Testing - Concrete Mix Design, Quality Control and Specification - page 142

The essential items are
1. A heavy, accurately machined steel mold into which to pour the sulfur
mixture
2. A guide along which to slide the cylinder to ensure the cap will be
perpendicular
3. A thermostatically controlled melting pot in which to heat the sulfur
mixture
4. A scoop holding an exactly suitable amount of the mixture to produce
a cap
There are a number of difficulties to be overcome by the uninitiated:
1. Neat (undiluted) sulfur is not suitable because it shrinks too much and
sets too quickly. A mixture with finely ground silica, fly ash, or other
inert material should be used. Proportions are trial and error, depend-
ing on the particular sulfur and the particular filler. Some like to
include a proportion of carbon black. Commercial blends are available.
2. The temperature of the mixture must be just right; too cool and it will
not flow and set too quickly giving a thick cap, too hot and it goes
rubbery and shrinks too much. Again it is trial and error.
3. The first cap is difficult because the mold is cold; later the mold gets
too hot and causes delay waiting for setting.
4. The mold must be very lightly oiled between each use.
5. The cap must be thin, preferably less than 3 mm.
6. Especially for high strength concrete, a sulfur cap will not overcome a
rough cylinder end. The cap will exhibit slight plastic flow under load
and allow load concentration on high spots.
7. The hot sulfur emits fumes and requires at least an exhaust fan and
preferably a fume hood.
All the above makes it quite clear why users of cubes are not tempted to
turn to cylinders but has no bearing on the question of which is the more
reliable test.
According to the U.S. Federal Highway Administration (FHWA-RD-97-
030), traditional sulfur capping is suitable for concrete strength up to about
50 MPa. However, Lessard et al. (1993) found that a capping compound with
a mini-cube strength of 55 to 62 MPa gave comparable results to grinding
when used for testing concrete strength up to 120 MPa. However, the cap-
ping layer was less than 2 mm thick, which ensured adequate confinement
of the capping compound. Under these conditions, the confined strength of
the capping material is two to three times the unconfined strength. Lessard
et al. suggested higher strength capping materials, and specialised prepara-
tion would only be required when testing strengths greater than 130 MPa.