EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World Concrete Construction
Field Techniques for Use in the Developing World
Jon Fripp PE1 , Phuc Vu PE2, and David Cruz PE3
Subject Key Word: Concrete Construction
Key Words: Concrete, Mix Design, Slump, Field Control, Field Test
Abstract: A variety of projects in the developing world involve construction with
concrete. This material is strong, cheap, durable, water tight, resistant to abrasion and
readily obtained in a variety of areas. Concrete is made up of three components:
Portland cement, water, and aggregate. While the components are simple; adequate
mix design and quality control during construction is critical to obtain a good project.
While all designers want as high a quality mix as possible, this goal needs to be
achieved as simply as possible with readily obtained materials and easily implemented
techniques when working in the developing world and in low or limited resource areas.
This paper summarizes concrete mix design with a particular focus on quality control
under field conditions. Rules of thumb and ad-hoc techniques, which are applicable in
resource limited areas, are presented. This paper is intended to be used as a field
reference sheet and will be of use to aid groups involved in concrete construction
projects especially where conventional first world techniques are not feasible.
Introduction
Concrete has been used to construct important structures for over 2000 years. Modern
concrete practice dates to almost 200 years ago and it remains an important building
material both in the developed as well as the undeveloped world. Concrete is the
second most utilized man made product on earth (Concrete Trust, 2013) and is used for
buildings, foundations, bridges, dams, pipes, highways, posts, and water storage
structures. It is favored because of its durability, strength, and applicability to a wide
variety of engineering projects. However, concrete must be mixed, placed, and cured
correctly to achieve the design goals of a project.
In the developed world, there are a variety of established practices and procedures
which, if followed, assure a consistent and quality product (ASTM C172, ASTM C14378, ASTM C231, ASTM C39). This guidance is taught in universities and is readily
1
Civil Engineer Technical Mentor, Texas A&M‐EWB, North Texas Professional EWB, and Southern Methodist University‐EWB 2
Civil Engineer Technical Mentor, Southern Methodist University‐EWB and Portland Professional EWB 3
Civil Engineer, Membership Coordinator, North Texas Professional EWB and Southern Methodist University‐EWB Page 1 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World available from a variety of sources. However, the use of these techniques and practices
requires equipment and testing that is often not available for short-term projects in the
developing world.
This paper addresses concrete mix design and control for small projects that include
concrete slabs and walls. It does not address mortar design. Throughout this paper,
reference is made to ‘the engineer’ as the person in charge. In some cases, an
experienced technician or mason may be a suitable person to direct aspects of a small
project.
The techniques and procedures presented in this paper are applicable where
conventional quality control and testing approaches are not possible. The focus of these
techniques is on achieving good enough results which can be appropriate for many
limited resource based construction projects. However, the short cuts presented are not
appropriate for high value and high-risk projects. Projects of more than one story or
greater than 10 feet in height require a more thorough treatment. An engineer with
experience in concrete work must make the decision as to when they are appropriate.
The use of any construction technique should never be put to a popular vote. It is
incumbent upon the responsible engineer to assure that the ad-hoc approaches are
good enough for the project being constructed.
Materials
Concrete is made up of water, cement, aggregate, and additives. In many developing
world situations, additives are not readily available. Therefore, the three components
discussed in this guidance paper are as follows:
1. Water: Relatively clean, clear water should be used. While the water does not
need to be potable, water that is contaminated with salts, tannic acid, algae,
sewage, oils and chemicals should be avoided. The pH of mixing water should
be between 4.5 and 8.0 which can be checked with a hand held pH meter.
2. Cement: This is usually obtained in 70 lb or 94 lb bags of Portland cement. The
larger size is approximately equivalent to one cubic foot. It is important to assure
that the cement is relatively pure and has not been pre hydrated. Figure 1 shows
photographs of cement that should be rejected.
Page 2 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World Figure 1: Reject concrete that has been hydrated (left photo) or material that contains
contaminants (right photo).
The engineer in charge should spot check some of the delivered material to assure that
it does not contain inappropriate materials such as dirt, saw dust and seed hulls. Such
contamination not only adversely affects the strength but also indicates that the overall
product is of substandard and often unknown quality.
The engineer in charge must also assure that the concrete has not been pre hydrated.
If the bags are wet or the concrete has a solid feel, the cement should be rejected and
not used for the project. Cement that has been hydrated cannot be ‘made right’ by
breaking it with hammers. A visual inspection is usually sufficient to assess the
adequacy of the cement. Be sure to protect delivered cement bags from rain. If the
project is being constructed in an area of high humidity (often characteristic of the
tropics) prolonged storage of greater than a week should be avoided.
