Yixin Shao

Studies on concrete containing ground waste glass

The possibility of using finely ground waste glass as partial cement replacement in concrete was examined through three sets of tests: the lime-glass tests to assess the pozzolanic activity of ground glass, the compressive strength tests of concrete having 30% cement replaced by ground glass to monitor the strength development, and the mortar bar tests to study the potential expansion. The results showed that ground glass having a particle size finer than 38 μm did exhibit a pozzolanic behavior. The compressive strength from lime-glass tests exceeded a threshold value of 4.1 MPa. The strength activity index was 91, 84, 96, and 108% at 3, 7, 28, and 90 days, respectively, exceeding 75% at all ages. The mortar bar tests demonstrated that the finely ground glass helped reduce the expansion by up to 50%. A size effect was observed; a smaller glass particle size led to a higher reactivity with lime, a higher compressive strength in concrete, and a lower expansion. Compared to fly ash concrete, concrete containing ground glass exhibited a higher strength at both early and late ages.

Mechanical properties of PVA fiber reinforced cement composites fabricated by extrusion processing

Discontinuous polyvinyl alcohol (PVA) fiber reinforced cement matrix composites fabricated by an extrusion process were investigated. The extruded composites have exhibited a postpeak strain hardening type of response with an enhanced composite strength. Tensile strength of extruded composites was dependent on the fiber fraction used, while the flexural strength and deflection at the peak load were influenced by both the fiber fraction and the matrix composition. Larger fiber volume fraction, longer fiber length, and higher cement content lead to a higher flexural strength, a larger deflection at peak, and a higher elastic modulus. Fracture process was studied using laser Moire interferometry and scanning electron microscopy. It was observed that sequential multiple cracking was associated with the strain hardening type of response and that the spatial distribution of fibers can control the sequence of multiple cracking. Interface between fiber and matrix was also studied using continuous, aligned fibers and with the help of Moire interferometry.

Microstructure of extruded cement-bonded fiberboard

The microstructure of cement-bonded fiberboard manufactured by extrusion process was studied using scanning electron microscope (SEM). Comparison between extruded and cast fiberboard revealed that the extruded products were better in strength, stiffness, toughness, fiber distribution, fiber orientation, and bond of fiber with matrix, even in the presence of a higher percent air voids. The dominant component in extruded fiberboard was the type of fiber. Extrusion was capable of incorporating both hydrophilic and hydrophobic fibers into fiberboard production. It was found the sand content had significant effect on toughness. The more the sand added, the less the toughness. Fiber dispersion seemed not to be critical. Fiberboard made by nondispersive mixing exhibited satisfactory performance. Accordingly, the mixing time and energy in extrusion production could therefore be reduced.

Carbonation Curing versus Steam Curing for Precast Concrete Production

AbstractAn investigation was conducted into the beneficial utilization of captured CO2 for early-age curing of precast concrete products. The performance of the carbonation-cured concrete was compared to that of steam curing to investigate the possibility of replacing steam curing with carbonation. The early-age carbonation curing was performed for a short period after an initial curing in a controlled environment. The effect of the carbonation curing was studied in terms of carbon uptake, accelerated strength, and durability. It was found that the short-term carbonation promoted early strength development, while subsequent hydration was essential to obtain later age properties. Durability performance of the carbonation-cured concrete was compared with steamed and normally hydrated references. The carbonation-cured concrete exhibited more resistance to chloride permeability, ion migration, sulfate attack, and freeze-thaw damage. The improved durability by carbonation is attributed to the significantly red...

Matrix cracking and interface debonding in fiber-reinforced cement-matrix composites

Abstract The processes of matrix cracking and interface debonding were studied using the high sensitivity Moire interferometry technique. The experiments were conducted with continuous steel fiber reinforced cementitious composites subjected to uniaxial tension. The initiation and propagation of cracking and debonding were observed during the tests with the specimens of different fiber-volume ratios. Based on the experiments, the fiber stress, the interface slip, the interface shear stress, and the matrix strain distribution were calculated. It was shown that interfacial frictional shear stresses were not constant either along the whole interface or at different loading levels. The strain localization was observed in the matrix where it was bonded to the fiber. The average contribution of the matrix was greater for the composites with the higher fiber-volume ratio.

Carbonation Curing of Precast Fly Ash Concrete

AbstractThe feasibility of carbonation curing of precast fly ash concrete is studied. If fly ash concrete can be produced by carbonation curing, the carbon footprint of the products can be significantly reduced. In this paper, the relationship between carbonation reaction and pozzolanic reaction was examined. After carbonation curing with different duration and fly ash content, the cement reaction degree was estimated through the equivalent nonevaporable water content, and the fly ash reaction degree was analyzed through a selective acid dissolution test. It was found that the pozzolanic reaction of fly ash in a fly ash–ordinary portland cement (OPC) system was hindered by early carbonation reaction. The higher the early carbonation degree, the lower the pozzolanic reaction of fly ash. In addition, fly ash–OPC paste was more reactive with carbon dioxide than plain cement paste. Therefore, controlled carbonation at an early age is necessary to trade off the carbon emission reduction with performance gain. ...

