Calcium Sulfoaluminate Research Articles

Durability of clayey soil stabilized with calcium sulfoaluminate cement and polypropylene fiber under extreme environment

Calcium Sulfo-Aluminate Cement (CSAC) is gaining interest among researchers, consultants, engineers, and environmentalists as a prospective replacement of Ordinary Portland Cement (OPC) across many construction industries. The main reason is that the production of CSAC generates a lower carbon footprint as compared to OPC cement production. Other benefits of CSAC include quick bonding & strength development, and less shrinkage. Many studies are available and ongoing on the advantages and limitations of CSAC in concrete. However, only a few studies are available on its application in soil stabilization. Therefore, the efficiency of CSAC for soil stabilization is a topic of discussion in geotechnical engineering. In this research, a locally sourced highly compressible clayey soil was stabilized with different combinations of CSAC and polypropylene fiber (e.g., 5.0%, 7.5% & 10.0% of CSAC and 0.5% & 1.0% polypropylene fiber), and its effectiveness was observed and evaluated in terms of durability (Wetting-Drying and Freezing-Thawing tests) against extreme environments and also checked Unconfined Compressive Strength (UCS) as per American Society of Testing and Materials (ASTM) guidelines. The findings of this research are evaluated in terms of percentage of soil loss, change in moisture content and volume, and loss of strength after durability exposure. Moisture content and bulk unit weight were determined after each stage in the Freezing-Thawing durability cycles. Samples prepared with 10.0% of CSAC and 1.0% of fiber were able to survive all durability cycles of Wetting-Drying with PCA (Portland Cement Association) soil-loss percentage criteria. On the other hand, samples prepared with 10.0% cement with 0.5% as well as 1.0% fiber were able to survive all durability cycles of Freezing-Thawing with PCA soil-loss percentage criteria. These survived samples showed a significant amount of unconfined compressive strength even after being subjected to these durability cycles.

A Method of Producing Low-Density, High-Strength Thin Cement Sheets: Pilot Run for a Glass-Free Solar Panel

This paper presents an innovative method of producing a low-density, high-strength, thin cement sheet. A seaweed powder was mixed with Portland cement, a foaming agent, calcium sulfoaluminate (CSA), and a quantity of water to create an A4-sized thin sheet with a thickness of 7 mm, which can withstand 1.5 kg in weight. This sheet was then covered with ethylene vinyl acetate and a backsheet to create a sandwiched cement sheet. The advantages of this sandwiched cement sheet are two-fold. First, it can support up to 13 kg in a static mechanical loading test, without bending, for over eight hours. Second, it can be quickly recovered at the end of its life cycle. This was a preliminary experiment to produce a large cement sheet that could satisfy the loading requirements for a solar panel. The purpose of the large, thin cement sheet is to replace the glass in a conventional solar panel and create a lightweight solar panel of less than 10 kg, which would mean that the installation of solar panels would become a one-person operation rather than a two-person operation. It would also increase the efficiency of the solar panel installation process.

Effects of M-value on long-term microstructural evolution of CSA cement blended with slag

In the present study, the effects of the calcium sulfate-to-ye'elimite molar ratio (M-value) on the microstructural evolution of calcium sulfoaluminate (CSA) cement blended with slag during the long-term hydration process were investigated. Blast-furnace slag (BFS) was substituted with a dosage of 50% relative to the binder mass, and anhydrite was used to control it with M-values of 0.75, 1.0, 1.5, and 2.0. The fabricated samples were cured for up to 360 d, and the phase evolution and physicochemical properties were explored using various analytical methods. The test results indicated that the formation of CAH10—a metastable phase—was suppressed at higher M-values, and the formation of strätlingite was suppressed as the M-value increased, owing to the reduced dissolution extent of belite at higher M-values. C-(A)-S-H was produced along with strätlingite after the formation of ettringite and monosulfate in samples with low M-values because belite can be dissolved significantly in the later stage of hydration of CSA cement-based materials. BFS did not substantially affect the microstructural evolution of the samples during long-term hydration, because of its low reaction degree.

