Structural Guidelines

8/6/2019 Structural Guidelines

1/44

2006 Engineers Without Borders USA. All Rights Reserved Page 2

Forward

The intention of the EWB-USA Guideline series is to provide the basic elements for makinginformed decisions when investigating, designing, planning, and construction of sustainable

structural systems in developing countries. This is intended to guide students and members of EWB

chapters with appropriate questions and procedures of a project outside of the developed world.

This document DOES NOT eliminate the necessity for having design reviewed by experiencedengineering professionals in the appropriate area.

These Structural Guideline were written by Rick Strittmater, P.E.

Table of Contents Page

Introduction to the Structural Guideline.... 3

Part I - Assessment/Information Gathering PhaseSection 1.0 Site Investigation. 3

Section 2.0 Structural Materials.. 6Section 3.0 Structural Design Criteria.... 42

Part II - Design/Planning Phase

Section 4.0 Design Calculations.....44Section 5.0 Design Drawings, Material Lists, & Specifications.44

Part III - Construction/Implementation PhaseSection 9.0 Construction Logistics.....45


2/44


Introduction to the Structural Guideline

This Guideline is intended to describe the essentials of investigation, design, and construction of thestructural elements and systems of permanent buildings, bridges (both pedestrian and vehicular),

and earth and water retaining systems.

The Guideline is not intended to apply to temporary structures, which generally do not require thesame level of investigation, completeness of overall fact-finding and design, and care in

construction, as do permanent structures.

The Guidelines do not address cultural, social, environmental, or economic appropriateness of

structures. As applied to buildings, the Guideline does not address the other systems that may be

required to make facility useable such as water supply and treatment, solid and other wastemanagement, electrical power, lighting, heating, ventilation and cooling.

Part I - Assessment/Information Gathering Phase

Section 1.0 - Site Investigation

This section addresses the investigation and determination of the project site information includingsurface, sub-surface, and other natural (and man-made as appropriate) factors that will affect the

design and/or configuration of the structural systems.

The EWB project team shall investigate the conditions of the project site before beginning the

design process. Ideally this would occur on an assessment trip and should be conducted or overseen

by individuals with experience in the appropriate areas.

1.1 - Site survey

The EWB team shall gather the necessary data to create an accurate site plan, including but notlimited to, existing structures, topography, vegetation, features such as roads and rivers. The site

plan should be drawn and included in all project reports and plans.

The level of sophistication of the site plan should be dependent on its use in the final design. In

most cases, prefabricated piece sizes will not depend on the survey as most work will be built in

place. For simple areas, such as flat building sites, it may be possible to complete the survey with atape and level. However for complicated topography that is important (such as a bridge site),

surveying equipment should be employed. In addition, abundant photographs (digital if possible)

should be taken looking at and away from the site in all directions.

It is important for the site plan to be as accurate as possible as much of the cost estimating may

depend on this. In addition, there are aspects of the analytical design that may also depend on the

features of the specific and general site. For example, wind design pressures are related to astructures exposure. Exposure is often described in terms that relate to the features at the site. The

same can be said for determining snow loads on a structure. For example, heavily forested areas

may be treated differently than open areas.


3/44


1.2 Soils Evaluation and Investigation

The EWB team shall investigate the soil properties of the area that is to be built upon. This is most

effectively done by digging test pits and characterizing the soil found. Test pits are generallyassumed to be approximately 4-0 deep. In all cases, the digging of test pits should proceed with

caution and due regard for safety with respect to the stability of the soil being excavated. The

results of this field investigation should be summarized in a geotechnical report.

Since this digging will likely be done by shovels, in many cases this is a futile effort as there may bea large strata of a completely different soil 20 below the surface. And the soil cannot be examined

as it exists in the ground (in situ) without sophisticated testing. However, test pits allow a quick,general characterization (clay or sand) of the soil near the surface. In addition, if the surface has

been modified by the local community or natural processes, such as adding soil to make fertile land,

a test pit will most likely penetrate this to the soil below. The test pit will also help characterize thedifficulty of digging in the region, and estimate the labor required.

