Effect of environmental conditions on field welding of precast concrete connections
Clay Naito, Jason Zimpfer, Richard Sause, and Eric Kaufmann
T
he use of precast concrete enables year-round, highquality construction of buildings and bridges. The individual components are typically manufactured in an environmentally controlled facility. This allows for consistency of concrete production, steel installation, and concrete placement. Connections between precast concrete elements are often made by welding steel plates embedded in the precast concrete components. Figure 1 illustrates a standard double tee–to–inverted tee connection. When designed in accordance with PCI recommendations, these connections (PCI Connections Manual for Precast and Prestressed Concrete Construction)1 provide stability during erection and strength for service and ultimate-load cases.
■ A research study was conducted to investigate the quality of welded connections between precast concrete components made under environmental conditions typically encountered in precast concrete construction. ■ The effects of wind, humidity, temperature, and surface moisture on the quality of shielded metal arc welds (SMAWs) on ASTM A36 Type 304 stainless steel and ASTM A36 galvanized steel plates were examined. ■ The results showed that good-quality SMAW welds can be made in wind up to 35 mph (56 kph), in temperatures as low as -10 °F (-23.3 °C), and under wet conditions.
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In most cases, certain connections are welded initially during erection of the precast concrete component to provide stability. The remaining embedded connections are completed later to provide the full service and strength load capacity. Welding of the connections needed for erection stability is performed in the field under a variety of wind, humidity, and temperature conditions. Current American Welding Society (AWS) specifications2,3 are either restrictive or unclear about the conditions under which field welds can be made. A research program sponsored by PCI was conducted to investigate the effects of environmental conditions on the quality of field-welded precast concrete connections.
Figure 1. Field welding of precast concrete connections.
Welding requirements for precast concrete systems Field welds of steel connection plates embedded in precast concrete components are commonly made using the shielded metal arc welding (SMAW) method. This method, also called stick welding, uses a flux-coated electrode rod. During welding, the wire transmits current, creating an electric arc to the base metal. The rod melts and then solidifies, becoming the weld metal (filler), while the flux shields the molten weld metal from the atmosphere. Flux core arc welding (FCAW) is also used in precast concrete construction; however, this method uses a continuous-feed electrode wire. The size of the equipment used to feed the wire often makes it difficult to use in multistory precast concrete construction. Due to the limited use of the FCAW method in precast concrete construction, the research study focused on the welds made using SMAW.
Limits on environmental conditions for welding The standards often used for welding in the United States are produced by AWS. Two specifications apply for connections used in precast concrete construction: Structural Welding Code–Steel (AWS D1.1)2 and Structural Welding Code–Stainless Steel (AWS D1.6).3 AWS D1.1 provides a summary of unacceptable environmental conditions for welding in section 5.12.2: “Welding shall not be done (1) when the ambient temperature (temperature in immediate vicinity of weld) is below 0 °F (-18 °C), or (2) when surfaces are wet or exposed to rain, snow, or (3) high wind velocities, or (4) when welding personnel are exposed to inclement conditions.” In accordance with AWS D1.1, preheat is required for ASTM A364 base metals with a thickness between 1/8 in. and 3/4 in. (3 mm and 19 mm) welded with low-hydrogen electrodes using the SMAW process. The minimum preheat
temperature is 32 °F (0 °C). If the base metal temperature is below 32 °F, the base metal must be preheated to at least 70 °F (21 °C). For high wind velocities, a suitable shelter must be used to protect the weld.2 High wind velocity is defined as 5 mph (8 kph) for weld processes that use a gas shield to protect the molten weld metal from the environment. These gasshielded processes require a low wind condition to maintain the shield. Because the SMAW process does not use a gas shield, a wind velocity limit is not directly prescribed by AWS. AWS D1.6 similarly states that welding should not be performed on surfaces that are wet or in wind that would adversely affect shielding of the molten weld metal in the welding process. The AWS D1.6 code does not quantify the wind velocity that would affect the shielding process. The American Petroleum Institute (API) has similar environmental restrictions within its document Welded Steel Tanks for Oil Storage.5 With respect to wind, field welding is not allowed during periods of high wind unless the welder and weld are sheltered adequately. With respect to moisture, welding is not allowed when surfaces are wet from any form of precipitation or when precipitation is falling. Finally, API requires preheat if the ambient temperature is between 0 °F and 32 °F (-18 °C and 0 °C), and welding is forbidden if the temperature is below 0 °F. To summarize, current welding codes prohibit welding at ambient temperatures under 0 °F (‑18 °C) and permit welding, with associated preheat, at ambient temperatures between 0 °F and 32 °F (-18 °C and 0 °C). Limitations on welding using the SMAW process under high wind conditions are ambiguous. Last, there are no limitations on ambient moisture, but welding is prohibited when surfaces to be welded are wet.
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Electrode exposure to environment Exposure of standard carbon steel electrodes is limited by AWS D1.1 to four hours outside a hermetically sealed container or holding oven at 250 °F (121 °C), and electrodes that have been wet are prohibited from use. AWS D1.1 also states that electrodes with the supplemental designation R (such as the E7018-H4R electrodes used in this study) are approved for nine hours of exposure to the environment. AWS D1.6 states that electrodes for the SMAW of stainless steel can be kept in a hermetically sealed container provided it is reclosed immediately after opening. Otherwise, electrodes must be stored in a holding oven. A maximum exposure time is not defined.
Three series of specimens were fabricated. The ASTM A36 (36 series) specimen and the stainless steel (SS series) specimen were forensically examined through sectioning and surface observations. The T series specimen was evaluated for strength. Zimpfer et al.6 provides further details on each specimen.
Research overview
Forensic evaluation of welds
A research program was initiated to evaluate the quality of welded connections made under various environmental conditions. Welds were made under a variety of temperature, humidity, and wind conditions simulating those encountered in precast concrete construction. Three steel types used in precast concrete construction, ASTM A36, ASTM A36 galvanized, and Type 304 stainless steel, were examined. The study focused on the most commonly used fillet weld sizes of 1/4 in. and 3/16 in. (6 mm and 5 mm) produced with the SMAW process. The welds were examined visually and microscopically and by destructive testing to evaluate their adequacy with respect to the AWS standards. The destructive testing was designed to evaluate the strength of welds made under base conditions (71 °F [22 °C], 35% relative humidity [RH], 0 mph wind) and less favorable environmental conditions.
Fillet welds are susceptible to a variety of discontinuities that can affect their strength. These include weld profile irregularities, slag inclusions, porosity, and discontinuities that are cracklike in nature. A brief description of each type of discontinuity follows.
