Washington (CNN) President Donald Trump on Tuesday will visit prototypes for his long-promised border wall, an opportunity to see in person what options his administration has tested for future construction -- if Congress approves any money to proceed.

But experts say Trump shouldn't appear to choose any prototype as his favorite, for fear of seeming to influence the rigid procurement process.

Customs and Border Protection picked eight proposals for construction along the San Diego border, which were finished in October and have been tested in a real-world border environment for evaluation since.

The eight prototypes fall into two categories -- four are designed to be 30-foot concrete walls, and four are made of other materials.

Photos: Prototypes for border wall construction This $344,000 prototype was built by Caddell Construction Co. of Montgomery, Alabama. Customs and Border Protection is evaluating eight potential barriers in San Diego and may use characteristics of them in future construction along the border. Hide Caption 1 of 8 Photos: Prototypes for border wall construction This $453,538 prototype was built by W.G. Yates & Sons of Philadelphia, Mississippi. Customs and Border Protection is evaluating eight potential barriers in San Diego and may use characteristics of them in future construction along the border. Hide Caption 2 of 8 Photos: Prototypes for border wall construction This $458,103 prototype was built by W.G. Yates & Sons of Philadelphia, Mississippi. Customs and Border Protection is evaluating eight potential barriers in San Diego and may use characteristics of them in future construction along the border. Hide Caption 3 of 8 Photos: Prototypes for border wall construction This $320,000 prototype was built by Caddell Construction Co. of Montgomery, Alabama. Customs and Border Protection is evaluating eight potential barriers in San Diego and may use characteristics of them in future construction along the border. Hide Caption 4 of 8 Photos: Prototypes for border wall construction This $365,000 prototype was built by Fisher Sand & Gravel of Tempe, Arizona. Customs and Border Protection is evaluating eight potential barriers in San Diego and may use characteristics of them in future construction along the border. Hide Caption 5 of 8 Photos: Prototypes for border wall construction This $486,411 prototype was built by KWR Construction of Sierra Vista, Arizona. Customs and Border Protection is evaluating eight potential barriers in San Diego and may use characteristics of them in future construction along the border. Hide Caption 6 of 8 Photos: Prototypes for border wall construction This $406,319 prototype was built by ELTA North America of Annapolis Junction, Maryland. Customs and Border Protection is evaluating eight potential barriers in San Diego and may use characteristics of them in future construction along the border. Hide Caption 7 of 8 Photos: Prototypes for border wall construction This $470,000 prototype was built by Texas Sterling Construction Co. of Houston, Texas. Customs and Border Protection is evaluating eight potential barriers in San Diego and may use characteristics of them in future construction along the border. Hide Caption 8 of 8

After construction was completed, the prototypes were tested on several fronts, including on withstanding breaches, climbing, digging, their ability to slow down and stop border traffic, and whether they are safe for Border Patrol agents. For example, officials in the past have said border barriers ideally allow agents to see what's happening on the other side, so adversaries can't hide in the shadow of a wall.

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