Wu-Jian Long

Shrinkage of Precast, Prestressed Self-Consolidating Concrete

Proper evaluation of shrinkage is critical for design of prestressed structural members. An experimental program was undertaken to evaluate autogenous and drying shrinkage of precast, prestressed self-consolidating concrete (SCC). 16 SCCs with a slump flow of 680 ± 20 mm (26.7 ± 0.8 in.) were evaluated. These mixtures were made with 440 to 500 kg/m³ (742 to 843 lb/yd³) of binder, Type MS cement or HE cement; and 20% Class F fly ash; 0.34 to 0.40 water-cementitious material ratio (w/cm); viscositymodifying admixture content of 0 to 100 mL/100 kg (0 to 1.53 fl oz/cwt) of binder, and 0.46 to 0.54 sand-to-total aggregate volume ratio. Two high-performance concretes (HPCs) with 0.34 and 0.38 w/cm and slump of 150 mm (5.9 in.) were also studied. The SCC developed 5-30% higher drying shrinkage at 300 days than the HPC but with similar autogenous shrinkage. The shrinkage was compared to prediction models proposed by AASHTO 2004 and 2007, CEB-FIP 90, GL 2000, and ACI 209R. The CEB-FIP 90 and a modified AASHTO 2004 model was found to provide an adequate prediction of shrinkage for precast, prestressed SCC.

Creep of Prestressed Self-Consolidating Concrete

This paper will discuss how time-dependent deformations caused by creep lead to the loss of prestressing force and changes in camber in the design of structural members. An experimental program was performed to evaluate the creep of prestressed self-consolidating concrete (SCC). A total of 16 SCC mixtures with a slump flow of 680 ± 20 mm (26.7 ± 0.8 in.) were prepared with various parameters, including the binder content and binder type, water-cementitious material ratio (w/cm), dosage of viscosity modifying admixture, and sand-to-total aggregate ratio (S/A). Two high-performance concrete (HPC) mixtures with 0.34 and 0.38 w/cm and a slump consistency of 150 mm (5.9 in.) were investigated for the control mixtures. The findings indicated that the evaluated SCC developed up to 20% higher creep than HPC of similar w/cm. The creep values of SCC were compared to the prediction models proposed by AASHTO 2004 and 2007, CEBFIP MC90, GL 2000, and ACI 209R-92. The CEB-FIP MC90 and modified AASHTO 2007 models are found to provide an adequate prediction of creep for prestressed SCC.

Performance-Based Specifications of Workability Characteristics of Prestressed, Precast Self-Consolidating Concrete—A North American Prospective

Adequate selection of material constituents and test methods are necessary for workability specifications and performance of hardened concrete. An experimental program was performed to evaluate the suitability of various test methods for workability assessment and to propose performance specifications of prestressed concrete. In total, 33 self-consolidating concrete (SCC) mixtures made with various mixture proportioning parameters, including maximum size and type of aggregate, type and content of binder, and w/cm were evaluated. Correlations among various test results used in evaluating the workability responses are established. It is recommended that SCC should have slump flow values of 635–760 mm. To ensure proper filling capacity greater than 80%, such concrete should have a passing ability that corresponds to L-box blocking ratio (h2/h1) ≥ 0.5, J-Ring flow of 570–685 mm, slump flow minus J-Ring flow diameter ≤75 mm. Moreover, Stable SCC should develop a column segregation index lower than 5%, and rate of settlement at 30 min of 0.27%/h for SCC proportioned with 12.5 or 9.5 mm MSA. It is recommended that SCC should have a plastic viscosity of 100–225 Pa·s and 100–400 Pa·s for concrete made with crushed aggregate and gravel, respectively, to ensure proper workability.

Dynamic Mechanical Properties and Microstructure of Graphene Oxide Nanosheets Reinforced Cement Composites

This paper presents an experimental investigation on the effect of uniformly dispersed graphene oxide (GO) nanosheets on dynamic mechanical properties of cement based composites prepared with recycled fine aggregate (RFA). Three different amounts of GO, 0.05%, 0.10%, and 0.20% in mass of cement, were used in the experiments. The visual inspections of GO nanosheets were also carried out after ultrasonication by transmission electron microscope (TEM) atomic force microscope (AFM), and Raman to characterize the dispersion effect of graphite oxide. Dynamic mechanical analyzer test showed that the maximum increased amount of loss factor and storage modulus, energy absorption was 125%, 53%, and 200% when compared to the control sample, respectively. The flexural and compressive strengths of GO-mortar increased up to 22% to 41.3% and 16.2% to 16.4% with 0.20 wt % GO at 14 and 28 days, respectively. However the workability decreased by 7.5% to 18.8% with 0.05% and 0.2% GO addition. Microstructural analysis with environmental scanning electron microscopy (ESEM)/backscattered mode (BSEM) showed that the GO-cement composites had a much denser structure and better crystallized hydration products, meanwhile mercury intrusion porosimetry (MIP) testing and image analysis demonstrated that the incorporation of GO in the composites can help in refining capillary pore structure and reducing the air voids content.

