Variation Of Concrete Strength Research Articles

Early hydration properties and performance evolution of phosphoaluminate cement concrete

As a novel cementitious material, phosphoaluminate cement (PAC) has the characteristics of early strength and high strength. The early hydration properties of PAC were investigated, and the influence of curing age on concrete strength, anti-permeability, surface ventilation and micro structure was analyzed. Moreover, the inherent correlation between hydration mechanism and process of PAC was explored deeply by different test methods. The research results indicate that the curing age has significant effects on the performance development and micro structure of the concrete. The variation of concrete strength with curing age can be expressed by a Boltzmann function. Besides, the changes of relative permeability coefficient of anti-water permeability and chloride diffusion coefficient with curing age meet an exponential function. There exists a good linear relationship between natural logarithm of permeability pressure of concrete surface and time. The early hydration process of PAC is consisted of four stages, i.e., dissolution, induction, acceleration and deceleration.

Analysis of a Large Database of Concrete Core Tests with Emphasis on Within-Structure Variability.

In reinforced concrete (RC) structures, the compressive strength of concrete can play a crucial role in seismic performance and is usually difficult to estimate. Major seismic codes prescribe that concrete strength must be determined essentially from in situ and laboratory tests. Mean values obtained from such tests are the reference design values when assessing existing structures under seismic actions. The variability of concrete strength can also play an important role, generally requiring that various homogeneous domains are identified in a single structure, in each of which a specific mean value should be assumed as representative. This study analyzes the inter- and intra-variability of the concrete strength of existing buildings using a very large database made up of approximately 1600 core tests extracted from RC buildings located in the Basilicata region (Southern Italy). The analysis highlighted that concrete strength variability was dependent on the structures’ dimensions as well as on the number of storeys. Moreover, the concrete strength of cores extracted from columns was found to be, on average, lower than that from beams, thus justifying the usual practice to extract cores mainly from columns, which results in a conservative approach as well as a more feasible one. Finally, some case studies were analyzed, specifically focusing on the effects of the within-storey variability. Conservative strength values, to be used especially in the case of vertical members subjected to high axial loads, are suggested.

PERILAKU PUNCHING SHEAR PADA HUBUNGAN KOLOM BULAT DENGAN FLAT SLAB AKIBAT BEBAN TEKAN AKSIAL

The aim of the tests was to investigate the influence of concrete strength, the eccentricity of the column and the use of shear reinforcement in flat slabs on punching shear. The research specimens are 8 units of flat slabs. Flat slab size 1400 x 1400 mm2 with thickness of 120 mm. Flat slabs were connected with circular column with dimension 225 mm of diameter and 200 mm of height. Flat slabs were made in to 2 variations of concrete strength, e.i. 30 MPa and 60 MPa, 2 variations of shear reinforcement, e.i. without shear reinforcement and with shear reinforcement and 2 variations of eccentricity that, e.i. without eccentricity and with eccentricity. Each treatment has 1 specimen. Each specimen has 6 cylinder specimens. Cylinder specimens used as a concrete strength control for main specimen (flat slab). The tests showed that the concrete strength had a strong influence on punching shear strength. This is shown by capacity increase of 42.78%; 54.00%; 46.59% and 0.02%. The value is ratio between the maximum load of the specimens with 60 MPa and 30 MPa at the same eccentricity and the same shear reinforcement. The eccentricity of column reduce the capacity of punching shear. This is shown by 3 specimens decrease in capacity of 3.70%; 36.75% and 7.30%. Only 1 specimen that increase in capacity of 9.27%. The value is ratio between the maximum load of the specimens with 40 mm eccentricity and 0 mm eccentricity at the same compressive strenght and the same shear reinforcement. The use of shear reinforcement does not always increase the punching shear capacity. There are 2 observations that increased capacity (52.07% and 65.37% at the centric load) and 2 observations decreased capacity (0.12% and 4.92% at the eccentric load). The value is ratio between the maximum load on the specimens using shear reinforcement with the specimens that do not use shear reinforcement at the same compressive strenght and the same eccentricity.The use of shear reinforcement increase punching shear capacity of flat slab at the centric load condition. The use of shear reinforcement decrease punching shear capacity of flat slab at the eccentric load condition.

