1、Standard Method of Test for Macrocell Slab Chloride Threshold AASHTO Designation: T 376M/T 376-171Technical Section: 4f, Metals Release: Group 2 (June 2017) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-4f T 376M
2、/T 376-1 AASHTO Standard Specification for Macrocell Slab Chloride Threshold AASHTO Designation: T 376M/T 376-171Technical Section: 4f, Metals Release: Group 2 (June 2017) 1. SCOPE 1.1. This test method describes procedures to measure the chloride threshold concentration to initiate active corrosion
3、 of reinforcing bars in concrete using a macrocell slab type specimen. 1.2. Test measurement of chloride threshold concentration can be used to project service life of various reinforced concrete structural members when placed in corrosive environments. 2. REFERENCED STANDARDS 2.1. AASHTO Standards:
4、 M 31M/M 31, Deformed and Plain Carbon and Low-Alloy Steel Bars for Concrete Reinforcement M 43, Sizes of Aggregate for Road and Bridge Construction M 85, Portland Cement M 334M/M 334, Uncoated, Corrosion-Resistant, Deformed and Plain Chromium Alloyed, Billet-Steel Bars for Concrete Reinforcement an
5、d Dowels R 39, Making and Curing Concrete Test Specimens in the Laboratory T 22, Compressive Strength of Cylindrical Concrete Specimens 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
6、 Mixed Concrete by the Pressure Method T 196M/T 196, Air Content of Freshly Mixed Concrete by the Volumetric Method T 277, Electrical Indication of Concretes Ability to Resist Chloride Ion Penetration 2.2. ASTM Standards: C33/C33M, Specification for Concrete Aggregates C1152/C1152M, Test Method for
7、Acid-Soluble Chloride in Mortar and Concrete G193, Standard Terminology and Acronyms Relating to Corrosion 3. TEST EQUIPMENT AND MATERIALS 3.1. VoltmeterHigh impedance voltmeter (at least one M) capable of measuring 0.001 mV. 3.2. Reference ElectrodeSaturated calomel electrode, as defined in ASTM G1
8、93. 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-4f T 376M/T 376-2 AASHTO 3.3. Resistor10-ohm ( 0.3 ) electrical resistor. Note 1Agency may also select a 100- resistor noting that a 100- resistor
9、will be more sensitive than a 10-, recognizing for a given current the measured voltage drop (the product of resistance and current) will be ten times higher. It should also be noted that a 100- resistor has a higher load on the circuit, so the current flows less easily. 3.4. Terminal BoxTerminal bo
10、xes house the electrical connection to the reinforcing bars. 3.5. Wire16-gauge 1.5-mm2 insulated copper wire is used to make the electrical connections to the bars. 3.6. Epoxy SealerA two part impermeable epoxy sealer shall be used to cover the sides of the specimens and the electrical connections.
11、It shall be applied in accordance with manufacturers recommendations. 3.7. Concrete MixCement shall be Type I or II per M 85. Coarse aggregate shall conform to ASTM C33 and M 43, with nominal maximum size between 9.5 and 12.5 mm 38and 1/2in. (See Section 4.9.) 3.8. Sodium Chloride Solution3.0 wt. pe
12、rcent sodium chloride solution is prepared by adding 30 g 1 oz. of NaCl to 970 g 34 oz. of distilled or deionized water. 3.9. Plastic Dams, 100-mm 4.0-in. wide and 150-mm 6.0-in. long with a minimum height of 75 mm 3.0 in. for placement on the test specimens. The wall thickness shall be 3 1 mm 18 1/
13、32in. 3.10. Marine (Medium- or High-Strength) Polyurethane Adhesivefor sealing the outside of the plastic dam to the top of the concrete specimen. Note 2The user shall determine an adhesive that is appropriate for a marine environment. 3.11. Rebar Test SpecimensThe tested rebars will include both th
14、e selected candidate test bar specimens conforming to M 334M/M 334, Table 1 and an uncoated comparator “black steel” M 31 bar. All rebar test specimens shall be No. 16 No. 5 steel bars as-received from the manufacturer under normal delivery conditions. Each bar shall be tapped to receive a 10-24 bol
15、t. Note 3All reinforcing bars shall be as sold and provided by the manufacturer, with or without mill scale. 4. TEST SPECIMEN PREPARATION 4.1. A minimum of three reinforced and three non-reinforced specimens shall be prepared. The two specimen types are identical except for reinforcement. 4.1.1. The
16、 test specimen shall be: 4.1.1.1. 114 mm 4.5 in. wide; 4.1.1.2. 152 mm 6.0 in. high; and 4.1.1.3. 280 mm 11.0 in. long 6 mm 0.25 in. for any dimension. 4.1.2. Reinforced test specimens shall contain one No. 16 No. 5 bar in the top mat electrically connected across a 10- ( 0.3 ) resistor to four No.