Effect of Initial Curing on Carbonation of Lightweight Concrete Masonry Units

The effect of initial curing on carbonation curing of lightweight concrete masonry units (CMUs) was examined. Initial curing was performed from 4 to 18 hours at a relative humidity (RH) of 50% and temperature of 25°C (77°F). Based on cement content, 4-hour carbonation curing allowed concretes to uptake 22 to 24% CO2 with initial curing and 8.5% without initial curing, while prolonged 4-day carbonation recorded an uptake of 35%. Carbonated CMUs exhibited higher early strength but lower 28-day strength due to water loss by initial curing in comparison to hydrated and steam-cured references. A water-spray mechanism was thus devised to compensate the water loss and, ultimately, made the late strength of carbonated CMUs comparable to references. Carbonation curing can replace steam in CMU production to accelerate hydration, improve durability, and recycle cement kiln CO2 in a beneficial manner.

Behavior of Reinforced Concrete Beams Strengthened in Shear Using L-Shaped CFRP Plates: Experimental Investigation

This paper presents the results of an experimental investigation on reinforced concrete (RC) T-beams retrofitted in shear with prefabricated L-shaped carbon fiber–reinforced polymer (CFRP) plates. Shear strengthening of RC beams with L-shaped fiber-reinforced polymer (FRP) plates has proved effective. In this method, grooves are made throughout the beam flange to fully embed the vertical leg of the L-shaped CFRP plate perpendicular to the longitudinal axis of the RC beam and in the RC beam web surface. However, in some cases, drilling grooves in the concrete flange might not be feasible because of the presence of obstacles such as longitudinal steel in the flange of the RC beams. Therefore, the main objective of this investigation was to evaluate the performance of the RC beams strengthened in shear with externally bonded (EB) L-shaped plates as affected by the embedment length of the L-shaped FRP plates. In total, six tests were performed on 2,500-mm long T-beams. Three specimens were strengthened in shear using epoxy-bonded L-shaped CFRP plates with different embedment lengths in the RC beam flange. One specimen was shear-strengthened with fully embedded CFRP plates in the concrete beam flange. The second specimen was strengthened with partial embedment of the L-shaped CFRP plate. This specimen is representative of the case where full penetration of the CFRP plate is not feasible because of an obstacle. In this specimen, the embedment length was set to 25 mm to simulate the minimum concrete cover thickness in RC beams. The third specimen was shear-strengthened with L-shaped CFRP plates with no embedment in the concrete beam flange. In addition, the performance of the beams strengthened with L-shaped CFRP plates was compared with that of a similar specimen strengthened with EB FRP sheets without embedment. Results show that the performance of the specimens strengthened with partially and fully embedded L-shaped CFRP plates in the beam flange was superior to that of the beams strengthened with EB FRP sheets and L-shaped CFRP plates with no embedment.

Reaction Products in Carbonation-Cured Lightweight Concrete

The effect of early-age carbonation curing on the microstructure and properties of lightweight concrete with expanded slag aggregates was examined. Carbonation was performed on concretes either immediately after casting or after 18-h air curing. Their corresponding carbon uptake was 8 and 23%, respectively, based on cement content. A process involving initial air curing, carbonation curing, water compensation, and subsequent hydration was developed to maximize the degree of carbonation and hydration. Reaction products of carbonation-cured concretes at early and late age were characterized by using thermogravimetrical (TG) analysis, X-ray diffraction analysis, and scanning electron microscopy. Although the presence of calcium carbonates was evident, the microstructure was nevertheless typical of amorphous. It was believed that early carbonation of concrete consumed calcium hydroxide, calcium silicate hydrates, and anhydrous calcium silicates while producing calcium carbonates of different polymorphs and amorphous calcium silicate hydrocarbonates.

Influence of moisture content on CO2 uptake in lightweight concrete subject to early carbonation

Early-age carbonation curing of fresh concretes has shown its capacity to accelerate the production, recycle CO2, and to improve the concrete performance. The challenge facing this technology is the moisture content in fresh concrete which impedes CO2 diffusion. This paper studies the effects of moisture content on carbonation reactivity of lightweight concretes (LWCs). The moisture content in LWC was adjusted by varying (1) the water to cement ratio, (2) the moisture carried in by lightweight aggregates, (3) the moisture removed by forced fan drying, and (4) the externally added water by surface spray as compensation of water loss. The performance of the carbonated concretes was evaluated by carbon uptake, strength gain, plastic shrinkage, and pH values. A process involving vibration compact forming, preconditioning, carbonation, water compensation, and subsequent hydration proved to be effective in achieving high degree of carbonation and hydration.