CURING AND PROTECTION OF O&M CONCRETE

Curing

Quite a little has been said about the damage of excess water
to concrete in mixing, placing, and finishing. As soon as concrete
hardens, the reverse is true. The more water in the concrete after
hardening, the better. This a another way of saying that it should not
be permitted to dry, internally or externally, In fact, it is better for
it to have a little extra water on the inside,

This strange reversal is because cement must have water to
become a hard substance capable of binding sand and gravel together.
But the more water cement has before it sets, the weaker the binding
of sand and gravel will be; and when cement has less water after it
sets, the binding will be still poorer, Whatever the water-cement mix
is to begin with, if it dries out soon after the concrete sets, and be-
fore the possible strength of that mix develops, its strength and service-
ability will be less than they would have been had it been kept wet.

So, it is important to see that new concrete does not dry out,
Drying is prevented when concrete surfaces are kept continually wet
for several days, preferably at least a week. In warm or moderate
weather, forms should be removed as soon as possible without damag-
ing the concrete, Forms do not provide as good curing as water or
damp backfill, In cold weather, when drying is slow, the protection
wood forms give from freezing may more than offset their shortcomings
as a method of curing,

A very practical and convenient means of curing is available
in the white-pigmented sealing compounds, When applied correctly,
these seal in enough of the original moisture to do a better-than-aver-
age job of curing, Unformed surfaces should be sprayed as soon as
they have become dull in appearance after finishing, Formed surfaces
should be kept damp, but not wet when sealed. The application is ade-
quate when the surface is completely covered and evenly white.

Several manufacturers supply portable equipment for the ap-
plication of curing compounds, For small work a "'back-pack" type
pressure tank similar to that used in weed spraying is adequate.

Protection

In warm weather, probably the main precaution on O&M con-
crete work is to make sure that newly placed concrete is not allowed
to become dry for at least several days. Any precautions that can be
taken to keep concrete temperatures down are beneficial. At lower
temperatures, less mixing water is required, strengths are better,
and shrinkage is less when the concrete later cools in winter. It helps
when possible, to keep materials in the shade; to cool them by sprin-
kling; to use cold water, maybe ice water in very hot locations; and to
avoid working in the heat of the day.

19


PLACING O&M CONCRETE

Placing

On the job, one usually hears of this as "pouring concrete".
There is no objection to this common expression except that it is
strongly suggestive of a mix that is far wetter and more fluid than it
should be to make good concrete. The fluid idea also suggests flow,
or flowing concrete from one location to ancther, which is contrary
to good practice. Flowing is objectionable because it destroys uni-
formity; the rocks stay behind; water and fines flow ahead filling the
corner, ends of walls and other parts which would then be the first to
fail in durability because they are composed of inferior concrete.

So, it is best to place concrete", whether we call it that or
not, Concrete for each part of a structure should be placed where it
is to remain. It should be a non-fluid but workable consistency that
will stay where it is placed, and should be placed directly in ends of
walls and corners of forms. Any unavoidable flow should be from
those parts rather than toward them.

rere
Fea

T
i
t
it
:

Bhi

° pea Soret

CONSISTENCY OF CONCRETE

2 F
PLACING IN DEEP NARROW IY GSSE: RABROS RORMS
WALL THRU PORT IN FORM

FIGURE 105- CONCRETE WILL SEPARATE SERIOUSLY UNLESS INTRO!

\NTO THE FORMS CORRECTLY

A fluid mix that really would be "'poured"' is sure to be much
weaker in strength, less durable, and certain to crack more when it
dries cut. Best concrete is a workable, mushy, plastic mixture that
will not flow unless placed on a steep slope or in a chute ona 2 tol
slope. Even stiffer consistencies will flow during vibration unless con-
crete is placed on a fairly level layer several feet ahead of where it is
vibrated,

Several figures have been borrowed from the Bureau's Con-
crete Manual, including the one above, to illustrate how easily well

13


sulfate is its bitter un-
pleasant taste. If sulfate
is present, there is prob-
ably evidence of its cor-
rosive attack on some
project concrete; bottoms
of drops and tops of out-
let wing walls are often
severely attacked and de-
teriorated by the alkali
sulfates.

For concrete that will
be exposed to such corro-
sive attack, best durabil-
ity can be assured only
with Type V, sulfate-
resisting cement. This
cement will cost a pre-
mium of as much as 30
cents a sack and then
sometimes is not readily
available. Calcium chlo-
ride should not be usedin

concrete which will be ex-

posed to alkali sulfate.

For other exposure
and ordinary service con-
ditions, any good stand-
ard cement is usually
capable of making a good
strong, serviceable con-
crete. Cement should be
in good condition, free of lumps and foreign matter. For larger work re-
quiring more than 100 sacks, it is safest to purchase cement under stand-
ard Bureau specifications, which are readily available. This is particu-
larly advisable if Type V, sulfate-resisting cement is to be used.

