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Development of Retrofit Scheme for Deficient Post Tensioned Flat Slab by Using Post Tensioned Carbon Laminates for Large Commercial Premises

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MangeshManish Yadav  Sagar PatilAbstract: Post tension flat slab construction is very popular to cover large areas with economy and speed, especially for commercial type of buildings like  IT Parks. Even though post tension slab is popular system if proper quality is not maintained during execution it may lead to major structural stability issue. This case study presents one such typical example attributed to poor workmanship at site and bad quality control at site. When this particular building was given for long term lease, just  before fit out large deflections were observed in slab panels. On closer look and after cleaning through cracks were observed from top around the column capital. Prime reason for this was the bad quality at site and wrong profile placement of post tension tendons inside the slab. Because of this slab was behaving like a regular RCC flat slab, leading to excessive sagging and cracks on top. Sanrachana Structural Strengthening Pvt. Ltd. (SSSPL) team was roped in to provide most efficient solution in terms of both time and cost as structure was to be handed over to tenants. After weighing all the available options it was decided to make use of   high strength pre-cured carbon fiber reinforced polymer (CFRP) laminate system with anchor plate by using both post tensioned and non-posttensioned CFRP Laminates.  The material systems was provided by Specialty Reinforced Polymer Composite Pvt. Ltd. After successful completion of the work, full scale load test conforming to IS 456 was conducted at site before handover. The load testing was carried out by Vijna Consulting Engineers Pvt. Ltd. The structural performance of the particular strengthened structure was satisfactory under the actual load test and it successfully met all the structural requirements with generous safety margin. Over 1.00 lac sq.ft of area was successfully retrofitted by using 12000 RMT (12KM) CFRP laminates in 30 days. The paper presents the evolution of the structural strengthening system, its application at site and details of load test conducted. Pre-cured Carbon fiber laminate have emerged as very efficient construction material systems, especially in the field of structural strengthening/retrofitting of existing structures. While there are many reasons why structures need strengthening/retrofitting, change in use, poor workmanship at site, errors during design or subsequent increase in imposed load demand is one of the major reasons for structural strengthening/retrofitting.

Introduction
Number of new building structures are deficient because of poor workmanship at site, bad quality or because of error in the design. This paper present one such case study.

Post tension flat slab construction is very popular to cover large areas with economy and speed, especially for commercial type of buildings like  IT Parks. Even though post tension slab is popular system if proper quality is not maintained during execution it may lead to major structural stability issue. When this particular building was given for long term lease, just before fit out large deflections were observed in slab panels. On closer look and after cleaning through cracks were observed from top around the column capital. Prime reason for this was the bad quality at site and wrong profile placement of post tension tendons inside the slab. Because of this slab was behaving like a regular RCC flat slab, leading to excessive sagging and cracks on top. It was observe that major cracks are at top of the slab along the periphery of the column capital and also the same cracks are propagating between columns in longitudinal as well as in transfer direction. It was also observe that there is a permanent sag in the slab which is around 40mm (minimum) to 120mm (Maximum). This was the alarming signal to the client.

This type of cracks were observed over on 2 floors having area of around 50000.00 sq.ft each. So it was decided to retrofit total area of 1 lac sq.ft.  As the property was to be given on lease and fit out period was of 60 days, the biggest challenge was to complete the project in 30 days and handed over to client. This would avoid loss of revenue to the client.

Development of Retrofit Scheme 2Different options for retrofitting like addition of structural steel, span shortening and stiffening were considered. However, it would have reduced the head room in addition to adding extra dead load on already weak structure and time required for completion was very high. Considering these limitation of traditional system it was decided to use CFRP Laminates for structural retrofitting by passive as well as active systems. CFRP systems are light weight, its profiles are thin so there is no loss of head room and its application is fast. Use of CFRP is now well establish in various field in construction it can be used by different methods as described in figure 1

Initail Condition of Slab

Development of Retrofit Scheme 3As discussed above post tensioning cable placement was wrong so although slab was designed as Post tensioned slab it was behaving as regular RCC flat slab. And in absense of lesser top reinforcement cracking was observed around the column capital and parallel to column strip. This is can be more clearly seen in Figure 2 a and 2b.

