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This paper presents a new evaluation of the strength compliance of concretes produced in Brazil. It is based on experimental results of over twenty-seven thousand concrete samples from different parts of the country.
This study also reveals the concrete produced in the South and Midwest regions have less variability than the ones produced in the other regions of the country.
These results emphasize the importance of a rigorous control in manufacturing and reception of concretes in order to reduce the nonconforming cases. This article presents an investigation about the strength compliance of concretes manufactured in different states from the five geographic regions of Brazil. It is based on results from axial compression strength tests at 28 days of age performed in more than 27 thousand cylindrical specimens of concrete molded in loco.
This study divides the data into 4 classes – C20, C30, C40 and C50 – in order to evaluate the conformance of the concrete as a function of its characteristic strength. The analysis are performed both at regional and national levels.
As part of this work, comparisons are made between these results and the results shared by Santiago and Beck five years ago [ 1 ]. It is important to emphasize the entire study is based on current national normalization, specially the codes for compression test of concrete cylindrical specimens [ 2 ], preparation, control and acceptance of concrete [ 3 ] and the design of concrete structures [ 4 ]. This work is part of a larger project about the calibration, based on structural reliability, of the partial factors from Brazilian design codes.
The calibration is being developed by the authors of this paper, though it has already been addressed in a preliminary way by Beck and Doria [ 5 ], Beck et al. The compressive strength of concrete is random by nature, its variability is a result from the constituent materials, test procedures, production and operation equipments. The necessary measures to reduce variability involves the quality control of each one of the variables and the more efficient it is, the more homogeneous is the concrete produced [ 9 ].
The concrete production caminao its dosage which is the process used to obtain the best proportion between cement, aggregates, water, additives and addition in order to meet certain specifications [ 10 ]. Once produced, the concrete is submitted to a quality control that test the cylindrical specimens at 28 days, in order to verify its compressive strength.
In the control by total sampling two specimens are molded for each concrete batch, while in the control by partial sampling at least six specimens are molded for each 50 or m3 of concrete used on the structure.
At the end of the quality control, a batch is considered conform when the estimate value of its characteristic strength fckestreferred to a known sampling of a unknown population, satisfies the relation presented in Eq.
Although the compressive strength test in specimens is an evidence of quality of the concrete, its result is restricted to indicate a potential strength of the population based on sampling. The issue of the conformity of structural concretes produced in Brazil has been the object of several studies, but it still needs to be further developed.
Some studies also indicate the dosing plants are centralizing the dosage strength in the characteristic strength and not in the average strength [ 116 ]; which is probably the main reason for the frequent occurrences of nonconforming concrete in Brazilian construction sites. In this scenario, this study present a statistic description that represents the set of concrete produced in Brazil, given the analysis are performed from samples collected and tested in the same conditions established by a code applied throughout the country.
The present work is subsidized with information about the compressive strength at 28 days of over 27 thousand cylindrical specimens – 10 cm diameter and 20 cm height – of concretes molded in loco in constructions sites from the five Brazilian geographical regions – Midwest COSouth SSoutheast SENorth N and Northeast NE – referring to the 15 federative units.
Table 1 shows a quantification of the specimens according to the different regions. In order to evaluate the behavior of the concrete in relation to its characteristic strength, the data were classified into 4 classes – C20, C30, C40 and C50 – according to the information available in the technological control reports. Table 2 presents a brief quantification of the tested specimens according to the evaluated strength classes.
Figure 1 shows the percentage distribution of the data according to the geographical regions of the country for each one of the four strength classes of concrete studied in this work.
The results expressed in the graphics have a strong relation with the strength classes usually used in each region. In addition, the data used were obtained from the following companies, educational institutes and laboratories: The randomness nature of concrete strength requires a statistical approach for the problem.
The variation coefficient was calculated through the ration between the standard deviation and the mean, see Eq. Results from widely known international works [ 1718 ] indicate that normal distribution is the one that best fits the description of concrete behavior, which was proven after successive fits using Easyfit software.
Caminhão Betoneira SENAI – Para choque traseiro
It should be mentioned that in the existence of an efficient control, the strength values are grouped close to the mean and the distribution curve is high and narrow; on the opposite scenario, the strength values spread out and the curve becomes low and wide.
