Shrinkage Compensation Research Articles

Shrinkage compensation design and mechanism of geopolymer pastes

Geopolymers (GI) possess excellent seawater erosion resistance because of stable hydration products and compact microstructures, and therefore, they have broad application prospects in marine engineering if their large volume shrinkage behavior can be mitigated. In this study, active MgO was used to compensate for different GI shrinkage stages and a scheme was proposed for staged compensation. Autogenous shrinkage was controlled by adjusting the content, activity, and combinations of active MgO with particle sizes of 1–100 µm. Based on the effects of MgO on shrinkage, GI paste composition and microstructure, the GI reaction process and mechanism of MgO-compensated volume shrinkage were examined and summarized. The results showed that the GI paste shrinkage decreased with MgO addition, and the higher the MgO activity, the smaller was the paste shrinkage in the early stages of hardening. Late-stage shrinkage of hardened paste was improved with the addition of low-activity MgO. Combining low and high activity MgO addition effectively ameliorated the paste hardening shrinkage at different stages. In the high-alkalinity environment of the liquid-phase GI reaction, the Mg(OH)2 product presented microcrystalline structures, and it was dispersed in the cement matrix, which resulted in uniform volume expansion. Meanwhile, other reaction products containing magnesium were produced, filling pores in the hardened paste and effectively compensating for volume shrinkage during the hardening process. Based on the effects of MgO on the composition structure, reaction process, and volume shrinkage performance of GI slurries, this study proposes a mechanism for MgO to compensate for the volume shrinkage of GI.

Ternary system Portland cement-calcium aluminate cement-calcium sulfate applied to self-leveling mortar: a literature review

The modernization of building systems is increasingly demanding products that improve efficiency during their processing. The development of self-leveling mortars for the regularization of flooring is a consequence of this evolution. The high contents of binder and water to obtain self-flowing property, plus the application of thin layers over large areas, make this type of mortar susceptible to high shrinkage levels. Thus, the ternary system Portland cement-calcium aluminate cement-calcium sulfate (OPC-CAC-CaSO4) is a viable solution for shrinkage compensation. The expansion mechanism based on ettringite formation aims to prevent the mortar from cracking. Considering its complexity and that international literature is still limited (while Brazilian is null), this article presents a literature review that covers diverse aspects related to the application of the system OPC-CAC-CaSO4. The influence of raw materials and mix proportions on rheological behavior, reaction kinetics, development of strength, and length change are discussed.

Mitigating autogenous shrinkage using a combination of superabsorbent polymer and magnesia-based expansive additive

Various strategies have been developed to mitigate the autogenous shrinkage of cement-based materials (CBMs), including internal curing and shrinkage compensation. However, the sole use of internal curing or shrinkage compensation is insufficient in many cases. In this study, magnesia-based expansive additives (MEAs) coupled with a superabsorbent polymer (SAP) were investigated as internal curing agents to mitigate the autogenous shrinkage of cement pastes. The internal relative humidity (RH), autogenous deformation and compressive strength of cement pastes containing two different types of MEAs and SAP were examined. The results showed that, with the incorporation of SAP, the RH in the cement paste was maintained at a higher level than that in cement pastes without SAP, and hence autogenous shrinkage was reduced. The combination of SAP and MEA provided very effective compensation for the autogenous shrinkage of cement paste. Moreover, the cement paste containing 8 wt% MEA and 0.2 wt% SAP produced a gentle expansion of 111 microstrain rather than shrinkage. This was attributed to the increased hydration degree of magnesia owing to the additional water supply provided by the SAP. This study provides a novel and efficient way to mitigate the autogenous shrinkage of CBMs.

Evaluation Method and Mitigation Strategies for Shrinkage Cracking of Modern Concrete

Abstract The complex compositions and large shrinkage of concrete, as well as the strong constraints of the structures, often lead to prominent shrinkage cracking problems in modern concrete structures. This paper first introduces a multi-field (hydro–thermo–hygro–constraint) coupling model with the hydration degree of cementitious materials as the basic state parameter to estimate the shrinkage cracking risk of hardening concrete under coupling effects. Second, three new key technologies are illustrated: temperature rise inhibition, full-stage shrinkage compensation, and shrinkage reduction technologies. These technologies can efficiently reduce the thermal, autogenous, and drying shrinkages of concrete. Thereafter, a design process based on the theoretical model and key technologies is proposed to control the cracking risk index below the threshold value. Finally, two engineering application examples are provided that demonstrate that concrete shrinkage cracking can be significantly mitigated by adopting the proposed methods and technologies.

