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An experimental investigation involving 287 concrete core specimens obtained from large beams cast from 45 to 90 MPa (6500 to 13,000 psi) ordinary portland cement concretes is described. The data indicate the effect of variations of core size and test moisture condition on the measured compressive strength of cores from elements made of high-performance concrete. The relationship between the flexural capacity of the beams and the strength of the cores obtained from the beams is investigated. The data indicate that the least-biased estimate of the in situ concrete compressive strength is found by increasing the strength of seated cores by 6 percent to account for damage sustained during drilling of the core. However, a reasonable estimate of the in situ concrete compressive strength may be obtained by correcting the strengths from tests of air-dried or soaked cores to account for both core damage and moisture condition factors.

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... It is not only in ASR-damaged concrete that the compressive strength is anisotropic. Several experimental studies have found the compressive strength of sound concrete to be anisotropic as well (Hughes and Ash, 1970;Johnston, 1973;Leshchinsky, 1990;Bartlett and MacGregor, 1995;Ergun and Kurklu, 2012;Den Uijl and Yang, 2009). The studies however, strongly disagree on the magnitude of the anisotropy; absolute as well as relative. ...

... The existence of strength anisotropy in sound concrete is often explained by segregation or water migration in the fresh concrete, which causes weak interfaces or initial microcracks between the cement paste and the undersurface of the large aggregate particles (Hughes and Ash, 1970;Johnston, 1973;Leshchinsky, 1990;Bartlett and MacGregor, 1995;Ergun and Kurklu, 2012;Van Mier, 1984, 1996. The compressive strength is therefore often indexed by its direction relative to the casting direction: the core compressive strength parallel to the casting direction (f c,0 ) and the core compressive strength perpendicular to the casting direction (f c,90 ), see also Figure 5.5. ...

... It is well known that f c has a spatial variation, see e.g. (Bartlett and MacGregor, 1995). In the slabs of Test Series 1, the drilling locations for core 90 and core 0 are sufficiently interspersed to neglect the influence on the measured anisotropy. ...

Alkali-silica reaction (ASR) is a deterioration mechanism that can occur in concrete structures. It is a chemical reaction between alkalis, silica minerals in the reactive aggregates and water. The reaction causes severe cracking of the concrete, which results insignificant reductions of the strength parameters. This material degradation has raised serious concerns regarding the safety of ASR-damaged structures; particularly structures, which may be sensitive to shear failure. The Danish Road Directorate has estimated that more than 600 Danish road bridges have the potential to develop ASR in the future. The majority of these bridges has been constructed as slabs without shear reinforcement, i.e. structures where the shear capacity relies entirely on the strength of the concrete. Unfortunately, there exists no satisfactory method to assess the residual shear capacity of ASR-damaged slabs without shear reinforcement - in spite of nearly 80 years of research on ASR.
The aim of this PhD project is therefore to develop an approach that can be used to determine the shear capacity of ASR-damaged slabs without shear reinforcement. The approach includes a shear model as well as recommendations and descriptions of how the relevant strength parameters should be determined by simple tests on samples taken from the structure. The works that have been undertaken to develop this approach are as follows.
In the first part of the project, a literature study on how ASR affects the parameters that are important for the shear capacity is conducted. One of the main findings here is that ASR affects slabs differently than other types of structures, e.g. the way that the ASR-induced cracks are orientated. The majority of the existing ASR research on material characteristics and/or residual capacity of reinforced members is therefore not directly applicable for this PhD project. Based on the findings as well as shortcomings in the existing literature, a number of research questions that need answers in order to develop a shear model for ASR-damaged slabs are formulated.
In the second part of the project, answers to the formulated research questions are found by means of a thorough experimental investigation, where the effects of ASR on the material properties as well as on the structural response are studied. The investigation includes a large shear testing campaign with specimens cut out from two ASR-damaged bridges. The material properties are investigated by means of standard test methods and Digital Image Correlation (DIC). By a critical examination of the results and an optical investigation of the underlying mechanisms, recommendations of testing methods to obtain the anisotropic residual compressive- and tensile strength are formulated.
In the last part of the project, a model to determine the shear capacity of ASR-damaged slabs without shear reinforcement is established. The model is based on the upper bound theorem of plasticity theory, where the specific solutions are derived with inspiration from the failure mechanisms observed in shear tests with the ASR-damaged slab bridge specimens. The calculated shear capacity correlates well with test results, both for simply supported members and for continuous members.
Based on the model, some recommendations are given for how practical assessment of members subjected to arbitrary loading can be carried out.

... Due to practical reasons, the cores are always drilled perpendicular to the surface of the structure. However, it is known that the core compressive strength is dependent on the drilling direction, [1][2][3][4][5][6]. Hughes and Ash [1], for example, found as much as 50 percent strength difference between cores drilled parallel and perpendicular to the casting direction. ...

