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UNITED STATES DEPARTMENT OF LABOR
L . B . Schw ellenbach, Secretary
B U R E A U O F L A B O R ST A T IST IC S
Ew an Clague, Commissioner
+
Labor Requirements
for Construction M aterials
PAR T IL—CONCRETE MASONRY UNITS
Bulletin H o. 888—
2
For sale by the Superintendent o f Documents, U. S. Governm ent Printing Office
Washington 25, D . C. - Price 10 cents
Letter of Transmittal
U n it e d S t a t e s D e p a r t m e n t of L a b o r ,
B u r e a u of L a b o r S t a t is t ic s ,
Washington , D . C., April 8 , 1947.
The S e c r e t a r y of L a b o r :
I have the honor to transmit herewith a report on the labor requirements in the
production and distribution of concrete masonry units, a summary of which was
published in the Monthly Labor Review for November 1946.
This is the second of a series of reports covering those industries which supply
essential building materials for residential construction. These surveys were
made in order to measure the amount of ‘ ‘behind-the-line” employment which
would result from any given level of construction activity.
The labor requirements series, under the direction of Brunswick A. Bagdon, is
based upon plant data collected by the field personnel assigned to this project in
the Bureau’s Regional Offices; the report was written by Alfred W. Collier and
Clyde Stone in the Bureau’s Division of Construction and Public Employment.
E w a n C l a g u e , Commissioner .
Hon. L.
B . SCHWELLENBACH,
Secretary of Labor.
Contents
Page
Preface___________________________________________
Introduction______________________________________
Definition of concrete masonry____________________
Development of the industry______________________
Scope of survey___________________________________
Manufacturing processes for concrete masonry_____
Man-hour requirements:
Summary_____________________________________
Production and transportation of raw materials
Manufacturing— plant operations_____________
Transportation of concrete blocks_____________
Variations in labor requirements____________ _
(in)
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B ulletin J^jo. 888-2 o f the
U n ited States Bureau o f Labor Statistics
[Reprinted from the M onthly L ab or R e v ie w , November 1946, with additional data.]
Preface
This study, the second of a series covering those industries which
supply essential building materials, has been made in order to measure
the amount of “ behind-the-line” employment which would result in
the concrete masonry industry from any given level of construction
activity.
Previous studies of man-hour requirements made by the Bureau
from 1933 to 1939, as a part of the program of the Federal Emergency
Administration of Public Works, included steel, cement, lumber,
plumbing and heating supplies, clay products, and electrical goods.
For these products, information was collected from the primary
sources for raw materials, transportation, manufacturing, and de
livery to the construction site. Today these studies, while of historical
significance, have several serious limitations; namely, (a) new products
have been developed which were not included in the previous report,
(6) manufacturing methods have, in several instances, changed con
siderably, and (c) variations in volume of output as between the period
of the thirties and the current time would result in marked variations
in man-hour requirements.
Building construction was greatly hindered during the period fol
lowing VJ-day, and by the middle of 1946 building activity still had
not shown marked headway. However, the forecasts, both public
and private, indicate peak activity in the months ahead. Housing
programs are under way. Federal subsidies have been appropriated
to speed up and increase the volume of the production of essential
building materials. Thus, everything points to a high level of activity
in the building construction industry for some time to come.
This series of reports will provide accurate data on the man-hours
required per unit of output and per unit of dollar value, for each of 50
important construction and building materials— both traditional
materials such as dimension lumber, cement, and reinforcing steel,
and newer materials such as plywood (included only incidentally in
the previous reports), insulating material, and the commoner fabri
cated steel products for residential buildings. For each of the prod
ucts included, comprehensive field data will be collected on the direct
(V)
VI
and overhead man-hours in production during a recent period, the
output during this period, the quantities or value of materials, sup
plies, and fuel consumed, and wherever possible, sales both directly to
contractors and through distributors and dealers. From these data,
total man-hour requirements, from extraction of raw materials to
delivery of completed materials at the construction site, will be ob
tained for an extensive series of materials representative of the require
ments for most types of construction; in addition, the data will permit
reasonable estimates of man-hour requirements for a large number
of other materials generally similar to those studied, but not sufficiently
important for individual study (primarily highly specialized materials,
and custom-order variants of common materials).
Labor Requirements for Construction
Materials
P art II.— C oncrete M asonry U nits
Introduction
Construction activity, especially in residential and G I hous
ing, continues at a high rate in spite of material and manpower short
ages. Employment at the construction site has increased markedly
in the past 12 months. A t the same time there has been increased
employment in most of the industries producing the building materials
used. The present study is the second in the series o f labor-require
ments studies of the more important building materials, undertaken
by the Bureau o f Labor Statistics, to determine the indirect labor
involved in any construction activity.
