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This study aims to investigate the effects of two superabsorbent polymer (SAP) types, S1 and S2 (with continual absorption and rapid desorption, respectively) as rheology-modified agent on 3D printing characteristics. Three base mixtures were selected with different thixotropic behaviors (i.e., mixtures with high re-flocculation (τ floc), high structuration rate (A thix), and both low τ floc and A thix , respectively). The initial mini-slump flow of 190±10 mm was secured by adjusting superplasticizer (SP) or water demand (increasing w/b by either 0.025 or 0.05). An extrudable region was determined within the range of τ floc (200–890 Pa) and A thix (15–60 Pa/min). The use of the S2 SAP significantly improved extruded performance for mixtures with τ floc higher than 1000 Pa. The use of the S1 SAP without increasing w/b (only with extra SP demand) significantly improved the resistance against plastic collapse for the mixtures with τ floc lower than 300 Pa. The reduction in 28-day compressive strength for printed specimens was correlated with τ floc. A high τ floc value led to inadequate interlayer bond and an increased propensity for surface defects of printed structure. The S2 SAP acted as a rheology-modified agent to reduce τ floc and improved the interlayer bond performance by 10 %-20 %. • SAPs were used as rheology-modified agent on 3DPC with different thixotropy behavior. • The SAP with rapid desorption improved extrudability for 3DPC with high re-flocculation. • The SAP with continual absorption increased the resistance against plastic collapse. • The reduction in compressive strength for printed specimens was related to re-flocculation. • The SAP can improve the interlayer bond performance by adjusting thixotropy. [ABSTRACT FROM AUTHOR]

This study investigates the effect of thixotropy and stiffness evolution on the extrudability and buildability of 3D printing concrete. Different types of supplementary cementitious materials and limestone filler were used to prepare mortars with varying levels of thixotropy and early-age stiffness. Mixtures with yield stress and flocculation (τ floc) of 180–400 and 420–950 Pa, respectively, exhibited adequate extrudability without plastic collapse. The printing was completed 8–15 min after mixing at a vertical build-up rate of 138 mm/min. Several process parameters, including rest time to secure a penetration resistance of 150 kPa (T 150) and penetration resistance at various rest times were developed to assess early-age stiffness. Results indicate that the penetration resistance at rest time of 10–30 min and T 150 can be correlated with the buildable height at elastic buckling. A prediction model of buildable height based on penetration resistance was proposed. The early-age hydration was analyzed to evaluate the early-age stiffness. [ABSTRACT FROM AUTHOR]

Fiber-reinforced high-strength grout (HSG) can secure exceptional mechanical properties; yet, case studies show that the interfacing layer to the existing substrate can be particularly vulnerable when used in specialty repair, precast, and retrofitting applications. Polymeric latex materials such as styrene-butadiene rubber (SBR) and acrylic ester (AE) are often incorporated to improve the bond properties and ensure monolithic behavior of the composite system. This paper assesses the concurrent effects of using steel fibers (SFs) and polymeric latexes on the flow and rheology of HSG, including their impact on mechanical properties and bond to existing concrete. The SF content varied from 0 to 5% by volume, while the mixing water was replaced by 10 to 20% of latex. Test results showed that the rheological properties of HSG increased with latex inclusion, given the coalescence of watersoluble polymers in the cementitious matrix that increased the viscosity of the interstitial liquid phase. The viscosity was aggravated with the addition of SF that accentuates the tendency of fiber grouping and interference between solid particles to hinder the ease of flow. The compressive strength slightly decreased when part of the mixing water was replaced by SBR or AE. Yet, in contrast, the flexural properties and pulloff bond strength were remarkably improved, which can be relevant to guarantee the integrity and monolithic behavior of the repair application. [ABSTRACT FROM AUTHOR]

Highlights • Delays in multilayer casting of thixotropic SCC can decrease the flexural strength. • Delays in placing successive lifts of thixotropic SCC can increase the permeability. • SCC with low level of thixotropy can improve the performance of multilayer casting. • The increase in static yield stress can decrease the roughness of bonded interphase. Abstract Multilayer casting of self-consolidating concrete (SCC) can be critical in situations involving delays between the placements of successive lifts. In the absence of mechanical consolidation, the increase in structural buildup at rest (SBR) of existing SCC lift prior to the placement of a successive lift can lead to lift lines. Furthermore, the bond strength deteriorates across the bonded interphases. In this study, eight SCC mixtures with different levels of SBR were considered. The thixotropy of concrete was determined using standard workability test methods and inclined plane test. Flexural strength and water permeability were determined using composite specimens cast in two SCC lifts. The second lift was placed after given periods of rest up to 60 min. Composite specimens developed residual flexural strengths and water permeability resistance with minimum values of 56% and 2%, respectively, compared to reference samples. To secure 90% residual flexural strength and impermeability, delays up to 25 and 10 min can be tolerated, respectively, depending on SBR level of the existing SCC layer. Statistical models were established to predict the residual flexural strength and water permeability resistance between successive lifts. SCC with low level of SBR, having static yield stress up to 250 Pa after 15 min of rest and rate of SBR up to 2.5 Pa/min evaluated using the inclined plane test, can secure relatively high level of bond strength and impermeability providing that the delay time before casting successive lifts is limited to 20 and 5 min, respectively. The interphases bond of successive SCC lifts was found to be affected by intermixing level of the layers. Both can be increased by the reduction in SBR of existing lift when concrete is cast with a given free fall height. [ABSTRACT FROM AUTHOR]