... strength anisotropy) is as dramatic as suggested by Hughes and Ash [1], then the current practice -as described above -to estimate the residual capacity of an existing structure may potentially be misleading. Despite the relevance and the potential impact on current practices for strength assessment of existing structures, the subject of compressive strength anisotropy has received very little attention in the literature, [1][2][3][4][5][6][7][8]. The few previous studies disagree strongly on the magnitude of the anisotropy; absolute as well as relative. ...

... The existence of strength anisotropy in concrete (without a previous load history) is often explained by segregation or water migration in the fresh concrete, which causes weak interfaces or initial micro cracks between the cement paste and the undersurface of the large aggregate particles [1][2][3][4][5]7,8]. The most commonly used measure for anisotropy is the ratio between the concrete core compressive strength parallel to the casting direction (f c,core ) and the core compressive strength perpendicular to the casting direction (f c,core ), see also Figure 1. ...

This paper offers a new and closer look into the strength anisotropy of concrete by presenting the so far largest experimental programme (290 tests) and by presenting an advanced statistical analysis of the results. The experimental investigation sheds light on the influence of several important design parameters and conditions on the anisotropy. This includes the influence of reinforcement, w/c-ratio, curing time, load history and structural geometry. For this purpose, cores were drilled out at different angles from beam-and slabs specimens for compressive testing. The main findings include: a) the reference cylinder strength (i.e. w/c-ratio) does not have a significant influence on the anisotropy when the anisotropy is quantified as an absolute difference between the strength of cores drilled in the two directions; b) the anisotropy in structural members without load history is less than 5 MPa; c) the anisotropy amounts to 5-10 MPa for members with load history.

... of 54.1 MPa (7850 psi) differed by 11% of their average (Bartlett and MacGregor 1994a). Microcracks can be present if the core is drilled from a region of the structure that has been subjected to stress resulting from either applied loads or restraint of imposed deformations. ...

... The single-operator coefficient of variation is a measure of the repeatability of the core test when performed in accordance with ASTM C 42/C 42M. A practical use of this measure is to check whether the difference between strength test results (Bartlett and MacGregor 1994a). ...

... Moisture condition-Different moisture-conditioning treatments have a considerable effect on the measured strengths. Air-dried cores are on average 10 to 14% (Neville 1981; Bartlett and MacGregor 1994a) stronger than soaked cores, although the actual ratio for cores from a specific concrete can differ considerably from these average values. Soaking causes the concrete at the surface of the specimen to swell, and restraint of this swelling by the interior region causes self-equilibrated stresses that reduce the measured compressive strength (Popovics 1986). ...

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.

... Several experimental programmes have been carried out to study a particular condition that the conversion formulas do not account for; namely the (presumed) anisotropy of the compressive core strength [2][3][4][5][6]. The studies conclude that the core strength depends on whether the cores are drilled parallel (in this paper named core∥) or perpendicular (in this paper named core⊥) to the casting direction, see figure 1. ...

... In 1995 Bartlett and MacGregor [5] studied the anisotropy of the compressive strength and the influence of three different concrete mixes. The test result shows that only one concrete mix has a significant anisotropy where fc,core∥ is 14% larger than fc,core⊥. ...

p>When the load carrying capacity of existing concrete structures is (re-)assessed it is often based on compressive strength of cores drilled out from the structure. Existing studies show that the core compressive strength is anisotropic; i.e. it depends on whether the cores are drilled parallel or perpendicular to the casting direction. Engineers may therefore misjudge the load carrying capacity. Thus structures may be strengthened or rebuilt unnecessarily or left in service with high failure probability.
This paper presents a literature review and an experimental study on the anisotropy and its correlation to the curing time. The experiments show no correlation between the anisotropy and the curing time and a small strength difference between the two drilling directions. The literature shows variations on which drilling direction that is strongest. Based on a Monto Carlo simulation of the expected variation it is argued that the variation of the anisotropy may be statistically coincidences.</p

... For practical convenience, these cores are typically drilled perpendicular to the concrete surface, under the assumption that the mechanical properties of the concrete are isotropic. Several experimental studies however showed that this assumption is not always valid [1][2][3][4][5][6], and strength differences up to 50% for different drilling directions have been reported [1]. These studies disagree on the magnitude and causes of the anisotropic behaviour of the compressive strength. ...

... The concrete compressive strength is known to be strongly affected by spatial variations [4]. The experiments (core drilling) must therefore be designed such that potential strength differences caused by spatial variations are not mistaken for anisotropy. ...

