Concrete Lab Test Report Preview

CNM2421: Materials for Construction I

Concrete Engineering Technology Lab Test Reports for C30 Concrete

Thomas Mifsud, David Mifsud Elena Gauci, Judy Hunter Valentina Vella Falzon

Contents 1

Mix Design and Fresh Concrete Testing

1.1 1.2 1.3 1.4 1.5 1.6

Constituent Proportions................................................................................................................. 3 Batch Mixing.................................................................................................................................. 5 Slump Test..................................................................................................................................... 7 Flow Test........................................................................................................................................11 Vebe Test.......................................................................................................................................13 Fresh Density Test.........................................................................................................................15

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Hardened Concete Tests

2.1 Compression Test..........................................................................................................................19 2.2 Non-Destructive Testing (A) Schmidt Hammer Test.............................................................................................................21 (B) Ultrasonic Pulse Velocity Test.................................................................................................22 2.3 Flexural Test..................................................................................................................................23 2.4 Split-Cylinder Test.........................................................................................................................24 3

Data and Results

3.1 C20 Results...................................................................................................................................27 3.2 C25 Results...................................................................................................................................28 3.2 C30 Results...................................................................................................................................29 4

Analysis of Results

4.1 Introduction: Required Characteristics for wet and dry concrete.................................................32 4.2 Introduction II: Properties of the Consituent Materials Used........................................................32 4.3 Comparing the Tests Results with Standard Data for the Three Grades......................................33 4.3 The influence of Safety Factors....................................................................................................33 4.4 The effects of Globigerina Limetone aggregates instead of Upper Coraline Limestone ............34 4.6 Comparing 7 and 28 day tests.....................................................................................................35 4.5 The Merits and limitations of Non-Destructive Testing..................................................................36

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Part I

Mix Design & Fresh Concrete Testing

Lab Session 1

Date: 17th November 2017 The first week involved the mixing of a C30 concrete batch and applying the appropriate tests to it to ensure that the mix satisfied the conditions needed to ensure that the batch mix was of adequate quality.

The following report records: • • • • •

Measurments for the ratios of different aggregates, water and cement used for the C30 mix.

Procedure and results for the slump test.

Procedure and results for the float test.

Procedure and results for the Vee-Bee test.

Procedure and results for the fresh density test.

A: Constituent Proportion Measurments.

Introduction: C30 Concrete: The strength of a concrete mix is measured in grades. There are different concrete grades for example C15, C20, C25, C30. The grade of the concrete refers to the concrete compression resistance after 28 days. It is measured in Newtons per square millimetre.

A concrete of Grade C30 means that the concrete will have a compresion resistance (withstanding a compression) of 30 N/mm2 per square millimetre in 28 days.

Mix Proportions: The mix constituent proportions required for the production of 1 metre3 Grade C30 concrete is as follows Portland Cement Water Fine Aggregate Coarse Aggregate: 20mm Coarse Aggregate: 10mm

350kg 210kg 695kg 650kg 320kg

Since the batch required for testing purposes required much less concrete than the 1 metre cubed, the proportions were amended accordingly in the same ratios. The proportions used were calculated to make a 0.33 metre cubed batch of concrete. The lab test mix constituent measuements were thus as follows: Portland Cement Water Fine Aggregate Coarse Aggregate: 20mm Coarse Aggregate: 20mm

11.55kg 6.93kg 22.935kg 21.56kg 10.56kg

The measurements were taken using a top pan balance, eliminating the weight of the container using the “tare” function.

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Fig. 1: Preparing the mix constituents for measurment

Fig. 2: Measuring mix constituents on the top-pan balance

Batch-mixing: Under the instructions of the lab supervisor, the concrete mixer was prepared for mixing by layering the internal surface of the mixer with lubricating oil, to aid the mixing process and ensure that all batch is evenly mixed.

Firstly the 20mm aggregate is mixed with the fine aggegate in the mixer. The mixer was switched on for a few seconds to evenly distribute the two aggregates. After this, half the requried water was also added to the mix. The cement was then added and the mixer started again.