One of the more important uses of portland cement is as a raw
material in the manufacture of concrete units, particularly concrete
block. This industry has had remarkable growth in the last 25 years.
It has progressed a long way since those days of unsightly, ill-made
and nonuniform products which were made in small plants, most of
which were in the so-called “ back yard” category.
Today, concrete block plants equipped with modern machinery are
manufacturing very attractive units that are of uniformly high
strength, and which are highly fire resistant. They are being used
on a large scale as “ back up” for many types of facing materials such
as stone, brick, etc., as well as for both load-bearing and non-load
bearing walls. Other uses are for partitions, for fireproofing as floor
fillers, and where masonry can be used to advantage in almost any
type of building construction.
Concrete masonry has some uses other than as a structural material.
Chief among these is its adaptability to any style of architecture. It
is widely used as a surface finish for both exterior and interior walls.
Units made with lightweight aggregates have excellent sound absorp
tion qualities and as a result are especially desirable for exposed walls
in auditoriums, gymnasiums, classrooms, and corridors.
Concrete units made by modern methods easily meet the require
ments of the national standard specifications of the American Society
for Testing Materials, Federal Master Specifications, and Under-
Cl)
2
writers* Laboratories. These specifications for hollow units require
face shells of 1){ inches and strength of 1,000 pounds per square inch
for use in exterior walls below grade and for unprotected exterior walls
above grade. Specifications for protected work above grade require
700 pounds per square inch. Absorption should not exceed 15 pounds
of moisture per cubic foot of concrete. The products of better plants
exceed these requirements two or three times.
The products enjoy wide acceptance from architects, builders, and
prospective home owners. According to estimates by the Civilian
Production Administration and reports of the Bureau of the Census,
60 percent of all masonry construction today uses concrete units.1
Various reports indicate spotty shortages in concrete block in some
areas and a fair to good supply in others. Current delivery dates vary
from 2 to 6 weeks on most orders. Demand for block is increasing
because it can be used as an alternate for brick and structural clay
tile, both of which are in extremely short supply. The backlog of
orders is increasing.2
Estimates on production and requirements for concrete block for
1946 prepared by Civilian Production Administration are as follows:
Millions ofblocks (8x8 x 16-inch equivalent)
1946
First
quarter
Estimated production
Requirements_______
-
Second
quarter
Third
quarter
Fourth
quarter
153
187
194
205
255
240
300
222
From the above statement, it appears that the peak of requirements
was reached in the third quarter of 1946, and that by the fourth quar
ter production met all requirements. According to the Civilian Pro
duction Administration, several thousand new plants began operations
in 1946 and block-making machines were currently being delivered at
the rate of 100 to 200 per month. Production of concrete block was
generally meeting all requirements.3
With the construction industry expecting to operate at continued
high levels for some time to come, the production of block is keeping
pace with this trend. The capacity of established plants is sufficient
to meet estimated peak requirements but production has been limited
by shortages of labor, materials, repair parts, and transportation.
1 Brick and Clay Record, October 1946 (p. 16).
3 U. S, Department of Commerce, Construction Division, Construction and Construction Materials, July
1946 (p. 65).
3U. S. Department of Commerce, Construction Division, Construction and Construction Materials,
November 1946 (p. 53).
3
D efinition o f Concrete M asonry
Concrete can be described as an artificial stone produced by mixing
together and hardening definite proportions of cement, aggregates,
and water. The type of cement generally used is portland cement or
some modified product thereof.
The initial plasticity of concrete permits it to be formed into any
desired shape and size. Its hardening qualities allow it to be manu
factured in structural units which may be stored for use as needed.
Units so manufactured are known as concrete products. Among the
concrete products manufactured are building blocks and shapes, pipes
and conduits, and similar products such as poles, piling, vaults, etc.
Figure 1.— Vibration equipment operating in modern high-production concrete masonry plant. Three
block machines with total capacity of 12,000 to 15,000 8 x 8 x 16-inch blocks per day (8 hours).
Note overhead bins, traveling weigh-batcher, and power block handling equipment.
The term “ concrete masonry” is applied to block, brick, and tile
building units molded from concrete and laid by masons in a wall.
Masonry units are durable, fire-resistive, and able to carry heavy loads.