Effect of different retarders on setting time and mechanical properties of hemihydrate phosphogypsum-calcium sulfoaluminate cement composite binder

The optimization of mechanical properties and regulation in setting time of beta-hemihydrate phosphogypsum (β-HPG) are the key to its utilization in building materials. Calcium sulfoaluminate cement (SAC) can significantly enhance the mechanical properties of β-HPG to form a gypsum-based-composite binder, but have little effect on regulating the setting time. Retarders can regulate the setting time of β-HPG but decrease its mechanical properties. However, the synergistic modification mechanism of retarders and SAC on β-HPG has not been clarified. In this paper, the effects of protein retarder (SC) and citric acid (CA) on the setting time, hydration heat, mechanical properties and microstructure of the β-HPG-SAC composite binder were studied systematically. The results show that incorporating SC and CA can effectively regulate the setting time, but the effects on the mechanical properties differ. SC has a certain optimization effect on mechanical properties at low content, but has a negative effect when the content is over 0.2%, while CA has a continuous negative effect with its increasing content. Specifically, SC hinders the formation of the crystalline structure to a certain extent, and makes the crystallization of β-HPG and the hydration of SAC synergistic, while CA delays the hydration process of β-HPG and SAC simultaneously. Different from SC, CA can inhibit the growth of dihydrate gypsum (DH) crystals in the C-axis direction, transforming long rod crystals into short columns, eventually forming a looser microstructure and decreasing mechanical properties. This work provides an effective method for the modification of β-HPG and the high value-added resource utilization of PG.

Comparative analysis of three different types of self-healing concrete via permeability testing and a quasi-steady-state chloride migration test

Concrete is the most used construction material in the world and with the growing world population, the demand for housing and infrastructure keeps increasing. Due to its low tensile strength, concrete cracks easily and these cracks have a negative effect on the durability. Different studies have already been conducted on the development of self-healing concrete, which has the ability to heal its own cracks. Unfortunately, due to a lack of standardized test methods, the boundary conditions in these studies are often different, making it difficult to do an accurate comparison. In this research, three different healing agents are tested using the same test methods under the same boundary conditions: a powder of organic/inorganic composite material (WM), solidified calcium sulfoaluminate capsules (SMU) and bacterial pellets (KU). The healing efficiency of the agents is tested with (1) a low-head water flow test on prismatic specimens cracked in a three-point bending test with subsequent active crack width control, (2) a Korean water permeability test on cylindrical specimens cracked by a Brazilian splitting test, (3) a chloride diffusion test and (4) a quasi-steady-state chloride migration test. The results showed that WM performed superior with respect to the reference mix and the other healing agents in the two water permeability tests and the quasi-steady-state chloride migration. The performance of SMU and KU was depending on the healing condition. When fully saturated during healing, the KU specimens had a good behaviour and SMU performed poorly. For the cylindrical specimens, with the specimen partly immersed in water, SMU and KU had a similar performance which was better in comparison to the reference.

Investigations on the Suitability of Alternative and Supplementary Cementitious Materials to produce High Performance Aerogel Concrete

AbstractHigh Performance Aerogel Concretes (HPAC) are produced by embedding silica aerogels in high‐performance matrices, which were previously based on finely ground Portland cement CEM I 52.5 R and microsilica. On the one hand, this ensured a comparatively high compressive strength, but on the other hand it was associated with large shrinkage strains and a high global warming potential (GWP). In the following, approaches are presented on how these disadvantages can be counteracted by replacing Portland cement with alternative (ACM) or supplementary cementitious materials (SCM). In the experimental investigations, calcium aluminate cement, calcium sulfoaluminate cement, calcium sulfoaluminate belite cement and a CSH phase binder were used as ACMs; limestone powder, calcined clay, fly ash, glass powder, rice husk ash, brick dust and ground granulated blast‐furnace slag were used as SCMs. Based on extensive fresh and hardened concrete tests, it could be shown that with the (partial) replacement of Portland cement by ACMs or SCMs, High Performance Aerogel Concretes can be produced that have significantly reduced shrinkage deformations and a lower GWP with an unchanged good relationship between compressive strength and thermal conductivity.

