INTRODUCTION TO AGRICULTURAL
ENGINEERING, SOI 203
By
Dr. M. K. Othman
NAERLS, Ahmadu Bello University, Zaria
AGRICULTRAL ENGINEERING
Agricultural engineering is the application of
engineering principles to any process associated with production, processing
and handling of agriculturally based goods and management of the natural
resources. The discipline focuses on the development of labour-saving farm
machines, farm buildings, irrigation and drainage systems, and processes for
preserving and converting agricultural products to useful food, feed, and fibre
products.
WHO ARE AGRICULTURAL ENGINEERS?
Agricultural
engineers are the broadly and intensively trained individuals who
•Devise
practical, efficient solutions for producing, storing, transporting,
processing, and packaging agricultural products
•Solve problems related to systems, Processes
and machines that interact with humans, plants, animals, microorganisms, and
biological materials
•Develop
solutions for responsible, alternative uses of agricultural products, by
products and wastes and of the natural resources - soil, water, air, and energy
and
•Do all
these with a constant eye toward improved protection of people, animals and
environment.
Agricultural engineers are those professionals who
are constantly striving to ensure that the growing world population has the
necessities for life: safe and abundant food and water to eat and drink; timber
and fibre for shelter and clothing; plentiful and renewable energy resources,
clean air and a safe, healthy environment in which to live.
One feature that distinguishes agricultural
engineers from other engineers is their interest and commitment to solving
problems and pursuing opportunities related to human and animal needs for food,
feed, and fibre and a sustainable, safe living and working environment. The
biological and economic constraints will continue to make this a challenging
career opportunity.
OPTIONS IN AGRICULTURAL AND BIOLOGICAL
ENGINEERING
Agricultural engineering embraces a variety of
specialty areas. As new challenges, technology and information emerge, specialty
areas are transformed, new ones are created with many overlapping with one or
more other areas. Here are descriptions of some of the exciting specialties
that have emerged and / or are emerging in agricultural engineering.
Forest Engineering
Forest engineering entails the application of
engineering principles to solving problems in natural resources and
environment, forest production systems and related manufacturing industries.
Engineering skills and expertise are needed to address problems related to
• Equipment design and manufacturing
• Forest access systems design and
construction
• Machine-soil interaction and erosion
control
• Operations analysis and improvement
• Wood product design and manufacturing
• Forest operations analyzation and
improvement
• Decision modeling
Forest
engineers are involved in a full range of activities in natural resource
management and forest production systems.
Aqua cultural Engineering
Aqua
cultural engineering involves the preservation of our natural fish populations
and habitats through improved aqua cultural practices. Agricultural engineers
concentrate on increasing production while decreasing costs and environmental
impact through:
• System design for fish farms
• Water quality, machinery, feeding,
and ventilation
• Pollution reduction and water
conservation
• Ecological re-use or disposal of
waste
• Aquatic animal harvesting, sorting
and processing
Agricultural engineers specializing in water
quality, biotechnology, power and machinery, natural resources, food
environment and sanitation are suited for careers in this ever-expanding field.
The demand for aqua cultural engineering will continue to grow as natural fish
supplies are threatened and continue to decline.
Safety, Health and Ergonomics
Safety,
health and ergonomics focuses on making agriculture safer, more efficient, and
more economical, Agriculture is one of the few industries in which entire
families - who often share the work and
live on the premises - are vested and are at risk for injuries, illness, and
death. Agricultural engineers are charged with the responsibility of constantly
improving the safe use of agricultural equipment. Their scope includes to:
• Compile and analyze health and injury
data
• Standardize equipment for component
compatibility
• Encourage safe use of machinery,
equipment, and materials through better design and better communication
Power Systems & Machinery Design
Agricultural engineers in this specialty area
focus on improving efficiency and conservation in agricultural, food, and
biological systems, Agricultural engineers are involved in designing and
developing advanced equipment that are mechanically sound and biologically
sensitive such as
• Agricultural tractors, combines,
implements, and transportation equipment
• Landscape equipment
• Equipment for special crops
• Irrigation equipment
• Farmstead equipment
• Food processing equipment
Their work remains challenging as technology
advances, production practices change and equipment manufacturers expand
globally,
Food and Bioprocess Engineering
Food and bioprocess engineers use
microbiological processes to develop useful products, treat municipal,
industrial, and agricultural wastes, and improve food safety and these
includes:
• Packaging, storage, transportation of
perishable products
• Pasteurization, sterilization,
irradiation techniques
• Food processing techniques &
technologies
• Biomass fuels
• Pharmaceuticals
• Biodegradable packaging materials
Food and bioprocess engineers work on the
boundary where biology meets engineering. Food, fibre, and timber are only the
beginning of a long list of products that benefit from efficient use of the
natural resources. The list is growing - it includes biomass fuels,
biodegradable packaging materials, pharmaceutical and other products and is
limited only by the creative vision of food and bioprocess engineers. These
engineers understand microbiological processes and use this expertise to
develop useful products, to treat municipal, industrial and agricultural
wastes, and to improve food safety. Increasing concerns about food safety and
environmental protection are creating a growing demand for food and bioprocess
engineers.