3. Aggregate: This is divided into coarse aggregate and fine aggregate.
a) Fine Aggregate: This is sand and/or small crushed stone material. It should
be less than 3/8 inch in size and can include rock powder or rock dust. Be
careful to avoid contaminants such as debris, mica, and soil.
b) Coarse Aggregate: This is gravel or crushed stone. Angular stone is
preferred over smooth stone. The coarse aggregate should be 3/8 to 1.5 inch
in size. However the maximum size of the coarse aggregate should not be
more than 1/5th the distance between form faces, 1/3 depth of slab or 3/4ths
the distance between reinforcing bars. The coarse aggregate must be sound
rock. If it is weak, the concrete will be weak. Avoid mudstone, coal, lignite, as
well as porous or sandstone rock. A field expedient test is to sharply strike a
sample rock with a hammer. If it readily shatters, do not use that type of rock.
If weak rock is the only material that is available, the designer should assume
that the resulting concrete will be weak.
Page 3 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World It is often necessary to separate the fine and coarse aggregate materials on site. A
screen frame can be fabricated on site and used to segregate materials (Figure 2).
Figure 2: Screens constructed on site to segregate aggregate materials
Particular care must be taken to remove any organic debris (wood, dirt clogs, etc) from
the aggregate. Construction debris and trash must also be avoided. It is recommended
that the aggregate be kept moist. This can be done by wetting the aggregate an hour
before mixing and will help maintain consistency of the mix. If the aggregate is dry,
some of the added water will absorb into the aggregate and not be available for the
hydration of the cement.
The engineer should visit the site where the aggregate material is to be obtained. The
engineer should confirm the quality and how it is to be mined. It is often useful to create
sample set of aggregate materials on site to be used as an example (Figure 3). Be sure
to note rock that is too large as it is common for inexperienced personnel in the
developing world to use larger aggregate than is appropriate. The aggregate guide can
be provided as a photo or in small plastic bags to the personnel who are obtaining the
materials since they may be more accustomed to following physical examples than
written specifications. The sample also facilitates matching size and consistency. While
aggregate color is not a property that is of concern, often nearby sources will be similar
in color and shade which can be useful when querying locals about potential sources.
Figure 3: Example gradation samples. Rock to the left of the red pen is too large and
should not be used.
Page 4 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World Mix
A typical, workable concrete mix design in the developing world is 3:2:1. This means 3
parts gravel, 2 parts sand and 1 part cement. The typical water to be added is ½ part
which results in a water/cement ratio of 0.5. Most mixing that is done on a site in the
developing world is accomplished by volume (referred to as volumetric mixing). This is
accomplished by measuring materials by volume – typically by buckets or shovels of
consistent size. It is important to keep careful count of the buckets (or shovels) during
the mixing process as often the work becomes fast and workers may lose count.
On site mixing is the norm for work in the developing world. Ideally, it is done in a
powered mixer (Figure 4) but can also be accomplished on the ground (Figure 5). If
mixing on ground or trough -mix the cement and sand in a dry condition; then add the
gravel. If mixing in a rotating drum, start with the gravel. The engineer should take
particular care with on ground mixing to avoid debris contaminating the mix.
After the dry materials are mixed, then slowly add enough water to make the mix
workable. Typically, the added water is about 1/2 the volume of the cement (a
water/cement ratio of 0.5). The mix should be the consistency of wet dough or stiff, dry
oatmeal. It should not be shiny or visibly wet. Do not add too much water or it will
adversely affect the strength of the concrete. The detrimental effect of too much water
can be dramatic. An increase of one gallon of water per 70 lb sack will generally
increase the water/cement ratio by 0.1 and decrease the strength by 1000 psi. It is
important to maintain close supervision of adding water when mixing on the ground.
Mixing on the ground often results in excess water being added which affects the
strength and water tightness of the concrete. A lower limit for a water/cement ratio is
generally 0.35. A water/cement ratio of 0.6 is the upper limit for water tight concrete.
Figure 4: Typical powered mixers. Inspect before use to assure they are clean and in
working order
Page 5 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World Figure 5: Typical ‘on ground’ mixing. Note excess water and the accumulation of debris
in the mix. Care should be taken to minimize this.