Concrete masonry may be classified into two general types—light
weight and heavyweight— according to the materials used as the ag
gregates in the mixing process. Among the lightweight aggregates are
cinders, expanded slag, expanded clay and shale, volcanic cinders,
crushed pumice rock, and certain other materials which are sold under
various trade names (Haydite, Waylite, Superock, etc.). The cinders
731394° — 47—— 2
4
are the waste product of industrial firing of either anthracite or bi
tuminous coal; desirable qualities are hard clinkers and low combus
tible content. Waylite and Superock are made from molten blast
furnace slag, expanded by controlled quantities of water and violent
agitation set up in an especially designed processing machine. Hay
dite is clay or shale expanded by a rapid rise of temperature of the
prepared material feeding through a rotary kiln. Natural aggregates
(sand, gravel, crushed stone, etc.) and air-cooled blast furnace slag
(unprocessed) are the principal heavyweight aggregates.
Figure 2.— Tamping equipment operating in small, modern concrete masonry plant. Block machine
has capacity of 2,000 8 x 8 x 16-inch blocks per 8-hour day. Note mixer located on machine floor
level, skip-hoist for elevating concrete to block machine, and plant lay-out designed for future
installation of overhead bins and additional block machines as business expands.
Solid units are defined as having an average core area of not more
than 25 percent of gross area, whereas hollow units usually have core
areas of approximately 45 percent of gross area.
Concrete masonry is usually identified by the 8 x 8 x 16-inch or
equivalent size. It is commonly manufactured in widths of 4, 6, 8, 10,
or 12 inches, and it is made in half heights, half lengths, and quarter
lengths to work out ashlar patterns in walls. Slight variations are
characteristic of localities and of individual producers. There are
special units which do not have standard sizes, such as corner units,
joist units, jamb units, half units, and fractional units.
The dimensions used by the construction industry for concrete
masonry units are in general nominal rather than in stated sizes. Ac
5
tual heights, widths, and lengths are usually % inch to % inch scant to
allow for the thickness of mortar joints. The industry was one of the
first to adopt the recommendation of the American Standards Associ
ation on modular sizes to provide the 4-inch increment to both vertical
and horizontal dimensions. This permits the nominal lay-out of
buildings in plan and elevation in multiples of 4 inches. Thus, the
modular unit 7% inches by 7% inches by 15% inches used with a %-inch
mortar joint occupies a space in the wall exactly 8 by 16 inches. This
gives perfect conformity with all other building materials that can be
used in multiples of 4 inches (such as clay or glass brick window and
door jambs, lintels, etc.).
Concrete block comprises a major segment of the concrete products
industry, but there are other important products in this field. Re
cently, because of shortages in lumber, concrete joists have been sub
stituted for wood in many structures. Likewise, the field of precast
slabs for roofs, partitions, and floors is relatively undeveloped and
great possibilities unquestionably lie ahead for these products.
Other specialty products are burial vaults, outdoor garbage re
ceptacles, laundry trays, and a variety of miscellaneous products, such
as lighting standards, fence posts, signal standards, battery boxes,
manhole and silo staves, and concrete ornamental products.
Development of the Industry
The growth and development of concrete products manufacture is
coincidental with the development of modern producing machinery.
Concrete blocks were produced by hand in the early part of the cen
tury. Typical of the early machines was one introduced about 1904.
It was wholly hand-operated and consisted of a mold with collapsible
sides and cores which were withdrawn through the bottom of the mold.
Compacting the material was accomplished by shoveling into the mold
box and tamping by hand. Under favorable conditions, two workers
could produce about 200 blocks per day, or 100 blocks per man per day.
Until 1914 the industry was primarily a “ back yard” industry and
producers were mainly small contractors and building material firms
which manufactured blocks as a subsidiary part of other business.
Rock-faced blocks were produced, which were nonuniform and un
sightly. They were used primarily for sheds, outbuildings, cellars,
garages, etc.
Mechanical improvements appeared and their use was generally
extended throughout the industry during the First World War. De
vices to provide mechanical tamping were quickly seized upon by
block producers to eliminate the laborious hand tamping heretofore
employed. The tampers were raised by chain or by eccentric crank
shaft that lifted and dropped the rods, to which the tampers were at-
6
tached. Over the years, the efficiency of these mechanical tamping de
vices was greatly improved, with the result that modern tamping ma
chines are widely used today.
Power feeders, another labor-saving device, developed coincidentally
with power tamping. Fresh concrete was carried to the mold box by
various methods, including drag elevators, small bucket elevators,
feeder belts, and feed drawers. The latter two methods are the most
widely used in today’s modern equipment.
Figure 3.— Modern concrete block machine with power off-bearer in operation.