Information & Electrical Systems
This is perhaps the most versatile specialty
area and it is applied to virtually all others from machinery design to soil
testing to food quality and safety control. Some of the exciting information
and electrical technologies being used today and being developed for the future
include:
• Geographic information systems [GIS]
• Global positioning systems [GPS]
• Machine instrumentation and controls
• Data
acquisition and bio-informatics: bio-robotics, machine vision, sensors,
spectroscopy
• Electromagnetic
Developments in information and electrical
technologies provide the agricultural community with opportunities for
increased efficiency and improved reliability, safety and productivity. In
modern agriculture, all engineering touches on one or more aspects of
information and electrical technologies in developing agricultural systems.
Structures & Environment
Structures and environments entail engineering
a healthy environment for living things. Agricultural engineers with expertise
in structures and environment design safe and economical structures such as
• Animal housing
• Greenhouses
• Storage structures for grain, fruits
and other food products
• Waste storage, recovery, re-use,
transportation
• Climate, ventilation, disease control
systems
Biological Engineering
Biological engineering is one of the most
rapidly growing specialties in agricultural engineering and this specialty
applies engineering practice to problems and opportunities presented by living
things and the environment. Biological engineers are involved in a variety of
exciting interests that continue to emerge as the understanding of science and
nature grows and these include:
• Pest control
• Hazardous waste treatment
• Environmental protection
• Bio-instrumentation
• Bio-imaging
• Medical implants and devices
• Plant-based pharmaceuticals and
packaging materials
This fast developing specialty has resulted in
many universities especially in the USA and Europe to re-designate their Agricultural
Engineering Departments to Biological and Agricultural Engineering [BAE]. In
Nigeria, University of Nigeria Nsukka and many other Universities have already
re-designated their Agricultural Engineering Department to Department of
Agricultural and Bio-resources Engineering. However, it is important that the
re-designation of the agricultural engineering programme should be adequately
harmonized to reflect the uniqueness of the agricultural engineering profession
and the emerging challenges in of bio-resources. Similarly, there should be
minimum standard for the programme in all institutions.
Natural Resources
The devastating droughts, fast desertification
processes, ravaging gully and coastal erosion, oil pollution and massive
deforestation are constant reminders that our land and water resources are
vulnerable to degradation by both natural and man-made force/. This specialty
area focuses on improving conservation by understanding the complex mechanics
of soil and water and its areas includes:
• Wetlands protection
• Water control structures: dams,
reservoirs, floodways
• Drainage
• Erosion control
• Pesticide and nutrient runoff
• Crop water requirements
• Water treatment systems
• Irrigation
Agricultural
mechanization
It is the art and scientific application of
mechanical aids for increase product and preservation of food and fibre with
less drudgery and increased efficiency.
Hindrances of agricultural mechanization:
1.
Lack
of capital
2.
Lack
level of capability of local industry
3.
Law
level Lack of training and extension personnel
4.
Small
filed size
5.
Irregular
filed shape
6.
Slow
industrial development
7.
Crop
varieties not amendable to mechanization.
Advantages of
mechanization
1.
Increases
land productivity
2.
Increases
return to famers
3.
Reduces
losses in storing and processing
4.
Improves
timelines
5.
Improves
appreciation of operation
6.
Reduces
drudgery
7.
Improves
work environment
Agricultural engineering is the application of
any and all branches of engineering to the extent that they may be used for
farm operation, rural processing of farm products and such allied activities such
as malaria control and wildlife conservation.
Objectives: The basic
objectives of agricultural engineering are:
1.
To
reduce the hazards of faming. These include too much water, effect of storms,
fires, pests and accidents around build.
2.
To
reduce production costs, this can be achieved through better farming methods,
more efficient and design of structures.
3.
To
improve and retain the quality of farm products. Better storage, ventilation,
refrigeration, pasteurization, grading and improved methods of handling can
achieve this objective.
