AIR BUBBLE STABILITY MECHANISM OF AIR-ENTRAINING ADMIXTURES AND AIR VOID ANALYSIS OF HARDENED CONCRETE Bei Ding, Jiaping Liu, Jianzhong Liu Jiangsu Academy of Building Science Co., Ltd, Nanjing, China Abstract Monolayer of two kinds of air-entraining agent GYQ and modified rosin R are formed in KSV 2000 Langmuir min-trough at air-water interface, respectively. The surface pressure of each monolayer is measured using the wilhelmy plate technique. The static surface elasticity of monolayer at air-water interface are discussed by analyzing the -A isotherm. The air void characteristics of hardened concrete are determined by automated linear traverse method with image processing system based on ASTM C 457. It s found that the performance of air bubble stability of air-entraining admixtures is determined directly by the strength of monolayer at air-water interface. Comparing with the air void characteristics of hardened concrete produced by air-entraining admixtures, higher strength of GYQ monolayer resulted in the smaller spacing factors and more excellent air void characteristics. It s indicated that durability of harden concrete produced by GYQ is better than modified rosin R. 1. INTRODUCTION There are more and more large-scale infrastructure construction period in china, such as long-span concrete bridges and high-level concrete structures. The durability of these major projects directly relates to the national economy and the people s live and has become one of hot research field in civil engineering. When air-entraining agents are added in the concrete mixture, the introduction of large number of uniform and stable micro-bubble can optimize the concrete pore structure and distribution effectively. It is one of the most significant technical measures to improve the concrete durability. Air-entraining admixture is a kind of amphiphilic surfactant, the amphiphilic molecule can form oriented adsorption layer at interface spontaneously and reduce the surface tension of solution when surfactant is dissolved in water. Such material can form a kind of directional monomolecular surface membrane through spreading on the water. The bubble stability of airentraining admixture is determined mainly by the property of monomolecular membrane. This paper presents the air bubble quality of air-entraining concrete with monolayer strength of air-entraining admixture at air-water interface. The parameters of the air-void 1005
system in hardened concrete mixing with two kinds of air-entraining agents analyzed. The result indicates that the performance of air bubble stability of air-entraining admixtures is determined directly by the strength of monolayers at air-water interface. The higher the performance of air bubble stability of air-entraining admixtures, the better the air void characteristics of hardened concrete. 2. EXPERIMENAL 2.1 materials Binders: type II Portland cement with 42.5 grade from Nanjing Jiangnan cement plant is used. Class I fly ash specified by JGJ 28-86 come from Nanjing Thermo Electrical Plant. The chemical and physical properties of the binders are listed in Table 1. Table 1: Chemical and physical properties of the binders Chemical composition (%) Specific Name Density Area (symbol) SiO 2 Al 2 O 3 CaO MgO Fe 2 O 3 SO 3 Loss (kg/m 3 ) (m 2 /kg) Cement 20.60 5.03 64.11 1.46 4.38 1.72 1.18 3150 450 Fly ash 49.39 33.36 4.13 0.85 4.92 1.96 2.49 2200 615 Fine aggregate: river sand with fineness modulus of 2.65 and density of 2650 kg/m 3 is used for concrete. Coarse aggregate: 5-25 mm continuous graded crushed basalt and density 2700 kg/m 3 Admixtures: Poly-naphthalene sulfonates superplasticizer (named JM-B, with 20% water reduction at dosage of 0.5 %) and concrete air-entraining admixture named GYQ. The LB film analytical instrument of KSV mini-trough, made by Finland KSV Instrument Ltd., with a 700 120 10 mm 3 Langmuir trough made of Teflon, is used for all measurements. Surface pressure is measured by Wilhelmy plate technique. 