Category Archives: CONCRETE

Reinforced cement concrete is a topic you will need to familiarize at some point, if you are into civil engineering field. The category explicitly presents it. Reinforced cement concrete (RCC), a composite material has been accepted worldwide as a construction material, bridges, retaining walls, docks and harbour, airfield pavements, flyovers,multi-storey building,complexes and simple houses etc., Concrete is good in resisting compression but is very weak in resisting tension. Hence reinforcement is provided in the concrete wherever tensile stress is expected. The best reinforcement is steel since the tensile strength of steel is quite high and the bond between steel and concrete is good. As the elastic modulus of steel is high, for the same extension the force resisted by steel is high compared to concrete. However in tensile zone, hair cracks in concrete are unavoidable. Reinforcements are usually in the form of mild steel or ribbed steel bars of 6 mm to 32 mm diameter. A cage of reinforcements is prepared as per the design requirements, kept in a formwork and then green concrete is poured. After the concrete hardens, the formwork is removed. The composite material of steel and concrete now called R.C.C. acts as a structural member and can resist tensile as well as compressive stresses very well. Aggregates are the inert or chemically inactive materials which form the bulk of cement concrete. These aggregates are bound together by means of a cement. They can be classified into two. The selection of aggregate is based on the purpose and its maximum size.

Admixture types | Mineral and Chemical Admixture

Types of admixture to be used in concrete depend on the structure’s intended use, design strength, pouring conditions, and other factors. The most commonly used material in construction is concrete. Admixture is a material that is added to concrete along with cement, sand, water, and aggregates to modify or enhance the properties of the concrete and make it more suitable for a specific environment.

Concrete is used in many applications and is exposed to extreme environmental conditions that can lead to severe performance and quality issues. Admixture and their types are important ingredients in today’s high-performance, long-lasting, durable, and beautiful concrete. Admixtures are broadly classified into chemical admixtures and Mineral Admixtures. This article discusses the most common types of admixture used in concrete.

Definition of Admixture and its types used in concrete

Admixtures are materials that enhance performance and alter the properties of concrete. Concrete admixtures are natural or manufactured chemicals or additives added during concrete mixing to enhance specific properties of the fresh or hardened concrete, such as workability, durability, or early and final strength and make it desirable for a certain condition. Also, Admixture reduces construction costs and makes the concrete more economical.

Types of Admixture
Types of admixture used in concrete

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Functions of different types of admixture

There are many applications for different types of admixture, including but not limited to the following ones:

  • Reduces the water-cement ratio.
  • Increases the strength and durability of concrete.
  • Improves the workability of concrete.
  • Provides particular properties to concrete to make it suitable under different weather conditions 
  • Limits the shrinkage cracks.
  • Avoids bleeding and segregation of concrete
  • Accelerates or retards the setting time of concrete 
  • Reduces corrosion of reinforcement in concrete or improves Resistance to chemical attack.
  • Decreases the heat of hydration.
  • Strengthens the bond between old and new concrete surfaces.

Types of Admixture

There are mainly two types of admixture 

  1. Mineral admixtures
  2. Chemical admixtures

Types of Mineral Admixture

Mineral admixtures are siliceous and insoluble materials other than cement and aggregate that are added to concrete in concentrations ranging from 20 to 70% by mass of cement. These are fine materials that have an impact on concrete via hydraulic and pozzolanic activity. Natural materials, processed natural materials, and artificial materials are all examples of mineral admixtures. The following are some types of mineral admixture commonly used in concrete.

