2.6 TESTING WET AND HARDENED CONCRETE

Transcription

1 2.6 TESTING WET AND HARDENED CONCRETE Workability Bleeding and segregation Air content Test cubes Other mechanical tests

2 Workability Workability is defined as: "The amount of useful internal work necessary to produce full compaction". Note that "consistence" is the European word for workability and is used in all the new European Standards etc.

3 Low Slump Concrete

4 Workability Tests Slump Test Degree of Compactability Test VB Test Flow Table Test Slump Meters Rheological Tests Self Compacting Concrete Tests Slump Loss

5 Slump Test

6 When carrying out the slump test Ensure that the base on which the test is to be carried out is flat, level, clean and free from vibration and of adequate size to stand the cone next to the slumped concrete for measurement. Check that the cone is not bent or dented and the inside surface is clean. Check that a length of 16 mm diameter steel bar is available for tamping. Obtain a representative sample. Do not use the first or last fraction from the mixer. Fill the cone carefully tamping it in three layers. Lift it off vertically. Record the result carefully with details of the date, concrete batch etc.

7 Slump Cone

8 When carrying out the slump test Ensure that the base on which the test is to be carried out is flat, level, clean and free from vibration and of adequate size to stand the cone next to the slumped concrete for measurement. Check that the cone is not bent or dented and the inside surface is clean. Check that a length of 16 mm diameter steel bar is available for tamping. Obtain a representative sample. Do not use the first or last fraction from the mixer. Fill the cone carefully tamping it in three layers. Lift it off vertically. Record the result carefully with details of the date, concrete batch etc.

9 How do you get a sample from this?

10 Concrete Sampling

11 When carrying out the slump test Ensure that the base on which the test is to be carried out is flat, level, clean and free from vibration and of adequate size to stand the cone next to the slumped concrete for measurement. Check that the cone is not bent or dented and the inside surface is clean. Check that a length of 16 mm diameter steel bar is available for tamping. Obtain a representative sample. Do not use the first or last fraction from the mixer. Fill the cone carefully tamping it in three layers. Lift it off vertically. Record the result carefully with details of the date, concrete batch etc.

12 Slump Test

13 Workability Tests Slump Test Degree of Compactability Test VB Test Flow Table Test Slump Meters Rheological Tests Self Compacting Concrete Tests Slump Loss

14 Degree of Compactability (compaction Index) test c h1 = 1 s h s h1

15 Workability Tests Slump Test Degree of Compactability Test VB Test Flow Table Test Slump Meters Rheological Tests Self Compacting Concrete Tests Slump Loss

16 The V-B Test

17 Workability Tests Slump Test Degree of Compactability Test VB Test Flow Table Test Slump Meters Rheological Tests Self Compacting Concrete Tests Slump Loss

18 Flow Table Test Flow Distance

19 Workability Tests Slump Test Degree of Compactability Test VB Test Flow Table Test Slump Meters Rheological Tests Self Compacting Concrete Tests Slump Loss

20 Problems with single-point tests

21 Viscosity (see section 1.6 of the notes) Shear Stress τ Fluid velocity V 1 δy Shear Stress τ Fluid velocity V 2 Bingham Fluid The viscosity e is defined as: e = τ δy Pa s v 1 - v 2 yield stress γ Newtonian Fluid Gradient of this line = viscosity (e) where τ is the shear stress Shear rate

22 Rheometers

23 Results from viscometer test Torque Shear Rate

24 Site Rheometer

25 Rheology of Concrete Mixes Effect of increasing: yield stress g %fines Viscosity Modifyer PFA, GGBS Water Plasticiser or vibration gradient h of shear stress vs shear rate (see fig 1.6.1)

26 Effects of Silica Fume on Rheology

27 Workability Tests Slump Test Degree of Compactability Test VB Test Flow Table Test Slump Meters Rheological Tests Self Compacting Concrete Tests Slump Loss

28 Tests for Self Compacting Concrete L Box test Normal concrete, SCC V Funnel test, Normal concrete, SCC

29 Workability Tests Slump Test Degree of Compactability Test VB Test Flow Table Test Slump Meters Rheological Tests Self Compacting Concrete Tests Slump Loss

30 Effect of Temperature on Slump

31 Slump Loss

32 2.6 TESTING WET AND HARDENED CONCRETE Workability Bleeding and segregation Air content Test cubes Other mechanical tests

33 Segregation of concrete When Poured One hour later Bleed water Depletion of coarse aggregate Accumulation of coarse aggregate

34 The consequences of Segregation Segregation will reduce the strength both in areas where there is a lack of aggregate and in areas where there is a surplus of aggregate. The lack of aggregate may produce cracking and the surplus may produce voidage. A small amount of bleeding will be harmless because it will reduce the effective water/cement ratio and provide some water on the surface for curing. Larger amounts of water loss will cause plastic settlement and cracking. If the rising water carries cement with it a dusty porous surface may result. Bleed water trapped under reinforcement etc. will reduce bond and cause voidage.

