Entrapped Air Content Research Articles

Plastic-sleeve test method to measure autogenous and drying shrinkage in paste, mortar, and concrete: Test Results

The Plastic-Sleeve Test (PST) method of molding cylindrical specimens in plastic sleeves enables the measurement of autogenous and total shrinkage of pastes, mortars, and concretes. Photogrammetric tests confirmed that the linear support of the specimens with a temporary bottom support mold does not restrain deformation. Tests were performed to compare the PST versus corrugated tube methods and similar relative humidity and autogenous shrinkage results were obtained. The PST shows comparable CoV values to those of the corrugated tube test. The method presented provides solid circular cross-section samples, enabling control over entrapped air content, specimen shape, and bleeding effects, thereby ensuring the measurement method’s high repeatability. The PST showed a tighter seal than prismatic samples molded for total shrinkage testing according to EN 1367-4. Results indicate that the PST can effectively be used for molding samples intended for total shrinkage measurement. It is concluded that the PST method is an effective method to measure autogenous shrinkage and total shrinkage of cementitious materials.

Effect of competitive adsorption between specialty admixtures and superplasticizer on structural build-up and hardened property of mortar phase of ultra-high-performance concrete

This study aims to enhance structural build-up of ultra-high-performance concrete (UHPC) without influencing the initial flowability, which is critical in repair applications (e.g., use of UHPC in thin bonded overlays for bridge deck rehabilitation). Specialty admixtures, such as a viscosity-modifying admixture (VMA), have been used to enhance the structural build-up at rest. However, the use of specialty admixtures can increase superplasticizer (SP) demand to maintain proper flowability; the synergistic effect of these admixtures on structural build-up is not well understood as the coupled admixture content can reverse the net effect on yield stress, viscosity, and thixotropy. In this study, VMAs including welan gum (WG), diutan gum (DG), and cellulose ether (CE) were used. Styrene-butadiene rubber (SBR) and acrylic ester (AE) latex polymers (LPs) that can enhance structural build-up and bond strength were also incorporated. The competitive adsorption between these specialty admixtures and SP and its effect on structural build-up, early-age hydration, compressive and pull-off strengths, porosity and entrapped air content of UHPC mortar (i.e., without fiber) were systematically investigated. Test results indicated that the incorporation of anionic WG and DG that exhibited high competitive adsorption with SP led to a 275%–450% enhancement in structural build-up despite the 55%–135% increase of SP demand. Such increase was limited to 130% when using CE, SBR, or AE that cannot effectively adsorb onto cement particles in the presence of SP. This was because the high competitive adsorption between specialty admixtures and SP strengthened the particle flocculation and promoted the cement hydration in the first hour; the latter resulted in 20%–40% enhancement in non-reversible component of structural build-up. Furthermore, the use of LP at low and moderate dosages secured high bond strength to normal strength mortar despite 8%–33% decrease in compressive strength given the increased capillary porosity. The use of VMAs increased the entrapped air and resulting 5%–15% lower compressive strength, while the bond strength was not influenced.

Determination of properties and environmental impact due to the inclusion of cigarette fibers in mortar: a new solution to mitigate the CB pollution.

Cigarette butts generated are one of the major sources of total solid waste production and lead to environmental issues. This article has the objective of evaluating the effects of cellulose acetate microfibers (CAFs) sourced from discarded cigarette filters (CFs) as fiber reinforcement on the physico-mechanical properties and thermal conductivity of cementitious materials. To do so, mortar samples were prepared using different incorporated quantities of fibers (0.5, 1, 1.5, 2, 2.5, and 5% compared to the quantity of sand added to the mixture) and subjected to different tests to characterize the influence of CAFs on the microstructure of elaborated materials, considering the changes in workability time, compressive strength, flexural strength, density, water absorption, and microstructural analysis. Furthermore, the life cycle assessment (LCA) of mortar mixes in terms of CO2 emissions is made. The results revealed that the increasing percentages of CAFs reduced the dry density and compressive strength, by approximately 1.62-51% and 37-69.64%, respectively, and a notable enhancement of insulation characteristics by about 5-47.5% was achieved. Microstructure analysis confirmed the experimental investigation and revealed that adding more than 1% of fibers resulted in a significantly low unit weight with greater entrapped air content. The studies prove the possibility of recycling cigarette butts for insulating cementitious matrix. In addition, applying mortar containing acetate cellulose fibers is recognized as a more environmentally friendly mixture in terms of reducing CO2 emissions and could participate significantly in the achievement of SDGs.