Assessment of the load-carrying capacity of existing concrete structures is often based on the concrete compressive strength obtained from drilled cores. These cores are typically drilled perpendicular to the concrete surface, under the assumption that the mechanical properties of the concrete are isotropic. Recent studies however showed that concrete may in fact be subject to anisotropic behaviour. These studies are limited to newly-cast concrete only, and little is known about the anisotropic behaviour of existing structures in-service. This paper presents the first results of a large experimental programme where the anisotropy of the compressive strength in existing concrete structures is investigated. For this, 195 cores, drilled from a large concrete bridge located in Denmark, are tested. Three drilling directions are considered. The results are analysed using statistical techniques. The results showed that there is a statistically significant difference between the compressive strength in longitudinal and transverse/vertical direction, with an average value of 4.5 MPa in the disadvantage of the longitudinal direction.

... Several experimental programmes have been carried out to study a particular condition that the conversion formulas do not account for; namely the (presumed) anisotropy of the compressive core strength [2][3][4][5][6]. The studies conclude that the core strength depends on whether the cores are drilled parallel (in this paper named core∥) or perpendicular (in this paper named core⊥) to the casting direction, see figure 1. ...

... In 1995 Bartlett and MacGregor [5] studied the anisotropy of the compressive strength and the influence of three different concrete mixes. The test result shows that only one concrete mix has a significant anisotropy where fc,core∥ is 14% larger than fc,core⊥. ...

When the load carrying capacity of existing concrete structures is (re-)assessed it is often based on compressive strength of cores drilled out from the structure. Existing studies show that the core compressive strength is anisotropic; i.e. it depends on whether the cores are drilled parallel or perpendicular to the casting direction. Engineers may therefore misjudge the load carrying capacity. Thus structures may be strengthened or rebuilt unnecessarily or left in service with high failure probability.
This paper presents a literature review and an experimental study on the anisotropy and its correlation to the curing time. The experiments show no correlation between the anisotropy and the curing time and a small strength difference between the two drilling directions. The literature shows variations on which drilling direction that is strongest. Based on a Monto Carlo simulation of the expected variation it is argued that the variation of the anisotropy may be statistically coincidences.

... It is generally agreed that the compressive strength of the extracted core can be obtained by dividing the ultimate load by the cross-sectional area of the core, calculated from the average diameter; however, the critical issue is to translate this result into cube strength. In fact, the compressive strength of the concrete cores is affected by a number of factors such as slenderness ratio of sample (ratio between sample length and sample diameter, λ = l/d), the direction of drilling (parallel or perpendicular to casting direction), diameter value (50 mm, 100 mm, or 150 mm), presence of reinforcement steel bars in the specimen, moisture condition of the core specimen and the effect of drilling damage to the sample [34][35][36][37][38][39]. Table 1 summarizes the factors considered in American code, European code, Egyptian code, British standards and Concrete society to interpret core strength to in-situ concrete strength. ...

The core-drilling method is a reliable and confident method to evaluate the in-situ compressive strength. Codes and standard relations are developed to evaluate the compressive strength of the traditional concrete. The fiber reinforced concrete (FRC) is different in its behavior and mode of failure when compared to traditional concrete. This paper presents an experimental study on the applicability of these features of FRC. 168 drilled specimens were extracted from 10 different concrete mixes then tested. The investigated parameters were: the fiber type (polypropylene, glass, steel); fiber volume fraction (0.25%, 0.50%, 0.75%); cutting direction (horizontal, vertical); core diameter (100 mm, 150 mm). Standard cubes and cylinders were taken from each mix to evaluate the compressive strength. The results of the drilled cores were verified to more than predicted by code. The results revealed that the existing relationships underestimate the core compressive strength for FRC. A new modification factor was proposed depending on the fiber type and fiber volume fraction to estimate accurately the in-situ core compressive strength.

... Therefore, the common way of determining the in-situ strength of concrete is accepted to be to drill and test concrete cores [3][4][5][6]. However, the strength results obtained on cores should be carefully interpreted because they are affected by several factors such as core diameter, core length-to-diameter ratio, moisture condition of the core specimen, the direction of drilling, the presence of reinforcement steel bars in the core and even the strength level of the concrete [7][8][9][10]. ...