  • Cementitious
  • Pozzolanic
  • Blast Furnace Slag
  • Flash
  • Silica Fume
  • Rice Husk
Type of Mineral Admixture - Blast furnace slag
Type of mineral Admixture – blast furnace slag

Types of Chemical Admixture

Chemical admixtures are chemical additives blended with concrete to modify their properties. It imparts special properties to concrete. The most common types of chemical admixture are listed below

  • Plasticizers
  • Super Plasticizers
  • Accelerators
  • Set Retarders

Types of concrete admixture based on applications

Admixture is classified into various types based on various applications

  • Water-reducing admixture
  • Retarding admixture
  • Accelerating admixture
  • Air entraining admixture
  • Pozzolanic admixture
  • Damp-proofing admixture
  • Gas forming admixture
  • Air detraining admixture
  • Anti-washout admixture
  • Corrosion inhibiting admixture
  • Bonding admixture

Water-reducing admixtures

Plasticizers are another name for water-reducing admixtures. Basically, by lowering the water-cement ratio, they assist in reducing the water content of the concrete mix by 5 to 20%, resulting in high-strength concrete. Workability is increased by water-reducing admixtures because they can even maintain a high slump without adding more water. Examples include polycarboxylates, multicarbovyl ethers, and acrylic polymers. etc.

Retarding admixtures/Retarders

Retarding admixtures or retarders decreases the setting rate of concrete. They are suitable in hot weather conditions where the high temperature drastically increases the setting rate of concrete. However, the fast setting rate of concrete affects its strength and durability. Generally, retarding admixtures are widely used to overcome this problem. Some examples of retarders are Starch, cellulose products, common sugar, acid salts, etc.

Accelerating admixture/Accelerators

Accelerating admixture decreases the initial hardening time of concrete. As a result the rate of hydration of cement increases. There are two types of accelerating admixtures.

  • Set accelerating admixture
  • Hardening accelerators

Accelerating admixture improves the concrete strength by increasing the rate of hydration. This type of admixture is suitable for early formwork removal, emergency repairs, buildings in low-temperature regions, etc. Some examples of accelerators are triethanolamine, calcium formate, active silica, calcium chloride, finely divided silica gel, etc.

Air entraining admixtures

During the mixing of concrete air bubbles are formed.

Air-entraining admixtures facilitate the development of a stable air-void system within the concrete that increases durability, workability of the concrete and cohesion. Similarly, these admixtures impart air entrainment that results in:

  • Increased resistance to deterioration from cyclic freezing and thawing 
  • Improved workability and cohesiveness of concrete placement
  • Reduced segregation and bleeding

Pozzolanic admixtures

These admixtures are suitable for hydraulic structures such as dams, reservoirs, etc. Pozzolana is a cementitious material that helps in preparing high-dense concrete mixes. Accordingly, It increases the strength, and reduces the cost of concrete and thermal shrinkage. Some examples of pozzolanic admixtures are fly ash, silica fume, rice husk ash, metakaolin, etc.

Damp proofing admixtures

Dampproofing admixtures make the concrete impervious and durable. It also helps in attaining the early stage of concrete hardening. Some examples of dam-proofing admixtures are hot bitumen, mastic asphalt, bituminous felt, etc.

Gas forming admixtures

During the hydration process of cement, we get hydroxide. The gas-forming admixture reacts with the hydroxides and forms hydrogen gas bubbles. This bubble helps in avoiding settlement and bleeding of concrete. Some examples of gas-forming admixtures are Aluminum powder, activated carbon, hydrogen peroxide, etc. 

Air detraining admixture

During the mixing of concrete, the air gets entrapped in the concrete. Furthermore, this air reduces the strength of the concrete. So to avoid this air content we use air-detraining admixtures. Some examples of this type of admixture are tributyl phosphate, silicones, water-insoluble alcohols, etc.

Anti-washout admixture

Generally, the main application of anti-wash-out admixtures is in underwater construction. Some examples are natural or synthetic rubbers, thickeners based on cellulose, etc. Similarly, It makes the concrete more cohesive and avoids washing out of concrete mixes underwater. 

Corrosion inhibiting admixture

Corrosion of reinforcement is one of the common problems in construction. So to decrease the corrosion this type of admixture is used. Basically, corrosion-inhibiting admixtures help in decreasing the corrosion rate and delaying the corrosion. Some examples of corrosion-inhibiting admixtures are sodium benzoate, sodium nitrate, sodium nitrite, etc.

