects in non structural elements etc.
• Identification of broad areas / locations in the structure requiring further detail investigation and for conducting various ND Tests

Conducting various Non Destructive Tests

• Ultrasonic Pulse Velocity test – Direct& Semi- direct Methods
• Rebound Hammer Test
• Core Test
• Carbonation Test
• Half-Cell Potential Test
• Chloride & Sulphate Content test

History of past Repairs / Modifications

• The structure was constructed in two stages. The first building was constructed in 1989 and an extension to this building on west side was done in in 2006.
• Some of the RCC members ( Columns ) were strengthened ( Jacketed ) in 2011-12.
• In the past, some changes in machinery / vessels were carried out, but no data / record was available to know whether the structural members were checked for its capacity for this new / additional loads

Observations

• One of the vessel resting on first floor beams was blasted. The roof slab over the blasted vessel was totally damaged, the adjoining slab near staircase was also totally damaged.
• Other than this slab no other RCC members at other locations and on other lower floors were having any visible crack / deflection caused due to the said blast.
• Due this blast, the plaster over some RCC slabs, beams, columns and walls had de-bonded. The RCC jali around the blast area was damaged at almost places.
• Majority of the RCC members were noticed to have moderate to severe corrosion cracks and spalling of cover concrete.
• Foundations could not be inspected, but the superstructure was not having any visible signs indicating possibility of the settlement of foundations.
• The other observations are as reported below.

Non Destructive Testing observations

• The original grade of concrete of both the wings ( old and new ) was not known. Considering the year of construction, probably the grade of concrete of old building could be M-15 and that of new building being constructed in 2006, should be M-20. But as there was no data available to confirm this, we had assumed the grade of concrete of both the wings as M-15
• The core test results indicated a very wide range in strength of concrete for both the wings as below –
  • Old building – M – 09 to M – 32
  • New building – M – 08 to M – 38
  • New building ( blasted area Grid A2-A3-B2-B3) – M – 14 to M – 30
• The Rebound Hammer & Ultra Sonic Pulse velocity test results also indicated a very wide range in strength of concrete for both the wings as below –
  • Old building – M – 17 to M – 32
  • New building – M – 06 to M – 26
  • New building ( blasted area Grid A2-A3-B2-B3) – M – 06 to M – 27

The strength of site concrete was predicted by carrying out a regression analysis on combined NDT methods – Rebound Hammer Test, UPV Test and Core test.

The qualitative assessment of concrete quality was done based on below table –

At CDC we have carried out extensive research on quality assessment and it has been observed that, the quality gradation as per IS – 13311 (part-1)- 1992 is valid ONLY for M – 15 grade concrete and only for direct probing method. For concrete with more than M – 20, we recommend to grade the quality of concrete as per below given table –

The Half-Cell Potential test indicated that at majority of the test locations, there was a more than 90 % possibility of corrosion

Interpretation of Hal-Cell Potential test results

• If potentials over an area are numerically less than – 200 mV Copper – Copper sulphate half cell, there is a greater than 90 % possibility that, no reinforcing steel corrosion is occurring in that area at the time of measurement.
• If potentials over an area are in the range of – 200 mV to – 350 mV Copper – Copper sulphate half cell, then the corrosion activity of reinforcing steel in that area is uncertain.
• If potentials over an area are numerically greater than – 350 mV Copper – Copper sulphate half cell, there is a greater than 90 % possibility that, reinforcing steel corrosion is occurring in that area at the time of measurement.