2.2 methods The slump of fresh concrete and the mechanical properties of hardened concrete are tested through the methods based on general concrete design technical requirements (JGJ 55-81) and general mechanical properties of concrete test method (GBJ81-85). The air void characteristics of hardened concrete are determined by automated linear traverse method with image processing system based on ASTM C 457. -A isotherm of monolayer: Two kinds of air-entraining admixtures are separately dissolved in chloroform at about 0.1 mmol/l concentration, and then the solution is evenly spread on the cleaned water surface at 21 C in the Langmuir trough with a micro-syringe, respectively. Once the solvent is complete evaporated after 20 min, the monolayer is formed. The monolayer then is compressed by two symmetrically approaching barriers at constant pressure () and the area of monolayer at that time (t) is automatically recorded by a computer connected to Wilhelmy plate. The mean molecular area is calculated and the -A isotherm is plotted. 1006
2.3 concrete mix design Test concretes are proportioned with cement, sand and gravel, incorporating with fly ash, superplasticizer, and air-entraining admixture. The design of concrete mix is presented in the Table 2. Table 2: design of concrete mix Airentraining admixture Cement Material quantity/(kg/m3) Fly ash Fine aggregate Coarse aggregate Water JM % Airentraining Admixture None 340 60 810 1030 170 1.2 0 0.43 GYQ 340 60 810 1030 165 1.2 0.3 0.41 R 340 60 810 1030 166 1.2 0.5 0.42 3. RESULTS AND DISCUSSIONS 3.1 properties of test concrete Three groups of concrete have good performance of cohesion and water retention. The slump and compressive strength of concrete with air content are listed in Table 3. It can be seen that the higher monolayer strength of GYQ results in better bubble stability and more excellent bubble quality. Concrete with GYQ additive has lower loss of air content and strength. Table 3: slump and compressive strength of concrete with air content W/B Air-entraining admixture Slump (cm) Air content () Loss rate of air content (%) Compressive strength (MPa) 0h 1h 0h 1h 1h 28d None 20.3 13 2.2 1.8 18.2 52.1 R 22 17.0 5.0 3.3 34 50.8 GYQ 22.5 16.5 4.8 3.8 21.1 52.6 3.2 Mmonolayer of air-entraining admixture at air-water interface In the monolayer study, the most fundamental and important is the curve of the film surface pressure and area of film-forming molecule, named -A isotherm curve. The -A isotherm curve indicated the nature and state of monolayer, which will vary with chemical composition and physical conditions of system. As air-entraining admixture has the hydrophilic group, the most important section of -A isotherm curve is the transition from expanding film in liquid state to transiting film in the research of static surface elasticity of monolayer at air-water interface. In the phase of expanding film in liquid state, monolayer at air-water interface is able to resist external force to collapse because of its static surface elasticity. When molecules of air entraining admixture at air-water interface desorb and enter into the water under external force, the monolayer start to collapse, corresponding external pressure is collapse pressure. The static surface elasticity of monolayer at air-water interface can be characterized by collapse pressure to a large extent, which reveals the air bubble stability mechanism of airentraining admixture. 1007
40 30 20 10 1 2 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Figure 1: - A isotherms of monolayer for GYQ and R at 25 C Fig. 1 shows the -A isotherms of monolayer for GYQ and R at air-water interface. It can be seen that the liquid state of monolayer for R has remarkable less expanded than that of GYQ. monolayer of GYQ collapses as the surface pressure reaches 39mN/m and the limited area is 0.36nm 2, while the limited area of monolayer for R is 0.27nm 2, and the surface pressure at collapse point is 33.1mN/m. GYQ air-entraining agent has more hydrophilic groups than R. it leads to a reduction in the repellency of the static electric charges between the nearby molecules. Furthermore, GYQ is less hydrophobic than R, the ability of resisting to external force for the monolayer is stronger, and it results in a higher collapse pressure for GYQ. 3.3 Air void characteristics of hardened concrete The principal purpose of entraining air in concrete can be accomplished by providing a sufficiently large number of bubbles