Studies of Certain Sierran Concrete Aggregates
Creators: name: Pray, Lloyd Charles
Publisher: name: California Institute of Technology
Year: 1943
Subjects: subject: Geology
subject: FOS: Earth and related environmental sciences

description: The purpose of this investigation has been to understand
more fully the behavior of certain Sierran rocks when used as
concrete aggregate. The granodioritic aggregate used in the
construction of the Florence Lake Dam was selected for detailed
examination.

Two lines of attack to this problem have been followed.
The aggregate was quantitatively separated into rock types,
and these were then studied petrographically to determine the
mineralogical composition, with particular emphasis being
placed on the alteration products present in the rocks. The
other phase of the investigation was the separation of the
aggregate into its major mineral constitutents, in order to
test the behavior of these isolated constituents as aggregate
in mortar bars. Since the length changes of the mortar bars
are to be observed over a two year period, only preliminary
results can be reported at this time.

On the basis of the known behavior of minerals, the only
constituent present in the granodioritic aggregate which might
be suspected of causing trouble is a zeolite, a hydrous sodium
and/or calcium aluminum silicate. The peculiar property of
base exchange whereby the sodium or calcium content of the
molecule may be easily exchanged for the other element is
worthy of attention in a problem as intimately connected with
alkali content as are many concrete failures. The bulk
percentage of this hydration product of plagioclase feldspar is
certainly not more than several per cent of the aggregate.

To separate the mineral constitutents of the Florence
Lake Dam aggregate, about 500 pounds of fresh appearing rock
was milled. Combinations of the separatory processes of sizing,
tabling, magnetic concentration, and heavy liquid concentration
yielded the following mineral concentrates:

Potash feldspar - 97 per cent purity

Quartz - plagioclase - 53 and 43 per cent respectively, yielding
a combined purity of 96 per cent

Biotite - 96 per cent purity

Hornblende - 98 per cent purity

Magnetite - 98 per cent purity

Sphene, spatite, and zircon - 89, 5, and 4 per cent respectively,
yielding a combined purity of 98 per cent

These concentrates, mixed with different amounts of inert
quartz, were used as aggregate for mortar test bars. Each
aggregate mixture was fabricated with high and low alkali
cements, and triplicate bars made for each condition. The mortar
test bars are as follows:

Refer to PDF for table

All bars are curing at a temperature of 100° F. in a sealed
chamber containing moisture. Length measurements to 0.001 mm.
were made at weekly intervals for the first month, and then
the interval increased to one month. In June 1943 the bars
range in age from one to four months.

So far, none of the bars made from granodioritic materials
has shown excessive expansion, or even significant expansion.
The highest expansion so far with these materials is about 0.04
per cent, as contrasted to an expansion of 0.22 per cent for
the high alkali cement and siliceous magnesian limestone
aggregate. Graphs have been made recording all expansions of the 
200 bars up to June 3, 1943.

RECOMMENDATIONS

1. The length measurement program should be continued as
planned for the full two years. The monthly measurement
interval might be increased to two months if all the
expansion-age curves flatten out in time.

2. If possible, restrict the usage of granodioritic aggregates
in the Big Creek area until the results of the current
testing program are known. If necessary to use any of these aggregates,
select only the freshest rock types, and reject the fines resulting
from crushing the aggregate.

3. The behavior of the zeolite found in minor quantities at 
the Florence Lake Dam aggregate stock pile should be tested
by using it as aggregate for mortar test bars. The only
way to achieve a concentration of this mineral is by hand picking
some of the remaining aggregate for rock fragments
containing veinlets of this light salmon colored mineral.
One of these fragments is submitted with this report. Careful
chipping of the rock fragments should yield a concentrate
with at least 10 per cent of the zeolite. Crush this 10 - 10 mesh and mix with different
amounts of inert quartz to yield three aggregate mixtures with a zeolite content of
1, 5, and 10 per cent. Fabricate each aggregate mixture
with a high and a low alkali cement.

Should the zeolite prove to be the deleterious constituent
of the aggregate, a quantitative determination of this
mineral in future aggregate would be invaluable. The base exchange
property of zeolites is utilized in water softening processes,
and it might be possible to at least semi-quantitatively soften
water and obtain rapid determinations of the zeolite content
of a crushed rock sample.

4. Should any of the bars show excessive expansion, 
petrographic investigations should be made with a view to determining
the mechanism of the reaction involved.