Rerofitting Approach

First slab was analysed by using FEM based sofwares for its designed load and to find the forces on section. It was found that if we don’t consider contribution of post tension forces there was defficiency of top steel. The same defficiency was compensated by applying CFRP laminates at designed spacing and loaction the details of the same are given in following section and figures. Prior to application of CFRP laminates all crackes were opened with V-Cut and are grouted with low viscosity monomer by gravity pouring at many locations grout could be seen dropping from bottom of the slab. After this slab was supported from bottom to release the load partially and in that state laminate application was done. The pannels where excessive deflection was there were strengthened usning post-tenstioned laminate sysytem and balance with regular anchoring.

The strengthening system was designed in reference to ACI 440.2R-08. Pre-cured CFRP laminates being produced in the factory under stringent quality norms, consistent and relatively higher mechanical properties are ensured. Ends were fixed in position by MS plate to avoid peeling.

Defficient Top Reinforcement / Steel

As per input data provided by client, a model is prepared on FEM based software.

Input Data

Grade of slab= M35 N/mm2
Grade of Steel = Fe 415 N/mm2
Depth of slab = 230 mm
Load considerations on the slab
Self-weight of Slab = 5.75 KN/m2
Floor Finish = 2KN/m2
Imposed Load = 5KN/m2

Development of Retrofit Scheme 4  Development of Retrofit Scheme 5  Development of Retrofit Scheme 6

Development of Retrofit Scheme 8  Development of Retrofit Scheme 9

Development of Retrofit Scheme 10   Development of Retrofit Scheme 11   Development of Retrofit Scheme 12

Development of Retrofit Scheme 13   Development of Retrofit Scheme 14

Development of Retrofit Scheme 15  Development of Retrofit Scheme 16

Development of Retrofit Scheme 17  Development of Retrofit Scheme 16

Development of Retrofit Scheme 19FEM Analysis Results and Proposed Strengthening Scheme

Actual Site Photograph of Execution

Material Specification used in Project

STR Strong Lam 100X1.4

Tensile Strength: 2800 MPa
Tensile Modulus: 165 Gpa
Elongation: 1.7 %
Width 100mm
Thickness 1.4 mm

Full Scale Load Testing

Upon completion of the strengthening work, as per the project specifications, full scale load test was carried as per the details below-

Objective of the Test

The objective of the load test was to verify the structural performance of retrofitted slab, which was strengthened using pre-cured carbon  laminate  strengthening system as per the approved design. Thus, with the load test, the structural performance of the strengthened slab was thoroughly verified as per Clause No. 17.6 of IS:456 2000.

Test Parameters

The test parameters were set as per the input data received from consultants as follows:

Design imposed load of 500 kg/m2.

Note: Since the displacements to be measured in the proposed load tests, the test conformed to limit state of serviceability. The imposed test load considered as per IS456 clause 17.6 as follows.

Dead Load + 1.25 times imposed load Therefore actual imposed test load = 200+ (1.25 x 500) = 825 kg/m2.

Test Specimen

Load test was carried out on the strengthened slab panel of typical 10.8 m wide x 10.8 m long patch, which was mutually selected by the consultants as per the test arrangement described herewith.

To apply design imposed load a water tank was constructed with brick block. The height of water level is fill upto 825mm (0.825m) to get the imposed load of 825Kg/m2

Test Equipments

The test facility consists of the following:

Development of Retrofit Scheme 24Test specimen 10.8 m wide x 10.8 m long patch of slab clearly demarcated.

And specially devised data acquisition system- TC-1600 FD in order remotely acquire the test reading with the help of 6 numbers of LSC 100 mm range linear displacement sensors.

Test Schedule

Development of Retrofit Scheme 23Following are the steps to be followed for load testing.

1.    Initially a tank had been made with brick block so that water can be stored over the selected slab panel and a measuring scale was fixed at center of the slab as shown in fig. no.
2.    Load increment will be 25%, 50%, 75% and 100% of the calculated imposed load.
3.    First a water level is fill till 150mm and reading was recorded
4.    Water level is filled till 400mm and reading was recorded
5.    Water level is filled till 600mm and reading was recorded
6.    And at last water level is filled till 825mm and reading was recorded.