Constant efforts were exerted by block producers and machinery
manufacturers to reduce the amount of hand labor involved in block
manufacture. Mechanical means w
rere developed to stop and start
the tampers and the feeders automatically. Another development
was mechanical troweling of the tops of the units. Still another
tedious back breaker was eliminated by the development of automatic
strippers for removing the block from the mold box. Machines were
developed in which blocks were made face down— a convenient means
of molding the then popular rock-faced block; and efforts were made
to produce two blocks per operation instead of one. By the early
1920’s production had been increased so that the daily output per
machine approached 1,400 blocks, with four men as a crew, or 350
blocks per man per day.
During the 1920’s, the face-down machines were superseded by
vertical stripper type machines, on which the blocks were manufac
7
tured with the cores formed in a vertical plane, in the same position
as laid in the wall. One type of stripper machine made blocks on plain
pallets by suspending the cores from the top of the mold box and
ejecting the blocks by means of a stripper head in a stationary position
while the mold was stripped up from the blocks.
Another type of stripper machine made blocks on cored pallets.
In this machine the cores were stationary and the block itself was
stripped upward from the cores and then removed from the machine.
Nearly all blocks today are made on improved stripper machines of
either the plain or cored pallet type.
Figure 4.— Concrete block machinery which combines the features of multiple-unit production per
operation, automatic operation, and compaction of concrete by vibration.
The next important development in the industry was compaction
of the concrete by vibration rather than by tamping. An extended
series of experiments and research tests, culminating the work of Kurt
F. Wendt and Paul M. Woodworth at the Materials Testing Labora
tory of the University of Wisconsin,4 demonstrated the efficiency of
vibration, with the result that equipment was made available by
machinery manufacturers to combine multiple-unit production, auto
matic operation, and vibration. Today’s high production machines
are the outgrowth of this development.
Machines are available today which produce from 9 to 15 blocks
per minute of the 8 x 8 x 16-inch equivalent size on either cored or
4 Tests on Concrete Masonry Units, in Journal of the American Concrete Institute, Vol. 36 (p. 121).
8
plain pallets, with operating crews of four men. As these machines
are entirely automatic, the operations of placing the pallet, feeding
concrete into the mold, vibrating and stripping and ejecting the block
from the machine are accomplished mechanically. Thus the use of
hand labor in the manufacturing operation in a block plant has largely
been eliminated. The direct production labor output has been in
creased to 1,500 blocks per man per day, and even better in the more
efficiently operated plants. It should be noted, however, that with
these high production records additional labor is required to handle
raw materials as well as the finished blocks in the curing room and
storage yard.
Along with the development of the molding machine, the use of
auxiliary mechanical equipment has increased. The various kinds of
such equipment which have resulted in the reduction of hand labor,
and a high degree of mechanical efficiency, include power offbearers for removing the block from the machine, power lift trucks
for handling the fresh block from the machine to the curing room and
from the curing room to the storage yard, and power yard handling
equipment to remove the cured block from the racks and load them
into the stock pile, or from the stock pile to the delivery trucks.
Various block manufacturers are developing ingenious devices for
mechanical handling of their product from the delivery trucks to the
building site, so that the time is rapidly approaching when blocks will
be “ untouched by human hands” until the mason actually lays them
in the wall.
Along with the block machinery improvement, the curing process
has made tremendous progress. Better methods of moisture and
heat application have greatly speeded up production and have resulted
in highly uniform concrete products.
The original curing process is known as the dry cure or natural
process. Many plants still use this process and it consists of sprinkling
the units with water and protecting them from temperatures of 50° F.
or less for about 3 days. The limitations of the process are that manu
facturing of block can be done only in the warmer months or in areas
where the temperature is moderate the year round, and that the
length of time required for the cure is 28 days.
The next development was the addition of moisture in the form of
steam and heat, which is applied in curing rooms or kilns for a mini
mum of 24 hours This necessitated the building of closed rooms or
kilns where temperature and humidity could be controlled, and into
which racks of units could be pushed or drawn by tractor. This
process is in wide use today in most modern plants.
To speed up the curing process, experimental work is currently
under way by the National Concrete Masonry Association on hightemperature curing, which reduces the 24-hour curing time to less
than 10 hours. Simultaneously, a number of plants employ autoclave
9
curing, in which the blocks are subjected to temperatures of 350° F. at
steam pressure of 125 pounds per square inch. A limiting factor in
high temperature and high pressure steam curing is, of course, the
added cost of these processes, and it is toward the reduction of these
costs that the research is aimed.
Improvements in the quality of general-use cement have resulted in
better products, and the development of special cements which have
quick-setting properties has reduced the time required in production.
