Tests of reinforced concrete slabs subjected to a line load and a concentrated load: Experimental data. Concept v. 25-10-2012

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Department of Design & Construction – Concrete Structures

October 25, 2012

Tests of reinforced concrete slabs subjected to a line load and a

concentrated load

Experimental data

CONCEPT v. 25-10-2012

Author:

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Delft University of Technology Report nr. 25.5-12-12

Faculty of Civil Engineering and Geosciences

Department of Design & Construction – Concrete Structures

October 25, 2012

Tests of reinforced concrete slabs subjected to a line load and a

concentrated load

Experimental data

Delft University of Technology

Faculty of Civil Engineering and Geosciences

Department of Design & Construction – Concrete Structures Stevinlaboratorium Postbus 5048 2600 GA Delft Telephone 015 2783990/4578 Telefax 015 2785895/7438 AUTEURSRECHTEN

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AANSPRAKELIJKHEID

De TU Delft en degenen die aan deze publicatie hebben meegewerkt, hebben een zo groot mogelijke zorgvuldigheid betracht bij het samenstellen van deze uitgave. Nochtans moet de mogelijkheid niet worden uitgesloten dat er toch fouten en onvolledigheden in deze uitgave voorkomen. Ieder gebruik van deze uitgave en gegevens daaruit is geheel voor eigen risico van de gebruiker en de TU Delft sluit, mede ten behoeve van al degenen die aan deze uitgave hebben meegewerkt, iedere aansprakelijkheid uit voor schade die mocht voortvloeien uit het gebruik van deze uitgave en de daarin opgenomen gegevens, hetzij de schade die mocht voortvloeien uit opzet of grove schuld zijdens de TU Delft en/of degenen die aan deze uitgave hebben meegewerkt.

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1. Introduction

This report contains the results of experiments on slabs subjected to a combination of a concentrated load and a line load. The goal of these experiments is to study if superposition of loading holds true when comparing to the results of slabs under concentrated loads only close to the support. A total of 8 slabs is used in this series of tests; some slabs are used for reference tests while the majority is used for testing under a combination of a line load and a concentrated load. All slabs are normal strength concrete.

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2. Material properties

2.1. Concrete mix

2.1.1. Cast 13: S19, S20

The 13th cast was executed on 26-05-2011. The mix was composed of blast furnace B cement and gravel aggregates with a particle size between 4mm and 16mm, Table 2.1 and Fig. 2.1.

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Fig. 2.1: Sieve analysis Cast 13. The mix composition is given in Table 2.2.

Table 2.2: Mix composition of cast 13.

The air content and the slump of the mix were not measured. The concrete was delivered by a truck mixer. 10,5m3 of concrete was delivered in a truck mixer of which 7,8 m3 used for this research. Each slab was cast in 5 layers. During casting poker vibrators were used to compact the concrete of the slabs. The cubes were vibrated on the vibration table at 150Hz during 10 seconds. For standard tests 36 cubes were cast on 26-05-2011. A concrete block was cast along with the slabs and stored in the same conditions as the slabs for drilling cores. After casting the slabs and cubes were covered with plastic sheets. The cubes were demoulded after one day and the slabs 5 days. The cubes were stored in the fog room (99% RH and 20°C) and

tested at an age of 7, 53, 85 and 179 days. The slabs were stored in the laboratory (65% RH and 15-20°C). S19 was tested at 89, 96, 179 days. S20 was tested at 176, 187, 189, 190, 195 and 196 days. The results of the standard tests on the cubes are given in Table 2.3, in which fcc is the concrete compressive strength of the cube and fcspl the concrete tensile splitting strength of the cube. The development of the concrete compression strength is given in Fig. 2.2 and of the splitting tensile strength in Fig. 2.3.

Table 2.3: Results of cube test for cast 13. cube age (days) fcc (MPa) fcspl (MPa)

1 7 24,16 2 7 24,43 3 7 25,06 4 7 2,62 5 7 2,56 6 7 2,58 7 53 50,31 8 53 51,49 9 53 50,72 10 53 4,23 11 53 4,07

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-7- 12 53 4,37 13 85 55,04 14 85 57,93 15 85 57,78 16 179 63,16 17 179 56,69 18 179 61,69 19 179 4,94 20 179 4,52 21 179 4,54

Fig. 2.2: Development of concrete compressive strength, cast 13.

0 10 20 30 40 50 60 70 0 20 40 60 80 100 120 140 160 180 fc ( M P a) time (days)

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Fig. 2.3: Development of splitting tensile strength, cast 13.

2.1.2. Cast 14: S21, S22

The 14th cast was executed on 09-06-2011. The mix was composed of blast furnace B cement and gravel aggregates with a particle size between 4mm and 16mm, Table 2.4 and Fig. 2.4. A retarder and superplastifier were added to the mixture.

Table 2.4: Sieve analysis Cast 14.