Determination of the compressive strength of concrete in existing reinforced concrete structures is important, particularly in some cases. Destructive and non-destructive test methods are used to determine the compressive strength in such structures. Amongst these, coring is the most widely used method as a destructive method in determining the compressive strength in existing reinforced concrete structures. However, even though numerous studies have been carried out on the variation of the determined compressive strength depending on the coring direction, the discussions continue. In this study, concrete blocks containing fly ash (FA) and silica fume (SF) and aggregates of different maximum sizes were produced. In the mixtures, cement was replaced by fly ash at ratios of 20%, 40%, and 60% and silica fume at ratios of 5%, 10%, and 15%, respectively. Two aggregates with the maximum aggregate sizes of 16 mm and 31.5 mm were used in the production of the concrete blocks. The concrete blocks were kept in a laboratory by covering them with burlaps moistened intermittently for 28 days and then cores of 10 cm diameter were taken and cut to have 20 cm height then capped and tested to determine compressive strength. Core drilling was carried out parallel and perpendicular to the casting direction and then the compressive strengths were determined. Cubes of 15 cm were also prepared and tested to determine the compressive strength level of concrete and to make comparisons accordingly. The compressive strength of mineral-added core samples taken parallel to the casting direction is higher than those of taken perpendicular to the casting direction, in all mixtures. The ANOVA test applied on the results obtained, it was found that the maximum aggregate size (Dmax) and the core drilling direction with respect to casting direction is statistically significant in terms of the compressive strength of concrete produced using fly ash and silica fume at certain substitution ratios.

... Engineers also perform destructive testing in which concrete cores are bored out from the final structure and are tested for compressive strength. This is one of the most reliable methods to estimate compressive strength [2][3][4] . However, the method is destructive and damages concrete integrity and might affect the reinforcement as well. ...

... In compliance with the British Standard Institution [30] Requirements, the International Union of Laboratories and Building Materials Experts, Structures and Systems (RILEM) [32] and the Japan Institute of Architecture (AIJ) [31] for NDT, a non-destructive test is proposed for the purpose of measuring the compressive strength of concrete members on the basis of the strength similarity obtained from core strength tests. In the use of a core sample, one of the most critical issues is that the high-speed drill bit during the extraction phase of the cores is quickly damaged [33]. ...

This paper aims to respond to these concerns through the identification and explanation of the most popular and effective NDT approaches in concrete structures and also their accuracies. The fundamentals of the non-destructive test methods are discussed in terms of their capacity, limits, inspection techniques and interpretations. Factors that affect the performance of NDT an approach are discussed and means of mediate their influence was suggested. Ultrasonic pulse velocity and SONREB methods of Non-destructive test are showed in this paper as past experiments of NDT. NDT of concrete was found to be increasingly recognized as a way of measuring the strength, integrity, resilience and other properties of existing concrete structures, Perceptions of NDT inadequacy are attributed to lack of knowledge of the building materials and the NDT approaches themselves. The goal of this paper is to resolve these issues reviewing some articles already done and defining and discussing the most common popular NDT methods applied to concrete structures.

... For this reason, the drilling-and-coring test method is commonly used for determining the in-situ concrete strength. Although the method suffers from expensive and time-consuming operations, the cores lead to reliable and useful results because they are mechanically tested until failure [8][9][10][11][12][13][14][15][16]. However, the test results should be carefully interpreted because the core strength is influenced by a number of factors such as: diameter, length-to-diameter ratio, core specimen moisture, drilling orientation, steelreinforcement elements, and strength of the concrete surface [17][18][19][20][21][22][23][24][25][26][27][28]. ...

The concrete compression strength is an effective characteristics among other properties of practical significance. Although coring testing on such members as columns is not recommended. But sometimes, for determining the concrete strength in the column, this method should be applied to the reinforced concrete one. Coring in a reinforced concrete column creates a cylindrical cavity in it, which has an apparently negative effect on the bearing capacity of the structural member. The effect of different sizes of cavities on uniaxial compression strength of concrete was investigated based on the experiment and numerical simulation (particle flow code). The results of the experiments show that the cavities have a great influence on the uniaxial compression strength. For example, if the
cavity volume is calculated about 14% of the sample volume, it can reduce of the uniaxial
compression strength down to 58%. If the cavity diameter is 60% of the sample width, the strength can go down to 74%.

... For this reason, the drilling-and-coring test method is commonly used for determining the in-situ concrete strength. Although the method suffers from expensive and time-consuming operations, the cores lead to reliable and useful results because they are mechanically tested until failure [8][9][10][11][12][13][14][15][16]. However, the test results should be carefully interpreted because the core strength is influenced by a number of factors such as: diameter, length-to-diameter ratio, core specimen moisture, drilling orientation, steelreinforcement elements, and strength of the concrete surface [17][18][19][20][21][22][23][24][25][26][27][28]. ...

