RESUMEN
En cualquier elemento estructural de concreto armado se necesita un espesor determinado de concreto alrededor de la barras para garantizar la transmisión de los esfuerzos de tracción de las barras de acero. Sin este recubrimiento, se presenta el fenómeno conocido como splitting o hendimiento del concreto. Para verificar la efectividad del confinamiento del concreto se puede utilizar el ensayo de arrancamiento, modificado a partir del ensayo Pull-Out, indicado en la recomendación de la RILEM (RC6).
En la primera etapa de este trabajo, se realizó una revisión bibliográfica de las investigaciones posteriores a la recomendación de la RILEM respecto a la longitud de adherencia (igual a cinco veces el diámetro de la barra), analizando el tipo de falla de los diversos especímenes ensayados. En la segunda etapa se desarrolló un programa experimental de ensayos con barras de 8 mm de diámetro, distintos valores de recubrimientos y resistencias medias a compresión del concreto.
Finalmente se realizó una comparación de los resultados experimentales y los valores de la bibliografía analizada, estableciéndose cuál es la relación óptima entre el recubrimiento de la barra y su diámetro, para garantizar el deslizamiento de la barra en el ensayo de arrancamiento, evitando el efecto splitting del concreto.
1. INTRODUCCION
La adherencia concreto-acero es el fenómeno básico sobre el que descansa el funcionamiento del concreto armado como material estructural. JIMÉNEZ MONTOYA et al (2009).
La adherencia de las barras de acero al concreto, está compuesto de diversas parcelas que provienen de diferentes fenómenos que intervienen en la unión de los dos materiales (Fusco 2013). Estas parcelas son: adherencia por adhesión, adherencia por rozamiento y adherencia mecánica.
Adherencia por Adhesión: este fenómeno se verifica por la resistencia a la separación de los dos materiales, cuando se trata de separar un bloque hormigonado directamente en contacto con una placa de acero.
Adherencia por Rozamiento: si tenemos una barra de acero embebida en una masa de concreto, como se muestra en la Figura 2, en la misma se presentaran unas fuerzas debido al rozamiento existente entre el concreto y el acero, que dependerá principalmente de la rugosidad de la barra de acero y de la existencia de una presión transversal ejercida por el concreto, proveniente de la retracción del mismo FUSCO, (2013).
Adherencia Mecánica: esta proviene principalmente de la prominencia que presentan las barras de acero en su superficie.
Estas protuberancias funcionan como piezas de apoyo, movilizando tensiones de compresión FUSCO, (2013). La figura 3 muestra la distribución de estas fuerzas en barras lisas y corrugadas.
Otra explicación detallada del fenómeno de la adherencia es presentada por Orangun et. al (1964).
Ellos han explicado el fenómeno de agrietamiento por adherencia, suponiendo que se crea una condición de esfuerzos semejante a la que existiría en un cilindro de concreto que rodeará a cada barra al actuar sobre él las componentes radiales normales a las barras mostradas en la Figura 4 c.
El diámetro interior del cilindro a considerar sería igual al diámetro de la barra, db , y su espesor, C, igual al menor de los siguientes valores: el recubrimiento libre respecto a la cara inferior (Cb) o la mitad de la distancia libre a la barra contigua (Cs) (figura 5).
La resistencia al agrietamiento depende de la resistencia a tensión de este cilindro.
Si Cs < Cb se presenta un agrietamiento como el de la figura 6 c. Si CS > Cb, inicialmente se originan grietas verticales en el recubrimiento inferior (6 a y 6 b).
Si Cs, es sólo ligeramente mayor que Cb, el agrietamiento secundario se presentará en el plano de las barras, prolongándose hasta las caras laterales exteriores.
Si Cs es considerablemente mayor que Cb, el agrietamiento secundario tendrá una configuración en cuña como la de la Figura 6 b.