0,00 0,50 1,00 1,50 2,00 2,50 3,00 3,50 4,00 4,50 5,00 0 20 40 60 80 100 120 140 160 180 sp li tt in g s tr en g th ( M P a) time (days)

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The air content and the slump of the mix were not measured. The concrete was delivered by a truck mixer. 11,5m3 of concrete was delivered in a truck mixer of which 7,8 m3 used for this research. Each slab was cast in 5 layers. During casting poker vibrators were used to compact the concrete of the slabs. The cubes were vibrated on the vibration table at 150Hz during 10 seconds. For standard tests 36 cubes were cast on 09-06-2011. A concrete block was cast along with the slabs and stored in the same conditions as the slabs for drilling cores. After casting the slabs and cubes were covered with plastic sheets. The cubes were demoulded after one day and the slabs after 1 week. The cubes were stored in the fog room (99% RH and 20°C) and tested at an age of 7, 28, 187, 221 days. The slabs were stored in the laboratory (65% RH and 15-20°C). S21 was tested at 187, 224, 253, 271 and 273 days. S22 was tested at 188, 190 and 221 days. The results of the standard tests on the cubes are given in Table 2.6, in which fcc is the concrete compressive strength of the cube and

fcspl the concrete tensile splitting strength of the cube. The development of the

concrete compression strength is given in Fig. 2.5 and of the splitting tensile strength in Fig. 2.6.

Table 2.6: Results of cube test for cast 14. cube age (days) fcc (MPa) fcspl (MPa)

1 7 26,07 2 7 26,41 3 7 27,04 4 7 2,42 5 7 2,73 6 7 7,70 7 28 37,34 8 28 42,74 9 28 43,73 10 28 3,59 11 28 3,71 12 28 3,57 13 187 55,52

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-11- 14 187 55,59 15 187 59,16 16 187 4,49 17 187 4,35 18 187 4,62 19 221 60,72 20 221 57,61 21 221 59,23 22 221 4,74 23 221 4,35 24 221 4,32

2.1.3. Cast 15: S23, S24

The 15th cast was executed on 20-06-2011. The mix was composed of blast furnace B cement, flyash and gravel aggregates with a particle size between 4mm and 16mm, Table 2.7 and Fig. 2.7. A retarder and superplastifier were added to the

mixture.

Table 2.7: Sieve analysis Cast 15. 0 10 20 30 40 50 60 0 50 100 150 200 fc ( M P a) time (days) 0,00 0,50 1,00 1,50 2,00 2,50 3,00 3,50 4,00 4,50 5,00 0 50 100 150 200 spl it ti ng s tr eng th (M P a) time (days)

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The air content and the slump of the mix were not measured. The concrete was

delivered by a truck mixer. 8,5 m3 of concrete was delivered in a truck mixer of which 7,8m3 was used for this research. Each slab was cast in 5 layers. During casting poker vibrators were used to compact the concrete of the slabs. The cubes were vibrated on the vibration table at 150Hz during 15 seconds. For standard tests 36 cubes were cast on 20-06-2011. A concrete block was cast along with the slabs and stored in the same conditions as the slabs for drilling cores. After casting the slabs and cubes were covered with plastic sheets. The cubes were demoulded after one day and the slabs 5 days. The cubes were stored in the fog room (99% RH and 20°C) and tested at an age of 28, 184, 203 days. The slabs were stored in the laboratory (65% RH and 15-20°C). S23 was tested at 197 and 203 days. S24 was tested at 183, 190, 204 and 206 days. The results of the standard tests on the cubes are given in Table 2.9, in which fcc is the concrete compressive strength of the cube and fcspl the concrete tensile splitting

strength of the cube. The development of the concrete compression strength is given in Fig. 2.8 and of the splitting tensile strength in Fig. 2.9.

1 28 41,47 2 28 42,29 3 28 41,34 4 28 3,81 5 28 3,91 6 28 3,97 7 184 58,95 8 184 58,63 9 184 58,80 10 184 4,90 11 184 4,64 12 184 4,54 13 203 58,87 14 203 58,08 15 203 59,90

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16 203 4,86

17 203 4,68

18 203 4,29

2.1.4. Cast 16: S25, S26

The 16th cast was executed on 03-08-2011. The mix was composed of blast furnace B cement, flyash and gravel aggregates with a particle size between 4mm and 16mm, Table 2.10 and Fig. 2.10. A retarder and superplastifier were added to the mixture.

Table 2.10: Sieve analysis Cast 16. 0 10 20 30 40 50 60 0 50 100 150 200 fc ( M P a) time (days) 0,00 0,50 1,00 1,50 2,00 2,50 3,00 3,50 4,00 4,50 5,00 0 50 100 150 200 spl it ti ng s tr eng th (M P a) time (days)

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The air content and the slump of the mix were not measured. The concrete was delivered by a truck mixer. 7,8 m3 of concrete was delivered in a truck mixer. Each slab was cast in 5 layers. During casting poker vibrators were used to compact the concrete of the slabs. The cubes were vibrated on the vibration table at 140Hz during 15 seconds. For standard tests 36 cubes were cast on 03-08-2011. A concrete block was cast along with the slabs and stored in the same conditions as the slabs for drilling cores. After casting the slabs and cubes were covered with plastic sheets. The cubes were demoulded after one day and the slabs after 3 months. The cubes were stored in the fog room (99% RH and 20°C) and tested at an age of 7, 28, 170 and 184 days. The slabs were stored in the laboratory (65% RH and 15-20°C). S25 was tested at 170, 184, 191 and 194 days. S26 was tested at 174, 176, 177, 180 and 181 days. The results of the standard tests on the cubes are given in Table 2.12, in which fcc is the concrete compressive strength of the cube and fcspl the concrete tensile splitting strength of the cube. The development of the concrete compression strength is given in Fig. 2.11 and of the splitting tensile strength in Fig. 2.12.