The environmental conditions used in the study represented extreme conditions encountered in U.S. construction. Three temperatures were chosen. Standard room temperature of 71 °F (22 °C) was used as a base condition, 32 °F (0 °C) was used as the temperature below which preheat is required by AWS, and 0 °F (-18 °C) was selected as a lower bound for practical construction conditions. Due to practical limitations, the ambient temperature varied marginally from the target value during welding. The humidity levels of 35%, 50%, and 95% RH were chosen to represent low-, average-, and near-saturation humidity. In addition, a surface wet condition was included to examine the effect of liquid or frozen water on the base metal plate surfaces. The surface wet condition was achieved by misting with a spray bottle, with the exception of one of the stainless specimens (SS-88), which achieved the surface wet condition by liquid droplets. Surface moisture was provided before welding only. No additional moisture was added during or after welding. Finally, the wind speeds were chosen as 0 mph, 5 mph, 10 mph, 20 mph, and 35 mph, (8 kph, 16 kph, 32 kph, and 36 kph) with 5 mph being the maximum allowable wind
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speed for many welding processes according to AWS D1.1. The greatest wind speed, 35 mph, was chosen as an upper bound under which welders would be willing to operate. The 10 mph and 20 mph conditions were chosen to provide adequate data to quantify the effects of wind on the welds. Table 1 summarizes the combinations of steel types and actual environmental conditions under which test welds were made.
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Weld profile irregularities include undercut, concavity or convexity, and overlap. Figure 2 illustrates samples of these irregularities. Undercut occurs parallel to the junction of weld metal and base metal at the top of the profile, and the associated stress concentration can reduce the strength of the weld. Convexity and concavity are specific forms of oversized or undersized welds, respectively. Concavity is detrimental from the reduction of the weld cross-section area, but weld passes can be added to increase the weld area. Oversized welds are not inherently harmful to weld quality or strength but might interfere with the assembly geometry and might produce excessive distortion of the base metal plates. Overlap (not shown) is usually caused by improper procedure or improper preparation of the base metal due to interference of surface oxides with the fusion process. Weld surface irregularities or ripples can be caused by improper technique or by excessive wind acting on the molten weld pool. However, variations in weld dimensions, depressions, nonuniformity of weld ripples, and other surface irregularities are not classified as weld discontinuities.7 Incomplete fusion is a lack of fusion between the weld metal and the base metal along one or more of the weld boundaries. It can result from improper preparation of the base metal before welding (insufficient cleaning) or insufficient welding current. It can result in crack formation (Fig. 3). Slag inclusions are nonmetallic solid materials trapped in the weld metal or at the interface of the weld metal and base metal. With proper welding procedures and technique, slag should rise to the surface of the molten weld metal.
Table 1. Test matrix Specimen identification
Base material
Temperature, °F
Relative humidity, %
Wind velocity, mph
Electrode condition
Plate surface condition*
36-1
ASTM A36
72.0
41.0
0
AWS D1.1†
Dry
36-3
ASTM A36
72.5
98.2
0
AWS D1.1
Dry
36-6
ASTM A36
76.6
94.3
20.0
AWS D1.1
Dry
36-7
ASTM A36
73.6
97.8
34.7
AWS D1.1
Dry
36-8
ASTM A36
78.3
92.4
0
AWS D1.1
Wet
36-14
ASTM A36
39.0
75.5
20.0
AWS D1.1
Dry
36-15
ASTM A36
31.0
100.0
32.4
AWS D1.1
Dry
36-22
ASTM A36
-5.0
99.9
21.3
AWS D1.1
Dry
36-23
ASTM A36
-13.0
100.0
27.0
AWS D1.1
Dry
36-17(95)(1)
ASTM A36
72.9
92.0
0
~4%
Dry
36-17(95)(2)
ASTM A36
77.1
88.6
0
~4%
Dry
36-C1
ASTM A36 Hi-C(1)
-6.0
100.0
0
AWS D1.1
Dry
36-C2
ASTM A36 Hi-C(1)
-4.0
66.7
0
AWS D1.1
Dry
36-PC1§
ASTM A36
88.9
43.4
0
AWS D1.1
1 wet, 1 dry
36-PC2§
ASTM A36
91.1
50.0
0
AWS D1.1
1 wet, 1 dry
§
36-PC3
ASTM A36
91.9
28.8
0
AWS D1.1
Dry
36-PC4§
ASTM A36
84.5
50.0
0
AWS D1.1
Wet
36-PC5
ASTM A36
15.0
85.3
0
AWS D1.1
Wet (ice)
36-PC6
ASTM A36 Hi-C(2)
74.2
17.6
0
AWS D1.1
Wet
36G-25
ASTM A36 galvanized
73.0
43.0
4.3
AWS D1.1
Dry
36G-33(1)
ASTM A36 galvanized
36.0
28.5
3.0
AWS D1.1
Dry
36G-33(2)
ASTM A36 galvanized
20.0
33.6
3.0
AWS D1.1
Dry
36G-17(95)
ASTM A36 galvanized
77.3
84.6
3.0
4%
Dry
SS-73
Stainless steel 304
73.0
35.7
0
AWS D1.6
Dry
SS-74
Stainless steel 304
73.7
47.7
0
AWS D1.6
Dry
SS-75
Stainless steel 304
77.0
100.0
0
AWS D1.6
Dry
SS-76
Stainless steel 304
71.4
100.0
5.1
AWS D1.6
Dry
SS-77
Stainless steel 304
74.8
95.7
10.1
AWS D1.6
Dry
SS-78
Stainless steel 304
75.5
94.8
20.1
AWS D1.6
Dry
SS-79
Stainless steel 304
75.8
90.9
33.2
AWS D1.6
Dry
SS-82
Stainless steel 304
45.5
48.8
0
AWS D1.6
Dry
SS-83
Stainless steel 304
35.6
99.2
0
AWS D1.6
Dry
SS-84
Stainless steel 304
43.4
100.0
5.1
AWS D1.6
Dry
SS-85
Stainless steel 304
39.8
100.0
10.0
AWS D1.6
Dry
SS-86
Stainless steel 304
37.2
100.0
19.3
AWS D1.6
Dry
SS-87
Stainless steel 304
33.8
100.0
33.1
AWS D1.6
Dry
SS-88
Stainless steel 304
35.7
99.9
~15.0
AWS D1.6
Wet
§ §
‡
†
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Table 1. Test matrix (cont.) Specimen identification
Base material
Temperature, °F
Relative humidity, %
Wind velocity, mph
Electrode condition
Plate surface condition*
SS-89
Stainless steel 304
-4.6
24.7
0
AWS D1.6
Dry
SS-90
Stainless steel 304
-5.0
49.5
0
AWS D1.6
Dry
SS-91
Stainless steel 304
-5.4
100.0
0
AWS D1.6
Dry
SS-92
Stainless steel 304
-2.2
95.5
5.5
AWS D1.6
Dry
SS-93
Stainless steel 304
-2.4
93.0
10.0
AWS D1.6
Dry
SS-94
Stainless steel 304
-3.0
100.0
20.6
AWS D1.6
Dry
SS-95
Stainless steel 304
-1.2
100.0
26 to 27
AWS D1.6
Dry
SS-96
Stainless steel 304
-2.0
99.9
0
AWS D1.6
Wet
SS-4(100)
Stainless steel 304
73.0
96.7
0
4 HR**
Dry
SS(1/4)-35
Stainless steel 304
73.0
94.6
32.0
AWS D1.6
Dry
SS(1/4)-0
Stainless steel 304
78.0
45.4
0
AWS D1.6
Dry
T-1
ASTM A36
84.0
15.4
0
AWS D1.1
Dry
T-2
ASTM A36 Hi-C(2)
77.9
26.4
0
AWS D1.1
Dry
T-3
ASTM A36 Hi-C(2)
-15.4
73.0
0
AWS D1.1
Dry
T-4
ASTM A36 Hi-C(1)
72.0
32.3
0
AWS D1.1
Wet
T-5
ASTM A36 Hi-C(2)
72.7
19.3
0
AWS D1.1
Wet
Wet indicates that the surface was intentionally wet before welding. Electrodes were stored and used in accordance with provisions outlined in AWS D1.1 and D1.6. ‡ Approximate percentage of moisture in electrode by weight (17-hour exposure to > 80% relative humidity). § Specimens were not sectioned according to standard procedure but were welded for the purpose of examination for porosity and cracking behavior and inspected as needed for these purposes. ** Refers to exposure of 308-16 electrodes for four hours to moist environment (within AWS D1.6 limits). Note: °C = (5/9)(°F – 32); 1 mph = 1.6 kph. *