Absorption and Strength Properties of Short Carbon Fiber Reinforced Mortar Composite

This paper presents the water absorption and strength properties of short carbon fiber reinforced mortar (CFRM) composite. Four CFRM composites with 1%, 2%, 3%, and 4% short pitch-based carbon fibers were produced in this study. Normal Portland cement mortar (NCPM) was also prepared for use as the control mortar. The freshly mixed mortar composites were tested for workability, wet density, and entrapped air content. In addition, the hardened mortar composites were examined for compressive strength, splitting tensile strength, flexural strength, and water absorption at the ages of 7 and 28 days. The effects of different carbon fiber contents on the tested properties were observed. Test results showed that the incorporation of carbon fibers decreased the workability and wet density, but increased the entrapped air content in mortar composite. Most interestingly, the compressive strength of CFRM composite increased up to 3% carbon fiber content and then it declined significantly for 4% fiber content, depending on the workability and compaction of the mortar. In contrast, the splitting tensile strength and flexural strength of the CFRM composite increased for all fiber contents due to the greater cracking resistance and improved bond strength of the carbon fibers in the mortar. The presence of short pitch-based carbon fibers significantly strengthened the mortar by bridging the microcracks, resisting the propagation of these minute cracks, and impeding the growth of macrocracks. Furthermore, the water absorption of CFRM composite decreased up to 3% carbon fiber content and then it increased substantially for 4% fiber content, depending on the entrapped air content of the mortar. The overall test results suggest that the mortar with 3% carbon fibers is the optimum CFRM composite based on the tested properties.

An attempt to quantitatively and qualitatively differentiate the pore structure originating from air naturally present in hardened concretes and introduced by an air-entraining admixture

The new generation superplasitcizers affect air-entraining stability and, consequently, the air void characteristics in hardened concrete. As part of research 6 air entrained and 6 non air entrained concretes with a variable water-cement ratio liquefied, using modified polycarboxylates. The air void characteristics in hardened concretes were determined using the automatic RapidAir 457 system according to EN 480-11. Porosity structure parameters change with the w/c ratio and were described by mathematical relationships. Water-cement ratio does not affect the content of favourable pores from air entraining, however is responsible for entrapped air (above 300 µm) content. The most favourable pore structure was obtained in concrete, which has the optimal viscosity allowing to escape large pores during compaction and preventing coalescence of fine pores, as well as the superplasticizer content that does not cause additional air entraining.

The Impact of Capillary Trapping of Air on Satiated Hydraulic Conductivity of Sands Interpreted by X-ray Microtomography

The relationship between entrapped air content and the corresponding hydraulic conductivity was investigated experimentally for two coarse sands. Two packed samples of 5 cm height were prepared for each sand. Air entrapment was created by repeated infiltration and drainage cycles. The value of K was determined using repetitive falling-head infiltration experiments, which were evaluated using Darcy’s law. The entrapped air content was determined gravimetrically after each infiltration run. The amount and distribution of air bubbles were quantified by micro-computed X-ray tomography (CT) for selected runs. The obtained relationship between entrapped air content and satiated hydraulic conductivity agreed well with Faybishenko’s (1995) formula. CT imaging revealed that entrapped air contents and bubbles sizes were increasing with the height of the sample. It was found that the size of the air bubbles and clusters increased with each experimental cycle. The relationship between initial and residual gas saturation was successfully fitted with a linear model. The combination of X-ray computed tomography and infiltration experiments has a large potential to explore the effects of entrapped air on water flow.

Effect of partial substitution of superplasticizer by silanes in Portland cement pastes

This work evaluated the effect of partially replacing a superplasticizer by silanes on the fresh and hardened properties of Portland cement pastes. For this, pastes with water/cement ratio of 0.20 and 25 and 50% replacement of a polycarboxylate-ester-based superplasticizer by tetraethoxysilane (TEOS), 3-glycidoxypropyltrimethoxysilane (GPTMS) and aminoethylaminopropyltrimethoxysilane (AEAPTMS) were produced. The specific gravity, entrapped air content and mini-slump of the pastes were determined in the fresh state. The hydration kinetics was evaluated by isothermal calorimetry up to 120 h. Finally, the compressive and flexural strength of the pastes, as well as the ratio between them (referred to as toughness) were determined at 7 and 28 days. The results showed that 25% replacement by either the silanes increased the mini-slump and decreased the air content of the pastes in comparison to the reference. In contrast, 50% incorporation of GPTMS and AEAPTMS increased the air content and decreased the mini slump of the pastes, while 50% TEOS did not significantly affected these properties. The isothermal calorimetry indicated that TEOS reduced the induction period of the pastes, while the other silanes increased it, delaying the acceleration period start in up to 53 h in comparison to the reference. In addition, the increase in both silane content progressively increased the cumulative heat up to 120 h, reaching values 11% higher than the reference. The incorporation of TEOS progressively increased the flexural strength, while did not significantly changed the compressive strength, both at 7 and 28-day. The same trend was found for the compressive strength of the pastes containing GPTMS and AEAPTMS, while the flexural strength of these pastes decreased as the replacement level increased. Finally, 7 and 28-days toughness of the pastes increased as the content of TEOS increased, while no clear trend was found for GPTMS and AEAPTMS.

