Shrinkage Reducing Admixture Research Articles

Alkali-Activated Slag as Sustainable Binder for Pervious Concrete and Structural Plaster: A Feasibility Study.

In the realm of sustainable construction materials, the quest for low-environmental-impact binders has gained momentum. Addressing the global demand for concrete, several alternatives have been proposed to mitigate the carbon footprint associated with traditional Portland cement production. Despite technological advancements, property inconsistencies and cost considerations, the wholesale replacement of Portland cement remains a challenge. This study investigates the feasibility of using alkali-activated slag (AAS)-based binders for two specific applications: structural plaster and pervious concrete. The research aims to develop an M10-grade AAS plaster with a 28-day compressive strength of at least 10 MPa for the retrofitting of masonry buildings. The plaster achieved suitable levels of workability and applicability by trowel as well as a 28-day compressive strength of 10.8 MPa, and the level shrinkage was reduced by up to 45% through the inclusion of shrinkage-reducing admixtures. Additionally, this study explores the use of tunnel muck as a recycled aggregate in AAS pervious concrete, achieving a compressive strength up to 20 MPa and a permeability rate from 500 to 3000 mm/min. The relationship between aggregate size and the physical and mechanical properties of no-fines concretes usually used for cement-based pervious concrete was also confirmed. Furthermore, the environmental impacts of these materials, including their global warming potential (GWP) and gross energy requirement (GER), are compared to those of conventional mortars and concretes. The findings highlight that AAS materials reduce the GWP from 50 to 75% and the GER by about 10-30% compared to their traditional counterparts.

Tensile Creep and Cracking Resistance of High-Strength Concrete with Shrinkage Reducing Admixture under Uniaxial Restrained Condition at Early Age

Tensile Creep and Cracking Resistance of High-Strength Concrete with Shrinkage Reducing Admixture under Uniaxial Restrained Condition at Early Age

Feasibility of Sprayable Engineered Cementitious Composites Using Natural Sand and Low Fiber Content: Effect of Hybrid Fiber System, Silica Fume, and Air-Entraining and Shrinkage-Reducing Admixtures

Deterioration of corrugated steel culverts (CSCs) is a significant problem for the Virginia Department of Transportation and other U.S. transportation agencies. Spray-on lining using shotcrete engineered cementitious composites (ECCs) is a promising structural repair strategy for deteriorated CSCs. This study evaluates the feasibility of practical sprayable ECCs using natural silica sand and low fiber content, while forgoing the use of commonly utilized calcium sulfoaluminate cement to allow for delivery of the materials via ready-mixed concrete trucks. Aspects of the compositional space evaluated include the effect of a hybrid polyvinyl alcohol/polyethylene fiber system, silica fume (SF), shrinkage reducing admixture (SRA), and air-entraining admixture (AEA) on the properties of the composites. Experimental results suggested that the development of the proposed materials is feasible, because flow characteristics associated with shotcrete ECCs and ECC-like tensile ductility were achieved in the fresh and hardened state, respectively. Notwithstanding, further mixture optimization may be required for satisfactory field performance, which should be assessed through field testing. Generally, the use of the hybrid fiber system tended to improve the strength and ductility of the composites, especially when coupled with the use of SF. Furthermore, SF was effective at mitigating the strength loss associated with the incorporation of AEA and SRA, and meaningfully improved surface resistivity. SF also tended to reduce the mixture’s flow, whereas the contrary occurred with the use of SRA and AEA. SRA contributed to the reduction in drying shrinkage at early ages. However, the effect of SRA in reducing shrinkage was negligible at later ages.