In addition, if there are wells nearby that are considerably deeper, the team may choose toinvestigate the inside walls of the well, taking care not to contaminate water supplies.

Questions of locals can also help to understand the stiffness of the soil, such as when digging awell, how deep can you dig without supporting the side walls?.

A general guide for the field evaluation of the soil type that is discovered through the digging of test

pits is as follows:

Material Size of Particles Means of Field Identification

Gravel 2.0 mm 60 mmCoarse pieces of rock, which are round or angular.

Material is generally not bound together.

Sand 0.06 mm 2.0 mmSand breaks down completely when dry, the particles

are visible to the naked eye and gritty to the fingers.

Silt 0.002 mm 0.06 mm

Particles are not visible to the naked eye, but slightly

gritty to the fingers. Moist lumps can be molded butnot rolled into threads. Dry lumps are fairly easy to

powder.

Clay < 0.002 mmSmooth and greasy to touch. Holds together when dry

and is sticky when moist.

Organic VariesSpongy or stingy appearance. The organic matter is

fibrous or rotted. Has an odor of wet decaying wood.

Often, the soil is a mixture of perhaps two, three or even four of these specific types of materials, a

blended matrix if you will. A simple field test to determine the percentage of materials is called theSedimentation Test. This test measures the proportions of clay, silt, and fine gravel/sand. For

increased accuracy, two tests can be performed. The sedimentation test consists of the following

steps:


4/44


Fill a jar up to 1/3 of its volume with dry soil;

Add clean water up the second-third of the jars height;

Mix the soil and water with a stick;

With the lid on, shake the jar vigorously until the soil particles are in suspension;

Let the jar sit for one hour;

Again, with the lid on, shake the jar vigorously, and allow it to sit for one minute;

After one minute, mark the height of the fine gravel and sand layer, which will be the firsttwo materials to readily settle to the bottom of the jar, as T1 in photo below;

After 30 minutes, add a second mark to the point where the fine gravel, sand and silt havesettled out of the water, as T2;

After 24 hours, add a mark at the highest level of the fine gravel, sand, silt, and clay havesettled out of the water, just where the water and earthen soil contents have separatedvisually, as T3; and,

Calculate the percentages of the ingredients of the soil by following the equations where T1= depth of fine gravel & sand, T3-T2 = depth of clay, T2-T1 = depth of silt, and where each

depth is divided by T3 and then multiplied by 100 to give a percentage.

Sedimentation Test Jar

1.3 - Climate & Weather

Many assumptions must be made in the design. These assumptions rely heavily on the climate of

the project site. The EWB team shall gather climate date including, but not limited to, extreme

temperatures, wind speeds and frequency, elevation, yearly precipitation amounts, precipitationrates for severe storms, and seismic information. If the project is a bridge, water level and velocitydata and river basin area should also be collected. If possible, a topographic map of the drainage

basin should be obtained. The results of this investigation shall be shown in a climate report and in

the case of bridge projects, a hydraulic report.

In some cases, the information will be easy to find, as weather data (rainfall, wind etc) is now kept

for any city with an airport. However, in many cases, the same history of 100+ years of records thatexists in the US will not be available elsewhere. Thus, it is important to confirm or quantify the

fairly recent and possibly somewhat far away data by asking lots of questions of older members of

T1

T3

T2


5/44


the community. These questions can be comparative in time or space, such as has it becomemore/less rainy or windy in the last 10 years, Is it colder here than in the Capital? Or the

questions should attempt to find a specific point of data that might be remembered, such as In the

Hurricane 5 years ago, what was the highest water level in that river/lake?, or have you ever feltan earthquake?. It is important to document as much as possible, even if it seems unimportant at

the time.

It is difficult to anticipate what questions the hydraulic analysis and surveys will bring up, so talk to

as many people as possible and ask as many questions as possible. The critical design questionsthat will need to be answered from this data include:

Freezing/frost depth (for foundation stability)

Maximum Wind Velocity/ Direction (generally for building design)

Maximum Snow (depth)/Rain load (depth) (generally for building design)

Seismic Determination (all types of structures)

Stream Velocity and Maximum Stream Height (generally for bridge structures)

Section 2.0 - Structural Materials

This section addresses investigation and determination of structural material shapes and properties

that may be available for a given project. The materials described may be used as part of the

permanent structure and/or as part of the temporary shoring/false work system.