Bonding admixture

This type of admixture helps to the bond between the new and old concrete surface. Basically, It is commonly used in floor overlays, screed over roofing, repair work, etc. Some examples of bonding admixtures are natural rubber, synthetic rubbers, and polymers such as polyvinyl chloride, polyvinyl acetate, etc.

Penetration Resistance Test – Significance and Procedure

Penetration resistance test is a concrete non-destructive test. This test determines the strength of concrete by penetrating a steel rod.

Non-destructive test (NDT) and destructive tests (DT) are the the tests done one hardened concrete. Concrete is the oldest and most important construction material in the world. Testing of the concrete plays and important role to know about the strength, durability and condition of the structure. This article is about the types of Destructive and non destructive tests done on concrete.

Also read : Flow table test for workability of concrete

Also read : Concrete mixing objective and types

Types of concrete tests

Tests on hardened concrete are classified into two types.

Non destructive tests and Destructive tests are for determining the important properties of concrete like compressive strength, flexural strength, tensile strength etc.

Non- Destructive tests

The standard method of testing hardened concrete is by testing the concrete specimens cast simultaneously with structural concrete. These tests can determine the compressive, flexural, and tensile strengths. The major disadvantage of these tests is the non availability of immediate results. Moreover, the properties of the concrete specimen may differ from what it is in the actual structure. This difference is due to pouring conditions, curing factor, compaction and concrete density, etc. These tests come under the category of destructive tests (DT). Destructive testing destroys or changes the part in some way such that even if it passes the test it is no longer fit for service.

Non Destructive test is for determining compressive strength and other properties of concrete from existing structures or buildings. NDT does not destroy or change the part such that it is still fit for service if it passes the test. The non-destructive test determines the properties of concrete or structures without their destruction. Moreover, we can study its changes over time, The measurements are repeated many times and validate in detail, and gets immediate results.

Objectives of Non destructive test

  • Quality check of the existing structure.
  • For determining the homogeneity, internal and external characteristics of the concrete. 
  • To detect the cracks and voids of the concrete structures. 
  • Assess the quality of concrete with the standard requirement. 
  • To study the ageing of concrete. 
  • For finding the compressive strength of structures.
  • Evaluation of elastic modulus

Types of Non Destructive test

Penetration resistance test is a concrete non-destructive test. Nowadays Non-destructive test is conducted to determine the relative strength of the concrete. The non-destructive test is the method of evaluating the strength and durability of concrete without loading the specimen.

The main Non Destructive test are

Penetration Resistance Test

Another name of penetration resistance test is the Windsor probe test. This test determines the strength of concrete by penetrating a steel rod. The equipment used for this test is the Windsor probe test machine. Also, the penetration resistance is inversely proportionate to the concrete strength. 

Relevance Codes

  • ASTM C 803/ 803M- 97
  • BS 1881 Part 207

Significance of Penetration resistance test

  • To determine the concrete strength.
  • For evaluating the uniformity of concrete.
  • To identify deterioration in concrete. 
  • To evaluate the in-place strength.
  • For investigating the presence of cracks and flaws.

Apparatus used for Penetration resistance test

The equipment to determine the strength using this test is Windsor probe equipment. It contains a gunpowder actuated driver, a loaded cartridge, a depth gauge and a hardened alloy rod. The gun powder actuated driver helps the alloy rod to penetrate the concrete surface. The probe with a blunt conical nose is made of hardened alloy. So that it should affect by corrosion. On the basis of the concrete aggregates, different probes are used. The test position should be at least 200mm apart from the edge. The maximum spacing between the probe should be 175mm. Also, the minimum spacing should be 100 mm. 

Penetration-resistance-test
Penetration-resistance-test

Test Procedure

  • Firstly, fix the equipment in an appropriate test position on the concrete surface.
  • Then attach the probe to the driver. 
  • Drive the probe into the surface of the concrete. 
  • Check whether the probe is set firmly. 
  • Now measure the length of exposed above the concrete surface. 
  • Take at least three measurements and average them to evaluate the result.