†
Slag inclusions reduce weld strength by reducing the weld cross section and by creating stress concentrations. Porosity is the presence of voids in the weld metal. These voids have a variety of appearances. The main types are uniformly scattered, clustered, linear, and wormhole (elongated). Porosity is caused by the presence of gases in concentrations above their solubility limits as the weld metal solidifies. Hydrogen, oxygen, and nitrogen gases are soluble in the weld metal. Hydrogen is the primary cause of porosity in welds. Hydrogen can enter the molten weld pool from moisture in the cellulose constituents of the electrode coating or through dissociation of water. Water can be present on the electrode, the base metal plates, or in the air surrounding the weld.8 Porosity reduces the weld cross section and creates stress concentrations, both of which reduce the weld strength. Results from slow bend tests show that scattered, unaligned, unclustered porosity has little effect on the static yield strength, the ultimate strength, and the ductility of welds when composing less than 5% of the cross section and in some cases up to 7%.9 Cracks in the weld or heat-affected zone of the base metal (adjacent to the weld) increase the propensity for abrupt weld fracture due to
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stress concentrations at the tip of the crack. AWS D1.1 does not allow cracking. The required field inspection for the presence of cracks in fillet welds, however, is limited to a visual observation of the weld surface. In most cases, no microscopic or nondestructive examination of the weld is required; therefore, cracks of concern include those visible to the naked eye. For visual identification, a minimum crack length of approximately 1/32 in. (0.8 mm) is needed. For this investigation, the term microcrack refers to cracks less than approximately 1/32 in. long, while the term crack refers to those longer than approximately 1/32 in. Microcracks are often present after welding but in most cases are stable and do not propagate. Figure 2 shows two microcracks visible in a magnified view of a weld. Welds made under the various environmental conditions were examined to assess compliance with AWS requirements. The examination and evaluation criteria were as follows: •
Profile: The concavity and convexity of the weld profile were measured. Convexity must be less than 1 /8 in. (3 mm) for 1/4 in. (6 mm) fillet welds, and 1 /16 in. (2 mm) for 3/16 in. (5 mm) fillet welds.
Figure 2. Welding discontinuities. Note: " = inch. 1 in. = 25.4 mm.
•
Undercut: Undercut was identified and measured on polished weld cross sections. The undercut must be less than 1/32 in. (0.8 mm).
•
Cracks: Visible cracks (longer than 1/32 in. [0.8 mm]) were identified on polished weld cross sections and weld surfaces. Such cracks were considered unaccept-
able. Microcracks (shorter than 1/32 in.) were noted when observed under the microscope but were acceptable. •
Porosity: Porosity was identified and measured on polished weld cross sections and weld surfaces. For statically loaded welds, the sum of the visible piping porosity 1/32 in. (0.8 mm) or greater in diameter
Figure 3. Crack formation due to incomplete fusion on specimen 36-22. Note: 1 mm = 0.0394 in. PCI Journal | S p r i n g 2012
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Figure 4. Forensic weld specimen setup. Note: 1 in. = 25.4 mm.
was not to exceed 3/8 in. (10 mm) in any linear inch of weld or 1/4 in. (6 mm) in each 4 in. (100 mm) of weld length. For cyclically loaded welds, the frequency of piping porosity was not to exceed one in each 4 in. of weld length and the maximum diameter was not to exceed 3/32 in. (2 mm). •
Slag inclusion: Slag inclusions were identified and measured on polished weld cross sections. The sum of greatest dimensions of the slag inclusions on a cross section must be less than 1/4 in. (6 mm).
Experimental program
Strength evaluation setup
The specimen configuration was similar to a doubletee–to–inverted-tee connection (Fig. 1). The specimen consisted of two base plates and one cover plate oriented in a horizontal position (Fig. 4). The base plates were recessed in a 4-in.-thick (100 mm) concrete block to replicate plate embedment and heat sink conditions typical of precast concrete construction. The plates were clamped at all four corners to simulate a fully restrained condition. This restraint allowed residual stresses in the welded joint to develop on cooling of the weld metal. The cover plate was held stationary as shown by a single, unobtrusive hold-down point in the center of the plate. This configuration was used for all 36 series and SS series forensic specimens.
The fabrication setup used for the T series strength specimens was modified from the forensic specimen setup to facilitate testing. The top plate was offset to accommodate the necessary grip length in the tensile testing machine and to ensure failure of the specimens through the weld metal rather than a base metal fracture. The specimen involved lapping one 4 in. × 6 in. (100 mm × 150 mm) cover plate over one 4 in. × 6 in. base plate, with both plates oriented in the same direction such that the weld metal was deposited along the 6 in. plate length. Restraint was maintained using the edge clamps as previously discussed. The start and stop portions of the weld were not included in the tested specimen because these portions typically contain a disproportionately high level of discontinuities and are not representative of the majority of the weld. Finally, a bolt was placed through holes drilled in the cover plate of these two halves, as well as through a central 3/4 in. (19 mm) plate, which served as a grip for the testing machine through which tension could be applied concentrically and distributed equally to the welds on either side. Figure 5 illustrates the specimen and testing configuration. The tests were conducted at a quasi-static rate of 9.2 kip/min (41 kN/min). The specimens were loaded until a complete loss in load-carrying capacity occurred.
The welding was performed in an environmentally controlled chamber. Within the chamber, ambient temperature and relative humidity were controlled, with the ability to create temperatures as low as ‑18 °F (‑28 °C) and relative humidity from approximately 35% to 100%. Wind was simulated with a variable powered centrifugal blower with air flow transverse to the fillet weld (normal to weld axis). The fan was configured to achieve wind speeds ranging from 0 mph to 35 mph (56 kph) at the weld, with the wind being applied at a nominal distance of 6 in. (150 mm) from the fillet weld.
