Concrete cores are essential for evaluating the actual properties of concrete in existing structures. These cores are used to conduct various tests, including strength, permeability, chemical analysis, carbonation, and more. While methods such as Rebound Hammer, CAPO/Pullout, Windsor probe, and ultrasonic pulse velocity tests provide indirect evidence of concrete quality, core sampling and testing offer a more direct assessment of concrete strength.
Core sampling involves extracting cylindrical samples from existing concrete structures for further testing. These samples are then subjected to various tests to determine their strength and other properties. This method provides a more accurate and reliable assessment of the concrete quality compared to indirect methods. It allows for a thorough evaluation of the actual condition of the concrete in the structure, providing valuable information for structural engineers and construction professionals.
Core testing is particularly useful in assessing the strength of the concrete, as it provides direct evidence of its compressive strength. This information is crucial for evaluating the structural integrity of the existing structure and determining its load-bearing capacity. Additionally, core testing can provide insights into other properties of the concrete, such as permeability, chemical composition, and carbonation, which are important factors in assessing the durability and long-term performance of the structure.
In summary, core sampling and testing are vital techniques used for evaluating the quality and properties of concrete in existing structures. Unlike indirect methods, core testing provides a more direct and accurate assessment of concrete strength and other properties. It is a valuable tool for assessing the condition of existing structures and making informed decisions about their structural integrity and performance.
Core Sampling and Testing of Concrete
Concrete cores are typically obtained by using a rotary cutting tool with diamond bits, resulting in a cylindrical specimen with uneven ends that are parallel and square, and may contain embedded pieces of reinforcement. The visual description and photography of the cores should pay specific attention to factors such as compaction, distribution of aggregates, and presence of steel. Subsequently, the cores need to be soaked in water and capped with molten sulphur to make their ends plane, parallel, and at right angles, before being tested for compression in a moist condition as per the standards BS 1881: Part 4: 1970 or ASTM C 42-77.
Apart from compressive strength and density determination, core samples can also be used for various other purposes, such as determining the depth of carbonation of concrete, conducting chemical analysis, evaluating water/gas permeability, performing petrographic analysis, and conducting ASHTO chloride permeability tests.
Fig: Instrument showing core cutting
Fig: Concrete Core
The strength of a concrete core test specimen is influenced by its shape, proportions, and size, particularly the height/diameter (H/D) ratio. It is well-established that the H/D ratio has an impact on the recorded strength of a cylinder, with a preferred ratio of near 2. When the H/D ratio is less than 1 or between 1 and 2, a correction factor must be applied. The use of cores with H/D ratios less than 1 is discouraged, with a minimum value of 0.95 specified in BS 1881: Part-4:1970. The standard also prescribes the use of 150mm or 100mm cores, but permits cores as small as 50mm. However, very small diameter cores, such as 50mm, tend to exhibit higher variability in results, making their use generally not recommended. In addition to the H/D ratio, the nominal size of stone aggregate is also considered when fixing the core size. The diameter of the core should be at least 3 times the maximum size of the stone aggregate. When the diameter of the core is less than 3 times the size of the stone aggregate, an increased number of cores must be tested.
Factors Affecting Strength of Concrete Cores
There are several factors that can have an impact on the compressive strength of extracted concrete cores. These factors include the age of the concrete, as the strength of concrete tends to increase over time due to continued hydration and curing. Another factor is the water-cement ratio used in the original mix, as a higher water-cement ratio can result in lower compressive strength due to increased porosity and reduced interlocking of the cementitious matrix. Additionally, the curing conditions, such as temperature and humidity during the curing period, can affect the development of concrete strength. Lower curing temperatures and higher humidity levels generally result in higher strength. The presence of any admixtures, such as accelerators or retarders, in the original mix can also impact the compressive strength of extracted concrete cores. Other factors that may influence core strength include the size and shape of the core, the presence of any defects or cracks in the core, and the quality of the coring process itself, including the drilling technique and equipment used. Proper handling, storage, transportation, and testing of the extracted cores are also critical factors that can affect the compressive strength results.
Size of stone aggregate
When the ratio of the diameter of the core to the maximum size of the stone aggregate in concrete is less than 3, a reduction in strength is observed. For instance, in a test conducted using a 50mm diameter core with 20mm size aggregate, the results were found to be 10% lower compared to tests conducted with 10mm diameter cores. This suggests that using larger diameter cores relative to the maximum size of the stone aggregate may result in decreased strength in concrete.
Presence of transverse reinforcement steel
According to reports, the inclusion of transverse steel in concrete cores can result in a reduction of compressive strength ranging from 5% to 15%. This reduction tends to be more significant in stronger concrete mixes, and is more pronounced when the transverse steel is located closer to the middle of the core rather than towards the ends. It should be noted that the presence of steel parallel to the axis of the core is generally considered undesirable due to its potential negative impact on the core’s performance.
H/D ratio
The previous discussion already covered the desired value range for the ratio, which should be between 0.95 and 2. If the ratio exceeds this range and becomes higher, it can result in a reduction in strength. It is important to keep the ratio within the specified limits to maintain optimal strength.
Age of concrete
The Concrete Society does not recommend age allowance as there is evidence that suggests in-situ concrete gains little strength after 28 days. However, there are other sources that propose an increase in strength after 28 days, with estimates ranging from 10% after 3 months to 15% after 6 months under average conditions. As a result, determining the effect of age on core strength is not straightforward due to conflicting opinions on the matter.
Strength of concrete
The reduction in core strength has been found to be more significant in stronger concretes, with a reported reduction of up to 15% in the case of 40 MPa concrete. However, a reduction of 5% to 7% is generally considered to be acceptable and reasonable.
Drilling operations
Cores are typically weaker than standard cylinders due in part to the potential disturbance caused by vibrations during drilling operations. Despite taking the best precautions during drilling, there is always a risk of slight damage to the cores. This is because even with proper precautions, vibrations generated during drilling can impact the structural integrity of the cores, resulting in reduced strength compared to standard cylinders. Therefore, it is important to be mindful of the potential for damage to cores during drilling operations, and take appropriate measures to minimize the risk and ensure their structural integrity.
Site conditions vis-a-vis standard specimens
Site curing of concrete is typically found to be less effective compared to the curing methods prescribed for standard specimens. As a result, the in-situ core strength, which is the strength of concrete samples taken from the actual construction site, tends to be lower than the strength of standard specimens that are tested during the concreting process. This discrepancy in strength can be attributed to the differences in curing techniques employed on site compared to those used for standard specimens.