Investigation of leaching behaviors of carbonation-cured CSA cements using an electrically accelerated leaching test

The present study provides the investigation of leaching behaviors of carbonation-cured calcium sulfoaluminate (CSA) cements with varying initial sulfate contents using an electrically accelerated leaching test. The CSA cement samples were normally or carbonation-cured for 28 days before subjecting them to an electrically accelerated leaching condition for 14 days. The samples were characterized experimentally before and after accelerated leaching. In addition, the leaching behaviors of the CSA cement samples over a wide range of cumulative leached water were thermodynamically modeled to determine their likely performance in repository environments. The results indicate that the reaction of ye'elimite was significantly promoted by accelerated leaching, resulting in the formation of ettringite after leaching, regardless of the curing regime used. Calcium sulfates and C-(A)-S-H formed in the carbonation-cured samples were the first phases to decompose upon accelerated leaching, which is in good agreement with the thermodynamic modeling calculations. For the normally cured samples, the use of higher calcium sulfate in the mixture is preferred, considering the amount of leached ions, yet the opposite holds true for the carbonation-cured samples. In particular, the changes in the calculated volumes of the reaction products upon simulated leaching were smaller for the carbonation-cured samples than for the normally cured samples.

Hydration mechanism of anhydrite and calcium sulfoaluminate co-activated slag cement: insights into the role of composition

Utilization of Portland cement to initiate the dissolution of blast furnace slag (BFS) in super-sulfated cement is limited by low slag substitution level and poor early strength. In this study, a new activator composed of anhydrite and calcium sulfoaluminate cement clinker was proposed to enhance the utilization of BFS in super-sulfated cement. Tests were conducted on the compressive strength, autogenous shrinkage, hydration heat, pH and conductivity of the composites. XRD, 29Si NMR, BSE and MIP were employed. Results reveal that ettringite, gypsum and C-S-H are the primary hydration products. The composites with 20% activator display the highest compressive strength and shrinkage accompanied by the highest hydration heat and degree, as well as the highest quantity of monomer silica tetrahedrons and the largest volume of fine pores. Thermodynamic modeling confirms the critical value for the activator-substitution level. Additionally, the activator-BFS composite exhibited significantly lower carbon emissions compared to Portland cement.

Chloride-induced corrosion patterns of reinforcements in simulated pore solutions of calcium sulfoaluminate cement concrete: An analytical study

The widespread use of calcium sulfoaluminate cement (CSA) for rapidly repairing marine structures exposed to high chloride levels has spurred limited investigation into the corrosion behavior of reinforcements in low-pH CSA systems. To address this knowledge gap, researchers conducted a comprehensive study on the impact of primary ions within CSA (OH−, SO42−, and Al(OH)4-) on steel rebars in simulated pore solutions (SPSs). The investigation employed various analytical methods such as Mott-Schottky/cyclic polarization curves, corrosion current, impedance spectrum analyses, and microstructural examinations. The results demonstrate that SO42− diminishes the corrosion resistance of reinforcements by depassivating them, consequently reducing the critical chloride concentration (Ccrit). However, this influence is mitigated with elevated OH− or Al(OH)4− concentrations. Furthermore, the presence of Al(OH)4- in the pore solution contributes to an increased Ccrit by stabilizing alkalinity via OH− hydrolysis. Based on the empirical findings, the researchers developed an empirical formula for Ccrit, offering insights into estimating the service life of CSA-based concrete structures in real marine environments.

