Theoretical models are developed for the prediction of the mechanical properties of mature concrete based on the morphology and interactions of its constituents. The models account for the morphology and interactions of cement hydration products, the capillary pores and microcracks. Concrete is modeled as a three-phase material composed of cement mortar, coarse aggregates and ITZ. The modulus of elasticity of concrete is determined using the Hashin's bound model for three-phase composites. Modeling of fracture toughness indicates that the frictional pull-out of coarse aggregates makes major contributions to the fracture energy of concrete. A tensile strength model is developed for concrete based on linear elastic fracture mechanics theories. The predicted theoretical models are in reasonable agreement with empirical models reflecting the experimental performance of concrete.
The scope of this study is twofold: first to present the current state-of-knowledge on lightweight aggregate self-compacting concrete (LWASCC) and second to discuss the development of pumice aggregate self-compacting concrete (PASCC) falling in the LC20/22 strength class and D1.4 density class (as per EN 206-1). Former studies have showed that the commonly accepted range of values derived from fresh-state tests on normal weight self-compacting concrete (NWSCC) is also attainable for LWASCC, the majority of which is produced using artificial lightweight aggregates. The second part of the paper presents the findings of an experimental program that aimed to assess the effect of coarse-to-fine pumice aggregates ratio on the rheological and mechanical properties of PASCC. Based on the results of this study it is verified that properly designed PASCC mixtures with a dry density in the order of 1400 kg/m3 may be evaluated while in fresh state using the same test methods applicable for normal weight self-compacting concrete (NWSCC) and yield scores that lie within the commonly accepted ranges. If pumice aggregates are introduced in the mix in a saturated state, PASCC exhibits self-compactness similar to the one typically characterizing most NWSCC mixtures. Surface coating of fine and coarse pumice aggregates - though difficult in practice - improves workability and prolongs its retention.
Concrete expansion due to alkali-silica reaction (ASR) is one of the serious deterioration mechanisms of concrete structures. However, no promising repair method for ASR has been established yet. In a bid to remedy this situation, an electrochemical technique to accelerate the penetration of the lithium ions (Li+) in a lithium-based electrolyte solution into concrete has been developed for the purpose of suppressing ASR-induced expansion due to Li+. From the results of past research work, the penetration area of Li+ is limited around the concrete surface and it is difficult to make Li+ penetrate into the deeper part of concrete. In this study, experimental investigation was carried out aiming to grasp the influence of the kinds of lithium salts and the temperature of the electrolyte solution on the migration properties of ions in concrete and ASR-induced expansion of concrete. The electrochemical migration of Li+ was found to accelerate with rises in temperature and the effective diffusion coefficient of Li+ increased three times with changes in temperature from 20°C to 40°C in the case of a Li2CO3 electrolyte solution. Moreover, ASR-induced expansion of concrete after this treatment was suppressed compared with the case of non-treated specimens.
In the pre-cast construction of concrete structures, the minimization of the joint size and simplification of its design can greatly improve construction efficiency. This paper reports the result of an experimental study on the use of high strength fiber reinforced cementitious composites (HSFRCC) for the joining of precast concrete slabs. In order for the joined slab to have similar bending behavior as a monolithic slab, sufficient bond length between the HSFRCC and reinforcing bars is required to ensure steel yielding before bond failure. To control the material cost, the fiber content in the HSFRCC is limited to 2%. To find a suitable bond length, direct Tension Pull-out Bond Test (DTP-BT) was first conducted on steel bars embedded in HSFRCC with compressive strength of 150 MPa. The testing parameters included the anchorage length (5 d or 8 d, where d is the steel bar diameter, 16 mm) and type of steel bar (straight, hooked and nut at the end). According to the test results, 8 d is sufficient for steel yielding to occur in a joint with 2% of micro-steel fiber. Monolithic slabs as well as slabs with HSFRCC joints were then prepared and tested in bending. Similar load displacement performance was observed for the two kinds of slabs, demonstrating the effectiveness of the HSFRCC joint.
