Sampling
Site Sampling and Testing of Concrete: Importance and Procedures
Concrete strength testing is a critical process in construction projects that must meet specific design requirements. Cubes play a vital role in such testing to ensure that the concrete strength complies with the specified requirements. However, for the results of cube testing to be meaningful and accurate, it is crucial to follow proper sampling and testing procedures. These procedures determine the accuracy of the results and ensure that the concrete is suitable for its intended purpose.
Procedure: Composite Sample
For accurate testing of a concrete truck load, it is crucial to obtain a representative sample that is indicative of the entire load. Standard sampling procedures involve collecting scoopfuls from four different parts of the load, which are then combined in buckets. The sampling process should take place as the concrete is discharged, with scoopfuls taken through the moving stream to ensure that the entire width and depth of the load is sampled. The size of the sample collected should be 1.5 times larger than the estimated volume required for testing. The sampling process should involve taking the first and last concrete, with scoopfuls taken from part 1 to part 4 in between. By following these procedures, the collected sample will be more representative, allowing for more accurate testing and analysis of the concrete's properties.
Alternative Method
In addition to standard sampling procedures, an alternative method of spot sampling can be used for an early check on the consistency of concrete delivered in a truck mixer. With this method, scoopfuls are taken through the moving stream, sampling the entire width and depth of the concrete. The sampling process involves letting the first concrete go, then taking six scoopfuls from the next 0.3 m3 of concrete discharged. After the first 0.3 m3 has been discharged, an additional six scoopfuls are taken. It is important to note that additional tolerances will be applicable for concrete sampled using this alternative method. While this method can provide an early indication of concrete consistency, it is not a substitute for standard sampling procedures to ensure that the sample is representative of the entire load.
Method for Making Concrete Cubes
Making concrete cubes involves following a specific procedure to ensure accurate testing of concrete strength. The first step involves collecting a composite sample using the standard method of sampling described above, and not using concrete sampled using the alternative spot method. The following steps outline the procedure for making concrete cubes:
Step 1:
Ensure that the moulds are clean and lightly oiled, with all bolts tightened to prevent leakage.
Verify that the correct halves of the moulds are used, and the corner lifting pins are located correctly.
Thoroughly remix the sample, as described for the slump test.
Step 2:
Fill the mould with concrete in 50 mm layers, using the tamping bar to compact the concrete.
Use no fewer than 25 tamps for each of the two layers in a 100 mm mould and no fewer than 35 tamps for each of the three layers in a 150 mm mould.
For high-workability concrete, fewer tamps may be needed.
Step 3:
After tamping each layer, tap the sides of the mould with a hide hammer until large bubbles of air cease to appear on the surface and the tamping bar holes are closed.
Step 4:
Remove surplus concrete and smooth it over with a float.
Wipe the mould edges clean, number the moulds for identification, and record details.
Step 5:
Cover each mould with a damp cloth and plastic sheet.
Store the moulds inside at room temperature (15-25°C), such as on top of the curing tank.
Protect the cube moulds at all times from high and low temperatures (especially sub-zero temperatures) and drying winds.
Complete the sampling and cube-making certificates.
By following this procedure, accurate testing of concrete strength can be achieved, ensuring the concrete's suitability for its intended purpose.
Method for Storing Cubes
When making concrete cubes, it is essential to store them properly for accurate testing results. Follow these steps for storing concrete cubes:
Step 1: After casting, remove the cubes from the moulds within 16 to 72 hours.
Step 2: Slacken all nuts and gently tap the sides of the mould to part them. Carefully lift off the moulds and avoid damaging the new cubes.
Step 3: Mark each cube with its identification number on two of its cast sides.
Step 4: Place the cubes in a curing tank and clean and reassemble the moulds.
Step 5: Make sure the water temperature is controlled at 20°C ± 2°C, and the cubes are completely covered with water. Ensure that the power supply is not switched off day or night. Check the temperature range daily, using the maximum/minimum thermometer, and keep a record of the readings.
Step 6: For despatch to the test laboratory, wrap the wet cubes in damp cloths, then plastic bags and pack them in trays. Attach the certificates for sampling, testing, cube making, and storage to the package, along with the order for testing. Proper storage of concrete cubes is crucial for accurate testing results.
Importance of sampling and testing concrete
Concrete testing and sampling are critical procedures performed daily at precast plants and construction sites. The collected samples are used to test the concrete’s slump, temperature, density, air content and prepare specimens for testing the concrete’s compressive strength. Obtaining a representative sample is critical for accurate testing and reliable results which are used to demonstrate product compliance. If the sample collected is not truly representative, then the test results will not reflect the characteristics of the product made with that concrete.
Various standards outline the procedure for obtaining a representative sample when using different types of mixing and conveyance equipment. ASTM C172, “Standard Practice for Sampling Freshly Mixed Concrete,” or EN 12350-1 are standard practices for sampling freshly mixed concrete.
Sampling should be performed when the fresh concrete is delivered from the mixer to the conveyance equipment used to deposit the concrete into the forms, and the sample should be protected from the elements like sun, wind, rain, snow, temperature fluctuations, sources of rapid evaporation or unintended added moisture, and sources of contamination.
A representative sample will consist of two or more portions of fresh concrete combined into a single composite sample, and the elapsed time between collecting the first and final portions of the composite sample must not exceed 15 minutes. Allowing more than 15 minutes could result in a non-representative sample due to segregation.
Collecting the portions of fresh concrete in a fluid motion helps ensure the concrete is homogeneous and representative of the entire batch. It is also crucial to ensure the concrete flow is not restricted when dispensing from the mixer, chute, or transportation equipment. This could result in segregation and may alter the homogeneity of the concrete collected.
In conclusion, collecting a representative sample of concrete is a critical procedure to ensure the validity and accuracy of test results. It is crucial to follow the procedures outlined in ASTM C172, EN 12350-1 and other relevant standards for obtaining a representative sample to avoid non-representative samples due to segregation or other factors. By ensuring the collection of a truly representative sample of concrete, we can guarantee the production of high-quality concrete products that meet the requirements of our clients.