Correlate the depth of penetration with the compressive strength. If the depth of penetration is low, then the strength of concrete is high and vice versa. 

Limitations

  • Tests only on through the thin layers of concrete surface. 
  • Fails to yield the absolute value of the strength. 
  • Damages the structures by making holes.
  • The accuracy of this test is ± 25 %.

Even though the penetration resistance test has some limitations, this method is apt for the on-site estimation of concrete stren

Methods of concrete curing – Top 3 curing methods explained

The concrete curing methods depend on the nature of the structure, site conditions, and ingredient parameters. In this article, we will go through some common curing methods adopted in constructions sites. For getting a better understanding of the topic, refer to our earlier article Curing of concrete – Process and significance.

Methods of concrete curing

Curing is the process that helps in maintaining moisture to allow fresh concrete to attain its desired strength in a planned manner through a hydration reaction. If the water evaporates quickly, the requisite amount of water will not be available for the hydration process. The curing of concrete helps to retain the concrete moisture until the hydration process is complete and concrete attains the requisite strength.

The curing method and time primarily depend on structure type, site conditions, and ingredient parameters. Some of the curing concepts adopted in constructions sites are as follows.

  • Maintaining a water layer of water over the concrete surfaces (water curing)
  • Reducing the loss of water from concrete
  • Accelerating the initial strength gain

Let us go through these three basic concepts and curing methods adopted for each of these criteria.

Also Read : Non destructive tests ( NDT } on hardened concrete

Maintaining water over the concrete surfaces -Methods of concrete curing

Maintaining water over concrete surfaces or water curing is the most popular and conventional method of concrete curing. The methods of curing depends on the type of structure basically. Let us go through the water curing methods adopted in construction sites .

  • Ponding
  • Spraying and fogging
  • Saturated or wet coverings
  • Immersion curing

Ponding

Ponding is the method of wetting the concrete surface by creating a temporary containment area around the concrete. 

The ponding method is one of the very well-known and widely adopted concrete curing methods. This method is possible only on horizontal or flat structures and is best suited for curing concrete slabs. 

Small bunds of clay or lean cement sand are placed over the concrete surface. Water is filled in the bunds, refilled, and maintained as and when the level goes down.

This method is very efficient but requires a huge amount of water. The ponding method of curing is not preferred for large areas and areas where water is scarce.

Spraying and fogging method of curing

Spraying and fogging are conventional water curing methods for horizontal and vertical concrete surfaces. These methods are very effective and efficient in supplying additional moisture during hot weather. The spraying and fogging methods help in reducing the temperature of the concrete.

Spraying is a manual method of simply spraying water over the concrete surface. Even though the spraying method is effective, it tends to waste a lot of water.

curing by water spraying
curing by water spraying

Fogging is applicable for areas where the temperature is above freezing point and with low humidity. This process raises the humidity higher than curing concrete by spraying a fine mist of water regularly across the concrete surface. Fogging, or fog spraying is effective in reducing the chances of plastic shrinkage cracking in mixes.

Saturated or wet coverings

In this method of curing, saturated or wet coverings are placed over the hardened concrete. The wet covering material includes hessian cloths, cotton mats,moisture-retaining fabrics, etc. Vertical and inclined structures like columns, beams, sloped roofs, etc are cured in this method.

Curing by wetting -methods of concrete curing
Curing by wetting

Immersion curing

Immersion method of curing is for curing concrete specimens.

Reducing Water loss from concrete- Methods of concrete curing

This concept of curing is by reducing the moisture loss from the concrete surface by wrapping it with an impermeable membrane or plastic sheets. This method is the most practical and efficient way to cure concrete nowadays and is a much-needed option for areas where there is water scarcity. The common methods are..