This was then followed by the rest of the required water.

Upon analyzing the fresh concrete tests, more water was added accordingly in steps of 100ml to achieve the correct workability. The extra water was noted,

Fig. 3: Adding constituents in mixer

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B: The Concrete Slump Test Aim The aim of this test is the obtain a concrete mix with the right workability. The concrete shoudl flow readily into the form and go around and cover the reinforcement, the mix should retain its consitstency and the aggregates should not segregate. The main factor affecting this workability is the water content ratio. For this reason, the slump test is used to determine whether the right W/C ratio is acheived.

Apparatus: • • • • •

Slump Cone Mould Metal Rod Smoothening Trowel Measuring Scale Non Porous Base Plate

Method 1.

2.

3.

4.

5.

6.

7.

8.

9.

The internal surface of the mould was coated in oil.

The mould was placed on a smooth horizontal non-porous base plate.

The mould was filled with the prepared concrete mix in 3 approximately equal layers .

Each layer was tapped 25 times using the metal rod, in a uniform manner over the cross section of the mould. (For the subsequent layers, the tamping penetrated into the underlying layer).

The excess concrete was removed and the surface was levelled with a smoothening trowel.

The water leaked out between the mould and the base plate was removed.

The mould from the concrete was raised immediately in a vertical direction and the unsupported concrete started to slump.

The decrease in height at the centre point was taken as the slump measurement.

In the case of unsatisfactory results, additional measured amounts of water was added to the mix and the test was repeated until the 30mm slump was achieved.

Results For the C30 Mix, an additional 300ml had to be added to the mix to achieve the right slump.

Upon testing again, the result was stil unsatisfactory and thus anotther 200ml of water was added. This yielded the right workability.

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Fig. 4: Pouring the Mix into the Slump Cone.

Fig. 5: Tapping the concrete mix in the slump cone

Fig. 6: First Slump Test - Unsatisfactory

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Fig 7: A satisfactory slump of 30mm

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C: The Wet Concrete Flow Test Aim The concrete flow test is a lab procedure carried out in order to determine the consistency of a fresh concrete mix. This test is important to evaluate whether the respective concrete mix is of the correct consistency, that is it is not too fluid.

Apparatus • • • • • • • • •

Flow table Abrams cone (bottom : 20cm) Water bucket Wooden stick Measuring tape Spirit Level Sponge Funnel C30 mix of concrete

Method 1. The flow table was ensured to be level using a spirit level.

2. The flow table was then cleaned using a sponge and water.

3. The dimensions of the abrams cone were noted and it was then placed in the centre of the flow table.

4. The mix was then poured until it filled half the height. Body weight was applied to keep down the abrams cone as the mix was being poured in, by means of placing ones feet on the handles. A funnel was placed at the top of the cone.

5. A wooden stick was used to gently tap the mix 10 times. This was repeated for the upper half of the mix.

6. The rod was also used to level out the surface. A few second were allowed to pass before lifting the abrams cone.

7. The flow table was then raised from one end and dropped 15 times to allow the concrete to spread.

8. The final diameter of the concrete mix was noted and compared to the original diameter (of the bottom of cone).

Results Initial Diameter: 0.2m Diameter after flow test for C30 mix: 0.25m Sources of Error • Concrete mix seeping out of the bottom to the cone.

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Fig. 8. The Flow Test after dropping.

Fig. 9: Measuring the Flow of Concrete Mix

D: The “Vebe” test.

Aim This dynamic test is used to measure the “Vebe time” of fresh concrete Its main advantage is that it can be used on concretes that are too stiff for a slump test Apparatus • • • •

Slump cone Cylindrical container Vibrating table A moving vertical rod connected to a clear plastic disc and a rotating arm • Stop clock • Metal rod

Method 1. The plastic disk, the moving vertical rod and the rotating arm were set up on the vibrating table as shown in the diagram.