Parametric study on the non-contact splices at drilled shaft to bridge column interface based on multiscale modeling approach

The structural performance of large-scale rectangular column-circular drilled shaft interface using non-contact splices is investigated through experimental study and numerical analysis. The influences of lap spacing and the length of overlapped longitudinal bars, the stirrup ratio and the variation of concrete strength on the global mechanical response and mesoscale cracking pattern are analyzed. According to the provisions in current design guidelines, a total of eleven specimens are designed for parametric study. The macroscopic load-displacement curves, the cracking pattern of concrete of designed specimens are comparatively discussed. The experimental results matched well with that of the FEM analysis. The research findings can provide valuable guidance on the construction and design of non-contact lap splices with geometric irregularities in the following Phase II of this project.

Geometry, material properties and bond performance of prototype titanium reinforcing bars

The use of titanium as a concrete reinforcing bar material has been proposed. This study summarizes measured geometric and experimentally determined material properties of 6Al-4V titanium reinforcing bars and comparable properties of ASTM A615 steel. All bars are nominally #5 bars. Bond characteristics of the titanium bars were assessed through ASTM D7913 pull-out tests, ASTM A944 beam-end tests and four concrete prism tension tests. The nature of reinforcing bar bond to concrete is such that deformed bars exhibit very similar patterns of bond stress-slip behaviour. Provided adequate deformations are provided, the bond-slip relationship is dominated by concrete behaviour. The bond performance of the 6Al-4V titanium bars was similar to that of A615 steel bars and, as expected, clearly affected by the rib ratio. The results presented reinforce the ASTM A615-implied lower limit for the rib ratio, Rr > 0.05. The implication of a similar bond-stress behaviour is that existing bond relationships for steel-reinforced concrete likely apply to titanium bars provided they meet the deformation requirements of ASTM A615 – the standard for which steel reinforcing bars, and therefore their bond characterisation – is calibrated. Both the pull-out and beam-end test results reinforce the conclusion that bond behaviour of titanium bars is essentially the same as that steel bars. The bond stresses, normalised to account for variation in concrete strength, are similar and the calculated development lengths are essentially in the ratio of yield strengths of the materials. The prism tension tests demonstrated that concrete crack width is proportional to modular ratio of the reinforcing material, while spacing is inversely proportional to the stiffness of the initial bond-slip response.

Predicting pull-out behaviour of 4D/5D hooked end fibres embedded in normal-high strength concrete

Previous research (Abdallah et al., 2016) conducted by the authors had proposed an analytical model to predict the pull-out behaviour of 4DH/5DH fibres embedded in ultra-high performance concrete. The present paper is an extension to the prior study and focuses on the pull-out load-slip behaviour of these fibres embedded in normal-high strength concrete combinations. Based on experimental investigations, an elastic-plastic moment expression has been proposed to represent the partially plastic hinge formed during pull-out of each circular steel fibre. This prediction takes into account the variation of concrete strength, geometrical and tensile properties of the fibres. Partial deformation in straightening of the hook as well as fibre rupture was also included in the analysis. The model developed is validated against experimental pull-out results of various hooked end fibres, for which reasonable to very good predictions are found.

Assessing concrete strength variability in existing structures based on the results of NDTs

One of the main factors affecting the survey of concrete strength in existing structures is the inherent material variability, which can only be fully characterized when destructive tests are performed, especially in older structures. Therefore, having a preliminary estimate of the concrete strength variability facilitates the planning of destructive testing campaigns. In light of this, the proposed study presents the development of general empirical expressions estimating the in situ concrete strength variability using non-destructive test (NDT) results. These expressions are defined by examining the correlation between statistical parameters of datasets of concrete core strength, rebound hammer and ultrasonic pulse velocity test results using several correlation models that are based on common conversion models and on a generalization of the bi-objective approach. Based on these analyses, empirical models that are able to provide a reliable estimate of concrete strength variability using NDT results are proposed.

