Concrete Mixing and Testing Lab Report

INTRODUCTION: This experiment was carried out to demonstrate mixing and compaction of concrete, carrying out a slump test and making cubes and prisms for later compressive strength and flexural tensile strength tests. During this experiment, BRE Manual Design of Normal Concrete Mixes was used as guide to proper mixing and testing procedure. Several tests were done for obtaining Compressive Strength (fcu) and Flexural Tensile Strength (fctm): Compressive Strength of the cubes was done by crashing the cubes in a crashing device whereby; for the prism, different procedures were done: Pundit Test, a non-destructive test, was used to measure the time taken by a pulse of ultrasound to pass through the material under test which were air cured and water cured, with these results; it is possible to detect the presence of cracks, voids and other imperfections. Furthermore; the pundit test also provides the elastic modulus of the prisms. Erudite Test: The Erudite is an electronic resonant frequency test system used to determine the dynamic modulus of elasticity and rigidity of sample blocks of concrete. The Erudite automatically measures the fundamental longitudinal, torsional and transverse resonant frequencies. It can also calculate the damping constant Q.

OBJECTIVE: The main objectives of this experiment were to:        

Design concrete mix for a given workability and 28 day compressive strength. Weigh out a total of 25kg of the required proportions of Ordinary Portland cement, water, fine aggregate, and 20mm size crushed gravel coarse aggregate. Batch and mix the materials in a pan mixer according to the instructions given by the laboratory technicians Carry out a slump test Make 4 100mm cubes, one by hand tamping and three on the vibrating table Make 2 500 x 100 x 100 mm prism on the vibrating table Label and Cover the specimens for curing And after 28 days; carry out Pundit test, Erudite test, Flexural test and crashing on the samples.

PROCEDURE: 

The experiment was done in a group of three and based on the calculated individual mix designs, the proportions of the mix were agreed upon.



The quantities of cement, fine and coarse aggregate, and water were weighed out (to the nearest 0.05kg) in vessels provided. The aggregates were placed in the pans first, then the cement. Cement and the aggregates were dry mixed for 1 minute and then water was added through the port holes, and wet mix for a further 3 minutes.



After mixing, a slump test was then carried out by filling the cone in 3 layer and each layer was hand tamped approximately 25 times with a rod provided. The slump was then recorded to the nearest 5mm. The concrete was then taped by the tamping rod so as to observe the cohesiveness of the mix and this observation was then recorded on a sheet.



The concrete used for slump test was returned to the mixer and was remixed for about 30 seconds



One 100mm cube was made by hand tamping, with the rod provided.



Three 100mm cubes and Two 500 x 100 x 100mm prisms were made using the vibrating table.



Labels were placed next to the specimens.



And finally, the specimens were placed in water for water curing except for one prism and one cube (vibrator) which were air cured.



It was also necessary to conduct a sieve analysis for fine and coarse aggregates and the result was plotted.

RESULTS: Table 1 below shows the general details of the four 100mm cubes Cube no.

Information

Mass (kg)

Volume (m3)

Density (kg/m3)

1

Water Cured, tested after 7days

2.331

0.001

2331

2

Air Cured

2.272

0.001

2272

3

Water Cured, Hand tamped

2.34

0.001

2340

4

Water Cured

2.333

0.001

2333

Table 2 below shows the general details of the 200 500 x 100 x 100mm prisms Prism no.

Information

Mass (kg)

Volume (m3)

Density (kg/m3)

1

Air cured

11.381

0.005

2276.2

2

Water cured

11.604

0.005

2320.8

PUNDIT TEST RESULTS: The results obtained from the Pundit test done on the prisms were tabulated as follows: Transit time (s) Prism no. 1

2

3

4

5

6

Average

1

130.2

130.7

130.5

130.1

130.6

131.5

130.6

2

122.6

123.3

123.4

123.8

123.9

122.9

123.32

Velocity (m/s) Prism no. 1

2

3

4

5

6

Average

1

3802

3779

3787

3776

3776

3790

3785

2

4022

4058

4048

4019

4016

4019

4030.33

Elastic Modulus (GN/m2) Prism no. 1

2

3

4

5

6

Average

1

28.8

28.5

28.6

28.8

28.3

28.3

28.55

2

33.2

33.8

34

33.5

33.8

35.1

33.9

Using the formulas Ec(t) =  V2

(1   ) (1  2 ) x 10-9 (kN/mm2) and Ecm = Ec(t) /1.05 (1   )