Experimental Setup

Development of Retrofit Scheme 20Structural Performance Test

a)     Displacement sensors were placed on the soffit of the slab panel to measure the displacements at predetermined locations. Each sensor was checked and initial reading was noted.
b)     Load was applied gradually.
c)     Displacement and residual displacements at the predetermined locations were captured by the data acquisition system and the deflection values were calculated by using in-house software.
d)     The test specimen was loaded to 100% design load in specific intervals. Each load interval was maintained for specific time interval.

Development of Retrofit Scheme 21Results of Load Testing

–    In the load test, it was found that within 24 hours of removal of the imposed load, much more than 75% of the recovery observed in the structure as described in the graphs presented above.
–    As per the Indian standard IS456: clause 17.6.3.1 If the maximum deflection in mm, shown during 24hr under load is less than 40l2/D, where  is the effective span in m and D the overall depth of the section in mm, it is not necessary for the recovery to be measured and the recovery provisions of clause 17.6.3 shall not apply. In the load test, the recovery of the structure took place with the generous margin as mentioned above. The Development of Retrofit Scheme 22maximum deflection observed in the slab panel = 22.1 mm, The test slab was subjected to a load equal to full dead load of it plus 1.25 times the imposed load for a period of 24 hours and then the imposed load were removed. It was found that the maximum deflection during 24 hour load period is 22.1 mm which is greater than the limit value of 40l2/D i.e. 20.28 mm. The recovery observed in the load test is 75% after unloading.

Thus, the strengthened structure suitably met the IS code requirements as per clause 17.6.3 of IS 456:2000.

Concluding Remark:

Over 1.00 lac sq.ft of area was successfully retrofitted by using 12000 RMT (12KM) CFRP laminates in 30 days. Pre-cured Carbon fiber laminate have emerged as very efficient construction material systems, especially in the field of structural strengthening/retrofitting of existing structures.

Acknowledgements:

Specialized Retrofit Designer & Contractor – Sanrachana Structural Strengthening Pvt. Ltd.

Material Supplier – Specialty Reinforced Matrix Pvt. Ltd.

Load Testing Agency – Vijna Consulting Engineers Pvt. Ltd.

Third Party Consultant – IIT Bombay

References

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2.    MANCARTI, G. D. Strengthening of California steel bridges by pre-stressing, Transportation Research Record No.950, Transportation Research Board,1984, 3-187.
3.    DUNKER, K. F., KLAIBER, F. W., BECK, B. L. and SANDERS, W. W. Jr. Strengthening of existing single span steel beam and concrete deck bridges, Report No. ISUERI- Ames-85231, Civil Eng, Iowa State University, Ames, 1984, pp.102.
4.    SWAMY, R. S., JONES, R. and BLOXHAM, J. W. Structural behavior of reinforced concrete beams strengthened by epoxy bonded steel plates, The Structural Engineer, 65A(2), 1987, pp. 59-68.
5.    MEIER, U. Carbon fibre-reinforced polymers: Modern material in bridge engineering, Structural Engineering International, 1, 1992, pp. 7-11.
6.    MUKHERJEE A. Repair and rehabilitation of structures- Strategies with nonmetallic fibres. Proceedings of the 1st National workshop on ageing and restoration of structures, IIT Kharagpur, 2001,13, pp. 1-12.
7.    GIBSON, R. F. Principles of composite material mechanics. McGraw Hill International edition, engineering mechanics series, 1994.
8.    Maitra, S. R. -2001 ‘‘Fiber-reinforced polymer composites in the rehabilitation and strengthening of reinforced concrete columns.’’ M.Tech. thesis, Dept. of Civil Engineering, Indian Institute of Technology, Bombay, Mumbai, India.
9.    Manfredi, G., and Realfonzo, R. – 2001 ‘‘Models of concrete confined by fiber composites.’’ Proc., 5th Int. Symp. on Fiber-Reinforced Polymer Reinforcement for Concrete Structures (FRPRCS–5), C. Burgoyne, ed., Thomas Telford, London, 865–874

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