CASE STUDY - 1

Fast Track Construction Leads to Fast Track Restoration

Introduction

A multistoried building for a German software company comprising of basement, ground plus five upper floors of total built-up area 32500sq.mt. (3,50,000 sq.ft.) was under construction. Time for completion of the building in all respects was given as 12 months. The columns and footings were of RCC with flat slab construction.

Different grades of concrete were used for different levels; columns up to second floor were of grade M40 and upper floors M35. All other structural members were of grade M25.

Typical photographs of the building at the time of investigation are shown in Fig - 1.1.

Quality control and quality assurance methods were strictly adopted as per company's systems and procedures based on ISO 9002-Quality Management System and relevant codes of practice for quality control of materials were followed. Good construction practices were adopted.

Fig - 1.1:  Typical Views and Plans of Building at the Time of Investigation

Problems Observed

During routine inspection of the construction activities few cracks were observed in:

• Two boundary columns at 2nd floor level in Block-X
• Two boundary columns at 5th floor level in Block-X

These cracks were observed almost after three months after concreting. Typical cracks noticed are shown in Fig - 1.2.

Fig - 1.2 :  Cracks Identified at 2^nd and 5^th Floor Levels

Reasons for the Existing Cracks

Expert Consulting Engineer after critical examination of the cracks declared that the cracks have appeared in horizontal direction mainly due to the plastic shrinkage of the slurry in the concrete at the lateral ties level, because the cracks were equally spaced at the level of lateral ties in the columns (Fig - 1.3 and 1.4).  The construction work had stopped at fifth floor level. Except self load and construction load, there was no other load on the structure.  Hence the cracks are not due to failure of columns due to crushing

However, a joint meeting of clients, engineers, project management group and design consultants declared that the columns have failed by crushing and attributed the following possible reasons for the failure of these columns.

• Crushing of concrete due to overloading
• Poor quality of concrete
• Deficient structural design
• Delayed concreting
• Lack inadequate curing
• Shrinkage
• Plastic shrinkage

Fig: Cracks at Second Floor Level in Another Column

Fig - 1.3 :  Cracks identified at 2^nd Floor level

Fig - 1.4 :  Cracks identified at 5^th Floor level

Core Test

It was decided to extract cores in these columns and the other columns cast on the same day using the same batch of concrete for estimating the probable compressive strength of hardened concrete in the columns. The cores were extracted and tested as per Cl. 17.4 of IS 456-2000.

The numbers of cores taken were 9 and of diameter 68.74 mm. The abstract of the core test results is given in the Table - 1.1.

Table - 1.1 : Abstract of the Core Test Results

As seen in the table only 4 out of 9 cores have passed the test as per Cl 17.4.3 of IS: 456-2000.

Due to the failure of large number of cores, the clients along with Project Engineer and Architects decided to go for core extraction in large number of columns to assess the existing strength of hardened concrete. The Project Engineer selected critical columns at each floor over the entire height of the existing blocks and instructed the contractors to extract one core in each column of diameter 68/74/100 mm.  It was instructed to extract cores in slabs also at 13 identified locations.

The total number of cores extracted amounted to 263 (170 from Block-X+93 from Block-Y).

As supplementary tests it was also decided to conduct Non destructive tests–Rebound hammer test and Ultrasonic pulse velocity test as per IS: 13311 (Part 1 & 2), on all the existing columns not all floors of both the blocks.

The number of columns amounted to 411 (253 from Block-X + 158 from Block-Y).

An abstract of core test and ultrasonic pulse velocity test results are given in Table - 1.2

Table - 1.2 :  Abstract of  Test Results at Second Floor Level

Core test results were correlated with the results of NDT-Rebound Hammer and UPV test.  A typical correlation is shown in Fig - 1.5.

Acceptance criteria of core test results

As per Cl.17.4.3 of IS: 456-2000 - "Concrete in the member represented by a core test shall be considered acceptable if the average equivalent cube strength of the cores is equal to atleast 85 percent of the cube strength of the grade of concrete specified for the corresponding age and no individual core has strength less than 75 percent". The acceptance value of different concrete grades used are given in the Table - 1.3, as per Cl. 17.4 of IS : 456-2000.

Fig - 1.5 :  Correlation of Core Test Results and NDT Results

Table - 1.3 : Acceptance Criteria as per IS : 456-2000 for Different Concrete Grades Used

However, the project consultant and client engineers classified the columns into categories A, B and C, depending on the existing equivalent cube strength of cores, so as to carry out with corresponding strengthening scheme (Table - 1.4).

Table - 1.4 : Classification of Columns by Project Consultant and Client

Based on the Core Test and NDT results (Table - 1.5), jointly, it was decided to strengthen the columns at different floor levels as per Table - 1.6.

Table - 1.5 :  Abstract of Core Test Results

Table - 1.6 :  Columns to be Strengthened at Different Floor Levels

An abstract of the work done is presented in Table - 1.7.

Table - 1.7 :  Abstract of the Work Done

Strengthening Scheme Proposed and Implemented

After long deliberations and discussions of the Project Structural Engineer with clients and strengthening consultant a mutually acceptable and feasible strengthening scheme was proposed. Even though the proposed scheme was tedious and costly the clients insisted that the scheme may be adopted keeping in mind the safety of the structure.

Methodology of Execution of Strengthening Scheme for Identified Columns

• Slab and beams to be adequately supported prior to and during retrofitting works
• Existing concrete surface to be cleaned and roughened.
• Grout the affected columns using low viscosity epoxy grout
• Fix MS plate and ISMC 150 using M24 high strength Hilti expansion bolts at top and bottom of slab
• Weld reinforcement steel (32-Y16) as indicated in the drawing
• Jacketing by using M50 grade free flow micro concrete as per specifications
• Grade of reinforcement steel-Fe 415
• Grade of steel plates, channels and angles 240 Mpa

Typical photographs of implementation of strengthening scheme of columns at different stages are given in Fig - 1.8.

Fig - 1.8 :  Views of different stages of Strengthening Scheme Executed

Inferences on the Present Case Study - Based on Fast Track Construction

The following inference can be drawn after a critical study of the existing condition of the existing of the building, test results and execution of the proposed strengthening scheme.                                 

Conclusion

In meeting the target of completion of construction and handing over the building to the client, do not forget to follow good construction practices

• In the name of Fast Track Construction do not forget QA / QC procedures to be implemented
• Identify the type of cement used-OPC or PPC or Blended Cement in the manufacture of concrete (RMC or SMC). Accordingly, give adequate time for cement to hydrate completely and allow concrete to attain its design strength
• Allow the concrete to gain its full strength in its natural way
• Implement adequate curing
• Give more importance for curing of vertical members, in particular columns
• Follow all codal provisions to achieve required durability of concrete
• Appoint qualified Civil Engineers as Project Managers and Site Supervisors
• For durability, produce concrete of: Dense
• Impermeable Required strength