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For each test specimen, measurements were taken and recorded inside the chamber to verify the wind speed, humidity, and temperature. Relative humidity was measured in the center of the chamber using a handheld meter with an accuracy of ±3%. The temperature was measured in the air in the vicinity of the weld, as well as on the surface of the steel plates near the weld joint and on the concrete surface about an inch away from the plate recess. The wind speed was measured approximately 6 in. (150 mm) from the opening of the blower with a device with an accuracy of ±3%.
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Figure 5. Weld strength evaluation.
Base metal and electrodes The steel types in the experimental program include ASTM A36 (nongalvanized), ASTM A36 galvanized, and stainless steel Type 304. Fillet welds on three types of A36 were examined, one with a moderate carbon content (A36-1) and two with relatively high carbon content (HC1 and HC2). This variation allowed for an assessment of the effect of carbon content on the potential for cracking. The material originated from three different manufacturers. Mill certificates were obtained for each steel type and a sample of each steel type was sent for independent chemical analysis. A spectrographic analysis was performed on each of the samples, and the results of the independent analyses were compared with mill certificate values. The sensitivity of the potential for cracking in the heataffected zone of the base metal to the steel carbon content can be summarized using the Graville diagram (Fig. 6). Steel materials in zone I are unlikely to crack except when high concentrations of hydrogen are introduced during welding and the weld joint is highly restrained against local deformation. Steel materials in zones II and III have a greater potential for cracking in the heat-affected zone, which can be mitigated by using proper energy input or preheat. The classification of steels presented in the Graville diagram depends on both the carbon content and the carbon equivalent (Fig. 6). All A36 steel materials used in the project are in zone II, meaning that there is some potential for cracking if proper energy input and/or preheat is not used. Standard welding electrodes typical of precast concrete building erection were used in the experimental program. For the A36 and A36 galvanized steel base plates, E7018-
H4R electrodes were used in accordance with AWS D1.1 Table 3.1. Electrodes were stored and used in accordance with the restrictions found in AWS D1.1 section 5.3.2, except in cases where the electrodes were exposed to the environment to evaluate the effects of this exposure on weld quality. Namely, the E7018-H4R electrodes were purchased in hermetically sealed containers, stored in a holding oven held at a nominal temperature of 250 °F (120 °C), and not exposed to the environment for at least nine hours. For the A36 and A36 galvanized welds, 5/32-in.-diameter (3.9 mm), E7018-H4R electrodes were used to make 1/4 in. (6 mm) fillet welds in a single pass. The E70 designation indicates a nominal tensile strength of 70 ksi (480 MPa) for the weld metal. The H4 designation indicates that the electrodes met the requirement of having less than 4 mL (0.135 oz) average diffusible hydrogen in 100 g (0.22 lb) of deposited weld metal when tested in the as-received condition. The R identifies electrodes that pass the absorbed moisture test after exposure to an environment of 80 °F (26.7 °C) and 80% relative humidity for a period of at least nine hours. One-eighth inch (3.2 mm) 308-16 electrodes were chosen at the beginning of the study for the purpose of producing 1 /4 in. (6 mm) fillet welds in a single pass on stainless steel specimens. The weld size was measured after the specimens were sectioned, and the stainless steel welds were found to have a nominal size of 3/16 in. (5 mm). This was not the specified weld size but is acceptable for the 3/8 in. (10 mm) plate thickness used according to AWS D1.1 Table 5.8. Rather than remake the specimens and welds, all remaining stainless steel welds were made using 1/8 in. (3 mm) electrodes, and data were analyzed with respect to a 3/16 in. weld instead of a 1/4 in. (6 mm) weld. PCI Journal | S p r i n g 2012
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Figure 6. Graville diagram heat affected zone (HAZ) crack susceptibility. Note: C = carbon; Cr = chromium; Cu = copper; Mn = manganese; Mo = molybdenum; Ni = nickel; Si = silicon; V = vanadium.
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The welding setup involved the use of a constant current welding power source. Grounding was provided directly to the restraint clamp, which was in contact with the plates. The energy input for the 1/4 in. (6 mm) welds on the plain A36 steel plates ranged from 33 kJ/in. to 48 kJ/in. (13 kJ/cm to 19 kJ/cm). The energy input for welds on the A36 galvanized plates ranged from 41 kJ/in. to 48 kJ/in. (16 kJ/cm to 19 kJ/cm), and the energy input for the smaller 3/16 in. (5 mm) welds on the stainless steel plates ranged from 30 kJ/in. to 33 kJ/in. (12 kJ/cm to 13 kJ/cm). The lower energy input for the stainless welds reflects the smaller electrode size and smaller weld sizes because less energy is required for smaller welds.
After the welds were sectioned and polished, inspection was performed on the cross sections with the naked eye and a magnifying glass, by measurements made on photographs taken of the cross section, and by microscope. Quantifiable weld discontinuities were measured using a magnifying glass and digital caliper. The largest dimensions of pores and inclusions were measured, and undercut was quantified as the distance from a line passing through the original plate edge to the deepest point of the undercut in the cross section. Discontinuities were further investigated under the microscope, measured when appropriate, and recorded. The acceptability of the weld profiles was determined from the cross-section photographs.
Evaluation procedure
Experimental results
Table 1 gives the environmental conditions under which the welds were made. The welds were sectioned after at least 24 hours to allow cracks to develop. The outer faces of the center cut sections were polished with a 1200-grit grinding surface, buffed with a 0.3 μm (0.0001 in.) particle solution, and etched using an appropriate acid etching agent. The final product was a clean surface with a clear view of the weld in cross section (Fig. 7).
Effect of wind speed on weld profile
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Examination of welds made under different wind speeds (Fig. 8) indicates that wind acting on the molten weld pool results in ripples in the welds made under high wind speeds. The effect is most noticeable for winds of 35 mph (56 kph). Figure 8 shows that the effect was greater for A36 steel than for stainless steel. In addition, the high
Figure 7. Specimen section procedure. Note: 1 mm = 0.0394 in.
wind caused the arc to behave erratically. Lowenburg et al.10 reported in the context of pipeline fabrication that weld failure can be initiated from ripples on the weld surface, which underscores the importance of maintaining the weld profile. In all cases, the wind direction was perpendicular to the longitudinal axis of the weld. High wind speed did not cause any significant profile skew in the sections examined. The sensitivity of wind direction on the weld was not examined; however, it is hypothesized that head wind or tail wind in the direction of the welding axis may have a greater effect on the formation of ripples and a lesser effect on concavity. This hypothesis should be verified through additional experimentation. Effect of wind speed on slag inclusions Figure 9 plots the sum of the greatest dimensions of slag inclusions on a cross section versus the wind speed under
which the weld was made. The temperature and humidity are held constant for each data set. Linear trends of the three data series conducted at 95% RH and three different temperatures (‑10 °F, 32 °F, and 71 °F [‑23 °C, 0 °C, and 22 °C]) are presented. The 32 °F condition (8 sections examined) has the highest correlation, followed by the 71 °F condition (12 sections examined). The ‑10 °F condition (8 sections examined) shows no correlation; however, this is attributed to the limited number of samples examined. In all cases the linear fit is poor. Slag inclusions were observed regularly in the sections taken from the stainless steel specimens. In fact, 62 of the 100 sections examined exhibited at least one inclusion. The presence and size of inclusions tended to increase with wind speed, as was the case for the A36 specimens (Fig. 9). The majority of the slag was observed at the root of the weld (Fig. 9). Three possible reasons for this are hypothesized:
Figure 8. Weld quality under various wind speeds. Note: 1 mph = 1.6 kph. PCI Journal | S p r i n g 2012
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Figure 9. Influence of wind on slag inclusions at 95% relative humidity. Note: 1 in. = 25.4 mm; 1 mph = 1.6 kph; °C = (5/9)(°F – 32).