Pre-compensation method for optimizing recording process of holographic optical element lenses with spherical wave reconstruction.

We propose a pre-compensated recording process of holographic optical element (HOE) lenses, where both of reference and signal waves have spherical wavefronts, for solving a wavelength mismatch problem between the recording and displaying process. Based on a localized approximation for aperiodic volume gratings, the wavelength mismatch and shrinkage effects are pre-compensated by optimizing the recording setup of HOE lenses, so that the Bragg condition of each local grating is satisfied. In order to realize the practical implementations of recording setup, complicated wavefronts to be required for the wavelength and shrinkage compensation are approximated into spherical waves. The simulation results using the volume hologram models of OpticStudio verify that the undesirable focal shift and color breakup problems in the HOE lens due to the wavelength mismatch are compensated. Displaying experiments using a full-color HOE lens with the field of view of 30° are presented, where the maximum wavelength mismatch between the recording and displaying process is 17 nm.

Manufacturing of a ceramic groove part based on additive and subtractive technologies

Ceramic stereolithography Three-Dimensional (SL-3D) printing has a unique advantage in forming ceramic parts with complex shapes. However, because of the effects of the slicing layer thickness, laser spot diameter, and curing inhomogeneity, its manufacturing dimensional accuracy still lags far behind traditional processing methods. In this study, a manufacturing process based on additive and subtractive technologies was used to prepare ceramic groove parts. The manufacturing process of ceramic groove parts is composed of SL-3D printing, debinding, micro-milling, and sintering processes. In order to improve the dimensional accuracy of ceramic groove parts, a model based on shrinkage compensation and micro-milling was proposed. The dimensional change pattern in the curing phase was modelled and a shrinkage compensation method was established to control the dimensional accuracy of the finished parts. A method to characterize the micro-chipping on machined edges of the workpiece was investigated by applying a micro-milling cutter to process the 3D-printed part pre-sintered at 1150 °C. The effectiveness of the proposed dimensional accuracy control method was verified by the case study of a ZrO2 ceramic part.

Effect of expansive agents on the workability, crack resistance and durability of shrinkage-compensating concrete with low contents of fibers

The effect of hybrid CaO- and ettringite-based expansive agents (HCSA) on the mechanical properties and durability of shrinkage-compensating concrete (SHRCC) is not well established. In this paper, SHRCC specimens were prepared incorporating with a high-performance calcium sulfoaluminate- hydrate calcium hydroxide expansive agent (HCSA) and three different types of fibers, i.e., polypropylene fiber (PP), polyvinyl alcohol fiber (PVA), and cellulose fiber (CF). The first part of the experimental program included material tests of the slump, compressive strength, restrained expansion ratio, early crack resistance, permeability and chloride penetration of SHRCC. The test results indicated that the HCSA-based SHRCC exhibited good workability, early crack resistance and durability. It is suggested that HCSA with a dosage of 7% − 8% of the weight of binder materials provided favorable shrinkage compensation and early crack resistance and had negligible impacts on the slump and compressive strength of the SHRCC. Although the content was low (approximately 0.7% by volume), PP fibers effectively improved the early crack resistance of the SHRCC. Adequate moist curing was also required to retain acceptable permeability and chloride penetration of the SHRCC. The second part of the experimental program involved tests on small-scale concrete basement slabs, which verified the effects of the expansive agent and fibers on the crack resistance of SHRCC at the component level.

Volumetric shrinkage estimation of benchmark parts developed by rapid tooling mold insert

This paper estimates the volumetric shrinkage for thermoplastic Polypropylene (PP) injection molded components made using digital Acrylonitrile butadiene styrene (ABS) mold. The parameters affecting volumetric shrinkage for the digital ABS mold are mold temperature and injection temperature, cooling time, hold pressure and injecting speed. Therefore, twelve standard benchmark CAD model were selected with different geometric attributes. Subsequently, simulation analysis was performed on all CAD model using Moldflow® (MFA) simulation software. Additionally, regression analysis is applied to identify the effect of injection molding parameters on the volumetric shrinkage of part made using rapid tooling mold insert of digital ABS material. It is found that maximum volumetric shrinkage (18.75%) is observed for square pyramid frustum, conical frustum, and solid torus. On the contrary, hollow rectangular prism shows minimum shrinkage effect having 12.61% of volumetric shrinkage. This study predicted that shrinkage is the main concern for these three geometric features (i.e., square pyramid frustum, conical frustum, and solid torus) and must be looked for its minimization. The results are experimentally validated, with 3D scanner integrated with COMET plus and Inspect plus softwares. Since shrinkage estimation for digital ABS mold using Rapid Tooling technique has not been attempted before, therefore, this study provides guidance for the optimum parameter selection and assigning suitable shrinkage compensation values for digital ABS mold made using direct rapid tooling.