35 Measurement of Bleed Bleed measurement with the bleed water replaced after measurement. It will normally all be reabsorbed within 24 hours. Bleed measurement with the water removed and not replaced. This is often a better simulation of site conditions where sun and wind will dry it off

36 2.6 TESTING WET AND HARDENED CONCRETE Workability Bleeding and segregation Air content Test cubes Other mechanical tests

37 Types of air void Accidental air voidage. Typical void size 1-3 mm but any size possible. Entrained air. Typical size 0.05 mm. Entrained to give resistance to freeze/thaw. Capillary pores. Typical size 10-7 m

38 Air Content Measurement

39 2.6 TESTING WET AND HARDENED CONCRETE Workability Bleeding and segregation Air content Test cubes Other mechanical tests

40 When preparing cubes check the following Check that the cube moulds are clean and properly bolted together. Note that moulds may be expensive but bolts are cheap so always have a box of spare bolts and nuts. Never use wooden moulds or steel moulds held together with tie wire. Oil the moulds carefully. Always use proper shutter oil. Check that a steel tamping bar is available. Get a good representative sample. (as for slump test). Always compact the concrete in layers.

41 Cube moulds, metal and plastic

42 When preparing cubes check the following Check that the cube moulds are clean and properly bolted together. Note that moulds may be expensive but bolts are cheap so always have a box of spare bolts and nuts. Never use wooden moulds or steel moulds held together with tie wire. Oil the moulds carefully. Always use proper shutter oil. Check that a steel tamping bar is available. Get a good representative sample. (as for slump test). Always compact the concrete in layers.

43 Storage and Despatch This part of cube testing is often overlooked and can lead to unnecessary failures. Always cover the cubes with wet hessian covered with polythene after casting. Note that hessian alone is virtually useless because it has to be continually wetted. Always ensure that some form of heating is provided in cold weather in the area where the cubes are stored overnight. Try to get the cubes collected by the day after casting. If this is not possible a heated curing tank must be used. Always keep good records of the cubes and ensure that they are properly labelled in at least two places.

44 Temperature matched curing

45 The observed cube strength will depend on: The original concrete (w/c ratio, age, curing conditions etc.) Factors in manufacture and storage (see above) The cleanliness of the platens The alignment of the platens (should be on hemispherical seating which locks on loading) The rate of loading. Faster loading gives higher strength. EN specifies a stress increase of N/mm 2 /s The mode of failure. The failed cube should be two pyramids.

46 Compression of cube

47 The observed cube strength will depend on: The original concrete (w/c ratio, age, curing conditions etc.) Factors in manufacture and storage (see above) The cleanliness of the platens The alignment of the platens (should be on hemispherical seating which locks on loading) The rate of loading. Faster loading gives higher strength. EN specifies a stress increase of N/mm 2 /s The mode of failure. The failed cube should be two pyramids.

48 Effect of loading rate on cube test Duration of test 10 seconds Stress 3 minutes 30 minutes Strain

49 The observed cube strength will depend on: The original concrete (w/c ratio, age, curing conditions etc.) Factors in manufacture and storage (see above) The cleanliness of the platens The alignment of the platens (should be on hemispherical seating which locks on loading) The rate of loading. Faster loading gives higher strength. EN specifies a stress increase of N/mm 2 /s The mode of failure. The failed cube should be two pyramids.

50 Effect of testing method on strength Observed strength loss Insufficient consolidation: 61% Seven days on site: 26% Plastic mould: 14% Eccentric loading: 12% Mix subject to vibration while setting: 5% Rebar tamping: 2%

51 Equivalent Cube Test

52 2.6 TESTING WET AND HARDENED CONCRETE Workability Bleeding and segregation Air content Test cubes Other mechanical tests

53 Tensile Strength Measurement Load P Hardboard pads Diameter D Stress =2P πld Concrete test cylinder Length L Crack forms here

54 Cylinder Splitting

55 Large scale splitting

56 Flexural Strength Measurement Load P L/3 L/3 L/3 depth d width b Span L Crack forms in this region Flexural stress = PL bd 2 Modulus of rupture = flexural stress at failure

57 Flexural strength (modulus of rupture) test