Improving the air bubble stability of air-entrained mortar in low air pressure environments via adopted rheology and surface tension methods

Air bubble stability in cementitious materials in low air pressure (AP) environments is influenced by the Ostwald ripening process, which causes coarsening behavior of air bubbles. This research reports the enhancement of air bubble stability in low AP via the adoption of rheology and surface tension methods. Besides, the evolution of air bubbles during the cement early hydration process is proposed. It was found that incorporating viscosity-modifying agents (VMAs) and shrinkage-reducing admixtures (SRAs) resulted in improving air bubble stability by 37.7 % and 23.4 %, upon undergoing AP change from 100 to 60 kPa. Calculation of air bubbles via ripening equations under low AP revealed that VMA induced a notable decrease in the effective diffusion flux within fresh cement mortar, leading to a reduction of 9.8 % in air bubble ripening within 120 min. Conversely, SRAs were primarily aimed at reducing the initial dimensions of air bubbles.

Contribution of Shrinkage-Reducing Admixture and Lightweight Sand to Moist-Curing Requirement for Fiber- Reinforced Ultra-High-Performance Concrete

Contribution of Shrinkage-Reducing Admixture and Lightweight Sand to Moist-Curing Requirement for Fiber- Reinforced Ultra-High-Performance Concrete

Increasing the Length of Concrete Pavement Slabs Using Shrinkage Reducing Admixture and Polypropylene Fiber

Pavement engineers frequently employ concrete pavements because of their benefits such as extended lifetime, superior performance and durability, and so on. However, there are some disadvantages of these pavements such as shrinkage which may lead to cracking, warping, and limiting the length of the concrete pavement slabs. Shrinkage reducing admixture (SRA) and polypropylene fibers can be employed to prevent or control shrinkage cracking. In this study, increasing the length of concrete pavement slabs using shrinkage reducing admixture and polypropylene fiber was investigated. For mix compositions, two water–cement ratios of 0.35 and 0.4 were employed, and the percentages of SRA and polypropylene fiber utilized in mixes were 2% and 1% by weight of cement, respectively. Slump, compressive strength, third point flexural strength, electrical resistance, free and restrained shrinkage tests were carried out as the experimental programming to investigate the effect of these materials on concrete behavior and evaluate the amount of concrete pavement design parameters. Statistical analysis and RSM were used to determine the significance of each parameter and their interactions on concrete properties. It was observed that the use of SRA had no influence on workability; however, polypropylene fibers reduced the slump flow of concrete. Also, the use of SRA resulted in a decrease in mechanical properties. In addition, the use of polypropylene fibers considerably enhanced the energy absorption of concrete. Furthermore, on concrete containing SRA and polypropylene fiber, the magnitude of free and restrained shrinkage and crack width were reduced. Finally, the length and thickness of concrete pavement slabs were evaluated using the experimental results on the Tehran-Shomal freeway as a case study. The slab length could be increased by about 20% without any significant change in the slab thickness using SRA and polypropylene fiber in concrete mix composition. This can lead to an increase in construction speed, improve the durability of pavement and generally increase the quality of the concrete pavement.

Use of hybrid fibers and shrinkage mitigating materials in SCC for repair applications