The concrete compressive strength is a good index of most other properties of practical significance. Although coring testing on members such as columns is not recommended. But sometimes, in order to determine the concrete strength in the column, we have to apply this method in the reinforced concrete column. Coring in a reinforced concrete column creates a cylindrical cavity in it, which seems to have a negative effect on the bearing capacity of the structural member. In this paper, the effect of different sizes of cavities on uniaxial compressive strength of concrete was investigated based on experimental tests and numerical modeling (particle flow code). The results of the experiments show that the cavities have a great influence on the uniaxial compressive strength. For example, if the cavity volume is calculated about 14% of the sample volume, it can reduce up to 58% of the uniaxial compressive strength. Or, if the cavity diameter is 60% of the sample width, it can reduce up to 74% of the uniaxial compressive strength.

... On the other hand the difference in experimental conditions and devises cause significant differences in concrete strength. Despite the relevance and the potential impact on current practices for strength assessment of existing structures, the subject of compressive strength has received little attention in the literature [1,[10][11][12][13][14][15][16][17]. ...

In Iran Concrete Code (ABA), the criteria for calculation of standard deviation (s) are comprehensive and holistic. However, if it would be determined separately for each geographical area, significant changes could occur due to the use of concrete as one of the common materials. This paper analyses the criteria and redefines the acceptance standards for concrete compressive strength in ABA using experimental data available in Kohgiluyeh and Boyer-Ahmad and Fars provinces. The main hypothesis of the study is that using the statistical analysis of the test specimens for three categories C21, C30 and C35 in various projects located in Kohgiluyeh and Boyer-Ahmad and Fars provinces, extracting standard deviations, mean and the compressive strength of the specimens and their comparison with ABA proposed relationships and values, it is possible to propose new amendments for these areas in line with economic savings in national and international projects. In this study using the quantitative Strategy, library - Internet studies, field studies and in cooperation with the concrete labs, required information for 4878 concrete specimens was collected from the above-mentioned areas. By analysing the acceptance regulations for the specimens based on ABA and comparing the standard deviation of these data with the formulas of the regulations, significant results were obtained for the standard deviation factor correction and finally some formulas were suggested for the acceptance of the concrete specimens.

... According to the specifications British Standard Institution (BSI) [21], International Union of laboratories and Experts in Construction Materials, Systems and Structures (RILEM) [5] and Architectural Institute of Japan (AIJ) [4] for NDT, a non-destructive test is recommended for estimating the compressive strength of concrete members, based on the correlation of the strength obtained by core strength tests. One of the most significant concerns in using a core specimen is that it is easily damaged by the high speed drill bit during the extracting process of the cores [41]. The core strength test essentially partially damages the target concrete member. ...

Estimating the compressive strength of high strength concrete (HSC) is an essential investigation for the maintenance of nuclear power plant (NPP) structures. This study intends to evaluate the compressive strength of HSC using two approaches: non-destructive tests and concrete core strength. For non-destructive tests, samples of HSC were mixed to a specified design strength of 40, 60 and 100 MPa. Based on a dual regression relation between ultrasonic pulse velocity (UPV) and rebound hammer (RH) measurements, an estimation expression is developed. In comparison to previously published estimation equations, the equation proposed in this study shows the highest accuracy and the lowest root mean square error (RMSE). For the estimation of compressive strength using concrete core specimens, three different concrete core diameters were examined: 30, 50, and 100 mm. Based on 61 measured compressive strengths of core specimens, a simple strength correction factor is investigated. The compressive strength of a concrete core specimen decreases as the core diameter reduces. Such a relation is associated with the internal damage of concrete cores and the degree of coarse aggregate within the core diameter from the extracting process of the cores. The strength estimation expressions was formulated using the non-destructive technique and the core strength estimation can be updated with further test results and utilized for the maintenance of NPP.

... 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.

... Bartlett and MacGregor [13] found that, on average, the strength of cores dried in the air for 7 days is 14% higher than the strength of cores soaked in water for, at least, 40 hours. Another research carried out by Bartlett and MacGregor [14]. They observed a more severe strength loss in 50 mm diameter cores compared with 100 mm diameter cores from the same element. ...

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.

... Test results of Bartlet et al. [1], indicated that the average compressive strength of cores with 50 mm diameter equaled to 92 and 94 percent of that of the cores with 150 mm and 100 mm diameter, respectively. It was concluded that concrete cores with smaller diameters have smaller compressive strength. ...

... Moreover, observations of compressive strength are contradictory, some showing that SAP addition increases strength, others that SAP decreases strength. The main factors include [1,4,5,6], see table 1: The objective of the present project is to study the development of strength and modulus of elasticity for concrete with SAP and see if results are in accordance with models that are already established for conventional concrete. Results concerning modulus of elasticity are described in another paper, see [7]. ...

