Air-entrained Concrete Research Articles

Effect of Atmospheric Pressure on Performance of AEA and Air Entraining Concrete

The effect of atmospheric pressure on performance of air entraining agent (AEA) and air entraining concrete was studied in Tibet and Hubei, China. The result shows that the reduced atmospheric pressure increased surface tension and lowered foaming property of solution. The AEA with excellent foaming ability and stability is preferred in low atmospheric pressure. The freeze‐thaw deterioration process of hardened pastes is relatively faster under low atmospheric pressure. The effect of air pressure on concrete frost resistance performance is more sensitive than the mechanical property. The bigger pores and poor uniformity of internal pore size distribution led to the deterioration of concrete macroscopic properties. Therefore, the AEA varieties should be preferred, the dosage of AEA should be increased, and pore structure of pastes should be optimized to ensure the durability of concrete frost resistance for construction in low‐pressure areas.

Plasticizer and Superplasticizer Compatibility with Cement with Synthetic and Natural Air-Entraining Admixtures

When increasing the degree of fluidity of previously aerated cementitious mixtures, there is a problem of maintaining their correct aeration. Most of the available superplasticizers cause a significant increase of the air content of concrete mixtures. The problem of compatibility of superplasticizer and air-entraining admixture increases in case of multicomponent Portland cement, due to different effects of these additives. It comes down to achieving a compatibility of the three variables mentioned in the title of the paper, due to the required air entrainment and consistency of mixture. Achieving compatibility of such a system requires a series of experimental studies that were presented in the paper together with their resulting indications. In case of previously air-entrained concrete that is made with an air-entraining cement, after the addition of new generation SP occurs very large increase in air entrainment. The air-content of mixture according to EN 480-1 may be higher than 13%.

The effect of calcium salts on air-void structure in air-entrained concrete – a statistical and simulated study

AbstractThis paper analyzed the air-void system in air-entrained concrete in the presence of calcium salts and developed a new approach method, random “parking” method, for studying it. The foam index test studied indicated that the introduction of calcium salts did result in a drop in foaming power at 0.5 min, but the foam ability after 60 min was enhanced in surfactant-water solution. The Powers spacing factor in the air-entrained hardened concrete decreased gradually in the presence of CaCl2 or Ca(NO3)2. The probability density function of air-void distribution showed that the air-void size in air-entraining concrete was reduced with the addition of CaCl2 and Ca(NO3)2. The random parking method developed herein was compared with the experimental Powers spacing factor, and the concept of the alternative spacing factor, defined as the average distance between the surfaces of the two nearest air voids in concrete, was used. It was indicated that the alternative spacing factor is in close proximity to the experimental Powers spacing factor.

Influence of freeze-thaw cycles on capillary absorption and chloride penetration into concrete

Combined mechanical and environmental actions reduce service life of reinforced concrete structures more than each of the participating actions alone. In this contribution, the influence of freeze-thaw cycles on capillary water absorption and chloride penetration into ordinary concrete with two different water-cement ratios and air-entrained concrete was investigated. Water profiles in concrete were quantified by neutron radiography. Capillary water absorption and chloride penetration are both slowed down considerably by air entrainment. Chloride ions dissolved in water are filtered out of the penetrating salt solution and remain concentrated near the surface. Capillary water absorption and chloride penetration into ordinary concrete are accelerated considerably by frost damage. After exposure to freeze-thaw cycles a damage gradient from the surface to the center of the samples was observed. Transport mechanisms were studied in some detail and the data obtained can serve as a basis for more realistic prediction of service life of reinforced concrete structures under the influence of combined freeze-thaw cycles and chloride penetration.

The Resistance of Chloride Ion Penetration and the Microstructure of Air-Entrained Concrete

It was investigated that the variation of compressive strength, chloride diffusion coefficient and microstructure of concrete with different water-cement ratio and air content. The experimental results indicate that the bubble distribution and the continuity of the pores in the concrete are greatly affected by the air content in it. It enhanced the discontinuity of the pores and increased the number of tiny bubbles which were independent, closed, stable and equally distributed, as the air content was appropriate. So that it improved the resistance of chloride ion penetration. The chloride diffusion coefficient of C30 concrete whose air content was 5% was only 69.52% of that of theC30 reference concrete. And this effect to C50 concrete was more apparent than C30 concrete.