Evaluation of time to shrinkage-induced crack initiation in OPC and slag cement matrices incorporating circulating fluidized bed combustion bottom ash

The effect of bottom ash produced from circulating fluidized bed combustion (CFBC) on the shrinkage-induced cracking potential of ordinary Portland cement (OPC) and slag cement mixtures were investigated by evaluating the time to crack initiation, predicted by comparing tensile strength obtained from direct tensile tests with shrinkage-induced stress obtained from shrinkage strain and elastic modulus results. Although replacing OPC and slag cement amounts with CFBC bottom ash delayed cracking times due to its shrinkage compensation, the effect varied with the drying condition. Therefore, for effective application of CFBC ash in cement mixtures, boundary conditions affecting drying rates need careful consideration.

Shrinkage-reducing measures and mechanisms analysis for alkali-activated coal gangue-slag mortar at room temperature

By using slag as a calcium source, high-strength alkali-activated coal gangue-slag (AACGS) cementitious material can be prepared. Given that higher drying shrinkage at room temperature is detrimental to the expanded engineering application of the material, this paper studies several methods for reducing drying shrinkage such as shrinkage-reducing admixture (SRA), polypropylene fiber (PPF), sulfate-enriched materials (gypsum, high-performance concrete expansive admixture (HCSA), U-type expansive admixture (UEA)) and water retaining admixture (WRA). Meanwhile, the mechanisms for drying shrinkage compensation were deeply analyzed by mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Experimental results show that the addition of admixtures to reduce the drying shrinkage of AACGS mortar is related to the change of pore size of 2–5 nm, and reduces the volume fraction of the minimum aperture. At room temperature, reducing the surface tension of pore solution, generating expansion products, increasing compressive strength and improving pore structure distribution by adding admixtures are important measures to reduce drying shrinkage. SRA exhibits the strongest capacity to compensate for drying shrinkage, followed by UEA, WRA, HCSA, PPF and gypsum. Despite better compensation for drying shrinkage with the increase in SRA dosage, the reduction of strength value becomes more drastic. The findings of this study provide substantial experimental foundations and in-depth theoretical analyses for the drying shrinkage compensation of AACGS binary cementitious materials.

Internally cured high performance concrete with magnesium based expansive agent using coal bottom ash particles as water reservoirs

Shrinkage is one of the main concerns related to high performance concrete (HPC) durability. Its high density paste, consequence of a low water to binder ratio, can be unprofitable if cracks appear due to excessive tension when volume changes are restrained. Therefore, volume stability is a priority. In this work, three different strategies have been studied with that purpose: integration of fly ash as a low reactive supplementary cementitious material, internal curing via coal bottom ash particles as water reservoirs and the use of an expansive agent based on magnesium oxide (MEA). Many research works address the three shrinkage reduction strategies individually. However, studies regarding their simultaneous use are not prevalent so this work proposes its combined application. Results indicate that internal curing and MEA have a synergistic effect in HPC. Internal curing enhances MEA expansion due to the lack of water in this kind of concrete, contributing to autogenous shrinkage compensation. When concrete is affected by air-drying conditions, the use of MEA, internal curing, or both together make shrinkage to increase. MEA effectively expands in wet cured HPC although internal curing is not effective in this condition due to the absence of self-desiccation and limited porous aggregate water desorption. Taking into account the effects of each shrinkage reduction strategy and curing condition, it has been concluded that the use of fly ash as supplementary cementitious material, internal curing and MEA is recommended together with prevention of water evaporation from HPC surface.

Combined use of superabsorbent polymers and nanosilica for reduction of restrained shrinkage and strength compensation in cementitious mortars

In cementitious mixtures with a low water-to-cement ratio, autogenous shrinkage and cracking due to restrained shrinkage pose a severe problem. To reduce the risk of autogenous shrinkage cracking, internal curing by means of superabsorbent polymers (SAP) was applied. The drawback is that macropores are created upon the release of water by the polymers. The latter reduces the mechanical properties of the material. To counteract this strength reduction, nanosilica is introduced to the internal curing mixture. The effect of additives on the restrained shrinkage of mortar was evaluated by means of the restrained shrinkage ring test accompanied by acoustic emission (AE). AE enabled the monitoring of the hydration process, indicating when the absorbed water was released by the SAPs, while it also pointed out the moment of fracture of the ring. Results showed that combining SAPs and nanosilica, a mixture was created that decreased shrinkage while maintaining the mechanical properties of the reference material.