Improved ettringite stabilization by calcium carbonate and calcium nitrate additions in ternary PC-CSA-C$ systems

Mixing Portland cement (PC) and calcium sulfoaluminate (CSA) cement can provide several advantages for specific applications. However, the stability of ettringite has a significant impact on ettringite-rich cements properties. This study aims to investigate the impact of calcium nitrate (Ca(NO3)2 = CN) and calcium carbonate (CaCO3 = CC) on the performance evolution and stabilization mechanism of PC-CSA-C$ systems. The experimental results demonstrate that the incorporation of CC and CN has a significant impact on the early hydration kinetics. A second exothermic peak appears indicates the depletion of the calcium sulfate, and its appearance time is related to the amount of CC and CN added. The blended system containing CN exhibited the highest early and late compressive strength, as well as excellent volume stability. Compared with CC, CN was confirmed to be more effective in stabilizing ettringite in the PC-CSA-C$ system and preventing its conversion into monosulfate. Furthermore, the analysis of the pore structure reveals a significant increase in the proportion of pores with diameters ≤50 nm in the systems containing CN. In particular, CN displays a stronger ability to stabilize ettringite to reduce chemical shrinkage and increase the content of C-(A-)S-H gel, and the 28-day compressive strength was approximately 10 MPa higher compared to the control group.

Effect of Na versus Ca Sulfate Salts on the Hydration of Calcium Sulfoaluminate Clinker.

This paper examined how the amount (5% or 20%) and type (CaSO4 or Na2SO4) of sulphate salt affect the hydration of calcium sulphoaluminate clinker (KCSA). The mechanical behavior of the pastes was determined at 1, 3, 28, and 90 days, the heat flow and total heat were measured with isothermal conduction calorimetry, and the reaction products were characterized using X-ray diffraction (XRD), differential thermal analysis/thermogravimetry (DTA/TG) and scanning electron microscopy (SEM). The results obtained indicated that both the amount of sulphate salt (5% or 20%) and its type (CaSO4 or Na2SO4) affect the hydration kinetics, type of reaction products formed, and development of mechanical strength. The incorporation of CaSO4 has a positive effect on the development of the mechanical strength of KCSA. However, Na2SO4 can also produce adverse side effects at older ages. The presence of Na2SO4 increases pH values, which partly destabilizes the ettringite formed, thereby favoring carbonation and thenardite precipitation, which can cause the specimens to crack and break.

Development of a novel cement-based grout with enhanced thermal and sealing performance for borehole heat exchangers

Filling a borehole with proper grout is important for maintaining the safety and performance of borehole heat exchangers (BHEs). The current grouts have problems of large shrinkage and low thermal conductivity, which may restrain BHE efficiency and pose safety risks. To address this, this study explores solutions to enhance the thermal and sealing performance of cement-based grout, specifically for BHE applications. A novel grout, composed of ordinary Portland cement (OPC)-calcium sulfoaluminate (CSA) cement-gypsum ternary system and graphite, was developed and thoroughly examined. The developed grout exhibited good flowability and volumetric stability, meeting the required compressive strength in one day. It displayed high thermal conductivity, ranging from 1.606 to 2.504 W/mK at oven-dry conditions and 2.173 to 3.532 W/mK at saturated conditions. At last, numerical modeling was conducted to compare the thermal performance of boreholes using the novel grout developed in this study and a conventional grout. Numerical models prove that using developed grout can improve the thermal performance of boreholes and decrease the borehole resistance, thus lessening the required borehole length, further reducing the costs and carbon emissions of borehole construction. Overall, the developed grouts showcased enhanced thermal and sealing performance. They can be applied in BHE systems not only to enhance operational efficiency and safety, but also to reduce costs and carbon footprints.