Self-consolidating concrete (SCC) is used to repair structural elements with congested reinforcement or restricted access to placement and consolidation. In addition to adequate flowability, passing ability, filling capacity, and stability, adequate bonding to the substrate and reinforcing bars, low shrinkage and cracking resistance are required for successful repair. This study employs various shrinkage mitigating materials, including expansive agent (EA), coupled use of EA with shrinkage reducing admixture (SRA), and EA with pre-saturated lightweight sand (LWS) to fulfill these requirements. The effect of these materials on the performance of SCC made with a hybrid synthetic (PP) fiber and a hybrid steel-synthetic (STPP) fiber is investigated. The investigation included key fresh and hardened concrete properties and flexural performance of composite beams repaired using these materials. Test results show that the investigated fiber-reinforced self-consolidating concrete (FRSCC) mixtures can secure adequate workability and mechanical properties and low shrinkage, limited to 600 µstrain after 224 days. High EA content (10% by mass of binder) can lead to excessive expansion at early ages resulting in microcracking and drop in mechanical properties. The FRSCC made with 5% EA and 0.5% SRA, by mass of binder (5EA0.5SRA mixture), had superior mechanical properties (50 MPa compressive strength and 36 GPa modulus of elasticity). The cracking potential due to restrained shrinkage of this mixture is very low. The 5EA0.5SRA mixture made with hybrid synthetic fibers is successfully used to repair reinforced concrete beams prepared with various reinforcement ratios in the tension zone. The results indicate a 48% increase in first crack load, 4% gain in peak load, and 33% increase in flexural toughness compared to the monolithic beam cast with the conventional concrete. A comparison between the cracking load and ultimate flexural capacity of the repaired beams with estimated values by ACI 544 reveals the estimated to experimental ratio of 0.36–0.70 for cracking load and 0.56–0.74 for the flexural capacity. This can stem from the promotion of FRSCC concrete by the effect of hybrid synthetic fibers and the development of internally induced stress in the presence of shrinkage mitigating materials and fibers that led to higher mechanical properties and enhanced performance of FRSCC for repair. Reinforced beams repaired using FRSCC made with 0.5% hybrid synthetic fibers and combination of EA-SRA can reduce the steel reinforcement ratio by 21% to 27%, depending on the primary steel reinforcement.

Drying shrinkage inhibition effect and mechanism of polyol shrinkagex reducing admixture on the metakaolin-based geopolymer

Geopolymers, environmentally friendly materials, face application limitations due to their high drying shrinkage and propensity for cracking. The shrinkage reducing admixture (SRA) has shown promise in mitigating drying shrinkage in cement-based materials, yet its impact and mechanism in geopolymers remain uncertain. This study examines the influence of the small molecule polyol SRA-2-methyl-2,4-pentanediol on metakaolin-based geopolymer (MKG) properties and its multiscale structural development, aiming to understand the drying shrinkage behavior. The findings reveal that SRA significantly curbs the drying shrinkage of MKG while also reducing its mechanical properties. Analyses of macroscopic and microscopic properties indicate that while SRA does not alter the hydration products of MKG, it can impede the hydration process, particularly in the early stages. Specifically, SRA inhibits the formation of N-A-S-H gel, thereby reducing shrinkage caused by late-stage polycondensation. Furthermore, the alkaline environment in MKG aids SRA in lowering pore solution surface tension and increasing its contact angle, reducing drying shrinkage forces. Additionally, SRA modifies MKG's micro-structure, decreasing pore volume, particularly in the 10–50 nm range, further mitigating shrinkage due to surface tension.

Continuous monitoring of the moisture, shrinkage, and carbonation effects on foam concrete performance

The carbonation of conventional concrete is a slow process that generally affects exposed surfaces, however, this reaction is significantly facilitated in foam concrete because of its highly cellularized cementitious matrix. Coupled with the rapid moisture loss, foam concrete tends to exhibit drastic compositional, dimensional, and mechanical changes over a short period of time, highly contingent on environmental conditions. In this study, we investigated the change in the shrinkage, resonant frequency, compressive strength, and carbonation degree of foam concrete at various density levels (2.0, 1.0, and 0.6 g/cm3), under 50% and 100% relative humidity conditions, over 84 days. Under each condition, a physical and a chemical approach (sand and shrinkage-reducing admixture, respectively) were compared to determine the most suitable shrinkage mitigation. The results show that foam concrete undergoes intensive shrinkage within a couple of days when exposed to ambient conditions, with higher values present for the lower density levels. While both mitigation approaches effectively restrain shrinkage of the 1.0 g/cm3 samples, the physical-based shrinkage mitigation can cause marked foam degradation to the 0.6 g/cm3 samples. Additionally, results indicate that, due to rapid carbonation of both Ca(OH)2 and C–S–H, substantial CO2 uptake can take place in foam concrete within 84 days.

