By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jun 14 2024A recent article published in Scientific Reports proposed a non-destructive cut and pull out (CAPO) test to estimate the in situ compressive strength of concrete made using machine-crushed brick aggregates. The results of the CAPO test were found to correlate with those from conventional core and cylindrical tests.
100 mm × 200 mm Mould Preparation for Cylinder. Image Credit: https://www.nature.com/articles/s41598-024-59560-z
Background
Various methods are used to evaluate the compressive strength of concrete in its applications, with pullout testing being one of the popular approaches. This method is divided into two main types: the LOK (punch out) test, which involves inserting a disc into freshly mixed concrete, and the CAPO test, which uses an enlarged ring positioned in an undercut recess in hardened concrete.
The CAPO test is considered non-destructive as it minimally damages the concrete surface and preserves its structural integrity. It employs an empirical correlation to indirectly determine the in situ compressive strength of concrete, based on parameters measured during the test.
In regions where stone chips are scarce and demand is high, crushed brick chips (BC) are often used in concrete construction as a cost-effective alternative. This study evaluates the effectiveness of the CAPO test in determining the strength of concrete made with machine-crushed brick aggregates, potentially reducing the manufacturing costs of high-quality concrete.
Methods
The researchers undertook a comprehensive examination prior to testing, which included reviewing the types of tests, the concrete's age at testing, the aggregates used, and the desired strength levels. They prepared three concrete mixes with target strengths of 20 MPa, 25 MPa, and 30 MPa and tested them at intervals of 14, 28, 56, and 90 days.
Two different categories of coarse aggregates were used to form six mix ratios. Clay-burnt bricks with an average strength of 25.3 MPa were selected for the brick aggregates. Adjustments were made to the concrete mixture to maintain the required slump before the freshly mixed concrete was poured into various molds.
For each target strength, twelve 100x200 mm2 concrete cylinders were cast from every batch, totaling 180 cylinders (48 for each brick aggregate mix) prepared for lab testing. Additionally, fifteen 600x600x225 mm^3 concrete blocks were cast for conducting both core and CAPO tests.
All concrete blocks and cylinder specimens underwent identical curing processes. They were covered with wet bags and polythene to retain moisture, compacted using a vibrator, and placed in an exposed area for curing. After curing, four CAPO tests were performed on each concrete block to extract four 100x200 mm2 cores for the core tests. The study also included a thorough investigation of material properties such as cement characteristics, particle size distribution, fineness modulus, specific gravity, and absorption capacity, using standard testing methods.
Results and Discussion
The established correlation from the test data revealed a direct relationship between the force applied by the CAPO device and the cylinder's compressive strength. A linear relationship was observed, with the R2 value ranging from 0.95 to 0.99 depending on the concrete's age.
The relationship derived from the test data defined the relationship between CAPO force and cylinder strength as Y=1.6208X−2.3643Y = 1.6208X - 2.3643Y=1.6208X−2.3643 and between CAPO force and core strength as Y=1.447X−4.3762Y = 1.447X - 4.3762Y=1.447X−4.3762. Consequently, the compressive strengths calculated for concrete containing brick chips were approximately 5-17 % higher than those measured using cylindrical specimens and about 0-6 % higher than those measured using core specimens.
Overall, the compressive strength of cylinders using brick chips was found to be approximately 0 to 5 % higher than the core strength of the concrete. However, these findings are contrary to previous studies, which suggested that the compressive strength determined from CAPO tests is lower than that of cylinders.
Moreover, core tests on concrete samples containing stone chips and brick chips showed that the former exhibited increases of approximately 8 %, 11 %, and 3 % over the latter, corresponding to the target strengths of 20 MPa, 25 MPa, and 30 MPa, respectively. Similarly, cylinder samples with these target strengths indicated that concrete made with stone chips had strengths that were 3 %, 2 %, and 4 % higher than those containing brick chips, respectively.
Conclusion
In conclusion, the study employed CAPO, core, and cylinder tests to assess the compressive strength of concrete using brick and stone aggregates, exploring various target strengths and ages. Comparative analyses established a strong correlation between the pullout force and compressive strength.
Notably, the CAPO test reported compressive strengths 5-17 % above those measured by cylinder tests and 0-6 % higher than core tests for concrete with brick chips. This divergence from previous findings was attributed to inconsistencies in workmanship, which were observed despite the operators' varying levels of CAPO test experience. The researchers recommend that enhancing the consistency and skills of the performers could significantly improve the CAPO test’s accuracy in estimating the in situ compressive strength of concrete containing brick chips.
Journal Reference
Nasrin, U. S., Khair, A., & Ahsan, R. (2024). Compressive strength assessment of concrete with brick chips using the CAPO-test. Scientific Reports, 14(1), 12881. https://doi.org/10.1038/s41598-024-59560-z, https://www.nature.com/articles/s41598-024-59560-z
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