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5 Methods for On-Site Evaluation of Concrete Strength

On-site Evaluation of Concrete Strength is a main challenge in the condition assessment of existing infrastructure, or the quality control of new construction. Owners, maintenance managers of such existing concrete structures normally prefer non-destructive and non-intrusive methods to avoid further damage to an already struggling structure. In construction projects, switching to non-destructive means less intervention, shorter down-times, and saving money. However, all parties agree that the strength of concrete is a critical parameter. In this article, we will review the potential options and practical solutions for on-site evaluation of concrete strength. 

On-site Evaluation of Concrete Strength

Concrete strength (compressive strength) is by far the most important property of concrete. It represents the mechanical characteristics of concrete; The 28 days compressive strength of concrete cylinders or cube samples has widely been accepted as the minimum specified concrete strength in most design codes (ACI 318-14, CSA A23.3-14). Concrete Strength is also considered a key factor in obtaining desired Durability Performance.

Evaluation of Concrete Strength is an important task:

  • Existing Structures: Concrete Strength is of special interest for engineers involved in the Structural Condition Assessment of concrete structures. It is used to assess the mechanical characteristics and durability performance of concrete.
  • New Construction: Concrete Strength is usually monitored during the construction process. Construction engineers, project managers, and Quality Control and Quality Assurance auditors depend on the compressive strength test results. When compression test on concrete cylinders yield low breaks, engineers require reliable tools to assess the actual strength of concrete.

Nondestructive testing (NDT) offer an interesting approach to evaluating the compressive strength of concrete. NDT methods provide access to material properties while remaining rapid and of moderate cost (Breysse, 2012).  The following article will take a quick look at some of the major non-destructive testing solutions for on-site evaluation of concrete strength. In the first part, we will present and discuss NDT methods for evaluating concrete strength in existing structures. In the second part, we will present and review the NDT methods for evaluation of early age strength of concrete.

Part I - Existing Structures

1- Compression Test On Concrete Cores


Extracting concrete samples (Read More: Challenges of Concrete Coring) and testing for compressive strength is often considered the most cost-effective and most reliable solution. In fact, many codes and guidelines consider this the only approved method for evaluating concrete strength. In this case, concrete core is taken from the existing structure.

The core needs cutting (sawing) and surface preparation. The core is then tested for compressive strength. However, the reality is far from this. There are certain questions that needs to be answered: Where to take concrete cores from? How to handle cores properly (maintain moisture, safe mobilization)? How many cores will yield reliable information?

Pros

  •  This is the most reliable method to estimate the compressive strength. The method is relatively fast.

Cons

  •  It is destructive. Not only it damages concrete integrity, it might affect reinforcing bars in RC structures. Rebar locating tools, such as Ground Penetrating Radar – GPR are needed to avoid this problem.
  •  Selecting test locations can be difficult. Selecting the best location of cores is relatively subjective.
  •  The locations of cores needs to be repaired.
  •  Coring is not an option for owners of important structures, especially when there are concerns about further damaging the structure.

Concrete Coring and Sampling

2- The Pull-Out Test

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, is related to the compressive strength of the concrete.The pull out test is normally used for early diagnosis of strength problems. However, it can be used to evaluate the strength of concrete in existing structures. Pull out testing involves attaching a small piece of equipment to the exterior bolt, nut, screw or fixing. This is then pulled to the designated stress load level to determine how strong and secure the fixing is.

Pros

  • Relatively easy to use
  • If relationship to strength is established, the mothod can deliver robust test results.

Cons

  • Pull-Out test often involves crushing and damaging concrete

3- Rebound Hammer For Concrete Strength 

The Rebound Number of Hardened Concrete (see ASTM C805) can be used to assess:

  • In-place uniformity of concrete,
  • to delineate variations in concrete quality throughout a structure, and
  • to estimate in-place strength if a correlation is developed (Read More)

The Rebound Hammer works based on the rebound principle, and consist of measuring the rebound of a spring driven hammer mass after its impact with concrete. New versions of the test have been commercialized and are used to help engineers and inspector with a wider range of material properties.

Due to its simplicity and low cost the rebound hammer is the
most widely used nondestructive test for concrete. It is frequently used-although by mistake- as a tool to assess the strength of concrete. Malhotra (2004) argues that “there is little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer. However, within limits, empirical correlations have been established between strength properties and the rebound number. ACI 228.1R describes a standard procedure to calibrate test results for every specific project, and use project-specific correlation to assess the strength. This will minimize the number of intrusive tests.

