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Equivalent specified concrete strength from core test data
Abstract
A procedure for the determination of concrete strength value from core strength data is described. The procedure involves (a) planning of the scope of the testing program, (b) procuring and testing of cores, (c) conversion of core strength to equivalent in-place strengths, (d) identification of low outliers in the set of equivalent in-place strengths, and (e) calculation of equivalent specified strength from the in-place strength data. A sample determination using the proposed procedure is presented.
... We can observe a general trend in which the core-to-cubic strength ratio decreases as the concrete strength increases. This is an unexpected outcome since most of the references (see [18] and [35] for a comprehensive review and [20] and [36] for an up-to-date code approach) on modern concrete shows that the core-to-cubic strength ratio increases as the concrete class increases, such as in figure 7 for [38] among the others. Figure 5. ...
... The usually accepted cubic-to-core strength, usually assumed in-between 0.80 to 0.85, in some cases may be rather different, ranging from 0.60 to 1.00 [36]. According to some results, [18] and [35] for example, the most common value should be 0.80 approximately. All these data refer to modern concrete, i.e. with adequate aggregate proportioning, proper aggregate quality and proper water/cement ratio. ...
... All these data refer to modern concrete, i.e. with adequate aggregate proportioning, proper aggregate quality and proper water/cement ratio. Looking at the test results from the scientific literature [18,[35][36][37][38][39][40][41] we can see that test data show a dispersion similar to the one showed in figures 7 and 8. ...
Coring is considered to provide the best estimate of concrete compressive strength in existing structures and is commonly used to calibrate Non-Destructive and Moderately Destructive Techniques. Historical concrete, produced in the pre-code period until the ‘20s, significantly differs from modern concrete due to lack of standardization, improper rules of thumbs and to aggregate shape (round, smooth and often excessively large aggregates) and proportioning. Therefore, the applicability of the procedures calibrated on modern concrete to a historical one, also coring, is an issue that needs to be discussed. In this paper, an experimental campaign on historical-like concrete, i.e. with the same defects as historical concrete, aims at identifying the reliability of drilled cores due to the effect of round aggregates. The results show that standard procedures commonly used on modern concrete cannot be directly applied to historical concrete: drilled cores suffer from scale effects (core diameter) and from cutting damage of the material much more than modern concrete. In detail, the core-to-cubic ratio, that modern codes assume in the range 0.70-0.85, due to the dimension and shape of the aggregates is found inside a larger range, 0.70-1.00, and, as opposed to modern concrete, is found to be decreasing as concrete strength increases. Besides, the diameter of the core is found to have a relevant effect on the estimate of the material compressive strength and on the core-to-cubic strength ratio, pointing out that the dimension of the core affects the results much more than for modern concrete. This latter result, which needs further research, points out that historical concretes may be rather different from modern ones and probably need larger cores to be drilled than modern concrete due to the larger dimension of aggregates that are often found in pre-code concrete.
... Estimates of the overall variability of in-place concrete strengths reported by Bartlett and MacGregor (1995) are presented in Table 2.1. The variability is expressed in terms of the coefficient of variation V WS , which is the ratio of the standard deviation of the in-place strength to the average inplace strength. ...
... If the investigator cannot find a physical reason to explain why a particular result is unusually low or unusually high, then statistical tests given in ASTM E 178 can be used to determine whether the observation is an "outlier." When the sample size is less than six, however, these tests do not consistently classify values as outliers that should be so classified (Bartlett and MacGregor 1995). An example calculation using ASTM E 178 criteria to check whether a low value is an outlier is presented in the Appendix. ...
... subsequently pop out during testing (Bartlett and MacGregor 1994d). Table 8.1 shows the mean values of the strength correction factors reported by Bartlett and MacGregor (1995) based on data for normalweight concrete with strengths between 14 and 92 MPa (2000 and 13,400 psi). The right-hand column shows coefficients of variation V that indicate the uncertainty of the mean value. ...
