Concrete testing is crucial in deciding various in-situ qualities, including durability, sturdiness, and condition before the product is used in construction. It provides critical information about the properties of the piece, which is essential for diagnostic planning as well as evaluating service performance.
Concrete Testing For Hardened Concrete
Measuring the compressive strength of concrete, its tensile strength, and its elasticity are crucial parts of the Quality Assurance process. These three aspects determine the safety and integrity of a structure.
Tensile Strength Testing
Tensile concrete testing prevents structural cracks by determining the maximum tensile force the material can withstand before cracking. While concrete is one of the most robust building materials available, it is also highly brittle and unable to withstand high tension levels. The brittle quality of concrete has a significant effect on the size and degree of fractures in buildings, resulting in tensile forces exceeding tensile strength.
Elasticity Testing
Elasticity testing is the measurement of concrete’s elasticity. Elastic modulus quantifies concrete’s resistance to non-permanent, or elastic, deformation. When under stress, concrete will first exhibit elastic properties: the stress causes them to deform, but it will return to its previous state after the stress is removed.
The tensile strength testing results can also be used to determine the sample’s modulus of elasticity. However, manufactures can also perform an independent flexure test to assess the sample’s resistance to bending forces.
During testing, the outcome is determined based on the consolidated impact of tensile, compressive, and shear stresses, as well as the shape of the sample and the rate that the pressure is added.
Compressive Strength Testing
Compressive strength testing gives a better understanding of the condition of a given sample. It is used to assess the mechanical characteristics and durability performance of concrete.
Compressive strength testing can be completed on both manufactured concrete samples and full-scale models, and can be done using destructive or non-destructive methods.
Destructive concrete testing methods are explained in detail in the next section.
Destructive Concrete Testing
The compressive strength of concrete is this material’s most critical quality because it identifies its mechanical properties. Testing this characteristic is an essential component in determining its durability when used in a structure. In developed countries, testing the compressive strength of concrete is standard practice for precast and site-cast concrete.
Destructive testing is just one method for analyzing the compressive strength of concrete and identifying any flaws that may not be obvious under ordinary circumstances. This method encompasses any process where samples are broken to determine the concrete’s strength and hardness.
Conventional destructive testing is the “gold standard” method for measuring the compressive strength of concrete and masonry. When testing the compressive strength of concrete, structures made from the same batch of concrete can reasonably be assumed to have approximately the same durability.
Destructive testing can also be used to measure the compressive strength of concrete in-situ. This process promotes:
- Routine monitoring of the infrastructure’s structural integrity
- Testing structures for which data about the strength of materials is incomplete, unavailable, or needs to be corroborated
- Assessment of structural integrity after a seismic event or other events
- Monitoring the effects of corrosion or other forms of degradation that happen with aging
- Detection and evaluation of manufacturing anomalies
- Inspections during and after construction projects to verify compliance with design specifications and standards
This method of testing can be applied directly to:
- Coupons or cores cut from larger concrete or masonry elements
- Testing cylinders from large batches
- Standard concrete and masonry units
Compressive Strength Testing Of Core Samples
Removing cores from a sample of concrete is widely considered one of the most dependable processes to evaluate the material’s compressive strength. However, this type of concrete testing has several drawbacks, with the risk of weakening the material’s integrity being one of the most significant.
Additionally, finding a location to perform the test can be challenging, since finding the most suitable place to cut the cores is largely subjective. And in the case of significant structures that could incur additional damage, coring may not be an option.
When removing a core to assess the compressive strength of concrete, the sample should be three times the nominal maximum size of the entire structure. Although guidelines regulating the size of cores vary throughout the world, 100 mm is the typical size in most concrete testing.
Compressive Strength Testing Of Cylinders
Concrete cylinders are a common way of performing concrete tests. These cylinders are prepared following ASTM C1758 and ASTM C39 (or similar standards), with at least four cylinders tested for every 150 cubic yards (115 cubic meters) of concrete or once per week, whichever occurs first. To find the compressive strength of concrete, cylinders must be capped or ground to ensure that the compression force is distributed evenly across the cylinder under test.
Two cylinders are tested after 7 days of curing, and the remaining two are tested after 28 days of curing.
Precast and site-cast concrete undergo similar quality assurance testing. However, the difference lies in the testing frequency. To test precast concrete, two cylinders cure for 1 week, while the other two are tested 28 days after curing. Conversely, site-cast concrete is tested every 3 days, 7 days, 28 days, and 90 days.
Compressive Strength Testing Of CMUs
Testing for concrete masonry units (CMUs) typically involves destroying a CMU sample consistent with ASTM standard C140 or similar. The testing process generally includes capping the CMU undergoing testing, so the compression machine presses on a smooth and flat surface, and the compressive force is distributed evenly across the sample under test.
Five CMUs are tested, and the average of the strength measurements is taken to represent the remainder of the CMUs. Prisms are constructed from multiple CMUs in some circumstances to measure the strength of CMUs in combination with mortar. In situations when the CMU is too large to fit into the compression machine, a “coupon” is cut from a CMU.
Advantages and Challenges of Destructive Concrete Testing
As stated earlier, destructive concrete testing is an industry-standard for adhering to Quality Assurance guidelines. Its advantages include:
- Affordability: Relative to non-destructive testing, destructive concrete test equipment is cost-efficient
- Accuracy: It exposes the material’s mechanical characteristics, including its modulus of elasticity, strength, and tensile strength
- Reliability: It lowers the frequency of structural failures, accidents, and expenses
However, conventional destructive concrete testing also has several drawbacks, namely:
- While it is an affordable option, it may not be cost-effective for remote or low-resource regions because of the cost of acquisition, installation, and maintenance. The cost starts at about US$10,000 per machine and goes up as additional features are added
- It requires operators to have specialized training and protective gear
- Selected samples must be transported to the testing facility, which may be far from the worksite
- Proper preparation requires capping or grinding of the sample under test to ensure consistent coupling between the piece and the compression machine, which requires a high degree of training and can be time-consuming
- The test samples are destroyed, when otherwise they could have been sold and used
- Debris from destroyed samples must be cleaned up and discarded on completion of each test
The disadvantages of destructive testing may mean it is not an appropriate method of testing in situations where a worksite is remote or has limited resources or smaller companies. In these cases, Non-Destructive Testing would be a better alternative for testing the compressive strength of concrete. Read more about Non-Destructive concrete testing in our blog on Measuring The Compressive Strength Of Concrete With Non-Destructive Testing Methods