1 7 25,07 2 7 24,74 3 7 26,74 4 7 2,37 5 7 2,52 6 7 2,48 7 28 41,78 8 28 41,72 9 28 42,29 10 28 3,61 11 28 3,73 12 28 3,50 13 170 59,27 14 170 59,82

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-17- 15 170 60,20 16 170 4,50 17 170 4,72 18 170 4,64 19 184 57,51 20 184 56,25 21 184 58,37 22 184 4,56 23 184 4.16 24 184 4,22

2.2. Steel

Ribbed reinforcing bars of 10 and 20mm diameter were used. The stress-strain diagrams as tested by Exova are given in Fig. 2.13 and Fig. 2.14.

0 10 20 30 40 50 60 0 50 100 150 200 fc ( M P a) time (days) 0,00 1,00 2,00 3,00 4,00 5,00 0 50 100 150 200 spl it ti ng s tr eng th (M P a) time (days)

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Fig. 2.13: Stress-strain diagram for ribbed bar of diameter 10mm.

Fig. 2.14: Stress-strain diagram for ribbed bar of 20mm diameter. 0 50 100 150 200 250 300 350 400 450 500 550 600 650 0, 0000 0, 0020 0, 0040 0, 0060 0, 0080 0, 0100 0, 0120 0, 0140 0, 0160 0, 0180 0, 0200 0, 0220 0, 0240 0, 0260 0, 0280 0, 0300 0, 0320 0, 0340 0, 0360 0, 0380 0, 0400 0, 0420 0, 0440 0, 0460 0, 0480 0, 0500 0, 0520 0, 0540 0, 0560 0, 0580 0, 0600 0, 0620 0, 0640 0, 0660 0, 0680 0, 0700 0, 0720 0, 0740 0, 0760 0, 0780 0, 0800 S tr es s [M P a] Strain [-] 0 50 100 150 200 250 300 350 400 450 500 550 600 650 0 ,0 0 0 0 0 ,0 0 5 0 0 ,0 1 0 0 0 ,0 1 5 0 0 ,0 2 0 0 0 ,0 2 5 0 0 ,0 3 0 0 0 ,0 3 5 0 0 ,0 4 0 0 0 ,0 4 5 0 0 ,0 5 0 0 0 ,0 5 5 0 0 ,0 6 0 0 0 ,0 6 5 0 0 ,0 7 0 0 0 ,0 7 5 0 0 ,0 8 0 0 0 ,0 8 5 0 0 ,0 9 0 0 0 ,0 9 5 0 0 ,1 0 0 0 0 ,1 0 5 0 0 ,1 1 0 0 0 ,1 1 5 0 0 ,1 2 0 0 0 ,1 2 5 0 0 ,1 3 0 0 0 ,1 3 5 0 0 ,1 4 0 0 0 ,1 4 5 0 0 ,1 5 0 0 S tr es s [ M P a] Strain [-]

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3. Specimens

In total 8 slabs with a width of 2,5m were cast, named S19 – S26. The dimensions of the slabs are presented in Fig. 3.1.

Fig. 3.1: Dimensions of slabs S19 – S26.

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4. Loading scheme and measuring devices

S19 is loaded through a point load only, while S20 – S26 are loaded through a concentrated load and a line load. The line load is applied through a HEM 1000 beam at 1200mm from the support. The size of the loading plate is 300mm x 300mm. The slabs are supported by a line of seven steel plates or rubber pads, Fig. 4.2. The bearings contain 3 layers of 8 mm natural rubber, 4 layers of 4 mm steel S235 and 2 layers of 2.5 mm chloroprene, resulting in a compression stiffness of 2361 kN/mm. S19 is supported by a steel plate in which the load cells fit, Fig. 4.1.

Fig. 4.1: Support conditions of S19.

Fig. 4.2: Overview of test setup, top view.

During the course of testing the forces in the load cells are measured. Crack widths are measured by using a crack width comparator (0,05mm – 10mm). The

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displacements are measured by using lasers. For the tests in which the load is placed in the middle of the width, the layout of the lasers is shown in Fig. 4.3, Fig. 4.4, Fig. 4.5 and Fig. 4.6. When the load is placed towards the east, the layout is as shown in Fig. 4.7, Fig. 4.8, Fig. 4.9 and Fig. 4.10. When the load is placed towards the west, the layout is as shown in Fig. 4.11, Fig. 4.12, Fig. 4.13 and Fig. 4.14.

Fig. 4.3: Locations of lasers on top face, load location: middle, South.

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Fig. 4.5: Locations of lasers on top face, load location: middle, North.

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Fig. 4.7: Locations of lasers on top face, load location: East, South.

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Fig. 4.9: Locations of lasers on top face, load location: East, North.

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Fig. 4.11: Locations of lasers on top face, load location: West, South

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Fig. 4.13: Locations of lasers on top face, load location: West, North

Fig. 4.14: Locations of lasers on bottom face, load location: West, North

During the testing, first the line load is applied force controlled. Then, the

concentrated load is applied deformation controlled at a constant rate. Both loads consists of a hydraulic jack with a 2000kN capacity. The measured failure load is the load at which the force decreases for an increasing displacement.

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5. Presentation of the results

5.1. Introduction

This chapter contains all the experimental results. For each specimen the same set of results is reported. A brief explanation of this procedure is included here. First the data covering all tests on one specimen are presented:

1. The basic variables of the specimen.

2. The compressive strength of the cores drilled out of the specimens.

Then, the following series of data is represented for every test carried out on the same specimen:

3. The basic variables of the considered test.

4. Description of the observations made during testing and pictures of the failure mode.

5. The force-time diagram. This diagram represents the loading scheme on the specimen: for the line load and the concentrated load.