2. ANTECEDENTES EXPERIMENTALES
2.1 Ensayo de Arrancamiento
González Cuevas (2004), señala que al deslizar la barra dentro de la masa de concreto y romperse la adhesión entre los dos materiales, las corrugaciones reaccionan contra el concreto. La falla en este tipo de espécimen ocurre al partirse longitudinalmente la masa del concreto en dos o tres segmentos (Figura 7 a). También se ha observado en algunos casos, especialmente en especímenes de concreto ligero, que la falla ocurre por cortante en una superficie cilíndrica, al desprenderse la zona de concreto que rodea a la barra (Figura 7b). La clase de concreto, el recubrimiento y el diámetro de la varilla, son los parámetros que más influyen en el tipo de falla.
2.2 Fisuración y Adherencia
Cuando las solicitaciones son suficientemente bajas, y el concreto resiste a la tracción, se dice que el concreto está en Estadio I. Cuando aumenta las tensiones, en las fibras más traccionadas es alcanzada a la tensión Oct=Fet de rotura del concreto a tracción, se da inicio a la fisuración de la pieza, y se dice que el concreto esta en Estadio II (Fusco 2013). En la Figura 8, se explica el fenómeno detallado.
El estudio más minucioso de los fenómenos de adherencia, muestra que hay una intensa microfisuración del concreto que envuelve a la barra de acero.
2.3 Adherencia y Hendimiento
La unión del acero y el concreto, genera un movimiento de tensiones diagonales de compresión que son acompañadas por tensiones transversales de tracción para el establecimiento del equilibrio, conforme se muestra en la figura a continuación.
La destrucción de la adherencia no se da por un simple proceso de deslizamiento progresivo de la barra de acero dentro del concreto. Los esfuerzos de tracción transversal movilizados por el fenómeno de adherencia tienden a provocar el hendimiento (splitting) del concreto paralelo al eje de la barra de acero FUSCO (2013).
3. ENSAYO DE ADHERENCIA
En Europa, tal vez la recomendación más reconocida es los diversos estudios sobre adherencia es la RILEM/CEB/FIP RC 6 BOND TEST FOR REINFORCEMENT STELL PULL-OUT TEST(1.983) que define un procedimiento estándar para la realización, presentándose de bastante simple realización, como lo muestra la figura 12, de ahí tal vez su masiva difusión.
A partir de este ensayo, diversos investigadores realizaron estudios de adherencia, con modificaciones en lo que refiere principalmente al volumen de hormigón que se encuentra confinando la varilla de acero durante el ensayo de adherencia.
Estudiando el tipo de falla, según sea hendimiento del concreto o deslizamiento de la armadura.
Uno de los grupos de investigación en esta línea es la que trabaja en el proyecto APULOT (An appropriate pull out test).
4. INVESTIGACIONES REALIZADAS
Se analizaron los resultados obtenidos en un conjunto de trabajos relacionados con estudios de distintos parámetros que intervienen en el fenómeno de la adherencia, que fueron realizados entre los años 2010 y 2013 por investigadores de Estados Unidos, Francia, España, Brasil y Paraguay. En conjunto incluye un total de 782 resultados de ensayos.
El análisis incluyó las relaciones entre recubrimientos y diámetros de barras empleadas, las resistencias del concreto ensayado y las formas de rotura observadas en los ensayos.
5. ANÁLISIS DE RESULTADOS
Del análisis realizado se verificó que en concretos de resistencia media comprendida entre 20 MPa y 40 MPa y superiores a 50 MPa, el fenómeno de splitting fue observado siempre que la relación entre el recubrimiento de concreto y el diámetro de la barra fue inferior a 4,5 como se puede comprobar en la Tabla 2.
Sin embargo, en concretos de resistencia media comprendida entre 40 y 50 MPa, el valor 4,5 se observa como límite de la relación recubrimiento-diámetro.
6. CONCLUSIONES
Del análisis de los resultados de las investigaciones, se puede concluir que con un valor mayor a 4,5 veces el diámetro de la armadura en el ensayo de adherencia, como valor del radio del concreto alrededor de la armadura, se puede garantizar que independientemente del valor de la resistencia a compresión del hormigón, el ensayo presentara deslizamiento puro, sin hendimiento de hormigón.