4.
To
utilize farm by products for profitable. By proper processing method, string
and handling, these can be made to serve a useful purpose.
5.
To
remove drudgery from farm operation this can be achieved through proper
handling of manure, carrying water and feed, haying and similar jobs.
6.
To
make farm jobs more enjoyable through provision of modern conveniences as
running water, electricity, sewage disposal.
7.
To
conserve soil and water for efficient utilization such as moisture
conservation, regulation of sub-surface water, planning of efficient farmstead
and farm houses.
Energy: Energy is the capacity to perform work
measured using the same unit of work. Energy comes in various forms e.g solar
energy, electrical energy, etc. the utilization of the latent chemical energy
in coal, oil and gas, released in the form of heat to drive internal
contribution engines, has been a major factor in the development of modern
civilization.
Mechanical Energy: In mechanics, there kinetic energy
and potential energy.
Potential energy is the energy which a body has by
reason of its position in a field of force or by its state. Kinetic energy is the energy a body
possessed when it is above the potential energy by reason of motion:
Work done = force x distance.
Power: it is defined as the rate of transfer of
energy.
Average power =
energy transferred
Time taken
The SI unit of power is called watt (W) and is
defined as the rate of transfer of energy of I joule per second. 1 W = joule /s
Example: Calculate the power of a pump
which can lift 1000 kg of water to a vertical height of 10m in 20s (assume g =
10m/s2).
Solution
Force of engine pump = 1000 x 10N
Distance = 10m
Work done (energy transferred) = 1000x10x10
Time taken = 20 s
Power = energy transferred/Time taken
=
1000x10x10
20
=
5000 W
=
5 kw
Example: A man of mass 50 kg runs up a 50
steps; each step is 10 cm high in 5s. find his power utilized. (Assume g = 10m/S2)
Solution:
Force overcome
= 50 kg x 10m/S2 = 500 N
Distance
= 50 steps x 10 = 500cm = 5m
Energy transferred = work done = force x
Distance = 500 N x 5
Power =
work done
Time taken = 500
N x 5m = 500 w
5
Example: A boy throws a stone of 400g
vertically upwards with a velocity of 20cm/s. Find: (a) the kinetic energy at
greatest height (b) the kinetic energy on reaching the ground. (Assume g =
10m/S2 and neglect air resistance).
Solution: V2
= U2 + 2as
U
= 20m/S, V = O m/s, a = g = - 10m/S2
Derived SI units
having special/names
Force: The Newton (N) is that force which
applied to a body having a mass of 1kg given it an acceleration of 1m/s2
Power: The watt (W) is a rate of energy
transfer of one joule per second
1 W = 1 J/s.
Frequency: The hertz (Hz) is the frequency of
periodic phenomenon of which the periodic time is 1s. 1 Hz = 1 cycle/second.
Units, equivalents and
conversion factor
1 acre = 0.40409 hectares
1 calorie = 0.001163 kilowatt – hour
1 calorie per second = 4.18617 kilowatts
1 centimeter = 10 millimeters.
1 gram per cubic centimeter = 1000
kilograms/cubic meter
1 hectare = 10,000 square meters = 0.01 square
kilometer
1 horsepower = 0.74565 kilowatt
1 inch = 0.0254 meter
1 kilogram = 1000 milligrams
1 kilometer = 1000 meters
1 kilowatt = 1000 watts = 1.34111 horse power
1 litre = 1000 cubic centimetres
1 mile square = 2.59 square kilometers
1 ton metric = 1000 kilograms
Mechanics
Mechanics: It is the science that treats of
forces and their effects.
Mass, force and weight
The mass of a body is the quantity of mater it
contains. A force is a push or a pull and may be measured by its effect on a
body. A force may change or tend to change the shape or size of a body; if
applied to a body at rest the force will more or tend to more it. If applied to
a body already moving the force will change the motion.
A particular force is that due to the effect of
gravity on a body i.e the weight of a body. The SI unit of mass is the kilogram
(kg) other units mass are:-
1 mega gram (mg) or tone (t) = 103
kg
1 gram (g) = 10-3 kg
1 milligram (mg) = 10-6 kg.
The derived SI units F= 1N, M = 1 kg and a =
1m/S2 i.e 1 (N) = 1 (kg) x 1 (on/S2).
