Entrained Air Voids Research Articles

Effect of fine fly ash and calcium hydroxide on air void structure in very-early-strength latex-modified concrete

Very-early-strength latex-modified concrete (VES-LMC) was developed for rapid repairs of distresses in concrete bridge decks and pavements, with the emphasis on early-age strength gain so that the repaired bridges and pavements can be opened to traffic within the time frame required in the specifications. However, there are two main concerns in the use of VES-LMC — early-age cracking and poor air void structure. The main objective of this study was to further improve VES-LMC to minimize early-age cracking and improve freeze–thaw durability, which included the use of fine fly ash (FFA) and calcium hydroxide (CH). Laboratory experiments were conducted on VES-LMC materials with cement replaced with FFA as well as CH, and various tests performed. Early-age drying shrinkages of VES-LMC containing both FFA and CH in the amounts evaluated in this study were smaller than that of VES-LMC with no replacements. It is expected that the use of FFA and CH in the range evaluated in this study will reduce the cracking potential of VES-LMC. Overall, the replacement of cement with FFA and CH improved the characteristics of entrained air void system, which will enhance the durability of VES-LMC against freeze–thaw damage. Scanning electron microscope and energy dispersive spectroscopy analysis indicate the primary mechanism of the generation of small sized air voids in concretes containing adequate amount of FFA and CH is the gas formation reaction between citric acid solutions and CH during concrete mixing. It is expected that the inclusion of adequate amounts of FFA and CH in VES-LMC will improve the performance of repaired bridge decks and pavements in terms of reduced cracking and improved freeze-thaw durability.

Analysis of Portland cement mortar under impact: A combined material characterization, micromechanics modeling, and dynamic testing approach

Toward the development of a constitutive model for describing high strain-rate behavior of cementitious materials, this study couples multiscale quantitative characterization of mortars with micromechanics modeling, validated against dynamic experiments. Blast or impact loads create a far-field compressive loading condition that drives local tensile crack growth from flaws. Two flaw types – round (e.g., entrained air voids) and slit (e.g., tandem effect of the interfacial transition zone and the aggregates) flaws – are considered in this flaw-driven, elastic model for predicting strength at high strain-rates. The model sheds light on the linkage between meso-scale fluctuations of the flaw populations (including size, shape, and spatial arrangement) and the material's overall dynamic response. Dynamic strength is interpreted in a statistical framework rather than a deterministic one. Micromechanics results agree with Kolsky bar tests on mortars with and without air entrainment. The model may be useful for engineering concrete for potentially at-risk elements.

Using Neutron Radiography to Quantify Water Transport and the Degree of Saturation in Entrained Air Cement Based Mortar

Air entrainment is commonly added to concrete to help in reducing the potential for freeze thaw damage. It is hypothesized that the entrained air voids remain unsaturated or partially saturated long after the smaller pores fill with water. Small gel and capillary pores in the cement matrix fill quickly on exposure to water, but larger pores (entrapped and entrained air voids) require longer times or other methods to achieve saturation. As such, it is important to quantitatively determine the water content and degree of saturation in air entrained cementitious materials. In order to further investigate properties of cement-based mortar, a model based on Beer's Law has been developed to interpret neutron radiographs. This model is a powerful tool for analyzing images acquired from neutron radiography. A mortar with a known volume of aggregate, water to cement ratio and degree of hydration can be imaged and the degree of saturation can be estimated.

An image analysis procedure to quantify the air void system of mortar and concrete

AbstractThe entrained air void system in cement paste and concrete directly affects both the fresh-state workability and the freeze-thaw durability of concrete pavements and structures. In order to effectively study the air void system, the volume, spacing, and size distribution of the air bubbles must be quantified. This manuscript describes an inexpensive and effective procedure to quantify the air void system using automatic image analysis. We compare various materials used for sample preparation and recommend a procedure to create more accurate images. We further implement a stereological analysis method in the form of a user-friendly and customizable Matlab script. The script was verified by comparing bulk density measurements between air-entrained and control samples with the air content computed by image analysis. As a result, the user can quantify the air void system in a variety of samples by computing the air content, specific surface, spacing, and size distribution of the bubbles.

Modelling Pore Structure of Cement Based Material According to Continuous Network System

In this study, a modified pore structure of cement based material with respect to a path for ingressive ions was established. Of pores in a concrete, gel pores and other entrapped air voids were excluded from modelling the pore structure as no interests are given due to the ions immobilization of cement paste media. To setup the pore structure, the linear traverse method (LTM) was used to distribute air voids along the traverse line in a hexahedron cement paste structure, followed by including entrained air voids to fill up the least space of the cement matrix and making a network of the air voids through capillary pores at the variation in the diameters. Then the mercury intrusion porosimetry (MIP) was used to iteratively approach an accordance rate with calculated one from the above way to get into appropriate convergence value. As a result, for the OPC specimen the developed model shows a somewhat relevant value of 42.4 % of the accordance rate compared to empirical one and 64.24 of the ratio of ionic path to original distance within a concrete.

