Pumping be not used in case of concreting works ?
During the pumping operation the pump exerted pressure must overcome any friction between the pumping pipes and the concrete, also the weight of the concrete and the pressure head when the concrete is placed above the pumps. Since only water is pump able, all the pressure generated is by the water that is present in the concrete. The major problem due to pumping are segregation effects and bleeding. In order to rectify and reduce these effects, generally the proportion of the cement is increased in order to increase the cohesion, which leads to the reduction of segregation and bleeding. Also if a proper selection of the aggregate grading can vastly improve the concrete pump ability.
Why is the compression test of concrete done only for 7, 14, 21, and 28 days?
It is not necessary to test concrete compressiion test on only 7, 14, 28 days. If you have enough resources, you can do it on daily basis. But you will not get any significant result. When you draw compressive strength vs time graph for concrete specimen, you will find that concrete attains strength rapidly but as time passes strength attainment rate decreases. For example based on various test, concrete attain strength in this manner if they get proper curing..
30 min – initial setting
10 hrs – final setting
1 day – 10 % of ultimate strength
7 day – 40%
14 day – 67%
21 day – 97%
28 day – 98%
1 year – 99%
and so on…
From this you can get an idea how much concrete gain strength with time passes.
Now coming to the main part why concrete testing procedure is designed in this manner. For this, you need to understand the practical use of this test. On sites during construction before placing concrete, we need to provide support and platform for it which is called shuttering. So before deshuttering, we need to ensure that concrete must attain that much minimum strength so that it can sustain its self load. For this purpose, minimum 67% of ultimate strength required. So to check this we need to carry out compressive test. From the above table, you can understand that you need to do it after 7 days atleast. Next when we deshutter the platform, we don’t remove the props which act as a column and transfer load to the grounnd. We can remove props only when concrete attain 90 % or more of ultimate strength which we will get atleast after 21 days so to check it we do one more test.
For every test, we need atleast 3 cubes. So it means a good amount of concrete is used for every test. So to make it reasonable and economical, IS Code suggest us to carry out test in this manner,
What Tor stands for in Tor steel ?
The owner’s last name or Twisted Ore Reinforced
How will trucks be checked on-site when receiving concrete?
– A. Revolution Counter B. Ticket Checked for Concrete Batching time and Slump Value at the plant. C. Slump Test. D. Temperature Test (32 C maximum @ Site). E. Assure Lab Technician is present at the site and performs testing. F. 6 Cubes or 4 Cylinders should be taken per 50cum or recommended in the project specification.
What is the purpose of the slump test?
To determine the consistency of the workability of the concrete mix and to check for the required slumps.
What are allowable slumps?
2.5cm to 7.5cm (for vibrated structure and without admixture ) and If with admixture and plasticizer, Slump depends upon the design mix. Slump shall be checked as per design mix.
What is a plasticizer?
Usually applied at a low water-cement ratio concrete to make it workable.
What is the maximum temperature of concrete that can be poured into a concrete structure?
32 Degrees C.
Min and Max atmospheric (ambient) temperature prior to paving?
Min is 5 Degree C in cold weather and 45 degrees C max for hot weather.
Min and Maximum temperature of concrete at the site prior to pouring?
– Types of Curing?
Water Curing – as in ponding, spraying, wet sand and wet earth
Membrane Curing – as in plastic film, liquid membrane curing compound, and reinforced paper
– When will you apply the curing of the concrete?
After the concrete has initially set (35 to 40 minutes is the initial setting time of concrete)
– How many times do you apply the curing membrane?
At least two times. The second application is perpendicular to the first and applied after the first application has set.
Within how many hours should concrete mix be discharged after leaving the batching plant or after all the aggregates, cement, and water are in the mixer?
– Within one hour if the mixer is an agitated mixer (Also depends on the design mix) And Within 30 minutes if the mixer is a non-agitated mixer.
– What is the initial setting time of Concrete Mix?
35 to 40 minutes.
– What is the mixing time of a stationary or central batching plant (concrete)?
– 50 to 90 seconds
– What is the agitating speed of the mixer?
– 2 to 6 revolutions per minute (rpm)
What is the mixing speed of the mixer?
– 6 to 18 rpm
– What is concrete fatigue?
It is the weakening of a material caused by repeated loads.
– What is creep?
– It is deformation due to the sustained load.
– How would you know if the concrete has gained its initial set?
– If there is no more water seen (brightness) on the surface of the concrete, or if there is no water on the surface of the concrete.
If agitated concrete is discharged after more than one hour, what will happen?
– The concrete is over mixed, becomes hot, and the strength is reduced
– How many layers are in the concrete cube?
– Concrete Cube is cast in 3 layers.
How many strokes are done in one layer in a concrete cube?
– 35 Strokes in one layer.
– What is the size of rode used in concrete cube casting and slump test?
Dia – 16mm and 600mm length.
What is the size of a concrete cube?
– 150mm X 150mm X 150mm.
– What is the size of slump cones?
– Height – 30cm, Top of Cone – 10cm, and Bottom of cone – 20cm.
How many layers are in the slump test??
3 Layers in ASTM and BS but 4 Layers in Indian standard.
How many strokes are done in one layer in a slump test?
– 25 Strokes