Study of the Behavior of Beam due to the Variation in Strength of Concrete and Steel Bars

RCC structures are primarily based with beams and columns. In general, the compressive strength of concrete and the yield strength of steel are specified in the design process. Compressive strength of concrete depends on a number of factors and generally shows some degree of variation from the desired strength. On the other hand, reinforcing bars with higher yield strength than that recommended in the Bangladesh National Building Code is available in the market and being used in construction. So, in practical cases, after casting the compressive strength as well as the tensile strength are not generally achieved as desired. Therefore, increase in yield strength of steel and decrease in compressive strength of concrete may have adverse effects on the flexural behavior of beams. This study includes the behavior of reinforced concrete beam due to this variation in strengths. This study shows that how certain beam behaves due to the variation of strength. Besides, reduction in ductility is also observed due to strength variation in the properties of the major constituent materials. A complete theoretical analysis along with some experimental investigation is presented in this paper.

Static behavior of stud connectors in bi-direction push-off tests

Headed stud shear connectors are commonly used to transfer longitudinal shear forces across the steel-concrete interface. In order to investigate the behavior of headed shear stud connectors, bi-direction push-off tests were conducted. Eighteen specimens, including six forward and twelve inverse push-off specimens, were tested to evaluate the failure modes, bearing capacity and load-displacement relationship of studs. An accurate numerical model was developed and validated against the experimental results, and based on which extensive parametric studies were performed to investigate the variation in concrete strength, stud yield strength, stud diameter and length-to-diameter ratio of stud on the structural performance. Based on the experimental and numerical results, new design equations of stud shear capacity and stud load-slip relationship were proposed and compared with the current code of practices GB 50017-2003, AISC, EN 1994-1-1, and relevant references. Comparing to the existing approaches, the proposed formulas have a better match with the experimental values.

Seismic behaviour of the old-type gravity load designed deteriorated RC buildings in Cyprus

Abstract Corrosion related deterioration of reinforced concrete buildings is often studied by the structural engineering community. The literature reveals the fact that modelling of deteriorated members requires a unique consideration due to brittle section behaviour. The late adaptation of seismic regulation rules and lack of decent quality construction material supply results a vulnerable building category in the island of Cyprus. This paper examines the vulnerable building category by combining concrete strength variation with different corrosion levels. A common building type representing 1980's was selected for analyses. By using the experimental data on local materials, the corrosion rate and concrete strength specific models were generated. The analyses on brittle low strength deteriorated models yielded to a reduction both in strength and ductility. The effects of considering different concrete strength on non-seismically designed deteriorated buildings were discussed herein.

Statistical analysis of concrete strength variability for quality assessment: Case study of a Saudi construction project

Statistical analysis of concrete strength variability for quality assessment: Case study of a Saudi construction project

Fracture energy-based model for average crack spacing of reinforced concrete considering size effect and concrete strength variation

The average crack spacing is a key parameter for an accurate evaluation of the crack width of reinforced concrete (RC) members. According to the test results in the existing literature, both the variations of the concrete strength and the size effect are critical factors influencing the average crack spacing. However, available prediction models for the average crack spacing cannot give satisfactory results in simulating both factors. Based on the finite-element (FE) analysis and the fracture-energy criterion, a theoretical method considering the influence of concrete strength variation and size effect is first proposed. It is assumed that a micro-crack will grow into a visible crack if and only if the energy release exceeds the fracture energy of the effective cracking area. Therefore, the average crack spacing can be predicted by equating the energy release, which is obtained by the three-dimensional FE model of concrete subjected to bond stress, to the fracture energy of the effective cracking area. In addition, from the proposed model, the characteristic length of concrete is found to be the most important material parameter for average crack spacing of RC members. Subsequently, a database including 136 test specimens is established to sufficiently validate the proposed model. The influence of various key factors on the average crack spacing is discussed in detail. Finally, simplified prediction formulas for average crack spacing of RC members are proposed considering both concrete strength variation and size effect. Comparisons indicate that both the proposed theoretical model and the simplified formulas have sufficient accuracy.