An investigation on the effect of styrene-butyl acrylate copolymer latex to improve the properties of polymer modified concrete

The effect of adding styrene-butyl acrylate (SBA) nano copolymer latex on the mechanical properties of concrete is investigated via performing scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX) analyses. For decreasing entrapped air content and increasing workability of the SBA modified concrete, silicone-based antifoam and superplasticizer were added to the mixture, respectively. Samples with different weigh proportions of SBA to cement, antifoam, and superplasticizer were fabricated to obtain proper ratios of these materials which had better effects on the mechanical properties of the concrete. Obtained results indicated that adding SBA along with antifoam and superplasticizer improved mechanical properties of the concrete significantly. On the other hand, the SEM analysis on the microstructure of the samples with the proper proportions showed that three-dimensional crystalline structures are forming in the structure of concrete and this can improve mechanical properties and durability of the sample.

Effects of Pitch-Based Short Carbon Fibers on the Workability, Unit Weight, and Air Content of Mortar Composite

Pitch is a viscoelastic polymer material consisting of aromatic hydrocarbons. It is used to produce carbon fibers with sheet-like crystal structures. The aim of the work presented in this paper was to evaluate the effects of pitch-based short carbon fibers on the workability, unit weight, and air content of freshly mixed mortar composite. Experimental investigation was carried out on five different types of mortar composite, including a control mortar. Four mortar composites were prepared including pitch-based short carbon fibers with 1–4% volume contents. The fresh mortar composites were tested to determine their slump, inverted slump cone flow (flow time, mass flow, and volume flow), unit weight, and air content. In addition, the correlation between the slump and flow time of various mortar composites was determined. It was found that the slump decreased with the increasing volume content of carbon fibers. The flow time of mortar composite increased, and therefore its mass flow and volume flow decreased with a greater volume content of carbon fibers. The slump was strongly correlated with the flow time, with a correlation coefficient of 0.9782. Furthermore, the unit weight of the fresh mortar composite decreased due to the incorporation of carbon fibers. However, amongst the different carbon fiber reinforced mortar composites, the mortar with 3% fiber volume content provided the highest unit weight. The air content results were consistent with the unit weight results. The change in air content of various mortar composites followed a trend reciprocal to that of unit weights. When the overall effects of carbon fibers were compared, it was observed that the fiber volume content higher than 3% resulted in a significantly low workability and provided a much lower unit weight with greater entrapped air content.

Relationship between Flowability, Entrapped Air Content and Strength of UHPC Mixtures Containing Different Dosage of Steel Fiber

This study investigated the effects of different fiber contents by volume (0%, 1%, 2%, and 3%) on flowability, entrapped air content, and compressive and flexural strength of ultra high performance concrete (UHPC). Four water-to-binder ratios (0.18, 0.20, 0.22, 0.24) were used and four different dosages of superplasticizer were introduced for preparing each water-to-binder ratio mixture. The experimental results reveal that the increased content of fiber decreases the flowability and entrapped air content of fresh UHPC mixtures. The compressive strength of UHPC increased with a greater addition of steel fiber, from 1% to 3%. The flexural strength was increased slightly by the addition of 1% steel fiber and increased remarkably with the addition of 2% and 3% steel fiber. For every water-to-binder ratio mixture, with and without steel fiber, a good linear relationship can be found between compressive strength and entrapped air content. Therefore, it is suggested that lowering the entrapped air provides a significant contribution to increasing compressive strength of UHPC containing higher steel fibers.

