The percentage gain in compressive strength was highest for concretes made with the 5 pct dicyclopentadiene modified sulfur blends, and lowest for the unmodified sulfur concretes.

Sulfur concretes made with modified sulfur and volcanic rock had compressive strengths equal to or less than those made with unmodified sulfur. With limestone rock, the compressive strengths of modified sulfur concretes were equal to or greater than those made with unmodified sulfur. These results indicate that there is a better bonding between sulfur concretes made with limestone aggregate than those made with volcanic type rocks.

In summary, the aging tests in dry and humid conditions have demonstrated the vulnerability of unmodified sulfur concretes to loss of compressive strength and to disintegration on weathering in moist atmospheres. It also demonstrated that the use of modified sulfur prevents compressive strength

Less modified sulfur than unmodified sulfur was required as the binding agent to give workable mixes. The cost of the modifier could be offset at least partially by the lower sulfur requirement. The modified sulfur concretes did not develop their maximum compressive strengths as rapidly as unmodified sulfur based on comparison of their initial and 28-day strengths.

Flexural Strength of Sulfur Concrete

A series of tests was made on both the sulfur concrete and modified sulfur concrete using the 10 aggregate blends listed in table 3 to determine their modulus of rupture. Their flexural strengths were determined in accordance with ASTM Method C 78-64 "Flexural Strength of Concrete (Using Simple Beam With Third-Point Loading)" using either 3- by 4- by 16- or 6- by 6- by 30-inch bars of the materials. The modulus of rupture values obtained along with their percentage of the compressive strength values are given in table 8. In general the flexural strengths of the unmodified sulfur concretes compared to their compressive strengths were in the range of what would be expected with regular Portland concretes. The flexural strengths of most of the modified concretes were greater than those of comparable unmodified concretes. The most notable examples of this were the sulfur concretes prepared using 5 pct dicyclopentadiene modified sulfur. These products had flexural strengths of approximately 20 pct of their compressive strengths which was double the values for unmodified concretes. The development of higher flexural strengths in sulfur concretes is important in making sulfur concretes more desirable for structural usage where greater flexural strength is more important than higher compressive strengths.

1

1Wt-pct of sulfur in the blends. The sulfur was either unmodified
or contained the amounts of dicyclopentadiene (DCPD) and
dipentene (DP) as shown.

Modulus of Elasticity

The modulus of elasticity of modified and unmodified sulfur concretes composed of sulfur and 1 to 1 desert sand and volcanic rock as well as sulfur and 1 to 1 desert sand and limestone rock was determined using a Baldwin Universal Test Machine. The modulus values were calculated from strain gage measurements and loads taken from the testing machine when loading the specimens in compression to approximately 50 pct of their ultimate strengths. Specimens, 3 inches in diameter by 6 inches high, were used as shown in figure 6 with the strain gages attached. The modulus values obtained are

FIGURE 6. Sulfur concrete cylinders for modulus of elasticity testing.

listed in table 9 and are comparable to published values for Portland cement concretes which normally range from 2 to 6 x 10 psi.

The stress-strain relationship between unmodified and modified sulfur concretes is quite similar. The materials did not undergo significant yielding before failure. The stress-strain curves for the five types of sulfur concrete are shown in figure 7. Specimen number 5 did not fail within the load capacity (60,000 lb) of the test machine.

FIGURE 7.- Stress-strain curves for sulfur concretes.

Freeze-Thaw Testing of
Sulfur Concretes

The ability of sulfur
concrete to withstand
freezing and thawing con-
ditions has been measured by
various methods. Beaudoin
and Sereda used a freezing
cycle of 6 hours in air at
0° F and a thawing cycle of
6 hours in water at 45° F in
testing a large number of
concretes made with sulfur
and sands (3). They also
tested some samples by
freezing in air to minus
10° F for 16 hours and

thawing for 8 hours at 70° F.
Malhotra used eight freeze-
thaw cycles per day in

accordance to ASTM

Method C 661-71 in testing
sulfur concretes (18).

He

30 suggested additional work be
done on slow freeze-thaw
cycling in testing sulfur
deleterious to sulfur
concretes, because of the

question on whether rapid freeze-thaw cycling is

concretes and does not represent freeze-thaw conditions found in nature.

Two methods of freeze-thaw testing have been used in this investigation. The first utilizes six cycles per day in a water-ice media at 40° to 0° F in accordance with ASTM Method C 215-60 "Test for Resistance of Concrete

Specimens to Rapid Freezing and Thawing in Water."

The second method was

Testing of both

Results

identical except only one freeze-thaw cycle per day was made.
modified and unmodified sulfur concretes are still in progress.
obtained so far show that sulfur concretes made with aggregate blends 1 to 6
all had poor freeze-thaw properties. The vesicular volcanic rock with desert
sand and sulfur had the lowest resistance to the freeze-thaw cycling used.
Desert blow sand and sulfur mixtures were not as resistant as those of con-

struction sand and sulfur.

blends improved their durability under freeze-thaw conditions. Using these blends and modified sulfur, the maximum number of freeze-thaw cycles before failure was 60 cycles.

Modified sulfur used as the binder for these

Blends of desert sand or construction sand with dense limestone aggregate

The

(blends 7 to 10) gave concretes with improved freeze-thaw properties.
best freeze-thaw resistance was for blend 9 with 26 pct sulfur; blend 9 with
23 pct sulfur modified with 5 pct dicyclopentadiene; and blend 10 with 23 pct
sulfur modified with 5 pct dicyclopentadiene. Using six freeze-thaw cycles

per day, these three blends went 165 cycles before failure. Results of testing using one freeze-thaw cycle per day in water gave similar results. In general, the use of modified sulfur as the binder improved the freeze-thaw properties of sulfur concretes made with aggregate blends 7 to 10. Blend 9, composed of construction sand and limestone rock, had the greatest durability of the unmodified sulfur concretes.

LARGER SCALE AND FIELD TESTING

Preparation of Sulfur Concrete Beams

As a means of determining potential problems when multiple pours of the sulfur concrete are made in practice, beams 6 by 6 by 30 inches were cast in one and four lifts. Aggregate blend 8 composed of 2 to 1 desert blow sand and 3/8-inch limestone rock was used with sulfur and modified sulfur as the binders. The beams poured with four lifts were allowed to solidify between additions of the next batch of sulfur concrete. Flexural strength determinations and compressive strength measurements were made on the beams, after aging 1 month, in accordance with ASTM Methods C 78-64 and C 116-49, respectively. The results listed in table 10 are the average of two determinations. Beams from test number 2 made with multiple lifts of unmodified sulfur concrete showed an indication of layering at the pour junctions on the fractured specimens. This was similiar to the results reported by Loov (15). There was no indication of layering on the fractured surface of the beams from tests 4 and 6. The unmodified sulfur concrete beams poured in multiple lifts had slightly lower modulus of rupture values than the beams made with one pour. Slightly higher flexural strengths were obtained with multilift beams made with modified sulfur compared to single lift beams. Higher flexural strengths were obtained with the beams made with the 5 pct dicyclopentadiene modified sulfur.

1 The wt-pct of dicyclopentadiene used in modifying the sulfur is
2 and 5 DC PD.

2 Compressive strength values: S side loading; T = top loading.

Based on the results obtained in these tests, there should be no problems in making multiple pours of modified sulfur concretes. Unmodified sulfur concrete beams had a tendency to layer at the junction of the lifts and de lamination might be a problem.