Non-traditional aluminosilicate based alkali-activated mortars - statistical optimization of solution parameters and processing conditions for optimal compressive strength, workability and setting time

The recent shortage of traditional fly ash and slag, commonly used precursors in alkali-activated binders, prompted a rigorous search for alternative materials. This study evaluated four groups of currently underused supplementary cementitious materials (SCMs) as precursors for producing alkali-activated mortars. The materials evaluated include three calcined clays, three ground bottom ashes, three volcanic ashes, and two fluidized bed combustion ashes, collectively named non-traditional and natural pozzolans (NNPs). Firstly, the compositions of the NNPs were optimized using trial-and-error methods to minimize the concentration of the activating solution. Then, the two solution parameters i.e., silica modulus and Na2O% were optimized for compressive strength using a central composite design. The compressive strength (ASTM C109), the workability (ASTM C1437), and the setting time (ASTM C191) of the mortar were determined. The results show that all 11 materials could produce mortar mixtures with compressive strength in the 20–40 MPa range at 28 days, satisfactory workability of at least 50 % (flow value), and at least 30 min of initial setting time without heat curing and without blending with conventional SCMs. Most of the materials were activated with lower solution parameters than those used in the literature. The binder optimization suggested Si/Al range 1.0–3.0 and a minimum Ca/Al of 0.25 as favorable molar ratios for potentially reactive aluminosilicate precursors. Multiple linear regression models validated by higher R2 and minimum RMSE and MAE could accurately predict the compressive strength of NNP mortar. Furthermore, calcium hydroxide and shrinkage-reducing admixtures were identified as set regulators in alkali-activated NNP mortars.

Early-age crack-free ultra-high-performance concrete under restrained ring test for large-scale production as an overlay

While UHPC overlays are known for their strength and durability, their high autogenous shrinkage limits its widespread nationwide use, as it can result in delamination and early age cracking. To overcome these issues, incorporating pre-saturated lightweight sand (LWS) combined with expansive agents (EA), and shrinkage reducing admixtures (SRA) in UHPC mixes is recommended. To achieve an optimal balanced proportion among these mitigation measures, a sequence optimization process was conducted for a series of mixtures produced at a mixing volume of 127 ∼ 140 Liters. This process aimed to develop a crack-free UHPC mix at an early age under restrained conditions (AASHTO T334). The findings indicate that crack-free UHPC can be produced by incorporating LWS (50 % replacement of fine aggregate), SRA (2 % by weight), and EA (2 % by weight). The optimized mixture demonstrates autogenous shrinkage of less than 100 µɛ and bond strength of 3 MPa within 3 days. Furthermore, three mixing protocols (mono-batching, two-batching, three-batching methods) were devised based on the fine aggregates (river sand) moisture content. These protocols enable the production of homogenous UHPC at larger volumes while ensuring sufficient flowability and desirable hardened properties. The results indicate that the mono-batching method yields the highest performance and is suitable for river sand moisture content of 1.6 % (by weight) or higher. On the other hand, the two- and three-batching methods are more suitable for river sand moisture contents below 1.6 %.

Design and performance of fiber-reinforced shrinkage compensating eco-friendly concrete