Factors affecting the reliability of assessing the concrete strength by rebound hammer and cores

To assess concrete strength in a structure, nondestructive technique (NDT) like rebound hammer is combined with destructive technique (coring tests) in order to implement a relationship ‘‘conversion model” between the compressive strength and, NDT measurements. The conversion model is used to estimate the local strength value at each test location using the corresponding NDT value. Then the estimated mean strength and/or estimated strength standard deviation (concrete strength variability) values are calculated. However, the reliability of these estimated values is always a questionable issue because of the uncertainties associated with the strength predictions based upon NDT measurements. To improve the reliability, the uncertainties must be reduced by specifying and controlling their influencing factors. The objective of this paper is to study the reliability of assessment by analyzing the effects of several influencing factors: number of test locations used to identify a conversion model between strength and rebound measurement NC (number of cores), true value of concrete strength variability, within-test variability of rebound measurements, accepted uncertainty level, quantity to be assessed (mean strength, strength variability), model identification approach (like regression) and the way of selection core locations (random or conditional i.e. selection based on NDT measurements from preliminary investigation). To this end, a large campaign of laboratory studies datasets (1700 test results) was considered for the analysis in the present study.Results show that NC, within-test variability of rebound measurements and true concrete strength variability have significant effects on the assessment reliability. Conditional selection of cores has also an important effect on improving the reliability so it is strongly recommended.

Non destructive assessment of in situ concrete strength: comparison of approaches through an international benchmark

Strength assessment of concrete in existing structures is a key issue. Many non destructive techniques (NDT) are available which can provide information about the material condition and several approaches can be used to derive strength estimates from NDT test results. However, common practice suffers from many drawbacks: (a) it is difficult to ascertain the level of accuracy/confidence of concrete strength estimates, (b) one lacks established guidelines for estimating the concrete strength variability, (c) the best way to combine NDT methods and coring remains an open issue. The RILEM TC 249 “Non destructive in situ strength assessment of concrete” is addressing these problems. A benchmark was carried out in order to compare (a) how experts define and can carry out an NDT investigation program and (b) how experts derive strength values from the NDT measurements. The benchmark was based on synthetic simulations which reproduced a synthetic data set corresponding to a grid of twenty 3 m-high columns defining a single-storey building structure. The comparison of the various strategies and the analysis of results enabled identification of the most influential parameters that define an investigation approach and influence its efficiency and accuracy. A special emphasis was placed on the magnitude of the measurement error. A model of the investigation strategy is proposed, that will be detailed in future guidelines. The reader is invited to refer to a companion paper (Alwash et al. in Mater Struct, 1. doi: 10.1617/s11527-016-0962-x ), based on more extensive Monte-Carlo simulations of the same case-study, which analyzes and quantifies the efficiency of the investigation strategies with respect to both average strength and strength variability assessment, in order to draw conclusions based on rational analyses rather than on the element of chance.

Using Monte-Carlo simulations to evaluate the efficiency of different strategies for nondestructive assessment of concrete strength

An international benchmark comparing a large variety of concrete strength assessment strategies was proposed by NDT experts of the RILEM committee TC-249. It was based on synthetic data and devoted to the estimation of average strength and concrete strength variability of a series of concrete columns of a single floor building using fixed budget. Based on information gathered after this benchmark, the main contribution of this paper is to simulate, using Monte Carlo simulation, the experts’ assessment strategies (i.e. NDT investigation program and data analysis process). These strategies are repeated a certain number of times in order to establish a representative assessment of each strategy. The quality of estimation by each strategy is evaluated using several indicators. The results show that this quality can be improved by using/changing several controlling factors: improving the quality of measurements, selecting core locations depending on the NDT test results (conditional cores), combining several NDT techniques and increasing the amount of resources.

Fire Performance of Columns Made of Normal and High Strength Concrete: A Comparative Analysis

The use of high strength concrete (HSC) in multi-story buildings has become increasingly popular. Selection of HSC over normal strength concrete (NSC) allows for reducing the dimensions of the columns sections. However, this reduction has consequences on the structural performance in case of fire, as smaller cross sections lead to faster temperature increase in the section core. Besides, HSC experiences higher rates of strength loss with temperature and a higher susceptibility to spalling than NSC. The fire performance of a column can thus be affected by selecting HSC over NSC. This research performs a comparison of the fire performance of HSC and NSC columns, based on numerical simulations by finite element method. The thermal and structural analyses of the columns are conducted with the software SAFIR®. The variation of concrete strength with temperature for the different concrete classes is adopted from Eurocode. Different configurations are compared, including columns with the same load bearing capacity and columns with the same cross section. The relative loss of load bearing capacity during the fire is found to be more pronounced for HSC columns than for NSC columns. The impact on fire resistance rating is discussed. These results suggest that consideration of fire loading limits the opportunities for use of HSC, especially when the objective is to reduce the dimensions of the columns sections.

