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Many critical concreting activities require the determination of the in-place concrete strength. It can be costly to wait too long to execute these activities, but acting too early can have a negative impact on structural performance. Conventional moulded concrete cylinders and alternative strength assessment methods such as the maturity method are commonly used to assess concrete's in-place strength. This study is focused on a special case when the maturity method is applied for evaluating concrete strength in cold weather at an early age. Specifically, the study proposes an alternative method for calibrating the activation energy, an input for the maturity method. Concretes cured at two different temperatures are used to calibrate the activation energy. The maturity method based on this activation energy predicted the cold weather concrete's compressive strength satisfactorily. It was also found that for a successful application of the proposed activation energy calibration, the two temperatures should represent the expected minimum and maximum temperature at the site. Furthermore, the results showed that the maturity method based on the proposed activation energy calibration method performed better than the conventional cylinder method. [ABSTRACT FROM AUTHOR]

Analytical models which can reliably predict the load–deformation behaviors and the ultimate strength of columns are indispensable tools for the practical design. In this paper, an analytical model is developed to predict the compressive load–deformation behavior and the ultimate capacity of engineered cementitious composite confined concrete encased steel (ECC‐CES) composite columns. The proposed analytical model employs the effective material stress and strain compatibility approaches for the predictions of compressive load–deformation response of ECC‐CES columns. The material constitutive models for ECC, concrete, and steel are first established, and then used in the analytical model. The effect of ECC and steel confinement on the concrete core and the softening behavior of steel column due to compression buckling are also considered. Based on the experimental results, appropriate strength reduction factors are proposed so that the effective material stresses can be calculated. The strength predictions from the proposed analytical methods and different design codes are compared with the experimental and numerical results. It was found that the proposed analytical models using effective material stresses gave better predictions than many commonly used design codes. [ABSTRACT FROM AUTHOR]

Accurate estimation of the in situ strength of concrete at early age is very important as it provides the necessary information required to start subsequent construction operations. Overestimation of the strength may cause serious safety hazards and underestimation may lead to unnecessary costly delays. This study investigates the performance of the maturity method in estimating the strength of in situ concrete subjected to cold weather at early age. Instrumented concrete slabs were subjected to cold weather conditions at early ages and their strengths were measured using drilled core samples from the slab. Sensors embedded in the slabs measured the temperature in the concrete which was used to estimate the strength using the maturity method. The measured core strengths at 24 and 72 h after casting are then compared with the estimated strengths using the maturity method and its performance is evaluated. The core strengths are also compared with the strength of standard cylinders cured at the same condition as the slabs. More than 250 cylinders from two slab thicknesses and four batches of concrete were used in the experiments to obtain statistically significant experimental data. The results show that the maturity method performed much better than the standard cylinder strength. On average the standard cylinder strength underestimated the core strength by more than 40% while the maturity method overestimated the strength by less than 10% with a lower variation. [ABSTRACT FROM AUTHOR]

For assessing the integrity of damaged hollow section joints, the plastic collapse loads of the joints containing cracks is an important parameter. Very little published information is available in the literature concerning the residual strength of square hollow section (SHS) joints containing defects or cracks. In this paper, an accurate numerical modeling of SHS T joints with cracks is proposed, whereby the entire finite element mesh is generated automatically. To validate this numerical model, several T joint models are studied and compared with the experimental data. A good agreement is obtained on the ultimate loads. Based on this model, the plastic collapse loads under different geometrical ratios are studied, and they are compared with the results calculated from the yield line theory. It is found that the yield line theory can give a reasonable prediction of the plastic collapse loads when the brace width to chord width ratio [beta] is less than 0.8. The failure assessment diagrams (FAD), constructed using the J-integral predictions, have confirmed that the standard BS7910 Level 2A FAD is appropriate for determining limits of safe loading of cracked SHS T joints, provided the plastic collapse load is calculated using the yield line formulae neglecting the influence of the welds. DOI: 10.1061/(ASCE)0733-9445(2006) 132:3(378) CE Database subject headings: Axial loads; Collapse loads; Failures: Hollow sections; Joints; Strength; Finite element method; Numerical models.

A series of static strength tests were performed on full-scale precracked square-to-square hollow section welded T joints. These joints were subjected under brace end axial load until failure using a special purpose designed test rig. An alternating current potential drop technique was employed to monitor the crack propagation during the tests, in addition, three sets of linear variable displacement transducers were used to record the crack mouth opening displacements. The tests were performed at room temperature; therefore, the material is on the upper shelf of the ductile-brittle transition curve where failure occurs in a ductile manner. In all these tests, a stable ductile tearing was observed with a small tearing region. Based on the yield line theory, the plastic collapse load is estimated, and then applied to the failure assessment diagram (FAD) of BS7910. The results show that BS7910 Level 2A FAD provides an adequate procedure for the assessment of cracked square-to-square hollow section T joints. DOI: 10.1061/(ASCE)0733-9445 (2006) 132:3(368) CE Database subject headings: Axial loads; Cracking; Fractures; Failures; Hollow sections; Joints; Strength.

In practice, tubular joints are always subjected to a combination of axial loading (AX), in-plane bending (IPB), and out-of-plane bending (OPB), and the initiation of surface cracks does not always start at the saddle or the crown position on the joints. A systematic finite element modeling of tubular Y joints with any surface crack and located at any position along the brace-chord intersection is developed and proposed in this paper. The weld details in the model conform to the American Welding Society codes. The contact surfaces have been defined carefully between the two crack surfaces that may arise due to some loading cases. Along the crack tip, a well-graded finite element mesh is generated to get the stress intensity factors. This modeling method is then used to analyze three tubular T joint specimens which were tested to failure earlier, and they were subjected to IPB only, combination of IPB and OPB, and a combination of AX, IPB, and OPB, respectively. The numerical results compared favorably with the experimental results based on the Paris' law, and all of these results demonstrate that the proposed model is consistent and reliable. CE Database subject headings: Fatigue; Cracking; T joints; Combined stress; Axial loads; Bending.

Experimental fatigue tests were carried out on three tubular T joints having the same geometrical parameters subjected to in-plane bending (IPB) only, combination of IPB and out-of-plane bending (OPB), and a combination of axial loading, IPB and OPB respectively. Static tests were first carried out to investigate the Hot Spot Stress (HSS) distribution along the intersection of chord and brace, and to determine the peak HSS and its location when the joints were subjected to these load cases. The peak HSS location was used to determine the placement of the probes. An alternating current potential drop technique was then employed to monitor the crack growth and crack shape development on the joints at a preset number of cycles. The experimental data were recorded, and the fatigue performance of the three T joints subjected to basic and combined load cases were investigated. The crack growth curves and crack growth rate on each specimen were presented. The results confirmed that the United Kingdom Department of Energy tubular joint fatigue design T curve derived from a single axis test data is conservative even for combined load cases. CE Database subject headings: Fatigue; Cracking; T joints; Combined stress; Bending; Axial loads; Tests.