Air-entrained concrete, obtained by the addition of air-entraining
agents to the concrete mix or to the cement when manufactured, has
expanded the field of uses for concrete products where durability and re
sistance to disintegration are important. More exacting specifications
in natural aggregates, as well as prepared aggregates which serve to
improve the insulation and sound absorption properties of the product,
have also been important factors in the development of the industry.
Scope of Survey
Data for man-hour requirements in plant operations were obtained
by direct reporting in the field. The sample consisted of 50 con
crete block plants which produced 8,674,284 concrete blocks during
one month in 1946. In selecting the sample, consideration was given
to size of plant, geographical location, type of aggregates used, and
diversity of products manufactured. The majority of plants surveyed
supplied data for a period during the second quarter of 1946. In some
cases it was necessary to collect data for periods, during the first quar
ter of the year in order to have a representative period of plant opera
tions.
Certain precast concrete products are not included in this survey
because of varying specifications and nonuniformity of production
methods. The study analyzed only units cast in plant operations
before delivery to the construction site, and is confined to concrete
blocks of varying sizes, brick, comer units, jamb units, chimney block,
etc. All block sizes are converted into 8 x 8 x 16-inch equivalent units
which are the standard measurements of the industry.
From previously published Bureau reports and secondary sources,
the Bureau estimated the man-hour requirements for the production
and transportation of raw materials, cement, aggregates, and electric
power. It was found practicable to omit estimates for materials used
in small quantities, such as lubricants, fuel, repair parts, curing agents,
etc. It is believed that the omission of man-hour estimates for these
items would not materially affect the total man-hour requirements.
Manufacturing Processes fo r Concrete M asonry
After the raw materials have been delivered to the plant they are
placed in storage bins from which they are fed by gravity, or by the
10
use of mechanical conveyors, into the processes of manufacture. The
first step in the manufacture of concrete masonry units is the propor
tioning and mixing of cement and aggregates with water. Proper
proportioning of raw materials in preparation for mixing is accom
plished by use of measuring devices. In most plants, batch mixers
are used into which a batch of materials is placed, mixed, and dis
charged before another lot is added. Continuous mixers are also used
in which the ingredients are added continuously and mixed during
passage through the machine.
The concrete flows from the mixing machine into molding machines
which form the units. From the machine, units are discharged on
pallets and placed on racks, usually by a mechanical hoist. The units
are then ready for curing.
All concrete work requires proper curing. Concrete hardens in the
presence of moisture and heat, causing a chemical reaction in the
cement. The processes employed are dry curing, moist steam curing,
and high temperature curing. The moist steam curing system is the
most widely used in modern plants.
After curing, the units are stacked in the storage yard and kept
damp until they develop the strength and absorption properties neces
sary. The units are stacked in such a manner that proper drying of
all units is permitted.
M an-Hour Requirements
SUMMARY
From basic plant records it was found that approximately 30.2
man-hours were required at the plant to manufacture 1,000 concrete
blocks. In addition to the man-hours necessary for plant manufac
ture, it was estimated that 38.8 man-hours were needed to extract the
raw materials used, to haul these materials to the plant, and to deliver
the finished product to the construction site. Similar estimates were
made for the production of block in which lightweight and heavyweight
aggregates, respectively, were used. Thus, the following statement
indicates that a total of approximately 69.0 man-hours of labor are
represented in the manufacture and delivery of 1,000 concrete blocks
to the construction site. The same operations require 64.8 man-hours
for lightweight blocks and 74.0 man-hours for heavyweight blocks.
Man-hours per 1,000 blocks
Heavy
All blocks
weight
(average)
Lightweight
Total production and transportation------------------- __ 69.0
64.8
74.0
Raw materials, production, and transportation 1__ - 29.2
Manufacturing___________________ ______________ . . . 30.2
Transportation, finished product__________ ______ — 9. 6
23.2
32.4
9 .2
36. 6
27.4
10.0
1Includes purchased electric power.
11
Significant variations were noted in man-hour requirements when
the data were analyzed according to rate of production, the number of
molding machines per plant, and type of aggregates used. Plants
producing at a monthly rate of less than 50,000 blocks required 74.5
man-hours per 1,000 blocks, while those producing at a rate of 350,000
blocks or more required only 23.1 man-hours for each 1,000 blocks
produced. Labor requirements in plant operations varied from 34.3
man-hours, for plants having one molding machine, to 24.9 man-hours,
for plants having 3 or more molding machines. The manufacture of
lightweight blocks required 32.4 man-hours per 1,000 blocks as com
pared with 27.4 man-hours necessary for the production of 1,000
heavyweight blocks.