Pore structure of mortars with cellulose ether additions – Mercury intrusion porosimetry study

In this study, multi-cycle mercury intrusion porosimetry (MIP) was employed to study the pore structure of model tile adhesive mortars containing different types (Methyl Cellulose, Hydroxyethyl- and Hydroxypropyl-Methyl Cellulose) and different dosages (0.3%, 0.8%) of cellulose ethers (CE). The two cycles of mercury intrusion and extrusion allowed resolving not only the total porosity of the mortar, but also the interconnected porosity of the matrix. Different application scenarios were tested: application on a substrate with no water absorption (plastic mold), on a moderately absorbing substrate (cement–fiber board) and on a substrate with high absorption (concrete slabs). These conditions allowed for determining the influence of different moisture regimes on the pore structure of the mortars. Finally, tensile adhesion strength tests with different open times were performed. It was observed that the addition of CE did not considerably affect the capillary and gel pores of the matrix. At the same time, it led to a substantial increase of the total porosity due to the entrainment of air, with slightly increasing size of entrained air voids at higher CE addition rates. A very pronounced coarsening of the porosity could be observed when mortars where applied on a substrate with high water absorption. An improvement in tensile adhesion strength respect to the reference mortar was observed thanks to CE addition, in particular for longer open times.

A probabilistic technique for entrained air void analysis in hardened concrete

A novel method that utilizes the lineal-path function to ascertain a probability density function for the three-dimensional size distribution of entrained air voids directly from plane polished sections of hardened concrete is proposed. The results then treat the spacing factor in terms of a probabilistic maximum distance from a random point in the cement paste matrix to the periphery of an air void, where air voids are treated as a polydispersed sphere system. The model was applied to concretes with various air entrainment admixture types and dosages. The results suggest that the model may offer a better assessment of the system for use in assessing durability and studying admixtures, as well as providing a new tool for spatial characterization of heterogeneous and porous materials.

Effect of entrained air voids on salt scaling resistance of concrete containing a new composite cement

Salt scaling is a major damage problem for concrete pavements in cold environments. Salt scaling is a type of superficial damage caused by freezing and thawing a saline solution on the surface of concrete. Effects of a new composite Portland cement and air void on deicer scaling resistance of concrete were investigated in this paper. Another objective is to investigate effects of compressive strength, tensile strength, abrasion resistance and water penetration on the freeze-thaw deicer salt scaling. Specimens were tested for freeze-thaw deicer salt scaling resistance in accordance with ASTM C672 test method. Surface strengths of concrete play an important role in salt scaling resistance. There is no appropriate relationship between compressive strength and salt scaling resistance, when concrete mixtures are made with various cementitious materials. Results reveal that the mixture containing composite Portland cement with entrained air bubbles has the best performance in salt scaling test.

Effect of porosity on thermal expansion coefficient of cement pastes and mortars

The thermal expansion coefficient (TEC) is studied for air-entrained cement pastes and mortars with different porosity. The results show that the porosity has significant effect on the thermal deformation and TEC decreases with porosity. The relation between TEC and porosity observes a power law, α d = α 0(1 − ϕ) C , and the exponents C for pastes/mortars are respectively 2.66/2.38 in terms of total porosity, and 3.74/2.69 in terms of air void content. The gravimetry and MIP measurements on the porosity and pore size distribution (PSD) indicate that the entrained air voids have not significant influence on the capillary porosity, but change the PSD curves measured by MIP. Three characteristic pore ranges are identified on these PSD curves and air voids are found to interfere with the mercury intrusion process above the scale of 50 nm. Thermal gravity analysis shows that the composition of hydration products and cement hydration degree for both pastes and mortars are almost the same. The decreased TEC with air-entrainment can probably be attributed to the existence of dense shell structures around the air voids.

Effect of entrained air voids on the microstructure and mass transport properties of concrete

The effects of entrained air on microstructure and transport properties of concrete with up to 11.5% air at different w/c ratios, curing and conditioning regimes were investigated. It was found that air voids disrupt the packing of cement and increase the heterogeneity of the microstructure. The width of the affected interface is around 30 μm. Gaseous diffusivity and permeability are increased by up to a factor of 2–3 at the highest air contents. This effect is similar to that due to increasing w/c ratio from 0.35 to 0.50 when samples are conditioned at 52% r.h or 50 °C. The effect on sorptivity is less consistent, while the effect on electrical conductivity is influenced by the moisture condition of the air voids. It is estimated that every 1% increase in air content increases transport by 10% or decreases it by 4%, depending on whether the air voids act as conductors or insulators.