Eco-Crete is an ecological and economical concrete that benefits from high packing density of solid materials and reduced paste. Eco-Crete can enhance the service life of structures by reducing the risk of shrinkage cracking. In this study, shrinkage mitigating materials included an expansive agent (EA), a shrinkage reducing admixture (SRA), and a lightweight sand (LWS), as well as steel and synthetic fibers were used to minimize the risk of cracking. A total of 35 fiber-reinforced Eco-Crete mixtures were prepared with 350 kg/m3 of cementitious materials and 55 % substitution of fly ash and slag. Key fresh and mechanical properties in addition to plastic, restrained, and drying shrinkage were evaluated. Results showed that 32-week drying shrinkage of Eco-Crete made with shrinkage mitigating strategies was limited to 400 µstrain, and none of the selected Eco-Crete mixtures experienced cracking up to 112 d under restrained shrinkage testing. Mixtures incorporating only EA and synthetic fibers exhibited excessive initial expansion and minor plastic shrinkage cracking. Eco-Crete mixtures developed at least 40 MPa compressive strength after 56 d. Mixtures made with EA and SRA presented the highest flowability and mechanical properties and the lowest drying shrinkage. The use of synthetic fibers exhibited a better flexural pre-cracking performance than steel fibers. However, steel fiber-reinforced mixtures showed 250 % greater flexural toughness. The use of steel fibers was more effective than synthetic fibers in reducing long-term shrinkage and limiting initial expansion caused by the high content of EA. Statistical analysis and embodied carbon assessment revealed the advantages of using Eco-Crete made with 5 % EA and 0.5 % SRA and 0.5 % synthetic fibers to secure low-shrinkage Eco-Crete.

Utilizing Ultra-High Performance Concrete Overlay for Road Pavement Repair and Strengthening Applications

This study aims to develop a new thixotropic ultra-high-performance concrete (UHPC) overlay for the repair and strengthening of damaged hot mix asphalt (HMA) pavements. The overlay is purposely designed to accommodate the roadway slope of up to 10% due to presence of viscosifying agent materials. The original UHPC materials are comprised of granite aggregate, ultra-fine calcium carbonate, shrinkage-reducing admixture, viscosifying agent, and expansive agent. The study is conducted with three sets of samples provided and considers thixotropic and mitigated shrinkage properties through comparing control (non-thixotropic) overlay 1 (thixotropic), and overlay 2 (thixotropic) mixtures. Based on the obtained results, only overlay 1 corresponds to the minimum requirement for pavement rehabilitation, with 160-200 mm flowability and -545.3 µm/m free shrinkage. As a result, an average 50 mm thick overlay 1 is selected to repair a damaged HMA pavement (1800 m2), while the field implementation procedures and drawing details are also presented in this paper.

Synergetic effect of superabsorbent polymer and shrinkage reducing admixture on the properties of alkali-activated slag binders

This research is to investigate the synergistic effects of superabsorbent polymer (SAP) and shrinkage-reducing admixture (SRA) on autogenous shrinkage, hydration and microstructure of alkali-activated slag system (AAS). Results revealed that the addition of SRA reduced the absorption capacity of SAP, resulting in a smaller size distribution of macrovoids throughout the AAS matrix. Nevertheless, the presence of SAP affected the ability of SRA to effectively reduce the surface tension of the pore solution of AAS. Instead, the addition of SAP resulted in an increase in surface tension by up to 14%. This phenomenon can be explained by the hydrophobic interactions between SAP networks and SRA molecules. The combination of SAP and SRA in AAS exhibited greater effectiveness in controlling autogenous shrinkage compared to using SRA alone, resulting in a reduction of around 53% compared to 46%. On the other hand, incorporating only SAP proved to be more effective, leading to a higher reduction of 90%. However, this combined effect is reported to be counteractive and dependent on the content of each component. Moreover, the hybrid system of SAP and SRA demonstrates a pronounced retardation effect on AAS hydration. The XRD patterns showed no new reaction products were formed. As revealed by SEM analysis, the microstructure of AAS containing SAP and SRA exhibited a distribution of microvoids ranging from 5 to 50 μm. This can explain the reduction of the compressive strength by incorporation a high content of SAP combined with SRA. However, incorporating a lower dosage of SAP is advantageous when combined with SRA in AAS systems. Finally, the integration of SAP and SRA had a positive influence on the properties of AAS mortars, thus compensating for the limitations of each constituent.