  • Membrane curing
  • Covering concrete with impervious sheets or plastic sheets

Membrane curing

The application of curing compounds is through spraying or painting directly on the concrete surface. The compound dries and forms an impermeable membrane that retards or reduces the moisture loss from the concrete. The membrane curing method is a flexible and easy method of curing.

membrane curing-methods of concrete curing
membrane curing /curing compound

Curing compounds are applied once the concrete is hardened, but still having water content on it. The application has to be done before the water evaporates fully from the concrete. The membrane curing may not produce desired results, if applied after the water evaporates. Ensure uniform thickness and coverage during the application process.
Curing compounds are available in acrylic-based and water-based forms.
The curing compound should comply with ASTM C3094 or ASTM C13155.
Always go through the manufacture specification before application of curing compounds.

Covering with plastic sheets

In this method, Impervious paper and plastic sheets is applied on thoroughly wetted concrete. The concrete surface should be hard enough to take the possible damages while placing the sheets.

Accelerating strength gain method – Methods of concrete curing

Accelerating strength gain using heat or additional moisture is done to speed up the early hardening of concrete and mortars by subjecting them to steam and humidity. Following are the methods adopted in this method of curing.

Steam curing for concrete

The steam curing method uses water vapor or steam for curing the concrete elements. This method of curing is for cold weather conditions where the concrete needs accelerated early strength gain and additional heat for hydration. Prestressing and precast factories use the steam curing method for early strength gain of components. The early strength gain can enable rapid removal and reuse of forms. 

The steam curing is done in two ways

  • live (or low pressure) steam at atmospheric pressure
  • high-pressure steam curing in autoclaves.

Steam curing at atmospheric pressure

Steam curing allows increased production due to rapid repetition and reuse of molds/forms in precast yards.

The steam temperature should be kept at around 140 degrees Fahrenheit or less for live steam at atmospheric pressure until the desired concrete strength is achieved.

By maintaining an optimum temperature, a 28 days strength of normal water cured concrete can be achieved in 3 days using the steam curing method.

High-pressure steam

High-pressure steam curing is carried out in a closed chamber at high temperature and high pressure. This process is also known as “Autoclaving”. Temperature should be maintained between 325°to 375°F and pressures should be around 80 to 170 psi. This method is used for manufacturing precast components, cellular concrete products, such as cellular blocks, precast panels, autoclaved aerated concrete (AAC) blocks, etc.

The main advantage of a high-pressure steam curing system is the capability of attaining 28 days of strength of normally cured concrete in 24 hours.

High-pressure steam cured concrete got less creep and shrinkage and better sulfate resistance. There will not be any efflorescence and moisture content after curing.

Heating coils

Heating coils are usually used as embedded elements near the surface of concrete elements. Their purpose is to protect concrete from freezing during cold weather concreting.

Conclusion

The type of curing to be suggested depend upon the structure, type, nature of the structure and climatic conditions. Curing is the most important activity to be done on concrete to maintain its design strength, durability,serviceability and life span.

Concrete Pumps Types – Applications and advantages

Concrete pumps are devices that convey concrete from a concrete source to a casting location. In this blog, I will show you the basic theory, forms, and selection of concrete pumps.

In a concrete pump, the concrete is drawn from the hopper (concrete source) by one piston and then pushes into the discharge outlet pipe by the other piston. A valve controls cylinders which must be opened to the hopper and the discharge pipe

Variations of this design were developed later. The designs helped for high-pressure pumping which were used when the concrete had to be pumped at various heights and lengths. This innovation is in high demand for the rapid construction of high-rise buildings.

Let’s see the types of Concrete Pump.

Types of Concrete Pumps

Different concrete pumps have been designed based on pumping pressure, ease of use, concrete placing height or vertical head etc. Concrete pumps are classified as follows:

  1. Boom Concrete Pump
  2. Line Concrete Pump/ Stationary pump

Boom Concrete Pumps

  • Boom pumps has a remote control operated hydraulic arm mounted on a truck. The concrete is fed through a hopper attached to the truck. The main benefit is that the arms are prefixed.
  • They can be operated remotely and the truck can be moved like and ordinary truck.
  • Boom concrete pumps are designed to pump concrete to any heights. They are mostly used in the construction of high rise buildings, chimneys, silos, industrial structures etc .
  • The flexibility of movement of pipe line in all directions and their ability to access and pour in congested locations make them a very preferred option over other methods
  • Compared to conventional method, concreting using boom pump is very fast. It also generates less wastage and maintains construction quality.
  • Due to prefixed arms, labour involvement is less in boom placer.
  • Boom placer can optimise construction schedules and mainly preferred for fast track projects.
Boom concrete pump
Boom concrete pump/placer