Changes in Entrapped Gas Content and Hydraulic Conductivity with Pressure

Water table fluctuations continuously introduce entrapped air bubbles into the otherwise saturated capillary fringe and groundwater zone, which reduces the effective (quasi-saturated) hydraulic conductivity, K(quasi), thus impacting groundwater flow, aquifer recharge and solute and contaminant transport. These entrapped gases will be susceptible to compression or expansion with changes in water pressure, as would be expected with water table (and barometric pressure) fluctuations. Here we undertake laboratory experiments using sand-packed columns to quantify the effect of water table changes of up to 250 cm on the entrapped gas content and the quasi-saturated hydraulic conductivity, and discuss our ability to account for these mechanisms in ground water models. Initial entrapped air contents ranged between 0.080 and 0.158, with a corresponding K(quasi) ranging between 2 and 6 times lower compared to the K(s) value. The application of 250 cm of water pressure caused an 18% to 26% reduction in the entrapped air content, resulting in an increase in K(quasi) by 1.16 to 1.57 times compared to its initial (0 cm water pressure) value. The change in entrapped air content measured at pressure step intervals of 50 cm, was essentially linear, and could be modeled according to the ideal gas law. Meanwhile, the changes in K(quasi) with compression-expansion of the bubbles because of pressure changes could be adequately captured with several current hydraulic conductivity models.

The Effect of Entrapped Air on the Quasi-Saturated Soil Hydraulic Conductivity and Comparison with the Unsaturated Hydraulic Conductivity

Entrapped air can greatly affect the hydraulic conductivity at or near saturation. In this study, we measured the hydraulic conductivity and volume of entrapped air in a quasi-saturated soil. Two soils, a Masa sandy loam soil from weathered granite rock and a TUAT light clay andisol from volcanic ash, were used. The soils, with three different dry bulk densities, were packed into a steel cylinder. To attain complete saturation, the packed soil samples were immersed in a 0.02 mol L −1 gypsum solution under vacuum conditions. The soil samples were then left on a sintered porous plate with suction of −17.0 kPa for different periods of time to allow drainage and air intrusion. After this drainage process, the samples were again immersed in water to permit air entrapment. The hydraulic conductivity was measured using the falling head method, and the amount of entrapped air was determined gravimetrically. The quasi-saturated hydraulic conductivity was found to decrease with increasing entrapped air content until the soil had the maximum fraction of entrapped air, approximately 10% of the bulk soil volume. A comparison of the quasi-saturated and unsaturated hydraulic conductivities of the soil samples at or near saturation, when the suction of soil water was greater than the air-entry value, showed that the quasi-saturated hydraulic conductivity was smaller than the unsaturated hydraulic conductivity.

Hydraulic Behavior of Quasi‐Saturated Soils in the Presence of Entrapped Air: Laboratory Experiments

Entrapped air in soils beneath the water table is one of the key factors controlling the hydraulic behavior under conditions of ponded infiltration, in perched waters, and in unconfined aquifers. The term quasi‐saturated soils defines the soils with entrapped air, and the term quasi‐saturated hydraulic conductivity defines the relationship between the hydraulic conductivity and entrapped air content. This paper focuses on an investigation of how entrapped air, along with other factors, affects the three‐stage temporal behavior of the quasi‐saturated hydraulic conductivity of soils. During the first stage the quasi‐saturated hydraulic conductivity of soils decreases by as much as 5–8 times, presumably because mobile entrapped air blocks the largest pores. During the second stage, as the mobile entrapped air is discharged from the core, the quasi‐saturated hydraulic conductivity of the soils slowly increases. When the mobile air is removed, the remaining immobile entrapped air is discharged as a dissolved phase, and the quasi‐saturated hydraulic conductivity increases rapidly by about 1–2 orders of magnitude, essentially reaching the value of the saturated hydraulic conductivity. During the third stage the hydraulic conductivity is decreased to minimum values. The effects of sealing at the soil surface and microbiological activities are assumed to be major factors in the final decrease of the hydraulic conductivity. This three‐stage temporal behavior of percolation in loam soils is repeatable. A new power law and an exponential relationship are proposed to describe the quasi‐saturated hydraulic conductivity of loams as a function of the entrapped air content.

Heating Rates of Thin-Profile Packages in Two Pilot-Scale Steam/Air Retorts

Heat transfer studies were carried out in two pilot scale steam/air retorts: a vertical positive-flow retort and a horizontal Lagarde retort, using rectangular test bricks of silicone rubber and nylon. The influences of steam content, temperature, flow-rate and flow-direction of the heating medium, package orientation, and entrapped air content in pouches on effective heating rates were evaluated. The study indicated the heating rate index, fh, to be significantly (p<0.05) influenced by steam content and temperature of the heating medium in the positive flow retort, and entrapped air in pouches under nonconstrained horizontal orientation while using both positive flow and Lagarde retorts. An air overpressure of 120kPa was found necessary to suppress the influence of entrapped gases during the processing. Heating of bricks in the vertical orientation resulted in higher fh values than in the horizontal orientation in some cases, while a reverse trend was observed in others.