Prediction of Restrained Shrinkage of New-to-old Concrete Interface

The focus of this study is on the shrinkage differences between new and old concrete, which can cause cracks at the bonding surface of the interface. The effects of various factors on shrinkage, such as the content of shrinkage reducing admixtures (SRA) the content of basalt fibers (BF) and the roughness at the new-to-old concrete interface (RI) are also examined. In this research, nine groups old-new concrete composite specimens were tested for shrinkage to gather experimental data. The restrained shrinkage mechanical model (RSMM) was deduced, and the restrained shrinkage model based on grey correlation analysis (RMGC) was proposed. The effectiveness and convenience of both prediction methods were evaluated by comparing their results to the experimental data. It was determined that the RMGC was more effective and convenient. Additionally, the research found that as the content of SRA and BF increases, shrinkage decreases. The effect of SRA was found to be more significant. As RI increases, shrinkage undergoes an initial increase and then decreases. The study can be used to inform the design and construction of structures that use both new and old concrete.

Effects of shrinkage-reducing admixtures and expansive agents on the self-sensing behavior of nanomodified cement-based materials

Shrinkage control is needed to ensure an appropriate installation of cementitious sensors used in the Structural Health Monitoring of concrete elements. However, the effects of shrinkage-reducing admixtures (SRAs) and expansive agents (EAs) on the self-sensing properties of the cementitious sensors is still unknown. Therefore, the present work evaluated the effects of a SRA and EAs based on calcium oxide (CaO) or calcium sulfoaluminate (CSA) on the shrinkage behavior, microstructure, self-sensing response and compressive strength of mortars containing carbon black nanoparticles (CBN). The SRA provided shrinkage reductions up to 47.4%. CaO-based EA and CSA-based EA provided an expansive behavior. The admixtures caused increases in electrical resistivity and gauge factor up to 37.1% and 71.0%, respectively. In contrast, they did not cause statistically significant changes in stress sensibility and compressive strength. A microstructural analysis suggested that portlandite and ettringite crystals provided by the inclusion of EAs caused interruptions in conductive paths inside the mortars, which decreased contact conduction mechanisms and increased the amount of insulating tunneling gaps between CBN, enhancing the gauge factor of the materials. Therefore, shrinking control admixtures were found to be promising alternatives to enable the dimensional compatibility between cement sensors and old concrete elements, and increase the self-sensing properties of smart cementitious composites.

SHRINKAGE MITIGATING STRATEGIES ON HIGH-PERFORMANCE SELF-CONSOLIDATING CONCRETE (HP-SCC)

Cracking in high-performance self-consolidating concrete (HP-SCC) is a significant concern that must be addressed during design and construction. HP-SCC is a concrete type reinforced with high-strength fibers, such as steel or synthetic fibers, to enhance its mechanical properties. HP-SCC is known for its high flowability and ability to self-consolidate without vibration, exceptional strength, durability, and crack resistance compared to conventional concrete. HP-SCC is highly durable and can withstand various environmental conditions and aggressive exposures. Combining high-strength fibers and advanced cementitious materials in HP-SCC improves resistance to chemical attacks, freeze-thaw cycles, abrasion, and impact. Utilizing an expansive agent (EA), shrinkage-reducing admixture (SRA), and saturated lightweight sand (LWS) ensures HP-SCC with a low risk of cracking. This study investigates using different types of EAs to improve compressive and flexural properties. Furthermore, the study investigates the combination of fiber, EA, and LWS in promoting high resistance to autogenous and total shrinkage. The dense microstructure of HP-SCC also contributes to its durability by reducing permeability and minimizing the ingress of moisture and harmful substances.