The development of mechanical properties has been studied in a test program comprising 15 different concrete mixes with 3 different w/c ratios and different additions of superabsorbent polymers (SAP). The degree of hydration is followed for 15 corresponding paste mixes. This paper concerns compressive strength. It shows that results agree well with a model based on the following:
1)
Concrete compressive strength is proportional to compressive strength of the paste phase
2)
Paste strength depends on gel space ratio, as suggested by Powers
3)
The influence of air voids created by SAP on compressive strength can be accounted for in the same way as when taking the air content into account in Bolomeys formula.
The implication of the model is that at low w/c ratios (w/c < 0.40) and moderate SAP additions, SAP increases the compressive strength at later ages (from 3 days after casting and onwards), because SAP increases the degree of hydration and therefore also the gel space ratio of the paste. However, at high w/c ratios (w/c > 0.45) and addition of large amounts of SAP, this effect cannot counterbalance the strength reducing effect of increased void volume. In these cases, SAP addition reduces the compressive strength.

... The moisture conditions of a sample influence its compressive strength. Bartlett and MacGregor [13] observed a 22% strength reduction when a specimen was soaked in water after drying at RH<60%. The higher strength at lower RH can be attributed to the strengthening action of the capillary forces in the pore fluid on the solid structure. ...

This paper deals with the compressive strength development of cement pastes and mortars with superabsorbent polymers, SAP. Addition of SAP to high performance cementitious mixtures is done to reduce or avoid self-desiccation and self-desiccation shrinkage, but also influences the compressive strength. This study shows that whereas the compressive strength of high-strength cement pastes is slightly reduced by SAP addition, the strength of mortars is almost unaffected. Additionally, the paper examines briefly the effect on strength of degree of hydration and porosity, moisture conditions and pore structure of the SAP composites.

... For these special situations, the core test is the most useful and reliable way to assess the properties of the concrete in the structure [1]. For these reasons, the common way of determining in-situ strength of concrete is to drill and test cores [1][2][3][4][5][6][7][8][9][10]. Although the method consists of expensive and time consuming operations, cores give reliable and useful results since they are mechanically tested to destruction [2]. ...

Core test is commonly required in the area of concrete industry to evaluate the concrete strength and sometimes it becomes the unique tool for safety assessment of existing concrete structures. Core test is therefore introduced in most codes. An extensive literature survey on different international codes’ provisions; including the Egyptian, British, European and ACI Codes, for core analysis is presented. All studied codes’ provisions seem to be unreliable for predicting the in-situ concrete cube strength from the results of core tests. A comprehensive experimental study was undertaken to examine the factors affecting the interpretation of core test results. The program involves four concrete mixes, three concrete grades (18, 30 and 48 MPa), five core diameters (1.5, 2, 3, 4 and 6 in.), five core aspect ratios (between 1 and 2), two types of coarse aggregates (pink lime stone and gravel), two coring directions, three moisture conditions and 18 different steel arrangements. Prototypes for concrete slabs and columns were constructed. More than 500 cores were prepared and tested in addition to tremendous number of concrete cubes and cylinders. Results indicate that the core strength reduces with the increase in aspect ratio, the reduction in core diameter, the presence of reinforcing steel, the incorporation of gravel in concrete, the increase in core moisture content, the drilling perpendicular to casting direction, and the reduction in concrete strength. The Egyptian code provision for core interpretation is critically examined. Based on the experimental evidences throughout this study, statistical analysis has been performed to determine reliable strength correction factors that account for the studied variables. A simple weighted regression analysis of a model without an intercept was carried out using the “SAS Software” package as well as “Data Fit” software. A new model for interpretation of core test results is proposed considering all factors affecting core strength. The model when calibrated against large number of test data shows good agreement. The proposed model can effectively estimate the in-situ concrete cube strength from core test results.

... Although the method consists of expensive and time consuming operations, cores give reliable and useful results since they are mechanically tested to destruction [2]. However, the test results should be carefully interpreted because core strengths are affected by a number of factors such as diameter, l/d ratio and moisture condition of the core specimen, the direction of drilling , presence of reinforcement steel bars in the specimen and even the strength level of the concrete111213141516. The diameter of the core plays an important role in the evaluation of core strength results. ...

... 13). The reduction of core strength due to microcracking was already examined by other researchers [19], thus explaining the difference of the average strengths of cores drilled from two ends of a beam made up of homogeneous concrete. core ...

p>Assessment of the load‐carrying capacity of existing concrete structures is often based on the concrete compressive strength obtained from drilled cores. These cores are typically drilled perpendicular to the concrete surface, under the assumption that the mechanical properties of the concrete are isotropic. Recent studies however showed that concrete may in fact be subject to anisotropic behaviour. These studies are limited to newly‐cast concrete only, and little is known about the anisotropic behaviour of existing structures in‐service. This paper presents the first results of a large experimental programme where the anisotropy of the compressive strength in existing concrete structures is investigated. For this, 195 cores, drilled from a large concrete bridge located in Denmark, are tested. Three drilling directions are considered. The results are analysed using statistical techniques. The results showed that there is a statistically significant difference between the compressive strength in longitudinal and transverse/vertical direction, with an average value of
4.5 MPa in the disadvantage of the longitudinal direction.</p