Design decisions in a structure are heavily dependent on the materials available locally. The EWB

team shall investigate the sizes, availability, quality, and cost of all materials that might be used inconstruction of the project. This information should be recorded and included in design documents

and cost estimates for the project.

2.1 Concrete

In construction, concrete is a composite building material made from the combination of aggregate

and a cement binder. The most common form of concrete cement binder is Portland cement. Aftermixing all of the ingredients; the water reacts with the cement in a chemical process known as

hydration. This water is absorbed by cement, which hardens, gluing the other components together

and eventually creating a stone-like material.

Concrete is used more than any other man-made material on the planet. It is used to make building

superstructures, foundations, roads, bridges, walls and bases for poles. As of 2005, about six billion

cubic meters of concrete are made each year, amounting to the equivalent of one cubic meter forevery person on Earth.

2.1.1 - Cement

Portland cement is the most common type of cement in general usage. It consists of a

mixture of oxides of calcium, silicon and aluminum. Portland cement and similar materials

are made by heating limestone (a source of calcium) with clay and grinding this product(called clinker), with a source of sulfate (most commonly gypsum). The resulting powder,

when mixed with water, will become a hydrated solid over time.


6/44


More than half of ready-mixed concrete contains fly ash, ground granulated blast furnaceslag, silica fume, metakaolin, or other pozzolanic materials. These materials are collectively

referred to as supplementary cementicious materials (SCMs). SCMs are very fine inorganic

materials that usually have pozzolanic or latent-hydraulic properties.

Fly ash: A by-product of coal-fired plants, it is used to partially replace Portlandcement by up to 60% by mass. The properties of fly ash depend on the type of coal

burnt. In general, silicious fly ash is pozzolanic, while calcareous fly ash has latent-

hydraulic properties. Ground granulated blast furnace slag (ggbs): A by-product of steel production, it is

used to partially replace Portland cement (by up to 80% by mass). It has latent-

hydraulic properties.

Silica fume: A byproduct of the production of silicon. Silica fume is similar to flyash, but has a particle size in the order of 100 times smaller. Silica fume is used to

increase strength and durability of concrete, but generally requires the use of

superplasticizers for workability.

2.1.2 - Aggregates

Aggregates are inert granular materials such as sand, gravel, or crushed stone that are anessential ingredient in concrete. For a good concrete mix, aggregates need to be clean, hard,

strong particles free of absorbed chemicals or coatings of clay and other fine materials thatcould cause the deterioration of concrete. Aggregates, which account for 60 to 75 percent of

the total volume of concrete, are divided into two distinct categories; fine aggregates and

coarse aggregates. Fine aggregates generally consist of natural sand or crushed stone with

most particles passing through a 3/8-inch (9.5-mm) sieve. Coarse aggregates are anyparticles greater than 0.19 inch (4.75 mm), but generally range between 3/8 and 1.5 inches

(9.5 mm to 37.5 mm) in diameter.

Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed.

Crushed aggregate is produced by crushing quarry rock, boulders, cobbles, or large-sizegravel. Recycled concrete is a viable source of aggregate. Aggregates strongly influenceconcrete's freshly mixed and hardened properties, mixture proportions, and economy.

Consequently, selection of aggregates is an important process.

Particle shape and surface texture influence the properties of freshly mixed concrete more

than the properties of hardened concrete. Rough-textured, angular, and elongated particles

require more water to produce workable concrete than smooth, rounded compact aggregate.

Consequently, the cement content must also be increased to maintain the water-cement ratio.Generally, flat and elongated particles should be avoided. The amount of water in the

concrete mixture must be adjusted to include the moisture conditions of the aggregate.