Commentaries are intended for guidance in planning, designing, executing, and inspecting construction. This document is intended for the use of individuals who are competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. The American Concrete Institute disclaims any and all responsibility for the stated principles. The Institute shall not be liable for any loss or damage arising therefrom. Reference to this document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer. It is the responsibility of the user of this document to establish health and safety practices appropriate to the specific circumstances involved with its use. ACI does not make any representations with regard to health and safety issues and the use of this document. The user must determine the applicability of all regulatory limitations before applying the document and must comply with all applicable laws and regulations, including but not limited to, United States Occupational Safety and Health Administration (OSHA) health and safety standards. Core testing is the most direct method to determine the compressive strength of concrete in a structure. Generally, cores are obtained either to assess whether suspect concrete in a new structure complies with strength-based acceptance criteria or to evaluate the structural capacity of an existing structure based on the actual in-place concrete strength. In either case, the process of obtaining core specimens and interpreting the strength test results is often confounded by various factors that affect either the in-place strength of the concrete or the measured strength of the test specimen. The scatter in strength test data, which is unavoidable given the inherent randomness of in-place concrete strengths and the additional uncertainty attributable to the preparation and testing of the specimen, may further complicate compliance and evaluation decisions. This guide summarizes current practices for obtaining cores and interpreting core compressive strength test results. Factors that affect the in-place concrete strength are reviewed so locations for sampling can be selected that are consistent with the objectives of the investigation. Strength correction factors are presented for converting the measured strength of non-standard core-test specimens to the strength of equivalent specimens with standard diameters, length-to-diameter ratios, and moisture conditioning. This guide also provides guidance for checking strength compliance of concrete in a structure under construction and methods for determining an equivalent specified strength to assess the capacity of an existing structure.
... To date, the concrete core testing, which is a destructive test (DT) that consists of the extraction of concrete cores from structural elements and the execution of laboratory compression tests on them, is the most reliable tool. In fact, after a calibration process aimed at taking into account the several extrinsic and intrinsic factors that may influence the strength values in the testing process [24,25,[32][33][34][35][36][37][38][39], the results of this DT are conventionally assumed as "exact", i.e. as the actual values of concrete strength at the measuring points. For this reason, in order to avoid an inaccurate strength estimation, the whole assessment protocol has to be carried out with great care, both during the execution phases and in interpreting data [2,22,[38][39][40][41][42]. ...
... In fact, after a calibration process aimed at taking into account the several extrinsic and intrinsic factors that may influence the strength values in the testing process [24,25,[32][33][34][35][36][37][38][39], the results of this DT are conventionally assumed as "exact", i.e. as the actual values of concrete strength at the measuring points. For this reason, in order to avoid an inaccurate strength estimation, the whole assessment protocol has to be carried out with great care, both during the execution phases and in interpreting data [2,22,[38][39][40][41][42]. However, in order to reduce costs and invasiveness, some Codes (e.g. ...
... Although it is the most direct and accurate method of estimating concrete strength, experience demonstrates that the strength measured on core specimens can be different from the in-situ strength, as discussed in many studies (e.g. [8,29]). As a consequence, some codes (e.g. ...
... An overview of the expected variability from non-destructive tests (rebound and ultrasonic pulse velocity) is reported in [21]. Further guidance concerning in-situ variability, including also core test, is given in [29,34]. Values around 5-15%, 2-6% and 8-13% are found for rebound, ultrasonic velocity and core test, respectively. ...
... Some studies (e.g. [10,14]) have discussed the difference between in situ and core strength. Size and geometry of core specimens, coring direction, presence of reinforcing bars or other inclusions, and effect of drilling damage can influence test results on core specimens and thus determine such difference. ...
... Generally speaking, some scatter in the test results may be expected, taking into account the normal within-concrete, withinmember and within-test variations. In [12,14] estimates of the overall variability of in situ concrete strength are provided in terms of coefficient of variation CV (Table 5), considering variability dependent on the number of structural members and concrete batches. It should be emphasized that the values reported in Table 5 are relevant to cast-in-place concrete produced and placed according to normal practice and standards. ...
