Durability of Mortars with Fly Ash Subject to Freezing and Thawing Cycles and Sulfate Attack

Destruction of cement composites occurs due to the alternate or simultaneous effects of aggressive media, resulting in the destruction of concrete under the influence of chemical and physical factors. This article presents the results of changes in the measurement of linear strains of samples and changes in the microstructure of cement after 30 freezing and thawing cycles and immersed in 5% sodium sulfate solution. The compressive strengths ratios were carried out at the moment when the samples were moved to the sulfate solution after 30 cycles and at the end of the study when the samples showed visual signs of damage caused by the effect of 5% Na2SO4. The composition of the mixtures was selected based on the Gibbs triangle covering the area up to 40% replacement of Portland cement with low and high-calcium fly ashes or their mixture. Air-entrained and non-air entrained mortars were made of OPC, in which 20%, 26.6%, and 40% of Portland cement were replaced with low and/or high-calcium fly ash. Initial, freezing and thawing cycles accelerated the destruction of non- air-entrained cement mortars immersed in 5% sodium sulfate solution. The sulfate resistance, after the preceding frost damage, decreased along with the increase in the amount of replaced fly ash in the binder. Air-entrained mortars in which 20% of cement was replaced with high-calcium fly ash showed the best resistance to the action of sodium sulfate after 30 freezing and thawing cycles.

Effect of Corn Straw Addition on Mechanical Properties of Polyvinyl Alcohol Hydrogel

In this study, PVA/CS composite hydrogels were prepared by means of freezing and thawing cycles of agricultural wastes, corn straw (CS) and polyvinyl alcohol (PVA). The mechanical properties of the composite hydrogels were analyzed by universal tensile device. The effects of CS on tensile strength and elongation at break of PVA/CS composite hydrogels were analyzed. On the other hand, PVA and PVA/CS composite hydrogels were also freeze-dried to investigate the mechanical properties of all hydrogels after drying.

Freezing and thawing cycles affect nitrous oxide emissions in rain-fed lucerne (Medicago sativa) grasslands of different ages

Lucerne (Medicago sativa L.) is a major component of the crops used in dry-land farming systems in China and its management is associated with notable nitrous oxide (N2O) emissions. A high proportion of these emissions is more likely to occur during periods when the soil undergoes freezing and thawing cycles. In this study, the effects of freeze/thaw cycles on N2O emissions and related factors were investigated in lucerne grasslands. The hypothesis was tested whether increased emissions resulted from a disruption of nitrification or denitrification caused by variations in soil temperatures and water contents. Three days (3 × 24 h) were chosen, where conditions represented freezing and thawing cycles. N2O emissions were measured for a fallow control (F) and two grasslands where lucerne had been cultivated for 4 and 11 years. Soil temperature, soil water content, soil microbial biomass carbon (MBC), soil microbial biomass nitrogen (MBN), soil ammonium nitrogen (NH4+-N), and soil nitrate nitrogen (NO3−-N) contents were measured. Moreover, the quantities of soil nitrification and denitrification microbes were assessed. Variations in N2O emissions were strongly affected by freeze/thaw cycles, and emissions of 0.0287 ± 0.0009, 0.0230 ± 0.0019, and 0.3522 ± 0.0029 mg m−2 h−1 were found for fallow, 4-year-old, and 11-year-old grasslands, respectively. Pearson correlation analyses indicated that N2O emissions were significantly correlated with the soil water content, temperature, NH4+-N content, and the number of nitrosobacteria and denitrifying bacteria at a soil depth of 0–100 mm. The numbers of nitrosobacteria and denitrifying bacteria correlated significantly with soil temperature at this soil depth. MBN and soil NH4+-N contents correlated significantly with soil water content at this depth. Principal component analysis highlighted the positive effects of the number of denitrifying bacteria on N2O emissions during the freeze/thaw period. Furthermore, soil temperature and the number of nitrosobacteria at the tested soil depth (0−100 mm) also played a significant role. This shows that soil freeze/thaw cycles strongly impacted both N2O emissions and the diurnal range, and the number of denitrifying bacteria was mainly influenced by soil temperature and soil NH4+-N content. The number of denitrifying bacteria was the dominant variable affecting N2O emissions from lucerne grasslands during the assessed soil freeze/thaw period on the Loess Plateau, China.

Effect of Lime Stabilization and Partial Clinoptilolite Zeolite Replacement on the Behavior of a Silt-Sized Low-Plasticity Soil Subjected to Freezing–Thawing Cycles

Soil stabilization, through either physical or chemical techniques of coating-cementation development, is a commonly adopted practice in geotechnical and transportation engineering projects used to strengthen soils and mitigate negative geo-environmental impacts. This can be particularly important in enhancing the mechanical properties of soils which are prone to degradation of their response because of freezing and thawing actions. It was attempted in this research study to examine the effect of lime–zeolite stabilization on the behavior of a silt-sized natural soil of low plasticity, by performing standard compaction tests as well as unconfined compressive experiments evaluating the strength of the composite samples. The natural soil mixed with various contents of lime and zeolite partial replacement was subjected to consecutive freezing and thawing cycles. The results from the study showed significant improvement of the mechanical performance of the treated soil when mixed with lime partially replaced with zeolite in terms of strength and durability and, based on scanning electron microscopic images, additional insights were attempted to be obtained on the microstructure of the specimens subjected to temperature changes.