Structural buildup at rest (SBR) is a rheological property that can affect the performance of self-consolidating concrete (SCC). Such a property should be optimized to intended placement condition. Adequate determination of SBR on the jobsite is important for quality control, thus necessitating the use of simple and robust testing methods. The ability of employing conventional workability test methods to evaluate SBR of SCC is discussed. Eight SCC mixtures designed to secure different SBR levels were investigated. SBR was determined using concrete rheometer and two field-oriented test methods: portable vane and inclined plane, as well as losses of slump flow, T50, and J-ring flow at rest. Correlations between SBR indexes determined from workability and rheological test methods were established. Statistical models to predict static yield stress and SBR indexes as a function of workability loss at rest are proposed, which indicate that standard workability test methods can be used to evaluate SBR of SCC. [ABSTRACT FROM AUTHOR]

The present research investigates the performance of mortar prepared with high-volume ground glass fiber (GGF) incorporated as partial replacement of Portland cement. Several binary and ternary mixtures with up to 50% cement substitution were investigated to evaluate rheological properties, heat of hydration, strength development, drying shrinkage, electrical resistivity, and carbonation. The incorporation of up to 50% GGF was found to reduce yield stress by up to 50% compared to control mortar without any GGF. On the other hand, this resulted in up to 100% increase of plastic viscosity of mortar in comparison with the Reference mortar cast with 100% Portland cement. The rate of structural build-up at rest of the tested mortars, that reflects the thixotropic nature of the mortar, decreased from 7.1 to 0.8 Pa/min with cement substitution by 50% of GGF. Reduction in 91-day compressive strength from 34 to 28 MPa was observed with 50% cement substitution by GGF. The coefficient of pozzolanic activity of mortar cast with 10% to 50% GGF ranged from 0.18 to 0.71 at 91 days, compared to mixtures containing 50% cement substitution with Class F fly ash (FA-F), Class C fly ash (FA-C), or blast furnace slag (SL) where the 91-day coefficient of pozzolanic activities were 1.80, 1.46, and 1.21, respectively. The incorporation of 10% to 50% GGF reduced the 91-day drying shrinkage by 0–20%. At 50% GGF replacement, the electrical resistivity was enhanced from 10 to 88 kΩ.cm at 91 days, while the carbonation coefficient increased by about 100%. The incorporation of 15% or 25% GGF in ternary systems containing either FA-C, FA-F, or SL was effective in enhancing compressive strength, with values ranging between 34 and 49 MPa. The best performance was observed in the case of the GGF/FA-C ternary binders, followed by the GGF/SL, and GGF/FA-F systems where 91-day compressive strength gains of up to 45%, 28%, and 10%, respectively, were observed compared to the Reference mixture with 100% Portland cement. [ABSTRACT FROM AUTHOR]

Multilayer casting of self-consolidating concrete (SCC) can be critical in situations involving casting of successive lifts. The increase in structural buildup at rest of freshly cast SCC material prior to the placement of a successive layer can result in lift lines and loss in interlayer bond strength. Delay in the casting of successive lifts without mechanical consolidation can further reduce bond. Eight SCC mixtures designed to develop different levels of structural buildup at rest were investigated. The structural buildup at rest was determined by multiplying the values of initial slump flow, T50, or J-ring flow by average rates of change in these properties with rest time. Bond between successive layers was determined using composite specimens cast with two lifts of SCC after rest periods of 17 to 52 minutes, which corresponds to 25 to 60 minutes of concrete age. Bond strength was determined using the slant shear and direct shear test setups. Compared to monolithically cast samples, composite specimens had residual bond strengths of 15 to 100%. The critical delay time to secure at least a 90% residual bond strength was found to vary between 5 and 55 minutes, depending on the structural buildup at rest of the concrete in the existing layer. Statistical models for predicting residual bond strength between successive lifts were established and account for the structural buildup at rest of the first lift and delay period between successive lifts. Based on the level of structural buildup at rest, three categories of SCC are proposed. Category III SCC with relatively low structural buildup at rest can develop high residual interlayer bond. Such concrete should have maximum slump flow filling ability index of 800 mm.mm/min (31.5 in.in./min), T50 viscosity index of 0.08 sec.sec/min, and J-ring passing ability index of 600 mm.mm/min (23.6 in.in./min). [ABSTRACT FROM AUTHOR]