Purchase of cement under Bureau specifications also has the ad-
vantage of insuring that a cement containing a desirably low amount of
alkalies is obtained when this is necessary to assure good durability with
certain aggregates. Any cement may be high or low in alkali and this has
nothing to do with its resistance to attack by alkali sulfates in soil or
ground water. Many sands and gravels in the western part of the United
States contain elements which cause concrete to swell inside and produce
severe map-type cracking on the surface as shown in the photograph on
the following page. This is particularly true when the cement contains
more than 0.6 percent of alkalies, For this reason it is safest to use a
"low-alkali" cement, unless the aggregates are known to be free of these


In cold weather new concrete must not be allowed to freeze.
In fall and spring, during weather that may freeze at night, but when
day and night high and low temperatures average above 40 degrees F,
no special precautions are needed except to make sure that the con-
crete does not freeze either of the first 2 nights after it is placed.
Wood forms are protection during such weather and bare surfaces can
be protected with light housing or canvas cover if it is not in contact
with the concrete and forms a shallow air space over it.

In weather colder than an average of 40 degrees F, several
precautions must be taken to be sure concrete will not be damaged by
freezing at the start, or have poor durability later, because it was not
protected long enough or well enough to begin with. In weather this
cold, air-entrained concrete should be kept at a temperature not less
than 50 degrees F. for 3 days and protected from freezing for the next
3 days, Without entrained air, the same durability cannot be attained
in any weather, but for proportionally as good durability without it in
cold weather, all 6 days of protection should be at 50 degrees F,

Additional maturity and strength is attained by the end of the
protection period when 1 percent of calcium chloride by weight of ce-
ment is used as mentioned earlier, This is advised during the colder
weather work except when the concrete must resist alkali sulfate attack.
As calcium chloride detracts from resistance of concrete to sulfate,
extra cement should be used instead, It not only provides extra early
strength but considerably increases resistance to sulfate,

The easiest way to hold the 50 degree F. temperature depends
on the shape and size of the work, It should not be attempted by mixing
the concrete at high temperature, This should be less than 70 degrees
F, Extensive flat work is best protected by reusable insulating mats,
thereby taking advantage of heat generated during setting and hardening
to keep the concrete warm enough, as long as the heat is conserved.
Formed concrete can be enclosed with heaters inside, If forms are to
be used repeatedly, they can be insulated with batts between the studs
so that no other protection is needed,

Occasional checking with a thermometer is advisable to be
sure necessary temperatures are being maintained, It should be re-
membered that corners and edges are the most vulnerable to freezing.
Covers should be held up off them. Extra insulation should be provided
at these places.

Usually hot mixing water is sufficient for heating materials to
secure concrete temperatures between 60 and 70 degrees F. However,
ice and frozen lumps in sand or gravel should be reduced to the point
where they do not exist in the concrete after mixing. Before concrete
is placed, frost should be removed from subgrade, forms and reinforcing
bars, If frost is not too deep, subgrade can be thawed by heat from the
earth below, by covering it for a few days with insulating blankets or a
12-inch layer of straw. Forms and bars can be defrosted by covering

and heating.
ORK OK OK

20


Also note that the screed is not being vibrated. To vibrate the screed
causes the concrete to flow under the screed and result ina wavy surface

7


improve ordinary concrete mixes is to add another sack of cement per
cubic yard. This is particularly true of O&M concrete and why it is
recommended that no less than 6 sacks of cement be used in each yard
of O&M concrete, or an equivalent amount of cement in each batch of
concrete,

With cement content decided, the concrete material next most
important to concrete quality is water; that is, the amount of water in
the mix with the cement, The more water used, the less strength and
durability the concrete will have; the less water, the better the concrete
quality, as long as the mix is workable enough to permit a good solid
job of molding it in place in the structure.

Several things affect the amount of water that will have to be
used in the mix to make workable fresh concrete:

1. More sand in the mix than necessary increases the amount
of mixing water. The correct amount of sand is best found by trial,
If a mix appears to have an abundant portion of sand, each succeed+
ing batch should be proportioned with less and less sand by some
fixed amount, until a proportion is arrived at which clearly cannot
be reduced without endangering sufficient workability. With each
reduction in sand in this procedure, gravel should be increased by
an equal amount to keep the batch the same size and cement con-
tent the same,

2, The colder concrete materials are mixed, the less water
will be required, However, when average day and night tempera-
tures are less than 40 degrees F, the temperature of freshly mixed
concrete should not be less than 50 degrees F.

3. More water is required to mix concrete to a fluid, sloppy
consistency than to a soft, mushy consistency that is plenty work-
able for placing even without vibration, Projects doing any amount
of concrete work should have and use a concrete vibrator to con-
solidate newly placed concrete. Then still less water can be used
in a mix of stiffer plastic consistency, which is amply workable and
responsive to good placement with vibration,

About 3,000 pounds of dry sand and gravel are required with 6
sacks of cement to make 1 cubic yard of air-entrained concrete of me-
dium consistency~-not too wet, not too dry.