Gas inclusions in the Antarctic ice sheet and their glaciological significance

Cores obtained to the bottom of the Antarctic Ice Sheet at Byrd Station have been used to analyze some physical properties of the air bubbles trapped in the ice. These bubbles constitute the remnant air that is retained when polar snow transforms into glacial ice. Parameters measured included size, shape, abundance, and spatial distribution of bubbles, gas volumes, and bubble pressures and their variations with depth in the ice sheet. Bubbles occur abundantly in the top 800 m of ice but then gradually disappear until they can no longer be detected optically below 1100 m. This disappearance is not accompanied by any significant loss of air from the ice, and the available evidence suggests that the air is retained in the form of a gas hydrate or clathrate. Because of the release of confining pressures following drilling, the hydrate begins to decompose soon after cores are pulled to the surface. This decomposition is accompanied by the growth of gas-filled bubblelike cavities that are easily distinguishable from original air bubbles. Bubble pressure measurements show that (1) bubbles with pressures exceeding about 16 bars begin to relax back to this value soon after in situ pressures are relieved by drilling, (2) further slow decompression will occur with time, and (3) the rate of decompression is controlled to some extent by the intrinsic structural properties of the ice and its thermal and deformational history. Only small variations were observed in the entrapped air content of the ice cores; they probably reflect variations in the temperature and/or pressure of the air at the time of its entrapment. Only in ice from the bottom 4.83 m was the air content observed to decrease to trace amounts. Since this virtual absence of air coincided precisely with the first appearance of stratified moraine in the cores, it is concluded that this ice originated from the refreezing of air-depleted water produced under pressure melting conditions at the bottom of the ice sheet.

New Modifications and Solutions of D. J. Korteweg's Equations to Determine Accurately the Sound Speed in Circular Elastic-Walled Liquid Columns

The purpose of this paper is to present modifications and new solutions of the three “Korteweg equations.” The equations define the velocity reduction of acoustic pressure waves propagating at low frequency through circular elastic-walled liquid columns. This theory uses a new mathematical model defining the sound speed in the liquid column as dependent on the time-varying boundaries that are affected by acoustic standing waves in the liquid. The theory replaces G. Wertheim's “Ratio of Strains,” used by D. J. Korteweg, with S. D. Poisson's theory as known today, and extends the solution to include “thin” or “thick-walled” circular tubes and incorporates an approximation for “extremely thick-walled” tubes. The tubes evaluated were four sizes of acrylic resin, four sizes of MC 901 Nylon, one size hard-drawn copper, and one size stainless steel. All tubes contained distilled water, and the entrapped air content and temperature were controlled. The average probable error of the measurement technique used was 2.40%. The average approximate error of the computer-calculated sound speed with the experimental determined value, for the frequency range investigated, was 1.4%. [Work supported by in-house independent research program at laboratory.]

Entrapped Air Content and Hydraulic Conductivity of Quartz Sand During Prolonged Liquid Flow

AbstractBoiled deionized water, with and without 0.1% phenol, was passed through permeameters filled with several types of quartz sand. In addition to measurements of hydraulic conductivity, the sand‐water system inside the permeameters was periodically subjected to a static gauge pressure ΔP, and the resulting volume change Δv was recorded by the movement of a mercury droplet in a capillary tube. The quantity Δv/ΔP was found to be a highly sensitive indicator of the air entrapped within the wet sand. For sand wetted at atmospheric pressure, Δv/ΔP was found to be high initially, and then to decline sharply as flow proceeded. When the hydraulic conductivity reached its maximum value, the decrease in Δv/ΔP flattened out abruptly, but a slow decline persisted as flow continued. When the sand was wetted under vacuum, the high initial value and subsequent rapid decline in Δv/ΔP were eliminated.Since Δv/ΔP was always observed to decline with continued flow, even when the hydraulic conductivity was undergoing a drastic decrease, it was concluded that flow‐associated reductions in hydraulic conductivity, observed in the present and in previous similar experiments, were not caused by an accumulation of entrapped air originating from dissolved air released by the flowing liquid.