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Fig. 3. Pad outlets. Weld "D" is the same as "A," except penetration is not required. Drill two -in. diameter holes, 180O apart, through weld for vent. Weld "E" plate must be bevelled distance equal to thickness of tank wall. Weld is to be built up above the surface of plate, peened and ground smooth and flush with plate. those enumerated for carbon steel and Portland cement concrete. Polymer Concretes Polymer concretes are a generation of Fig. 4. Corner of rectangular tank. Weld "F" should be burned into plate so welds meet from opposite sides, thereby excluding air pockets. Welds must be peened and ground flush. materials that have rapidly matured because of their outstanding chemical resistance and physical properties. They are not to be confused with Portland cement concrete or polymer-modified Portland cement concrete. The only similarity to Portland cement concrete or polymer-modified Portland cement concrete is the use of properly graded and sized aggregate in order to opti- mize workability and physical proper- ties of the composition. Polymer concretes utilize inert siliceous aggregates with binding systems based on such resins as furan, epoxy, polyester, vinyl ester, and acrylic. (See Table I for typical physical properties of polymer concretes.) The advantages to be derived from polymer-modified Portland cement concrete when compared with Portland cement concrete are: 1. Permits placement of concrete in thinner cross-sections. 2. Excellent bonding to existing concrete substrates. 3. Increased impact resistance. 4. Reduced porosity. 5. Faster set and cure. 6. Improved resistance to salt. It does not improve resistance to chemicals. Polymer modifiers are generally based on various resins and latexes, such as natural rubber, styrene-buta- diene, acrylic, polyvinyl acetate, epoxy, and urethane. 624 Fig. 5. Butt joint. Weld "G" to be laid in V in beads not exceeding -in. deep. After "G" is built up above plate on outside, the inner surface must be gouged out sufficiently to remove all scale and slag. Weld "H" is to be built up above the surface of plate, peened and ground flush.