Verifying Mortar Proportions
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> Proportions
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| Sand additions to the mixer can be verified
with a cubic foot box. |
Mortars contain three ingredients: cement, sand, and water. If
we recognize that the water will be adjusted with every batch to
meet job site conditions, we see that one of the key tasks of batching
mortar properly is accurately and consistently proportioning the
amount of sand relative to cement. This sounds simple, and it can
be. But many factors come into play, and we need to know what they
are.
For instance, sand volume changes with changes in moisture. An
oven-dry sand and a saturated sand each occupy less volume than
sand with moisture in intermediate ranges. At 4% to 8% moisture
content, sand fluffs up or bulks. To aid in correct batching of
sand, sand piles should be maintained in a damp, loose condition.
This might entail covering them to prevent evaporation or even sprinkling
them with a hose in hot or windy conditions.
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| Sand increases in volume at intermediate
moisture contents. |
Furthermore, it is good practice to check the quantity of sand
being added to the mixer by periodically verifying volume proportions
with a know volume measure. A cubic-foot box is one measure that
can be easily constructed.
The Mortar Aggregate Ratio Test
An effective way to check proportions of fresh mortar is called
the mortar aggregate ratio test. It is contained in Annex 4 of ASTM
C 780, Preconstruction and Construction Evaluation of Mortars for
Plain and Reinforced Unit Masonry. Portland Cement Association is
currently sponsoring research into this test method to verify that
the test produces reliable and meaningful results. So far, some
clarifications of the test method have been identified to yield
the most consistent results possible. PCA staff participates in
the American Society for Testing and Materials committee overseeing
the document, is sharing its research results with that group, and
hopes to modify the test method in the near future.
The mortar aggregate ratio is a quick test. It involves sampling
the fresh mortar as mixed in order to calculate how much sand it
contains relative to cement. As long as the individual ingredients
like cement and sand meet their own standards, the main thing to
check to assure mortar quality is its proportions. This is exactly
what the mortar-aggregate ratio does and the process is described
in ASTM C 780. Download article on mortar
testing.
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| Fresh mortar is sampled at the mixer and
placed into jars of alcohol to suspend hydration. |
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Fresh mortar is wet-sieved to determine relative
amounts of sand and cement.
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