When choosing a methodformonitoringthe compressive strength of concrete,it’s importantfor project managerstoconsider theimpacteachtechnique will have ontheirschedule.While some testing processes can be done directly onsite, others require extra time for third-party facilities to deliver strength data. Time is not the only factor thatcontributes to project managers’decisions.The accuracy of the testing process is just as important, as it directlyeffects the quality of the concrete structure.
The most common methodformonitoring the strength of in-situ concrete is the use of field-cured cylinders. This practice has remained generally unchanged since theearly 19thcentury. These samples arecast and cured according to ASTM C31andtested for compressive strengthby a third-party labat various stages.Usually, if theslabhasreached 75% of its designed strength, engineers will give the go aheadtotheir team to move on tothe next steps in the construction process.
There have been many developments to speed up the curing process since this testing method was first introduced. This includes the use of heating blankets, additives and vapor retarders. However, contractors still wait three days after placement before testing for strength, even though their targets are oftenreached much earlier than that.
Despite knowing that, many project managers prefer to stick to this testing practice because it’s “the way its always been done.” Thatdoesn’t meanthis techniqueis the fastest and most accurate method, however, for testing the strength of all placements.In fact, there are many different practices aside from cylinder break tests that can be used.Here are seven differentapproachesto consider when choosing a method of strength testing.
Methods for Testing Compressive Strength of Concrete
1. Rebound Hammeror Schmidt Hammer (ASTM C805)
Method: Aspring release mechanismis used to activate a hammer whichimpacts a plunger to drive into the surface of the concrete. The rebound distance from the hammer to the surface of the concreteis given a value from 10 to 100. This measurement is then correlated to the concretes’ strength.
Pros:Relatively easy to use andcan be done directly onsite.
Cons:Pre-calibration using cored samples is required for accurate measurements.Test results can be skewed by surface conditionsandthe presence of large aggregatesor rebarbelow the testing location.
2. Penetration Resistance Test (ASTM C803)
Method:To complete a penetration resistance test,a device drives a small pinor probe intothe surface oftheconcrete.Theforce used topenetrate the surface,andthedepth of the hole, is correlated to the strength of the in-place concrete.
Pros: Relatively easy to use and can be done directly onsite.
Cons:Data is significantly affected by surface conditions as well as the type of form and aggregates used.Requires pre-calibration usingmultiple concrete samples for accurate strength measurements.
3. Ultrasonic Pulse Velocity (ASTM C597)
Method:Thistechniquedetermines thevelocity of a pulse of vibrational energy throughaslab. Theease at whichthisenergymakes its’ way through the slabprovides measurements regarding theconcrete’s elasticity, resistance to deformation or stress, and density. This data is then correlated tothe slab’sstrength.
Pros:This is a non-destructive testing techniquewhichcanalsobe used to detect flaws within the concrete, such as cracks and honeycombing.
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)
Method:The main principal behind thistestis to pull the concrete using a metal rod that is cast-in-place or post-installed in the concrete. The pulled conical shape, in combination with the force required to pull the concrete,iscorrelated to compressive strength.
Pros:Easy to use and can be performed on both new and old constructions.
Cons:This testinvolves crushing or damagingthe concrete. A large number oftest samples are needed at different locations of the slab for accurate results.
5.Drilled Core (ASTM C42)
Method:A core drill is used to extracthardened concretefrom the slab.These samples are then compressedin a machine to monitor the strength of the in-situ concrete.
Pros:These samples are considered more accurate than field-cured specimens because theconcrete that is tested for strength has been subjected to the actual thermal history and curing conditions of the in-place slab.
Cons: This is adestructivetechnique that requires damaging the structural integrity of theslab. The locations of the cores need to be repaired afterwards. A lab must be used to obtain strength data.
6.Cast-in-place Cylinders (ASTM C873)
Method:Cylinder molds are placed in the location of the pour. Fresh concrete is poured intothese molds whichremainin the slab. Once hardened, these specimens are removed and compressed for strength.
Pros: Is considered more accurate than field-cured specimens because the concrete issubjected to the same curing conditionsof 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.WirelessMaturity Sensors(ASTM C1074)
Method:This technique isbased 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 collectedby the sensoranduploaded to any smart device withinanappusing a wireless connection. This information is used to calculate the compressivestrength of the in-situ concrete element based on the maturity equation that is set up in the app.
Pros: Compressive strength data isgiven 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 wastedonsitewaiting for results from a third-party lab.
Cons: Requires a one-time calibration for each concrete mix to establish a maturity curveusing cylinder break tests.
More On Concrete Testing
How to Get the Best Results from Concrete Core Testing - Understanding the factors that affect core testing results will help you achieve accurate in-place concrete strengths.
What Contractors Need to Know about Field Testing Concrete - Project delays can be avoided by correctly field testing concrete products using appropriate methods and procedures.
Who Pays for Additional Testing? When an engineer, owner or general contractor requests extra testing for defects on a concrete job, know the facts on contractor responsibility for payment of those tests.
Concrete Densifiers Performance Test - Read about an industrial densifier/hardener performance test conducted by the CTL Group and Mark Wetherell. The test measured abrasion resistances of four different densifier types in accordance with ASTM C779-05.
Four ways concrete testing software - or a construction materials testing platform - can make the concrete testing process easier and more accurate for concrete contractors.
Combined Methods of Strength Testing
A combination of these methods for measuring the compressive strength is sometimes used to ensure quality control and quality assurance of a concrete structure. A combined method results in a more comprehensive overview of your slab, allowing you to confirm strength data by using more than one testing method. The accuracy of your strength data will also increase as using multiple methods will help account for influencing factors, such as cement type, aggregate size, and curing conditions. For example,a combination of the ultrasonic pulse velocity method and the rebound hammer testhas been studied. Similarly, when using the maturity method on your jobsite to test compressive strength, it is recommended to perform cylinder break tests on day-28 of your concrete’s lifecycle for acceptance purposes and to confirm the strength of your in-situ slab.
Choosing a Compressive Strength Method
Tests like the rebound hammer and penetration resistance technique, while easy to perform, are considered less accurate than other testing methods (Science Direct). This is because they do not examine the center of the concrete element, only the curing conditions directly below the surface of the slab. Practices, such as the ultrasonic pulse velocity method and the pullout test, are more difficult to perform as their calibration process is lengthy, requiring a large number of sample specimens in order to obtain accurate data.
Your decision in choosing a testing method may simply come down to what you know and are used to. However, the accuracy of these tests and the time they take to obtain strength data are significant factors that are not always taken into consideration as heavily as they should. Think about where all of your time and money goes during the construction of a project. How much of that is spent on repairs, fees for testing labs and extra labor to make sure your project finishes on time? The accuracy of the technique you choose can lead to future durability and performance issues of your concrete structure. Furthermore, choosing a technique that takes additional time to receive strength data can be detrimental to your project deadlines, negatively impacting productivity on your jobsite. Conversely, choosing the right tool can positively impact project timelines and allow you to finish the project below budget. How do you decide which strength testing method to use?
Editor’s Note: This article was contributed by Giatec Scientific Inc.