7 most effective methods for Concrete Test

7 MOST EFFECTIVE METHODS FOR ON-SITE CONCRETE TEST

Concrete tests, always be considered interesting and important for Project engineers, Contractors or for Insurance Surveyors.

From an Insurance viewpoint, there are many times the situation of Total or Partial loss of any Concrete structure followed by any major incident like fire, flood etc.

Now the actual problem arises at the time of Claim to assess whether the affected structure after the incident or due to affected by similar peril should be considered Partial loss or Total loss.

Although the building structure stands out after the incident, the question is about the strength of that structure.

Most of the time from the Insured viewpoint, it should be considered a total loss because it cannot refurbish the same by carrying out necessary repairs, where there will always question about its strength in comparison to pre-incident situations.

Whereas from Insured point of view, it will be considered as Partial loss, which can be reused by performing only necessary repair work, which will also further reduce insurer liability of claim amount.

In such a scenario, the best way out is to check the Concrete strength of the remnant structure and then decide by the competent authority to consider it as partial loss or Total loss.

The insured also prefers non-destructive techniques(NDT) and non-intrusive methods to further damage an already struggling structure. 

Evaluation of Concrete strengthening is an important task. Mainly, this process is done in two stages as per requirement.

• For any Existing structures
• and, for any New construction

For Existing Structures

There are many methods for evaluating concrete strength for existing structures. But out of all of them, below seven methods are the most popular and common methods for this. 

Competent Engineers or technical Surveyors further analyze the results of these tests, and then the decision on concrete strength is taken. Cost, time and ease in adoption are some of deciding factors for the adoption of the below methods. 

These major tests or methods are as below.

1.  Rebound Hammer or Schmidt Hammer (ASTM C805)
2.  Penetration Resistance Test (ASTM C803) 
3.  Ultrasonic Pulse Velocity (ASTM C597) 
4. Pullout Test (ASTM C900) 
5. Drilled Core (ASTM C42) 
6. Cast-in-place Cylinders (ASTM C873)
7. Wireless Maturity Sensors (ASTM C1074)

These tests are followed as per ASTM standards and guidelines.

ASTM International, formerly known as the American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for various materials, products, systems, and services. The organization’s headquarters is in West Conshohocken, Pennsylvania, about 5 mi (8.0 km) northwest of Philadelphia. (source: Wikipedia)

1. Rebound Hammer or Schmidt Hammer (ASTM C805)

This method is also called as ‘Schmidt hammer’. It is the simplest non-destructive method for concrete strength testing method.Method: It works based on the rebound principle and consists of measuring the rebound of a spring-driven hammer mass after its impact with concrete.

In this method, a spring release mechanism is used to activate a hammer that impacts a plunger to drive into the surface of the concrete.

The rebound distance from the hammer to the surface of the concrete is given a value from 10 to 100. This measurement is then correlated to the concretes strength.  

Pros: 

• Relatively easy to use and can be done directly on-site
• It can be used for the uniformity of concrete

Cons: 

• Pre-calibration using cored samples is required for accurate measurements. 

Test results can be affected by surface conditions, large aggregates or rebar below the testing location. 

2.Penetration Resistance Test (ASTM C803) 

Method: To complete a penetration resistance test, a device drives a small pin or probe into the surface of the concrete.

The force used to penetrate the surface and the depth of the hole are correlated to the in-place concrete’s strength.

Pros: 

• Relatively easy to use and can be done directly on-site. 

Cons: 

• Data is significantly affected by surface conditions and the type of form and aggregates used.
• Requires pre-calibration using multiple concrete samples for accurate strength measurements.  

3.Ultrasonic Pulse Velocity(UPV) (ASTM C597) 

Method: This technique determines the velocity of a pulse of vibrational energy through a slab.

It is an effective method for quality control of concrete materials and detecting damages in structural components.

In this, the analysis is done on travel time taken by acoustic waves in a medium in correlation with elastic property and density of the material.

Some researchers have tried to develop a relationship between strength and wave speed.

Pros: 

• This is a non-destructive testing technique that can also be used to detect flaws within the concrete, such as cracks and honeycombing. 
• It can also be used to detect other sub-surface deficiencies.

Cons: 

• This technique is highly influenced by the presence of reinforcements, aggregates, and moisture in the concrete element.
• It also requires calibration with multiple samples for accurate testing.

4.Pullout Test (ASTM C900) 

The concept behind Pull-Out Test is that the tensile force required to pull a metal disk, together with a layer of concrete, from the surface to which it is attached related to the compressive strength of the concrete.

However, it can be used to evaluate the strength of concrete in existing structures.

Method: The main principle behind this test is the tensile force required to pull a metal disk, together with the concrete layer that is cast-in-place or post-installed in the concrete.

The pullout test is normally used for the early diagnosis of strength problems. 

Pullout testing involves attaching a small piece of equipment to the exterior bolt, nut, screw or fixing.

It is then pulled to the designated stress load level to determine how strong and secure the fixing is.