Non-destructive assessment of both mean strength and variability of concrete: A new bi-objective approach

Using non-destructive techniques (NDT) like rebound hammer in combination with destructive techniques (DT) like core test is a common practice. Two approaches are widely used to produce a model for assessing the concrete strength. The first approach consists in fitting a specific model between NDT measurements and cores using the regression analysis. The second approach uses a prior model which is calibrated according to measured core strengths. The EN 13791 and ACI standards require a large number of cores to estimate mean concrete strength and concrete strength variability and consequently to calculate the characteristic strength value which depends on these two inputs. In this work, we propose a new approach for identifying the models based on NDT and DT tests in order to capture both mean strength and concrete strength variability. This approach is first illustrated by synthetic simulations which are a good way to study a problem having many degrees of freedom. The proposed approach is then tested on a real data set. In both cases, it is confirmed that the common approaches are able to estimate the mean strength but they fail, even with a large number of cores, to accurately estimate the concrete variability and hence the characteristic strength. Reversely, the new approach shows its high efficiency in capturing the concrete variability (in addition to the mean strength) with a number of cores lower than that prescribed by the standards.

Combined effects of axial load and concrete strength variation on the seismic performance of existing RC buildings

It is well known that the axial load plays an important role in the evaluation of the structural capacity of RC columns. In existing buildings this problem can be even more significant than in new ones, since the material can easily present poor mechanical properties. The paper is aimed at the investigation of the role of the axial load variation on the seismic performance of RC columns of a case-study, i.e. a doubly symmetric 4-storey RC building. The effects of the axial load variation have been checked on the first storey columns, by comparing the seismic response, measured in terms of chord rotation and shear force, with the corresponding capacity. The sensitivity of the seismic performance to the axial load is evaluated with special attention on the type of analysis adopted to determine the seismic response and on considering a wide range of values for the concrete strength. The study points out a non-negligible effect of the axial load variation on the seismic response of the case-study building, especially when combined to concrete strength variability.

REINFORCED CONCRETE BEAMS USING LOCALLY MANUFACTURED STEEL BARS: INVESTIGATION OF FLEXURAL BEHAVIOR

While much attention has been and continue deservedly to be given to the effect of the variability of concrete strength and properties on the response of reinforced concrete structures, there is little, if any, information on the effects of variability of steel strength. Steel mechanical properties, including its yield strength, can significantly exceed the minimum nominal strength values for a specific grade of steel depending on the steel manufacturing processes. Such an increase in yield strength can have negative effects on the flexural behavior of beams designed as tension controlled, and reduce their ductility, an essential property in seismic resisting structures. An experimental and analytical study of the flexural behavior of RC beams was conducted through the investigation of the Moment-Curvature relationships and the ultimate steel strains. The main variable was the level of the actual steel yield stress as compared to the nominal value. It was found that unexpectedly high values of steel yield stress reduce the beam ductility and violate the tension-control condition, which was enforced in the design stage. Appropriate design corrections are proposed to account for high yield stress values in order to achieve the desired ductility of beams while maintaining the moment capacities.

Seismic performance sensitivity to concrete strength variability: a case-study

Existing building structures can easily present material mechanical properties which can largely vary even within a single structure. The current European Technical Code, Eurocode 8, does not provide specific instructions to account for high variability in mechanical properties. As a consequence of the high strength variability, at the occurrence of seismic events, the structure may evidence unexpected phenomena, like torsional effects, with larger experienced deformations and, in turn, with reduced seismic performance. This work is focused on the reduction in seismic performance due to the concrete strength variability. The analysis has been performed on a case-study, i.e., a 3D RC framed 4 storey building. A Normal distribution, compatible to a large available database, has been taken to represent the concrete strength domain. Different plan layouts, representative of realistic strength distributions, have been considered, and a statistical analysis has been performed on the induced reduction in seismic performance. The obtained results have been compared to the standard analysis as provided by Eurocode 8 for existing buildings. The comparison has shown that the Eurocode 8 provisions are not conservative for existing buildings having a large variability in concrete strength.