2. The slump cone was placed in the cylindrical container.

3. Concrete was compacted into the slump cone.

4. After each layer was filled, the concrete mix in the slump was tapped 25 times using a metal rod.

5. The container with the filled up slump cone was placed onto the vibrating table.

6. The slump cone was removed, revealing the concrete cone.

7. The metal arm was rotated, placing the clear plastic disk above the concrete.

8. The vibrating table was switched on, and the stop clock was started.

9. The time taken for the clear plastic disk to be fully in contact with the concrete (Vebe time) was taken on the stop clock and recorded.

Results Vebe time for C30 Mix: 9.41s

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Fig. 10: The Vebe Test

Fig. 11: The mix after finding the vebe time.

E. Fresh Density Test Aim: To determine the density of freshly mixed concrete.

Apparatus: • • • •

Concrete Plastic Container Vibrating Table Smoothening Trowel

Method: 1.

2.

3.

4.

5.

6.

7.

8.

The container was weighed using a weighing scale Vernier calipers were used to measure the height and diameter of the container.

These results were used to calculate the volume of the container.

The container was filled with 2 layers of concrete and placed on the vibrating table.

This process was repeated until the container was filled.

The excess concrete was removed and the surface was levelled with a smoothening trowel.

The concrete filled container was placed on the weighing scale to determine its mass in kg The density was then calculated by dividing the recorded mass by the volume of the container in cubic meters.

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Part II

Hardened Concrete Sample Tests At Age 7 Days and 28 Days

Lab Sessions 2 & 3

Date: 24th November and 11th December 2017 (Concrete Aged 7 Days and 28 Days respectively) The second week involved the testing of the C30 Concrete samples which were prepared in the previous week. The test is done at age 7 days and 28 days to study how the strength of concrete develops along a variation of ages.

The following report records: • • • •

Report for the Destructive Cube Compression Test Report for Split Cylinder Test Report for Flexural Test Report for Non Destructive Testing i.e. Ultrasonic Pulse Velocity Test and Schmidt Hammer Tet

A: Destructive Cube Compression Test Aim: To measure the compressive strength of the concrete mix at different stages of curing. This depends on many factors, such as the water to cement ratio, the cement strength and the quality of the concrete material.

Apparatus used: 100mm x 100mm cube specimen, Vernier calipers, compression testing machine Method: 1. The cube specimen was removed from the water after the specified curing time, and the excess water was wiped off the surface.

2. The dimensions of the cube were measured using the Vernier calipers.

3. The specimen was placed in the machine in such a manner that the load was applied to opposite sides of the cube.

4. The specimen was aligned centrally on the base plate.

5. The movable part of the machine was rotated by hand so that it made contact with the top surface of the specimen.

6. The load was applied gradually without shock and continuously at a known rate until specimen failure.

7. The maximum load was recorded, and any unusual features in the type of failure were also recorded.

Errors: Only one specimen was tested for every curing time – no repeated readings.

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B: Non-Destructive Tests (i) The Schmidt Hammer Test Apparatus: Schmidt Hammer Principle of the schmidt hammer test: The rebound of an elastic mass depends on the hardness of the surface against which its mass strikes Aim

To find out the compressive strength of concrete and to assess the quality of concrete in relation to standard requirements.

Method 1. Light pressure was applied on the plunger 2. The plunger was allowed to extend to the ready position for the test to begin 3. The plunger was pressed against the surface of the concrete. It was made sure that the instrument remained perpendicular to the test surface.

4. A gradual increase in pressure was applied until the hammer impacted the concrete 5. An average of 5 readings were taken

Errors: • Errors by not placing the instrument perpendicular to the surface • Errors by touching the button while depressing the plunger

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B: Non-Destructive Tests (ii) Ultrasonic Pulse Velocity Test Aim: To test the uniformity of concrete, and to test for cavities, cracks and defects. A pulse is generated, which is transmitted through the concrete and received at the opposite end of the concrete. The time taken for the pulse to pass through the concrete indicates whether any defects are present in the specimen.

Apparatus: Pulse generation circuit, stop clock, 100mm x 100mm specimen.

Method: 1.

2.

3.

The transducer, clock, oscillation circuit, and power source are assembled for use.