•
Negative pressure caused by wind blowing over and around plate surfaces provides suction at the root of the joint, trapping slag.
•
At higher speeds, the wind may push the slag ahead of the molten weld metal pool, trapping the slag under the advancing weld bead.
•
Higher wind speeds decrease the welding arc stability, which influences the uniformity of the molten weld metal pool and results in slag inclusions.
Regardless of the cause, the sizes of the inclusions are well below a level (about 5% cross-sectional area) that has an effect on weld strength and are within the acceptability limits from AWS D1.1. Effect of moisture on porosity Porosity was observed and quantified in two ways. Surface (piping) porosity was measured as the sum of the diameters of surface pores for a 4-in.-long (100 mm) weld. Section porosity was measured as the sum of the diameters of pores in a polished cross section. Because the second method examines only two discrete sections of the 4-in.-long welds made on each specimen, the likelihood of sectioning through a pore is low. Consequently, surface porosity is used to represent the porosity conditions. It was expected that moisture would have the greatest effect on porosity, as discussed earlier. Because welds are
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often made in the field under wet conditions (for example, from falling rain), it was decided that attention should be given to the cases of surface wetness. Six additional specimens (specimens 36-PC1 through 36-PC6) were made with wet surface conditions. For specimen 36-8, surface wetness was created by misting the clamped plate assembly at the weld joint before welding. Specimens 36-PC1 to 36-PC6, were wetted before laying the cover plate on top of the base plates. After the cover plate was in place, the assembly was further moistened using a misting bottle, and in some cases, pouring water over the plates until a pool of water was visible on plate surfaces. Figure 10 plots the total surface porosity of welds made on A36 steel and stainless steel against the conditions of the plates and electrode. The plot includes 24 welds conducted with dry electrodes on a dry A36 steel surface, 9 welds conducted with dry electrodes on a wet A36 steel surface, 4 welds conducted with moist electrodes on a dry A36 steel surface, and 49 welds conducted with dry electrodes on a dry stainless steel surface. The moist electrodes generated the greatest surface porosity, followed by the surface wet and dry conditions. The surface wet conditions did not generate appreciable surface porosity. For the wet conditions, initiation of the weld was sometimes difficult, but once it was started, moisture was driven off ahead of the welding arc. The dry welding conditions for the A36 and stainless steel material did not generate significant porosity.
Figure 10. Total surface porosity versus plate and electrode condition. Note: The area of the light-shaded circles in the plot represents the number of occurrences of a given level of measured surface porosity. The dark-shaded circles and black bars represent the average level of surface porosity measured for the indicated plate and electrode conditions. 1 in. = 25.4 mm.
All welds made under the various environmental conditions met the surface porosity limits of AWS for statically loaded nontubular connections. The porosity observed was less than the static connection limits but exceeded the requirements for cyclically loaded welds. For this case, the frequency of surface porosity exceeded the AWS limit of one in each 4 in. (100 mm) of weld length. In precast concrete building system applications, fillet welds of the type investigated in this study are not typically subjected to high cycle fatigue loading. However, precast concrete members in bridge applications may be subjected to high cycle fatigue loading. In cases where fatigue is a concern, care should be taken to ensure that the electrodes are dry.
in the cross sections. Furthermore, crack formation in the heat-affected zone (HAZ), though not observed, is related to the presence of hydrogen; therefore, moisture should be avoided. It is recommended that electrodes be used according to manufacturer guidelines and AWS requirements and surface wetness, whenever practical, should be eliminated using preheat. This preheat might not need to match AWS D1.1 requirements. The goal is to drive off the moisture on plate surfaces before welding. In cases where preheat is not practical, moisture should be wiped off with a cloth.
The results indicate that surface wetness does not generate unacceptably high surface porosity, while moist electrodes generate greater but acceptable (for static loading) porosity. Radiographic examination might, however, reveal subsurface pores. Some subsurface pores were observed
Visual inspection of the surfaces of welds made under various conditions did not reveal any cracks longer than 1 /32 in. (0.8 mm). Following visual inspection, the welds were sectioned and examined using an optical microscope. For A36 steel specimens, cracks on the order of
Effect of environmental conditions on crack formation
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Figure 11. Plot of Vickers hardness from microhardness tests. Note: 1 in. = 25.4 mm; °C = (5/9)(°F – 32).
/64 in. to 1/16 in. (0.4 mm to 1.6 mm) long were observed where the weld metal meets the HAZ and near the root or toe of the welds, at discontinuities where the stress concentration is high. 1
Crack formation in welded joints is related to the hardness of the HAZ. Cracking tends to occur in areas of greater hardness where ductility is low and residual stresses are high. Hardness measurements were made on several specimens. The Vickers microhardness test was used, which has the capacity to measure the hardness across regions of the HAZ.7 The test is performed using a pyramid-shaped diamond indenter that is pressed into the specimen with a given load for 10 seconds. The indentation is measured using a microscope, and the dimensions of the indentation are used to calculate the hardness on the Vickers scale. The hardness across the weld cross section is controlled by the steel microstructure created during welding and subsequent cooling. Depending on chemical composition and the thermal history of the weld metal and base metal, including the carbon content and cooling rate of the base metal, martensite can form in the HAZ, increasing hardness and susceptibility to cracking. The hardness of the HAZ is a good indicator of the amount of martensite present and susceptibility to cracking. Crack-
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ing occurs rarely when the Vickers hardness is less than 265 HV (Vickers pyramid number) but is common when the Vickers hardness approaches 470 HV if precautions to prevent cracking are not taken.7 Preheat treatment is a precaution against a high cooling rate that might promote martensite formation in the HAZ. The thermal mass of the specimens used in the present study is relatively low; thus, welding tends to heat the entire specimen, resulting in a relatively slow cooling rate. Preheat treatment was not used in this study. Microhardness tests were performed on five specimens (Fig. 11). The approximate crack-susceptibility threshold of 265 HV is indicated in the figure, as are the approximate zones (weld metal, HAZ, and base metal) where readings were taken for the tested specimens. Figure 11 shows that the Vickers hardness of the specimens is not sensitive to the ambient temperature but might be sensitive to the carbon content. The variation in hardness in the weld metal and HAZ region of the base metal was similar for all specimens examined. The hardness was highest in the HAZ at the interface with the weld metal. At this location the hardness was closest to, but below, the cracking-susceptibility threshold of 265 HV for all cases. Elevated carbon content resulted in two of the three highest hardness levels at the HAZ boundary. The weld made on
