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Análisis de reactividad de rocas mediante petrografía

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Se evalúa la potencial reactividad de los principales minerales y rocas utilizados como agregado para hormigón. Para tal fin, puede calificarse a las rocas como: graníticas (ígneas y metamórficas granudas), volcánicas y sedimentarias (litificadas), a las que deben agregarse conglomerados y arenas. Se debe analizar las características físicas, mineralógicas y texturales que definirán su comportamiento en el hormigón. Muchas de las partículas que constituyen los agregados pétreos están recubiertas parcial o totalmente por carbonatos, sílice, material arcilloso/limoso, materia orgánica, sulfatos o por otros sedimentos los que dificultan seriamente su determinación. Aún con ayuda de estereomicroscopio, sólo podrá observarse la superficie externa cubierta y no se identificará la composición litológica de la partícula. Por ello debe recurrirse al uso de microscopio petrográfico sobre secciones delgadas. Los componentes líticos de gravas y arena pueden estar enmascarados superficialmente por sílice criptocristalina o por óxidos de hierro asociados con sílice, tornándolos potencialmente reactivos frente a los álcalis lo que podrá ocasionar reacciones deletéreas en el hormigón que tiendan a disminuir sus prestaciones durante la vida útil de la obra.

The main minerals and rocks of the earth’s crust that are used as aggregates for concrete will be considered in this work. For this purpose, rocks can be classified as granitic (grainy igneous and metamorphic) rocks, volcanic and sedimentary (lithified) rocks, as well as conglomerates, sands, silts and clays (non-consolidated). The great part of metamorphic rocks have been excluded from this simplification, in particular those presenting fissility (shales), schistocity (schists in general) and mortar textures (cataclasites). They must be all rigorously treated since their physical, mineralogical and textural characteristics will define specific behaviors that would be damaging if they are added to concrete. Many of the particles making part of the rock aggregates are partially or totally covered by the deposition of carbonates, silica, clay/siltstone material, organic material, sulphates or by other sediments that makes difficult their determination. Even using a stereomicroscope only the external covered surface will be observed and the lithological composition of the particles will not be identified. If granitic rock or quartz are covered there will be no problem, but if soft clods, clay clods, volcanic glass, calcedonie or opalized sandstone are hidden there will be a serious problem. Deposits from river, glacial river or marine coastal origin show lithological composition highly varied. Lithic compounds of gravel and sand can be masked in the surface by criptocrystalline silica or by iron oxide associated with silica, making them potentially reactive in the alkali presence what can cause deleterious reactions in concrete tending to decrease their features during the lifetime of the construction.

Palabras clave
agregado para hormigón
microscopio petrográfico
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