Effect of Salt Admixtures on Air Content in Fresh Air-Entrained Concretes

Combined with the foam stability of air-entraining admixtures (AEAs), the effect of salt admixtures (including CaCl2, Ca(NO3)2, Na2SO4 and NaNO2) on morphology of air-voids in cement paste with a couple kinds of air-entraining admixtures (including SJ-2 and DH-9) was examined by means of SEM. In accordance with foam stability and the morphology, air content of fresh concrete was studied and analyzed more deeply. The results pointed out that these salts mitigated the foam stability of SJ-2 solution, and the deteriorated effect of CaCl2 and Ca(NO3)2 was more obvious. By contrast, these salts seemed to make no difference to the foam stability, but CaCl2 and Ca(NO3)2 reduced the foaming power of DH-9 solution. The walls of air voids in cement paste without using AEAs were compact. Differently, it was a distinct shell that formed around most of the entrained air voids with AEAs. This shell connected with capillary pores. For SJ-2 mixed, the air content of fresh air-entrained concretes was decreased by CaCl2 and Ca(NO3)2, but was enhanced by Na2SO4 and NaNO2; for DH-9 mixed, with the increasing usage of CaCl2, Ca(NO3)2, Na2SO4 or NaNO2, the air content was rising gradually.

Assessment of Air Entrainment in Fresh Cement Paste Using Ultrasonic Nondestructive Testing

Abstract It is understood that the frost protection afforded by entrained air voids in cement-based materials is dependent on their size and distribution or spacing factor. The common practice of adding air-entraining admixtures (AEAs) to concretes and mortars demands economical quality control measures of the air-entrained voids. However, conventional methods for qualifying air content in fresh cement-based materials, such as the pressure, volume, and gravimetric methods, measure only total air volume and cannot assess size (i.e., allow discrimination between entrained and entrapped air voids) or spacing. Ultrasonic monitoring may present an alternative in situ approach for these measurements. In this investigation, using matched pairs of transducers, ultrasonic pulses were transmitted through fresh cement paste specimens (containing 0.0 % up to 0.6 % AEA by weight of cement). The received signals were recorded every 5 min during the first 6 h and then every 15 min thereafter. Analysis of the signals shows strong distinctions between specimens with and those without the AEA. In general, the addition of AEA suppresses the peak-to-peak signal strength, pulse velocity, and peak frequency of the signal transmissions through the specimens. The data also suggest correlations between Vicat setting times, heat of hydration, and autogenous strain and ultrasonic metrics. The findings of this research should be most appropriate as a foundation for an inversion process and improved air-entrainment detection methods.

Investigations on the influence of fly ash on the formation and stability of artificially entrained air voids in concrete

In concrete, fly ash is applied to a task-oriented improvement of different properties. Besides the advantages, e.g. the improvement of the rheology of the fresh concrete or the density of the hardened concrete, some investigations and the experience from practice indicate that some fly ashes probably influence the formation and stability of artificially entrained air voids. The reason lies presumably in the fraction of unburned carbon, a minor component of the fly ash. To identify the causes, seven fly ashes from European power plants were investigated. The fly ashes were characterized and mortar and concrete tests were conducted to identify specific fly ash parameters which might be responsible for the impaired formation and stability of the air voids. Furthermore, it was examined whether the foam index test is applicable for the assessment of the air entraining agent demand and whether an adequate accuracy of the results is given. On the basis of the results it was also examined whether the mortar tests or a fly ash specific parameter can be applied as an alternative prediction tool to assess the air entraining agent demand for an air entrained concrete.

The morphology of entrained air voids in hardened cement paste generated with different anionic surfactants

The surface structures of entrained air voids in hardened cement paste were examined by means of an environmental scanning electron microscope (ESEM). Three different pure anionic surfactants were used: sodium dodecyl sulfonate (SDS), sodium dodecyl-benzene sulfonate (SDBS), and sodium oleate (SO). It was observed that a distinct shell forms around most of the entrained air voids regardless of the surfactant used. It was also shown that some hydration products can grow into the air voids at very early ages and may fill considerable amount of the void volume. In addition to the microscopic evaluation, a modified foam index test and surface tension measurements were also conducted on the solutions with different mixtures. It was determined that since oleate molecules coming from SO were precipitated in the mix as calcium oleate, exiguous amount of air bubbles were stabilized in the paste; however, when SDS and SDBS were used, an acceptable amount of air bubbles were stabilized.

