Earthquake Engineering and Disaster Management

 Dear Students 

In this blog series  introduction to disaster management and brief of earthquack engineering has been explored  . i hopes it will surely help not only to undergraduates students but also to research scholars and post graduate students 

https://tarungehlots.blogspot.com/2024/11/disaster-management-introduction.html

https://tarungehlots.blogspot.com/2024/11/earthquake-introduction.html

https://tarungehlots.blogspot.com/2024/11/indian-standards-on-earthquake.html

https://tarungehlots.blogspot.com/2024/11/basic-concepts-of-earthquake-resistant.html

https://tarungehlots.blogspot.com/2024/11/some-of-important-considerations.html

https://tarungehlots.blogspot.com/2024/11/seismicity-concept-of-seismic-zoning.html

https://tarungehlots.blogspot.com/2024/11/earthquake-intensity-their-scales.html

https://tarungehlots.blogspot.com/2024/11/structural-systems.html

https://tarungehlots.blogspot.com/2024/11/elastic-rebound-theory-of-rupture.html

https://tarungehlots.blogspot.com/2024/11/measurement-of-ground-motion.html

https://tarungehlots.blogspot.com/2024/11/magnitude-and-energy-of-earthquake.html

https://tarungehlots.blogspot.com/2024/11/cause-of-earthquake.html

https://tarungehlots.blogspot.com/2024/11/base-shear.html

https://tarungehlots.blogspot.com/2024/11/fundamental-period-of-buildings.html

Correlation between Compressive Strength of Concrete and Rebound Number

 The Schmidt rebound hammer is basically a surface hardness test with little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer. Rebound hammers test the surface hardness of concrete, which cannot be converted directly to compressive strength. The method basically measures the modulus of elasticity of the near surface concrete.. The distance travelled by the mass, expressed as a percentage of the initial extension of the spring, is called the Rebound number. This is simple, handy tool, which can be used to provide a convenient and rapid indication of the compressive strength of concrete. It consists of a spring controlled mass that slides on a plunger within a tubular housing. The method is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which mass strikes. When the plunger of rebound hammer is pressed against the surface of the concrete, the spring controlled mass rebounds and the extent of such rebound depends upon the surface hardness of concrete. The surface hardness and therefore the rebound is taken to be related to the compressive strength of the concrete. The rebound value is read off along a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer.

Table 1 : Impact Energy of Rebound Hammers (As per IS 13311 Part 2)

S. No.	Applications	Approximate       impact       energy required for rebound hammers (N- m)
1.	For testing normal weight concrete	2.25
2.	For light weight concrete or small and impact sensitive part of concrete	0.75
3.	For testing mass concrete   i.e. in roads ,airfield pavements and hydraulic structures	30.00

                              Table 2 :  Rebound Hammer types, impact energy and grade of concrete

 

Hammers type	Grade /type of  concrete	Impact energy (N-m)
N	M-15  to M-45	2.2
L	Light weight concrete	0.75
M	Mass concrete	30
P	Below M-15	<2.2

 

  Correlation  between  Compressive  Strength  of  Concrete  and  Rebound Number: The most satisfactory way of establishing a correlation between compressive strength of concrete and its rebound number is to measure both the properties simultaneously on concrete cubes. The concrete cubes specimens are held in a compression testing machine under a fixed load, measurements of rebound number taken and then the compressive strength determined as per IS 516: 1959. The fixed load required is of the order of 7 N/mm2  when the impact energy of the hammer is about 2.2 N-m. The load should be increased for calibrating rebound hammers of greater impact energy and decreased for calibrating rebound hammers of lesser impact energy. The test specimens should be as large a mass as possible in order to minimize the size effect on the test result of a full scale structure. 150 mm cube specimens are preferred for calibrating rebound hammers of lower impact energy (2.2 N-m), whereas for rebound hammers of higher impact energy, for example 30 N-m, the test cubes should not be smaller than 300 mm. If the specimens are wet cured, they should be removed from wet storage and kept in the laboratory atmosphere for about 24 hours before testing. To obtain a correlation between rebound numbers and strength of wet cured and wet tested cubes, it is necessary to establish a correlation between the strength of wet tested cubes and the strength of dry tested cubes on which rebound readings are taken. A direct correlation between rebound numbers on wet cubes and the strength of wet cubes is not recommended. Only the vertical faces of the cubes as cast should be tested. At least nine readings should be taken on each of the two vertical faces accessible in the compression testing machine when using the rebound hammers. The points of impact on the specimen must not be nearer an edge than 20mm and should be not less than 20 mm from each other. The same points must not be impacted more than once.

Test on fresh and Hardened Concrete

 Dear Students 

In this blog series i have include various test for fresh and hardened concrete . i hopes it will surely help not only to undergraduates students but also to research scholars and post graduate students 

To find the strength characteristic of concrete with varying water cement ratio and plat a curve between strength and w/c ratio.

 

OBJECT: To find the strength characteristic of concrete with varying water cement ratio and plat a curve between strength and w/c ratio.

OBSERVATION: - (1:2:4)

Weight of cement…………… Kg.

Weight of fine aggregate…………...Kg.

Weight of coarse aggregate………… Kg.

Size of cube………...= 15x15x15 cm.

 

S.No.	W/C ratio	Weight of Water Kg.	Identification mark	Age of test days.	Cube crushing Load Kg.	Cube Crushing strength Kg./m2
1.	0.50
2.	0.55
3.	0.60

 

RESULTS: The plat of strength V/s water cement ratio is enclosed

 

DISCUSSION: Cover the following points:

I. Water cement ratio law.

II. Effect of variation of water cement ratio on workability and corresponding effect on

strength.

III. Comments on results obtained

IV. Expected strength for an assumed w/c ratio, Ref. IS 456-1978

 

NOTE:

1. Prepare 15 cm cube for each water-cement ratio.

2. Keep the proportion of mix 1:2:4

3. Test all the cubes at an age of 28 days