Other units of force used are
1 kilo Newton (KN) = 103N
1 mega Newton (MN) = 106N and 1 Giga
Newton (GN) = 109N
The acceleration of a body towards earth in
free fall is g =9.81m/s2. Hence weight (w) of a body of mass in SI
unit is kg m/s2 (W = mg), this is equal to force which is product of
mass and acceleration (mass x acceleration)
Example If the mass m is in mega grams (tones)
then W= m x 1000 x 9.8N = m x 9.8KN
Proper specifications of a force requires
knowledge of three quantities
1.
Its
magnitude
2.
Its
point of application
3.
Its
line of action
Since a force has magnitude, direction and
sense, it is a vector quantity and may be represented by a straight line of a
definite length.
Electrical resistivity: Some materials allow electricity
to pass through them very easily and are called electrical conductors. These
include carbon and most of the metals such as aluminium, cropper, gross and
silver. Other materials offer a high resistance to the flow of electricity and
are called bad electrical conductors or insulators. These include non-metallic
materials such as plastics, rubber, wood, ceramics and glass. The resistance of
electricity and the dimensions of the conductor as well as the material
conductor is measured in ohms, which depend on the dimensions of the conductor
as well as the material from which it is made. The resistance to the flow of
electricity can be found from the following equation knowing the area and
length of the conductor and its resistivity.
R =
Where R= resistance, in ohms
L= length of conductor, in metres
A= cross sartorial area of conductor in, in
square metres
S= resistivity of the conductor material, in
ohms metres
Example: what is the resistance of an
electrical conductor I mm diameter and 20 maters long whose resistivity is
2.5410-8 ohm metres?
Solution:
Area of conductor = (22/7)X(1)2= 0.7854mm2
4
Since the other valves are expressed in metre
area in m2 = 0.7854 x 10-6 m2
1000 x 1000
Resistance is given by slla which is
2.5 x 10-6 x 20
0.7854 x 10-8 = 0.637 ohms.
Construction materials
The strength, durability and service of a farm
implement depend largely upon the kind and quality of material used in building
it. The material may be metallic and non-metallic.
Non-metallic
materials: They
include wood, rubber, leather vegetable fibre and rubber.
Rubber: Rubber is both derived from trees
and can be made synthetically. There are different grades of rubber materials
varying in hardness, flexibility, bonding properties and chemical resistance.
The uses of rubber on farm equipment include
implement tyres, tubes, flat and V-belts.
Plastics: A plastic material is an organic
solid, polymerized to a high molecular weight, that is capable of being moulded,
with the aid of heat or pressure or both, plastic product is used for seed
hoppers and chemical tanks, plough handles, bearings, tubing, conveyor belting,
windows and machine panels.
Leather and vegetable fibre: Leather is largely
belting material, vegetable fibre upholstery padding.
Nonferrous metals: They are copper and its alloys (such
as brass and bronze), aluminium magnesium, lead, zinc and tin.
Alloy: An alloy is a substance that has
metallic properties and is composed of two or more chemical elements of which
at least one is a metal. Examples of alloys are bronze, brass, alloy steels and
aluminium alloys.
Copper: It is soft enough to be rolled or hammered
into sheets or drawn into fine. It is used for ignition and electric wires on
engines, in generator and electric motors.
Brass: Ordinary brass is an alloy of
copper and zinc. Some commercial brass contains small percentages of lead, tin,
and iron. It is used for making radiators, pipe, welding rods, instrument parts
and fittings.
Aluminum: It is a white metal with a bluish
tinge which is resistant to corrosion. It is used to make light castings and
for coating chemical tanks.
Zinc: It is crystalline, metallic
element, brittle when cold and malleable at 110 to 210oc. It is used
as a coating and sheet iron and die castings as a protection against corrosion.
Ferrous metals: The ferrous metals include cast
iron wrought iron and steel. They are produced by the reduction of iron ore
into pig iron by various manufacturing processes.
Cast Iron: There are five types of cast iron
and they are white, chilled malleable and ductile cast iron.
Wrought iron: It is nearly pure iron and is used
in forge work since it is readily welded and easy to work, commercial form. It
is obtained by rolling the hot iron into bars or plates from which nails,
belts, wire and chains are made.
Steel: It is made from pig iron with
manufacturing processes different from that of cast iron. It may be classified
by:
(1) The manufacturing process as.
(2) The carbon content
(3) Alloy steel where other metals are
added
(4) Uses such as structural or tool
steel and
(5) Methods of forming, such as rolled,
forced and cast
Definitions
1.
Harwood: Conventionally, the timber of broad-leaved
trees belonging to the botanical group angiosperms.