If sand and gravel are well graded, as described earlier, 1,100
pounds of sand should be a sufficient portion of the 3,000 pounds to make
a workable concrete. At least it is about average for a starting mix.
More or less can be used as seems necessary after the mix is tried out
on the job,

Subtracting 1,100 from 3,000 leaves 1, 900 pounds of gravel,

950 pounds of small gravel 3/16 to 3/4 inch in size and 950 pounds of me-
dium gravel 3/4 to1-1/2 inchinsize, required for each cubic yard of concrete.

9


1. Sand and gravel should be clean and free of dirt and clay
when closely examined, It should be kept clean and uncontami-
nated on the job,

2. It should also be reasonably free of weak particles that can
be broken easily; or pieces of foreign materials like wood, coal
clay lumps, shale, mica; or pieces with heavy coatings that will
break off in handling.

3. Sand should appear to be well-graded and contain some of
all sizes up to 3/16 or 1/4 inch. No one size should appear to
predominate, particularly among the fine sizes. If sand is too fine,
lower concrete strength and durability will result unless additional
cement is used.

4. Gravel also should be well-graded and contain a fair share
of each size larger than sand and as little as possible of sizes like
sand,

5. For most structure and lining work the best size of gravel
is that graded up to 1-1/2 inches for the largest pieces. For the
gravel in each concrete mixer batch to be well-graded, it is best to
have the gravel screened into two sizes:

a. Small gravel usually ranges in size from 3/16 to 3/4
inch, Pea gravel smaller than 3/8 inch should be less than 20
percent of this gravel for easiest finishing of unformed concrete,
and less than 40 percent for formed concrete.

b. Medium gravel should range in size from 3/4 inch to
1-1/2 inch, It 1s well-graded when about one third of it is in
each 1/4-inch range in the 3/4- to 1-1/2-inch size range, that
is about one-third in 3/4 to 1 inch, one-third 1 inch to 1-1/4
inch, and one-third 1-1/4 inch to 1-1/2 inch,

c. About the same amount each of small gravel and of me-
dium gravel should be used in concrete mixes,

6. If an attempt is to be made to produce uniform and durable
concrete, pit run mixed sand and gravel should not be used, Although
such concrete may have sufficient serviceability, it can be very dis-
appointing. It will not be uniform and part of the structure will not
be durable. Pit run sand and gravel should be used only ina real
emergency and then only with addition of at least one more sack of
cement than usual per cubic yard of concrete,

Admixtures

Air-entraining agents. --A discussion of materials for good
concrete would be Tmcomptete without mention of certain admixtures
which have been found to be unusually effective in improving workability
and durability. These are the air-entraining agents. Since 1945 an air-

entraining agent has been used in all concrete in new construction on

6


dangerous elements, For the same reason many cements are now made
with little if any alkali more than 0.6 percent. When low-alkali cement
is specified, a premium of as much as 7-1/2 cents per sack may be
charged.

Sand and Gravel

In most cases sand and gravel sources in Bureau of Reclama-
tion project areas have been tested and their concrete-making capabili-
ties are a matter of record in the office of the Assistant Commissioner
and Chief Engineer in Denver, Colorado, and in the various Region Of-
fices, located in Boise, Idaho; Sacramento, California; Boulder city,
Nevada; Salt Lake City, Utah; Amarillo, Texas; Billings, Montana; and
Denver, Colorado, Existing information is available on inquiry, Un-
tested sources intended for use should be investigated. At least an ef-
fort should be made to see whether local concrete made with the sand
and gravel has been serviceable. If such local evidence is lacking on
a source to be used, arrangements should be made with the Assistant
Commissioner and Chief Engineer or Regional Directors of the several
regions for tests to confirm its reliability,

So much for the general quality of sand and gravel sources,
but there are several factors which can be and should be considered in
making a choice of materials available on the project.


Unvibrated oy

For placing unformed concrete concretes RON
on slopes slipform screed 7 1 pein.
should be steel faced, Immersion i MA,
weighted and unvibrated. type vibrator,
Concrete should be vibrated <_2"x 4" Side
ahead of slipform. form

“If a pipe stiffener
is used over the bridle rope,
the slip form will move
Steel plate -- more evenly up the slope.

Concrete for “No shoes at riding ends as form

weight ---- rides high when gravel gets under
shoes; keep riding edges sharp.
Vibrated
concrete SLIPFORM SCREED
surface--,,

FOR PLACING
UNFORMED CONCRETE ON SLOPES

Finishing

With few exceptions outdoor concrete should not be steel
troweled in finishing. Such surfaces are not only less safe but often
less durable, An exception is in canal lining where once or twice
over with a long-handled steel fresno trowel often provides all the
finish worth doing after good work with a steel-faced slip-form, A
neat, even job of floating with a wood float is all that is necessary for
unformed surfaces around structures.