Effect of CaO and MgO based expansive agent on deformation and mechanical properties of concrete-filled steel tubes

The separation problem between core concrete and steel tube is one of the main bottlenecks restricting the construction of super large-span concrete-filled steel tubular arch bridges. One of the main measures to solve this problem is to prepare micro expansive core concrete by using the shrinkage compensation property of expansive agent. This study investigated the deformation and mechanical properties of core concrete containing CaO-based and MgO-based expansive agent under both the constant temperature and the varying temperature. The results showed that concrete containing CaO-based and MgO-based expansive agents can produce remarkable expansion which increases with the increase of CaO-MgO ratios under the constant temperature of 20 °C, but decreases with the increase of CaO-MgO ratios under the varying temperature. Through the restraint from the steel tube can reduce the expansion value, proper selection of CaO-MgO ratio can still produce enough expansion in core concrete to compensate autogenous and thermal shrinkage, with the concrete still in expansion state when the temperature finally stabilizes. Thus it is beneficial to maintain the integrity between the core concrete and the steel tube. The results obtained will provide guidance for the application of this kind of expansive agent in concrete-filled steel tube structure.

Design-dependent shrinkage compensation modeling and mechanical property targeting of metal FFF

Metal-fused filament fabrication is gaining traction due to its low cost and high availability compared to metal powder bed fusion. However, the achievable mechanical properties and effects of shrinkage of this process should be understood thoroughly before it can be implemented as a direct digital manufacturing technology. This study investigates the influence of infill levels and different build orientations on the mechanical properties and shrinkage behavior of 3D-printed, debinded, and sintered components made from BASF Ultrafuse 316LX. The final objective of the work is to define a function for multi-directional shrinkage prediction for any given part geometry to achieve parts with a high degree of dimensional conformity by modifying the original designs accordingly. The Design of Experiment includes tensile and compression testing according to ASTM E8 M-04 and ASTM D695-15, respectively. Tensile testing samples are manufactured in three different build directions and compression testing pins are made with six infill levels. Furthermore, a complex part is printed and its dimensional shrinkage analyzed using 3D scanning. Finally, the multi-directional shrinkage behavior is measured for all samples to establish a shrinkage predictability function by applying linear regression models. Results show that material infill levels have no effect on the shrinkage behavior of printed components. Compressive strength increases with infill level and ultimate tensile strength of parts printed flat indicates the highest tensile testing results, followed by flipped and vertically printed parts. A complex part was manufactured successfully for spare part production, which helped to establish a function with moderate confidence levels for shrinkage predictability.

The Study on Early‐Age Expansion and Shrinkage Model of Massive Self‐Compacting Concrete Pumped in Steel Tube Column

Massive self‐compacting concrete pumped in steel tube columns has been used more and more widely in super high‐rise buildings and bridge engineering at present. The early‐age expansion and shrinkage performance of its core mass concrete is an important index to ensure the stress state of triaxial compression and structural safety. However, no relevant reports have been found. In view of the actual building with the height of 265.15 meters, the early‐age expansion and shrinkage tests of the massive self‐compacting concrete pumped in full‐scale columns with the height of 12.54 m and 12.24 m and diameter of 1.3 m and 1.6 m were carried out by means of strain gauges embedded in concrete‐filled steel tubes (CFSTs). The early‐age variation regularity of the vertical and horizontal expansion and shrinkage strains for the core concrete with the diameter of steel tube, development time, temperature, the pouring pressure, expansion stress, and so on is given. The calculation model of its early‐age deformation strains is presented in this paper, which is in good agreement with the experimental results. It provides the basis of experimental and theoretical analyses for shrinkage compensation of massive self‐compacting concrete pumped in steel tube columns.

Investigation of the effects of pre-cross-linked thermoset molding compounds on weld line strength in injection molding

In this study, epoxy molding compounds (EMC) filled with glass beads were fabricated with a twin-screw-extruder (TSE) and tempered before further processing to generate different degrees of cross-linking (0–15%) in the compounds. After that, the compounds were injection-molded into tensile samples with and without weld lines to investigate the influence of pre-cross-linking of compounds on the weld line strength. Both the referent fracture strength and the weld line strength were improved with the increase of pre-cross-linking in compounds. Especially, cavities in the weld line area were reduced presumably due to the improved venting with the increased viscosity. For the resized samples without notches, their weld line strength are higher than those with notches. The in-mold pressure of more pre-cross-linked compounds can be hold longer than less pre-cross-linked compounds. This could lead to a better compensation of the material shrinkage. This might be the reason for the improved weld line strength for more pre-cross-linked compounds.