2.
Sapwood: The outer layers of wood which in the growing
tree, contained living cells and reserve materials (e.g starch). It is
generally lighter in colour
3.
Softwood: Conventionally timber of coniferous trees
belonging to the botanical group gymnosperms.
4.
Timber: Wood in a form suitable for construction or
carpentry, joinery or for re-conversion for many manufacturing purposes.
5.
Wood: The principal strengthening and
water-conducting tissue of stems and it is a substance of which trees and
shrubs are largely composed.
6.
Woodworking: It is the forming and shaping of
wood to make useful and decorative furniture.
7.
Seasoning: The process of drying timber to a
moisture range appropriate to the tins and purposes for which it is to be used.
8.
Air-seasoning: The process of drying timber by
expositive to natural atmospheric conditions.
9.
Kiln-seasoning: The process of drying timber in a
kiln.
Moisture-content: The amount of moisture timber or
other material expressed as a percentage of its wet-dry weight moisture content
(%)
Weight wet wood-weight of dry wood and 100%
Weight of dry wood
Nigeria has abundant timber recourse and wood
has always been a major construction material. Its low cost and availability in
various forms and sizes, together with such properties as relatively great.
Ease of shaping and fastening, low heart conductivity and round deadening
qualities, have made if the outstanding building material. Even today, despite
the extensive use of other structural materials, the railroads, high way
departments, telephone, agricultural, building, mining and aviation companies
depend upon wood to till much of their construction materials.
Wood workshop tools
There are some fundamental tools that should be
available in wood workshop.
The tools include:
Handsaw: It is used for cutting large
pieces of wood.
Planes: Tin plane wood to make it
straight, flat, and square and to make it smooth.
Chisels: Firmer chisel is the bench tool
used for general purposes. It is robustly made so that it can stand up to the
work involved in chopping
Brace: It enables carpenter to bore a
large hole and wood with ease.
Hammer: It is useful for general indoor
woodwork furniture making. It has the cross metal head and handy for starting
or driving nails into the wood the handle is always made of wood.
Mallet: It is used for tight hitting or
driving of an object into the wood. It has round head with a Tapered handle
that fits the head with a wedge fit to prevent it from flying.
Screw driver: It is needed for driving in or
removing screws or nails.
File: It is pushed forward while resting
on wood. It takes on lumps and removes saw marks.
Clamp: It is used when wood is rained in
its thickness. It can be used in holding two or more pieces of wood together to
apply pressure.
Steps in wood working:
1.
Planning and
design: careful planning can prevent
mistakes and save time and materials. A scale drawing of the object being built
should be made before starting any woodworking project. This drawing includes
exact measurements of the object.
2.
Cutting: Cutting wood to the right size and shape can
be done with a variety of hand and power tools, including saws, chisels, and
planes. Power tools can do a job for more quickly, easily and accurately than
hand tools.
3.
Drilling: Drilling enables a wood work to connect
sections of wood with screws metal plates and hinges. Drilling may some joints.
Portable electric of the machines that are always used for drilling operation.
4.
Fastening: Sections of wood are fastened
together with metal fasteners such as screws and nails. Tools for fastening
include screw drivers and hammers.
5.
Sanding and finishing: Sanding removes tool marks and
makes wood surfaces smooth for finishing. Sanding should not begin, until the
wood has been cut to its final size.
Safety precautions in
woodworking machinery
1.
Driving belts: Machines that are driven by belts
from an overhead shafting. Belts not to be touched by hands when moving and
they should be guided.
2.
Clothing’s: Loose clothing may be entangled in
a moving part and because some of accident. So, overalls without loose ends are
recommended for use in woodworking.
3.
Wheel guards: They should be used on high speed
power-driven grinders. They keep participles of wood away from flying outward
toward the operator
4.
Firm
clamping of jobs, when using a power drill, the wood being drilled should be clamped firmly to availed the wood
being thrown off the table and injuring the operator.
5.
Electrical accidents: Electrical parts of wood working
machinery should be treated carefully. If the wires are loose they should be
corrected keeping, keep work spaces free from obstructions keep work benches
and workshop tools clean and in good order.
6.
Keep
a first-aid kit in the wood workshop. Use it even for apparent minor cuts and
injuries.
Woodwork and farm
carpentry
Bill of material: A bill of material is an itemized
list of the number of pieces needed and the dimensions of each.
