1、Standard Test Method for Air-Void Characteristics of Freshly Mixed Concrete by Buoyancy Change AASHTO Designation: T 348-131American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-3b T 348-1 AASHTO Standard Test Method for
2、 Air-Void Characteristics of Freshly Mixed Concrete by Buoyancy Change AASHTO Designation: T 348-1311. SCOPE 1.1. This test method covers the determination of characteristics of the air-void system of fresh concrete using a sample of mortar. Spacing factor, specific surface, and entrained air conten
3、t are determined by capturing air bubbles released from a mortar sample. 1.2. The sample will only be representative of the depth of the concrete within approximately 60 mm (2.5 in.) below the level at which the sampling is begun. This method is applicable to fresh concrete with a minimum slump of 1
4、0 mm (0.4 in.) and air content between 3.5 and 10 percent by volume. Only air voids less than 3 mm (0.1 in.) in diameter are measured by this method.2The test must be performed in sheltered, stable conditions. 1.3. The values stated in SI units are to be regarded as the standard. 1.4. This standard
5、does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. 2. REFERENCED DOCUMENTS 2.1. AAS
6、HTO Standards: M 231, Weighing Devices Used in the Testing of Materials T 119M/T 119, Slump of Hydraulic Cement Concrete T 121M/T 121, Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete T 152, Air Content of Freshly Mixed Concrete by the Pressure Method T 196M/T 196, Air Content
7、 of Freshly Mixed Concrete by the Volumetric Method 2.2. ASTM Standard: C457/C457M, Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete 3. SUMMARY OF TEST METHOD33.1. This method determines the air-void characteristics of fresh concrete by e
8、xpelling all air bubbles present in a given mortar sample, collecting the air bubbles and recording their quantities, and calculating their size distribution. According to Stokes Law, larger bubbles rise faster than smaller ones. Thus, for bubbles rising a known distance, the size of the bubbles can
9、 be determined from the time of their arrival at the surface of the liquid. The air voids of a sample of fresh concrete mortar are released as bubbles by mixing the mortar with a viscous liquid. The bubbles then emerge from the viscous liquid, rise through an overlying column of water, and collect u
10、nder 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3b T 348-2 AASHTO a submerged dish. As the bubbles accumulate under the dish, the buoyancy of the dish changes. The change in buoyancy of the dish,
11、 as measured by a change in weight and recorded as a function of time, can be related to the number of bubbles of different sizes by an empirical correlation. Specific surface, spacing factor, and air content as specified by ASTM C457/C457M may be calculated from this data with the use of an algorit
12、hm. 4. SIGNIFICANCE AND USE 4.1. An adequate air-void system in hardened concrete protects the cement paste from damage during freezing and thawing cycles under moist conditions. This air-void system can be characterized by the volume of entrained air, spacing between air voids, specific surface, an
13、d void-size distribution. 4.2. This buoyancy change test method is capable of testing the air-void system of concrete in situ, reflecting the history of the concrete as it is in place, not as it is prepared in a sample for testing. 4.3. The primary function of the buoyancy change method is to provid
14、e air-void size and distribution information for concrete mixture designs. This test method could be used by the mix designer to evaluate various mix proportion options during prequalification. The effect of admixture combinations and admixture dosages on the air-void system can be evaluated. It can
15、 also be used by the approving agency as a quick laboratory check on mixes offered to them for approval. 4.4. During production, the adequacy of the air-void system can be verified for acceptance and feedback can be provided for manufacturing control. This method also allows rapid assessment of the
16、effect of production changes in the mixture or equipment or variations in placement conditions such as temperature, slump, and pumping on the air-void system. Characterization of the air-void system of the concrete shortly after production provides an assessment of the durability of the cement paste
