Field Note: Retempering
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Retempering
Anyone who has worked with mortar knows how it dries out on a board.
Summer’s heat and winds accelerate the drying effect. Retempering,
the practice of adding water to mortar to restore workability, is
considered good practice because it keeps mortar in the right condition
for laying brick, block, and stone.
Retempering is done on the mortar board by the mason, usually by
dribbling water into the mortar pile, then reworking with a trowel
or shovel. This replaces water lost by evaporation. It keeps the
mortar plastic so that units can be laid and positioned in it.
 To
those familiar with concrete, adding water seems like it ought to
be prohibited. It’s true that concrete is not supposed to
have water added to it—except for a one-time adjustment if
the water-cement ratio isn’t exceeded. Producers can hold
back some water to allow field adjusting the mixture. But with mortar,
the approach is different. Mortar will be placed into contact with
absorptive units. As the mortar contacts the units, an immediate
“tug of war” begins for the water: the mortar tries
to hold it and the units try to draw it out. This process works
best when both the mortar and unit are trying to get the water.
It actually improves bond.
The characteristic of mortar that describes its ability to hold
water is water retentivity. The measured property is water retention
(WR). All mortars specified by ASTM C270, Standard Specification
for Mortar for Unit Masonry, are supposed to have a minimum WR of
75%. When mortar loses its water too fast, it becomes stiff and
unworkable. Then units can’t be positioned or adjusted and
bond can be impaired.
The characteristic of units that describes how water is drawn into
them is absorption or initial rate of absorption (IRA). If water
isn’t drawn into the units, good bond may not develop. Balancing
the water equation is essential to getting good results on masonry
projects.
Water additions to mortar are permitted only when time limits have
not been exceeded (beginning from the initial introduction of water).
When water and cement come into contact, there is a delay before
hydration, or chemical combining of cement and water, begins. Masons
use time limits to help guide them on mortar’s acceptability
rather than try to guess if mortar is unworkable because water has
evaporated or cement hydration has occurred. A general recommendation
is to limit a mortar’s useable life to 2-1/2 hours from the
introduction of water, but this can be shorter in hot or dry weather.
Old mortar should be discarded.
Of course, good practice also means mixing mortar quantities to
match the mason’s pace, and preventing evaporation of moisture
from the mortar. This could mean covering the mortar on the board,
working at cooler times of the day, or leaving the mortar in a covered
tub. Although retempering is allowed, it can affect mortar strength
and color, and is not the first or only defense against water loss.
Colored Mortars and Retempering
While a reasonable amount of retempering can be good practice, a
difference of opinion exists on the topic of retempering colored
mortar. Many specifications prohibit this in order to maintain consistent
color. Adding water can lighten the color. However, two contractors
we spoke to believe that retempering colored mortar within reason
can be good practice. Jay Jacob of J. Construction Company, Inc.,
Cincinnati, Ohio, and Jerry Painter, Painter Masonry, Inc., Gainesville,
Florida, weighed in on the subject. They both said that good practice
is necessary to minimize all of the various influences that play
into the resulting mortar color.
When water evaporates from mortar, it does not carry color pigments
with it. The water that is added back during retempering simply
replaces lost water. But it is possible that water could leak off
the board before being mixed in with the mortar. Or pigment could
bleed onto the board. So the first “good practice” step
is to work with a reasonable amount of water and avoid the need
to retemper.
That said, it seems reasonable that adding water to maintain workability
should lead to good placeability and bond. This added water should
be thoroughly remixed into the mortar to get uniform consistency
on the board. As already noted, preferred practice is to prevent
as much evaporation as possible to eliminate the need for retempering.
Abusing the practice of retempering can lead to problems. Manufacturer
recommendations printed on bags of color mortar mixes might have
statements to the effect of “avoid excessive retempering.”
 Other
factors play into the joint color as well, such as temperature and
humidity conditions during construction and time of tooling. To
demonstrate the effect of tooling time on joint color, this masonry
prism was constructed using one batch of mortar. The top mortar
joint was tooled immediately after placement of the units. Remaining
mortar joints were tooled at progressively greater time intervals
and thus stiffer consistency. The relationship between mortar consistency
when tooled and mortar color is quite apparent; earlier
tooling leads to lighter joint color. The higher water
content means that there is a greater water-cement ratio.
Both contractors note that the timing of striking mortar joints
impacts the overall appearance. It affects how much paste is brought
to the surface and how sand is exposed. These factors affect the
texture, which affects how light is reflected from the joint, and
what its apparent color is. Once the mortar is in place, color differences
can be quite noticeable on newly constructed masonry. The contractors
note that cleaning and sealing of the masonry can impact the wall’s
appearance. As time goes on, they say color differences may even
out.
 Mortar
accounts for a large part of a wall surface. Colored mortars, in
particular, can have a dramatic impact on masonry’s appearance.
To get the best results, both plain and colored mortars should remain
plastic and workable while being used to lay units. During hot or
dry weather, good practice becomes even more important, and that
means preventing evaporation of water from mortar, and perhaps,
retempering to maintain workability.
Additional References on retempering mortar from PCA:
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