Effects of curing temperature and superabsorbent polymers on hydration of early-age cement paste containing a CaO-based expansive additive

The early-age shrinkage can easily result in crack of concrete in the early age, which seriously affects the durability of concrete. The CaO-based expansion additive is a normally used and effective material for the shrinkage compensation and the reduction of early-age crack risk in concrete. The hydration is a factor that significantly influences the mechanical and durability properties of concrete, and the study on the hydration of the cement paste containing a CaO-based expansive additive under different curing temperature and with superabsorbent polymers addition is lacking. In this study, tests were conducted using the low-field nuclear magnetic resonance to assess the effect of curing temperature and superabsorbent polymers on the hydration of early-age cement paste containing a CaO-based expansion additive. The experimental results show that the incorporation of CaO-based expansive additive can promote the hydration of cement paste, and 2% dosage of CaO-based expansive additive has a better promotion than that of 3% dosage. High curing temperature can effectively accelerate the hydration of cement paste containing the CaO-based expansion additive. In addition, superabsorbent polymers can also effectively promote hydration.

Experimental Study on the Shrinkage Behavior and Mechanical Properties of AAM Mortar Mixed with CSA Expansive Additive.

In this study, a calcium sulfoaluminate-based expansive additive (0%, 2.5%, 5.0%, and 7.5% by the mass of the binder) was added to compensate for the shrinkage of alkali-activated material (AAM) mortar. Modulus of elasticity curves based on the ACI 209 model were derived for the AAM mortar mixed with the additive by measuring the compressive strength and modulus of elasticity. Moreover, autogenous shrinkage and total shrinkage were measured for 150 days, and drying shrinkage was calculated by excluding autogenous shrinkage from total shrinkage. For the autogenous and drying shrinkage of AAM mortar, shrinkage curves by age were obtained by deriving material constants using the exponential function model. Finally, shrinkage stress was calculated using the modulus of elasticity of the AAM mortar and the curves obtained using the shrinkage model. The results showed that the calcium sulfoaluminate-based expansive additive had an excellent compensation effect on the drying shrinkage of AAM mortar, but the effect was observed only at early ages when the modulus of elasticity was low. From a long-term perspective, the shrinkage compensation effect was low when the modulus of elasticity was high, and thus, shrinkage stress could not be reduced.

Investigation on expansion effect of the expansive agents in ultra-high performance concrete

In this study, the expansion mechanisms of calcium-sulfoaluminates -CaO based expansive agent (CSA-CaO EA) on the volume stability of ultra-high performance concrete (UHPC) were systematically studied. The test results show that the incorporation of CSA-CaO EA is beneficial to reduce the autogenous shrinkage of UHPC effectively due to the formation of additional Ca(OH)2 and ettringite. However, the reduction of early autogenous shrinkage is limited and the cracking risk is still very high when the addition of CSA-CaO EA reaches 15% of binders. According to the results, the factors hindering the expansion effect of CSA-CaO EA include the low w/b ratio, compact microstructure, out-sync of time “window” of expansion and shrinkage and the potential exchange of water between solids. Strategies and experimental validations on improving the shrinkage compensation are discussed. It indicates that the expansive UHPC can be prepared by using an CaO based EA with high reactivity and low water consumption via controlling the time “window” of expansion.

Effect of La on hot cracking susceptibility of Al-Cu-Mg alloy

Aluminum-copper alloys have wide crystallization range, leading to a strong hot cracking tendency because of dendrites and mushy solidification characteristics. In this study, the effect of La content (0–0.5 wt%) on the hot cracking susceptibility of Al-Cu-Mg alloy (Al-4.4Cu-1.5Mg-0.15Zr) was investigated. Results show that the grains size is nearly 160 ± 2 μm in La-free alloy with inhomogeneous structure. With the addition of La into alloy (0.1–0.3 wt%), grains size decreased from 120 ± 2 μm to 67 ± 2 μm and the morphology of grains gradually changed from dendrite to equiaxed crystal. The grains size was 40 ± 2 μm in La-0.4 wt% alloy with uniform equiaxed grains. When the addition of La element continued to increase to 0.5 wt%, the grain size increased to 80 ± 2 μm. Hot cracking sensitivity coefficient of the alloy is related to La content. The alloy with La-0.4 wt% represented good hot cracking resistance and mechanical properties. The hot cracking mechanisms of the alloys are discussed in detail, based on liquid film theory and solidification shrinkage compensation theory.