moderate carbon content base metal at room temperature produced the second highest hardness level. The welds made on higher carbon steel, namely specimens 36-C1 and 36-C2, while having slightly higher peak values (10% on average compared with moderate carbon samples), did not approach hardness levels that indicate that the HAZ is crack susceptible. The study used rather thin plates (3/8 in. [10 mm]). A thicker plate would produce an increased cooling rate due to its greater thermal mass. Contact with a thick concrete slab would also promote more rapid cooling, but the effect of the concrete mass on the cooling rate would be less than that of a thicker steel plate. The occurrence of root microcracks was not affected by environmental conditions. Root microcracking was observed in 15 sections (three sections from 36-C2; two sections from 36-8, 36-23, 36-C1, and 36-PC6; and one section from 36-1, 36-14, 36-22, and 36-17HR[1]). These specimens were welded in temperatures ranging from 74.2 °F to -13 °F (23.4 °C to -25 °C), 41% RH to 100% RH, wind speeds from 0 mph to 27 mph (0 kph to 43.5 kph), with dry and wet electrodes, and with dry and wet surface conditions. The root microcracking observed was widespread among specimens with no apparent correlation to any specific environmental conditions. The effect of surface wetness on crack formation was examined in specimen PC-6. This specimen was fabricated with surface water present at a temperature of 74.2 °F (23.4 °C), 17.6% RH, and under a 0 mph (0 kph) wind condition. The specimen was restrained for 24 hours in the concrete test block before removal. Four sections were taken from the specimen, and two of them were observed to have microcracks. One of the sections had microcracks through a slag inclusion near the root. Another section had root microcracks, as well as toe microcracks. Although only these four sections from a single specimen were studied, it appears that surface wetness may affect the potential for cracking. Thus, welding over surface wetness is a concern because the moisture might increase the presence of dissolved hydrogen in the microstructure and increase the potential for cracking. Microcracks were more numerous when discontinuities such as slag inclusions were present. These local discontinuities can generate a high stress concentration resulting in cracking through the inclusion (Fig. 12). This cause of cracking is important because those factors contributing to slag inclusions, even if the inclusions are small and acceptable, can contribute to microcracking. Other discontinuities, such as porosity or undercut, can also serve as initiation points for microcracks.
Figure 12. Root of specimen T-3.
Effect of environmental conditions on weld strength Five conditions were examined to determine the influence of environment on the weld strength. Each weld was performed on an ASTM A36 plate with no wind and with dry electrodes. Temperature, humidity, and surface conditions were varied. Table 2 presents the details of the five test specimens fabricated. The maximum loads at failure were recorded, and the sections were examined forensically to assess whether any unexpected failure mode occurred. All failures occurred in the weld metal. The failure was characterized by yielding and significant plastic deformation in the weld region followed by fracture on a plane approximately 45° from the root. Figure 13 illustrates examples of failure modes. Fracture surfaces were examined under an optical microscope to determine whether any discontinuities were present that may have influenced their ultimate strength. The nominal capacity was predicted in accordance with the American Institute of Steel Construction’s (AISC’s) Steel Construction Manual11 formulation for strength of a fillet weld in transverse tension (Eq. [1]). The nominal capacity was computed with the nominal electrode strength and with measured electrode strength. For both cases, the measured throat and weld length were used. Because measured values were used, no strength reduction factors are included. P = FT(2l)(1.5) (1) P = nominal tensile strength F = tensile strength of the weld metal T = minimum throat thickness l = weld length
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Relative humidity, %
Throat size, in.
Average hardness, Rockwell B
Estimated tensile strength FUTS, ksi
Predicted FEXX capacity, kip
Predicted FUTS capacity, kip
Measured strength, kip
Factor of safety for FEXX
Factor of safety for FUTS
Dry
84.0
15.4
0.186
92.0
92.0
23.4
30.8
33.1
1.41
1.07
T-2
HC2
Dry
77.9
26.4
0.215
89.7
88.7
26.9
34.1
33.8
1.26
0.99
T-3
HC2
Dry
-15.4
73.0
0.244
87.1
84.2
32.4
38.9
42.8
1.32
1.10
T-4
HC1
Wet
72.0
32.3
0.195
90.4
89.4
24.8
31.7
30.2
1.22
0.95
T-5
HC2
Wet
72.7
19.3
0.215
87.9
85.8
26.3
32.3
35.1
1.33
1.09
Plate surface condition
A36-1
Base metal
T-1
Specimen
Temperature, °F
Table 2. Strength performance of welds
Note: FEXX = nominal weld metal strength; FUTS = ultimate tensile strength as determined from Rockwell B hardness measurements. 1 in. = 25.4 mm; 1 kip = 4.448 kN; 1 ksi = 6.895 MPa; °C = (5/9)(°F – 32).
Two values are used for F: the nominal weld metal strength FEXX, 70 ksi (480 kN), and the ultimate tensile strength as determined from Rockwell B hardness measurements FUTS. A minimum of four hardness measurements were taken and averaged to determine the tensile strength of the weld metal. The minimum throat thickness T and weld length l were measured for each weld. Because the weld length is for only one side, a multiplier of 2 is included in the formulation. Table 2 presents the hardness measurements, ultimate tensile strength, and length and throat dimensions for all of the test specimens. The limited study indicated that the weld strength is not affected by variations in temperature or humidity. The measured strength exceeded the predicted strength, computed using the nominal weld metal strength, by an average of 30%. When the measured weld tensile strength was used, the AISC formulation predicted the tensile strength within 10%. Based on these
Figure 13. Failure modes for tension tests of specimen T-1 and specimen T-5. 156
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results, the tensile strength was not affected by any of the environmental conditions examined. Furthermore, the AISC formulations for strength conservatively estimate strength when the nominal strength of the filler metal is used. One failure surface was further examined under a scanning electron microscope, and an image taken with this microscope revealed the ductile nature of the shear fracture. Figure 14 shows a high-magnification secondary electron image of the ductile shear fracture surface. The surface is composed of regions shaped like elongated ovals, each of which surrounds a small inclusion, appearing as a white dot in the image. These microvoids elongate and coalesce under shear loading as the weld metal deforms plastically to form a ductile shear failure surface. The small slag inclusions are well below a size that would have any effect on weld strength and are only detectable at a level of magnification such as that in the secondary electron image.
Figure 14. Weld failure surface magnification. Note: 1 mm = 0.0394 in.