17、16 No. 5 bars in the bottom mat. The 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-4f T 376M/T 376-3 AASHTO concrete cover to the top and bottom steel shall be 25 1 mm 1 0.04 in. Figure 1 schematic
18、ally illustrates the geometry for reinforced specimens. 4.2. No. 16 No. 5 bars shall be cut to length of 381 mm 15.0 in. 4.3. Sharp edges on the ends of the bars shall be removed. 4.4. The ends of the bars shall be drilled and tapped to receive a 10-mm 38-in. long 10-24 threaded stainless steel bolt
19、. The bolt is used to make an electrical connection during the testing period. Figure 1Macroslab Reinforced Test Specimen (Source: MMFX Technologies Corp.) 4.5. The bars shall be cleaned with hexane to remove dust and oil. 4.6. If form release oil is to be used, it should be applied to the test spec
20、imen forms prior to the placement of the bars in the forms. 4.7. The bars shall be attached to the forms using marine adhesive per Section 3.10 with the specimens cast with concrete mix as indicated in Section 4.9. Place the bars in the molds so that approximately 40 mm 1.5 in. of the bars are prote
21、cted within each exit end from the concrete (minimizes edge effects). Both ends of each bar shall be coated with a two-part epoxy for a distance of 62 mm 2.5 in., except sufficient space shall be left uncoated at one end of each bar to attach the grounding clamps shown in Figure 1. This will expose
22、200 mm 8.0 in. of steel. Place the bars in the specimen formwork with the longitudinal ribs so that they are nearer the side of the beam, with both ridges equidistant from the top or bottom of the specimen. Note 4Caution should be taken not to introduce other metals that may stay in the concrete. 4.
23、8. Make the concrete specimens in accordance with R 39, using the same source of materials. Determine the air content, using either T 152 or T 196M/T 196. The watercement ratio (w/c) shall not exceed 0.5. The minimum slump is 50 mm 2.0 in. (See T 119M/T 119.) Place and consolidate the concrete in th
24、e molds in accordance with R 39. 4.9. Macrocell Slab Concrete Mix Design and CastingThe concrete used for the macrocell slab specimens will meet the requirements of the reference mix design below or a mix design set by the specifying agency. Macrocell slab casting and curing shall be in accordance w
25、ith Section 4.9.3. 4.9.1. Reference Concrete Mix: 4.9.1.1. 288 kg 635 lb Type I/II cement. (Not to Scale)Sealed withEpoxyAnode RebarCathode RebarsGroundClampEpoxySealer3% NaClSolutionPlexiglass DamRebarEpoxySealerConcrete Specimen1 in.1 in.3 in.3 in.VR = 10R6 in.4.5 in.2 in.1.3 in.6 in.8 in.11 in.15
26、 in. 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-4f T 376M/T 376-4 AASHTO 4.9.1.2. 408 kg 900 lb 9.5 to 12.5 mm 3/8- to 1/2-in. coarse fraction limestone aggregate. 4.9.1.3. 408 kg. 900 lb 4.75 t
27、o 9.5 mm No. 4 to 3/8-in. coarse fraction limestone aggregate. 4.9.1.4. 544 kg 1200 lb fine aggregate Ottawa graded sand. 4.9.1.5. 136 kg 300 lb water. 4.9.1.6. 6 percent 1 percent air content. 4.9.1.7. Permeability range greater than 4000 C (0.0415 F). Note 5Chloride-containing admixtures are not r
28、ecommended for incorporation into the reference concrete mix design protocol. 4.9.2. Agency Mix DesignAn agency should select a representative mix for their geographic and climatic region. The watercement ratio for the agency should be chosen to represent the highest anticipated acceptable w/c for a
29、 project to yield realistic results. Note 6The better the quality of the concrete mix (i.e., lower permeability slows chloride ion penetration), the longer the projected time until corrosion is initiated. Therefore, it is recommended to not incorporate fly ash, granulated iron blast-furnace slag, si
30、lica fume, or metakaolin into the mix design. 4.9.3. Macrocell Slab Casting and CuringThe concrete shall be cast and cured using the following procedure. In addition to casting macrocell slab test specimens, three 100-mm 4.0-in. by 200-mm 8.0-in. cylinder specimens shall also be cast for: 28-day com
31、pressive strength (T 22), density, air content (T 121M/T 121), and permeability (T 277) testing to confirm the concrete quality. 4.9.3.1. The concrete shall be mixed according to R 39. 4.9.3.2. The specimens shall be cast in an inverted position in two layers, so that the surface to be exposed is th
32、e bottom cast surface. Each layer shall be vibrated for 30 s on a vibrating table with an amplitude of 0.15 mm 0.0006 in. and a frequency of 60 Hz. 4.9.3.3. After the second layer is vibrated, the surface of the specimen shall be finished using a wooden float. 4.9.3.4. The specimens shall be cured i
33、n the forms for 24 h. 4.9.3.5. Following the form curing period, the specimens shall be removed from the molds. The specimens shall be placed in a plastic container with distilled or tap water for 25 days to affect a 100 percent relative humidity exposure condition. After the 25 days curing in the c
