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ASTM International · Concrete · Laboratory test
ASTM C1231: Use of Unbonded Caps in Determination of Compressive Strength of Hardened Cylindrical Concrete Specimens
The practice that qualifies neoprene pad caps for cylinder breaks, replacing sulfur capping for most strength ranges while keeping results comparable to ASTM C39.
| Designation | ASTM C1231 |
| Setting | Laboratory |
| Service line | Construction Materials Testing |
Testing is performed to the current edition of ASTM C1231 referenced by your project specification. This page is a plain-language overview, not a substitute for the published standard.
What ASTM C1231 covers
ASTM C1231 is the practice for capping concrete cylinders with unbonded neoprene pads held in steel retainers, instead of bonded sulfur or gypsum caps. Cylinder ends are never perfectly flat or square, and a cap is what spreads the machine load evenly across the specimen. Pad caps do that job faster, without the fumes and hot sulfur of the older method, and most compressive strength testing under ASTM C39 today uses them.
The practice matters because a bad cap produces a bad break. Load concentrated on a high spot fails the cylinder early and the reported strength is low.
What the practice requires
Pads have a specified Shore A durometer hardness chosen for the expected strength range, and each hardness has a maximum number of uses before it is retired. Retainers must fit the cylinder diameter closely and be checked for wear. Cylinder ends must meet a perpendicularity and planeness tolerance, and a cylinder with an end outside tolerance is ground or sulfur capped instead.
Above about 12,000 psi, and for any pad-and-strength combination not already qualified, the practice requires a correlation study showing pad-capped strengths equal to or above bonded-cap strengths before pads can be used for acceptance.
Why it matters to your results
Worn pads, cracked retainers, or pads used beyond their rated count lower the apparent strength of otherwise sound concrete. Because the effect always runs in one direction, it shows up as unexplained low breaks and expensive investigation. A laboratory that tracks pad use counts and retainer condition is protecting your acceptance data.
Common questions about ASTM C1231
Are pad caps as accurate as sulfur caps?
Within the qualified strength range, yes. The practice exists to define that range and the pad hardness that goes with it.
Do you still use sulfur capping?
For cylinders with damaged ends, very high strength concrete, and where a project specification requires it. Both methods are available in our laboratory.
How do I know the pads were within their use limit?
Use counts are logged per pad set. Ask for the record with your break report if it is ever in question.
How we apply ASTM C1231
Our laboratory maintains pad sets by durometer with logged use counts, inspects retainers on a schedule, and checks cylinder end condition before every break. Cylinders outside end tolerance are ground or capped so the break reflects the concrete, not the specimen.
Scheduling & proposals
Need ASTM C1231 testing?
Call for same-day dispatch questions, or send project documents for a written proposal.