17、. Results are usually obtained within 2 h, allowing adjustments in the subsequent production. 4.5. This method yields results that generally correlate well with the results of a linear traverse measurement on hardened concrete, as prescribed in ASTM C457/C457M for characteristics of the air-void sys
18、tem. Discrepancies between the results of this method and the results of ASTM C457/C457M may be due to coalescence of bubbles in the analysis liquid or due to errors in the ASTM C457/C457M test. The buoyancy change method does not give a total air content result that can be directly correlated with
19、the results of T 152 and T 196M/T 196. 4.6. For further discussion of the significance of characteristics of the air-void system, see ASTM C457, Section 5, Significance and Use. 5. APPARATUS45.1. Analysis and Data Collection ApparatusThis assembly, the sampling equipment, and materials are designed
20、and built to function as an integrated system that has been demonstrated by the manufacturer to accurately measure and calculate air-void distribution in fresh air-entrained concrete. 5.1.1. Riser CylinderA clear plastic cylinder with a base and a collar approximately as shown in Figure 1. The base
21、shall have an integral heating element capable of maintaining the analysis liquid at 23 2C (73 4F) and entry holes for the plastic rod and the sample syringe with gaskets to make a watertight seal. 5.1.2. Magnetic StirrerA magnetic stirrer capable of maintaining 300 rpm during mixing. 2015 by the Am
22、erican Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3b T 348-3 AASHTO Figure 1Riser Cylinder 5.1.3. BalanceThe electronic balance shall meet the requirements of M 231, Class G 1. The balance shall also have an integral
23、 arm from which the dish can be suspended. 5.1.4. CabinetThe cabinet shall house the riser cylinder, magnetic stirrer, and balance as shown in Figure 2. Figure 2Typical Apparatus with Riser Cylinder, Cabinet, and Computer 2015 by the American Association of State Highway and Transportation Officials
24、.All rights reserved. Duplication is a violation of applicable law.TS-3b T 348-4 AASHTO 5.1.5. Stirrer RodA ferromagnetic steel rod approximately 6 mm (0.25 in.) in diameter and 62 mm (2.5 in.) in length. 5.1.6. Temperature SensorThe temperature sensor shall detect the temperature of the analysis li
25、quid at the bottom of the cylinder. The temperature sensor shall be capable of measuring the temperature to within 0.5C (1.0F) in the range of 15 to 30C (59 to 86F) and of transmitting such measurements to the computer through an appropriate interface. 5.1.7. Syringes20-mL plastic syringes, with the
26、 tapered end removed, calibrated, and marked for collecting the specified sample volume as shown in Figure 3. 5.1.8. Plastic RodThe cylindrical plastic rod shall be at least 35 mm (1.5 in.) longer than the width of the base. The outside diameter of the body of the rod is the same as the syringes use
27、d in the test. A 1-mm length at the end of the rod shall have a reduced diameter that fits tightly within the inside diameter of the syringe as shown in Figure 3. 5.1.9. DishThe clear, shallow dish shall be large enough to cover the entire area of the cylinder, retain the rising bubbles, and fit wit
28、hin the collar. The dish shall have an opening on the side to allow entrapped air to be removed. Note 1An inverted Petri dish with an appropriate slot, as shown in Figure 3, can fulfill these requirements. 5.1.10. Suspension DeviceA device to suspend the dish from a balance arm by a single wire as s
29、hown in Figure 3. Figure 3Petri Dish, 20 mL Syringe, and Temperature Sensor 5.1.11. Control SystemA computer, software, and interface system capable of controlling the test, recording data, and displaying data at least once per minute during the test. It shall also calculate, display, and record the
30、 air content(s), air-void spacing factor, and specific surface of the air-void system. 5.2. Sampling Equipment: 5.2.1. Sampling AssemblyThe sampling assembly shall hold the syringe and a wire cage and vibrate at approximately 50 Hz with an amplitude that allows the mortar to flow into the wire cage.