Influence of Freezing-Thawing Cycles on the Glycemic Index of the Iraqi White Bread after Oral Ingestion

Background: The quality of bread depends not only on the quality of its ingredients, proper techniques during preparation, and storage also played a role in the rate of staling. Aims: The present study aims to investigate the impact of freezing and thawing on the glycemic response of Iraqi local white bread. Methods: In this prospective cross-over study, twelve healthy subjects (seven males, five females), aged 21–53 years, were recruited from Al-Rafidain University Campus and the local community. After overnight fasting, commercial Iraqi local white bread (200 g) was administered as fresh bread, following 1-week or 2-week freezing and thawing. Peak glucose response, 2 hr incremental area under the glucose response curve (AUC0-120) was evaluated as an outcome. Results: The different freezing and thawing conditions resulted in lower blood glucose AUC values compared to fresh white bread. In particular, compared to the fresh bread (AUC 14176±1134 mg min/dl), AUC was significantly lower when the bread was 1-week frozen and thawed (13205±660 mg min/dl, P<0.01), or 2-week frozen and thawed (12828±642 mg min/dl, P<0.01). Meanwhile, compared to the 1-week frozen bread, the 2-week freezing cycle did not produce a significantly lower AUC value. Conclusion: One or two freezing and thawing cycles decreased the glycemic response of the fresh Iraqi local white bread in healthy non-obese volunteers.

Effect of Polypropylene Fibers on some Mechanical Properties of Concrete and Durability against Freezing and Thawing Cycles

This investigation aims to improve some of the mechanical properties of concrete such as compressive strength, flexural strength, and modulus of elasticity, by using different percentages of polypropylene fibers, and also studying the durability of concrete for freeze-thaw cycles. the study shows a small increment in compressive strength due to adding fibers which were 28.3% compared with increment in flexural strength which was perfect (about 191%), modulus of elasticity also increased by adding fibers. The durability of concrete against freezing-thaw cycles for all mixes was studied. Fiber-reinforced concrete shows more durability against freezing-thawing cycles and less reduction in strength compared with reference mixes without fibers,21.5% reduction in strength for optimum polypropylene fiber concrete while the reduction in strength for normal concrete was found 54.2% in this study.

Experimental investigation on bond strength between BFRP bars and concrete under freeze-thaw cycles

To investigate the bond strength between BFRP bars and concrete under freezing and thawing cycles, a total of 36 specimens for freezing and thawing cycles tests and center pull-out tests were carried out with different times of freezing and thawing cycles (0, 10, 20, and 40 times) and different concrete strength grades (C30, C35, and C40). The results of this study showed that the specimens without freezing and thawing cycle (0 times) and specimens of C30 with freeze-thaw for 40 times were pulled out, and the remaining specimens were split. With the increase of the concrete strength grade, the debonding strength increases gradually and the ultimate bond strength does not increase in proportion. The debonding strength of BFRP bars and concrete decreases gradually with the times of freezing and thawing cycles. The ultimate bond strength and peak slip indicated a trend of increasing and then decreasing with the increasing times of freezing and thawing, while the peak slip of specimens of C30 with 40 times freeze-thaw increases slightly.

Effects of Post-Harvest Conditions on Sorption Isotherms of Soybeans

This study focuses on the modeling of sorption characteristics of three varieties of soybeans (Akras R2, Lono R2, and Podaga R2). Three pretreatments related to post-harvest conditions were tested on the soybean varieties: (1) freshly harvested soybeans, (2) soybeans subjected to three drying and wetting cycles, and (3) soybeans subjected to three freezing and thawing cycles. The adsorption and desorption experiments were conducted at 5°C, 10°C, 15°C, 20°C, 25°C, and 30°C using a dynamic equilibrium relative humidity (ERH) apparatus. Equilibrium moisture content (EMC) and the corresponding ERH were measured. The parameters calculated for the modified Halsey equation are applicable for storage temperatures above 10°C in the relative humidity (RH) ranges of 10% to 80% for desorption and 30% to 80% for adsorption. No significant differences were found in sorption isotherms among the soybean varieties. However, the soybean varieties responded differently to the different pretreatments (i.e., drying/wetting and freezing/thawing cycles). The adsorption isotherms of Akras and Lono soybeans showed significant differences at 10°C to 30°C when subjected to drying and wetting cycles, while Akras and Podaga soybeans showed significant differences in the same temperature range when subjected to freezing and thawing cycles. The effect of drying and wetting cycles on the desorption isotherms was found only for Akras soybeans at 10°C and 15°C below 63% and 71% RH, respectively, and for Lono soybeans at 25°C and 30°C above 69% RH for both temperatures. In general, the effect of both pretreatments on the sorption isotherms of soybeans was a reduction in EMC of up to 20%, when compared to fresh samples at selected storage temperatures. The findings of this study serve as a primary tool for developing a lookup table for safe storage guidelines for soybeans. Keywords: Equilibrium moisture content, Equilibrium relative humidity, Halsey equation, Oswin equation, Soybeans.