6. The force-displacement diagram as measured: for the line load and the concentrated load.

7. The force-time diagram at the prestressing bars.

8. The deflection profiles at selected points in time over the span length. 9. The deflection profiles at selected points in time over the simple support. 10.The deflection profiles at selected points in time over the continuous support. 11.The forces over the support at selected points in time. These forces were

measured by load cells placed underneath the elastomeric bearings. 12.The deflections profiles at selected points in time close to the load.

The selected points in time are typically determined as following: five equal intervals in time until failure at the concentrated load is reached, and a sixth point half-way in the post-peak regime.

Finally the following data representing all tests on one specimen are presented: 13.The measured cracked width along the support.

14.The measured crack widths.

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5.2. Test results

5.2.1. S19

1. The basic variables of the specimen

S19 was cast on 26-05-2011. S19 has a longitudinal reinforcement of φ20 – 125mm bars and a non-principal longitudinal reinforcement of φ10 –125mm bars. In other words, ρl = 0,996% and ρt = 0,258%. The average compressive strength at the age of testing was fc’ 56,9= MPa. A load plate of 300mm x 300mm was used. A steel strip was used for the support.

2. The compressive strength of the cores drilled out of the specimen.

Cores were drilled out of a reference concrete cube stored under the same conditions as the specimens. The cores were drilled and tested at 85 days.

Table 5.1: Results from concrete cores corresponding to S19.

Number Size Stress

cm MPa Cil 1.1 10*10 25,07 Cil 1.2 10*10 25,78 Cil 2.1 10*10 12,54 Cil 2.2 10*10 17,27

5.2.1.1 S19T1

3. The basic variables of the considered test.

Date: 23-08-2011

Load position: a = 600mm, br = 1250mm, at continuous support. The initial prestressing was 3x15kN.

4. The observations made during testing.

On 17/08/2011 a test was carried out the check the functioning of the line load. On 18/08/2011 a test with a single concentrated load was carried out. At 200kN, no cracks were observed. At 400kN, two small cracks at the bottom face were visible. However, the visibility on the bottom face of the slab was not sufficient as the bottom face turned out to be quite sandy and covered with darker patches. At 575kN the test was stopped because orientation of the jack became skewed. On 19/08/2011 the test was repeated. The slab was loaded to 600kN. Some cracks were visible at the bottom face. At 800kN, the maximum crack width was 0,1mm for the first crack in the north-south direction and for some cracks in the east-west direction. The majority of the observed cracks ran in the east-west direction. At 975kN, the test was stopped due to the position of the jack being skewed again. It was then observed that the loading

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frame was swaying. Bracing was provided, and on 23/08/2011 another test was carried out. At 800kN more cracks were observed in the span. The maximum load was reached at 1568kN. At failure, a grid-like pattern had developed on the bottom face and at the east side face a starting shear crack was visible. Afterwards, the cracking pattern did not indicate a clear effective width. The maximum measured crack widths were: 0,05mm at the front face; 0,15mm at the east side face for the shear crack; 0,3mm at the east side face for a crack above the support; 0,35mm at the bottom face for a crack in the north-south direction close to the load; 0,1mm for the same crack but close to the support; 0,25mm for a crack at the bottom face running in the east-west direction close to the support and 0,05mm for a crack in the east-west direction close to the load.

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Fig. 5.2: Inclined crack at side face S19T1.

5. The force-time diagram.

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6. The force-displacement diagram.

Fig. 5.4: Force – displacement diagram S19T1.

7. The force-time diagram at the prestressing bars.

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8. The deflection profiles at selected points in time over the span length.

Fig. 5.6: Deflection plot S19T1.

9. The deflection profiles at selected points in time over the simple support.

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10.The deflection profiles at selected points in time over the continuous support.

Fig. 5.8: Deflection at continuous support S19T1.

11.Forces over the support at selected points in time

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12.Deflections close to the load

Fig. 5.10: Deflections close to the load S19T1.

5.2.1.2 S19T2

Date: 01-09-2011

Load position: a = 600mm, br = 1250mm, at simple support. The initial prestressing was 3x15kN.

The first S19T2 was executed on 30/08/2011. At 200kN, no cracks were observed. At 400kN, the maximum measured crack width was 0,1mm for a crack in the north-south direction. No cracks in the east-west direction were observed. At 600kN the

maximum crack widths were 0,2mm for a crack running in the north-south direction and 0,1mm for a crack running in the east-west direction perpendicular to the north-south crack. At 800kN the maximum crack widths are: 0,1mm for a crack at the bottom face in the east-west direction; 0,25mm at the bottom face for a crack in the north-south direction and 0,25mm at the front face for a middle crack. At 1100kN, the test was stopped as the bracing system of the frame had displaced. The test was repeated on 31/08/2011. The bracing was altered: the bracing was now prestressed to the floor and a larger contact surface with the floor was used. The initial prestressing

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was now 3 x 17kN. At 800kN, the westernmost bracing element started sliding again. Consequently, additional bracing elements were added to form a triangle. At 1484kN the maximum load was reached slowly, after which the load decreased slowly again. It was observed that the loading plates had been moving, and afterwards it was observed that some parts of the frame had yielded. At failure, large cracks at the front face were visible. The effective width was measured as 1,58mm. The maximum measured crack widths after failure are 0,35mm at the front face for a middle crack; 0,25mm for a crack in the north-south direction on the bottom face and 0,05mm for a crack running in the east-west direction.

Fig. 5.11: Cracks at bottom face S19T2.

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Fig. 5.13: Damage to the frame after S19T2.

