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137 Vol 03, Issue 03, Sep-Dec 2012

http://technicaljournals.org

International Journal of Civil Engineering applications Research – IJCEAR

ISSN: 2249- 653X

Physical Load Test of a Poorly Built Concrete Slab Panel of a Frame Structure ALI M. SYED1, MOHAMMAD JAVED2, BASHIR ALAM3 1

Department of Civil Engineering, University of Engineering & Technology Peshawar, Peshawar, PAKISTAN Department of Civil Engineering, University of Engineering & Technology Peshawar, Peshawar, PAKISTAN 3 Department of Civil Engineering, University of Engineering & Technology Peshawar, Peshawar, PAKISTAN 1 [email protected] , [email protected] , [email protected] 2

ABSTRACT A three story RC frame building having column-beam frame structure was designed as public office building. The concrete strength of one of the floor was found to be much lower than the required strength. Concrete cores were cut which also showed poor concrete strength. The owner asked to undertake physical load test of concrete slab before any decision was to be made. Thus, physical load test as specified by the Building Code of American Concrete Institute was conducted. A numerical model was also prepared to correlate the field test results. In light of in-situ concrete strength results, physical field load test and numerical model, recommendations were made in which the structure slab was permitted despite low strength, however it was strongly discouraged to avoid such poor practice that resulted in poor quality concrete. Key words: Physical Load Test, Poor Strength Concrete, Slab, Frame Structure, Cores. 1. INTRODUCTION A three story reinforced concrete (RC) frame building having column-beam frame structure was designed as public office building in Peshawar and was under-construction (shown in Figure-1) when low strength of second floor was pointed out by the supervisory staff. The average height of the building is 3.66 meter (m) thus making the total height of 10.98 m. The owner tested six concrete cubes for quality assurance that were obtained during the pouring of concrete from second floor and gave poor strength. The average 28 days crushing strength of 152 millimeter (mm) cubes when tested in universal testing machine (UTM) was 13.74 Mega Pascal (MPa) instead the required crushing strength of 20.7 MPa. The owner stopped the work and asked the contractor to take out concrete cores to reconfirm the strength results obtained earlier. Six cores were cut from various panels of second floor according to American Society of Testing and Materials (ASTM) C42 [1] and tested as per ASTM C39 [2] which gave an average strength of 11.03 MPa. A technical committee ordered to conduct a physical load test in light of Building Code of American Concrete Institute (ACI) Chapter 20 [3] to ascertain the structural behavior of the under-consideration RC slab. The design loads specified by the owner were used to calculate the test load. A corner panel was selected for physical load test due to its relatively less stiffness. Nine displacement transducers were installed with one in center of selected panel. Sand bags were weighed and prepared as test load for loading the RC panel. Physical load test was performed [3] and response readings were taken by a digital data acquisition system. At various stages of loading the displacement was measured and noted for safety purpose. After unloading, the elastic recovery was measured. Data obtained from the physical load test was used to compare with the ACI criteria [3]. A numerical study was also performed and results of the test were compared with results obtained from numerical study and final recommendations were made thence. The overall project was divided in following three parts: * Physical Load Test * Numerical Study * Recommendations

Figure-1: Front view of under-construction three story building.

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ISSN: 2249- 653X

2. METHODOLOGY AND PHYSICAL LOAD TESTING It was decided to use test methodology specified in Chapter 20 as physical load testing by Building Code of ACI [3]. The methodology followed for conducting the test is explained in detail in the following paragraphs. 2.1 Preparation of Site for Physical Load Test The first step in physical load test was to select the test panel. A corner panel was selected for physical load test due to its relatively less stiffness, as seen in Figure-2. Necessary preparations were done before the test could be started which included the removal of waste material from slab, removing one layer of bricks below the beam soffit so as to allow free deflection, and preparation of platform. Wooden platform was used for working and for providing base for displacement transducers below the panel.

Figure-2: Corner panel on second floor selected for physical load test. 2.2 Visual Inspections After the installation of platform the underside of the slab was inspected. Visual inspections were done before and after the application of test load to see any abnormal physical state of the slab or distress such as crushing, spalling etc. The prime object of undertaking the visual inspection was to ascertain the reliability of the existing slab to be fully loaded as required under ACI loading criteria and to see the state of the RC slab after the load test is finished. No cracks and concrete spalling was observed before the start of the test. 2.3 Test Load The owner specified the design basis load for the RC slab as provided below.  Live load on intermediate floors = 292.95 kg / m2 *  Dead load of RC slab = 292.95 kg / m2 *  Dead load of floor finish = 170.89 kg / m2 * * kg / m2 = kilogram per square meter As per ACI [3], the total test load calculated for the RC panel is 12,194 kg. This load is calculated on the basis of taking into account the actual dimensions of the RC panel. The slab thickness of the panel is 127 mm. The size of RC panel is 3,658 mm x 4,293 mm. This test load was applied in the form of sand and crush filled in bags, each bag weighing 35 kg. Thus total number of bags required were 348. During the process of loading the bags on the RC panel, each bag was verified for its 35 kg weight. Any deviation from 35 kg was noted and was adjusted using extra bags. 2.4 In-situ Material Properties Six cubes were prepared during the concrete pouring and the strength results are provided in Table-1 below. Table-1: Strength test results of concrete cubes prepared during pouring of concrete. S. No.

