Cement and Concrete Composites

Effects of the strain rate and fiber blending ratio on the tensile behavior of hooked steel fiber and polyvinyl alcohol fiber hybrid reinforced cementitious composites.

 

Contents

1. Effects of the strain rate and fiber blending ratio on the tensile behavior of learn more and polyvinyl alcohol fiber hybrid reinforced cementitious composites

2.Mechanism of long-term capillary water uptake in cementitious materials

3.Modeling the effect of temperature gradient on moisture and ionic transportin concrete

4.Effects of heating and drying on the strength and stiffness ofhigh-early-strength Portland cement pastes

 

Passage 1

Title: Effects of the strain rate and fiber blending ratio on the tensile behavior of hooked steel fiber and polyvinyl alcohol fiber hybrid reinforced cementitious composites

 

The effect of strain rate on the tensile properties of high-performance hybrid fiber-reinforced cementitious composites, as well as the tensile strength, strain capacity, peak toughness and softening toughness of the composites, was investigated based on the mixing ratio of hooked steel fibers (HSF) and polyvinyl alcohol fibers (PVA).

According to the experimental results, the specimens with 1.5 vol% HSF and 0.5 vol% PVA (HSF1.5PVA0.5) exhibited the highest tensile strength and softening toughness. However, their strain capacity and peak toughness were lower than those of the 2.0 vol% HSF (HSF2.0) specimens. Nevertheless, HSF 1.5PVA 0.5 had the highest dynamic growth factors (DIFs) for tensile strength, strain capacity, and softening toughness. However, at high strain rates (>101s-1), the peak toughness of HSF1.5PVA0.5 exhibited higher DIFs than HSF2.0 due to the highest strain rate sensitivity of its tensile strength and strain capacity.

 

Fig. Schematic diagram of the specimen and setup for the static test.

 

Fig. Tensile stress–strain curve and summary of the tensile properties.

 

Passage 2

Title: Mechanism of long-term capillary water uptake in cementitious materials

 

Capillary self-absorption or water absorption tests are practical and they provide a good characterization of the properties of the gelling material. However, most of these tests are performed in a short period of time (within a week), and once the water level covers the total height of the sample, excess water absorption is recorded. This process occurs at a very low rate, and it can provide additional information about the pore structure.

We investigated the long-term capillary water absorption phenomenon in mortars and concretes with and without auxiliary cementitious materials. In this paper, we report our results as well as relevant results in the literature and present a phenomenological description of the process.

The trends observed in the long-term tests are consistent for different water-cement ratios, types of cementitious materials, fine and coarse aggregates, age of curing, and curing conditions. The long-term measurements reveal primary and secondary periods of capillary self-priming, which are bilinearly related to the quadratic root of time.

 

Fig. Long-term capillary water uptake results of FA concrete samples at 28 (a) and 90 (b) days.

 

Fig. Schematic representation of the primary and secondary capillary imbibition phenomena in cementitious materials. The C-S-H structure is based on Jennings C-S-H model [18] and adjusted for bilinear flow during capillary imbibition.

 

Passage 3

Title: Modeling the effect of temperature gradient on moisture and ionic transportin concrete

 

Most reinforced concrete structures are subject to multiple environmental conditions simultaneously, such as temperature fluctuations, humidity and chloride ion variations. Therefore, water transfer in concrete is driven by a combination of moisture and temperature gradients. Similarly, ion migration in concrete is driven by ion concentration gradients and temperature gradients.

Considering the effect of temperature gradients, an analytical solution that can be used for water or chloride ion transport in concrete was developed in this study. The problem is reduced to a one-way coupling problem since the effect of mass transfer on heat conduction is not considered. The validity of the analytical model was verified by two examples, in which real material parameters were used for the concrete material parameters, and the parameters were derived from experimental data in the literature. By comparing the analytical results with the experimental results, it is found that the two can be in good agreement. This indicates that the model can better characterize the effect of temperature gradient on water transfer in concrete and the effect of chloride ion migration.

 

Fig. Schematic illustration of the experimental set-up

 

Fig. Comparisons of experimental data and simulated %Cl distributionsof present model.

 

Passage 4

Title: Effects of heating and drying on the strength and stiffness ofhigh-early-strength Portland cement pastes

 

The changes of flexural strength and Young's modulus of early-strength silicate cement paste with water/cement ratio (W/C) of 0.30 and 0.55 were measured after drying and heating to 90°C.

When the W/C was 0.55 and the relative humidity decreased to 40%, the flexural strength decreased continuously and began to increase when the relative humidity decreased to below 40%; when the W/C was 0.30 and the relative humidity was above 70%, the flexural strength changed only slightly and began to increase significantly when the relative humidity was below 70%. Based on microstructural characteristics, when W/C is 0.30, the change in strength of the slurry can be assessed by the change in bond strength and of the solid phase skeleton composed of C-S-H agglomerates and macropores, which is related to the strength characteristics of porous materials; when W/C is 0.55, the change in solid phase strength, macropores or mesopores affects the change in strength of the slurry, while the effect of mesopores on strength is higher than that of macropores. Because higher relative humidity plays a dominant role in the increase of solid phase strength for slurries with lower W/C, the starting point of strength increase (SPSI) for low W/C slurries appears at higher relative humidity conditions compared to those with higher W/C. Despite the relatively large dispersion in the experimental results, the standard deviations of flexural strength and Young's modulus of HCP were 16% (W/C) and 10% (W/C), respectively, and the trend of the results was more consistent and reliable.

 

Fig. (A) samples dried at different RHs (20℃), (B) samples heated at 40℃–90℃. Data are plotted in the x-axis direction with an offset value, 100.

 

Fig. Calculated paste bending strength ratios following Eq. (A) is evaluated by a difference of surface area and change in macropore, (B) is evaluated by a difference of surface area and change in mesopore. (C) is fitted by combinations of a difference of surface area and changes in macro-/mesopore, while parameter A=2.91(H30), 2.41(H55), B=0.17(H30), 1.73(H55), and C=1.32(H30),  -0.70(H55).