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Fig. 5.15: Force – displacement diagram S19T2.

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Fig. 5.17: Deflection plot S19T2.

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11.Forces over the support at selected points in time

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13.Deflections close to the load

Fig. 5.21: Deflections close to load S19T2.

13.The measured cracked width along the support.

Table 5.2: Effective width S19.

beff

S19T2 1,58m S19T1 nn

14.The measured crack widths.

Table 5.3: Crack widths S19.

F (kN) wmax (mm) where?

S19T1 800 0,10 bottom face - crack in NS direction

0,10 bottom face - crack in EW direction close to support

Fail 0,05 front face

0,15 side face east - shear crack

0,30 side face east – crack above support

0,35 bottom face - NS close to load

0,10 bottom face - NS close to support

0,25 bottom face - EW close to support

0,05 bottom face - EW close to load

S19T2 400 0,10 bottom face - NS close to load

600 0,10 bottom face - EW close to load

800 0,10 bottom face - EW close to load

0,25 front face - middle crack

Fail 0,35 front face - middle crack

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0,05 bottom face - EW close to load

5.2.2. S20

S20 was cast on 26-05-2011. S20 has a longitudinal reinforcement of φ20 – 125mm bars and a non-principal longitudinal reinforcement of φ10 –125mm bars. In other words, ρl = 0,996% and ρt = 0,258%. The average compressive strength at the age of

testing was fc’ 60,51= MPa. A load plate of 300mm x 300mm was used. Steel

bearings were used for the support.

Cores were drilled out of a reference concrete cube stored under the same conditions as the specimens. The cores were drilled and tested at 180 days.

Table 5.4: Results from concrete cores corresponding to S20.

Number size fc' cm Mpa Cil 1.1 10*10*10 18,74 Cil 1.2 10*10*10 15,59 Cil 2.1 10*10*10 26,02 Cil 2.2 10*10*10 27,31

5.2.2.1 S20T1

Date: 18-11-2011

Load position concentrated load: a = 600mm, br = 1250mm , at simple support.

Load position line load: a = 1200mm. The initial prestressing was 3x15kN.

First, the line load was applied. The load was increased to 600kN, force-controlled, and then was kept constant. This resulted in flexural cracks, with the largest crack width 0,15mm wide at the east side face. Next, the concentrated load was applied deformation-controlled. Up to 600kN, the crack width of the flexural crack at the east side face remained constant at 0,15mm. Afterwards, the load was increased

continuously, and cracks were not measured anymore for safety reasons. The maximum load was reached at 1506kN. Then, the point load was removed. After removal of the point load, failure was induced at the line load. This failure was brittle, sudden and unexpected. A shear crack over the full width resulted. After failure, the following crack widths were measured: at the east side face: a shear crack of roughly

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50mm wide, a crack above the support of 0,7mm and a flexural crack of 0,6mm; at the bottom face: a crack in the north-south direction of 0,1mm wide, an 8mm wide east-west crack right next to the support and a 1mm wide north south crack at 6cm from the west side; at the front face only shrinkage cracks and at the west side face: a crack of 0,5mm above the support and a more than 100mm wide shear crack.

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Fig. 5.24: Failure at top face S20T1.

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Fig. 5.26: Loading scheme S20T1, line load.

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Fig. 5.28: Force – displacement diagram S20T1, line load.

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Fig. 5.30: Loading scheme at prestressing bars S20T1.

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Fig. 5.32: Deflection over simple support S20T1.

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11.Deflection over width at the location of the point load at selected points in time

Fig. 5.34: Deflection parallel to line load S20T1.

14.Forces over the support at selected points in time.

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5.2.2.2 S20T2

Date: 29-11-2011

Load position concentrated load: a = 600mm, br = 1250mm , at continuous support.

Load position line load: a = 1200mm. The initial prestressing was 3x15kN.

During a first trial, the line load was increased up to 600kN, after which problems with the circuits of the point load arose. After repair, the test was repeated. The line load was then again loaded up to 600kN. Then, the load on the concentrated load was increased. At 200kN the following cracks were observed: an inclined crack between the prestressing bars and the support of 0,1mm wide and a vertical crack above the support of 0,1mm wide. Failure occurred when the point load reached 1262kN.

However, the slab failed by opening the shear crack of S20T1. After failure, no cracks were found in the area of the concentrated load. The maximum measured crack widths were: 0,1mm for a crack above the west side support and 0,2mm for an inclined crack at the west side face.

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Fig. 5.37: After S20T2, the slab was lifted off the simple support.

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Fig. 5.39: Loading scheme of concentrated load S20T2.

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Fig. 5.41: Force – displacement diagram concentrated load S20T2.

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Fig. 5.43: Deflection plot S20T2.

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11.Forces over the continuous support at selected points in time

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15.Deflections close to the load at selected points in time

Fig. 5.47: Deflection along line load S20T2.

5.2.2.3 S20T2D

Date: 02-12-2011

Load position concentrated load: a = 600mm, br = 1250mm , at continuous support.

The span length was reduced to 2400mm. Load position line load: a = 1200mm.

The initial prestressing was ± 3x80kN, as the slab was not resting on the temporary support but on the prestressing bars and the continuous support.

The size of the loading plate was 200mm x 200mm.

After S20T2, which failed at the simple support, it was decided to use a temporary support to reduce the span. The temporary support consists of a steel strip of 2,5m x 0,1m x 0,015m and a layer of felt N100 of 2,5m x 0,1m x 0,0075m.

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Fig. 5.48: Temporary support.

