Tag Archives: Compressive strength test

Building materials, CEMENT

Cement tests – Laboratory tests and field tests

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Cement tests are significant because cement is the most important and highly recognized binding material used in construction. Cement is an integral part of all types of construction ranging from huge skyscrapers, bridges, tunnels, etc to small residential buildings. For industrial structures like power plants, refineries, steel plants, cement plants, bridges, roads, etc. cement is the main component. Cement, when mixed with sand and aggregates, forms concrete and with sand makes mortar. The serviceability, strength, and durability of a structure are related to cement used in various works.

This article is about various tests conducted on cement to check its quality.

Cement tests – Categories

Cement is one of the oldest and most used binding materials and an integral ingredient used in the construction sector. The quality, serviceability, and stability of a structure are directly related to the quality of cement used in it. Hence it is required to analyse the quality of cement before using it for works. The tests for determining cement quality are split into two major categories.

Cement got physical as well as chemical properties. Normally lab tests are conducted to ascertain the properties of cement. Lab tests require time, special equipment, and professionals for testing and interpreting the results. It may not be possible to check all the properties of cement at the site. To overcome this difficulty cement tests are categorised into field tests and laboratory tests.

Cement tests

Field tests on Cement

The quality of cement is determined using some simple field tests. These tests do not require any sophisticated types of equipment and professional skills and get the results very quickly. By conducting these simple tests and analyzing the results we will get an idea about the cement quality and can immediately decide on accepting or rejecting it.
Following are the common field tests conducted to ensure the quality of cement.

These are first look tests and quality of cement is ensured by its smoothness to touch, the colour of cement, etc.

  • Checking manufacturing date
  • Visual checking of lumps
  • Feel test
  • Heat of cement
  • colour
  • Water float test
  • Setting test

These basic tests give an approximate characteristic of cement. These are easy and quick but not accurate, however help in concluding the acceptance of cement for works.

Also read : Field tests for cement

Laboratory tests for Cement

The laboratory tests are conducted to define the physical and chemical properties of cement. It is not possible to check all the cement properties at the site. The main laboratory tests conducted on cement is as follows.

  • Fineness Test
  • Compressive Strength Test
  • Consistency Test on cement
  • Setting time
  • Soundness Test
  • Tensile strength Test
  • Heat of hydration

Lab tests require time. But it provides accurate results.

Fineness test

The fineness test determines the size of the cement grains. Smaller the diameter of grains finer the cement is. Finer cement grains enhance the strength and cohesiveness of concrete.
The fineness of cement grains plays an important role in the hydration process and directly impacts the strength of concrete. Cement fineness is determined with the help of the following tests.

  • Sieve test
  • Blain’s air permeability test
  • Wagner turbidimeter method.

Blain’s air permeability test is the more reliable than sieve analysis test. The apparatus for this test is the permeability apparatus.

Finer cement grains above the permissible limits are also not preferred because excess finer grains increase the surface area. An increase in the surface area requires more water and results in the quick setting of cement. The standard guidelines for the Fineness test are IS 4031-PART1-1996, IS 4031(Part2)-1999, ASTM-204-05, ASTMC-115-96a (re-approved 2003).

Compressive Strength Test

The compressive strength of cement is the prime data to be determined before selecting the cement at the site. Concrete imparts strength to the structure and cement is the main ingredient in concrete. The apparatus for this test is a Compression testing machine. Gradual load is applied to the cement specimen. The load at which the specimen breaks and the area of the specimen define its strength. Certain factors like improper mixing, curing, proportioning, etc also affect the strength of cement. The cement with low compressive strength is not recommended in construction. The standard guidelines available for this test are IS4031 (Part 6)-1988, ASTM C 109, BS EN 196 – 1:2005.