Weakening of concrete is largely affected by the design of the structures, construction procedure and activities of the construction site, and the length of experiences the structure is exposed to during its service life. However, proper maintenance and timely inspection of existing building can minimize the possible structural failures. The concrete strength of an existing building plays a vital role in overall performance of the building. The estimation of strength properties of concrete can be performed by several methods: visual, destructive and non-destructive test methods. The core test (destructive) is considered as the most reliable test method since it provides the direct com-pressive strength of the concrete, but often it is difficult of extracting large diameter concrete core due to the presence and interference of reinforcement bars inside of concrete. Therefore, the core diameter varies significantly, even in the same building, in order to extract the cores without interfering the inside main reinforcements. In this study, six existing buildings are studied in the field to estimate the in-situ concrete strength using extracted core test. Different sizes of cores are collected and tested in the laboratory to investigate concrete strength. No consistent trend is found from the collected samples. In these type of cases, more core samples with a smaller diameter (51 mm) might be an option to estimate the existing concrete strength of a building with more consistent manner, and as well as safe for the building during the safety evaluation. To justify the effectiveness of smaller diameter cores, the concrete strength is calculated for different core diameters using finite element analysis in Abaqus environment. Variation in core diameter does not show any significant deviation in estimating concrete strength, which is a promising indication of using cores with 51 mm (2 in.) diameter to estimate existing concrete strength.

There is no universal relation between the compressive strength of cores drilled from concrete elements and molded cylinder and cube concrete specimens. The strength of concrete cores depends on several parameters. The strength correction factors account for these parameters. In this study, the effects of diameters, length to core diameter ratio (λ=L/D), test age, and coring orientation on the compressive strength of cores were analyzed with respect to the molded cylinder and cube concrete specimens. According to the test results; the compressive strength changes in 100 and 75 mm diameter cores were found to be more significant and reliable compared to those of 50 mm diameter cores. The strength decreased by 10% and 6% in 100 and 75 mm diameter cores drilled perpendicular and parallel to the direction of casting due to drilling damage. The compressive strength of cores with λ=1.0 was equivalent to 92% of that of cores with λ=2.0. Furthermore, it was found that the cores drilled perpendicular to the direction of casting and having a λ=2.0 ratio was 83% and 71% of that of 28-day standard cylinder and cube, respectively. The correction factors between cores and standard cylinder and cube specimens were determined to assess the in-site compressive concrete strength.

The variation of in-place strength in a structure is due to within-batch variation, batch-to-batch variation, systematic within-member strength variation, and systematic between-member strength Variation. Batch-to-batch variation is particularly significant for cast-inflate structures and may either inflate the within-member variation if each member is cast from many batches or inflate the between-member variation if each member is cast from a single batch. Values of coefficients of variation that represent the overall variation of the in-place concrete strength in a structure vary from 7 percent for one member cast from one batch of concrete to 13 percent for a structure consisting of many members cast from many batches of cast-in-place concrete. Multiple regression analysis techniques are used to assess the systematic variation of the strength of concretes in laboratory specimens cast from one batch of concrete. Statistically significant systematic strength variation is detected over the height of 32 of 43 columns with average strengths from 2200 to 5200 psi. Typically, the top region was 3 to 14 percent weaker than the region in the middle, and the bottom region was 3 to 9 percent stronger than the region in the middle. Significant systematic variation of the in-place strength is also detected in 20 of 26 beams, blocks, slabs, and walls with average strengths from 2200 to 17,000 psi. investigation of ultrasonic pulse velocity and pull-off test data from building columns and bridge girders corroborates the findings of the investigation of elements cast in the laboratory.

A pulse-shaped split Hopkinson pressure (SHPB) was employed to determine the dynamic compressive mechanical responses of concrete cores. The loading pulses in SHPB experiments were precisely controlled to ensure that the core specimen deforms at a nearly constant strain rate under dynamically equilibrated stress during compression. A modified two-parameter Weibull distribution was used to analyze the test data. The Kolmogorov-Smirnov goodness-of-fit test was used to decide whether test data come from a population with this distribution. On the basis of the test data, Kolmogorov-Smirnov goodness-of-fit test, and probability plot, it is found that the modified Weibull model can be applied to compressive strength for concrete cores. In addition, the strain rate effect on the compressive strength of cores can be accurately predicted from the modified Weibull model. (C) 2014 American Society of Civil Engineers.