Reinforced Concrete (RC) buildings can require the determination of in situ concrete strength during the execution of new structures to investigate low-strength results from acceptance tests and, especially, in the capacity assessment of existing structures. Most structural codes and technical recommendations indicate that in situ concrete strength should be estimated by means of drilled cores (Destructive Test, DT), possibly supplemented by non-destructive tests (NDTs). Besides contributing to the identification
of homogenous concrete areas and thus indicating locations where cores have to be extracted, NDTs
can significantly reduce the total amount of cores needed to adequately estimate concrete strength in
an entire structure. The paper firstly reports a brief review of the most usual NDT methods (rebound
number and ultrasonic pulse) and DT methods (cores) and discusses the role of the main factors influencing
in situ strength estimation. It then reports and analyses the results of a wide investigation carried out
on an RC beam member extracted from an existing structure. The results show a low variability of rebound number and direct velocity values along the beam, and a high variability for surface velocity values
and, especially, core strengths. The wide scatter in some test results has been seen in relationship to the micro-cracking condition arising from past applied loads. Suggestions for test location and interpretation of in situ and laboratory test results are provided.
... Although it is the most direct and accurate method of estimating concrete strength, experience demonstrates that the strength measured on core specimens can be different from the in-situ strength, as discussed in many studies (e.g. [8,29]). As a consequence, some codes (e.g. ...
... An overview of the expected variability from non-destructive tests (rebound and ultrasonic pulse velocity) is reported in [21]. Further guidance concerning in-situ variability, including also core test, is given in [29,34]. Values around 5-15%, 2-6% and 8-13% are found for rebound, ultrasonic velocity and core test, respectively. ...
... To eradicate this problem, technicians need to cut concrete core without having reinforcement by scanning through magnate. Bartlett and MacGregor proposed corrections for concrete core strength if cores contain reinforcement [9]. The corrections are shown in Table 1. Bartlett and Macgregor adjustments are for the reinforcement which runs at right angles to the direction of drilling. ...
An experimental study has been carried out to observe the influence of different positions that affect the interpretation of core test results of a beam. For this purpose a number of concrete cores have been drilled from RC beams. Six beams were tested under two-point loading to produce the first crack. The beams and blocks were cast for different mix ratios with different coarse aggregates (stone chips and brick chips- ¾ inch downgraded). The minimum reinforcement was provided in beams just to resist the temperature, shrinkage, and
stresses due to handling. For each mix ratio, 4inch diameter cores were drilled from different locations of each beam which experience different stress conditions under the bending test. Height to diameter was maintained as 2.0. The w/c ratio was 0.42 to maintain a slump within 3 to 4 inches. The concrete cores and standard cylinders were tested in the laboratory following the standard method specified in ASTM. It was found that the compressive strength of concrete cores drilled from the RC beam is lower than the corresponding standard
cylinder strength. The cores in the compression zone experienced higher strength than the core in the neutral zone, and the cores in the neutral zone experienced higher strength than the core in the tension zone.
... Este método fue propuesto por Barlett y MacGregor [11] en 1995. Estos investigadores planteaban que el Método del Factor de Tolerancia resultaba demasiado conservador en la práctica ya que los ensayos de testigos sobreestimaban la verdadera variabilidad del hormigón in-situ; es decir, el valor de f ck,is calculado según (4) resultaba muy bajo porque el valor de la desviación estándar (s) empleado era muy alto. ...
Este trabajo resume una investigación sobre los métodos para determinar la resistencia
característica a compresión del hormigón en estructuras existentes (fck,is), a partir de la
extracción y ensayo de testigos; cuando se evalúa la seguridad de la estructura ya sea por la
presencia de patologías o por el aumento de la carga de uso sobre la misma, etc. El estudio
está motivado por la ausencia en las normativas cubanas de un método que cumpla con dicho
fin. En consecuencia el objetivo principal de este trabajo es establecer el estado del arte sobre
el tema, como el primer para la resolución del problema planteado. La principal conclusión de
este estudio es que se exigen diferentes niveles de confianza en el cálculo de fck,is en las
normativas norteamericanas y europeas lo que va a significar evidentes diferencias en los
valores obtenidos al usar los diferentes métodos.
... These Authors stated that the tolerance factor approach may be too conservative mostly for two reasons. First of all, in their opinion the measured core test values overestimate the actual variability of the in-place concrete strength [16], furthermore they believe that this approach is too precise for the requirements of actual design practice. ...