2.1.3 - Water

Water suitable for human or animal consumption can be used for the manufacture of

concrete. The water to cement (w/c) ratio (mass ratio of water to cement) is the key factor

that determines the strength of concrete. It is also a key factor in the viscosity of wet

concrete, which directly affects its workability during placement. A lower w/c ratio willyield a concrete which is stronger but all else being equal, more difficult to work with. A

higher w/c ratio yields a concrete with a lower strength, but is very easy to work with.


7/44


Concrete can be produced in two general ways; ready-mixed and site-mixed. Ready-mixed

concrete is produced (batched) at a plant, placed into a delivery truck and sent to the job site for

placement. Site-mixed concrete is produced by hand at the job site either through the use of a smallmixing device or mixing tub. Site-mixed concrete is proportioned by the crew at the job site.

2.1.4 Ready-mixed Concrete

During a site assessment trip, it is very important to investigate the availability of ready-

mixed concrete. Remote locations may not always have such material readily available. Itis important to also note that it is necessary to have all ready-mixed concrete out of the

delivery truck and in-place no longer than 90 minutes after the mix is batched at the plant.

So, delivery routes, distances, temperatures, site access, size of crew, and many other factorscan influence the time it takes to get the concrete out of the delivery truck.

If ready-mixed concrete is available and deemed suitable for use on the project, there areadditional items that should be investigated during a site assessment trip. Generally all

ready-mix concrete producers have a variety of mixes that they can provide. The need for a

variety of mixes is a result of many factors that include; strength, admixtures, air content,gradation of aggregate and even color. Depending on the nature of the concrete item that is

being constructed, the EWB chapter should determine a general range of concrete mix

characteristics that would be suitable. The intent of these guidelines is not to provide an

exhaustive explanation of all aspects of concrete mix design. In the complex world whichincludes all structure types, all exposures, all conditions, there are literally thousands of

considerations to be made when designing a suitable mix. In general, for EWB projects it

would be suitable to work within the following:

Strength. Concrete strength is expressed as the compressive strength attained at anage of 28-days. Higher strength concretes are usually more durable and more

expensive. The structural engineering design of a concrete element uses the 20-daystrength, expressed as fc. Common 28-day strengths range from 2,000 psi (13.8

MPa) to 5,000 psi (34.4 MPa). If the structural engineering design of a concrete

element is based upon a concrete strength which is greater than 3,000 psi (20.7MPa), then the EWB Technical Advisory Committee (TAC) may require the EWB

chapter to devise some means of assuring that this strength has been attained.

Generally, this may mean that on-site material testing could be required. A guidelinefor concrete strengths for ready-mixed concrete is as follows:

o Walks, Slabs-on-Grade 2,000 psi (13.8 MPa)o Foundations, Footing, Grade Beams 2,500 psi (17.2 MPa)

o Water Containment Tanks, Retaining Walls 3,000 psi (20.7 MPa)

In the U.S.A., ready-mixed producers keep records of test results for the mostcommon mixes that they provide. Producers can simply be asked to provide some

test data for various mixes and a suitable mix is then requested. During a site

assessment trip, the EWB chapter might seriously consider whether any local ready-mixed producers have this type of historical data to substantiate strength, as well as

many other parameters about the mix.


8/44


Water:Cement Ratio. The water-cement ratio is a convenient measurement whosevalue is well correlated with concretes strength and durability. In general, lower

water-cement ratios produce stronger, more durable concrete. If natural pozzolans

(or other SCMs see above) are used in the mix then the ratio becomes a water-

cementicious material ratio (cementicious material = Portland cement + pozzolonicmaterial). A guideline for water:cement ratios for ready-mixed concrete is as

follows:

o 2,000 psi (13.8 MPa) = 0.82o 2,500 psi (17.2 MPa) = 0.75o 3,000 psi (20.7 MPa) = 0.68o 4,000 psi (27.6 MPa) = 0.57