This paper describes the effect of curing conditions on the mechanical properties of mortars containing superabsorbent polymers (SAP). Curing temperature and relative humidity were varied from 20 to 40 oC and 30 to 95%, respectively, in mixes with different water/cement and cement/aggregate ratios. Tensile and compressive strength tests were performed at several ages. Weight loss over time was measured and related to curing conditions and strength. The addition of SAPs was found to effectively maintain cement-based mortar strength under extreme curing conditions.

This paper concerns a new concept for the prevention of self-desiccation in hardening cement-based materials. The concept is based on using fine, superabsorbent polymer (hydrogel, SAP) particles as a concrete admixture. This permits a controlled formation of water-filled macropore inclusions—water entrainment—in the fresh concrete. Consequently, the pore structure is actively designed to control self-desiccation. In the paper, experimental observations in relation to this technique are described and discussed. The observations show that self-desiccation can be controlled by water entrainment. The paper forms the second part of a series. In the first part, the theoretical background was presented [Cem. Concr. Res. 31(4) (2001) 647].

This paper concerns a new concept for the prevention of self-desiccation in hardening cement-based materials. The concept is based on using fine, superabsorbent polymer (hydrogel, SAP) particles as a concrete admixture. This permits a controlled formation of water-filled macropore inclusions—water entrainment—in the fresh concrete. Consequently, the pore structure is actively designed to control self-desiccation. In the paper, experimental observations in relation to this technique are described and discussed. The observations show that self-desiccation can be controlled by water entrainment. The paper forms the second part of a series. In the first part, the theoretical background was presented [Cem. Concr. Res. 31(4) (2001) 647].

This paper deals with the mechanical properties of mortars with internal curing, by means of water-entrainment with super
absorbent particles (SAP). The use of SAP in high performance concrete is focusing on the mitigation of autogenous deformation,
arising from self-desiccation. The effect of SAP in mortars subjected to several curing conditions and different water/cement
ratio was analysed. The curing conditions ranged between 30 and 100%RH at 20°C temperature. Water/cement ratio ranged between
0.25 and 0.35 for both reference and internal cured mixtures. Tensile and compressive strength tests were performed at different
ages. Results include weight loss measurements with time and its relation to environmental conditions and strength.
Internal curing by means of SAP was efficient in maintaining the mechanical properties of mortars regardless the variation
of external relative humidity.

A two-step method for converting a concrete core compression test result to the in-place strength of the corresponding volume of concrete is presented. The strength of a non-standard core is first converted to the equivalent strength of a standard core, and then the standard core strength is converted to the equivalent in-place strength. Strength correction factors required for these conversions, obtained from weighted linear and nonlinear regression analyses presented elsewhere, are summarized. The accuracy of the predicted in-place strength is affected by the inherent error of the core strength measurement itself, and by the uncertainty of the various strength correction factors. It is shown that confidence intervals on the estimates of the strength correction factors obtained by regression analysis underestimate the true model error because the underlying models are imperfect. Instead, the accuracy of the strength correction factors is determined by a weighted regression analysis of ratios of observed-to-predicted values which accounts for the non-uniform variances of the dependent and independent variables. The coefficient of variation of the in-place strength predicted from a test of a 100 or 150 mm diameter core is between 4 and 5.5.%. If the in-place strength is predicted from a test of a 50 mm diameter core, the coefficient of variation of the predicted in-place strength is approximately 12.5%. These error estimates do not account for possible variation of in-place strength throughout the volume of the element being cored.

This paper describes the effect of curing conditions on the mechanical properties of mortars containing superabsorbent polymers (SAP). Curing temperature and relative humidity were varied from 20 to 40 ºC and 30 to 95%, respectively, in mixes with different water/cement and cement/aggregate ratios. Tensile and compressive strength tests were performed at several ages. Weight loss over time was measured and related to curing conditions and strength. The addition of SAPs was found to effectively maintain cement-based mortar strength under extreme curing conditions.
Este artículo presenta el efecto de las condiciones de curado sobre las propiedades mecánicas de los morteros a los que se han añadido polímeros superabsorbentes como agentes de curado interno. Los morteros se curaron a dos temperaturas, 20 y 40 °C, y a varios valores de la humedad relativa entre el 30 y el 95%. Se estudió asimismo dicho efecto en función de la relación agua/cemento. Se realizaron pruebas de resistencia a la compresión y a la tracción a distintas edades. Los resultados incluyen la evolución de la pérdida de masa y su relación tanto con las condiciones ambientales como con el comportamiento resistente de los morteros. El curado interno de éstos con polímeros superabsorbentes permitió el mantenimiento de sus propiedades mecánicas en condiciones de curado extremas.

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