Where: Ec(t) – Dynamic Modulus Ecm – Static Modulus  - Density of the samples V – Velocity  - Dynamic Poisson’s ratio  ≈ 0.25 The Dynamic and Static Moduli were obtained for the two cases (Air and Water curing) Case 1 (Air curing): (

)

()

(

(

) )

()

And hence; Ecm = 25.88kN/mm2

Case 2 (Water Curing): ( ()

()

And hence; Ecm = 29.92kN/mm2

) (

(

) )

ERUDITE TEST RESULTS: The results obtained from the Erudite test done on the prisms were tabulated as follows: First Natural Resonant Frequency, n (kHz)

Prism no.

1

2

3

4

5

1

1.9

2.42

1

4.425

1.45

2

1

1

1.54

2.65

3.09

6

7

4.685 2.025 1

1

8

9

10

Average

1

3.65

4.64

2.72

1

2.52

3.65

1.84

Using the formulas Ec(t) = 4 n2 L2 x 10-15 (kN/mm2) and Ecm = Ec(t) /1.05 Where: Ec(t) – Dynamic Modulus Ecm – Static Modulus  - Density of the samples L – Length of the samples (500mm) n – Natural resonant frequency (Hz) The Dynamic and Static Moduli were obtained for the two cases (Air and Water curing) Case 1 (Air curing): ( )

()

And hence; Ecm = 16.04kN/mm2

Case 2 (Water curing): ( )

()

And hence; Ecm = 7.48kN/mm2

Flexural Test Results The table below shows the results obtained from the flexural test done on the two prisms Prism no.

Flexural Strength (kN)

1

8.0

2

9.0

Concrete Cube Test Results Cube no.

Compressive Strength (N/mm2)

Information

1

Water Cured (Vibrated), Compressive test done after 7days

10.0

2

Air Cured (Vibrated), Compressive test done after 28days

13.66

3

Water Cured (Hand tampered), Compressive test done after 28days

15.6

4

Water Cured (Vibrated), Compressive test done after 28days

14.35

Direct Static Method

Prism no.

Compressive strength, fcu (N/mm2)

Static modulus, Ecm (kN/mm2)

1

13.06

26.4

2

12.70

26.3

Prism no.

Flexural strength (kN)

Flexural tensile strength, fct (kN/mm2)

1

8.0

8 x 10-4

2

9.0

9 x 10-4

Flexural Tensile Strength

DISCUSSION: It was observed that Water Cured samples had higher compressive strength values (15.6 & 14.35kN/mm2) than the Air Cured sample (13.66kN/mm2); this is probably because concrete strength tends to increase with time if complimentary temperature and efficient moisture are present to hydrate the cement.

Concrete yields more compressive strength when it is tampered by vibrating machine than when tampered by hand because with vibrating machine, air void are efficiently removed. However, according to the results obtained, it was found out that the compressive strength of the hand tampered sample (15.6kN/mm2) was greater than the sample that was vibrated (14.35kN/mm2).

A graph of the Compr. Strength vs. no. of the days of the cube sample 16

Compressive strength

14 12 10 8 6 4 2 0 0

5

10

15

20

25

30

Days

The graph above shows how the concrete strength increases with time. At day 7, the sample had a compressive strength of 10kN/mm2 and by day 28, it finally had strength of 14.35kN/mm2. This however; derails from our expectations because, from the design, the sample was to reach approximately 70% of its design strength, which was 35kN/mm2 by day 7.

It was noted that the experiment was not 100% efficient. The inaccuracies may have been caused by the lack of technical know-how of the individuals who carried out the experiment and it is because of it: 

They may have been more voids in the concrete samples than the allowable amount because the samples were not vibrated properly.



There might have been some errors during the mixing, not getting the ratios correctly, or not following the procedures correctly

Other noticed predicament faced during the experiment was that the target slump of 50mm was not achieved.

CONCLUSION: To conclude, as expected, it was obtained that Water cured samples yielded higher compressive strength than the Air cured samples. However, it was obtained that the hand tampered sample had a greater compressive strength than the vibrated sample which should not be the case. Furthermore the target design strength of 35kN/mm2 was not achieved and….