The transducers and the concrete specimen were covered in oil to ensure full contact for accurate results.

The pulse generation circuit was switched on, and the time taken for the pulse to pass through the concrete was recorded.

Limitations: With this test, only the presence of defects can be attained. The compressive strength of the specimen needs to be determined with other tests.

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C: Flexural Test The Flexural test is performed in order to evaluate the tensile strength of concrete. This is done by observing the ability of the unreinforced concrete to withstand failure in bending. The test was conducted by means of the centre point load test.

Apparatus: • Beam Mould • Tamping bar • Flexural test machine

Method: The beam was loaded at its centre along its length. The load was increased until fractures were observed indicating that the beam had reached maximum stress. The modulus of rupture was computed by means of the standard flexure formula:

Cause of errors: • • • • •

Preparation of concrete specimen The size of the specimen The moisture content of the specimen Curing conditions Mode of sizing of the specimen, moulding or sawing (moulding in this case)

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D: Split Cylinder Test The split cylinder test is done to determine the tensile strength of concrete in an indirect way.

Apparatus: • Compression testing machine • Cylindrical concrete specimen

Method: A standard test cylinder of a concrete specimen, with measurements of 300mm by 150mm ϕ, was placed horizontally between the loading test machine. Steel plates were placed between the loading of the plates of the machine in order to allow uniform distribution of the load and to also reduce the high compressive stresses near the point of application The compression load was then applied diametrically as well as uniformly along the length of the cylinder until the cylinder failed along its vertical diameter. The concrete cylinders split into two halves along the vertical plane, this was due to the indirect tensile stress which was generated due to poisson’s effect.

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Part III

Test Results

C20 Concrete 7-Day Age Tests Variable

Value Variations

Values

Average

Volume Measurement

Width Length Height

100.19mm

Submerged Weight SSD Weight Ultrasonic Pulse Velocity Schmidt Hammer Test

N/A N/A Calibration Reading N/A

Compressive Test

N/A

100.82mm 100.49mm 100.10mm 100.09mm 100.87mm 1229.08g 2246.6g 24.9ms 22.2ms 26kN 28kN 24kN 30kN 235.7kN

Variable

Value Variations

Values

Average

Volume Measurement

Width Length Height

99.75 mm

Submerged Weight SSD Weight Ultrasonic Pulse Velocity Schmidt Hammer Test

N/A N/A Calibration Reading N/A

Compressive Test Flexural Test Tensile Test

N/A N/A N/A

92.88 mm 103.10 mm 94.41 mm 96.19 mm 89.83 mm 1232.68g 2240.7g 24.9 ms 22.2 ms 26 30 24 294kN 12.19kN 170kN

1229.08g 2246.6g 24.9 ms 22.2 ms 27kN

235.7kN

28-Day Age Tests

294kN 12.19kN 170kN

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C25 Concrete 7-Day Age Tests Variable

Value Variations

Values

Average

Volume Measurement

Width Length Height

99.88mm

Submerged Weight SSD Weight Ultrasonic Pulse Velocity

1199.87g 2183.48g 25.4ms 23.2ms

Schmidt Hammer Test

N/A N/A Calibration Reading N/A

Compressive Test

N/A

100.5mm 100.38mm 98.45mm 99.08mm 99.11mm 1199.87g 2183.48g 25.4ms 23.2ms 26kN 31kN 28kN 29kN 280.5kN

Variable

Value Variations

Values

Average

Volume Measurement

Width Length Height

101.96 mm

Submerged Weight SSD Weight Ultrasonic Pulse Velocity Schmidt Hammer Test

N/A N/A Calibration Reading N/A

Compressive Test Flexural Test Tensile Test

N/A N/A N/A

100.59 mm 103.51 mm 102.45 mm 102.9 mm 100.52 mm 1237.37 g 2254.16 g 25.4 ms 22.9 ms 31kN 28kN 23kN 311.40kN 13.06kN 159.6kN

280.5kN

28-Day Age Tests

311.3kN 13.06kN 159.6kN

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C30 Concrete 7-Day Age Tests Variable