Effects of High Temperature on the Properties of High Performance Concrete (HPC)

Recent years have seen a tendency towards increasingly slender structures designed with the use of high performance concrete. Application of high performance concrete is also growing in tunnel construction. One of the most important challenges is to control explosive spalling of concrete and the method recommended by Eurocode 2 is addition of polypropylene fibres to the mix. The purpose of the research described in this paper was to evaluate the changes of the physical and mechanical properties of HPC exposed to the effect of high temperature. The tests were carried out on three types of high-performance concrete: air-entrained concrete, polypropylene fibre-reinforced concrete and reference concrete having constant water/cement ratio. The properties of hardened concrete including compressive strength, tensile splitting strength, flexural strength and E-modulus were studied. The latter tests were carried out on both on concrete cured at 20̊C and concrete subjected to high-temperature conditions at 300̊C, 450̊C and 600̊C. The results enabled us to evaluate the effect of high-temperature conditions on the properties of high-performance concrete and compare the effectiveness of the two methods designed to improve the high-temperature performance of the concrete: addition of polypropylene fibres and entrainment of air.

Surface Scaling Resistance of Concrete Subjected to Freeze-thaw Cycles and Sustained Load

The concrete resistance to surface scaling under combined freeze-thaw cycles, sodium chloride solution and external flexural load was experimentally investigated. The test were carried out for non-air-entrained, air-entrained concretes and for concrete with steel fiber reinforcement. The test method for evaluation the resistance to scaling under combined environmental and mechanical actions was proposed. The analysis of the results obtained revealed that including the interaction of freezing and thawing and external flexural load makes change both in the character and the range of surface scaling process. The statistically confirmed synergistic effect of interaction of cyclic freezing and thawing and flexural stress which caused the acceleration of damage process was found. The beneficial effects of an air-entraining agent and steel fibers used for improving the resistance to scaling were stated.

Mechanical and Failure Criteria of Air-Entrained Concrete under Triaxial Compression Load after Rapid Freeze-Thaw Cycles

The experiment study on the air-entrained concrete of 100 mm cubes under triaxial compression with different intermediate stress ratioα2=σ2D : σ3Dwas carried out using a hydraulic-servo testing system. The influence of rapid freeze-thaw cycles and intermediate stress ratio on the triaxial compressive strengthσ3Dwas analyzed according to the experimental results, respectively. The experimental results of air-entrained concrete obtained from the study in this paper and the triaxial compression experimental results of plain concrete got through the same triaxial-testing-system were compared and analyzed. The conclusion was that the triaxial compressive strength is greater than the biaxial and uniaxial compressive strength after the same rapid freeze-thaw cycles, and the increased percentage of triaxial compressive strength over biaxial compressive strength or uniaxial compressive strength is dependent on the middle stress. The experimental data is useful for precise analysis of concrete member or concrete structure under the action complex stress state.

Effect of sodium sulfate and sodium nitrite on air-void system in air-entrained concrete

The purpose of this work was to explore the effect of sodium salts on the air-void structure in air-entrained concrete, and to observe the morphology of the air voids in the presence of sodium sulfate (Na2SO4) and sodium nitrite (NaNO2) as well as the use of sodium abietate. The foam index test was used in analysing the foam stability of sodium abietate, microscopic determinations of the Powers spacing factor in hardened air-entrained concrete and the air-void distribution were characterised, and a microscopic evaluation was carried out using scanning electron microscopy for the morphology of air voids in cement paste. The foam index test indicated that the introduction of either sodium sulfate or sodium nitrite results in a drop in foaming power, but has no effect on foam stability. The addition of sodium salt was found to lead to a decrease in the pore size of air voids in air-entrained concrete. Adding sodium sulfate in conjunction with sodium abietate was found to gradually increase the Powers spacing factor, while the corresponding results for sodium nitrite were the reverse. Regardless of the sodium salt used, a distinct shell was observed to form around the entrained air voids. However, with the addition of sodium sulfate, more needle-like crystals were formed on the surface.