Forecasting unconfined compressive strength of calcium sulfoaluminate cement mixtures using ensemble machine learning techniques integrated with shapely-additive explanations

Calcium sulfoaluminate (CSA) cement mixture design is challenging due to the influence of multiple features on its unconfined compressive strength (UCS). Consequently, the relationships between input features and the UCS exhibit non-linear behavior, making it difficult to understand using experimental methods alone. Therefore, for the first time, this study constructed non-linear ensemble machine learning (ML) models on a dataset compiled from experimental literature to accurately predict the UCS of CSA cement mixtures. After applying feature selection techniques, four different ensemble models were built on the modified datasets to predict the UCS. The extreme gradient boosting model built on the dataset modified by the least absolute shrinkage and selection operator method achieved the best prediction accuracy (coefficient of determination; R2 = 0.95) on testing data. Finally, the SHapely Additive exPlanations analysis could interpret the selected ML model both quantitatively and qualitatively, by explaining the independent relationships between each input feature and UCS.

Processing of calcium sulfoaluminate eco-cement coatings containing microencapsulated phase change materials

On the one hand, calcium sulfoaluminate (CSA) eco-cements release about 40% less carbon dioxide (CO2) than Portland cement during fabrication; on the other hand, phase change materials dispersed in a cementitious matrix can help to optimise the indoor temperature of buildings, reducing carbon dioxide emissions related to heating/air conditioning. However, this is only economically viable if it is used as a thin layer (a coating). In addition, the combination of both materials supposes a double environmental benefit. Consequently, the main objective of this work is the preparation of a suitable homogeneous and well-adhered bilayer sample, composed of CSA and CSA-MPCM. To achieve this, in the first step, the effect of pH, temperature and stirring was studied for microencapsulated phase change material (MPCM) aqueous suspensions (47.3 wt%); second, the MPCM (45 wt% with respect to dry cement) was dispersed in a CSA paste; then, in a third step, a homogeneous well-adhered coating of CSA-MPCM, with undamaged MPCM, was obtained on a CSA matrix. This was achieved through rheological measurements and checked by microscopy. Finally, the corresponding CSA and CSA-MPCM mortars were characterised through their mechanical properties (compression) (70 and 13 MPa at 7 days, respectively) and thermal conductivity (2.06 and 1.19 W/mK, respectively).

Frost-Heaving Behavior and Enhancement Approaches of Cement-Based Grout Materials under Freeze–Thaw Conditions

In the seasonally frozen regions, during the grouting of prestressed bridge ducts in low-temperature environments, incompletely cured grout materials undergo volumetric changes due to freeze–thaw cycling, resulting in structural cracks along the prestressing ducts of the bridge, thereby diminishing the bridge’s operational lifespan. In order to investigate the freeze–thaw characteristics of grouting materials under the influence of freeze–thaw cycles and propose improvement measures, the influence of various additives on the freeze–thaw stress characteristics of mortar under freeze–thaw cycle conditions was elucidated through freeze–thaw stress tests. The mechanisms for improving the freeze–thaw characteristics of grouting materials were explored through analyses of free water content, setting time, compressive strength, XRD, and SEM. In light of the requirements for comprehensive performance of grouting materials, composite additives are employed to enhance the freeze–thaw performance of the grout. The results indicate that reducing the water-cement ratio, incorporating calcium formate, sulfoaluminate cement, air-entraining agents, and carbamide all have a positive impact on mitigating frost-heaving stress in grout materials. However, the improvement mechanisms differ, and employing a single measure alone is insufficient to effectively reduce frost-heaving stress while meeting performance criteria such as compressive strength, setting time, and flowability. Free water content emerges as a crucial indicator determining the magnitude of frost-heaving stress in grout materials, with 11.5% of free water content representing the critical threshold for frost heaving in grout materials. Utilizing composite admixtures can simultaneously decrease free water content, lower the freezing point of free water, and alleviate frost-heaving deformation, resulting in a more efficient reduction of frost-heaving stress. When the admixture content reaches 9.9%, frost-heaving stress is eliminated, and the comprehensive performance parameters, including compressive strength, setting time, and flowability, meet the specified requirements. Overall, the conclusions of this research will offer a scientific foundation for the choice of cold-resistant grouting materials, the mitigation of grout material freeze–thaw risk, and the improvement of quality assurance levels in bridge construction within seasonally frozen areas.