Crushing Strength (MPa)

1 10.68 2 8.06 3 10.00 4 21.95 5 12.49 6 19.25 Average = 13.74 Six cores were cut from the second floor RC slab as per ASTM C42 [1]. From the testing of six cores as per ASTM C39 [2], the results are established and provided in Table-2 below.





ISSN: 2249- 653X

Table-2: Strength test results of concrete cores cut from RC slab. S. No.

Crushing Strength (MPa)

1 2 3 4 5 6 Average =

11.09 9.49 10.82 11.71 12.25 10.84 11.03

2.4 Instrumentation The positions of displacement transducers were marked on the underside of the slab at the locations determined. The transducers were installed at mid point and third points of the slab panel, as shown in Figure-3. The displacement transducers were placed on rigid platform and connected with the channels of the Data Logger. All the displacement transducers were calibrated before the load test.

Figure-3: Installation of displacement transducers. 2.5 Loading and Unloading the RC Panel The load was placed in equal increments according to ACI Chapter 20 [3] specifications. Figure-4 shows an intermediate stage of loading. A set of response measurements was made after each load increment was applied and 24 hours after the application of 100% test load. The load was removed immediately after all response measurements were made and a set of final measurements were recorded 24 hours after the load was removed.

Figure-4: Loading of sand and crush bags on to the RC slab panel. 2.6 Response Measurement The deflections measured at all locations of the slab were found to be appreciably small. The maximum deflection measured was 3.068 mm which is 1.75 times less than the allowed maximum deflection of 5.379 mm as per ACI Building Code Chapter 20 [3]. This shows that the structure is stiff. The maximum measured deflections under full test load were well below the maximum deflection permitted by the ACI Code. For details please refer to Table 1 for measured maximum deflection at various locations. As per ACI Code Commentary recovery measurements are waived if the maximum measured deflection is less than

lt2 20000h

whereas,





ISSN: 2249- 653X

“lt” is span length and “h” is slab thickness. Table-3 below shows the response measurement at the nine channels for the 100% of the test load. Table-3: Maximum measured deflections after 24 hours of 100% test load. Channel No.

Maximum Deflection , Δmax, (mm)

1 (mid-span) 3.068 2 3.000 3 2.987 4 1.905 5 1.833 6 2.013 7 1.836 8 2.036 9 1.879 The displacement time history of the mid-span is shown in the Figure-5, below which shows the maximum deflection at 24 hours after the application of 100% test load and the residual measured deflection after 24 hours of unloading.

Figure-5: Displacement time history of mid-span deflection. 3. Numerical Study A numerical model of the RC slab of the test floor is prepared. This model in conjunction with physical load test is used to arrive to conclusions and recommendations. The thickness of slab is taken 127 mm. Finite Element Method (FEM) software SAP2000 is used to prepare the numerical model of the RC slab. Loads provided as discussed above are used to analyze the numerical model. The material properties used in the numerical model are those obtained from core tests done for the RC slab in question, and is based on its average strength which is 11.03 MPa. For the nine points on which displacement transducers are installed during the physical load test, the deflections are checked and compared with numerical model results. Table-4 gives the results of the deflection calculated from numerical model. Table-4: Deflections computed from Numerical Model. Maximum Deflection, Δmax, (mm) Channel No. 1 (mid-span) 2 3 4 5 6 7 8 9

4.259 4.013 4.109 3.539 3.793 3.651 3.570 3.805 3.633





ISSN: 2249- 653X

4. CONCLUSIONS Based on the physical load test, numerical model and strength test results, following conclusions are drawn and accordingly recommendations are made. 1. Required 28 days concrete cylinder strength as per specifications provided in the design document was 20.7 MPa. Whereas, the cubes tested and core tests have shown average concrete strength of 13.73 MPa and 11 MPa respectively, which is less than the required 28 days cylinder strength of 20.7 MPa. 2. The owner of the building has requested to undertake a physical load test to check the response of RC slab only, and provide recommendations, for which physical loading test of one panel was done according to ACI318 [3]. 3. The maximum mid-span deflections observed during the loading test is 3.068 mm which is less than the deflection criteria set by ACI-318. The elastic rebound recovery is 78.75% which also satisfies the elastic recovery criteria set by ACI-318 [3]. 4. The maximum mid-span deflections measured from the physical load test are less than that from the calculated deflections from numerical model (4.259 mm). Thus concluding that the actual structure is much stiffer than the numerical model. 5. No abnormal behavior such as cracks, crushing or spalling of concrete is observed after the completion physical load test. 5. RECOMMENDATIONS On the basis of conclusions drawn from the field testing, following recommendations are made: 1. For all RC works, a mix design should always be done. The supervisory staff and contractor shall make sure that the mix design is followed properly so that the desired concrete strength as per specifications is achieved. Concrete which is under specification is strongly discouraged and owner, Consultant and Contractor shall ensure good quality concrete as per specifications, which is 20.7 MPa cylinder strength at 28 days. 2. Although the criteria set by physical Load test is qualified by the RC panel tested, all measures should be taken to avoid the practice which results in low strength concrete. ACKNOWLEDGEMENTS The authors wish to thank the University of Engineering & Technology, Peshawar for supporting and facilitating this research work. REFERENCES [1] ASTM C42/C42M-10, Standard Test Method for Obtaining and Testing Drilled Cores and Sawed Beams of Concrete, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. [2] ASTM C 39/C 39M- 03, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. [3] Building Code Requirements for Structural Concrete and Commentary, 318, 2008, American Concrete Institute, Farmington Hills, MI 48331, U.S.A.


Preparation of papers in single column format

Preparation of papers in single column format

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