Line Concrete Pumps/Stationary concrete pump

  • It is a concrete pumping device that can only be fixed on one point till the concrete pour ends with a hopper to receive concrete from source
  • The pipe line is fixed at site. It consists of 1 Meter, 2 Meter pieces and long and shot bends.
  • There will be a hose fixed at the end of pipe line which is 2 Meter in length
  • Pump will be in one position. Only pipe lines can be extended as per requirements
  • In this method, labour involvement is very high due to fixing of pipe lines at site . These pumps are used for mass concrete and in areas where there is minimal location changes.
  • One major drawback with Line Concrete pump are chances of pipe line getting choked due to concrete consistency or extreme weather conditions. In that case the pipe lines has to be removed, cleaned and refitted to restart the concrete. This process generates huge wastage and chances of rejection of concrete.
Concrete Pump
Concrete pump

How to Choose a Concrete Pump?

The concrete pump is chosen based on the construction project’s specifications. The following criteria are used to determine a successful and appropriate concrete pump:

  • The Concrete Pump’s manufacturer
  • Concrete Pump’s Function
  • Construction Projects’ Schedule
  • Price and economic factors

Concrete Pump’s manufacturer:

The following are important considerations to consider when choosing a successful concrete pump manufacturer:

  • Choose the best concrete pump manufacturer in the region. For this, consult local contractors and seek assistance from any recognised manufacturers.
  • A wide range of options are now available online. Check out the top-rated concrete pumps and the feedback they’ve received.
  • Under high pressure, the concrete pumping procedure is carried out. This long-term pressure must be able to withstand a strong concrete pump. 
  • If the concrete pump’s output is below average, building projects can break down and suffer losses.

Price and economic factors

The concrete pump chosen must be practical and work at the rate for which it was purchased. Avoid the risk of paying a high price for an underperforming concrete pump. Often choose a concrete pump based on its efficiency and cost. Compare the price and the results.

Concrete Pump Selection Based on Work

When the concrete pump is chosen on the basis of work, two basic factors are considered:

  1. Volumetric Concrete Output
  2. The Pumping Pressure

The work determines the maximum volumetric concrete output required per hour for concrete construction. The pump’s pressure is specified based on this. This pressure is the criterion for choosing a concrete pump. 

The strength of the concrete pump’s driving unit is determined by the desired concrete output (Q) and the pumping pressure (P). Then the  hydraulic output (H) is given by:

H = Q x P 

The concrete pump described above can be chosen based on the various construction projects.

  • For the building of large road and highway projects, a boom or truck-mounted concrete pump is ideal.
  • A boom concrete pump is the best option for concrete pouring at various altitudes and where there is a small area for concrete pouring.
  • Boom concrete pumps can mount two or three robotic pump lines that function as a multi-purpose robotic arm, reducing the number of workers needed.
  • For the building of sidewalks or slabs, a line concrete pump or a stationary concrete pump is used. The stationary pump is used if only a small amount of concrete needs to be pumped.

Concrete Pumps Advantages

  • Ability to position concrete in difficult-to-reach areas of a site, especially at heights.
  • The ability to alter the pouring spot location.
  • Regular concrete in small quantities is needed at different locations in some projects such as bridges and dams.
  • There is a very low reliance on labourers.
  • Concrete is transported quickly from the output point (transit mixer/mixing m/c) to the pouring point.
  • Where other concrete carrying mechanisms, such as a crane or a trolley, are impractical.
  • The pump does not require light for service, while other methods of moving concrete to placement points, such as cranes or trolleys, do.
  • The potential for placement is adaptable.