Consistency test on cement

The consistency test is performed to determine the water necessary for attaining standard consistency or normal consistency. Water content is an important factor in making cement mortar. On mixing water with cement the chemical reaction or hydration is initiated. Excess water in cement results in an increase in the water-cement ratio. An increase in the water-cement ratio leads to a loss of strength when cement hardens. Less water content reduces the hydration process leads to loss of strength. Vicat apparatus is used to determine the consistency. The standard consistency of cement is when the Vicat plunger penetrates to a point 5 to 7 mm from the bottom of the Vicat mould. The consistency of cement ranges from 26% to 33%.

Vicats apparatus-Consistency test on cement
Vicats apparatus-Consistency test on cement

An increase in the amount of water content creates problems like bleeding, segregation in concrete. The standard guidelines about cement consistency are available in IS 4031 (Part 4)-1988, ASTM C 187, BS EN 196-3:2005, etc.

Setting time

Setting time is the time at which the cement hardens after gaining strength. It is necessary for the transportation, placing, and compaction of cement. The setting time of cement is measured at two stages – Initial and final. The time at which the setting begins is the initial setting time. At this stage, the cement loses its plasticity. The time at which the setting completes is the final setting time. This helps in the removal of scaffolding. We use the Vicat apparatus for determining setting time. The initial setting time of cement is 30 minutes and the final setting time is 600 minutes. The code which gives the guidelines about this test are IS 4031 (Part 5)-1988, ASTM C 191, BS EN 196-3:2005.

Soundness Test

A soundness test determines the capacity of cement to retain its volume after hardening. It also determines the additional lime present in the cement. Lime is one of the main ingredients of concrete. The deficiency of lime affects the setting time of cement. If the amount of lime is high, the cement will become unsound. Large expansion in the cement may produce cracks in concrete. Thus the disintegration, corrosion, and distortion occur in the concrete. So unsound cement should not be used in construction. This test can be done using the Le Chateler method and Autoclave method. The standard guidelines about this test are available in IS4031 (Part 3)-1988, ASTM C 151-09, BS EN 196-3: 2005.

Cement tests - Le Chatelier apparatus
Cement tests – Le Chatelier apparatus

Tensile strength Test

The tensile strength test is done using the Briquette test method or by the split tensile strength test. The tensile strength of cement is less compared to the compressive strength. Tensile cracking occurs due to dynamic loading and temperature variation. In this test, uni axial loading is done to determine the tensile strength. The standard guidelines for this test are available in ASTM C307 and EN 196-1.

Heat of Hydration

Hydration is an exothermic chemical reaction between cement and water involving the release of Heat. The hydration process increases the strength of cement. The heat of hydration is signified in terms of kilo joules per kilogram.
A calorimeter is an apparatus used for determining the hydration of cement. Important factors that influence the heat of hydration are the proportion of C3S and C3A, water-cement ratio, the curing temperature, fineness of cement, etc. An increase in heat of hydration produces undesirable stresses. The standard guidelines for this test are available in ASTM C 186 and IS4031 (Part 9 ) – 1988.

Conclusion

Tests on cement is the most essential activity to be included before starting any construction activity. The quality of cement determines the strength, serviceability, stability and life of a structure. Any compromise on quality can lead to serious construction defects and failures.

CONCRETE, quality tests on concrete

Compressive strength test of concrete | Cube test for Concrete

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Compressive strength of concrete is the ability of the concrete to withstand loads without cracking or deformation. Compressive Strength of concrete is defined as the Characteristic strength of 150 mm size concrete cubes @28 days.  Marginal variations in water to cement ratio, ingredient proportioning, increase in a slump, etc impacts the desired concrete strength which in turn affects the strength and stability of structures.

Quality tests are to be conducted on concrete at various stages starting from the production stage to the hardened stage, and on structures. Quality tests play an important role in ensuring the quality of a particular construction. This article covers the types of tests conducted on concrete at various stages.