Many countries are experiencing an increasing need of checking the safety of existing structures. The assessment of structural capacity of RC structures strictly depends on the in situ compressive strength of concrete. The evaluation of this property is typically carried out by means of destructive tests on concrete cores taken from the structure. The experimental data is then interpreted using a relevant code to obtain a design strength value according to the required percentile and confidence. In this paper the principal international standards that deal with the statistical interpretation of data from concrete core tests are presented. Since it is reasonable to assume that concrete strength is a realization of a random field, the assumption of statistical independence of core test data is questioned. An extension of the classical theory of tolerance limits in the case of normally distributed correlated samples is thus proposed. Finally, application examples of this methodology are provided to illustrate some important implications of the spatial correlation of core test values on concrete strength estimations.
... On the other hand, to drill the core specimens from an existing structure and to determine the strength from these specimens bring additional variation parameters. The diameter and length of the core specimen, length to diameter ratio, humidity condition, rebars in the core specimens, damage caused during the drilling operation are the main causes of the additional variation [3][4][5][6][7][8]. ...
In this article, using the databases of private firms working for Ministry of Defense and Ministry of Public Works and Settlement, compressive strength of 4647 core specimens taken from 693 buildings, mostly in İstanbul are analyzed. Analyses include the distribution of concrete compressive strengths of public, residential and military buildings. The correlation of concrete compressive strengths found by testing cored specimens and Schmidt hammer rebound readings is also sought. The effects of several factors on the concrete compressive strength of existing reinforced concrete structures; origin, construction date, number of stories, and the correction factors utilized in several standards, and the reliability of the Schmidt hammer rebound testing compared by destructive test methods are also discussed.
... The value of 85% in ACI 318 is not very different from the value used in BS 6089 [9] , which require the "estimated in-situ cube strength" to be 83% of the specified characteristic strength of concrete, to which the partial safety factor for design strength is applied. Bartlett and MacGregor [10] submitted a relationship between the average in situ strength and the specified strength is determined from the analysis of result test data. The average in situ strength at 28 days, as corrected for the effect of the core moisture condition and damage sustained during drilling, approximately equals the average cylinder strength. ...
Assessment of in-situ concrete strength by means of
cores cut from hardened concrete is accepted as the most common
in-situ nondestructive method, however the assessing of the
concrete in the existing buildings, particularly in the
troubleshooting of problems with new construction, If the
strength of standard compression test specimens found to be
below the specified 28 days value, frequently, cores tests are
undertaken at later ages exceeding the 28 days. This study
includes an attempt to find the influence of the long-term concrete
age and strength level on the compressive strength development
for the standard concrete core.
This study involves laboratory investigation were number of
specimens including concrete panels and cubes with specified
compressive strength ranging from 25-55 MPa were prepared
and tested at concrete age of 28, 60, 90, 120, 180,and 270 days by
in-situ nondestructive tests (cores) and destructive tests (cubes).
The test results obtained from core specimen were compared with
those of standard specimens.
The test results showed that the core compressive strength
increases as the age of concrete increase, but the core strength is
somewhat higher than 28-day cube compressive strength even up
to the age 270 days in moderate concrete, while the core
compressive strength remains lower than 28-day cube
compressive strength in the higher strength level even up to the
age 270 days.
... Az MSZ EN 13791:2007 szabványt tanulmányozva észrevehetjük, hogy annak alkalmazási területe csak azokra a vizsgálati módszerekre terjed ki, amelyek során a roncsolásmentes módszerekkel kapott közvetett szilárdságjellemzők és a szerke- Bartlett, MacGregor, 1994a;1994b;1994c;1995;1996;Neville, 2001). Az alapgörbe MSZ EN 13791:2007 szerinti függvény-transzformációját sematikusan a 2. ábrán mutatjuk be. ...
... Megállapítható, hogy a beton nyomószilárdságának szórása a minőség-ellenőrzés szigorúságának függvényében gyakorlatilag konstans, σ = 2,0 -8,0 N/mm 2 értékűnek tételezhető föl. Ezzel szemben más, újabb keletű szakirodalmi források alapján a szerkezeti beton nyomószilárdságának variációs együtthatója tekinthető konstansnak, értéke: V = 7 -12% (Bartlett, MacGregor, 1995). A vita végére továbbra sem tehetünk pontot, ugyanis pl. ...