Mechanism underlying effect of expansive agent and shrinkage reducing admixture on mechanical properties and fiber-matrix bonding of fiber-reinforced mortar

Expansive agent (EA) and shrinkage reducing admixture (SRA) are utilized to reduce shrinkage and risk of cracking in concrete. EA compensates shrinkage by initial expansion, and SRA reduces surface tension in the pore fluid. Although EA and SRA effectively reduce shrinkage, they can impair micro-structure of concrete at high contents. The shrinkage reduction effect of EA and SRA is well known; however, there is limited knowledge about their negative effect on microstructure and fiber matrix interfacial transition zone (ITZ). The current study explores the effect of using 10 % CaO-based EA, 2 % SRA, and their combination on mechanical, shrinkage, and microstructural properties of fiber-reinforced mortar (FRM) made with 0.5 % steel fiber. The hydration products are characterized by X-ray diffraction and derivative thermogravimetric. Backscatter scanning electron microscopy, energy dispersive spectroscopy (EDS), and micro-hardness experiments are employed to characterize the microstructure of fiber-matrix ITZ at two distances of 50 and 100 μm surrounding the fibers. Experiment results indicate that the quality of ITZ and fiber-matrix bonding is impaired by using either EA or SRA. The highest drop is obtained by the combination of EA and SRA. The element analysis by EDS line and point scanning reveals the presence of longer silicate chain and stronger fiber-matrix ITZ in the reference mixture. Micro-hardness measurements confirm higher mechanical properties at the ITZ between 50 and 100 μm surrounding the fibers. Thermodynamic simulations and phase assemblages interpret the influence of EA and SRA on mechanical properties and shrinkage.

Evaluation of the Fresh Properties of Cement Pastes: Part I-Response Models Applied to Changes in the One-Variable-at-a-Time Method.

The workability of cement-based materials is one of the features that makes these construction materials the most used worldwide. Measuring and understanding how cement-based constituent materials affect fresh properties depends on the experimental plans. The experimental plans deal with the constituent materials used, the tests carried out, and the run of experiments. Here, the fresh properties (workability) of cement-based pastes are evaluated based on the diameter in the mini-slump test and the time in the Marsh funnel test being measured. This overall study is composed of two parts. In Part I, tests were carried out on several cement-based paste compositions incorporating distinct constituent materials. The effects of the distinct constituent materials on the workability were analyzed. Furthermore, this work deals with an approach to the run of experiments. A typical run of experiments was applied, with basic sets of mixed compositions being studied by only changing one input parameter at a time. This approach used in Part I is faced with a more scientific approach applied in Part II of the work where, based on the design of experiments, multiple input parameters were changed at a time. This work showed that a basic run of experiments is quick and easy to apply and leads to results for simple analyses; conversely, it lacks information for advanced analyses and scientific conclusions. The tests carried out included studies on the effect on the workability caused by changes in the limestone filler content, the type of cement, the water-to-cement ratio, distinct superplasticizers, and shrinkage-reducing admixture.

Effect of internal curing and shrinkage-mitigating materials on microstructural characteristics of fiber-reinforced mortar

Internal curing (IC) and other shrinkage mitigating materials are employed to reduce shrinkage and risk of cracking. This study investigates the efficiency of individual versus combined use of IC and shrinkage mitigating materials on key characteristics of fiber-reinforced mortar (FRM). The investigated mixtures include a 25% pre-saturated lightweight sand (LWS) that is added individually and combined with 10% CaO-based expansive agent (EA) and 2% shrinkage reducing admixture (SRA). This elucidates the synergistic effect of high content IC and EA/SRA on macro- and micro-mechanical characteristics of FRM, especially at the interface of the matrix with fibers. Mechanical properties, microstructural characteristics, and fiber–matrix bonding of FRM made with 0.5% steel fibers are investigated. The results show that the use of IC with EA and SRA completely compensates for shrinkage at 56 days. The highest compressive and fiber pull out strengths are observed for FRM with IC and without EA/SRA due to the densification of the interfacial transition zone (ITZ) confirmed by microstructural analysis. Such improvement is associated with the lower porosity of the cement paste and longer silicate chain—higher Si/Ca—obtained by FRM made with LWS. Although the combination of both EA and SRA with IC leads to 180 μstrain expansion after 56 days, the corresponding mixture presents the weakest ITZ and inferior mechanical properties.