Quality tests on concrete

Quality tests are done on different stages as listed below,

  • Production stage quality tests ( On fresh concrete before placing)
  • Hardened stage quality tests ( hardened concrete specimens)
  • On structures ( tests done on the structures )

Quality tests on Fresh concrete

Quality tests on hardened concrete

Non destructive tests on concrete

  • Rebound hammer
  • Penetration resistance test
  • Ultrasonic pulse velocity test

This article is about compressive strength test of concrete, its significance and procedure.

Compressive strength test of concrete – significance

As a construction material, concrete should be able to withstand heavy loads. The concrete material beneath compression tends to reduce its size. Concrete gains its strength over time. The compressive strength depends on cement strength, water-cement ratio, concrete quality etc. The test gives an idea of the overall strength and above-mentioned factors. Through conducting this test, one can easily judge the concrete strength and quality of concrete produced. We can calculate the compressive strength from the failure load and the cross-sectional area of the specimen.

Compressive strength test of concrete

Concrete compressive strength for general construction varies from 15 MPa (2200 psi) to 30 MPa (4400 psi) and more than that in case of commercial, industrial structures and special structures.

The compressive strength helps in determining

  • Quality control
  • Acceptance of concrete
  • Evaluation of curing
  • For determining the in-place concrete strength.
  • Age of concrete, etc.

Formula for compressive strength

Compressive strength formula for any material is the load applied at the point of failure to the cross-section area of the face of the concrete specimen (cube or cylinder) on which load was applied.

Compressive Strength = Load / Cross-sectional Area

Relevant IS code

IS: 516-1959

Apparatus used

  • Compression testing machine or Universal testing machine
  • Cube or cylinder specimen
  • Weighing machine
  • Vibrating machine
  • Trowel
Compression test - Apparatus

Compressive strength test of concrete

The specimen for this test can be cubic or cylindrical. The dimension of the specimens is as follows.

  • Cube = 150mm x 150mm x 150mm
  • Cylinder = 150mm diameter, 300mm height

The mould is made up of cast iron or steel. Cubical moulds are preferred for most works.

Compression testing of cubes – Procedure

Mixing of materials.

  • The first step is to prepare the test specimen by mixing the ingredients.
  • Mix the cement, coarse aggregate and fine aggregate in dry condition uniformly.
  • Mixing of the materials is by using batch mixer or by hand mixing.
  • Ingredients shall be bought to room temperature, before commencing the test.

Preparation of Test specimen

  • Clean the cube mould and apply the lubricant inside the mould.
  • Make at least three specimens from each batch.
  • After mixing the concrete, fill the specimen in the mould in three-layer at 50 mm thickness. 
  • After that stroke 35 times and compact the concrete using a vibrating machine. 
  • Then remove the excess concrete using a trowel. 
  • After that mark the date, grade of concrete etc and immerse the specimen in freshwater.
  • Then store the specimen at a temperature of 27-degree Celsius for 24 hours.

Procedure for testing

  • The specimens should be tested using the compression testing machine at 3, 7 and 28 days. 
  •  Before starting the test take the specimen from water and remove the mould.
  • The test should be conducted in the wet condition of the specimen.
  • Record the weight and dimension.
  • Then place the specimen in the testing machine. 
  • Gradually apply load on the specimen at a rate of 140 kg/ cm2 per minutes.
  • Note down the load at which the specimen breaks.
Compressive strength test of concrete
Compressive strength test of concrete

Calculation for Compressive strength test of concrete

The compressive strength of the concrete = Load at which the concrete breaks / Cross-sectional area of the specimen.

The compressive strength is expressed in N/mm2. The cube specimen is tested at 7, 14 & 28 days.

Calculation of compressive strength

Size of cube = 15cm x 15cm x 15cm

Area of specimen = 225 cm2

Expected maximum load = fck x area x f.s

Calculation shall be repeated for 7, and 28 days.

Results shall be furnished as average compressive strength of cube = …………… N/mm2 (7 days and 28 days)

Conclusion

The compressive strength of concrete cube gives and idea about the characteristics of concrete. With this single test we can judge the concrete quality and hence preferred for major construction works.