Coarse Aggregate. The maximum coarse aggregate size will affect such parametersas amount of cement paste, workability and strength. In general, the maximumcoarse aggregate size should be limited to 1/3 of the depth of any slab or 3/4 of the

minimum clear space between reinforcing bars. Coarse aggregate larger than these

dimensions may be difficult to consolidate and compact resulting in a honeycombedstructure or large air pockets. As stated above, coarse aggregates are generally well

graded, meaning they have a variety of sizes. In the U.S.A., there is a guideline foraggregates called ASTM C33. This specification has a number associated with a

specific gradation of aggregates. Of course this may not be the case in the thirdworld. But it is most likely that there is a similarity in the way in which the

aggregate is specified with respect to size of particles. Since there are a litany of

different types of structures, a guideline for two coarse aggregate gradations thatwould be suitable for the widest variety of structures are in the following table. The

upper gradation would be for larger items such as footings, grade beams, retaining

walls, tanks etc. and the lower gradation would be for walks, slabs, and the like.

Percent (%) passing by weight

1 inch(37.5 mm)

1 inch(25.0 mm)

inch(19.0 mm)

inch(12.5 mm)

3/8 inch(9.5 mm)

No. 4(4.75 mm)

No. 8(2.36 mm)

95-100 - 35-70 - 10-30 0-5 -

- 100 90-100 - 25-55 0-10 0-5

When expressed as a percent (%) passing, by weight, this specification would be

applicable in any location since the U.S. and metric equivalent sizes are given.

Fine Aggregate: Fine aggregates are generally taken to mean clean, durable sand. Inthe ready-mixed world, this is generally just a ready, reliable source of silica sand or

ground sand.

Thus, guidelines for the four most basic characteristics of a ready-mixed concrete are given

above. Ready-mixed concrete producers may inquire or ask you about other characteristicsof a proposed mix that they are being asked to provide. If this occurs, the EWB chapter

members should be made aware of these characteristics, a very brief discussion of three of

these additional characteristics is as follows:

Slump. Often a ready-mixed provider will ask. What is the desired slump? Thisrefers to workability can be described as a combination of several different, but

related properties such as:


9/44


o Ease of mixingo Ease of placingo Ease of compaction / consolidationo Ease of finishing

Generally, mixes of the stiffest consistency, that can still be placed adequately, and

practically should be used. A guideline of typical slumps is:o Reinforced Foundation Walls and Footings: 1-3 inches (25 75 mm)o Reinforced Beam and Columns: 1-4 inches (25 100 mm)o Slabs: 1-3 inches (25 75 mm) at a quicker pace than other concretes.

Air Content. Ready-mixed providers may ask, What is the desired air content?One of the greatest advances in concrete technology was the development of air-

entrained concrete in the late 1930s. Today, air entrainment is recommended for

nearly all concretes, principally to improve resistance to freezing and thawing.

However, there are other important benefits of entrained air in both freshly mixed

and hardened concrete. Air-entrained concrete is generally produced by introducing

air-entraining admixtures into the ready-mixed concrete. The amount of entrainedair is usually between 5 percent and 8 percent of the volume of the concrete, but may

be varied as required by special conditions. Air content is not mandatory. Air-

entraining admixtures will have an effect on the cost of the ready-mixed concrete.

Air content is generally specified on the basis of 2 variables: the maximum coarse

aggregate size and exposure environment. Relative to the two gradations for coarse

aggregates shown above, guidelines for air content are as follows:

Air Content, percent (%)Nominal Maximum

Size of Aggregate

in. (mm)Severe Exposure Moderate Exposure Mild Exposure

1 in. (37.5 mm) 5.5 4.5 2.5

in. (19.0 mm) 6.0 5.0 3.5


10/44


Type of Cement. Ready-mixed providers may ask, What is the desired type ofcement? The description of cement, contained above, generally referred to Portland

cement in a generic term. Portland cement actually has different types. For the most

part, the EWB chapter really not be concerned all that much with the type of cement.

But, in the event that a ready-mixed provider brings this topic to light, the chaptershould realize the differences between the three most common types of cement

which are explained below.

o Type I. This is normal Portland cement and it is for all general uses. It is alsothe most common cement.

o Type II. This type of cement is used for structures which contain water or are incontact with soil which contains moderate amounts of sulfate.

o Type III. This cement is used when high strength is desired in a very short time.Keep in mind that you will have to place concrete with Type III cement at aquicker pace than other concretes.

o Note that there is a type of cement that is considered to be Type I/II, which isalso very common.