The position of the lasers remained the same, with lasers 11, 12 and 13 above the simple support. The zero measurement of the load cells over the continuous support was done before the start of the test.

First S20T2b was carried out, in which problems arose with the circuits of the

concentrated load. This experiment was stopped, and repeated as S20T2c the next day. A loading plate of 300mm x 300mm was used. After reaching the maximum

deformation capacity of the jack of the concentrated load, the test was stopped. A load of 1600kN was reached, while the maximum observed crack width at the bottom of the slab was 0,05mm. Then, the test was repeated as S20T2d, with a loading plate of 200mm x 200mm. In this experiment, the line load reached 600kN and the

concentrated load 1552 kN. Failure occurred, but no signs of failure were visible. After testing, the maximum measured crack widths were: 0,05mm for a flexural crack at the bottom of the slab; 0,15mm at the east side face and 0,25mm at the west side face; both for an inclined crack between the concentrated load and the support. This crack might indicate shear, but the crack was not developed enough to be categorized as a shear crack. The failure mode was –most likely, but not observably- punching inside the slab depth.

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Fig. 5.49: Cracks at bottom face S20T2D.

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Fig. 5.51: Loading scheme of line load S20T2D.

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Fig. 5.53: Force – displacement diagram of line load S20T2D.

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Fig. 5.55: Loading scheme at prestressing bars S20T2D.

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Fig. 5.57: Deflection over simple support S20T2D.

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11.Forces over the continuous support at selected points in time

Fig. 5.59: Forces plot at continuous support S20T2D.

12.Deflection profiles close to the load at selected points in time

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5.2.2.4 S20T3

Date: 07-12-2011

Load position concentrated load: a = 600mm, br = 438mm east , at continuous support.

A few changes were made to the setup prior to S20T3: the direction of the supporting boxes of the temporary support were moved, such that lasers 11, 12 and 13 could be placed over the temporary support. No soft pads were placed, the lasers were measuring directly on the concrete surface, as experience in the previous tests showed that these pads could slide during the experiment. The zero measurement at the beginning of the experiment did not affect the measurements of the jacks and the load cells of the prestressing bars.

The line load was applied force-controlled at 1kN/s up to 601kN, after which the concentrated load was applied deformation-controlled at 0,02mm/sec until failure at 1337kN. A shear crack formed at the east side face. After failure, the following cracks were measured: at the bottom face: 0,35mm for a north-south crack parallel to the east side face at 7,5cm from the east side face; 0,25mm at the bottom face close to the support indicating possible punching around the two easternmost supports; at the side face: 4mm for a shear crack between the concentrated load and the support; 3,5mm for the branch of the shear crack going to the line load. Also, a fan of smaller cracks was observed at the east side face. No cracks were observed at the front face or on the bottom face between the prestressing and the front face. Overall, very few cracks were observed at the bottom face, only locally in the vicinity of the crack.

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Fig. 5.61: Shear crack S20T3.

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Fig. 5.63: Loading scheme of line load S20T3.

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Fig. 5.65: Force – displacement diagram of line load S20T3.

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11.Forces over the simple support at selected points in time

Fig. 5.71: Forces plot at continuous support S20T3.

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5.2.2.5 S20T4

Date: 08-12-2011

Load position concentrated load: a = 600mm, br = 438mm west, at continuous support.

First, the line load was applied until a load of 601kN was achieved. The largest crack width at the west side face was then 0,35mm wide. Then, the concentrated load was applied. At 1100kN some sliding and moving of the steel frame was audible. At 1400kN the cracking of the concrete was audible. A shear crack led to failure at 1449kN for the concentrated load. After the experiment, the following cracks were observed: at the bottom face: 0,1mm for a north south crack at 54cm from the west side face; 0,05 – 0,1mm for cracks in the east-west direction, some of which were inclined towards the second bearing from the west side; at the side face: 0,2mm for a crack above the temporary support; 1mm for the first branch of the shear crack (before the concentrated load); 0,7mm for the second branch of the shear crack towards the concentrated load; 0,6mm for the third branch of the shear crack towards the line load and 0,1mm for a crack between the continuous support and the location of the prestressing bars.

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Fig. 5.75: Loading scheme line load S20T4.

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Fig. 5.77: Force – displacement diagram line load S20T4.

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11.Forces at the continuous support at selected points in time

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beff S20T1 2,5m S20T2 2,5m S20t2b nn S20T3 0,99m S20T4 1,04m

Fpunt (kN)

wmax

(mm) where?

S20T1 200 0,15 side face east - flexural crack from line load

300 0,15 side face east - flexural crack from line load

Fail 50,00 east side face - shear crack

0,70 east side face - crack above support

0,60 east side face - flexural crack

0,10 bottom face - north east crack at middle

8,00 bottom face - east west crack next to support

1,00 bottom face west - north south crack 6 cm from west side

0,50 west side face - crack above support

>100 west side face - shear crack

S20T2 200 0,10 side face west - inclined crack between support and prestressing

0,10 side face west - crack above support

Fail 0,10 side face west - crack above support

0,20 side face west - inclined crack

S20T2

b Fail 0,05 bottom face - grid of cracks

0,15 side face east - inclined crack between load and support

0,25 side face west - inclined crack between load and support

S20T3 Fail 0,35 bottom face - NS crack at 7,5cm from E edge

0,25 bottom face - close to support, 1st two supports from E side

4,00 side face - shear crack

3,50 side face - shear crack towards line load

S20T4 Fail 0,10 bottom face - NS crack at 54cm from W-side

0,05 - 0,1

bottom face - EW cracks, inclining EW crack towards 2nd support from W-side

0,20 side face W - crack above temporary support

1,00 side face W - shear crack before point load, 1st branch

0,70 side face W - shear crack towards point load

0,60 side face W - shear crack towards line load

0,10 side face W - inclined crack between support and prestressing

15.The dimensions of the dominant shear cracks at the side face.