Concrete Pumps Disadvantages

The disadvantages of using concrete pumps in building projects are as follows:

  • The net cost would be higher.
  • Due to the huge initial cost, these pumps are not feasible for structures with minor concrete volume.

That’s it about concrete pumps. Let us know in comments if you found this insightful.

Non Destructive test (NDT)- Hardened Concrete

Non-destructive test (NDT) and destructive tests (DT) are the the tests done one hardened concrete. Concrete is the oldest and most important construction material in the world. Testing of the concrete plays and important role to know about the strength, durability and condition of the structure. This article is about the types of Destructive and non destructive tests done on concrete.

Also read : Flow table test for workability of concrete

Also read : Concrete mixing objective and types

Types of concrete tests

Tests on hardened concrete are classified into two types.

  • Non destructive tests (NDT)
  • Destructive test (DT)

Non destructive tests and Destructive tests are done to determine the important properties of concrete like compressive strength, flexural strength, tensile strength etc.

Non- Destructive tests

The standard method of testing hardened concrete is by testing the concrete specimens cast simultaneously with structural concrete. These tests can determine the compressive, flexural, and tensile strengths. The major disadvantage of these tests is the non availability of immediate results. Moreover, the properties of the concrete specimen may differ from what it is in the actual structure. This difference is due to pouring conditions, curing factor, compaction and concrete density, etc. These tests come under the category of destructive tests (DT). Destructive testing destroys or changes the part in some way such that even if it passes the test it is no longer fit for service.

Non Destructive test is for determining compressive strength and other properties of concrete from existing structures or buildings. NDT does not destroy or change the part such that it is still fit for service if it passes the test. The non-destructive test determines the properties of concrete or structures without their destruction. Moreover, we can study its changes over time, The measurements are repeated many times and validate in detail, and gets immediate results.

Objectives of Non destructive test

  • Quality check of the existing structure.
  • For determining the homogeneity, internal and external characteristics of the concrete. 
  • To detect the cracks and voids of the concrete structures. 
  • Assess the quality of concrete with the standard requirement. 
  • To study the ageing of concrete. 
  • For finding the compressive strength of structures.
  • Evaluation of elastic modulus

Classification of Non-Destructive test

Non destructive tests are the following types.

Penetration Resistance Test

The penetration resistance test is an NDT that determines the relative strength of the concrete structures. The Windsor probe is the equipment for conducting this test. This equipment includes of powder-actuates driver or gun, probes, loaded cartridges and a gauge that measures the penetration.

The penetration depth indicates the compressive strength of the concrete. However, this depends on the aggregate type and size. This test evaluates the poor quality and deterioration of concrete

The major disadvantage of this test is getting variable results. Several probes are often shot to achieve a solid average depth for arriving a final conclusion. So we cannot determine the exact strength. But it is a quick method to evaluate the quality and maturity of concrete. Care should be taken to calibrate the instrument before taking readings.

Rebound Hammer Test

The rebound hammer is also known as Schmidt’s Hammer test. It determines the strength of concrete based on the hardness of the concrete surface. It is a surface hardness tester. The equipment consists of a spring-controlled plunger, a hammer that weighs 1.8 kg, and a graduated scale. By pressing the hammer on the concrete surface, the graduate scale measures the rebound number. A low rebound number means the concrete has low compressive strength and stiffness.

Non destructive test - Rebound hammer test
Rebound hammer test

An accuracy of 15 to 20% is possible through this test. It is a simple and quick method. Also, the result relies on the surface smoothness, water content, type and size of aggregate and carbonation of the surface.

Ultrasonic Pulse Velocity Test

Ultrasonic pulse velocity test is another type of Non-destructive test. This test measures the time of travel of ultrasonic pulse waves for evaluating the concrete quality. The UPV test units consists of a pulse generator and pulse receiver. The frequency of the wave is 50-55 kHz. The pulse generator produces the pulses and is allowed to pass through the concrete. Then we calculate the velocity, by measuring the traversing distance and the time. Higher velocity means the concrete has a higher elastic modulus and density. It also determines the cracks and flaws in the structure. Large differences in pulse velocity values indicates a defective and deteriorated concrete.