PRODUCTION AND TRANSPORTATION OF RAW MATERIALS
The principal raw materials used in the production of concrete
block are (1) cement and (2) aggregates, which fall into two classifi
cations, lightweight and heavyweight. Cinders, expanded slag, and
burned clay and shale are the principal lightweight aggregates, whereas
sand and gravel or crushed stone are the most commonly used heavy
weight aggregates.
The quantities of raw materials, including electric power, and the
man-hour requirements for these materials are shown below, by type of
block, for the production of 1,000 blocks.
Requirements per 1,000 blocks
Light
All blocks
Heavy
weight
weight
Raw materials:
(average)
Cement___________________
Sand and gravel__________ _____________ to n s .. 13. 3
Cinders___________________ _____________ to n s.- 7. 6
Electric power____________ ___________ k w .hr_- 56.3
Man-hours:
Cement__________________ ___________________ 10.9
Sand and gravel__________ ___________________ 12.0
Cinders___________________ ....... ............................ 6. 1
.2
Electric power____________ __________________ _
Total_____________ _____ ....... ............................29. 2
12.2
---.
13. 6
51. 2
9 .2
30. 2
12.2
---10.8
.2
9 .2
27. 2
23.2
36. 6
___
62. 7
.2
The quantities of cement and electric power shown above for all
types of block are average quantities required for all plants (producing
either lightweight or heavyweight blocks) included in the survey.
Similarly, the quantities of sand and gravel and cinders are weighted
averages of heavyweight and lightweight aggregates required for the
production of the two types of block. The quantities shown sepa
rately for lightweight and heavyweight blocks are those required
when production is confined to each respective product.
In 1945-46 the man-hour requirements for the production and trans
portation of 100 barrels of cement were 100.49, and 3.12 man-hours
12
were required to produce 1,000 kilowatt-hours of electric power.*
6
In the manufacture of 1,000 blocks it was determined from these
figures that the labor requirements were 10.9 man-hours for the 10.9
barrels of cement used, and 0.2 man-hour was expended in providing
the 56.3 kilowatt-hours of electric power consumed.
The production and distribution of sand and gravel, the principal
heavyweight aggregates, required an average of 0.9 man-hour in 1937.6
In the absence of information for the current period, these data were
used as a basis for estimating the labor requirements for sand and
gravel. It was therefore estimated that 12.0 man-hours were needed
for the production and transportation of the 13.3 tons of sand and
gravel used in the production of 1,000 blocks. While other heavy
weight aggregates, such as crushed stone, untreated blast-furnace
slag, etc., are sometimes used, sand and gravel are predominant in
use, and are considered as representative of all heavyweight aggregates
in this study.
Since cinders are a waste product resulting from the combustion of
coal, no man-hours were estimated for the production of this material.
The estimate of man-hour requirements for this material include those
necessary to transport the cinders to the plant and its preparation
at the plant after delivery. The labor requirements for the transpor
tation of cinders are similar to those for sand and gravel. From a
previous Bureau of Labor Statistics study, it was found that 0.6 man
hour was necessary to transport 1 ton of sand and gravel in 1939.6
This figure was used as the labor requirement for the transportation
of 1 ton of cinders. Thus it was estimated that 4.6 man-hours were
required to transport 7.6 tons of cinders to the plant. By use of basic
plant data it was determined that 1.5 man-hours were needed to
prepare the cinders at the plant after delivery, making a total of 6.1
man-hours for transportation and preparation of the 7.6 tons of cin
ders. Analysis of the data collected in this study shows that cinders
were the aggregate used for 67 percent of the total production of
lightweight blocks. Since no available data were found for estimating
the man-hour requirements for other lightweight aggregates, labor
requirements for the transportation and preparation of cinders, based
on data collected in this survey, were used as representative of all
lightweight aggregates.
The labor requirement estimate for lightweight aggregates appears
to be understated in view of the fact that no allowance is made for
labor expended in the production of raw cinders. It is not to be
imputed that lower man-hour requirements for lightweight aggregates,
as compared with total requirements for heavyweight aggregates,
are indicative of lower costs for the lightweight block manufacturer.
* See Labor Requirements in Cement Production, in Monthly Labor Review, September 1946.
6 See Labor Requirements in Production and Distribution of Sand and Gravel, in Monthly Labor Review,
July 1939 (reprinted, with additional data, as Serial No. R. 944).
13
On the contrary, it has been noted in some areas that the cost of raw
cinders (due, at least in part, to inadequate local supplies) exceeded
the cost of prepared heavyweight aggregates. In addition, the cinderblock manufacturer had to bear the cost of the extra operations in
preparing the raw cinders for use.