In this research project the behaviour of strain-hardening cement-based composites (SHCC) subjected to low and high strain rates was studied. Uniaxial tension tests on dumbbell-shaped SHCC specimens were performed at rates ranging from 10-5s-1 to 50s-1. For the tests performed at strain rates of 10-2s-1 and below, SHCC yielded a moderate increase in tensile strength and simultaneous decrease in strain capacity with increasing strain rate. When tested for higher strain rates from 10 to 50s-1 a considerable increase in tensile strain and strain capacity was measured. Microscopic investigation of the fracture surfaces showed that almost no fibre failure and an average pullout length of 2.5mm were found in the high strain rate test. This observation is in contrast to that of rapid quasi-static testing, where the average fibre pullout length of 300μm was much shorter. Furthermore, the fibres on the fracture surfaces produced in the high rate tests exhibited pronounced plastic deformations. Finally, quasi-static and high-speed tension tests on individual fibres and single fibre pullout tests were performed. While the increase in the tensile strength of the fibre was only moderate in the range of strain rates investigated, a considerable increase in bond strength between fibre and matrix was determined.
When designing blast-resistant reinforced concrete (RC) structures, reducing spall damage due to reflected tensile stress waves is a major problem. To investigate the applicability of polyethylene fiber reinforced concrete (PEFRC) for use in blast-resistant RC structures, experimental investigations were conducted to evaluate the damage to PEFRC slabs subjected to contact detonation. As a result, it was shown that PEFRC was effective in reducing the spall damage due to contact detonation as compared with normal RC. Moreover, an equation for estimating damage depth to the PEFRC slab subjected to contact detonation was derived based on the test results.
The shear capacity and post-peak ductility of reinforced high-strength concrete (HSC) beams, which are greatly affected by both autogenous shrinkage and a notable reduction in shear transfer along HSC crack planes, are simulated using nonlinear finite element (FE) analysis. The volumetric change caused by autogenous shrinkage is incorporated into the analysis by introducing an effective shrinkage strain related to the initial stress that develops in the reinforcement. The computed capacity, loading/unloading stiffness, crack pattern, and mode of failure replicate data obtained from systematic experiments. Approximately 50% of the plain concrete early-age shrinkage is observed to be consistent with self-induced stresses in structural concrete. The impact of autogenous shrinkage is further emphasized in assessing shear performance together with reduced crack shear transfer. Shrinkage changes the stress transfer path and may also alter the failure mode. The multi-directional fixed crack approach is verified as a reliable structural concrete model in the case of high autogenous shrinkage as well.
Reinforcing bar corrosion induces splitting cracks in concrete along the bar axis and leads to bond deterioration. This can adversely affect the crack spacing in an RC member and have a serious effect on its serviceability. This study looks at axial nonlinearity in corroded RC members under tension and shows that fewer transverse cracks with greater spacing occur as steel corrosion progresses. The open-slip coupled model, which takes into account the transverse action associated with longitudinal bond stress transfer in the bond transition zone, is extended to cover corroded reinforcement and is successfully used to simulate the behavior of RC members in tension. Modeling of the bond transition zone and of the layer of corrosion products is found to be crucial to understanding residual bond performance after corrosion has occurred.
Pipe-cooling has been widely used for reducing hydration heat and controlling cracking in massive concrete structures. Therefore, the heat transfer coefficients in flow convections, which represent the thermal transfer between the inner stream of the pipe and the concrete, must be estimated accurately. In this paper, a device measuring the heat transfer coefficient is developed based on the concept of internal forced convection. The main influencing factors on the heat transfer coefficient in the flow convection are the flow velocity, pipe diameter and thickness, and the pipe material. Using experimental results obtained from the developed device, a general prediction model for heat transfer coefficients is suggested. The proposed prediction model was found to estimate the heat transfer coefficient correctly with respect to the properties of the flow and pipe in comparisons of measured data and the numerical results of a heat transfer analysis conducted on an actual massive concrete structure.
One of the largest contributors of the greenhouse gas emission is the production of cement for use in concrete. However, concrete is well-known for its carbon dioxide (CO2) uptake by carbonation. The purpose of this study was to consider the CO2 uptake in demolished and crushed concrete. In this study, three kinds of experiments and survey were carried out, including (1) an experiment using mortar specimens made in the laboratory so as to identify the conditions that accelerate CO2 uptake, (2) an experiment using concrete obtained from the demolition site and (3) a survey on the CO2 uptake in recycled crusher-run stone obtained from recycling plants. The experiment on new mortar and demolished concrete pointed out that the CO2 uptake in cement hydrate increases significantly when the particles are relatively small and when they are alternately wetted and dried. Furthermore, the survey on concrete at recycling plants found the amount of CO2 uptake in one ton of recycled crusher-run stone to be approximately 11 kilograms. Finally, using this value, the life cycle of CO2 of concrete structures was calculated and shown to be approximately 5.5% less when the CO2 uptake is taken into account compared to when it is not taken into account.
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