Thus, these are three (3) additional characteristics of ready-mixed concrete can be discussed

with the ready-mixed provider if need be.

2.1.5 Site-mixed Concrete

In the case where the EWB chapter has determined that ready-mixed concrete is not

available or suitable, then the only other alternative for concrete is to mix it at the site. The

discussions above that were relative to the components of ready-mixed concrete are still

applicable and need not be repeated. It should be obvious that the quality control of site-mixed concrete will definitely be less than that which can be attained by the use of ready-

mixed concrete. Also, several characteristics of ready-mixed concrete will not necessarily

be available for site-mixed concrete.

Taking into consideration that the quality control is diminished, it is suggested that the

structural engineering design of concrete items that are site-mixed should be altered. Mostimportant among these is the value used for the 28-day strength of the concrete. If the

structural engineering design of a concrete element is based upon a concrete strength which

is greater than 2,500 psi (17.2 MPa), then the EWB Technical Advisory Committee (TAC)may require the EWB chapter to devise some means of assuring that this strength has been

attained. Note that this is 500 psi (3.4 MPa) less than that which was suggested for ready-

mixed concrete. A guideline for concrete strengths for site-mixed concrete is as follows:

Walks, Slabs-on-Grade 1,500 psi (10.3 MPa) Foundations, Footing, Grade Beams 2,000 psi (13.8 MPa)

Water Containment Tanks, Retaining Walls 2,500 psi (17.2 MPa)

For site-mixed concrete, it is generally assumed that the ingredients for the concrete willhave to be delivered in bulk. During any site assessment trip, the EWB chapter should

thoroughly investigate the availability of the separate ingredients, as well as means to

deliver and stockpile them at the site. Keep in mind that the ingredients are very sensitive tomoisture and will need to be covered and protected.


11/44


Bulk Cement. Bulk generally connotes the image of a loose pile of material.Cement is not conducive to this sort of delivery method. In any location where site-

mixing of concrete is to be considered, it is likely that the cement is going to be

obtained in bag form. The most common bag form is the similar to the product

produced by Holcim, http://www.usmix.com/dp_holcim_portland_cement.phtml .This bag is 92.6 lbs. (42 kg) of Type I/II Portland cement. Just a side note in that

while cement is a very prevalent product in the U.S.A., so much so that one might be

tempted to think it is a ubiquitous, American Product. The truth is that 85 percent of

the cement plants operating in the U.S.A. are owned by foreign companies. The topthree manufacturers of cement in the world are:

o LaFarge (France): http://www.lafarge.com In 42 countries on 4 continentsyou can buy products from our Cement Division. For locations where

cement products can be purchased in any on of these 42 countries, seehttp://www.lafarge.com/cgi-

bin/lafcom/jsp/directory.do?function=directory&BV_SessionID=@@@@14

83169320.1157917402@@@@&BV_EngineID=ccccaddijmfgfegcfngcfkmd

hgfdggg.0

o Holcim (Switzerland): http://www.holcim.com For locations worldwide, see

http://www.holcim.com/CORP/EN/id/1610640774/mod/gnm0/page/location_global.html

o Cemex (Mexico): http://www.cemex.com From this website you can findlocations in several countries on several continents from which to purchase

their products. See the Cemex Worldwide pull down menu on the homepage.

Cement in this form is very heavy and delivery of the material is something that must

be addressed. Typically, one could estimate that you will need approximately 515 550 lbs. (233 249 kg) of cement for 1 cubic yard (0.76 cubic meters) of concrete

placed. Make sure and take into account some waste, perhaps 5%. The sacks that

contain the cement do offer some protection from humidity and very light, but un-sustained rain. The sacks are not very easy to manipulate and transport by hand and

breakage is common. Also, if buying the sacks directly, you might want to check

first to see if the sacks feel overly hard or lumpy as they may have been exposed tosome moisture and the cement within the bag has already hydrated and set up.