MANUFACTURING—PLANT OPERATIONS
The 50 plants in this study represent a monthly rate of production
of 8,674,284 lightweight and heavyweight concrete blocks of 8 x 8 x 16inch equivalent size. Total man-hour requirements for this produc
tion and the man-hours per thousand blocks are shown below by
plant operation I
Man-hours required in
manufacture of con
crete block, 19461
To produce
Total
1,000 blocks
Total, plant operations______
Proportioning and mixing___
Machine molding___________
Hauling____________________
Yard_______________________
Maintenance________________
Superintendents and foremen
Miscellaneous labor_________
Administration______________
262, 044
20,
46,
24,
85,
21,
17,
9,
35,
876
382
872
958
377
704
114
761
30. 2
2. 4
5. 3
2. 9
9. 9
2. 5
2 .0
1. 1
4. 1
i Does not include man-hours required for preparation of aggregates or transportation of finished product.
The first operation is the proportioning and mixing of the aggre
gates and cement before the units are molded. In modem plants the
materials are batched by measuring devices and placed into the con
crete mixer.
Many aggregates, such as sand and gravel used for heavyweight
block, and Haydite and Waylite for lightweight block, do not need
additional preparation. They are delivered to the plant ready for
use. Cinders, however, sometimes need preparation at the plant,
such as crushing and removing foreign particles (metal, sulphur, etc.).
For this reason, the man-hours for aggregate preparation are not
shown separately above but are included in the raw materials section
of the summary statement (see p. 11).
The key operation in the plant is the machine molding operation.
The machine forms the units, vibrates or tamps the concrete mixture,
and discharges the formed units onto pallets which are lifted to racks
by air hoists. Modern machines are almost fully automatic, so that
usually only one worker is necessary to operate air hoists and to tend
the machine as it is in production.
After the blocks have been molded on the machine and placed on
racks, they are hauled by tractor or by hand to the curing rooms or kilns.
In any plant the largest single group of employees is the yard gang.
This group numbered from 3 to 7 employees per plant. As concrete
14
units are bulky and must be moved many times, considerable labor is
expended in this operation. The yard employees remove the blocks
from the curing rooms, stack them in the yard for stock piles or ad
ditional curing, sometimes assist with the loading of the finished
blocks for shipment, and perform other unclassified tasks. The labor
is generally unskilled and nearly a third of the entire plant employ
ment is engaged in yard operations. For this operation a total of
85,958 man-hours was required, or an average of 9.9 man-hours per
thousand blocks produced.
Maintenance includes the labor necessary to make repairs on
machinery and plant equipment. During the period surveyed, plants
were operating at a high rate of production and a considerable range
in the man-hours required for this function was noted between plants
having relatively new machinery and those where the machinery was
older. This range was from 0.5 man-hour per thousand blocks
produced in the newer, modern plants to 11.3 man-hours in the older
plants.
Plant supervision functions are performed by superintendents and
foremen. Administration includes the executive, clerical, and sales
force. Miscellaneous labor includes watchmen, janitors, boilermen, etc.
TRANSPORTATION OF CONCRETE BLOCKS
High transportation costs limit the area which can be economically
served by the concrete products manufacturer and for this reason
plants are widely dispersed. Since they serve relatively small local
areas, the shipment of the finished product is usually by motortruck.
Most plants deliver varying proportions of their production and this
function may be considered as a plant operation. In many plants
surveyed, deliveries were made by trucking companies on a contract
basis for all or part of the finished product. In addition, considerable
proportions were transported from the plant by the purchaser. For
the purpose of establishing a basis for comparison, the data for the
transportation of concrete blocks to the construction site were not
included in plant operations. From plant records it was found that
the delivery of 3,244,000 blocks required 31,000 man-hours and it
was estimated that 9.6 man-hours were required for the transporta
tion of 1,000 blocks. The transportation of lightweight blocks re
quired 9.2 man-hours for 1,000 units, whereas 10.0 man-hours were
needed for heavyweight blocks.
VARIATIONS IN LABOR REQUIREMENTS
B y M onthly Rate o f Production and M olding M achines in U se
Among the significant factors in determining the number of man
hours required to produce 1,000 blocks, the monthly rate of production
and the number of molding machines in the plants are taken into con
sideration. Table 1 shows that, in terms of total man-hours, plants
15
producing over 350,000 blocks were about 3 times as efficient as those
producing less than 50,000 blocks monthly. The greatest variation
appeared in the manufacturing process. The 9 plants producing less
than 50,000 blocks required 27.8 man-hours per thousand as compared
with 7.6 man-hours for the 5 plants which produced in excess of
350,000 blocks during the same period. The next greatest variation
(24.1 man-hours compared with 6.7) occurred in the yard operation.
