Concrete Engineering Tests

Civil Diary Online

Split tensile strength test of concrete

Split tensile strength test is a method of determining the tensile strength of concrete using a cylinder which splits across the vertical diameter. It is an indirect method of testing tensile strength of concrete.

The tensile strength of concrete is one of the basic and important properties. Splitting tensile strength test on concrete cylinder is a method to determine the tensile strength of concrete.

The concrete is very weak in tension due to its brittle nature and is not expected to resist the direct tension. The concrete develops cracks when subjected to tensile forces. Thus, it is necessary to determine the tensile strength of concrete to determine the load at which the concrete members may crack.

AIM:

To determine the splitting tensile strength of cylindrical concrete specimens.

Apparatus :

Testing Machine - The testing machine may be of any reliable type, of sufficient capacity for the tests and capable of applying the load at the rate specified in 5.5. The permissible error shall be not greater than ± 2 percent of the maximum load.

Cylinders -The cylindrical mould shall be of 150 mm diameter and 300 mm height conforming to IS:10086-1982.

Weights and weighing device, Tools and containers for mixing, Tamper (square in cross section) etc.

FIGURE:

PROCEDURE:

1. SAMPLING OF MATERIALS –

Samples of aggregates for each batch of concrete shall be of the desired grading and shall be in an air-dried condition. The cement samples, on arrival at the laboratory, shall be thoroughly mixed dry either by hand or in a suitable mixer in such a manner as to ensure the greatest possible blending and uniformity in the material.

2. PROPORTIONING –

The proportions of the materials, including water, in concrete mixes used for determining the suitability of the materials available, shall be similar in all respects to those to be employed in the work.

3. WEIGHING –

The quantities of cement, each size of aggregate, and water for each batch shall be determined by weight, to an accuracy of 0.1 percent of the total weight of the batch.

4. MIXING CONCRETE –

The concrete shall be mixed by hand, or preferably, in a laboratory batch mixer, in such a manner as to avoid loss of water or other materials. Each batch of concrete shall be of such a size as to leave about 10 percent excess after moulding the desired number of test specimens.

5. MOULD –

The cylindrical mould shall be of 150 mm diameter and 300 mm height conforming to IS: 10086-1982.

6. COMPACTING –

The test specimens shall be made as soon as practicable after mixing, and in such a way as to produce full compaction of the concrete with neither segregation nor excessive laitance.

7. CURING –

The test specimens shall be stored in a place, free from vibration, in moist air of at least 90 percent relative humidity and at a temperature of 27° ± 2°C for 24 hours ± ½ hour from the time of addition of water to the dry ingredients.

8. PLACING THE SPECIMEN IN THE TESTING MACHINE –

The bearing surfaces of the supporting and loading rollers shall be wiped clean, and any loose sand or other material removed from the surfaces of the specimen where they are to make contact with the rollers.

9.           Two bearings strip of nominal (1/8 in i.e 3.175 mm) thick plywood, free of imperfections, approximately (25 mm) wide, and of length equal to or slightly longer than that of the specimen should be provided for each specimen.

10.        The bearing strips are placed between the specimen and both upper and lower bearing blocks of the testing machine or between the specimen and the supplemental bars or plates.

11.        Draw diametric lines an each end of the specimen using a suitable device that will ensure that they are in the same axial plane. Centre one of the plywood strips along the centre of the lower bearing block.

12.        Place the specimen on the plywood strip and align so that the lines marked on the ends of the specimen are vertical and cantered over the plywood strip.

13.        Place a second plywood strip lengthwise on the cylinder, cantered on the lines marked on the ends of the cylinder. Apply the load continuously and without shock, at a constant rate within, the range of 689 to 1380 kPa/min splitting tensile stress until failure of the specimen

14.        Record the maximum applied load indicated by the testing machine at failure. Note the type of failure and appearance of fracture.

OBSERVATIONS:

Calculations of Mix Proportion

Mix Proportion of Concrete	For 1 m3 of concrete	For one batch of concrete
Coarse aggregate (kg)
Fine aggregate (kg)
Cement (kg)
Water (kg)
S/A
w/c ratio
Admixture

Sr. No.	Age of Spec imen	Identi-fication Mark	Dia. of Spec- imen (mm)	Depth (mm)	Maximum Load (N)	Tensile Strength (MPa)	Avg Tensile Strength (Mpa)
1	7 days
2
3
4	28 days
5
6

CALCULATION:

Calculate the splitting tensile strength of the specimen as follows:

T = 2P/πLD

Where

T = Splitting tensile strength

P = Maximum applied load

L = Length, m

D = Diameter

CONCLUSION:

Record the following things

1. The average 7 Days Tensile Strength of concrete sample is found to be…..…..
2. The average 28 Days Tensile Strength of concrete sample is found to be…..…..
   

   Rebound hammer test

   Rebound hammer test (Schmidt Hammer) is 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 rebound of an elastic mass depends on the hardness of the surface against which its mass strikes. When the plunger of the rebound hammer is pressed against the surface of the concrete, the Spring-controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the 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 from 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.

   

   What is Rebound Hammer?

   Rebound hammer is an instrument or a device, which is used to assess the relative compressive strength of concrete based on the hardness at or near its exposed surface. Rebound hammer is also known as Schmidt’s Hammer or Swiss Hammer as it is invented by Ernst Schmidt, a Swiss engineer.

   The following points should be observed during testing:

   (a) The concrete surface should be smooth, clean and dry.

   (b) Ant loose particles should be rubbed off from the concrete surface with a grinding wheel or stone, before hammer testing.

   (c) Rebound hammer test should not be conducted on rough surfaces as a result of incomplete compaction, loss of grout, spalled or tooled concrete surface.

   (d) The point of impact of rebound hammer on concrete surface should be at least 20mm away from edge or shape discontinuity.

   Procedure to determine strength of hardened concrete by rebound hammer.

   1.   Before commencement of a test, the rebound hammer should be tested against the test anvil, to get reliable results, for which the manufacturer of the rebound hammer indicates the range of readings on the anvil suitable for different types of rebound hammer.

   2.   Apply light pressure on the plunger – it will release it from the locked position and allow it to extend to the ready position for the test.

   3.   Press the plunger against the surface of the concrete, keeping the instrument perpendicular to the test surface. Apply a gradual increase in pressure until the hammer impacts. (Do not touch the button while depressing the plunger. Press the button after impact, in case it is not convenient to note the rebound reading in that position.)

   4.   Take the average of about 15 readings.

   
   

   The rebound hammer test method is used for the following purposes:

   1)   To find out the likely compressive strength of concrete with the help of suitable co-relations between rebound index and compressive strength.

   2)   To assess the uniformity of concrete.

   3)   To assess the quality of concrete in relation to standard requirements.

   4)   To assess the quality of one element of concrete in relation to another.

   How to Check the Quality of Concrete Based on Rebound Number?

   The table below shows the quality of concrete based on the average rebound number or rebound index.

   
   

   Average Rebound Number	Quality of Concrete
   > 40	Very Good
   30 -40	Good
   20-30	Fair
   < 20	Poor and/or Delaminated
   0	Very Poor and/or Delaminated