Bond behavior of steel bar in air-entrained RCAC in fresh water and sea water after fast freeze-thaw cycles

An experimental study on the bond behavior of steel bar in air-entrained recycled coarse aggregate concrete (RCAC) in fresh water and sea water under the action of fast freeze-thaw (F-T) cycles was carried out. A total of 63 center pull-out specimens, including 9 unfreezed specimens and 54 freezed specimens which suffered from 25, 50 or 75cycles of fast F-T according to standard GB/T 50082-2009, were constructed and tested to investigate the effect of the following parameters: (a) number of fast F-T cycles, (b) solution, (c) diameter of steel bar on the bond behavior of steel bar in air-entrained RCAC. The experimental results were compared with bond behavior of steel bar in RCAC conducted by the author. By comparison, the bond behavior is found to be improved greatly through mixed air-entraining agent into RCAC. A calculation expression of bond strength of steel bar in air-entrained RCAC considering number of fast F-T cycles is proposed based on the experimental results obtained in this paper.

Sulfate attack resistance of air-entrained silica fume concrete under dry-wet cycle condition

Based on the erosion resistant coefficient, the effects of water-cement ratio, air-entrained, silica fume content and sand ratio on the sulfate attack resistance of air-entrained silica fume concrete were studied by orthogonal experiments in order to explore its sulfate attack resistance under dry-wet condition. A more significant model of concrete resistance to sulfate attack was also established, thus this work provided a strategy reference for quantitative design of sulfate attack resistant concrete. The experimental results show that dry-wet cycle deteriorates the concrete resistance to the sulfate attack, and leads to the remarkable declines of concrete strength and sulfate resistance. Air bubbles in the air-entrained silica fume concrete lower and delay the damage resulted from the crystallization sulfate salt. However this delay gradually disappears when most of the close bubbles are breached by the alternative running of the sulfate salt crystallization and the permeating pressure, and then the air bubbles are filled with sulfate salt crystallization. The concrete is provided with the strongest sulfate resistance when it is prepared with the 0.47 water-binder ratio, 6.0% air-entrained, 5% silica fume and 30% sand ratio. The erosion resistant coefficients K80 and K150 of this concrete are increased by 9%, 7%, 9%, and 5% respectively as compared with those of concretes without silica fume and air entraining.

Design principles and testing of internal frost resistance of concrete for road structures - critical review

We present a review of freeze-thaw durability requirements laid down in both European standards and in the national specifications in relation to concrete for road structures. The principles of material selection for concrete mix are presented. We discuss local variations in the severity of the winter season and the extent of frost action in the components of road structures. The characteristic parameters of the test procedure for direct assessment of the resistance of concrete to internal damage are analysed, with particular attention paid to the specimen cooling rate. The temperature distribution in specimens was determined in standard freeze-thaw resistance tests carried out at two accredited laboratories. We discuss the criteria used in indirect assessment of freeze-thaw durability of air-entrained concrete on the basis of air void characteristics. Based on the determined temperature distribution in concrete specimens we postulate an improvement of the standard test procedure to clearly specify the cooling rate. It would be beneficial to distinguish frost impact zones depending on the severity of action of frost and de-icing salts on concrete in road structures.