... This query was generated by an automatic reference checking system. Bartlett and MacGregor (1995) could not be located in the databases used by the system. While the reference may be correct, we ask that you check it so we can provide as many links to the referenced articles as possible. ...
Precast prestressed double-tee girders are considered as crucial element of modern infrastructure used to accelerate building construction worldwide. These girders can be deficient because of local damages and developed cracks as a result of improper transportation and handling. Therefore, it is very important to develop an efficient retrofitting technique in order to restore the lost capacity and/or even outperform it. The objective of this paper is to develop retrofit strategy of the deficient girders, using externally bonded carbon fiber reinforced polymer (EB-CFRP) technique, and verify it using field data. A field test was conducted on three actual-size precast pretensioned double-tee (DT) girders, having different levels of damage and retrofitted using CFFP sheets. The girder stems for two of them were strengthened in flexure using uni-directional U–shaped CFRP sheets, while all girders were shear-strengthened at their dapped-ends. Each girder was loaded incrementally up to collapse while, the deflection of the girder and developed normal strains on both concrete surface and the CFRP sheets were recorded at each load increment. Test results assured the adequacy of the adopted strengthening technique with respect to both ultimate capacity and ductility. It addition, correlation between the experimental findings and available equations in design codes showed that the experimental flexural and shear resistances of the retrofitted girders are far greater than those obtained from equations available in ACI 440.2R-08 and CSA S806-02 codes by at least 60%.
... 5(a and b), it appears that the lognormal distribution provided a better fit to the 25 MPa (3.6 ksi) concrete. Change of probability distribution function from lognormal to normal distribution as the specified strength of concrete increased was noted previously (Stewart 1995;Tabsh and Aswad 1997), with the consensus among researchers (Mirza et al. 1979; ACI Committee 214 2010; ACI Committee 228 2003; Bartlett and MacGregor 1995) being that concrete strengths were normally distributed if control was above average and lognormally distributed if control was below average. However, because of its analytic tractability, the normal distribution was used for all concrete strengths in the present investigation, except as noted in the section on Percent Low Tests. ...
This investigation presents results of the statistical and probabilistic analyses of 3,269 normal weight concrete cylinder compression tests used for recently constructed highway bridges in California. A new model for in-place strength of concrete structures is proposed as a function of the specified compressive strength, the normalized 28-day cylinder strength, and age of the structure based on a realistic strength–age
relation for hardened concrete. The model prediction indicates that concretes in cast-in-place bridge structures designed with specified compressive strengths of 25 MPa (3.6 ksi), 28 MPa (4.0 ksi), and 35 MPa (5.0 ksi) reach their maximum strengths at about 40 years, with approximately 98% of the maximum strength occurring during the first 10 years. Also, through significance testing on the 28-day cylinder strengths, it was established that the California Department of Transportation practice of using an expected concrete strength instead of the specified strength for seismic design of bridge components is justified. An expression for predicting the 28-day strength of concrete cylinders as a function of the strength of companion cylinders, also proposed herein, could prove a useful tool for quality control of concrete during construction.
... It is also assumed that such test results have been converted to the corresponding in-place concrete strength. As an extensive discussion on such procedures is beyond the scope of this work, the reader is referred, for example, to [15,[18][19][20] for further details. In terms of the number of tests, and based on Sect. ...
A probabilistic framework is defined to evaluate the values of the Confidence Factors (CFs) proposed in Eurocode 8 Part 3 (EC8-3) for the characterization of material properties. This evaluation is presented for the concrete compressive strength but its validity for other material properties can also be inferred from the results obtained. The number of material tests and the existence of prior knowledge are the essential aspects for the CF quantification. The probabilistic framework proposed in the first part of the study does not consider the existence of prior knowledge and is based on the concept of confidence intervals. In the second part of the study, the effects of prior knowledge are considered using a Bayesian framework. The combination of testing data obtained from different types of tests is also addressed as an extension of the referred Bayesian approach. Results indicate that the EC8-3 proposed CFs for KL1 and KL2 are adequate, but for KL3 it is suggested that a larger value should be used.
