Benefits of fresh concrete to be vibrated

Fresh concrete that should be vibrated provides huge benefits. Freshly placed, unconsolidated concrete normally contains full of entrapped air bubbles. Concrete can attain a greater compressive strength and density with least air content mechanically i. e. with the help of high frequency (fast, repetitive) vibrations.

Thus, the concrete fulfill the present needs with regards to strength and density. The vibrator produces high frequency vibrations which are delivered to the components of the fresh concrete.

• Frictional forces among individual particles are significantly minimized.
• A flow process is formed.
• Air bubbles, excess water and paste are congested via capillary action and surface tensions are discharged and evaded to the surface.

Benefits of concrete vibration

• Superior density and consistency
• Superior compressive strength
• Superior durability (e. g. de-icing salt)
• Perfect bond with rebar, specifically in thickly reinforced sections

• Greater bond among the individually “wet” on “wet” placed layers
• Better quality exposed (fair-faced) surfaces
• Utilization of drier mixtures results in necessitating less cement.

Recognize your effective compaction diameter: For practical uses and as rule of thumb, approximate value should be taken for the operating diameter i.e. 10 times the vibrator head housing diameter.

Proper spacing should be retained while compacting large surfaces.

Proper spacing should be maintained while vibrating fresh concrete in walls.

Internal vibrators with dia (Ø) 30 (1.2”), 38 (1 ½”) and 45 mm (1.8”) should not be utilized because of poor or non-existing overlapping of the effective compaction Ø. It will result in producing a defective or inexpensive vibration method.

For superior effective compaction Ø and for more suitable, inexpensive compaction (less insertions, less time) preferably the following should be utilized :-

• Vibrators with diameters 57 or 65 mm, on the condition that the rebar allows it.
• The concrete will be compacted flawlessly due to the overlapping.

It is found that consistently and horizontally extended layers offer the superior outcomes in concrete compaction with approximately 50 cm (20”) thickness.



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Published By
Rajib Dey
www.constructioncost.co
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Some vital tests for checking compressive strength of cement

If water is included in the cement, the compressive strength of the hardened cement is considered as the vital factor. Cement hydrates and exposes cohesion and consistency. It joins all the components like cement, sand, aggregate etc. collectively. The hardiness of cement-based compound like mortar/concrete is based on the type and nature of cement.

Owing to strength, nature of cement, both mortar and concrete contain high strength against compression and less strength against tension. Therefore, testing of cement for compressive strength is very crucial. Cement is examined for compressive strength so that the strength and stability of the structure is retained.

Initial strength gain is affected by high lime or high alumina content. The firmness of cement is also impacted with the degree of burning, the fineness of grinding, and the aeration it obtains once the final grinding is completed.

Under the strength test, cement mortar is applied since neat cement produces shrinkage and cracking and it becomes complicated for testing.

Primarily, cement is recognized with its compressive strength. Cement is designated with its grade like 53 grade, 43 grade, 33 grade of cement. This grade signifies the compressive strength of cement, i.e. 53 grade of cement species that compressive strength of cement cube after 28 days of curing should be 53 N/mm2 (MPa) or 530 kg/cm2.

Compressive Strength Test of Cement according to IS 4031 (Part 6) 1988.

The following equipments are required for conducting the test :

• Compression Testing Machine or Universal Testing Machine.
• Cube Mould: 70.6 mm*70.6 mm*70.6 mm size
• Vibrating Machine
• Weighing Machine
• Gauging Trowel
• Measuring Cylinder
• Tray

Method: Arrangement Of Test Samples

• Testing material is cement, sand and water.
• The necessary material for each cube is as follows:

1. Cement – 200gm,
2. Sand – 600gm,
3. Water quantity (P/4 + 3) % of the total mass of cement and sand. Where P denotes the standard consistency of cement.
• Initially, blend cement and sand in dry condition with a trowel for one minute and then include water and blend unless homogeneous colour is produced.
• The time of mixing should not be under 3 minutes and not in excess of 5 minutes.

Moulding Samples
• Once the blending is completed, mortar is provided in the cube mould. Prior to arrange the mortar, use oil on the inside surface of cube mould.
• To discharge the entrained air and get rid of honeycombing, the mortar should be stimulated 20 times in about 8 s and then consolidated with vibration.
• The vibration period should be 2 minutes at the certain speed of 12 000 ± 400 vibrations per minute.
• Then, the top surface of the cube in the mould should be completed by leveling the surface with the blade of a trowel.

Curing Samples
• Once the vibration is finished, retain the filled moulds in a moist closet or moist room for 24 hours.
• As soon as that period is ended, detach mortar cube from the moulds and instantly immerse in clean, fresh water and put there unless it is removed before testing.
• The cubes should not get dried once they are removed and unless they are tested.

Testing
• The testing is done with compression testing machine or universal testing machine.
• Test 3 cubes for compressive strength for every time period according to detailed specifications. As for instance 3 cubes for 3 days test, 3 cubes for 7 days test and 3 cubes for 28 days test.
• The cubes testing should be performed on their sides. No packing among the cube is allowed and the steel plates of the compression testing machine while testing is going on.
• The load shall be gradually and consistently employed and the rate of loading should remain 35 N/mm2/min.

Points to be considered
• Clean appliances should be applied for the tests.
• Test for temperature and humidity should be carried out at 27 ± 2°C temperature and 65 ± 5 percent of the relative humidity of the laboratory.
• The water in which the cubes are immersed should be changed every 7 days and retained at a temperature of 27 ± 2°C
• While determining the compressive strength, the faulty samples should not be utilized.
• The samples which produce strength differing by above 10 % from the average value of all the test samples.



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Published By
Rajib Dey
www.constructioncost.co
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