Pros

  • It is easy to use for most field applications.
  • The test can be used to study the uniformity of concrete

Cons

  • surface condition, presence of rebar, presence of sub-surface voids can affect the test results

4- Ultrasonic Pulse Velocity


Ultrasonic Pulse Velocity (UPV) is an effective method for quality control of concrete materials, and detecting damages in structural components.

The UPV methods have been traditionally used for the quality control of materials, mostly homogeneous materials such as metals and welded connections. With the recent advancement in transducer technology, the test has been widely accepted in testing concrete materials. The test procedure has been standardized as “Standard Test Method for Pulse Velocity through Concrete” (ASTM C 597, 2016).

The concept behind the technology is measuring the travel time of acoustic waves in a medium, and correlating them to the elastic properties and density of the material. Travel time of ultrasonic waves reflects internal condition of test area. Some researchers have tried to develop a relationship between the strength and wave speed.

Pros

  • UPV can be used to detect other sub-surface deficiencies

Cons

  • The method is affected by presence of rebar, voids, and cracks.
  • There is no enough results for assessing the reliability of the method in the field.

5- Combined NDT Methods

As we discussed above, Rebound Hammer and Ultrasonic Pulse Velocity are  the most widely used NDT methods for evaluating concrete strength in existing structures (Malhotra, 2004)

Combined methods involves a combination of NDT methods for predicting the on-site strength of concrete. The combination of UPV and Rebound hammer has been studied by several researcher. Breysse, 2012 have conducted a comprehensive literature review on the combined methods.

The improvement of the accuracy of the strength prediction according is achieved by the use of correction factors taking into account the influence of cement type, cement content, petrologic aggregate type, fine aggregate fraction, and aggregate maximum size. The accuracy of the combination of rebound hammer and ultrasonic pulse velocity results in improved accuracy in estimating the compressive strength of concrete (Hannachi and Guetteche, 2012).

It is very important to consider that the accuracy of each and every relationship depends on the calibration and correlation that is made with destructive tests (core samples). While combined methods still rely on intrusive tests to yield accurate outcome, they have a huge potential to reduce the number of destructive tests on a job site.

Part 2 - New Construction

1- The Maturity Method

The maturity method is a technique to account for the combined effects of time and temperature on the strength development of concrete.” (Carino and Lew, 2001). 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

Maturity method uses the history of temperature variation in concrete elements. Termocouples (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 Maturity test to compressive strength obtained from cylinder samples cured in laboratory condition. The relationship can be used to monitor strength development in fresh and early age concrete.

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28 thoughts on “5 Methods for On-Site Evaluation of Concrete Strength”

  1. Francesc servos Ros

    Dear Mr.
    For a long time, I have been studying the easiest and most reliable way to control the strength of the concrete and come to the following conclusions:
    -The test pieces will be cubic 10x10x10cm, very manageable, light weight, take up little space, do not have to modify and little final residue.
    -The machine for the breaks, may be less capacity, but cheaper.
    -Use heat curing with wet temperature and you can create a curve of resistances in the time obtaining a very sure estimate of the resistances at each age.
    Regards

    1. The UPV is not suitable on its own. It is more closely related to the stiffness and density of the concrete than it is to strength (Chirgwin, Thesis for Masters of Engineering Science, University of Sydney, 1988). It is also affected by the saturation of the concrete. Also, the correlation to strength has such wide error bands that it is difficult to tell a 50 MPa concrete from a 30 MPa concrete.
      The other, unmentioned issue with coring is the restriction on coring sites. Congested areas may be the most problematic for the concrete quality, but the most problematic for strength.
      You have omitted a critical method for estimating the concrete strength gain in-situ during construction: The Maturity Method. Preferably, a series of strength curves are set up ahead of time to check the strength gain under standard, accelerated and retarded conditions over 28 or 91 days. The maturity of the concrete is then assessed using a maturity meter (See J. J. Carino “CN Tower” circa 1976). Samples should be taken, cured and tested at suitable intervals (1, 3 and 7 days) to check that the concrete is the approved mix.

  2. The windsor probe worked well for Florida Aggregates–Very accurate
    When we as a Ready Mix Producer had suspect strengths, we used the tested the concrete with a windsor probe. If the windsor probe told us the concrete met design strength it met design strength.