In this study, the author’s experience in the estimation of concrete strength by rebound hammer (RH) and ultrasonic pulse velocity (UPV) test is summarized and compared with destructive laboratory tests. In the destructive testing of concrete, only the impact resistance, ductility, yield strength can be obtained, whereas, through the non-destructive techniques (NDT) testing, discontinuities such as voids, cracks, and differences in material characteristics such as high strength materials or low strength materials can be more effectively attained and material under test can still be utilized after inspection. A various selection of NDT testing is available which can be used to provide information regarding the condition of the material and several other approaches can be used to derive the strength of material through NDT testing. To perform the test, samples of concrete blocks were prepared and kept under various curing conditions for assorted periods of time. Multiple tests were carried out under various conditions of various aged samples. Measurements and results from NDT are indicative of the properties of concrete such as porosity, the complexity of the pore network, water content, and strength. Samples that returned with the highest rebound number and peak compressive strength values using the RH test came from samples that were left for the first 14 days under adequate curing conditions with additional 14 days of dry conditions. Another condition that obtained peak results was the samples that were buried in the earth for 14 days as a means of curing. In the UPV testing, the strength depends on the aging of the concrete rather than the total curing days. In comparison with samples containing voids tested, it is found that as the voids are larger the UPV reading was more accurate which is indicative that NDTs have flaws that need to be taken into consideration.
Keywords:
Non-destructive testing Varied curing Buried moisture curing Hammer test Pulse velocity
Resumen
Este trabajo está motivado por la ausencia en las normativas cubanas de un método para estimar la resistencia característica a compresión del hormigón en estructuras existentes a partir de la extracción de testigos. Se establece el estado del arte sobre el tema y se utilizan los resultados de ensayos a compresión de testigos extraídos de 15 estructuras existentes de hormigón armado, para evaluar los diversos métodos encontrados en la literatura. Los resultados evidencian una gran variación en los valores determinados por los diferentes métodos encontrados. Finalmente se efectúa la comparación entre dichos valores, se cuantifican las diferencias promedio entre los valores calculados por los diferentes métodos y se emiten criterios sobre la sensibilidad de sus resultados ante las características del lote de hormigón.
Innovative Monitoringstrategien für Bestandsbauwerke sind entscheidend für die Bewertung von Lebenszyklen. Die Ergebnisse dienen als Entscheidungsgrößen zur Prognose der Lebensdauer sowie der Sicherheitsniveaus von Bestandsbauwerken. In diesem Beitrag wird der aktuelle Stand des Wissens im Bereich zerstörungsfreier Baustoffuntersuchung und des Monitorings dargelegt. Dabei wird den statistischen Auswertungsmethoden eine besondere Aufmerksamkeit geschenkt. Innovativ ist die holistische Lebenszyklusbetrachtung, wo jedem Bauteil und Bauwerk eine bestimmte Lebensdauer zugeordnet wird. Dynamische Monitoringsysteme mit digitaler Signalverarbeitung sowie Näherungsverfahren zur automatisierten Bestimmung der Dämpfung eignen sich, um die Verformungen von Bestandsbrücken zu bestimmen. Mit der satellitengestützten Radarinterferometrie kann großflächig Setzungsmonitoring von Gebäuden im Submillimeterbereich durchgeführt werden. An Bestandsbrücken kann mit Georadarmessungen auf die Ermüdung geschlossen und das Hinterfüllmaterial von Bogenbrücken bewertet werden. Bestimmte Monitoringsysteme sollten bereits bei der Planung von neuen Bauteilen mitbetrachtet und in den frühen Bauphasen eingesetzt werden.
Trata-se de discutir a complexa problemática de medida e avaliação da resistência do concreto em estruturas acabadas, ou seja, em estruturas ou componentes estruturais já moldados in loco ou pré-fabricados, em obras em construção ou construídas há anos, para fins de revisão da segurança dessa estrutura. Inicia-se por uma sintética revisão dos conceitos de introdução da segurança no projeto das estruturas de concreto com o significado dos coeficientes de minoração da resistência dos materiais. A partir de resultados experimentais obtidos em teses de doutoramento discute-se a ordem de grandeza da influência de certas variáveis aleatórias principais. Na seqüências trata-se da representatividade da amostragem e cuidados com a extração dos testemunhos cilíndricos. A questão do crescimento da resistência com a idade e do decréscimo dessa resistência com a carga de longa duração (efeito Rüsch) também são abordados para encerrar propondo um procedimento adequado de obtenção do fck para fins de revisão da segurança do projeto estrutural.
