5.2 Retainer Rings & Equipment Dimensions
Key Takeaways
- Retaining rings are metal fixtures that constrain the neoprene pad, preventing lateral extrusion and ensuring uniform compressive stress transfer.
- The inside diameter of the retaining ring must be between 102% and 107% of the cylinder's actual diameter to accommodate slight variances in specimen size.
- The minimum thickness of the retaining ring wall and baseplate is 12 mm [0.47 in.] for specimens larger than 100 mm [4 in.] in diameter (such as standard 6x12 inch cylinders).
- The bottom surface of the retainer baseplate must be flat to within a tolerance of 0.05 mm [0.002 in.] to ensure uniform contact with the testing machine's bearing blocks.
- Depressions or gouges on the inside bearing surfaces of the retainers must not exceed 0.25 mm [0.010 in.] in depth or 32 mm² [0.05 in.²] in surface area.
Function of Retaining Rings in Unbonded Capping
Retaining rings (often referred to simply as retainers) are critical components of the unbonded capping system. They are circular metal cups designed to house the elastomeric pads and align the concrete cylinder during compression testing. The primary engineering function of the retainer is to provide lateral restraint to the elastoprene pad. Under high compressive loads, neoprene behaves like a highly viscous fluid, attempting to flow laterally. If the pad is not restricted, it will expand outwards (extrude), causing two major issues. First, the thickness of the pad under the cylinder will rapidly decrease, preventing it from cushioning the end irregularities of the specimen. Second, the lateral expansion of the pad will generate a radial tensile force on the end of the concrete cylinder. Concrete is notoriously weak in tension (typically only 10% of its compressive strength), meaning this induced radial tension would cause the cylinder to split or fail prematurely at a load significantly lower than its true compressive strength.
By confining the pad within a rigid metal ring, the retainer prevents lateral extrusion. This forces the pad to compress vertically and distribute the load uniformly across the end of the cylinder, simulating a true uniaxial compressive state. The design of the retainer must ensure that the metal itself does not undergo plastic deformation or bending under these high lateral forces.
Inside Diameter Tolerances and Fit
The fit between the concrete cylinder and the retaining ring is critical. If the ring is too tight, the cylinder end may bind against the metal wall, causing direct metal-to-concrete contact and invalidating the test. If the ring is too loose, the neoprene pad will extrude into the gap between the cylinder and the retainer wall, resulting in rapid pad destruction and uneven loading.
ASTM C1231 specifies that the inside diameter of the retaining ring must be at least 102% and no greater than 107% of the nominal or actual diameter of the concrete cylinder. This range allows for minor variations in the diameter of molded concrete cylinders while maintaining sufficient confinement.
The table below shows the calculated inside diameter ranges for standard cylinder sizes:
Inside Diameter Limits for Standard Retaining Rings
| Nominal Cylinder Diameter | Nominal Cylinder Diameter (mm) | Minimum Inside Diameter (102%) | Maximum Inside Diameter (107%) |
|---|---|---|---|
| 3 inches | 76 mm | 3.06 in. [78 mm] | 3.21 in. [82 mm] |
| 4 inches | 102 mm | 4.08 in. [104 mm] | 4.28 in. [109 mm] |
| 6 inches | 152 mm | 6.12 in. [155 mm] | 6.42 in. [163 mm] |
Technicians must measure new retaining rings upon delivery to verify they fall within these limits. If the inside diameter exceeds 107% of the cylinder diameter, the retainer must not be used.
Retainer Thickness and Structural Rigidity
Because retainers must withstand immense lateral pressures during testing (up to several thousand pounds per square inch of lateral pressure exerted by the flowing neoprene), they must be constructed with sufficient wall and baseplate thickness to prevent bending or warping. ASTM C1231 establishes two sets of thickness requirements based on specimen size:
Retainer Wall and Baseplate Thickness Table
| Specimen Diameter | Minimum Retaining Ring Wall Thickness | Minimum Baseplate Thickness | Common Specimen Examples |
|---|---|---|---|
| 100 mm [4 in.] or less | 9 mm [0.35 in.] | 8 mm [0.30 in.] | 3x6 in. and 4x8 in. Cylinders |
| Greater than 100 mm [4 in.] | 12 mm [0.47 in.] | 12 mm [0.47 in.] | 6x12 in. Cylinders |
These components are typically machined from high-strength steel or a structural aluminum alloy. Steel is preferred due to its superior durability and resistance to wear from repeated contact with concrete edges. The baseplate and the retaining ring are usually welded or machined from a single piece of steel to ensure maximum structural integrity and prevent leakage of the pad.
Planeness and Surface Tolerances
The base of the retainer must transmit the load from the testing machine's bearing blocks to the neoprene pad without introducing any eccentricities. Consequently, the bottom surface of the retainer baseplate (the surface in direct contact with the machine's platens) must be extremely flat. The bottom surface of the baseplate must be plane to within 0.05 mm [0.002 in.]. A baseplate that is warped or out of plane will wobble or point-load the testing machine's platens, causing asymmetric stress distribution in the concrete specimen. This planeness must be checked periodically with a precision straightedge and feeler gauge. Additionally, the inside surface of the retainer that contacts the neoprene pad must also be flat and smooth. It must not contain gouges, grooves, protrusions, or indentations greater than 0.25 mm [0.010 in.] in depth, or greater than 32 mm² [0.05 in.²] in surface area. Minor imperfections can cause the pad to wear prematurely or pinch, leading to localized pad failure.
Centering and Perpendicularity
For a valid test, the cylinder must be aligned along the vertical axis of the testing machine. The cylinder must be placed in the center of the retainer, and both the upper and lower retainers must be centered on the testing machine's platens. The center of the retaining rings must align within 1/16 inch [2 mm] of the center of the spherically seated bearing block. Furthermore, the ends of the cylinder must be perpendicular to the longitudinal axis of the cylinder. While the unbonded capping pad can accommodate minor end slope, ASTM C39 and C1231 state that the ends of the specimen must not depart from perpendicularity by more than 0.5 degrees (approximately 1/8 inch in 12 inches, or 1 mm in 100 mm). If a cylinder's ends exceed this slope, it must be saw-cut or ground before testing, as the unbonded cap cannot safely redistribute such severe slopes without sliding or causing eccentric loading.
What is the permissible range for the inside diameter of a steel retaining ring relative to the nominal diameter of the concrete cylinder specimen?
For standard 6-inch (150 mm) diameter cylinders, what is the minimum required thickness for both the retaining ring wall and the retainer baseplate?
The surface of the retainer baseplate that contacts the bearing block of the testing machine must be plane to within what tolerance?