Insight into the role of early C3A hydration in structural build-up of cement paste

This research sheds light on the two origins of C3A hydration in structural build-up of cement paste. The largest critical strain is associated with colloidal interaction, whereas the smallest critical strain originates from the cohesion between calcium sulfoaluminate hydrate (CA$H) particles. Interestingly, we have found that the structural build-up primarily relies on the total colloidal interaction within 2 h of hydration rather than the cohesion between CA$H particles. In C3A-gypsum-CaCO3 pastes, a two-stage structural build-up was observed. The swift progression within the first tens of minutes can be traced back to the augmentation in the number of bonds (solid volume fraction), which then proceeds to the bond strength (colloidal interaction between two flocculated particles). The subsequent slow, linear development is attributed to the strengthening of bonds. As a result of this analysis, a time-dependent model for structural build-up was established. Two rates, namely Acol and ACA$H, were employed to illustrate the impact of the strength and number of bonds. This model is proven to be well-suited for analyzing cement paste within the initial tens of minutes.

Effect of polyvinyl alcohol powder on the impermeability, frost resistance and pore structure of calcium sulfoaluminate cement concrete

The pore structure has close tie with the performances of concrete, especially the impermeability and frost resistance. In this paper, Polyvinyl alcohol (PVA) powder was added into calcium sulfoaluminate cement (CSA) concrete to improve the pore structure. The effect of PVA on the impermeability, frost resistance and pore structure of CSA concrete were studied, and the relationship between impermeability and pore structure was revealed. The results show that PVA significantly improves the impermeability and frost resistance of CSA concrete. The seepage height reduces from 3.06 mm to 1.43 mm, the relative permeability coefficient reduces from 4.82 × 10-6 mm·h−1 to 1.05 × 10-6 mm·h−1, and the frost resistance increases from F100 to F150. PVA can reduce the porosity of CSA concrete, and the critical pore size and pore connectivity also gradually decrease, while the fractal dimension gradually increases. The relative permeability coefficient of CSA concrete has a good linear relationship with porosity and capillary porosity, an exponential relationship with critical pore size, a negative linear relationship with fractal dimension, and a logarithmic relationship with pore connectivity. Therefore, the addition of PVA reduces the porosity of CSA concrete and improves the impermeability and frost resistance.

STRUCTURE AND PROPERTIES OF THE ETRINGITE PHASE

Problem statement. Aluminate and sulfoaluminate cements are not produced in Ukraine, despite a rather significant need for binders of this class. The use of imported raw materials is limited by the high cost and certain disadvantages occurring during exploitation including rapid hardening, significant heat generation, which is associated with instability of some sulfoaluminates. At the same time, it is possible to highlight the following problems in the direction of expanding possibilities of using special cements of this type: stabilization over time and operating conditions of the hydrosulfate phase based on alumina cement, as well as modification of the compositions of mineral binders based on calcium sulfate dihydrate (CaSO4·2H2O) and development of binders of this class based on secondary production products. The main factor is that during hydration of sulfoaluminates and aluminates in the presence of gypsum (CaSO4·2H2O), a hydrosulfoaluminate phase is formed, which makes it possible to obtain a hardened cement paste structure with special properties. Then monocalcium hydrosulfoaluminate turns into hydrosulfoaluminate of the low-sulfate form С3А·СaSO4·12H2O with the release of gibbsite Al2O3·3H2O. Also, ettringite С3А·СaSO4·nH2O and 2(С2S)·nH2O is formed and hydrated calcium silicate CSH(B) can be formed. When gypsum is added to cement, ettringite is formed in this system. Etringite is one of the components. It is not formed initially, but through intermediate structures. During hydration reactions, ettringite is rearranged, neoplasms are formed, which can lead to gypsum corrosion. Etringite loses its stability. Herewith, the problem of primary and secondary ettringite arises. Primary ettringite creates conditions for strength. Secondary ettringite is formed already in the hardened system and leads to internal stresses. Formation of secondary ettringite can have both positive and negative consequences. The purpose of the atrticle is to investigate structure and properties of the ettringite phase. Conclusions. The hydration process depends on the Gibbs surface energy. Change in surface energy depends on the CaO/Al2O3 ratio. It was established that the surface energy increases with an increase in the CaO/Al2O3 ratio. The paper has studied influence caused by the ettringite phase on the main characteristics of alumina cement and gypsum in a modified gypsum binder. Studies have been conducted on formation of the maximum amount of ettringite phase. We have calculated the maximum amount of the ratio of alumina cement and gypsum to obtain the maximum amount of mineral – 70 % alumina cement and 30 % gypsum. The highest effect is achieved during the simultaneous use of С6АṤ3Н32 and АН3, which occurs during hydration of С4А3Ṥ. Taking into account the above, it is advisable to obtain clinker containing calcium sulfoaluminate and cements based on it.