Slump
A slump test is done to check the consistency of the mix. A slump test involves filling an
inverted, bottomless cone with the concrete mixture. The concrete is placed in three
lifts and compacted with 25 strikes of a smooth tamping rod. The cone is removed and
the amount that the concrete ‘slump’ is measured from the top of the cone and
compared to the specifications. This is illustrated in Figure 6.
Figure 6: Three step process for conducting a slump cone test
An alternative field expediency approach can make use of a smooth sided, disposable
cup. Place packed layers of the concrete mix into the cup then turn the cup over and
measure the slump. The target is a slump of 25% to 50% of the original height. If there
is no slump, the concrete will be too dry to work. More than 50% slump is too wet and
the mix will not have the necessary strength. Insufficient slump can be remedied by
adding more water but excessive slump usually requires that the mix be discarded.
Page 6 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World Figure 7: Slump cup approach
Once the proper mix has been achieved, it is a good idea to mark buckets with the
amount of each component (Figure 8). This facilitates both the work and the monitoring
of the mixing. As the work progresses and especially if the day temperatures change, it
is strongly recommended that the slump be rechecked. It may be necessary to change
the water volume that is added especially if the moisture content of the aggregate
changes.
Weight of Concrete
The specific gravity of Portland cement is 3.15. If the specific gravity of the aggregates
is 2.65, it is generally assumed that a 3:2:1 mix will result in concrete that weighs 150
lb/ft3 or 4050 lb/yd3. Often the number of sacks of Portland cement required per yard
of concrete are specified as part of a design. A 3:2:1 mix requires seven, 94 lb sacks of
cement per yard of concrete.
Figure 8: Marked buckets for material mixing
Page 7 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World Forms
Concrete placed as a slab or walls generally requires forms to establish the necessary
final shape. Excavation should be larger than the slab to allow for this form work. Forms
must be very strong to resist the weight of the concrete. A form failure during
construction can not only be very expensive but dangerous as well.
In the developing world, the most common material for forms is wood. It is important
that this wood is a sound material. The surface of the wood that is in contact with the
concrete should be smooth and free of holes, dents, sags, or other irregularities. The
wood should be cleaned and free of dirt and dust before placing the concrete. A typical
approach for walls is to use ¾ inch plywood and 2x4s on a 2-foot spacing. It is
important to secure with sufficient bracing both inside and outside. Put spacers between
the forms and use a level to assure the forms are straight. The construction of the forms
must not be rushed.
Coat the wood with plastic, diesel fuel, or vegetable oil to assure that the concrete does
not stick to the forms. This also reduces moisture from the concrete from being
absorbed into the wooden forms. Vegetable oil is preferred if the concrete project is to
contain potable water. Figure 9 shows forms under construction for a water storage tank
and the coating of a form with vegetable oil as a release agent. For most small projects
of the type addressed by this paper, leave the forms on for 24 hours (7 days if possible).
Reinforcement
Most concrete construction requires steel reinforcement. This steel reinforcing bar
(rebar) is available by commercial manufacture. Rebar is typically round and is
patterned to create a firm bond with the concrete. The size and spacing of the rebar is
determined in the design and is usually tabulated in a ‘reinforcement schedule’ on the
plans. It is not generally something that is determined in the field. Rebar is referred to
by number which is in increments of 1/8 of an inch (#3 is 3/8 inch diameter) or in
millimeters (25 = 25 mm = 1 inch = no. 8). It must be cut and/or bent rebar as specified.
Do not use rebar that is coated in oil or excessive rust.
Figure 9: Concrete forms under construction
Page 8 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World In a slab, the steel should be in the lower 1/3rd of the depth and with at least 1.5 inches
(2 inch optimum) between the bar and the subgrade (ground). Use chairs to hold the
rebar in place and at the depth required. These must be spaced so that the weight of
the concrete does not push the steel down during placement. Typically these chairs are
wire or plastic. However, they can be made in the field with concrete and short pieces
or scrap rebar. Examples are shown in Figure 10. Rock, wood, and dirt clogs should
not be used to hold the rebar in place.
Figure 10: Commercially available plastic chairs and field constructed concrete chairs.
For most applications, the steel bar that is spliced should be overlapped by 18 inches.
The overlap should be secured in at least two places with tie wire. On right angle or
cross overlaps, the installers should double loop with tie wire in a saddle type
arrangement. A simple wire tie tool is often used to facilitate this operation (Figure 11).
Tie wire may also be used in walls to assure that the steel remains in the center of the
forms. For seismic applications, additional issues related to rebar placement are
necessary.