As the objective of this study was to obtain statistical descriptions that represent the sets of concrete produced in the country, the data from different construction sites were grouped and analyzed in blocks. Therefore, the national statistics were obtained from regional statistical weights, which were applied according to the number of samples available for each class in each region of the country.
It is worth mentioning this works also involved a previous statistical analysis of the samples in order to eliminate data that does not belong to the group outliners.
Figure 2 shows box-and-whiskers representations for the strength class samples studied in order to highlight the lower and upper zenai used to exclude spurious results. It should be mentioned the results from each studied classes came from the same population, which they propose to represent. Concrete from classes C40 and C50, on the other hand, presents mean strength f cm value lower than the predicted value in the same norm. Figure 5 presents the probability distribution curves obtained for concretes produced in different geographic regions of the country.
Concrete produced in the South and Center-West regions, for example, tend to present less variability than those produced in the Southeast and Northeast regions. Figure 7 presents the probability distribution curves based on database from this studyas well as the probability distribution curves based on database from Santiago and Beck betoneia 1 ] This figure let clear that in the interval of five years the concrete produced in Brazil presented an increase of mean strength, although the variability also increased.
At first, this result is an indication that the dosing plants have centered the dosage of concrete on the mean strength and not in the characteristic strength anymore, as pointed out in previous works [ 116 ].
The results show that, in general, there has been an increase of strength and a decrease in the percentage of nonconforming samples. This paper presented a study of concrete strength conformance from four distinct classes – C20, C30, C40 and C50 – produced in Brazil. This study was accomplished using a database of over twenty-seven thousand cylindrical specimens molded in loco and tested at 28 days with instantaneous and monotonic load. In relation to the regionalized results, concretes produced in the South and Center-West regions presented aenai lower variability than the ones produced in the Southeast and Northeast region.
The authors of this work are sdnai in a bigger project that aims the calibration, based on structural bdtoneira, of the partial factors from Brazilian design codes. Therefore, this study of strength conformance of the concrete produced in the country is fundamental to achieve the final goal. The authors are grateful to CNPq for the founding, as well as the professionals who provided the necessary data for the accomplishment of this work: A study of Brazilian concrete strength non- compliance and its effects on reliability of short columns.
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Projeto de estruturas de concreto – Procedimento. Reliability analysis of Ebtoneira stell columns designed according to new Brazilian building codes. J, of the Braz. Reliability-based evaluation of design code provisions for circular concrete-filled steel columns.
Engineer Ing Structures, Elsevier, v. Reliability-based evaluation of design guidelines for cold-formed steel I-concrete composite beams. A first attempt towards reliability-based calibration of Brazilian structural design codes.
The problem of non-compliant concrete and its influence on the reliability of reinforced concrete columns. Materials and Structures, v. Probability-based criteria for structural design. Structural Safety 1, p. Calibration of caminjao code for buildings ACI Part 1 – Statistical models for resistance. ACI Structural Journal, v.
Ca,inhao is an open-access article distributed under the terms of the Creative Commons Attribution License. Services on Demand Journal. Articles A new study of Brazilian concrete strength conformance. Introduction This article presents an investigation about the strength compliance of concretes manufactured in different states from the five geographic regions of Brazil. Compressive strength of concrete The compressive strength of concrete is random by nature, its variability is a result from the constituent materials, test procedures, production and operation equipments.
Origin and database classification The present work is subsidized with information about the compressive strength at 28 days of over 27 thousand cylindrical specimens – 10 cm diameter and 20 cm height – of concretes molded in loco in constructions sites from the five Brazilian geographical regions – Betoneiar COSouth SSoutheast SENorth N and Northeast NE – referring to the 15 federative units.
Database treatment The randomness nature of concrete strength requires a statistical approach for the problem. Regional results Figure 5 presents the probability distribution curves obtained for concretes produced in different geographic regions of the country. Comparison with previous results Figure 7 presents the probability distribution curves based on database from this studyas well as the probability distribution curves based on database from Santiago and Beck [ 1 ] Conclusions This paper presented a study of concrete strength conformance from four distinct classes – C20, C30, C40 and C50 – produced in Brazil.
Acknowledgments The authors are grateful to CNPq for the founding, as well as the professionals who provided the necessary data for the accomplishment of this work: September 23, ; Accepted: How to cite this article.