Durability of GFRP and BFRP reinforcing bars in simulated seawater sea sand calcium sulfoaluminate cement concrete pore solution

Thanks to the much smaller carbon footprint of calcium sulfoaluminate cement (SAC) concrete and the corrosion resistance of fiber reinforced polymer (FRP) reinforcement, FRP-reinforced SAC concrete can possibly replace conventional reinforced concrete in some cases. In this study, for the first time, the durability of glass FRP (GFRP) and basalt FRP (BFRP) bars, made with epoxy resin, is investigated in simulated pore solutions of concretes made with (a) SAC and fresh water, (b) SAC and seawater and sea sand. The bars were immersed in each solution and kept at constant temperature of 30 °C, 45 °C and 60 °C for up to 180 days. Bar samples were tested for their retained tensile strength and physiochemical degradation after 30, 60, 90, and 180 days of immersion. Scanning electron microscopy (SEM), X-ray dispersive spectroscopy (EDS), and Fourier transform infrared (FTIR) spectroscopy were used to identify the underlying damage mechanisms. Results show that solution (b) is less harmful to BFRP than solution (a), while the opposite is true in the case of GFRP. Both solutions caused the dissolution of the epoxy resin and the leaching of the Fe in the basalt fiber. Overall, GFRP exhibited higher durability than BFRP. Based on image analysis, the deteriorated zone within the bar cross-section is not a uniform ring, but its area correlates with the loss of tensile strength. Inspired by the Avrami-Erofe'ev Equation and the Eyring's Transition State Theory, a novel time-temperature-dependent FRP bar degradation model is proposed. The proposed model shows close agreement between the predicted and measured tensile strengths.

Assessment and mitigation of early-age cracking for on-site concrete structures by a combined scheme using Temperature Stress Testing Machine (TSTM) and long-span restraining frame

From the field survey, thermal cracking frequently occurs in substructural concrete walls, although a high dosage of Calcium Sulfoaluminate (CSA) expansive additive is widely utilized. Conventional assessment methods fail to characterize the failure of CSA expansion under coupled realistic conditions. Therefore, an assessment scheme is proposed in this study: Firstly, mix proportions, temperature, restraint and curing conditions are used as input for a developed Temperature Stress Testing Machine (TSTM). By testing, limited expansions of CSA additive under low water-to-binder ratio, high temperature or drying condition, is unfolded. Corresponding countermeasures are also proposed. Secondly, long-span restraining frames are constructed to validate, and upscale findings from TSTM. Eventually, normal and optimized mixtures are applied and compared in real substructural walls. This combined scheme newly reveals that the lack of free water is the mechanism for CSA's stagnant expansion and a combination of CSA and lightweight aggregates shows promising potential as a countermeasure.