Premature debonding failure is a serious problem for reinforced concrete (RC) members strengthened with carbon fibre reinforced polymer (CFRP) laminates. Although various mechanical anchorage systems have been proposed to prevent debonding between CFRP and RC beam, these systems are generally difficult to apply to narrow RC beams due to width limitations. In this study, to resolve the debonding problems, an innovative hybrid end-anchorage system was developed, which achieves self-locking of the externally bonded CFRP sheets. To achieve this hybrid anchorage, both ends of the CFRP sheet were wrapped around twin rods, and the sheet was bonded along the length of the tension side of the beam. Then, to fix the twin rods, two L-shaped brackets were bolted on the side of the beams. In total, five RC beam specimens with rectangular cross sections were prepared and strengthened in different manners and then subjected to four-point bending tests. The enhancement effect of the hybrid anchorage on the flexural behaviour of the strengthened RC beams was validated. With a larger CFRP width, the strengthening effect was more notable, especially when the concrete strength grade was higher. Existing formulas were used to calculate the characteristic moments of the strengthened RC beams, including the cracking moment, yield moment and ultimate moment, and the applicability of the models was examined. Based on the test results, the formula for the ultimate moment of the specimens with the end anchorage requires further investigation.
Composite binders were prepared as novel compositions, on the basis of which fiber reinforced self-compacting concretes (FRSCC) with high rheological, mechanical, dynamical, and impermeability behaviors are created. Rice husk ash, quartz sand, and limestone crushing waste within CEM I and different ratio of superplasticizer were investigated as components of composite binders. Central Composite Face Centered method was used to design the number of experiments and randomizations and then screened by Response Surface Methodology. The validity of models was projected by ANOVA. Replacement of cement by waste composite binders as a supplementary cementitious material has a positive effect on mechanical properties of FRSCC from 30% to around 35% of cement replacement. Adding the superplasticizer improved the performance of FRSCC in all aspects in the given range. Increasing the waste composite binders decreased the vapor permeability and effective diffusion coefficient of FRSCC by improving the hydration rate and decreasing the pores of the concrete matrix. Designed FRSCC has the potential to be used as a material for protective structures, because it is able to provide comprehensive protection against the dynamic effects and penetration of gases.
Since the durability of concrete depends on the paste matrix, a sound knowledge of its pore structure is required. The most common methods to investigate the microstructure like mercury intrusion porosimetry (MIP) and gas sorption require the removal of free water in the pore space. However, the different drying methods used for this task either do not remove all free water or remove also chemically bound water and thus alter the microstructure. Small-angle X-ray scattering (SAXS) is one of the rare methods which allows to investigate the microstructure without complete water removal. Therefore, in order to compare the effect of different drying methods on the pore structure, hardened OPC paste samples (w/c = 0.5) were investigated with MIP, nitrogen sorption and for the first time with SAXS, after different drying was carried out. Since specific surfaces obtained for SAXS showed a similar trend as for MIP and nitrogen sorption, it can be concluded that SAXS can be utilized to predict the influence of drying on these methods as well. Solvent exchange appears to be a drying method that preserves the microstructure well, since for this the largest specific surface was obtained.
In view of the life-cycle assessment of concrete pavement, three-dimensional behavioral simulation was conducted by integrating constitutive laws for cracked/uncracked reinforced concrete, modeling of pore water inside cracks and multi-yield surface plasticity for soil to investigate the fatigue life under high-cycle moving loads. Presented is a discussion that the balanced pavement requires thicker slabs than those required by current design codes on medium compacted soil foundations, but for too much looser foundation, it is over-designed, conversely. Then, a new design approach is proposed in terms of the slab thickness by considering nonlinear coupling with soil foundation. When the stagnant water exists on the slab with cracks, the numerical analysis shows the excessive pore water pressure in cracks at the upper layers of the concrete pavement. The engineering experience is proved such that the deterioration of concrete slabs is accelerated by water and the fatigue life is dramatically shortened compared to the dry situation.
Application of corrosion inhibitors is considered as one of the most economical and effective solutions for the protection of reinforced concrete structures under severe environment. The aim of this paper is to investigate the protection performance and mechanism of an internal blending organic inhibitor for carbon steel in chloride contaminated simulated concrete pore solution. The influences of concentration of inhibitor, chloride content and pH value of the solutions on the protection efficiency were investigated. Both weight loss and electrochemical were used to evaluate the protection efficiency of the inhibitor. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) analyses were performed to reveal the mechanism of corrosion resistance of the inhibitor. Results indicated that the corrosion of carbon steel in chloride contaminated simulated concrete pore solution was highly resisted in presence of inhibitor, the inhibition efficiency of 3.5% NaCl saturated Ca(OH)2 solution with 4% inhibitor reached 89%. With lower pH value, corrosion of steel will happen easier even with addition of inhibitor. However, inhibitor will still have effective even to protect carbon steel in fully carbonated concrete. The organic film with characteristics of aromatic groups can be detected on the carbon steel surface in presence of inhibitor, confirming the protection effects for steel.
High-magnesium nickel slag (HMNS) and phosphogypsum (PG) are hazardous industrial solid waste. The present work focuses on the feasibility of using the two materials as supplementary cementitious materials (SCM) added to cement. The slag and gypsum were ground into different specific surface area (2235 cm2/g, 3040 cm2/g and 3900 cm2/g), then mixed into ten different samples. The effect of SCM addition was detected by isothermal conduction calorimetry (ICC) testing, compressive strength, mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), scanning electron microscope (SEM) and linear shrinkage. The results showed that the HMNS-PG composite cementitious material with suitable specific surface area has relatively high activity, and a small amount of gel and alumina-ferric oxide-trisulfate (AFt or ettringite) are detected in HMNS-PG composite cementitious material. According to the results, the high-magnesium nickel slag could be together with phosphogypsum as substitutes for cement, improved the utilization rate of industrial waste and reduced environmental pollution.
Superabsorbent polymers (SAPs) can effectively reduce the risk of concrete cracks by acting as an internal curing material. In this study, 1H low-field NMR was used to investigate the influence of the pre-absorbed water amount and the water-cement ratio of paste on the water absorption-release behavior of SAPs in fresh cement paste. Results show that when the water-cement ratio was 0.30, the SAPs without pre-absorption absorbed water from fresh cement paste and retained them with relative stability. The SAPs with pre-absorption would rapidly release the pre-absorbed water into a paste, and the water releasing speed increased with the amount of pre-absorbed water. Additionally, the water amount absorbed by the SAPs without pre-absorption increased with the water-cement ratio from 0.30 to 0.40. However, the water absorption of SAPs from the paste was also found to be severely inhibited when the water-cement ratio reached 0.50. SAP swelling in fresh cement paste is affected by the synergistic effects of multiple ions.