Evaluation of rebar corrosion in reinforced concrete under freeze-thaw environment and protection measures

Purpose – Rebar corrosion in reinforced concrete is the major reason for the durability degradation, especially under harsh environment. This paper presents an experiment conducted to investigate the influence of freeze-thaw cycles on the rebar corrosion in reinforced concrete. The purpose of this paper is to provide fundamental information about rebar corrosion under frost environment and improvement measures. Design/methodology/approach – The related elastic modulus and compressive strength of different concrete specimens were measured after different freeze-thaw cycles. The accelerated rebar corrosion test was carried out after different freeze-thaw cycles; additionally, the value of calomel half-cell potential was determined. The actual rebar corrosion appearance was checked to prove the accuracy of the results of calomel half-cell potential. Findings – The results show that frost damage aggravates the rebar corrosion rate and degree under freeze-thaw environment; furthermore, the results become more obvious with the freeze-thaw cycles increasing. Mixing the air-entrained agent into fresh concrete to prepare air-entrained concrete, increasing the cover thickness and processing the surface of concrete with a waterproofing agent can significantly improve the resistance to rebar corrosion. From the actual appearance of rebar corrosion, the results of calomel half-cell potential can well reflect the actual rebar corrosion in reinforced concrete. Originality/value – The durability of reinforced concrete is mainly determined on chloride penetration that brings about rebar corrosion in chloride environments. Furthermore, the degradation of concrete durability becomes more serious in the harsh environment. As the concrete exposure to the freeze-thaw cycles environment, the freeze-thaw cycles accelerate the concrete damage, and the penetration of chloride into the concrete becomes easier because of the growing pore and crack sizes. In addition, rebar corrosion caused by chloride is one of the major forms of environmental attack on reinforced concrete. The tests conducted in this paper will describe the rebar corrosion in reinforced concrete under freeze-thaw environment.

Protected Paste Volume (PPV) as a parameter linking the air-pore structure in concrete with the frost resistance results

A new description is proposed of the air void structure in air-entrained concretes using a PPV (Protected Paste Volume) parameter. The PPV is determined as a ratio of the paste area protected by air voids to the total paste area, obtained from the 2D surface measurements. This approach, although constitutes a development of the Philleo concept, takes into account the number, size and distribution of not only air voids but also aggregate grains, often disregarded by other researchers. The obtained results show that the standard spacing factor L⩽0.20mm fails to guarantee that 100 percent of the paste volume is protected against the harmful effects of frost, which is the basic assumption in the Powers spacing factor model. The preliminary verification was carried out of the relationships between the parameters L and PPV and the results of freeze–thaw experiments. According to the authors, at this stage of the study the PPV provides better correlation with the frost resistance results than the standard spacing factor L. Unlike other indicators representing a statistical estimation of the average spacing of air bubbles, the PPV much better reflects the spatial displacement of the air pores in concrete and can thus be considered as an improved factor of air voids spatial dispersion.

Simulation of Mass, Linear Momentum, and Energy Transport in Concrete with Varying Moisture Content during Cooling to Cryogenic Temperatures

A set of governing equations comprising linear momentum, mass, and heat transfer is presented for thermoelastic freezing of a porous material. The theory of unsaturated freezing porous media is introduced to model deformation of concrete, a traditional building material, whose pore network is pressurized by the wet air, frozen ice, and unfrozen water. A general solution scheme is provided for the appropriate boundary conditions pertaining to the primary concrete containment in a liquefied natural gas tank, and simulated results are analyzed for fully and partially saturated non-air-entrained concrete and fully saturated air-entrained concrete. Effect of cooling rate is also demonstrated. It is found that high cooling rate results in high expansion provoked by high hydraulic pore pressure and the corresponding suppression of pore liquid freezing temperature. It is also revealed that air-entrained concrete, by allowing quick dissipation of the displaced pore water and accommodating the ensuing ice formation, shows less contraction and subsequently less crack initiating stresses than the high-porosity, non-air-entrained concrete. Similar outcomes are observed near the concrete surfaces subjected to evaporation prior to cryogenic freezing. High hydraulic pressure, induced by the delayed dissipation of excess pore water, is likely to generate at the center of surface-dried concrete walls.