Effect of welding through galvanized steel Two welds were conducted under the ambient outdoor environmental conditions typical of winter in Pennsylvania. The conditions at the time of welding were 37 °F (2.8 °C), 30% RH, 0 mph (0 kph) wind. Two specimens were welded to compare results between welds and because the conditions outdoors were sufficiently cold to replicate a set of conditions from the test matrix. To assess the effect of moist electrodes on the galvanized steel, a specimen was welded using E7018-H4R electrodes that had been exposed to a moist environment for approximately 17 hours, resulting in electrode moisture content near 4.0% by weight. Welding through a galvanized plate is prohibited by current codes, including AWS D1.1 and the PCI Design Handbook: Precast and Prestressed Concrete.12 Removal of galvanizing in the area of the weld joint is recommended. Removal is often accomplished by grinding or burning. Because welding through galvanizing is prohibited and generates a health hazard by creating zinc oxide fumes, only a small sample was examined in this study. Additional tests are required to generate definitive conclusions. A PCI survey13 reports that 70% of reporting PCI Producer Members remove galvanizing on plates before welding, and 73% remove galvanizing on reinforcement before welding. In addition, 63% of the respondents have developed an associated welding procedure specification for welding galvanized components. Welding through a hot-dip galvanized zinc coating requires the volatilization of the zinc coating as the electrode passes along the weld joint. The melting point of zinc, the primary component of hot-dip galvanized coatings, is approximately 788 °F (420 °C), and the temperature at which zinc vaporizes is approximately 1665 °F (907 °C). The melting point of steel is approximately 2500 °F (510 °C), and the temperature of an arc in the SMAW process can be as high as 10,500 °F (5800 °C). As a result, it is possible that some of the zinc coating is vaporized as the arc approaches. Sperko14 reported that it is possible to weld through a galvanized coating without affecting weld strength. The AWS D19.0 document Welding Zinc-Coated Steel15 also reports that the same practices used for uncoated steel can
generally be used for galvanized steel for the SMAW process. The document notes that slower travel speeds and a whipping action of the electrode should be used to volatilize as much of the zinc coating as possible and avoid its introduction into the molten weld pool. The issue of welding through zinc galvanization, however, does not appear to be settled when considering research, current practice, and codification surrounding the issue. Problems with arc stability may be encountered by welding through the irregular zinc coating and generating vapors. The zinc coating, when vaporizing, can create porosity if gases become trapped in the weld joint between two coated surfaces. In addition, if zinc is present in solution in the molten weld pool, it could create a cracking hazard as metal cools around the lower melting point zinc compounds and restrains their cooling contraction, forming tears in the weld metal. The profiles of the welds made through the galvanized coatings tended to have a higher rate of unacceptability than those welded on nongalvanized carbon steel in the study. Fourteen out of the sixteen sections examined, or 87.5%, failed to meet the profile acceptability criteria. This may be a function of poor arc stability or low visibility when the galvanization is vaporized. Microcracking was observed at a higher percentage in galvanized specimens compared with nongalvanized specimens. In addition, a solidification crack was detected by visual observation unaided by microscopy on one specimen. The crack was located at the root of a section taken from specimen 36G17HR (77.3 °F [25.2 °C], wet electrode, 4 mph [6.4 kph] wind). The welds made through galvanization were free from porosity both on the weld surfaces and in cross sections. Only one of the 16 sections exhibited a small (0.008 in. [0.2 mm]) slag inclusion. Two sections exhibited undercut, and one example of undercut exceeded the 1/32 in. (0.8 mm) limit set forth in AWS D1.1. The relative lack of discontinuities in the sections examined indicates that welding through the galvanizing might not have a great effect on porosity or slag inclusions. Due to the small sample size, further study of welding through zinc coatings and more thorough nondestructive evaluation of such welds is necessary before drawing a conclusion.
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Figure 15. Root gap and microcracks in galvanized specimens. Note: 1 mm = 0.0394 in.
A problem that arises from welding galvanized plates is that the gap between the base plates and cover plate appears susceptible to cracking at the root of the weld. Because hot-dip galvanized coating is inherently irregular and creates a plate surface that is not smooth, large gaps result. The poor fit between plates produces inclusions or voids at the root, which can lead to microcracks (Fig. 15).
Research findings •
Wind tends to produce rippling of the molten weld pool and poor profiles. Increases in wind speeds tend to increase the presence and severity of slag inclusions, but not beyond AWS acceptability up to 35 mph (56 kph).
Based on the limited results obtained in the study, further research is recommended before conclusions can be drawn on the susceptibility to cracking and discontinuities in welds made through galvanized coatings on A36 plates. If weld quality is a concern, the procedures of the American Galvanizers Association can be followed, namely, the removal of galvanic coatings 1 in. to 4 in. (25 mm to 100 mm) away from the weld joint before welding.
•
Undercut was observed in several specimens but does not appear to correlate strongly with a specific environmental parameter. All observed undercut was within the AWS limit of 1/32 in. (0.8 mm).
•
Porosity increases when the electrodes used are exposed to a moist environment beyond AWS D1.1 code recommendations (nine-hour exposure limit to moist environment).
•
Surface porosity is not significantly affected by welding through surface wetness. Welding through surface wetness has the potential to increase microdiscontinuities and create visible cracking and should be further investigated. Cracking as a result of welding through surface wetness merits further investigation. In addition, subsurface porosity was generated in the case of the surface wet specimen T-5 but did not reduce the strength of the specimen.
•
Microcracking was widely observed and was more prevalent in specimens welded using higher carbon plate material. The presence of these microcracks was not correlated with environmental conditions.
•
Poor fit-up of plates as a result of the rough galvanization coating can contribute to root cracks where large plate gaps exist. Microcracking was observed at a higher percentage in galvanized specimens compared with nongalvanized specimens. Few discontinuities were observed in the sections made on galvanized welds, indicating that the galvanized coating does not significantly increase porosity or slag inclusions.
•
Surface porosity was more widely observed in welds made on stainless steel plate than on carbon steel plate
Conclusions and recommendations The research examined the influence of wind, temperature, humidity, and moisture on the integrity of welded connections used in precast concrete construction. The conclusions and recommendations are limited in application to the scope of the research study and may not be applicable to variables beyond those examined. The bounds of the study include:
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•
single-pass fillet welds made with the SMAW process low-hydrogen electrodes (E7018-H4R) for Astm A36 steel
•
308-16 electrodes for Type 304 stainless steel
•
plate thicknesses of /8 in. (10 mm)
•
plate sizes typical of precast concrete connections on the order of 4 in. × 6 in. (100 mm × 150 mm)
•
static loading
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but was acceptably low in all cases. Few microcracks were observed in stainless steel specimens. Microcracking was not correlated with any environmental parameter. •
The transverse shear strength of fillet welds made with the SMAW process on A36 plates was not sensitive to the environmental conditions studied and was accurately approximated using the AISC formulations. Microcracking of the size and shape observed in the specimens did not have a significant effect on weld strength.
Research conclusions The following conclusions are derived from the results of the research and relate to the effect of environmental conditions on the quality of welds simulating the welds used in precast concrete construction. Humidity Ambient humidity is not correlated with the presence of weld discontinuities. High humidity increases the presence of hydrogen in the vicinity of the weld; however, it was not found to affect the quality of the welds. The exposure of electrodes to humid conditions, however, did affect weld quality, increasing the potential for porosity and cracking. The guidelines and restrictions in AWS D1.1 regarding exposure of electrodes should be closely followed. Surface wetness Welds made on wet plates did not exhibit greater surface porosity. The moisture was driven away from the weld joint as the weld metal was deposited. Welding through surface wetness also has the potential to increase microcracking and create visible cracking and should be further investigated. The effect of moisture in the form of falling rain entering the weld pool was not studied, and this condition should be examined further. Until such research is performed, it is recommended that welding not be performed when the weld pool is subject to falling precipitation and, whenever possible, that surface moisture be eliminated from the plate surfaces before welding. Temperature Temperatures as low as -13 °F (-25 °C) were examined and found to have no effect on porosity or slag inclusions. Low temperatures have a tendency to increase cooling rates, increasing the propensity for high hardness and crack formation. The hardness levels measured in the specimens, however, were below a level that would increase the propensity for cracking. Microcracks were observed in welds made over a variety of temperatures and are more sensitive to base metal composition, restraint, and hydrogen present than the ambient temperature during welding.