Value Variations

Values

Average

Volume Measurement

Width Length Height

Submerged Weight SSD Weight Ultrasonic Pulse Velocity

1176.93g 2172.18g

Schmidt Hammer Test

N/A N/A Calibration Reading N/A

100.45 mm 100.62 mm 100.52 mm 99.29 mm 101.73 mm 1176.93 g 2172.18 g

Compressive Test

N/A

32kN 31kN 34kN 32kN 247.9kN

247.9kN

Variable

Value Variations

Values

Average

Volume Measurement

Width Length Height

101.98 mm 102.7 mm 101.94

102.20mm

Submerged Weight SSD Weight Ultrasonic Pulse Velocity Schmidt Hammer Test

N/A N/A Calibration Reading N/A

Compressive Test Flexural Test Tensile Test

N/A N/A N/A

1234.75g 2237.7 g 25.4 ms 24.2 ms 31 30 36 310kN 10.35kN 155.30kN

28-Day Age Tests

25.4ms 24.2ms 32.33kN

310kN 10.35kN 155kN

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Part IV

Analysis of Results

Analysis of Results A. Required Properties of Fresh Concrete The main requirement for fresh concrete is to have adequate workability. Workability depends on three main characteristics, which are compactibility, mobility and consistency. In the long term, these characteristics influence the ultimate strength, the elastic modulus, the creep and the durability of the concrete.

The importance of workability in fresh concrete is shown in its plastic state, as the concrete should be sufficiently fluid to flow and fill all parts of the formwork, it should do so without segregation and separation of constituent particles, it should be possible to fully compact the concrete when placed in position, and obtain the required finish.

B. Required Properties of Hardened Concrete Properties of hardened concrete The principal properties of hardened concrete are the following: 1.

Strength - The strength of a concrete specimen which is prepared, cured and tested under specific conditions depends on the w/c ratio and the degree of compaction.

2. Permeability & Durability - Permeability determines the extent of penetration of aggressive solutions which may result in the seepage of Ca(OH)2 which highly effects the durability of concrete. Similarly, moisture penetration (dependant on permeability), causes the concrete to become saturated and thus make it more liable to frost-action. When considering structural members such as dams and water retaining tanks, permeability is also of great importance.

Durability in concrete determines its ability to withstand the effects of service conditions to which it will be subjected. Both external and internal factors affect the durability of concrete.

3. Shrinkage and creep deformations - Hardened concrete should experience least shrinkage possible, this property is a function of the water-cement ratio. The lower the ratio, the less shrinkage occurs. Creep occurs when a load is maintained for a considerable period of time resulting in deformations in addition to initial deformation caused by primary loading. Creep is highly effected by: • • • • •

concrete mix proportions Aggregate properties Age at Loading Conditions during curing Temperature during Curing

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C. Constituent Material Properties The materials used for testing included Portland cement, crushed fine and coarse aggregate and tap water.

Ordinary Portland cement Portland cement consists of a number of raw materials including lime, silica, alumina and iron oxide. These constituents are crushed and blended in the correct proportions and burnt in a rotary kiln. Cement forms when the clinker is cooled, mixed with gypsum and ground to fine powder. A cement paste is formed when water is added to the powder. As time passes, the mix becomes stiffer. In order to control the setting time of the mix, ensuring that the concrete does not set too quickly, gypsum is added as previously mentioned; acting as a retarder.

Aggregates Aggregate is usually found in the form of sand and gravel and can be classed as either coarse or fine aggregate, depending on the particle size. Coarse aggregate usually takes the form of gravel or crushed rock 5mm or larger whilst fine aggregate takes the form of sand less than 5mm.

Natural Aggregates are classified according to the rock type from which they form. The purity of aggregates is crucial for the hydration and the bond between the cement and the aggregate. The presence of silica in some aggregates may cause a reaction with alkali in the cement causing the concrete to disintegrate. On the other hand, corrosion of the steel reinforcement could be caused by the presence of chlorides in aggregates. A dense, strong concrete can be achieved with minimal use of cement, where the cement paste fills the voids of the fine aggregate whilst the fine aggregate and cement paste together fill the voids of the coarse aggregate.