The important thing in finishing is not to overwork the sur-
face or to work the surface when it is wet. Overworking removes en-
trained air, brings excess water and fines to the surface, and results
in poor durability and scaling, Finishers should wait until the wet shine
is gone from the surface before performing floating or troweling opera-
tions, Even though concrete is not placed too wet, water may cover the
surface of flat work after strike-off and floating operations, This water
should be removed by draining, blotting with burlap, or dragging off in
a loop of hose. Dusting of the surface with dry cement to soak up the
water often results in a weak surface; a surface that is less resistant
to weather, erosion, etc.

18


Cambridge, Massachusetts, and commercial solutions of Vinsol
resin, produced by Hercules Powder Company, Wilmington, Delaware.
Any of the Vinsol resin solutions are satisfactory provided their strength
is known and their dosage is correct to produce 5 percent of entrained
air in the concrete. Protex is a widely used commercial solution of
Vinsol resin made in Denver, Colorado. All fluid agents should pref-
erably be protected from freezing and containers should be wellstirred
each day before use. Good reliable air-entraining agents are available
at prices making them cost only a few cents per yard of concrete.

Calcium Chloride. --The only other admixture which should be
used at certain times in O&M concrete is calcium chloride. AsO&M
construction is often done in cold weather between operating seasons,
need for calcium chloride might arise in a larger than usual fraction
of the work, When day and night high and low temperatures average
less than 40 degrees F, 1 percent of calcium chloride, by weight of ce-
ment, should be used in the concrete, except, as previously emphasized,
when it will be exposed to alkali-sulfate. Where exposed to alkali-sulfate,
instead of using calcium chloride, another sack of Type V, sulfate-
resisting cement, should be used in each yard of concrete or about 17
pounds for a one sack mix,

Calcium chloride is used in cold weather concrete because it
makes the concretc more mature by the end of its period of protection
from freezing (this protection will be discussed later), It is emphasized,
however, that the use of calcium chloride does not eliminate the need for
protection, Because calcium chloride causes concrete to gain strength
more rapidly, its use is sometimes advantageous when time for form
stripping, backfilling, or time for completion is short, This is oftenthe
case on O&M construction, For this purpose, no more than 2 pounds per
sack of cement should be used and it should not be used in warm weather.
Calcium chloride always should be dissolved first in a portion of the mix-
ing water before placing it in the mixer,

Mixing Water

Any water that is drinkable is suitable for mixing concrete.
Water that is too salty or brackish to taste well, should not be used.
Stream water with some cloudiness or suspended sediment in it may be
used as this is no different than clear water will be when mixed with ag~
gregate considered amply clean,

PROPORTIONING AND BATCHING MATERIALS
FOR O&M CONCRETE

Proportioning
Concrete is more dependent on cement for quality than any other

of its materials, Other things being equal, more cement inthe mix means
higher quality. For this reason the cheapest and most effective way to


)

LAST COPY - DO NOT REMOVE

LAST COPY - DO NOT RENGVE

Department of the Interior

Bureau of Reclamation

OPERATION AND MAINTENANCE

EQUIPMENT AND PROCEDURES

RELEASE NO. 11
January, February and March 1955

VIS

Adf

TANT ION A

CONTENTS

Good Concrete on Operating Projects

LAST COPY - DO NOT RENOVE


For a 2 sack batch use twice the amounts given for the 1-
sack mix above, for a 3-sack batch, 3 times the amounts, etc.

Batching

Uniformity of concrete mentioned earlier depends primarily
on getting the same amount of the same materials in each mixer batch,
The same amount is best assured when weighting is used. Batching
based on weight is best because weight of sand varies considerably less
than does volume of sand when the moisture content of the sand varies.
This does not mean that all the material has to be weighed for eachbatch,
although specially designed
platform scales are available
for doing so. Such a scale
is the so-called wheelbarrow
scale in the photograph at
left.

The scale is easily trans-
ported, is light enough for
convenient handling, and can
be adjusted and leveled quickly,
The scale shown has only two
weighing beams, one for sand
and one for gravel, The top
beam shown is a tare weight
beam for campensating for the
weight of anempty wheelbarrow,
In our work, where three sizes
of aggregates are used, it is bet-
ter to obtain a scale with three
weighing beams in addition
to the tare beam, one weigh-
ing beam for each size of ag-
gregate; that is, sand, small gravel, and medium gravel. The tell-tale
device or balance indicator on top of the beam box is a convenient means
for indicating when the correct amount of material has been added, In
operation, the weigh-beam poises are set to correspond with the desired
gross weights (including moisture of the aggregates), and when any size
of aggregate is to be weighed, the proper beam is brought into engagement
by releasing the trigger on the outside of the box. It is not essential that
the scale be at the stockpiles, for by keeping a small supply of each ag-
gregate at the scale, the wheelbarrow or cart can be trimmed or added
to as required, With a little experience, workmen become proficient in
approximating the correct load.

