Cause of errors in flat concrete surface

While concreting is going on the job site, construction errors may happen because of failure to certain methods, improper practice as well as casual attitude.

All of these errors do not contribute to failure or impairment of concrete, but they may provide harmful effect on the structure in due course.

These errors are not present throughout new construction, but also occur throughout repair or recondition works.

Given below, mostly found inappropriate finishing methods which can significantly affect the strength of flat concrete surface:

a. Adding water to the surface: It is found that the water is included to the surface and it is verified with the existence of a large paint brush, together with the other finishing tools. The brush is soaked in water and water is heaved onto the surface that is already completed.

b. Timing of finishing: Final finishing operations should be carried on as soon as the concrete has accommodated its initial set and bleeding does not occur. The waiting period is based on the amounts of water, cement, and admixtures in the mixture but mainly the temperature of the concrete surface. On a incompletely shaded slab, the portion in the sun will normally be prepared to finish prior to the part in the shade.

c. Inclusion of cement to the surface: It is frequently performed to dehydrate bleed water so that finishing can be started and it produces a thin cement-rich coating to craze or flake off easily.

e. Use of tamper: A tamper or “jitterbug” is utilized on several jobs needlessly. This tool drives the coarse aggregate far from the surface and the finishing becomes smooth. This method, however, forms a cement-rich mortar surface layer having the ability for scale or craze. A jitterbug should be un-allowed with a well designed mixture. If it is required to complete a rough mixture, a jitterbug could be useful by applying it sensibly.

e. Jointing: If the joints are placed and located improperly, it may lead to cracking in flatwork frequently.



Read more

~~~~~~~~~~~~~~~~~~~~~~~~
Published By
Rajib Dey
www.constructioncost.co
~~~~~~~~~~~~~~~~~~~~~~~~

How concrete is mixed in different ways

Concrete belongs to a composite material formed by cement, sand, coarse aggregate, water and chemical admixtures (if necessary). To maintain the superior quality of concrete, the materials of the concrete should be blended properly so that the quality of concrete is not impacted.

A well-mixed concrete is formed on the basis of the following conditions:

• The color of the concrete should be consistent.
• Admixture of all concrete materials like cement, fine aggregate, coarse aggregate and water should be uniform.
• Cement paste should wrap all the surface of the aggregate.
• Segregation should not happen as soon as the mixing of concrete is completed.

Mixing concrete is dependent on the following three options:

• Hand mixing (Mixing concrete devoid of a mixer)
• Machine mixing (Mixing concrete with a mixer)
• Ready mix concrete

Hand mixing (Mixing Concrete without a Mixer): Hand mixing stands for the method of mixing the different materials of concrete by hand. Mixing concrete devoid of a mixer is suitable for small works. Mixing of materials is executed on masonry platform or flat iron sheet plates.

The hand mixing concrete is performed as follow:

• Expand the measured quantity of sand on the platform, and then unload the cement on the sand.
• The sand and cement should be blended thoroughly with the help of shovels in the dry state.
• The measured amount of coarse aggregate should be extended, and the mixture of sand & cement should spread on it and mixed in an exact manner.
• Depression is provided at the centre of the mixed materials.

• Include 75% of the required quantity of water in the depression and blend well by the shovels.
• Include the leftover amount of water and the mixing method should be carried on unless a uniform colour and consistency of concrete is procured. Time of mixing concrete should not be in excess of 3 minutes.
• The mixing platform should be washed at the end of the day’s work.

Machine Mixing (Mixing Concrete with a Mixer): Machine mixing is mostly suitable for bigger projects where huge masses of concrete are necessary. The machine mixing can retain the persistent uniformity of concrete. Besides, the machine mixing can significantly reduce the mixing time. In recent times, different types of concrete mixers are available which run with petrol/diesel or electricity.

The machine mixing is performed in the following ways:

• Initially, the concrete mixer should be drenched inside of the drum.
• After that Cement, sand and coarse aggregate should be arranged in the portable concrete mixer in desired ratio.
• The dry materials should be blended in the mixing machine. After this, exact quantity of water should be added slowly when the machine is running.
• The concrete should be blended for minimum two minutes after placing all materials in the drum.
• If segregation occurs, the concrete should be remixed after unloading from the mixer.

Ready Mix Concrete: Ready Mixed Concrete (RMC) is developed in the factory or in a batching plant and supplied in a ready-to-use manner. The quality of the consequential concrete is superior as compared to the site-mixed concrete.

Less time is necessary for ready mix concrete as compared to site mixing (hand or machine mixing) and quality of concrete is also greater than the site mixing.

Read Continue

~~~~~~~~~~~~~~~~~~~~~~~~

Published By

Rajib Dey

www.constructioncost.co

~~~~~~~~~~~~~~~~~~~~~~~~