Effects of parent concrete and mixing method on the resistance to freezing and thawing of air-entrained recycled aggregate concrete

We elucidated the effects of the parent concrete and mixing approach used on the freezing/thawing resistance of air-entrained recycled aggregate concrete (ARAC). Three non-air-entrained concretes and one air-entrained concrete were used to prepare recycled coarse aggregate (RCA) samples. Three mixing approaches were also investigated. The frost resistances of the ARAC samples produced using an RCA obtained from the non-air-entrained concrete with high strength as well as the air-entrained one were close to that of conventional concrete. The mixing approach used had no effect on the frost resistance. A possible mechanism for the failure of ARAC is proposed.

Effect of air entrainment on shrinkage of blended cements concretes

Shrinkage strains of air-entrained (AE) concretes were bigger than strains of non-air-entrained concretes made with the same cements. The results clearly showed an increase in strains with increasing content of air voids, regardless of cement type. Higher shrinkage strains of AE concretes may be most probably related to their microstructure of complex porosity and in particular, to porous cement paste adjacent to air void. Occurrence of air void–cement paste interfacial transition zones of higher porosity as well as their overlapping and interconnection may increase vapour diffusion accelerating moisture loss and drying shrinkage. Another likely reason for shrinkage increase may be increase in the volume of cement paste and decrease in the volume of aggregate in AE concrete.

Properties of Air-Entrained Pumice Lightweight Aggregate Concrete and a Freezing-Resistance Forecasting Model

AbstractThis study investigates the impact of air content on the mechanical properties and resistance of natural pumice lightweight aggregate concrete to freezing. The distributional characteristics of bubbles in pumice lightweight aggregate concrete are analyzed, and a freezing-resistance forecasting model for air-entrained pumice lightweight aggregate concrete is established. Experimental results show that the distribution of bubbles in natural pumice lightweight aggregate concrete is different from that in ordinary concrete. More bubbles are found in pumice, and fewer bubbles are found in mortar. The air content of non-air-entrained pumice lightweight aggregate concrete is almost the optimal air content of ordinary concrete. However, aggregate concrete has poor resistance to freezing. The optimal air content of ordinary concrete does not apply to pumice lightweight aggregate concrete. Significantly improved freezing resistance is observed after air entrainment. However, the cube compressive strength of...

Freezing and Thawing Behavior of Internally Cured Concrete

Abstract The paper presents results from an experimental investigation on the freezing and thawing behavior of internally cured concrete. Air-entrained internally cured concrete was tested in a series of mixtures. The internally cured concrete was designed where a portion of the normal-weight fine aggregate was replaced with an equivalent volume of pre-wetted lightweight fine aggregate. The internally cured concrete was then exposed to cyclic freezing and thawing following the ASTM C666A procedure. This work was performed to determine whether properly designed, internally cured, air-entrained concrete may have an increased susceptibility to freeze–thaw damage. It is shown that when concrete is designed with a low water/cement ratio (w/c) (approximately 0.42 or lower) and a volume of internal curing water is added to match the chemical shrinkage volume, the internally cured concrete mixtures showed a very low potential for freeze–thaw damage (comparable to that of conventional concrete). The work did show that when water remains in the lightweight aggregates (because of a higher w/c that may not draw the water from the aggregate or when substantially more internal curing water is used than necessary), freeze–thaw damage may occur.

The influence of the air-entraining water-reducing agent on the mechanical properties of the recycled concrete

Taking the concrete as the matrix, and the air holes produced in concrete as the inclusions, we studied the decreasing influence of the air content on the Young's modulus of the concrete by using the Mori–Tanaka estimation method. We used the variation structure method and derived the estimation formula of effective potential energy of the air-entraining concrete. The relation curve between the effective stress and the effective strain for the air-entraining concrete was obtained by differentiating the equivalent strain. Assuming the concrete matrix follows the Drucker–Prager yield criterion, the macroscopic Drucker–Prager yield criterion of air-entraining concrete was derived by introducing the energy density support function. Based on the macroscopic Drucker–Prager yield criterion, we studied the decreasing influence of the air content on the inner friction coefficient of concrete. The variation of the effective properties of these mechanical parameters with the air content was found to match the experiment case roughly.