There are many different practices aside from cylinder/cube break tests that can be used.
1. Rebound Hammer or Schmidt Hammer [IS 13311-2 (1992)]
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.
Cons: Pre-calibration using cored samples is required for accurate measurements. Test results can be skewed by surface conditions and the presence of large aggregates or rebar below the testing location.
2. Penetration Resistance Test (IS: 8142 – 1976, Reaffirmed 2002)
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 is correlated to the strength of the in-place concrete.
Pros: Relatively easy to use and can be done directly on site.
Cons: Data is significantly affected by surface conditions as well as the type of form and aggregates used. Requires pre-calibration using multiple concrete samples for accurate strength measurements.
3. Ultrasonic Pulse Velocity [IS 13311 (Part1):1992, Reaffirmed 2004]
Method: This technique determines the velocity of a pulse of vibrational energy through a slab. The ease at which this energy makes its’ way through the slab provides measurements regarding the concrete’s elasticity, resistance to deformation or stress, and density. This data is then correlated to the slab’s strength.
Pros: This is a non-destructive testing technique that can also be 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. Pull-out Test [IS 11309 (1985)]
Method: The main principle behind this test is 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, is correlated to compressive strength.
Pros: Easy to use and can be performed on both new and old constructions.
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.
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. 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: Is 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, and 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.