For considerably smaller volumes of concrete, a standard plat-
form scale can be arranged to weigh a wheelbarrow of sand or gravel,
It need be used only at the start to determine the level to which each
wheelbarrow must be filled, and to verify or correct these levels if the
moisture content of the sand or either size of gravel appears tohave changed,
Wooden pieces can be made to strike off each wheelbarrow at the same

Ld


Since mixing is often done in mixers smaller than 1 cubic yard,
these batch quantities can be reduced to weights of dry sand and gravel
for 1, 2, 3, or any other number of sacks of cement, according to the
size of the mixer,

Sacks Cement Sand 3/16"=3/4" 3/4"-1-1/2"

per batch lbs, gravel-lbs. gravel-lbs.
1 183 158 158
2 367 317 317
3 550 475 475
6* 1100 950 950

*Amount for 1 cubic yard of concrete, approximately.

Batches should be based on a certain number of whole sacks of
cement, Fractional-sack batches are rarely uniform unless the cement
for the fractional sack is weighed for each batch, Cement ina full sack
weighs 94 pounds.

If, with a little experience with project materials, it is found
that these quantities of sand and gravel make appreciably more or less
than a yard of concrete in place with 6 sacks of cement, proportionate
decreases or increases in them can be made, Mainly, larger yield
batches should be avoided because the concrete from, such a batch con-
tains less than the desired 6 sacks of cement per cubic yard, Smaller
yield batches, although more expensive in cement, will be amply strong
and durable.

If materials are wet, particularly the sand, and no correction
is made for the weight of water in a batch weight of sand or gravel, the
batch yield will be small, Also, a mix may appear to be undersanded
if no correction is made for considerable moisture in sand used, For
instance, if the sand contains 5 percent moisture, the 1100 pounds
weighed out for a yard of concrete would actually be only 1047 pounds
of sand and 53 pounds of water, That lack of 53 pounds of sand might
make a noticeable reduction in workability. Actually, to get 1100 pounds
of sand in the batch, 1155 pounds of this wet sand should be used, So,
when sand is wet, even though a test of the exact amount is not made, a
few more pounds of it must be used as may be necessary to keep the con-
crete at the same workability and volume yield per batch.

Summarizing proportioning, to start with at least, materials
for a 1 sack mixture of good concrete should contain:-

1 sack of cement
183 pounds of fairly dry, well-graded sand
158 pounds of 3/16" to 3/4", well-graded small
gravel
158 pounds of 3/4" to 1-1/2", well-graded medium
gravel
Recommended dosage of an air-entraining agent
Only enough mixing water to produce a medium
plastic consistency.
If in winter--add 1 pound of calcium chloride previously
dissolved to make 1 quart of solution.

10


Bureau projects, Results have been excellent and no field concrete with
entrained air has yet shown susceptibility to freezing and weathering,
For these reasons all O&M concrete placed, no matter how small the
quantity, should include a proper dosage of an air-entraining agent and
have the benefit of entrained air,

It is best and most convenient on small work to get these agents
in liquid form and measure into each batch by volume the quantity pre-
scribed by the manufacturer, Under average conditions this will entrain
about 5 percent of air, for the most part in millions of very tiny bubbles.
Four to 6 percent air in concrete is a desirable amount, More air re-
duces strength; less does not take full advantage of air-entrainment,

There are many factors in the composition, mixing, and place-
ment of concrete that will change the amount of air that can be entrained
with a given air-entraining agent. The principal factors are:

1, Temperature - At high temperatures air-entraining agent
is less effective. More agent must be used as the temperature in-
creases to secure the same amount of entrained air,

2, Fines - Fine material in a concrete mix, whether it be ce-
ment pozzolan, very fine sand, dirt, etc., also increases the
amount of agent required, The finer the ingredients the more agent
needed to produce the desired 4 to 6 percent entrained air,

3, Long mixing ~ The effectiveness of the agent is also reduced
by long mixing of the concrete, As this is not normally done for
economy reasons, it probably is not too important, but if in emer-
gency a batch of concrete must be mixed for a long period, additional
agent is needed to secure the desired air content.

The same volume of concrete without air entrainment should
weigh 4 percent more than concrete with 5 percent of entrained air. (Con-
crete contains about 1 percent of air without using an air-entraining
agent.) The weighing of representative samples of each fresh concrete,
solidly filling the same strong bucket and carefully leveled across the
top, will serve as similar volumes for this rough test of the amount of
entrained air in air-entrained concrete. Because of the factors listed
above, such a test should be made occasionally to determine the amount
of air entrained in the concrete,

Air-entraining agents are now relatively standard products and
are available from several reliable sources. Most widely available and
used are Darex, produced by the Dewey and Almy Chemical Company,


MATERIALS FOR GOOD O&M CONCRETE

One might think that cement is just cement, and that sand and
gravel are just sand and gravel. Actually cements differ materially
from plant to plant and in types, and sand and gravel differ from place
to place. Often there are several choices that can be made in available
materials. Correct use or choice of these materials can often make the
difference between a serviceable, long lasting concrete job and one that
has poor durability and short life.