For more details : Ultrasonic pulse velocity test || UPV Test – Methods and procedure

Non destructive test - pulse velocity test
Ultra sonic pulse velocity test

Pullout Test and Pull-off Test

The pullout test in concrete is done using the LOK test and CAPO test. It defines the strength of concrete by measuring the force to pull the embedded disc from the structure. This can be done for both fresh and hardened concrete. The pullout test equipment induces a pullout force toward the concrete. This pullout force is related to the concrete strength. The dimension of the ring, the orientation of the embedded insert, type of aggregate are some factors that influence this test. In addition, this test terminates the curing of concrete and measures the time for form removal. 

Non destructive test-Pull out test
Pull out test

Concrete Core Testing

The Concrete core testing is a non-destructive test of concrete. Concrete cores cut using a rotary cutting tool as shown in the fig. We get an uneven cylindrical core specimen. By testing this specimen in the compression testing machine in a moist condition, we get the strength. This sample also determines the density, depth of concrete carbonation, permeability, chemical analysis, etc. 

Concrete core cutting
Concrete core cutting

Thus through Non-destructive test, it is possible to investigate the variation in concrete quality with time and external factors. When compared to other destructive test, there is no wastage of materials. Therefore, NDT is quick and most effective test of concrete.

Rebound hammer test on Concrete structures

Rebound hammer test is one of the most popular and simple non-destructive concrete tests. The Non-destructive test is the process of detection of defects in concrete without its destruction. The determination of concrete quality is necessary for both fresh concrete and hardened concrete. The rebound hammer test is the common test for determining quality for existing structures.

The rebound hammer is also known as the Schmidt hammer. It determines the hardness of the concrete, helps in repairing and quality checks of RCC structures. Moreover, It provides a simple and quick inspection of concrete structures.

Also Read : Non destructive tests on hardened concrete

Also Read : Ultra sonic pulse velocity test – UPV Test – Methods and procedures

Relevant Codes – Rebound hammer test

  • IS 13311 (Part 2): 1992
  • BS 6089-81 
  • BS 1881-202

Objectives of Rebound hammer test

The main objectives of the Rebound hammer test include

  • This determines the compressive strength of the concrete in terms of the rebound index.
  • Also finds the uniformity of concrete.
  • evaluates the concrete quality with standard requirements.

Test Procedure

  • The concrete surface should be smooth and dry. 
  • The plunger is pushed against the concrete surface. 
  • The spring inside the device rebounds. It depends on the hardness of the concrete surface. 
  • The graduated scale shows the reading of extending.
  • This is the rebound number or rebound index. 
  • The lower rebound value means low compressive strength. ie, they are weak structures.
  • An average of six reading should be taken. 

The correlation between the concrete strength and the rebound hammer shown in a graph is called the rebound hammer graph. 

Correlation between compressive strength of concrete and rebound index.

It is essential to calibrate the rebound hammer before testing. For this, we test a specimen of 150mm * 150mm * 150mm size using a compression testing machine (CTM). Fix the load rate at 7N/mm2, when the hammer result is 2.2 Nm. Test the vertical surface of the specimen using a rebound hammer. The results from the CMT and the rebound hammer should not have a major difference.

Rebound hammer test
Rebound hammer test

Result Calculation of Rebound Hammer tests

The rebound number assess the strength of the structure. As the rebound number increases, the strength also increases. This depends on the 

  • Cement type
  • Aggregate type
  • Surface condition
  • Age of concrete
  • Moisture content, etc.

The quality of concrete concerning the rebound concrete is as follows.

Average Rebound NumberQuality of concrete
>40Very Good
30-40Good
20-30Fair
<20Poor / Delaminated
0Very Poor / Delaminated
Concrete quality and rebound test values

Thus the rebound number predicts the quality of concrete. The rebound hammer apparatus is very easy to use. Also, it determines the uniformity of the concrete surface. Thus it can be used for the rehabilitation of old monuments. It produces a convenient indication of compressive strength