Pros: 

• Easy to use and can be performed on both new and old constructions.
• If a relationship to strength is established, the method can deliver the best test results.

Cons: 

• This test involves crushing or damaging the concrete.
• A large number of test samples are needed at different locations of the slab for accurate results.   

5. Drilled Core (ASTM C42) 

Method: A core drill is used to extract hardened concrete from the slab. These samples are then compressed in a machine to monitor the strength of the in-situ concrete. 

Pros: 

• These samples are considered more accurate than field-cured specimens because the concrete that is tested for strength has been subjected to the actual thermal history and curing conditions of the in-place slab. 
• The most reliable method with fast results generation.

Cons: 

• This is a destructive technique that requires damaging the structural integrity of the slab.
• The locations of the cores need to be repaired afterwards.
• Selecting test locations can be difficult. Selecting the best location of cores is relatively subjective.
• A lab must be used to obtain strength data. 

6.Cast-in-place Cylinders (ASTM C873)

Method: Cylinder moulds are placed in the location of the pour. Fresh concrete is poured into these moulds, which remain in the slab. Once hardened, these specimens are removed and compressed for strength.  

Pros:

• Results are considered more accurate than field-cured specimens because the concrete is subjected to the same curing conditions of the in-place slab, unlike field-cured specimens. 

Cons: 

• This is a destructive technique that requires damaging the structural integrity of the slab. The locations of the holes need to be repaired afterwards.
• A lab must be used to obtain strength data.

7.Wireless Maturity Sensors (ASTM C1074)

Method: This technique is based on the principle that concrete strength is directly related to its hydration temperature history. Wireless sensors are placed within the concrete formwork, secured on the rebar before pouring.

Temperature data is collected by the sensor and uploaded to any smart device within an app using a wireless connection.

This information is used to calculate the compressive strength of the in-situ concrete element based on the maturity equation that is set up in the app.  

Pros: 

• Compressive strength data is given in real-time and updated every 15 minutes.
• As a result, the data is considered more accurate and reliable as the sensors are embedded directly in the formwork, meaning they are subject to the same curing conditions as the in-situ concrete element.
• This also means no time is wasted on-site waiting for results from a third-party lab. 

Cons: 

• Requires a one-time calibration for each concrete mix to establish a maturity curve using cylinder brake tests.  

Combined Methods for testing:

Sometimes the combination of the above methods is also being used to ensure quality control and quality assurance of a concrete structure. Due to the use of combined methods, the accuracy of data strength also increases, as various parameters have taken into account for test and getting results out of them. 

For New Constructions

The concrete testing is also monitored and tested at the time of any new construction.

Construction engineers, project managers, and Quality Control and Quality Assurance auditors depend on the compressive strength test results.

When compression tests on concrete cylinders yield low breaks, engineers require reliable tools to assess the actual strength of concrete.

Maturity Method

The Maturity method is most common and adopted in this.

The maturity method is a technique that combined the effects of time and temperature on the strength development of concrete.

The maturity method provides a simple approach for evaluating the strength of cement-based materials in real-time, i.e., during construction.

The test procedure has been standardized in the ASTM C1074 – 19

The maturity method uses the history of temperature variation in concrete elements. Thermocouples (wired or wireless) are embedded within concrete, and the temperature variation of concrete during the curing process is monitored in real-time.

The Maturity Index is used to correlate test results from the Maturity test to compressive strength obtained from cylinder samples cured in laboratory conditions.

The relationship can be used to monitor strength development in fresh and early age concrete.

Choosing a Compressive Strength Method 

It is very difficult to select one of the best methods for the evaluation of concrete strength. As a result of all different methods are based on the use of different parameters, conditions, time is taken, and difficulty levels.

Tests like the rebound hammer and penetration resistance methods are easy to perform.

But these are considered less accurate in comparison to other testing methods.

This is because they do not examine the centre of the concrete element, only the curing conditions directly below the surface of the slab.

In addition, practices, whereas the ultrasonic pulse velocity method and the pullout test, are more difficult to perform as their calibration process is lengthy, requiring many sample specimens to obtain accurate data. 

So choosing out of one of the best and economical methods for testing is depends on many factors.

For example, Project timelines, the cost involved in the evaluation method, performance and durability of concrete structures or Insurer intention for undertaking these tests.

Once careful go through in all these will only help us in making a decision of best method adoption which will fulfil our purpose.

List of NABL accredited laboratories for Concrete test Click here

Also, read 

Slump Test

A slump test is a laboratory or on-site test, which is used to measure the consistency of concrete.

The slump test shows an indication of the uniformity of concrete in different batches.

The shape of the concrete slumps shows the information on the workability and quality of concrete. The characteristics of concrete with respect to the tendency of segregation can be also judged by making a few tamping or blows by tapping a rod on the base plate.

Cube Test

The concrete cube test is performed for the purpose of determining the compressive strength of a concrete element. The cubes used for this test have a dimension of 150 x 150 x 150 mm as long as the largest aggregate does not exceed 20 mm.

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