Exposed ends or rebar must be covered with a rebar cap until they are covered with the
concrete pour for safety of the workers. This important protection is commercially
available as plastic caps but they can also be made in the field by drilling a hole into a
block of wood. These are especially important for use on vertical pieces or rebar since
a fall onto this can result in impalement and serious injury. The engineer in charge
should periodically inspect the site to be sure that caps have not been knocked off or
stolen.
Page 9 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World Figure 11: Tie wire being installed
Placement
Care must be taken when placing concrete to assure a quality product. If concrete is
haphazardly placed, voids may form, components in the concrete can separate and
weak areas (cold joints) can occur between successive placements.
Concrete should be placed a soon as possible after it has been mixed. The time for
concrete to set or harden varies by temperature. At 100 degrees, a mix can set in 1 to 2
hours while at 70 degrees (an ideal temperature for concrete) it can take 6 hours. It is
generally good to avoid mixing more than can be placed in 30 minutes.
Each batch of concrete should be installed right after each other to avoid cold joints.
Place each batch on a ‘wet’ or non hardened edge. Do not dump concrete from a
height of over 5 feet or the mix can segregate. If a longer distance is necessary, and
improvised treme can be used as shown in Figure 12. In all cases be sure placement
does not result in damage to the formwork.
Compact the concrete in the walls with tamping each batch by poking (rodding) with a
pole. This squeezes out air bubbles, works concrete into gaps, and mixes each
successive batch that is place. This action also acts to compress the concrete. Often a
piece of rebar is used for this operation but it is strongly recommended that a protective
cap be placed on the exposed end. Do not allow workers to over rod (over work) a
section or segregation can occur. Over rodding will be evident when excessive water
appears at the top of the mix. In developed areas, an electric concrete vibrator may be
available.
Page 10 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World Figure 12: Improvised treme being used to place concrete from height
For slab construction, the concrete should be installed over a damp bed or compacted
earth, sand, or gravel (Figure 13). The base should be free of organic material. Fill from
one edge of the form to full height. Compact the concrete with a shovel, then compact
by screeding. To screed, first move a straight timber up and down across the wet
concrete in a tamping motion. Then drag the timber across the slab in a sawing motion,
level with formwork, to move excess concrete and bring the top surface to the design
grade.
Figure 13: Finishing concrete on a slab
Once the concrete in the slab has hardened enough to support a person leaving only
shallow footprints, finish with a float and trowel. This is done to remove slight humps
and voids, compact the surface, and achieve a final finish. Push the float lightly across
the surface in an even motion with the leading edge angled up. Similarly, a hand trowel
can be used in a sweeping motion across smaller slabs or at the edges of a large slab.
It should be noted that inexperienced concrete workers will often want to add excess
water to achieve a ‘mirror’ finish. This should be avoided for exterior applications as it
Page 11 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World will adversely affect the ultimate strength of the concrete surface. The nature of final
finishing depends on the ultimate purpose of the project. For example, a slip resistant
finish can be achieved by sweeping the concrete with a stiff broom to achieve light
scour lines (referred to as broom finishing) before the concrete has completely
hardened.
Curing
It is important to not allow a placed concrete batch to dry too fast. The water must
combine with the cement to form the concrete. If the water evaporates before the
concrete has formed (been cured) the resulting mix may be under strength. This is often
a particular concern with slab construction since so much of the slab is exposed to the
air and the sun.
Curing is the process of keeping the concrete damp to allow hardening at the proper
rate. This is a critical concern in hot, dry climates. An approach that is typically
employed on limited resource projects is to cover the placed concrete with plastic,
burlap bags, or damp straw. Jute or burlap sacks can be wrapped around columns and
wetted. In damp areas such as rainforests, the ambient moisture may be sufficient.
Within 24 hours, the slab can be walked on. Spray water on concrete walls 3 times a
day after removing forms. Curing is generally complete in 3 to 7 days (Figure 14).
Figure 14: A finished slab can be walked on without any foot imprints and is ready for
further work
Generally, a properly mixed concrete will be waterproof. However a sealant is often
applied especially if there is a concern about small cracks forming during curing. It is
applied after curing is complete. If a sealant is used, it is important to assure that it is
not toxic. Proper ventilation is also necessary if sealing within a closed tank. A cementsand slurry can be used to seal small cracks if a commercial sealant is not available.
