2" diameter Aluminum Survey Caps for Rebar
All Berntsen survey caps for rebar are forged from 6000 series aluminum or silicon bronze alloy that won't chip, crack, or break. They feature the SureGripT plastic insulator that locks the cap tight on to the rebar or pipe and helps prevent dissimilar metal corrosion. They feature a deep hex socket that won't "wobble" when tapped on to the rebar.
A good survey cap for rebar or pipe must virtually eliminate dissimilar metal corrosion. What typically happens is the survey cap loosens over a short period of time. This confirms metallurgical theory. Aluminum is less "noble" than steel and an electrical current will actually flow from the aluminum (+) to the steel (-). Corrosion will occur in the anode (+) area of the battery like combination of metals in the presence of moisture (air or soil acting like an electrolyte).
Berntsen originated the SureGripT insulator/wedge which effectively isolates the survey cap from the steel and de-forms around the ribs of the rebar. The hex shape fills with the deformed plastic material and the survey cap is locked on with a super tight compression fit. Great force is required to remove the cap.
FTD5200 Survey Marker Installation Instructions