Timber dealers have books for writing orders
for Timber. An order contains the name and address of the buyer, the date, the
name of the project, the number of pieces of lumber, the kind of wood, the name
of each pieces, and the cost. A complete bill of materials contains the name
and address of the buyer, the date, the name of the project, the number of
pieces of lumber, the kind of wood, the name of each piece, the dimensions of
each piece, and the cost. A complete bill of materials contains a list of all
the necessary materials for building a project unit of measure in selling timber:
while the unit of measure in timber is a board fort, timber is generally priced
on the basis of 1000 board feet (per. m).
Concrete work
Concrete is a mass of sand and gravel held
together by a cement paste. The cement paste is made of Portland cement and
water, concrete is economical, strong, durable, sanitary and attractive in
appearance. The cement and water forms a binder which holds the sand, gravel or
rock, commonly called aggregates together.
Uses of concrete on
farms: Concrete has
become an extensively used material in the construction of buildings. Other
common uses of concrete on farms are: feeding troughs, watering troughs,
foundations and walls, floors, milk houses, septic tanks, manure pits, building
blocks, inverts and fence posts.
Reason for wide uses
of concrete
1.
It
is adaptable and serviceable in a great variety of situations
2.
It
is permanent when properly made
3.
It
is sanitary and easily cleaned
4.
It
is more nearly fireproof than other building materials
5.
It
is rat and other rodent proof
6.
It
can be used for almost any purpose when properly reinforced
7.
It
any be made attractive and decorative
8.
It
is economical in installation and in maintenance
Concrete as Material
Plain concrete is obtained by adequately mixing
in certain proportions of aggregates (gravel and sand), Portland cement, and
water. Plain or reinforced concrete is used in livestock housing for
structures, foundations, floor, and walls. It is a durable material that can
resist attack by water, animal manure, chemicals, and fibre. High-quality
concrete is recommended for milk-, silage-, or manure-containing structures.
Properties of Concrete
Two main properties of concrete are strength
and workability. The strength of concrete depends on various factors, mainly
the proportion of sand, quality of the ingredients and the temperature and
moisture under which it is placed and cured. The methods for proportioning and
placing concrete to achieve a preset required strength can be found in the
literature. Concrete can develop a very high compressive strength equivalent to
two to five times that of wood. Compressive strength of a concrete increases
with the age of the concrete. It is measured by crushing cubes or cylinders of
standard sizes. Concrete design is based on the characteristics, strength values
at 28 days of age. Its tensile strength remains weak, however, about one tenth
of its compressive.
For this reason, steel rods (re-bars) are combined with concrete. In reinforced concrete, the area and position in of steel bars determined according to applicable standard codes.
For this reason, steel rods (re-bars) are combined with concrete. In reinforced concrete, the area and position in of steel bars determined according to applicable standard codes.
Workability of concrete relates to its ability
to be poured in forms and to properly flow around steel bars. This measured by a
slump test. Concrete can be placed in forms. This operation is undertaken
either on the construction site or in a pre-fabrication plant.
Aggregates: These are the materials used to
give bulk and body to concrete. They are in two categories such as fine
aggregate and coarse aggregate.
Fine Aggregate: They are materials which will pass
through a one to fourth inch in mesh screen, sand or crushed stones screenings
are usually used as the fine aggregate
Coarse Aggregate: They are materials like gravel pebbles or crushed rock,
ranging from fourth inch up.
All aggregate should be clean and hard and free
from dust, loam, day or vegetables matter foreign materials keep the cement
from adhering to the aggregate materials or particles and weaken the concrete.
Proportions and
quantities for concrete mixtures
Guessing is poor economy in handing concrete.
Without experience with concrete, persons who guess at the quantities of
cement, sand and gravel needed for concrete jobs are likely to have surplus of
some of the ingredients, which cannot be used economically. A shortage of one
of more ingredients results in delays in finishing jobs. To disregard recommended
mixtures or careless application may result in excessive costs in addition to
an inferior unsatisfactory product. The proper mixture is workable and contains
enough fine aggregate to fill the air spaces around the coarse aggregates and
enough cement paste around each particle to bind the whole mixture together in
a dense, plastic solid mad. It is a common practice to have concrete mixtures
in terms which indicate the proportions of the material. For example a 1: 2: 3 mixtures
consist of 1 part of cement by volume, 2 parts of sand and 3 parts of
pebbles.
A workable mixture can be placed in forms
readily and with spading will result in a dense concrete. It contains enough
sand and cement (1) to give smooth surfaces and (2) to bind the pieces of
coarse aggregate into the mass that will separate when the mixture is handled.
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