Uniformly Dispersed and Re-Agglomerated Graphene Oxide-Based Cement Pastes: A Comparison of Rheological Properties, Mechanical Properties and Microstructure

The properties of graphene oxide (GO)-based cement paste can be significantly affected by the state of GO dispersion. In this study, the effects of uniformly dispersed and re-agglomerated GO on the rheological, mechanical properties and microstructure of cement paste were systematically investigated. Two distinct dispersion states can be achieved by altering the mixing sequence: Polycarboxylate-ether (PCE) mixed with GO-cement or cement mixed with GO-PCE. The experimental results showed that the yield stress and plastic viscosity increased with the uniformly dispersed GO when compared to those of re-agglomerated GO cement paste. Moreover, the 3-day compressive and flexural strengths of uniformly dispersed GO paste were 8% and 27%, respectively, higher than those of re-agglomerated GO pastes. The results of X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy analyses demonstrated that uniformly dispersed GO more effectively promotes the formation of hydration products in hardened cement paste. Furthermore, a porosity analysis using mercury intrusion porosimetry revealed that the homogeneous dispersion of GO can better inhibit the formation of large-size pores and optimize the pore size distribution at 3 and 7 days than the re-agglomerated GO.

Pull-Out Strength and Bond Behavior of Prestressing Strands in Prestressed Self-Consolidating Concrete

With the extensive use of self-consolidating concrete (SCC) worldwide, it is important to ensure that such concrete can secure uniform in-situ mechanical properties that are similar to those obtained with properly consolidated concrete of conventional fluidity. Ensuring proper stability of SCC is essential to enhance the uniformity of in-situ mechanical properties, including bond to embedded reinforcement, which is critical for structural engineers considering the specification of SCC for prestressed applications. In this investigation, Six wall elements measuring 1540 mm × 2150 mm × 200 mm were cast using five SCC mixtures and one reference high-performance concrete (HPC) of normal consistency to evaluate the uniformity of bond strength between prestressing strands and concrete as well as the distribution of compressive strength obtained from cores along wall elements. The evaluated SCC mixtures used for casting wall elements were proportioned to achieve a slump flow consistency of 680 ± 15 mm and minimum caisson filling capacity of 80%, and visual stability index of 0.5 to 1. Given the spreads in viscosity and static stability of the SCC mixtures, the five wall elements exhibited different levels of homogeneity in in-situ compressive strength and pull-out bond strength. Test results also indicate that despite the high fluidity of SCC, stable concrete can lead to more homogenous in-situ properties than HPC of normal consistency subjected to mechanical vibration.

Autogenous Shrinkage of Prestressed Self-Consolidating Concrete

Shrinkage can be critical factor for the design of structural members due to the length changes by the time- dependent deformation. Given the high fluidity and different mixture proportions of SCC, the shrinkage of such concrete can differ from those of conventional concrete or HPC of normal consistency. Proper estimate of autogenous shrinkage of self-consolidating concrete (SCC) can provide engineers with the information necessary for producing high quality products manufactured with SCC. An experimental program was undertaken to evaluate autogenous shrinkage of precast, prestressed SCC. Sixteen SCC with slump flow of 680 ± 20 mm were evaluated. These mixtures were made with 440 to 500 kg/m 3 of binder, Type MS cement or HE cement and 20% Class F fly ash, 0.34 to 0.40 w/cm, viscosity-modifying admixture content of 0 to 100 mL/100 kg of binder, and 0.46 to 0.54 sand-to-total aggregate volume ratio. Two high- performance concretes (HPC) with 0.34 and 0.38 w/cm and slump of 150 mm were also investigated. Based on the test results, the HPC developed similar autogenous shrinkage at 56 days compared to SCC made of a given binder type. Shrinkage was compared to prediction models proposed by Tawaza and Miyazawa 1997, Jonasson and Hedlund 2000, and CEB-FIP 1999. The Tazawa and Miyazawa model was modified to provide adequate prediction of autogenous shrinkage for precast, prestressed SCC.

Statistical Models to Predict Fresh Properties of Self-Consolidating Concrete

In order to understand the influence of mixture parameters on concrete behaviour, a factorial design was employed in this investigation to identify the relative significance of primary mixture parameters and their coupled effects (interactions) on fresh properties of SCC that are of special interest to precast, prestressed applications. In addition to the 16 SCC mixtures employed, three SCC mixtures corresponding to the central point of the factorial design were prepared to estimate the degree of the experimental error for each of the modeled responses. The mixtures were evaluated to determine several key responses that affect the fresh properties of precast, prestressed concrete, including filling ability, passing ability, filling capacity, surface settlement, and column segregation. Mixture parameters modeled in this investigation included the binder content, binder type, w/cm, sand-to-total aggregate ratio (S/A), and dosage of thickening-type, viscosity-modifying admixture (VMA). The factorial design can identify potential mixtures with a given set of performance criteria that can be tried in the laboratory, hence simplifying the test protocol needed to optimize SCC.

Research on the internal pores in alkali-activated slag cementing material via X-CT three-dimensional imaging microscopy

X-Ray computed tomography (X-CT) three-dimensional imaging microscopy was used to scan the test specimens made with hardening alkali-activated slag cementing material, and the distribution and morphology characteristics of internal micron pores were obtained accurately; meanwhile the test specimens at the age of 1 day, 7 days, 14 days and 28 days were also scanned, and the data collected by X-CT were processed accordingly by use of images and data processing software Avizo for three-dimensional reconstruction study and comparison on the change conditions of internal pores. The experimental results showed that the proportion of micron pores firstly increased and then decreased, the pore volume was basically stable over the age of 14 days, and the diameter of most pores was 200µm or less.

Mechanical Properties of Fiber Reinforced Self-Compacting Concrete

The mechanical strengths of self-compacting concrete (SCC) with different strengths and different fibers were investigated. By mechanics performance testing on concrete samples, it shows that the fiber can significantly reduce strength of the self-compacting concrete during curing period. The 28d tensile strength of self-compacting concrete can be improved when steel fiber, polypropylene fiber, or polyethylene fiber were used. Moreover, steel fiber can improve the 28d compressive strength; contrarily, polypropylene fiber and polyethylene fiber can reduce the 28d compressive strength.