  3. The fact is that all 5 methods are not any good to evaluate the Concrete Strength. Now one is good enough !
    In year 2017 soon coming is it a shame that we do not have better methods in construction.
    We need to try harder in the coming years. (Did not 3 men go to the moon already in 1969?)
    Let´s hope everyone working in the concrete business will start to “think outside the box” and that we try to improve many type of tests that we today use. The Average Strength, now known in the middle of a structure, is only one parameter. Surface or the upper one inch concrete quality is also important, some times more important. How do we measure that in a simple way in a bridge deck or in an other wearing surface? Any idea?
    Happy new year! Yngve

    Yngve

    RolliT

    1. Dear Yngve,
      Many engineers in Europe and USA are useing the pull-out method to assess strength of existing structures (slabs, piers, beams, columns) succesfully. It is recomended to use the method by making correlations to cores of the same structure (as this publication specifies). However, there is documentation on how the method is very reliable even when using the manufacturer given correlation, wich was done by testing data of all sorts of concrtes and different laboratories. In my mind, pull-out is very well correlated to cylinder/cores because the failure mecanism is very similar.

  4. Designing concrete structures requires substituting f’c in empirical formulae. There is only one way to get this value – compressive test on samples. All other methods may only be correlated with compressive test but their results cannot be substituted in the formulae. It is a matter of engineering judgement if f’c from NDT correlations may be used to verify design assumptions. The best design may fail. In that case, if the Engineer of Record (EOR) has used method prescribed by Codes to assess f’c, then the EOR will not pay. The engineering judgement will be questioned by the court, and EOR may pay. Hence, any NDT, both existing and developed in the future, have very limited value in formal cases. We can use them in many cases to get general feeling of the situation, but not more than that. Publication in a good journal may be considered by a court, but a good attorney may always question its validity. And EOR may pay. To risk paying a judgement or not to risk is personal choice of the EOR. So in the world of real construction NDT is useful in very limited cases and never for formal assessment f’c. In academic world, NDT is very useful for getting points for publications. But this is a very diffirent story.

  5. It seems really important to me that you would want a site evaluation, especially when it comes to the strength of your concrete. A couple of years ago, I was walking on the sidewalk of privately owned property and it ended up caving in once I stepped on it! That is definitely a no-go and I can imagine how you would want to prevent that with a good evaluation first!

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  8. Dear All,

    Should be there a correlation and calibration of the rebound hammer device with each specific type of concrete mix for the evaluation of strength ? Is there any specific method for this type of calibration linking rebound hammer device and a concrete mix design ?

    Thank you in advance.

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  12. I like your suggestion about concrete strength evaluation. It’s a good thing that you mentioned ways on how to do it, especially that the strength is considered as a key factor for durability performance. If I were to work in a building construction project, I’ll hire a reputable concrete delivery service that can make our work easier. There will be no need for us to do the mixing process and other stuff. I will also do your suggested concrete evaluation methods. Thanks!

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  15. Hi,
    Some of our structures compressive strength result was failed. When we do the hammer test its also fail, then we went for ultra sonic test .Based on ultra sonic test most of the structures is passed.

    Can we trust the ultra sonic test?

    If some one have experience please guide.

  16. this all are after the construcion work done.so is there any method by which we can determine the strength of concrete in 30 minutes or one hour.

  17. I think your ‘cons’ concerning the Rebound Hammer may be a bit overstated. Using the rebound hammer over a large area ensures that there is a lager data set to sample

  18. Kumavat Hemraj Ramdas

    In case of rebound hammer strength, it is essential to calibration and reliabikt of results considered for exact relationship between rebound number snd compression strength. Otherwise more variability obtained in results due to influence factor ( cement type, compaction and curing methods, temperature variations, water to cement ratio)

  19. This blog happens to be one of the best blog on on-site evaluation of concrete strength, and both limitations and delimitations of it. It is just a proper critique blog, would suggest to others as well. This is a great addition tips blog, thank you for sharing this article.

  20. I’ve been planning to hire a service that will be able to run an array ultrasonic examination on the apartment building that is currently being constructed. I agree with you that concrete strength revelation is beneficial in terms of assessing mechanical characteristics. It’s also interesting to learn that the rebound number of hardened concrete could also help estimate the concrete’s strength.

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