Cement

Choice of cement is most important when white alkali sulfate
is prominent in the structure area, The photographs below, show a
structure that has been in service about 20 years in such a locality. The
invert in the drop structure of the lateral in the right foreground of the
upper photograph has
been completely disin-
tegrated by the corro-
sive attack of sulfate
laden soils and water in
the area. In the lower
photograph on this page
and the upper one onthe
following page, the ad-
vanced stage of disin-
tegration of the lower
portions of the structure
also are apparent,

In the second photo-
graph on the following
page, the disintegration
of a concrete check on
another of our projects
in the southwest is shown,
In service about 25 years,
the chemical attack of the
sulfates on the cementing
material that holds the
aggregate together is
clearly illustrated.

We have learned con-
siderable about cement
and aggregates and know
that the use of the right
cement and aggregate can
delay the disintegrationof
concrete used under such
conditions and prolong,
materially, the useful life of such concrete structures. One test for this



DISCHARGING

08
HAVING TWO DISCHARGE

DISCHARGE OF HOPPERS FOR LOADING CONCRETE BUGGIES

FIGURE 103- PROPER METHODS FOR LOADING CONCRETE BUCKETS,
HOPPERS, AND BUGGIES AVOID SEPARATION OF THE COARSE

CONCRETE

AGGREGATE FROM THE MORTAR

mixed concrete can be
UNMIXED by improper
handling and placing.
Beginning with the dis-
charge of concrete from
the mixer, as shown at
left, each time the con-
crete falls freely into a
buggy, wheelbarrow,
crane bucket, hopper, or
into the forms for a struc-
ture, it is likely to become
unmixed. Unless care is
taken to avoid this separa-
tion, rock tends to go to
one side and mortar to

the cther. The wetter

the mix the worse this
will happen. The more

it happens, particularly
as concrete goes into the
forms of a structure, the
more chance there is that
some of the separated
gravel will not get worked
back into mortar again,
and rock pockets will show
when the forms are stripped.

Separation is least with
air-entrained concrete of
medium consistency, When
discharging and falling con-
crete is baffled and con-
trolled in drop chutes so it
falls straight down and not
at an angle, there is little
or no separation. A little
experimenting will quickly
demonstrate this fact. Fill
buggies by vertical fall from
a gate in the bottom of a

hopper in comparison with separation in buggies filled from a sloping gate
Compare separation in concrete caught ina ver-
tical piece of drop chute and made to fall vertically from the end of the

in the side of the hopper.

chute, as in the figure on the facing next page.

Clusters of separated

gravel occurring as concrete is placed in the structure should be scattered
before they are covered with the next batch, Even vibration does not always
succeed in remixing unscattered, separated rock.

Concrete should be placed in forms in horizontal layers not over 1

14


CORRECT

START PLACING AT 8!
SLOPE SO THAT COMPA,
|S INCREASED BY WEIGHT OF
NEWLY ADDED CONCRETE,
VIBRATION CONSOLIDATES.

TION STARTS FLOW AND REMOVES
SUPPORT FROM CONCRETE ABOVE

WHEN CONCRETE MUST BE PLACED IN
A SLOPING LIFT

CORRECT

VERTICAL PENETRATION OF MAPHAZARD RANDOM PENETRA-
VIBRATOR & FEW INCHES INTO TION OF THE VIBRATOR AT ALL
PREVIOUS LIFT (WHICH SHOULD ANGLES PACINGS WITHOUT
NOT YET BE RIGIO) AT SYSTEM= SUFFICIENT DEPTH TO ASSURE
MONOLITHIC COMBINATION OF
THE TWO LAYERS

INCORRECT

ATIC REGULAR INTERVALS
FOUND TO GiVE ADEQUATE
CONSOLIDATION

SYSTEMATIC VIBRATION OF EACH NEW LIFT

CORRECT

|NCORRECT

SHOVEL ROCKS FROM ROCK
POCKET ONTO SOFTER, AMPLY
SANDED AREA AND TRAMP OR
VIBRATE on

ATTEMPTING TO CORRECT
ROCK POCKET BY SHOVELING
MORTAR AND SOFT CONCRETE

TREATMENT OF ROCK POCKET WHEN PLACING
CONCRETE

FIGURE 106 - PROPER METHODS OF VIBRATING AND WORKING

OF CONCRETE INSURE THOROUGH CONSOLIDATION

Several examples
of the proper methods
of vibrating and working
concrete to insure
thorough consolidation
are shown in the figure
at left. Placing unformed
concrete on slopes in ca-
nal lining and elsewhere
usually represents a large
part of O&M concrete work.
Too often this concrete is
poorly consolidated be-
cause efforts to do so re-
sult in causing it to flow
down the slope, Also too
often, to hold it on the
slope, concrete is mixed
too stiff for effective con-
solidation in that position,
Poorly consolidated con-
crete is either wasteful
of material for results ob-
tained or makes a weak
slab which may fail its
purpose, Such a failure
is shown in the photograph
on the opposite page, where
it was necessary to remove
the concrete lining. The
lack of consolidation is ap-
parent and the fines in the
mix were not distributed
throughout the mass.
Neither was the reinforce-
ment steel properly em-
bedded within the concrete,
Better vibration would have
resulted in better distribu-
tion of the mortar through-
out the mass,