The importance of the reliable evaluation of concrete quality is well accepted. The evaluation of concrete quality is a very wide topic. The first part of this paper in the current issue of BFT focuses on two topics, namely, specifying and monitoring concrete strength, and assessing concrete strength in structures by testing cores. The quality of testing and the retrogression of concrete strength will be discussed in BFT 12/2002.
Serious accidents of the last decades have drawn attention to investigate the effect of tunnel fires, tunnelling materials as well as to increase the residual safety of structures after the fire. To moderate the effects of heat on structural materials efficiently, it is necessary to understand the physical and chemical changes of the concrete during and after fire. The change of properties of the hardened cement pastes has been investigated at the Department of Construction Materials and Engineering Geology at Budapest University of Technology and Economics. The aim of our study was to define the change of physical and mechanical parameters of hardened cement pastes caused by thermal shock. Numerous (nearly 4500 pieces) 30 mm cubes have been made with various types of test parameters (e.g. specific surface, water/cement ratio, type and mass of different additives). The specimens have been heat treated in a preheated electrical furnace for 120 minutes. 11 different temperature steps were used (from the base laboratory condition up to 900 °C). Present paper summarises our results of these series of tests.
The aim of the paper is to introduce the statistical definition of the specified compressive strength of the concrete to be used for safety evaluation of the existing structure in domestic practice and to present the practical method to obtain the specified strength by utilizing the non-destructive test data as well as the limited number of core test data. The statistical definition of the specified compressive strength of concrete in the design codes is reviewed and the consistent formulations to statistically estimate the specified strength for assessment are described. In order to prevent estimating an unrealistically small value of the specified strength due to limited number of data, it is proposed that the information from the non-destructive test data is combined to that of the minimum core test data. The the sample mean, standard deviation and total number of concrete test are obtained from combined test data. The proposed procedures are applied to an example test data composed of the artificial numerical values and the actual evaluation data collected from the bridge assessment reports. The calculation results show that the proposed statistical estimation procedures yield reasonable values of the specified strength for assessment by applying the non-destructive test data in addition to the limited number of core test data.
Cores with diameters of 74 mm and length-to-diameter (l/d) ratios of 0.5, 1.0, 1.5, 2.0, 3.0, and 4.0, were removed from concrete beams. Compressive strength tests were conducted on 300 core specimens. The compressive strengths of concrete cores were fitted to the modified Weibull distribution. This information is useful in the theoretical description of concrete failure. Furthermore, on the basis of the Kolmogorov-Smirnov goodness-of-fit tests, the test data have been verified to follow this modified Weibull distribution. In addition, the modified Weibull model can predict the length effect more accurately than the conventional model. (C) 2014 American Society of Civil Engineers.
The evaluation of the structural capacity of existing structures and infrastructures requires the ssessment of the actual in-place concrete strength. Direct and indirect methods to evaluate the oncrete strength have been developed though the primary technique is the core test.
In order to estimate the structural capacity of an existing structure the experimental data obtained by esting of cores were used to evaluate the specified in-place compressive strength according to the ain standards such as EN 13791 and ACI 214.4R. The results obtained were critically evaluated, ompared, and analyzed to assess the in-place concrete quality according also to the statistical ethods proposed in ISO 12491.
The procedures proposed by the standards studied give similar results though they are obtained ollowing different approaches.
The facility performance requirements for the Confederation Bridge specified that the bridge be designed using load and resistance factors developed specifically for the site. This paper reviews the derivation of these factors for both the ultimate limit states and the serviceability limit states.
Practitioners were invited to respond to a problem statement published in the November 2000 issue of the Journal of the Performance of Constructed Facilities asking for an estimate of the compressive strength of in situ concrete based on hypothetical results of compressive tests on ten cores. Responses were received from 23 persons. Several respondents provided more than one estimate. It was found that there were wide variations in practice for determination of compressive strength from cores. Estimates varied from 3,000 to 5,000 psi. The most frequent estimate was 4,000 psi. The paper presents background information on code provisions relating to core testing, summarizes and discusses the responses, and offers observations based on the responses regarding investigations where compressive strength of concrete in an existing structure is determined from test of cores.
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