Evaluación de las propiedades de permeabilidad al aire y durabilidad de concretos adicionados con caliza y arcillas calcinadas
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Peña Cruz, Luis Eduardo
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Abstract
In this research, the properties of permeability to air and water, resistance to chloride penetration, electrical impedance, porosity, density, compressive strength, and elasticity modulus of five concrete mixtures blended with different proportions of calcined clay and limestone were evaluated (M01) to (M05), and a reference mixture without any addition (MR). Engineer Luis Felipe Salazar designed all the mixtures (Salazar Mayorga, 2023), a member of the research and analysis group led by Engineer Juan Manuel Lizarazo Marriaga, as the basis for carrying out a series of investigations on cements blended with calcined clay and limestone.
All the mixes have the same water/cement ratio equal to 0.50. The calcined clay used is a low-grade clay, with a kaolinite content equal to 40%. The cement used as a reference is Alión brand of high early resistance (ART) because it is a cement with a minimum of additions and close to what is considered a pórtland cement.
The specimens were cured and tested at ages greater than 90 days in the Engineering laboratory of the National University of Colombia, Bogotá headquarters. The Salt Ponding Test - SPT test; It was carried out according to the standards NT-443 for the conditioning of test tubes and NT-492 for evaluation of the depth of penetration. The specimens remained submerged in saline solution for an additional 148 days after 90 days of curing.
When analyzing the results corresponding to the mechanical properties, it was found that the compressive strength in the mixtures blended with up to 45% of calcined clay plus limestone (M01 to M04) is like that of the reference mixture (MR). For a 50% substitution of cement (M05), the compressive strength decreased by 17%.
The modulus of elasticity in the added mixtures M01 to M04 has a value like that of the reference mixture MR. For the mix M05 with 50% substitution of the cement, the value of the modulus of elasticity decreased by 15% compared to that of the reference mix MR.
The density of all the mixtures is similar and no appreciable variation was found. The porosity of the blended mixtures was lower than that of the reference mixture MR, except for the M01 mixture, which presented a slightly higher value.
Regarding the properties related to durability, the permeability to air and water, the resistance to diffusion and migration of chlorides and the electrical impedance were evaluated.
All the mixtures had low air permeability. It was evidenced that the addition of calcined clays and limestone, as expected, effectively decrease the air permeability due to a refinement in the pore structure of the concrete. The optimal content of additions corresponds to the mixture M02 with 20% of calcined clay and 10% of limestone.
The water permeability of all blends was evaluated as low. A reliable quantitative relationship between the mixtures could not be established due to deficiencies in the quality of the epoxy paint used, which distorted the results by allowing the water flow to not be unidirectional.
There is no quantitative relationship between air and water permeability due to the nature and behavior of fluids.
The resistance to chloride penetration in all the blended mixtures was lower than that of the reference mixture. According to the Nordtest Build 492 (NT Build-492) test, the optimum cement substitution percentage is 60% (Clinker factor 40%) corresponding to the M03 mixture. The SPT test resulted in an optimal content of additions equal to 45% of the Clinker content, corresponding to the M02 mix, but with very little difference with the most added samples M03, M04 and M05.
The electrical impedance test resulted in all the blended mixtures having a higher electrical impedance than the reference mixture. This is due to the increase in the alkalinity of the pore solution due to the calcined clay. The behavior of the mixtures is consistent with what was found in the chloride migration and diffusion tests.
As a conclusion of the investigation, it is found that the concretes blended with calcined clays and limestone improve their mechanical and durability properties with substitution percentages of cement from 40% to 60% (Clinker factor 60% to 40%). This elevated level of substitution must be viewed with great care since reducing the percentage of pórtland cement also reduces the availability of carbonated material and then durability is compromised by corrosion issues.
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