Table 5.7: Dimensions of shear cracks S20.

S20T3 linclined 55

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5.2.3. S21

S21 was cast on 09-06-2011. S21 has a longitudinal reinforcement of φ20 – 125mm bars and a non-principal longitudinal reinforcement of φ10 –125mm bars. In other words, ρl = 0,996% and ρt = 0,258%. The average compressive strength at the age of testing was fc’ = 56,76 MPa. A load plate of 300mm x 300mm was used. Steel

bearings were used for the support. On top of the steel bearing, 7 steel strips of

100mm x 15mm x 350mm and 7 strips of N100 felt of 100 mm x 350mm x 5mm were used.

Fig. 5.85: Detail of the support as used in S21.

Cores were drilled out of a reference concrete cube stored under the same conditions as the specimens. The cores were drilled and tested at 190 and 221 days.

Table 5.8: Results from concrete cores corresponding to cast 14.

Number size fc' cm Mpa Cil 1.1 10*10*10 22,20 Cil 1.2 10*10*10 35,90 Cil 2.1 10*10*10 28,51 Cil 2.2 10*10*10 30,31

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5.2.3.1 S21T1

Date: 13-12-2011

Load position concentrated load: a = 600mm, br = 1250mm west, at continuous

support.

The span length was 3600mm.

Load position line load: a = 1200mm. The initial prestressing was ± 3x15kN.

As compared to S20T2, the support conditions were altered: a steel strip and a layer of felt were added.

First, the line load was applied until a force of 600kN was achieved. Flexural cracks were observed, mostly at the east side of the specimen. This also corresponded to the observations of the load cells, which showed that more load was transferred to the east side. A large shrinkage crack was observed at the east side of the front face of

0,15mm wide. Next, the concentrated load was applied. At 930kN a shear crack developed, which led to a short drop in the load. The loading was resumed, and the load increased until 1165kN at which failure occurred. The cracking of the slab was audible. After failure, the following crack widths were observed: at the west side face: 0,3mm for a shear crack between the concentrated load and the support; 0,1mm for the part of shear crack near the support; at the front face: the shrinkage crack of 0,15mm wide; at the east side face: a shear crack between the support and the

concentrated load of 0,25mm wide and a shear crack between the support and the line load of 0,6mm wide; at the bottom face: flexural cracks at the east side in the east-west direction of maximum 0,2mm wide, close to the load grid-like cracks of

maximum 0,05mm wide. No cracks were observed at the west side of the bottom face, and in general very few cracks were visible at the bottom face.

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Fig. 5.86: Sketch of cracking top face S21T1.

Fig. 5.87: Shear crack at west side face S21T1.

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Fig. 5.89: Cracks at front face S21T1.

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Fig. 5.91: Loading scheme line load S21T1.

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Fig. 5.93: Force – displacement diagram line load S21T1.

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5.2.3.2 S21T2

Date: 19-01-2012

Load position concentrated load: a = 600mm, br = 1250mm, at simple support.

The span length was 3600mm.

Load position line load: a = 1200mm. The initial prestressing was ± 3x15kN.

First, the line load was applied until 603kN. Flexural crack of maximum 0,05mm wide were visible at the side face. A crack from the bottom face of 0,1mm wide was seen at the front face. Next, the concentrated load was applied. Around 1250kN, the shear crack was visible at the west side face. The load increased until 1386kN, after which the slab failed in shear. The failure was rather ductile, and after failure the load dropped back to 1320kN. After failure, the following cracks were measured:

- At the west side face: 0,2mm for the shear crack between the point load and the line load; 0,15mm for the second branch of the shear crack, above the main shear crack.

- At the front face: 0,1mm for a crack over the full depth, in the middle of the width. Smaller cracks from the bottom face were also visible.

- At the east side face: 0,15mm for a flexure shear crack between the

concentrated load and the line load. Crushing due to stress concentrations at the bottom of the line load was also visible.

- At the bottom face: 0,05mm for a short crack in the north south direction from the support; 0,05mm for an east-west crack closer to the support in the middle of the width and farther from the support close to the free edge, creating a V-shape; 0,05mm for a few shorter east-west cracks close to the free edge.

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Fig. 5.101: Shear crack at west side face S21T2.

Fig. 5.102: Cracks at east side face S21T2.

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Fig. 5.106: Loading scheme concentrated load S21T2.

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Fig. 5.108: Force – displacement diagram concentrated load S21T2.

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Fig. 5.114: Deflection plot at load S21T2.

5.2.3.3 S21T3

Date: 17-02-2012

Load position concentrated load: a = 1390mm, br = 438mm east, at simple support but north side (without prestressing)

The line load and prestressing are not used. The size of the loading plate is 300mm x 300mm.

In this experiment, lasers 6 and 7 have switched positions. Laser 3 was broken and not replaced. The shear capacity and force distribution at the support are studied in this experiment for loads at larger distances from the support. At 450kN the existing shear crack from S21T2 opened. The crack width of the shear crack from S21T2 increased until failure at 730kN. Since S21T3 failed at the shear crack from S21T1, this test result cannot be used to study the shear capacity and force distribution at larger

distances from the support. This observation was also reflected by the crack pattern on the bottom face: mainly existing cracks opened and connected, and cracks near the

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support. Barely any cracks near the load could be observed. After the experiment, the following crack widths are measured:

- on the bottom face: 0,2mm for a crack in the east-west direction close to the support; 0,3mm for a crack in the north-south direction close to the free edge; 0,05mm for a crack close to the center of the load.