Este valor resulta inferior al rango de valores sugeridos por la recomendación RC6 de la RILEM.
Al determinar un valor inferior al sugerido por la recomendación, se trabaja con menores volumen de probetas para los ensayos para obtener iguales resultados satisfactorios en lo que refiere a estudios de adherencia, por deslizamiento de la barra.
REFERENCIAS BIBLIOGRÁFICAS
* Vale Silva, B. Barbosa, M. Pinto da Silva, L. C. Lorrain, M.; Dal Bosco, V. (2011). Correlação entre a tensão última de adherência e a resistencia do concreto usando ensaio APULOT: avaliação do efeito de variação do cobrimento de concreto ao redor da barra de aço. 53º CBC Ibracon. Florianópolis. Brasil.
* Vale Silva, B. (2010). Investigação do potencial dos ensaios APULOT e pull-out para estimativa da resistência a compressão do concreto. Tese de mestrado. Faculdade de Engenharia de Ilha Solteira. Universidade Estadual Paulista. Sao Paulo. Brasil.
* Vale Silva, B.. Barbosa, M. Pinto da Silva, L. C. Lorrain, M. (2013). Investigação experimental sobre o uso de ensaios de aderência aço-concreto para estimativa da resistência á compressão axial do concreto: Parte 1. IBRACON, Estruturas e Materiais. Volume 6.
* Soudais, P. Barbosa, M. Pinto da Silva, L. C. Lorrain, M. (2011). Estimativa da resistência à compressão a partir de resultados de ensaios de aderência tipo Pull-Out para controle da qualidade do concreto na obra. IBRACON de estruturas e materiais. Volume 4.
* García-Taengua, E. Martí-Vargas, R. Serna-Ros, P.(2013). Bond of reinforcement in concrete applied to concrete quality control: The Bottle Bond Test. Strain 2013; 50: 57-67.
* Torre-Casanova, J, A. Davenne, L. Pinelli, X. (2012) Confinement effects on the steel–concrete bond strength and pull-out failure. Engineering Fractura Mechanics.
* Xu, F. Wu, Z. Zheng, J; Hu, Y. Li, Q. (2012). Experimental Study on the Bond Behavior of Reinforcing Bars Embedded in Concrete Subjected to Lateral Pressure. ASCE.
* Lovera, H. Frutos, A. (2013) Evaluación de la Tensión de Adherencia entre el Hormigón y el Acero ante la Variación de la Resistencia a Compresión mediante el Ensayo APULOT. Tesis de Ingeniería Civil, Universidad Nacional de Asunción. Asunción, Paraguay.
* Gavilán, S. (2013). Influência do diâmetro da barra e o cobrimento nas características aderentes de concreto armado. Disciplina Analise Experimental de Estruturas e Materiais. Programa de Pos Graduação em Engenharia Civil. Universidade Federal do Rio Grande do Sul.
* Orangun, C.D., Jirsa, J.O. Breen, J.E. (1964). A Reevaluation of test data on development Length and splices. Journal pf the American Concrete Institute. Detroit. Estados Unidos.
* American Concrete Institute (2003). ACI 408R: Bond and development of straight reinforcing bars in tension. Farmington Hills: ACI, Farmington Hills, MI. Estados Unidos.
* Comité International du Beton. (1983). RILEM/CEB/FIP RC6: Bond test for reinforcing teel: pull-out test. CEB, Paris, Francia.
* Fusco, P. B. (2013) Técnica de armar as estruturas de concreto. 2ºedicion. PINI. São Paulo. Brasil.
* González Cuevas, O. (2004) Concreto Reforzado. 3ra.ed. LIMUSA. Ciudad de México. México.
* P. Moran Cabré, F. García Meseguer, A. Arroyo, J. C. (2009) Hormigón armado. 15ª edición. Gustavo Gili, Madrid, España.
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