Page 12 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World Concrete Strength
A 3:2:1 mix with a strong aggregate and low w/c ratio will produce a very strong (4000
to 5000 psi) concrete. However, designers are discouraged from assuming this high
strength is readily accomplished in resource limited conditions. Given the inherent
uncertainties in cement quality and propensity for inexperienced construction crews to
add more water than is ideal, it is generally assumed that concrete strength is in the
3000 psi range for design purposes. Where resources are very limited and quality
control on both the aggregate and mix can be especially problematic, it is safer to
assume that an even lower concrete strength is achieved.
The implications of low strength aggregate on the ultimate strength of a concrete mix
was demonstrated on a 2012 EWB project in Guatemala. The project included the
construction of a concrete water storage tank. Test cylinders were formed, sealed, and
hand carried to the United States for testing. These test cylinders showed a 1500 psi
strength (Figure 15).
Figure 15: Test cylinders of field mixed concrete. Indication of breaks showed a deficit in
the aggregate strength. Note break across the aggregate.
The mix was accomplished on site and the aggregate material was hand carried in from
a nearby stream. It was not feasible to test the aggregate and a visual inspection did
indicate that it was not ideal. Careful slump tests were performed during the entire
construction which showed good control and a relatively low w/c ratio. Since this
empirical series of tests where consistent across several cylinders, it can be assumed
that the mix was controlled and the low strength is likely due to poor quality aggregate.
Fortunately, the engineers for this effort had assumed a low value for the concrete
strength and the design accommodated this approach with thicker elements. This
example is provided as an illustration of why it is potentially dangerous for designers to
assume that concrete strengths expected in the developed world under ideal conditions
can be achieved in resource limited conditions.
Safety and Management
Safety is always the first priority on any construction job. Construction is inherently
dangerous in any environment, but the remote locations that are often characteristic of
work in the developing world can delay obtaining medical care in the event of an
Page 13 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World accident. The engineer in charge should conduct a safety meeting every morning to
make all participants aware of the day’s activities and any associated safety issues. All
sites should have a designated safety person who is trained in first responder aid as an
adequate first aid kit including eyewash. Traditional safety equipment such as hard
hats, steel toe boots, safety vests, goggle and gloves are preferred for all workers.
However it is recognized that it is sometimes difficult to enforce the universal use of
developed world personnel safety gear in limited resource communities and still
maintain local involvement. The engineer in charge may have to make some
compromises and distribute limited safety equipment to workers who are involved in the
most risky aspects of the job. Regardless of limitations, under no circumstances should
work proceed without adequate rebar caps on all vertical rebar. Serious injury including
impalement has resulted from falls onto exposed rebar during construction operations.
Clear records should be recorded of developments during construction. Modification
and adjustments should be clearly documented. Every site should have a person who is
in charge of safety and construction procedures. This person should have and maintain
in their possession all the relevant design documentation for the project. Democratic
votes are good for planning phases of a project but when a site is under way, a single
point of contact (POC) is needed for effective and safe construction (Figure 16).
Figure 16: Every site needs an experience construction engineer to be ultimately
responsible for safety and project management
Conclusion
Concrete is a durable, strong, and cheap building material that is adaptable to most any
form and suitable for a variety of purposes. The simple constituents of conventional
concrete are readily obtained in most areas and it can be successfully placed under
Page 14 EWB‐USA Technical Paper 103: Concrete Construction ~ Field Techniques for Use in the Developing World primitive conditions. However, adequate mix design and quality control during
construction is critical to obtain a quality project. This paper has outlined several dos
and don’ts that are useful for engineers who are using this material for projects in the
developing world.
References:
ASTM C143-78 Standard Test Method for Slump of Portland Cement Concrete.
Developed by subcommittee C09.60, American Standard for Testing and Material
International.
ASTM C231 Standard Test method for Air Content of Freshly Mixed Concrete by the
Pressure Method. Developed by subcommittee C09.60, American Standard for Testing
and Material International.
ASTM C172 Standard Practice for sampling Freshly Mixed Concrete. Developed by
subcommittee C09.60, American Standard for Testing and Material International.
ASTM C39 Standard Test Method for Compressive Strength of Cylindrical Concrete
Specimens. Developed by subcommittee C09.61, American Standard for Testing and
Material International.
ACI 350, 2006, American Concrete Institute. Code Requirements for Environmental
Engineering Concrete Structures and Commentary
Cement Trust (2013) “What is the Development Impact of Concrete?” Cement Trust
Symposium, McMinnville, Oregon
Page 15