Usually the mushiest concrete that will stay on a slope is not too
wet for such work. It soon stiffens and in the meantime, screeding and

shaping the concrete will do much to consolidate it.
tained if a slip-form strike-off screed is used.

Best results are ob-
This saves a lot of hard

work and holds the concrete in place while it is being spaded or vibrated
along the headboard as the slip-form slowly moves up the slope, Such a
device is shown in the lower photograph on the facing page, and a sketch

of a similar device is shown on page 18.

Note the position of the vibrator

consolidating and working the concrete just ahead of the strike-off screed.

16


This, like all issues of the O&M Equipment and Procedures
Bulletin, is circulated for the benefit of irrigation project operation
and maintenance people, Its principal purpose is to serve as a me~
dium of exchange of operating and maintenance information, Refer-
ence to a trade name does not constitute an endorsement of a partic-
ular product and omission of any commercially available item does
not imply discrimination against any manufacturer, It is hoped that
labor saving devices or less costly equipment developed by the re-
sourceful water users willbe astep toward commercial developement
of equipment for use on irrigation projects in continued effort to re-
duce costs and increase operating efficiency,

Hook kK


INTRODUCTION

This release, No, 11 of the Operation and Maintenance
Equipment and Procedures Bulletin, is devoted to suggestions that
may help in obtaining good concrete on operating projects. Several
years ago an O&M Superintendent who had some engineering con-
struction background and who happened to be familiar with what it
takes to make good concrete was aware that concrete placed by his
maintenance crews could be considerably better with very little
added cost for either materials or labor, Accordingly, he took ad-
vantage of a little slack time to teach his foremen and principal
workmen how they could improve their concrete work, Because of
his obvious belief in what he was teaching them and its evident prac-
ticability, the men became truly interested and looked forward to
trying out the new ideas on their next concrete job,

The day came and late that afternoon the Superintendent
met them as they drove into the yard and asked how the concrete
had gone, "'Do you know, Harold," the foreman said in amazement
no less than if he had just that day discovered the law of gravity,
"When we did those things you told us to do, that Gol Darn concrete
was UNIFORM," ~~

Of course it was uniform and that unusual uniformity was
one indication that concrete placed that day was of better quality
than usual, But why was it uniform? What were the things Harold
told them to do? What things concerning concrete materials also
affect concrete quality? What other precautions are necessary to
be sure that good well-placed concrete will become strong and dura-
ble? After all, what is good concrete?

For the benefit of O& M people on other projects some of
these questions are being answered in this issue of the Bulletin by
L. H, Tuthill, Concrete Engineer, Commissioner's Denver Office,
Bureau of Reclamation, Not only the rules are given but an attempt
is made to explain them and tell why they are necessary and how
they help in obtaining good, uniform concrete at little, if any, addi-
tional cost,

Equipment like that shown on the cover, although used by
a contractor in construction could well be adapted in some instances
to O&M use, The truck bed has been divided into several compart-
ments; each compartment of a size suitable for the prebatched ce-
ment, sand, and gravel for about a half cubic yard of concrete, The
batches can be dumped directly into the loading skip for the 16S (16-
cubic foot) mixer, Prebatching can in some circumstances save
hauling of excessive materials, avoid wasting of cement, sand and
gravel, and provide for uniformity of batches, resulting in a better
and more uniform concrete, Mr, Tuthill has promised that he will
help prepare additional articles on O&M concrete for future releases
of the Bulletin,


level each time it is filled. The weight then will be the same as long
as moisture content remains the same.

The arrangement of equip-
ment on a job where small
amounts of concrete were being
placed in scattered areas, is
shown in the sketch at the left.
A supply of cement, sand and
gravel was always available
and made for ease in weighing
these materials, By towing
the mixer behind the truck,
rapid movement from place to
place was possible, For most
O&M work, a 1-sack mixer is
preferable.

On another project the
mixer and a water tank also
were placed ona "low-boy"
trailer and moved from place
to place.