- on the east side face: 5mm for the upper branch of the main shear crack and 6mm for the lower branch of the mian shear crack.

- on the front face: no new cracks.

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8. The deflection profiles at selected points in time over the south support.

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9. The deflection profiles at selected points in time over the north support.

Fig. 5.121: Deflection at north support S21T3.

10.Forces over the south support at selected points in time

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11.Deflection profiles close to the loads at selected points in time

5.2.3.4 S21T4

Date: 17-02-2012

Load position concentrated load: a = 1390mm, br = 438mm west, at simple support but north side (without prestressing)

In this experiment, lasers 6 and 7 have switched positions. Laser 3 was broken and not replaced. As S21T3 did not lead to a satisfactory, the test was repeated at the west side as S21T4. Unfortunately, again failure occurred at an existing crack from S21T2 at a load of 753kN. No crack close to the load could be seen on the bottom face and there were no signs indicating that the specimen failed as a result of loading at the location of the concentrated load. After the experiment, the following crack widths are measured:

- on the bottom face: 0,15mm for a crack in the east-west direction parallel to the support,

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- on the west side face: 5mm for the main shear crack and 0,1mm for a second branch underneath the main shear crack.

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5.2.3.5 S21T5

Date: 06-03-2012

Load position concentrated load: a = 870mm, br = 438mm west, at simple support but north side, slab is turned around (without prestressing)

In this experiment, lasers 6 and 7 have switched positions. Laser 3 was replaced. A correction for noise on the measurement of laser 2 was made. In this experiment the shear crack is first observed at 680 kN. At 730kN, the noise of the cracking is very loud. Failure is reached at 853kN. After failure, only inclined cracks were visible on the bottom face. The measured crack widths are:

- on the bottom face: 0,5mm for the largest crack in the northwest corner; 0,1mm for the other inclined cracks on the bottom.

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- on the west side face: 0,35mm for an existing crack from S21T2 above the support; 0,25mm for the first inclined crack; 0,1mm for the second

inclined crack; 0,05mm for the 3rd inclined crack; 0,15mm for the upper branch of the main shear crack; 0,2mm for the lower branch of the main shear crack and 0,15mm for the 2nd lower branch of the main shear crack. The main shear crack consisted of existing cracks from S21T2 and newly formed parts.

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5.2.3.6 S21T6

Date: 08-03-2012

Load position concentrated load: a = 1130mm, br = 438mm east, at simple support but north side, slab is turned around (without prestressing)

In this experiment, lasers 6 and 7 have switched positions. Laser 3 was replaced. A correction for noise on the measurement of laser 2 was made. In this experiment, a shear crack from S21T2 existed at the side face. The maximum capacity at the jack was 780kN, after which an immediate and explosive failure occurred. After failure, only very few new cracks were visible at the bottom face, none of which were in the vicinity of the center of the loading plate, which indicates that the failure occurred at the existing crack and the experiment cannot be considered as representative for this tested a/d ratio. The measured crack widths are:

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- on the bottom face: 0,5mm for an existing inclined crack close to the support, and maximum 0,1mm for new inclined cracks at the northeast corner of the slab.

- on the east side face: 6mm for the main shear crack; 1,5mm for a branch above and 0,3mm for a branch below the main shear crack.

- on the front face: 0,1mm for a crack in the northeast corner.

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beff S21T1 0,6m S21T2 2,5m S21T3 1,2m S21T4 1,3m S21T5 0,75m S21T6 1,24m

Flijn (kN) Fpunt (kN) wmax (mm) where?

S21T1 600 0 0,15 front face - shrinkage crack

600 Fail 0,30 west side face - shear crack between point load and support

0,10 west side face - shear crack at bottom close to support

0,15 front face - shrinkage crack near east side

0,25 east side face - shear crack between point load and support

0,60 east side face - shear crack between line load and support

0,20 bottom face - flexural crack EW direction towards E side

0,05 bottom face - close to load NS and EW direction

Tests of reinforced concrete slabs subjected to a line load and a concentrated load: Experimental data. Concept v. 25-10-2012

Tests of reinforced concrete slabs subjected to a line load and a

concentrated load

Experimental data

Tests of reinforced concrete slabs subjected to a line load and a

concentrated load

Experimental data

Contents

1. Introduction

2.

Material properties

2.1. Concrete mix

2.1.1. Cast 13: S19, S20

2.1.2. Cast 14: S21, S22

2.1.3. Cast 15: S23, S24

2.1.4. Cast 16: S25, S26

2.2. Steel

3.

Specimens

4.

Loading scheme and measuring devices

5. Presentation of the results

5.1. Introduction

5.2. Test results

5.2.1. S19

5.2.1.1

S19T1

5.2.1.2

S19T2

5.2.2. S20

5.2.2.1

S20T1

5.2.2.2

S20T2

5.2.2.3

S20T2D

5.2.2.4

S20T3

5.2.2.5

S20T4

5.2.3. S21

5.2.3.1

S21T1

5.2.3.2

S21T2

5.2.3.3

S21T3

5.2.3.4

S21T4

5.2.3.5

S21T5

5.2.3.6

S21T6