Similarly, the man-hours required for the administrative function
varied considerably—from 15.2 man-hours for the small to 3.0 man
hours for the large producers. The least variation was in overhead,
with the smallest producers requiring 7.4 man-hours, and the largest
5.8 man-hours per thousand concrete blocks.
.
T a b l e 1 — Average Num ber o f M an-H ours Required To Produce 19
000 Concrete Blocks,
1946, 1 by Rate o f Production
Man-hours per 1,000 blocks
Monthly rate of production
(8 z 8 x 16 inch equivalent
units)
Number
of plants
Total
Manufac
turing
Yard
Over
head
Adminis
trative
All plants
50
30.2
10.6
9.9
5.6
4.1
T n ftp.*5 ,0 0units
Td
00
__
*50,000-140,000 units
.
___
1/50,000-240,000units___
9*50,000-340,000 units
350,000 units and over_________ _______
9
17
14
5
5
74.5
38.0
30.9
26.9
23.1
27.8
13.6
10.9
9.8
7.6
24.1
12.8
10.6
9.5
6.7
7.4
6.0
5.5
4.5
5.8
15.2
5.6
3.9
3.1
3.0
i Does not include man-hours required for preparation of aggregates or transportation of finished product.
Molding machines.— The most important producing unit in any
plant is the molding machine, and the capacity of a plant is based on
the number of machines in use. In this study important variations
in man-hours were found to exist between plants having one, two,
and three or more of these machines. Table 2 shows a break-down of
man-hours required to produce 1,000 blocks for 45 plants, by number
of machines. A significant difference in total man-hours required
,
T a b l e 2.— Variation in M an-H ours P er 1,000 Blocks 1 94 6 ,1 b y Num ber o f M olding
M achines
Machines in use
Num Number of
ber of
blocks
plants produced
Man-hours per 1,000 blocks
Total
Manufac
turing
Yard
Over
head
Admin
istrative
All plants_______ ___________
45
7,723,000
28.7
10.7
9.5
4.7
3.8
1 molding machine....................
2 molding machines..................
3 or more molding machines___
20
19
6
2,025,000
3,836,000
1,862,000
34.3
27.5
24.9
11.5
10.3
10.3
11.9
8.7
8.6
6.2
4.4
3.5
4.7
4.1
2.5
1Does not include man-hours required for preparation of aggregates or transportation of finished product.
existed in those plants having one machine as compared with plants
having three or more machines. As table 2 indicates, the manufac
turing operation showed the least variation because of the automatic
16
features of most machines, while the yard and overhead functions
showed the greatest variations.
It should be noted that during this period, 6 large plants, each
with 3 or more machines, produced almost as many units as 20 plants
with 1 machine.
B y Lightweight and Heavyweight Aggregates
Concrete blocks are divided by the industry into two major
classifications—lightweight and heavyweight. A lightweight block
on the average will weigh about 30 pounds, while a heavy
weight block will weigh approximately 40 pounds for the same
size— 8 x 8 x 16 inches. The range is from 27 to 33 pounds per block
for the lightweight, and from 38 to 44 pounds for the heavyweight
blocks.
Table 3 shows the average number of man-hours required to produce
1,000 blocks by the two types of aggregates used. In general, 5 more
man-hours per 1,000 blocks were required to produce lightweight block
than heavyweight block. Some manufacturers contend that no appre
ciable variation should occur when production factors are equal. It is
probable that the variation indicated is due to an unbalanced propor
tion of efficient producers among the manufacturers of the two types
of blocks as represented by the sample. A sample sufficiently large in
size would tend to eliminate the chance occurrence of improper pro
portions and provide more conclusive evidence of variations according
to type of aggregates used.
T a b l e 3.— Average Num ber o f M an-H ours Required To Produce 1,000 Concrete Blocks,
1946, 1 b y T yp e o f Aggregates Used
Man-hours per 1,000 blocks
Type of aggregates used
Number
of plants
Total
Manu
facturing
Yard
Admin
Overhead istrative
All types...................................................
50
30.2
10.6
9.9
5.6
4.1
Lightweight...............................................
Heavyweight.............................................
31
32
32.4
27.4
11.2
10.0
10.1
9.6
6.8
3.9
4.3
3.9
1 Does not include man-hours required for preparation of aggregates or transportation of finished product.