34、ontainer, cure the specimens an additional 45 days in an 80 percent (minimum) humidity curing room to assure maturity of the concrete. Alternatively, wrap seal the specimens in plastic ensuring moisture retention and cure for 60 days outdoors protected from sunlight and within a temperature range of
35、 7C to 38C (45F to 100F) in lieu of the high humidity curing room. 4.10. Prior to the start of the testing period, 1.5 mm216-gauge insulated copper wire shall be attached to one end of each bar in reinforced specimens using 10-mm 38-in. long 10-24 threaded stainless steel bolts. 4.11. The vertical s
36、ides of the specimens shall then be coated with two layers of the epoxy sealer. The electrical connections at the bar ends shall also be thoroughly coated to prevent crevice corrosion or galvanic corrosion from occurring. 2017 by the American Association of State Highway and Transportation Officials
37、. All rights reserved. Duplication is a violation of applicable law.TS-4f T 376M/T 376-5 AASHTO 4.12. The top of the specimens shall be lightly sanded and a plastic dam per Section 3.9 shall be placed on the top of each test specimen and sealed with silicone caulk per Section 3.10. 4.13. The specime
38、ns shall be supported on two pieces of wood, at least 50 mm 2.0 in. thick, to allow air to flow under the specimens. 4.14. The top layer of steel of reinforced specimens shall then be connected to a red binding post on the terminal box, while each of the four bottom bars are connected to a black bin
39、ding post on the terminal box. The top and four bottom bars are interconnected (Figure 2) through a 10- resistor at the terminal box. Figure 2Macroslab Test Set-Up (Source: Florida DOT) 5. PROCEDURE 5.1. The test procedure for the four specimens shall proceed as follows after casting and curing: 5.1
40、.1. All specimens are to be ponded initially with either distilled or tap water for 14 days at room temperature 20 to 24C 68 to 76F. Subsequently, all specimens shall be ponded with a 3.0 percent sodium chloride solution, also at room temperature, for three days. 5.1.2. At the completion of this tim
41、e (approximately 72 h), potential of the electrically interconnected bars shall be measured using a saturated calomel electrode (SCE) positioned in the ponding solution and a high impedance voltmeter. 5.1.3. The voltage drop across the 10- resistor connecting the three mats of steel shall also be re
42、corded for each reinforced specimen at completion of the three-day ponding cycle. The negative terminal of the voltmeter shall be connected to the black binding post and the positive terminal of the voltmeter shall be connected to the red binding post. 2017 by the American Association of State Highw
43、ay and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS-4f T 376M/T 376-6 AASHTO 5.1.4. The voltage drop obtained from the macrocell readings shall be converted to a corrosion rate (in m/yr) using Equation 1. 11600Corrosion Rate 11.6cViAR= =(1) where: ic
44、= corrosion current density (A/ cm2), V = voltage drop across resistor (mV), R = resistance of the resistor (), and A = area of exposed metal at the anode bar (cm2) Note 7Owner may select the following alternate corrosion rate equation: mpy = (1.44/) (1.117n)/(atomic wt. of iron) 105A/cm2where: = de
45、nsity of iron, 7.87 g/cm3, n = number of electrons (2), and Atomic wt. of iron = 55.85 The surface area can be determined by use of either nominal bar diameter, or by an equivalent bar diameter, based on a volume measurement, that takes into account the protrusions of the bar deformations. 5.1.4.1.
46、A No. 5 bar has a nominal bar diameter of 1.59 cm with a length of 20 cm and therefore has a surface area of 99.9 cm2 100 cm2. 5.1.4.2. To determine surface area, including the deformations, cut a No. 5 bar to a specific length (for example, 20 cm) and measure its volume to the nearest cm3. The equi
47、valent bar diameter is therefore dequiv= (4V/L) 0.5, where V is the volume and L is the length of the bar. The surface area A of the bar, including deformations, is A = (dequiv)L. 5.1.5. After the readings have been taken, a heat tent shall be placed over the specimens to maintain a temperature of 3
48、8 2C 100 3F. The specimens shall remain under the heat tent for 4 days (approximately 96 h). This cycle (3 days chloride solution ponded followed by 4 days dry at 38 2C 100 3F), along with recording of potential and voltage drop measurements, is repeated for 52 weeks or longer at the discretion of t
49、he testing agency. 5.1.6. If a potential of 0.280V (SCE) or more negative is recorded for two consecutive measurements, then a 3-in. (nominal) diameter core is to be taken from one of the non-reinforced specimens and acid soluble chloride content measured at the cover depth of the top bar in the reinforced specimens using ASTM C1152/C1152M. The determined chloride concentration defin