per unit volume of paste to produce a cellular structure in which the cell walls, composed of hardened paste, are only a few thousands of an inch thick. For a given percentage of air, the smaller the average thickness of the walls between bubbles is, the smaller the mean bubble diameter and hence the larger the number of bubbles, therefore, this paper is concerned not only with quantity of the entrained air, but also with the characteristics of the air void system, particularly the bubble size distribution, spacing factor and specific surface area. The volume of air required to give optimum frost resistance has been found to be about 9% by volume of the mortar fraction, or practically 4-8% by volume of concrete. Most of the entrained air are greater than 10m and less than 1.25mm in diameter and are uniformly distributed throughout the concrete. The critical parameter of the air-entrained paste is the spacing factor, which is defined as the average maximum distance from any point in the paste 1008
to the edge of a void. The spacing factor should be around 0.2mm to ensure adequate frost protection, the smaller the spacing factor, and the more durable the concrete. The method of ASTM C457 is based on the spacing factor equation of T. C. POWERS. If p/a > 4.342: 3 1 p L = 1.4 1 + 3 1 α A If p/a 4.342: (1) L = P 400n (2) where, p: Paste Content (%), A: Air Content, A=100Sa/St, Sa: number of stops in air voids, St: total number of stops, : Specific Surface, =4/l, l: Average chord length,l=a/100n, n: Void Frequency, n=n/t, T: Calculate the total traverse length, N: total number of air voids intersected, L: spacing factor. Table. 4 air void characteristics of hardened concrete Serial number Sample Name Average Chord Length (mm) Paste (Air) Air Content () Spacing Factor (mm) Specific Surface (mm2/mm3) Void Frequency (number/mm) 1 GYQ 0.1760 5.1059 5.4254 0.2058 22.7273 0.3081 2 R 0.2648 5.2590 5.3885 0.3138 15.1041 0.2035 100 80 88 100 75 GYQ R air void number 60 40 20 0 56 53 55 35 33 16 14 3 1 < 60 60-100 100-200 200-500 500-1000 > 1000 air void size( um) Figure 2: air void distribution of hardened concrete 1009
Air void characteristics of hardened concrete with GYQ and R, as computed from the data in hardened state, are given in table 4. Fig 2 compares the bubble sizes and their distribution for GYQ and R. The automated image analysis system is employed for the determination of air bubble parameters in hardened state based on the linear traverse methodology. The linear traverse procedure is repeated three times for each sample to assure repeatability. Table 4 and Fig 2 show that when the air content in concrete is within the range of 5% to 5.5%, the air bubble sizes of hardened concrete with GYQ are mostly under 500m, Spacing Factor is only 0.2058mm. Comparing with the air bubble sizes, quantity and spacing factor of hardened concrete with R, the air void quantity of GYQ is obviously greater than of R, and spacing factor of GYQ is less than of R. the result indicates that the entrained air void by GYQ is more small, the air void distribution is more uniform than by R. By associating monolayer strength of air-entraining admixture at air-water interface with performance of fresh concrete and air void characteristics of hardened concrete, higher monolayer strength resulted in better bubble quality and more excellent air void characteristics. 4. CONCLUSIONS The mechanism of air bubble stability of air-entraining admixtures is revealed profoundly by using for reference of monolayer theory. The air bubble stability of air-entraining admixtures is determined directly by the strength of monolayer at air-water interface. Higher strength of GYQ monolayer resulted in better air bubble stability and more excellent air void characteristics. REFERENCES [1] G. L. Gaines, Jr.,Insoluble Monolayer at Liquid-Gas Interfaces, Interscience,New York,1966 [2] Ishiguro Ryo, Sasaki Darryl Y., Karihara Kazue,Colloids And Surfaces A:Physicohemical And Engineering Aspect,146,329,1999 [3] K.A.Snyder, A Numerical Test of Air Viod Spacing Equations, Advanced Cement Based Materials 1998,8:28-44 [4] D.J.Con, Air Void Morphology in Fresh Cement Pastes, Cement and Concrete Research, 2002, 32: 1025-10318. 1010