[vc_row][vc_column][vc_column_text]Para el control de la resistencia deben tenerse en cuenta las especificaciones de la normativa adoptada en el proyecto. Esta determina como considerar los lotes o unidades de control en función al tiempo de trabajo, al volumen de producción o a la superficie construida o al número de plantas, a partir de ello se determina la cantidad mínima de probetas a moldear para los ensayos de control.
Esto es, si el cálculo de la estructura se realizó bajo los lineamientos de la normativa española, el control del hormigón, debiera realizarse bajo la misma normativa, ya que si bien en esencia las normas son similares en sus principios, existen diferencias que podrían llevar a discusiones en obra, sobre el cumplimiento o no de la resistencia especificada. Por ejemplo, la norma brasilera NBR 5739 establece que en la rotura de 2 ejemplares de probetas, debe adoptarse como resistencia el mayor valor de los 2 ejemplares, situación no considerada en otras normativas.[/vc_column_text][vc_column_text]A nivel nacional, si bien no contamos con una norma para el diseño de estructuras de hormigón armado, en lo que refiere al control del hormigón, si contamos con la NP 17 058 08 para Hormigon Elaborado: especificaciones, transporte, colocación, compactación y curado, la misma está basada en la norma española (EHE-99). Esta normativa indica la cantidad mínima de muestras a extraer de acuerdo a la cantidad de amasadas elaboradas por día, considerando que de cada muestra se moldean como mínimo dos probetas para cada ensayo. En cuanto a los ensayos a compresión simple, indica que la resistencia a compresión de una amasada de hormigón se obtiene a partir de los resultados de al menos 3 probetas cilindricas ensayadas a los 28 días de edad.
Los tamaños de lote de acuerdo a la norma española EHE-08, se presenta en el cuadro a continuación:
1- Control total: cuando las muestras se toman de todas las amasadas.
2- Control estadístico a nivel normal: cuando las muestras se toman al azar, para esto se divide a obra en lotes. Cada lote corresponde a un solo un tipo de elemento estructural, de un mismo proveedor, con igual dosificación y elaboradadas en las mismas condiciones. Finalmente se determina la resistencia de un número N de amasadas por cada lote. Para fck < 25 MPa N>2, para 25 MPa<fck< 35 MPa N>4 y para fck>35 MPa N>6.
3- Control estadístico a nivel reducido: se realiza con mediciones de la consistencia del hormigón, con una frecuencia mínima de 4 veces a lo largo del día. Se emplea únicamente en estructuras pequeñas, con elementos de luces inferiores a 6 m y resistencias de cálculo no superiores a 10 MPa.
Figura 4. Conjunto de probetas moldeadas.
Finalmente, podemos concluir que NO EXISTE UN NUMERO UNICO que garantice un correcto control del hormigón, para todas las situaciones, ya que, conforme a los elementos, el volumen y el tipo de control que se llevará adelante, se podrá determinar la cantidad correspondiente de probetas a moldear, para finalmente ensayarlas a compresión.[/vc_column_text][/vc_column][/vc_row]
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