In shoveling gravel from stockpiles it will be noticed that in
some parts the gravel is small and in others it is large. In some parts
there is considerable undersize material, Best uniformity and best
workability is obtained in the concrete if workmen loading the wheel-
barrows see that some of each of these different sized materials are
included in each wheelbarrow load,

Once the amount of water required for the correct consistency
of the mix is determined at the start, this same amount should be meas-
ured by volume, in so many buckets or gallons for each batch thereafter.
It should not be done by counting while a hose is running into the mixer.
If measured by hand, water should be available in an open barrel or tank
from which it can be dipped using the measuring container, However,
the consistency of the concrete should be watched and if it changes, the
water volume should be adjusted accordingly, Drier sand or gravel in-
side the stockpile, finer sand or more fines in the gravel will make more
added water necessary for the same consitency. If the change in con-
sistency is due to extra fines, that will for a time have to be used, per-
haps a reduction in the proportion of sand is in order rather than more
water, Maybe the sand pile has been drying. As the wet interior is
reached, a reduction in the mixing water volume will be necessary.

Liquid air-entraining agent is best batched by volume, dipping
from an open container with a can cut down, or with holes punched in
the sides, so that it holds only the correct amount for the batch, The
same is true if calcium chloride is to be used, First it should be dis-
solved so that each quart of the solution holds 1 pound of flake calcium
chloride. Then from an open drum, 1 quart of the solution should be
measured into the mixer for each sack of cement in the batch,

12


COUNTER WEIGHTED

INCH" aii, RUBBER SCRAPER

NO SEPARATION
MORTAR

CORRECT

INCORRECT
THE ABOVE ARRANGEMENT pRE- IMPROPER OR COMPLETE Lacx oF
VENTS SEPARATION OF CONCRETE CONTROL AT END OF BELT,
WHETHER IT IS BEING DISCHARGED. USUALLY & BAFFLE OR SHALLOW
INTO HOPPERS, BUCKETS , CARS,TRUCKS, HOPPER MERELY CHANGES THE
OR FORMS. DIRECTION OF SEPARATION

CONTROL OF SEPARATION OF CONCRETE AT THE

END OF CONVEYOR BELT

cnure cuure

CORRECT

INCORRECT

TO DISCHARGE coNcRETE

A FREE ENO CHUTE
BE PAVED, ROCK IS SEPaR,

PLACE BAFFLE AND ROP at
ENO OF CHUTE SO THAT SEPARATION
|S AVOIDED AND CONCRETE REMAINS
ON SLOPE

€
VELOCITY TENDS To con:
CRETE DOWN SLOPE

PLACING CONCRETE ON A SLOPING SURFACE

A barns

|p {Roving 2e-

[5 iow wininom
\:/ 7 weaproow

«| FoR ooWNPRE

No SepaRamioN

Fe : MORTAR

rs
in
NCORRECT

IMPROPER OR LACK OF CONTROL
AT END OF ANY CONCRETE CHUTE,
NO MATTER HOW SHORT.

CORRECT

THE ABOVE BRRANGEMENT PRE
VENTS SEPARATION, NO MATTER HOW
SHORT THE CHUTE, WHETHER CONCRETE
S BEING DISCHARGED INTO HOPPERS, LY a BAFFLE
BUCKETS, CARS, TRUCKS, OR FORMS. CHANGES DIRECTION 0

CONTROL OF SEPARATION AT THE END OF CONCRETE CHUTES

THIS APPLIES TO SLOPING DISCHARGES FROM MIXERS,TRUCK MIKE
TO LONGER CHUTES, BUT NOT WHEN CONCRETE 1S DISCHARGED
THER CHUTE OR ONTO A CONVEYOR BELT.

FIGURE 104 - CORRECT METHODS MUST BE USED IF SEPARATION
AT ENDS OF CONVEYORS AND CHUTES 1S TO BE CONTROLLED

bration increases strength and reduces settleme

foot deep if vibrators

are not used, and not

over 2 feet deep if they

are used. Deeper layers
increase chances that
some concrete will not

be properly consolidated.
Rodding or vibration should
follow just far enough be-
hind to prevent the concrete
from flowing out of place.

Vibration should be
used to consolidate con-
crete in place. In view
of the great improvement
it makes in the quality of
concrete that can be placed,
and in the kind of job it does
in placing it, few O&M
forces can afford to work
without a concrete vibrator.
Soft, near fluid concrete
should not be vibrated as
it will make the rock sink
and the fines and water
come to the top, thus fur-
ther weakening the upper
layer. Less water should
be used in such a mix un-
til a fairly stiff mushy con-
sistency is obtained. This
kind of concrete will not be
damaged by the ample vi-
bration necessary to con-
solidate it and get good
formed surfaces,

Concrete can be vibrated
to advantage any time until
setting has progressed to
the point that vibration will
no longer again make the
concrete plastic, Late vi-
nt cracking. For this rea-

son, concrete delayed in placing can be used, regardless of the length of
the delay or the amount it has stiffened, as long as it can be consolidated
satisfactorily in the structure. Additional water should not be remixed
with such concrete unless cement also is added at the rate of 17 pounds of
cement per gallon of water, or a sack for each 5-1/2 gallons.

15