31、 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3b T 348-5 AASHTO Note 2A drill operating at 3000 rpm with an eccentrically weighted, forked assembly as shown in Figure 4 can fulfill these requiremen
32、ts. The hammering function of the drill can be used as needed in stiffer concrete mixes. Figure 4Wire Cage and Funnel 5.2.2. Wire CageThe cage shall be of sufficient size to obtain a sample of fresh concrete mortar, similar to Figure 4. The cage wires shall have a clear spacing of 6 mm (0.24 in.). 5
33、.2.3. Plastic PlateA rigid, clear plastic plate approximately 250 by 250 by 3 mm (10 by 10 by 1/8in.) with a center hole of a diameter approximately 3 mm (1/8in.) greater than that of the wire cage. 5.3. Miscellaneous Tools: 5.3.1. FunnelA calibrated funnel marked for measuring a specified amount of
34、 analysis liquid similar to that shown in Figure 4. The funnel shall be capable of introducing the analysis liquid into the bottom of the water-filled riser cylinder with a minimum of mixing. 5.3.2. SpatulaA spatula to trim the mortar sample flush with the end of the syringe. 5.3.3. Water ContainerA
35、 container with a 4-L (2-gal) minimum capacity. Note 3A 19-L (5-gal) portable insulated drinking water cooler is useful for repeated testing. 5.3.4. Heating ElementAn immersible heating element capable of maintaining the water in the container at approximately 23 2C (73 4F). 5.3.5. ThermometerA ther
36、mometer accurate to 0.5C (1.0F) over the range of 10 to 30C (50 to 86F). 5.3.6. BrushA brush with an angled head and a handle longer than the riser cylinder is tall. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of appl
37、icable law.TS-3b T 348-6 AASHTO 5.3.7. Insulated Box. Note 4An insulated “cooler-type” lunchbox is useful. 5.3.8. Sealable Plastic BagsCommercially available in pint and quart sizes. 6. MATERIALS 6.1. Analysis LiquidThe analysis liquid shall have physical and chemical properties such that the air-vo
38、id bubbles remain discrete. The viscosity of the analysis liquid must remain constant over the range of temperatures found in the test and be compatible with the apparatus and the control system. The viscosity of the analysis liquid used shall provide a measurable separation in time between the arri
39、vals of bubbles of different sizes at the top of the water column. The analysis liquid and its viscosity shall be specified by the equipment manufacturer. Note 5A commercially available solution of glycerol in water can fulfill these requirements. A mixture of 4 parts glycerol to 1 part distilled wa
40、ter has been known to work well. 6.2. Deaerated WaterThe water shall be potable and shall have been maintained at atmospheric pressure and approximately 23 2C (73 4F) for a minimum of 12 h before use. Note 6Properly deaerated water is crucial to this test. The solubility of air in water increases as
41、 pressure increases and temperature decreases. The change in dissolved air content due to temperature occurs slowly; thus, the water must be maintained at constant temperature for a minimum of 12 h before use. Deaerated water also reabsorbs air when cooled. If the water is not deaerated correctly or
42、 if it is used shortly after reheating, air may be liberated in the riser cylinder. Air bubbles may form in the riser cylinder and on the dish and may have a considerable effect on the specific surface and spacing factor results. 6.3. Ice or Freezer PacksIce as needed in cubes or chips or frozen ref
43、reezable ice packs or cubes. 7. SAMPLING 7.1. Take samples as soon as possible after the concrete is in the desired state. The sampling location depends on the purpose of the test. Samples can be extracted from concrete in situ (pavements, structural members, decks, etc.), from concrete sampling con
44、tainers such as unit weight buckets, beam molds, or cylinder molds, or from other locations. 7.2. Insert a syringe into the sampling assembly and mount the wire cage onto the sampling assembly. Fully collapse the syringe. 7.3. Place the plastic plate in good contact with the surface of the concrete
45、to be sampled. Begin the vibration of the sampling assembly. Lower the wire cage through the hole in the plastic plate into the concrete. The vibration will cause the mortar fraction of the concrete to flow into the wire cage. Advance the wire cage into the concrete at a rate such that the concrete
46、surface under the plate and the surface of the mortar within the cage remain at approximately the same level at all times. Avoid filling the cage with surface mortar by pressing the plastic plate against the fresh concrete. The pressure is adequate when the air bubbles under the plastic plate do not
47、 move towards the hole while sampling. 7.4. Advance the wire cage into the concrete until the end of the syringe plunger is in full contact with the surface of the mortar. While maintaining the vibration, push the syringe cylinder smoothly into the mortar at such a rate that the wire cage remains fu
48、ll of mortar until the syringe is fully extended. Stop the vibration and withdraw the wire cage and syringe from the concrete. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-3b T 348-7 AASHTO 7.5. Re
49、move the wire cage and the syringe from the sampling assembly, saving the excess mortar from the wire cage. Pack this excess mortar around the end of the syringe to be used to displace any large air bubbles from the syringe. 7.6. Immediately place the sample in a plastic bag on ice or freezer packs in the insulated box to retard the onset of initial set. Testing must begin before the initial set of the concrete. 7.7. If large air bubbles are present at the base of the syringe, remove the plunger and pack en
