Testing - Concrete Mix Design, Quality Control and Specification - page 143

Figure 7.1 Rubber cap and restraining ring.
A significant improvement is that of rubber caps. The rubber cap system
(consisting of a restraining cap and a rubber pad insert to accommodate
irregularities) is a practical and cost-effective alternative to sulfur capping,
which can be used by any laboratory. A suitable side clearance is essential
since, under the high pressure, the rubber behaves almost like a fluid. If
the clearance is too great, the neoprene will be extruded and will provide
excessive side restraint. The mold is illustrated in Figure 7.1. A study by
Carrasquillo and Carrasquillo (1988) showed that for concrete strengths
up to 75 MPa, the use of neoprene inserts with steel restraining caps yielded
average test results within 3% of cylinders using sulfur caps. However, the
use of a neoprene capping system became a problem at higher strength.
A relatively recent development that could be very important is a new
capping technique called the “sand box”, although Day has heard no more
of this development since including it in the previous edition of this topic.
The test was developed by Claude Bouley and Francois de Larrard, and was
reported in Concrete International (Bouley and de Larrard, 1982). The
“box” in question is a circular cup, very similar in appearance and function
to the restraining ring used in the rubber cap test but deeper (30 mm). The
rest of the apparatus is a positioning frame and guide similar to that used
in sulfur capping, except that a small, air-driven vibrator is incorporated.
The technique is to place a 10 mm layer of dry sand in the cup, position the
cylinder in the frame, and vibrate so that the cylinder compacts the sand
(20 seconds). The cylinder is then sealed into the cup by filling around the
periphery with molten paraffin wax.
The test may initially look unattractive compared to sulfur or rubber
caps, since it involves a capping process with molten material, a vibrator,
and does not permit reuse of the mold before testing. However, it does