Formwork lateral pressure exerted by concrete is affected by mixture composition, placement conditions, and formwork characteristics. The mixture composition includes binder type and content, supplementary cementitious materials, fillers, water-to-cementitious ratio, content and characteristics of coarse aggregate, paste volume, chemical admixtures, concrete unit weight, concrete consistency, and temperature. Thixotropy of concrete expressed here in terms of structural build-up at rest is employed to collectively reflect the effect of combined mix design factors on formwork pressure. This research demonstrates relationships between 12 indices that can be used to evaluate the structural build-up at rest of concrete, determined using portable vane, inclined plane, and concrete rheometer tests, and formwork pressure characteristics. These characteristics include the initial maximum pressure after casting and pressure decay. In total, 12 SCC batches and one conventional vibrated concrete were prepared with various consistency levels, coarse aggregate contents, sand-to-total aggregate ratios, and paste volumes to produce wide-range of thixotropic properties and formwork pressure. The results showed that concrete of greater level of thixotropy can develop greater structural build-up when left in rest and displays lower lateral pressure during placement and rapid decay of pressure thereafter. Statistical ranking of the 12 structural build-up indices suggested that the portable vane test can be the best to reflect structural build-up at rest of concrete. [ABSTRACT FROM AUTHOR]

In the case of casting deep elements, self-consolidating concrete (SCC) should be designed with high thixotropy to reduce lateral pressure on the formwork system and with low thixotropy in multilayer casting to avoid the formation of a weak interface between successive lifts. Therefore, thixotropy determination of SCC on site may be necessary. Thixotropy can be determined by evaluating the rate of structural buildup at rest. Quality control of SCC on site necessitates the use of a simple, robust, and inexpensive test. A field-oriented test (portable vane device) enabling the evaluation of the structural buildup at rest of concrete is highlighted in this paper. Relative errors between 4 and 20% are estimated for this device. Good correlations are observed between six indexes of structural buildup determined from the portable vane and concrete rheometer using 60 SCC mixtures. The classification of SCC mixtures according to thixotropic values and the ranking of the six structural buildup indexes are also introduced in this paper. [ABSTRACT FROM AUTHOR]

The article focuses on the evaluation of the rheological parameters and the flow properties of self-consolidating concrete (SCC) through concrete-equivalent mortar (CEM). It presents an analysis on the behavior of the fresh SCC using mortar that contains 2 mm sand particles. It investigates the yield stress of the concrete and its plastic viscosity through the consideration of the micro-mortar phase of concrete mixtures. It also dwells on the discussion on the proposed relationship between SCC's slump flow and CEM. Moreover, it explores several factors that affect the performance of SCC including the reduction in free water content, paste volume increase and high-range water-reducer (HRWR) dosage. It proposes a new method to understand SCC's demand for HRWR from its corresponding CEM.

The article evaluates the impact of thixotropy-enhancing agent (TEA) on the variations in thixotropy and formwork lateral pressure of self-consolidating concrete with slump flow consistency. The study involves the testing of six mixtures containing various TEA concentrations along with either a melamine- or polycarboxylate-based high-range water-reducing admixture. Results indicated that the use of TEA can significantly increase the degree of thixotropy and reduce the framework pressure compared with similar mixtures containing conventional viscosity-enhancing admixtures.

The article discusses a study which determines the effect of viscosity-enhancing admixtures (VEA) on the formwork pressure of self-consolidating concrete (SCC). The study used in the experimentation process both liquid and powder polysaccharide and cellulose-based VEA and engaged the high-range water-reducing admixtures (HRWRA). The result of the study showed that VEA-HRWRA has its effect in the formwork pressure of SCC.

This article reports on the experimental program undertaken to evaluate the effect of water-cementitious material ratio and type of high range water-reducing admixture on the development of form-work pressure that can be exerted when using self-consolidating concrete (SCC). It can be concluded from the test results that the variations in lateral pressure and thixotropy of SCC are significantly affected by water-cementitious material ratio.

Reports on an experimental program to determine the effect of binder type and content on variations in lateral pressure of self-consolidating concrete (SCC). Preparation of the mixtures with five binder types; Influence of thixotropy on the variations of lateral pressure development; Increase in binder content resulting to a greater rate of gain in cohesiveness and a sharper drop in lateral pressure with time.