A Novel Approach to Characterize Entrained Air Content in Pervious Concrete

Abstract The current pervious concrete placed in cold climates generally contains air entrainment but, unlike traditional concrete, the evaluation of entrained air is not performed. This paper presents results from a study that characterized the entrained air voids in pervious concrete using a RapidAir system. The RapidAir system is an automatic device that determines the air void properties of hardened concrete according to ASTM C457, “Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete.” Two types of aggregates, i.e., crushed limestone and rounded river gravel (pea gravel), and two types of air entraining agents, i.e., natural and synthetic, were used. The air entrainment dosage rates varied from zero to double the manufacturer’s recommended dosage. Compressive strength, tensile strength, and freeze-thaw durability (ASTM C666A) of the pervious concrete were tested. The results show that use of air entrainment improves workability of pervious concrete, thus reducing the overall porosity and increasing unit weight of the pervious concrete. The strength and freeze-thaw durability also increases with the level of entrained air in pervious concrete.

Characterization of multi-scale porosity in cement paste by advanced ultrasonic techniques

The effectiveness of advanced ultrasonic techniques to quantitatively characterize the capillary porosity and entrained air content in hardened cement paste is examined. Direct measurements of ultrasonic attenuation are used to measure the volume fraction and average size of entrained air voids and to assess variations in intrinsic porosity – as influenced by water-to-cement ratio (w/c) – in hardened cement paste samples. For the air entrained specimens, an inversion procedure based on a theoretical attenuation model is used to predict the average size and volume fraction of entrained air voids in each specimen, producing results in very good agreement with results obtained by standard petrographic methods and by gravimetric analysis. In addition, ultrasonic attenuation measurements are related to w/c to quantify the relationship between increasing porosity (with increasing w/c) and ultrasonic wave characteristics.

Influence of entrained air voids on pore water pressure and volume change of concrete before and during setting

The paper discusses the role of entrained air voids on the transformation from liquid to solid of cement based materials (paste, mortar, concrete). The discussion is based on pore water pressure and volume change measurements. Air pores seem to contribute to increased rate of autogenous shrinkage in the time before the “knee-point” (i.e. in the few hours before autogenous shrinkage rate becomes significantly lower than the chemical shrinkage rate), and an earlier appearance of the knee-point. Through setting and shortly after, air pores alter the pore water pressure evolution in that they act as a buffer, and thereby reduce the pressure decrease and the subsequent autogenous shrinkage, as well as friction against panels in slipforming. The influence on plastic shrinkage does not seem to be significant.

Characterization of entrained air voids in cement paste with scattered ultrasound

This research develops a technique that uses the attenuation of ultrasonic waves to characterize the average size and volume fraction of entrained air voids in hardened cement paste. Quantitative knowledge of entrained air void size and distribution helps ensure that an adequate design strength is developed, while maintaining resistance to freeze-thaw damage in cement-based materials. Ultrasonic attenuation coefficients obtained from pulse-burst signals are measured in the frequency range of 500 kHz–5 MHz. From these parameters, the average size and the volume fraction of the entrained air voids are determined using a combination of an ultrasonic scattering model and an inversion algorithm. Experiments are performed on specimens produced with and without entrained air voids. There is a good agreement between the model prediction and the experiments in these systems that contained < 10 % by volume of entrained air voids.

Fracture mechanics of air-entrained concrete subjected to compression

Air voids are entrained in concrete for protection of constructed elements, especially highway pavements, against freeze–thaw damage. Entrained air void systems inadvertently reduce the compressive strength of the concrete. The present study describes development of an analytical model for evaluation of the effects of entrained air void system on the compressive strength of concrete. The model developed here will assist in predicting the compressive strength of concrete for specified mix designs. The constitutive relationships for air-entrained concrete were established by considering a micro cracked porous material with randomly distributed circular air voids and uniformly oriented cracks from the air voids. Linear elastic fracture mechanics was employed to explain the evolution of damage due to the individual voids and cracks that emanate from such voids. The damage model considers the interactions among the voids and cracks during various stages of loading. The analytical results from this study were evaluated through an experimental program for comparison of the computed and measured compressive strengths. A wide range of samples were examined that included concretes with air contents ranging from 2% to 13% air by volume of concrete. The experiments involved microscopic determination of air content and spacing factors as well as compressive strength tests for all the concrete samples.

Estimation of the structure of air entrained concrete using a flatbed scanner

This paper describes the possibilities of estimating the structure of concrete with the help of automatic image analysis using a high-resolution flatbed scanner. Sample preparation techniques and components of the image analysis system are described. Concerning the structure of the entrained air voids system, it was found that the results obtained using a high-resolution flatbed scanner were comparable to those obtained by conventional methods, i.e. using a system equipped with stereomicroscope. The possibility of automatically measuring the paste content in hardened concrete by analysis of plane sections, taking into account information available in RGB histograms, is also presented. A new procedure is proposed for the identification of the distance from the cement paste to periphery of the nearest air void, which enables rapid evaluation of the quality of the air entrainment treatment The procedure also allows some characterization of the aggregate component.