MAKING AND CURING CONCRETE CUBES AS PER IS:516-1959

Testing of Cement before concreting at the site / What are the site test you can do  of cement before concreting

The initial Setting time of the Cement testing procedure in details

CRAZING OF CONCRETE AND THE REMEDIAL MEASURES FOR CRAZING OF CONCRETE

SUBRATA PAUL | CRAZING | 12-01-2019

 

What is Crazing of Concrete?

Crazing is the development of random fine cracks on the surface layer of concrete or mortar due to shrinkage of the surface layer. These cracks are intensively seen in floor and slab surface.

These cracks generally about 3mm deep and length, not more than 40mm. Sometimes these cracks are not visible in dry condition but when they are wetted, they become visible. These cracks do not affect the durability of the concrete nor the permeability and water resistance. Also, there is no effect on structural integrity due to crazing.

 Preventive Measures for Crazing of concrete:

v Troweling the surface as little as possible particularly avoiding using of steel float for finishing. Instead of steel float use Wooden float for finishing.

v Avoiding the use of neat cement powder for finishing of the concrete surface would prevent the crazing of concrete.

v Also avoid the rubbing the surface with neat cement paste, after finishing.

v Sometimes over vibrating the fresh concrete may result in bringing too much cement slurry on the top surface of concrete and this has a tendency of shrinkage which causes of crazing of concrete.

v Using of the low water-cement ration as much as possible to avoid crazing. 

What is Concrete and what properties Fresh concrete should have and should not have

What is Concrete?

Sometimes suitable admixtures are mixed with concrete to enhance certain properties of concrete like workability, setting time and heat production.

Properties of Fresh Concrete Should have –

• Workability:  

  Fresh Concrete should have good workability. Workability is defined as the ease with which it can be mixed, transported and placed in position in a homogeneous state. The workability of concrete depends on the quantity of water, percentage of fine aggregate, and grading of aggregates. For heavily reinforced concrete more workable concrete required than lightly reinforced concrete.

Properties of Fresh Concrete Should not have –

• Segregation:

• Bleeding :

Plastic shrinkage cracks in concrete and It's Preventive measures 

Plastic Shrinkage Crack 

Causes of Plastic Shrinkage Crack :

How to calculate Concrete Raw Materials Measurement box sizes at the site

For manual batching of concrete raw materials at a site in correct proportion, concrete raw material measuring boxes are used. This method of batching is generally used where a small quantity of concrete to be prepared using stacked raw materials. This is a volume batching and it should not be practised when weight batching is available to avoid less deviation.

General Requirments: 

Calculations:

now for sand x is =0.07412/2*(0.3*0.3)= 0.411m = 411 mm
and for the coarse aggregate of 20mm size x is = 0.082069/2*(0.3*0.3)= 0.455m = 455mm
and for coarse aggregate of 10mm size is = 0.044191/2*(0.3*0.3)=0.245m = 245 mm


So the sizes of the measuring boxes and mix proportion for 1 bag of cement will be - 
For Sand - 300mm x 300mm x 411mm (2 box)
For 20 mm CA - 300mm x 300mm x 455mm (2 box)
for 10 mm CA - 300mm x 300mm x 245mm (2box)

Batching with concrete raw materials measuring boxes

What Properties Harden Concrete should and should not have / Properties of Concrete at Harden State

In the 👉 Previous article we have learned the properties of fresh concrete, now we will know about the properties that harden concrete should and should not have.

After the final setting of concrete and sufficient curing of the concrete should achieve some specific properties to sustain the structure. Some of the properties that harden concrete should have are listed below –

1. Comprehensive Strength :

Concrete at the Harden state must achieve minimum compressive strength to resist the stress coming on it and resist the heavy loads of the structure. Concrete is designed to achieve some strength category Like M15, M20, M25, M30 etc. (MX) where M stands for Mix and ‘X’ means compressive strength of concrete after 28 days of curing. In any case, the strength of Concrete should not be less than 15.5 N/mm2

2. Durability :

The Harden Concrete must be durable enough to resist the effects of harsh environmental like rain, forest action also wear and tear. The property of fresh concrete that has the most impact over Durability of concrete is water cement ration.

3. Impermeability :

Impermeability of concrete that property which makes the concrete water resistance i.e. resist the entry of water into the concrete and structure. Sometimes a water tightness criteria of concrete is enhanced with suitable admixture mixed with concrete.

Now let’s talk about the Properties of Harden concrete that it should not have –

1. Shrinkage and Creep :

When fresh concrete undergoes the hardening process it has a tendency of reduction in its volume this results in shrinkage. Shrinkage is generally observed with the shrinkage crack which leads to the less bonding, less strength and finally failure of the structure.

The process of drying (like no curing) and high water-cement ration is the main reasons for shrinkage.

     Creep is the deformation of the hardened concrete when it is subjected to continuous strain or by its own weight. Creep is a time dependence i.e. when concrete is undergone strain for a considerable period of time. Creep is observed by a deep crack in the concrete. Loading condition is generally responsible for creep.

2. Thermal Shrinkage :

Like every material concrete also expand in volume when it is subjected to heat, but harden concrete is not an elastic material so when it undergoes in the heat it cracks itself, also known as thermal shrinkage. The hardened concrete should have a minimum thermal expansion to resist fire and heat.