Studies the effect of thixotropy on the development of formwork pressure of self-consolidating concrete and concrete-equivalent-mortar. Effects of different types of binders on concrete mixtures; Protocols to quantify the degree of thixotropy.

This paper aims at assessing the lateral pressure exerted by self-consolidating concrete (SCC) on single-sided vertical formwork during repair work. Reinforced concrete repair wall elements measuring 5.6 ± 0.3 m in height and 180 mm in width were cast using SCC mixtures having different thixotropic characteristics at casting rates varying from 6 to 9.5 m/h. Results showed that the actual field pressure values are substantially lower than the equivalent hydrostatic pressure, varying from 60% to as low as 34% at the end of casting. This was attributed to the SCC structural build-up coupled with the presence of vertical/transverse reinforcing bars, arching effect in confined and tall repair sections, and increased friction between the concrete and existing repair substrate. The use of a 1.1-m high experimental PVC column was found appropriate to mimic the decay of lateral pressure observed in the repair wall elements, making it relevant to predict the effect of concrete restructuring on casting rate. Such data can be of interest to tailor the mixture proportions and enhance safety aspects related to casting slender repair sections with SCC. [ABSTRACT FROM AUTHOR]

Changes of rheological properties in ultra-high performance concrete (UHPC) suspending mortar incorporated superabsorbent polymer (SAP) were investigated. The effect of chemical composition of two SAP types (S1, acrylamide/acrylic sodium copolymer and S2, acrylamide polymer) and the S1 SAP with mean particle sizes of 95.1 and 471.3 μm on water absorption was evaluated. The effect of these parameter for dry SAP used at content of 0–0.6%, by mass of cementitious materials, on the rheology of the UHPC mortar was evaluated. The S1 SAP was also employed at 0.4% content in a pre-absorbed state to investigate the effect of the preconditioning method on the rheological properties. Test results show that the rheology of UHPC made with SAP depend on the water absorption/desorption properties. The water desorption after reaching the maximum water absorption limit for the S1 SAP can led to a decrease or stagnation of yield stress between 15 and 30 min of age. The increase in particle size of the SAP was shown to delay this stage of decrease or stagnation. The variation in yield stress of UHPC mortar made with the S2 SAP over time increased from 1 to 4 times compared to the reference mixture given the longer time for water absorption. The addition of the S1 SAP reduced the plastic viscosity and thixotropy, regardless of the size, content, and preconditioning method. The use of dry S2 SAP increased plastic viscosity by approximately 10 Pa.s and the thixotropy. The preconditioning method of SAP had no significant influence on thixotropy. [ABSTRACT FROM AUTHOR]

The structural build-up of cement-based materials is often characterized by the determination of the evolution of static yield stress with time. This property is crucial to many applications of concrete. However, the measurement of static yield stress may disturb the structure of cement-based material. As a consequence, the true structural build-up of the materials may not be detected. In this paper, the evolution of static yield stress and storage modulus (G′) determined by small amplitude oscillatory test was determined to characterize the evolution of the structural build-up of cement pastes. Results showed that G′ and static yield stress developed similarly with time. When the tests were carried out on the same sample, the measurement of static yield stress disturbed the structure of cement paste and had a significant effect on the following measurement of G′, but a slight effect on the following measurement of static yield stress. [ABSTRACT FROM AUTHOR]

This study evaluates the effect of welan gum (WG) and nanoclay (NC) on thixotropy of ultra-high-performance concrete (UHPC) matrix made with 0.20 water-to-binder ratio. The mortar mixtures were proportioned with a fixed superplasticizer (SP) content and WG and NC contents varying between 0 and 0.09% and 0 and 0.4%, respectively. The dynamic yield stress of these mixtures varied from 40 to 170 Pa. Similarly, UHPC made with different WG and NC contents were prepared with variable SP dosages to maintain initial mini-slump flow of 200 ± 5 mm. Test results indicate that thixotropy can be enhanced by using WG or NC in UHPC with a fixed SP dosage. On the other hand, for UHPC made with variable SP, the incorporation of WG increased thixotropy, whereas the use of NC had limited contribution to thixotropy. Particle flocculation was correlated to the water film thickness (WFT) onto solid particles in UHPC. The rapid increase in static yield stress immediately after mixing (τ floc) showed a linear growth with the decrease of WFT due to enhancement of particle flocculation. A qualitative approach relating particle flocculation and bridging effect of nucleation of early-age hydration products was proposed to describe the rate of linear increase in static yield stress with rest time (A thix). The model yielded good prediction for UHPC prepared with NC, which was not the case for mixtures made with WG where other factors, such as polymer entanglement and association, can play a major role in controlling thixotropy. [ABSTRACT FROM AUTHOR]