Aggregates can also be classified by grades by means of a sieve analysis. This grading affects the workability of concrete. Good aggregate grading allows for a lower water-to-cement ratio, thus increasing the strength. Such a property also saves on cement content used as well as contributes to the prevention of segregation during placing, ensuring a good finish.

D. Comparing the Three Grades

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E. Strength Values - Normal Distribution The compressive strength of concrete is one of the most significant properties of concrete which is given in terms of the characteristic compressive strength. This value is based upon the testing of 150mm cubes over a duration of 28days. The characteristic strength can be defined as the strength of concrete below which not more than 5% of the test results are expected to fall. This concept assumes a normal distribution of the strengths of the respective specimens of concrete.

The graph shows an idealized distribution of values of compressive strength attained for a number of test specimens. The horizontal axis shows values of compressive strength in MPa whilst the vertical axis represents the number of test samples for a particular compressive strength, also termed frequency.

The mean strength obtained from the graph is 40Mpa. The characteristic strength (fck) is the value on the x-axis below which 5% of the total area under the curve falls. However, from the graph it is clearly visible that the characteristic strength of the concrete mix is 30MPa. The value of fck is lower than fcm which is equivalent to (40Mpa – mean strength) bby 1.64Ďƒ. Ďƒ is the standard deviation of the normal distribution.

F. The Influence of Globigerina Limestone 20mm Aggregate instead of Upper Coraline Limestone From the results attained by these tests, it was noticed that the incorrect aggregate mixture was used for the C30 concrete. 10mm of upper caroline limestone was used along with 20mm of globigerina limestone, instead of upper coralline limestone.

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This error was realised visually as well, as globigerina limestone is white/ yellow in colour and is softer which caused an inaccurate result due to the differences in their properties. The expected resulting values of the schmidth test was between 37-40 KN however the results obtained gave an average reading of 32.22KN. Coralline Limestone and globerina limestone are both porous in nature. However upper coralline limestone’s water absorption after 24 hours immersion is 2 to 3 per cent of their bulk, whilst globigerina limestone’s water absorption after 24 and 72 hours is about 25 per cent 6 and 31 per cent 7 respectively. When comparing the crushing value one can see a significant difference. The crushing value for upper coraline limestone varies from 5.9 to 22MPa and that of globalerina limestone varies from 8.2 to 14.3 MPa. Despite these inaccuracies during the tests, globigerina limestone is used as an aggregate. Limestone is reported from different quarries located in different areas in Malta, and also within the same quarry with depth. The Upper and Lower Coralline limestone geological formations are mainly exploited for the production of aggregate and civil engineering applications.

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G. Comparing 7 and 28 Day Tests The concrete specimens are tested after allowing it to cure after 7 and 28 days.

As the results of these tests where recorded and compared, one can see how the strength of the concrete varied between 7 days and 28 days. It is estimated that concrete reaches 75% of this 28-day compressive strength in seven days, and its strength will remain stable or even increase over time. Apart from time, different curing conditions also effect the strength of the concrete. Comparing moist cured with air cured it is clear how much stronger moist cured concrete is. This occurs as when the concrete is submerged in water it is under constant pressure which aids in keeping the concrete compact, in turn improving the strength of the concrete.

H. The Merits of Destructive and Non- Destructive Testing Non-destructive testing of concrete has two main advantages over destructive testing, which are that internal concrete defects can be identified without causing damage to the member’s structure, and the function of the member is uninterrupted during in-situ testing. However, these tests also have their limitations. For example, the results of the Schmidt Hammer Test are only valid within one year of concrete production, as it is based on surface hardness, and concrete begins to carbonise and becomes harder at the surface with time, therefore giving unaccurate results.

Due to this, the error of the Schmidt Hammer test within the first year can be up to Âą15%. Destructive testing, on the other hand, tests the strengths and limits of concrete directly by pushing them to the point of failure, therefore obtaining more accurate results for the mechanical properties of concrete.

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References

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