Wind High wind speed had a negative effect on the profile and surface geometry of the welds and increased the presence of slag inclusions. The amount of slag included, however, was below AWS limits. In addition, successful SMAW welds were made in wind up to 35 mph (56 kph). If the correct weld profile and good weld surface conditions can be achieved by a welder in a high-wind condition, then welding should be permitted. The resulting profile must be verified in accordance with the details set forth in AWS D1.1. Strength The welds that were evaluated for strength performed adequately and predictably. There was no appreciable reduction in strength for welds exhibiting discontinuities of the type and severity seen in the forensic examination. Design codes are conservative with regard to the prediction of strength for 1/4 in. (6 mm) fillet field welds made with the SMAW process. Recommendations for precast concrete construction Based on the results presented in this report, 1/4 in. (6.3 mm) fillet welds made on 3/8-in.-thick (9.5 mm) A36 base plates using E7018-H4R electrodes and 3/16 in. (4.8 mm) fillet welds made on 3/8-in.-thick Type 304 stainless steel base plates using E308-16 electrodes can be performed under any of the following environmental conditions as long as the welder is able to create a weld meeting the AWS acceptability criteria: •
wind up to 35 mph (56 kph)
•
ambient temperature of 0 °F (-18 °C) and above without preheat treatment
•
relative humidity up to 100%
SMAW electrodes should be stored, handled, and used in accordance with manufacturer guidelines and AWS D1.1 and D1.6 requirements. Failure to follow these requirements might result in excess porosity and crack formation in the weld. Due to limitations of the study, welding through surface wetness or in falling rain is not recommended. Excess moisture should be removed from the base metal before welding. All of the ambient environmental conditions will have a direct effect on the welder and might decrease his or her ability to successfully deposit a weld. Furthermore, skill level varies among welders; therefore, it is imperative that welders operate within their abilities. This may require the fabrication of a wind shield or covered structure in certain environmental conditions.
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References 1. PCI Details Committee. 2008. PCI Connections Manual for Precast and Prestressed Concrete Construction. MNL 138-08. 1st ed. Chicago, IL: PCI. 2. American Welding Society (AWS) Committee on Structural Welding. 2008. Structural Welding Code— Steel. AWS D1.1/D1.1M:2008. Miami, FL: AWS. 3. AWS Committee on Structural Welding. 2007. Structural Welding Code—Stainless Steel. AWS D1.6/ D1.6M:2007. Miami, FL: AWS. 4. ASTM Subcommittee A01.02. 2005. ASTM A36/ A36M-08 Standard Specification for Carbon Structural Steel. doi:10.1520/A0036_A0036M-08, www .astm.org/Standards/A36.htm. West Conshohocken, PA: ASTM International. 5. American Petroleum Institute (API). 1980. Welded Steel Tanks for Oil Storage. 7th ed. Washington, DC: API. 6. Zimpfer, J., C. Naito, R. Sause, and E. Kaufmann. 2008. Investigation of the Impact of Environmental Conditions on Field Welding of Precast Concrete Connections. ATLSS report no. 07-03. Bethlehem, PA: Lehigh University. 7. Conner, L. P. 1987. Welding Handbook: Welding Technology. 8th ed. V. 1. Miami, FL: AWS. 8. Lundin, C. D. 1984. Fundamentals of Weld Discontinuities and Their Significance. Welding Research Council (WRC) bulletin 295. New York, NY: WRC. 9. Lundin, C. D. 1976. The Significance of Weld Discontinuities—A Review of Current Literature. WRC bulletin 222. New York, NY: WRC. 10. Lowenburg, A. L., E. B. Norris, and A. R. Whiting. 1968. Evaluation of Discontinuities in Pipeline Weld Joints—Summary Report No. 1. Pressure Vessel Research Committee of WRC, Southwest Research Institute, San Antonio, TX. 11. American Institute of Steel Construction (AISC) Committee on Manuals and Textbooks. 2005. Steel Construction Manual. 13th ed. Chicago, IL: AISC Inc. 12. PCI Industry Handbook Committee. 2004. PCI Design Handbook: Precast and Prestressed Concrete. MNL120. 6th ed. Chicago, IL: PCI.
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13. PCI. 2006. Survey Results on the Use of Galvanizing for Precast Concrete Structures. PCI Journal, V. 51, No. 4 (July–August): pp. 106–110. 14. Sperko Engineering Services Inc. 1999. Welding Galvanized Steel—Safely. www.sperkoengineering.com/ html/articles/WeldingGalvanized.pdf (accessed July 7, 2011). 15. Bland, Jay, and AWS Technical Department. 1972. Welding Zinc-Coated Steel. AWS WZC/D19.0-72. Miami, FL: AWS.
Notation F
= ultimate tensile strength of the weld metal
FEXX = nominal weld metal strength FUTS = ultimate tensile strength as determined from Rockwell B hardness measurements l
= weld length
P
= nominal strength of connection
T
= minimum throat thickness
About the authors Clay Naito is an associate professor for the Department of Civil and Environmental Engineering at Lehigh University in Bethlehem, Pa. Jason Zimpfer, MSCE, is a former graduate research assistant for Lehigh University. He is currently a structural engineer at AECOM in Horsham, Pa. Richard Sause is the ATLSS director and Joseph T. Stuart Professor of Structural Engineering for Lehigh University.
metal arc welds (SMAWs) were examined. The study focused on ASTM A36 Type 304 stainless steel and ASTM A36 galvanized steel plates. Weld surfaces and cross sections were examined visually and with optical microscopy. The results of the examinations were compared with limits for various weld discontinuities in accordance with American Welding Society specifications D1.1 and D1.6. In addition, tests were performed to assess the impact of environmental conditions on strength. The results showed that good quality SMAW welds can be made in wind up to 35 mph (56 kph), in temperatures as low as -10 °F (-23.3 °C), and under wet conditions. In general, acceptable welds were fabricated under the variety of environmental conditions examined. Various types of discontinuities were observed but were not found to cause a significant reduction in the transverse shear strength of the welds.
Keywords Connection, humidity, temperature, welding, wind.
Review policy Eric Kaufmann is a senior research scientist for the ATLSS Research Center at Lehigh University.
This paper was reviewed in accordance with the Precast/Prestressed Concrete Institute’s peer-review process.
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A research study was conducted to investigate the quality of welded connections between precast concrete components made under environmental conditions typically encountered in precast concrete construction. The effects of wind, humidity, temperature, and surface moisture on the quality of shielded
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