Hydrated concrete can not simply be re-pulverized and used again. Hydrated

concrete is simply wasted.


12/44


Bulk Coarse Aggregate. Bulk coarse aggregate is no different that the coarseaggregate that is used by the ready-mixed concrete producers at a batch plant. The

coarse aggregate is simply delivered to the job site and stockpiled. Certainly the

aggregate will not deteriorate in the rain but it is important that the aggregate be as

clean as practical. If rain produces a flowing mud and the mud is allowed to coat thestockpiled aggregate, this could prove to be problematic when the concrete is site-

mixed. Placing the coarse aggregate on a tarp and covering the stockpile is always

best. Since it is not always clear what sort of delivery trucks or vehicles (carts,

wagons, etc.) are available, it is not easy to plan for how to facilitate delivery of bulkcoarse aggregate. For approximate planning purposes, you will generally need

between 1,600 1,700 lbs. (726 771 kg) of coarse aggregate for 1 cubic yard (0.76cubic meters) of concrete placed. Make sure and take into account some waste,

perhaps 5%.

Bulk Fine Aggregate. Bulk fine aggregate is no different that the fine aggregate thatis used by the ready-mixed concrete producers at a batch plant. Rain and moisture,

which does not adversely affect coarse aggregate, can have a large effect on fine

aggregate. First of all, the fine aggregate can simply wash and/or blow away if notprotected. Secondly, if the fine aggregate is allowed to become overly moist or wet,

the amount of water that is to be used in the mixing of the concrete can bedramatically altered. For approximate planning purposes, you will generally need

between 1,450 1,525 lbs. (658 692 kg) of coarse aggregate for 1 cubic yard (0.76cubic meters) of concrete placed. Dry sand will generally weigh 100 lbs./cu. ft.

(1602 kg./cu. m.). The voids contained in dry sand account for a tremendous amount

of volume. Most damp loose sands contain anywhere from 1/2 - 1 gallon (1.9 3.8l) of water per cu ft of sand. Typically, one might use approximately 30.0 35.0

gallons (113.6 132.5 l) of water for 1 cubic yard (0.76 cubic meters) of concrete

placed. Thus, if the sand above were simply damp loose sand instead of dry sand,you might already have anywhere from 7.2 15.2 gallons (27.3 57.5 l) of water

simply contained in the voids of the sand. This is significant! This might account

for anywhere from 20% to 50% of the water need! Remember, you can always add alittle water, but you cant take it out of the mix. Once it is in, it is in, period! And,as with the coarse aggregate, delivery of this material must be addressed. It is

moderately heavy. Site-mixed concrete can be prepared with simple hand tools such

as:

Square nosed shovel, see(http://www.homegardenandpatio.com/cat.cgi?s=2625093&c=garden_tools_shovels )

Garden hoe, see(http://www.homegardenandpatio.com/cat.cgi?s=2611648&c=garden_tools_hoes )

Mixing Tubs at 2.5 cu. ft. (70.7 l), see

(http://www.capcityequipment.com/mbmothers0244.html ) or even larger at 8.8 cu.ft. (250 l), see (http://www.plasgad.com/html/super_tub.htm)

Or, the concrete can be prepared with a portable concrete mixer, which can be seen at;

(http://www.constructioncomplete.com/ConcreteMasonryEquipment/ConcreteMortarMixers

.html). Portable concrete mixers can be powered by hand, electricity or small gas or dieselpowered engines. There are literally thousands of types of portable concrete mixers. If this

method is to be entertained, the EWB chapter must locate a reliable source for this

equipment on a site assessment trip. Use would generally be through rental. Consider theunits capacity, proximity to electrical power, security, and quality.


13/44


The final topic to be discussed for site-mixed concrete is the mix design, or recipe. In its

most simple form, the proportioning method has evolved from the arbitrary volumetric

method, (1:2:3 cement:sand:coarse aggregate) to the present-day weight and absolute-

volume method. Based upon years of reviewing ready-mixed producers mix designs and

thorough investigation with several ready-mixed concrete providers, the following are

examples of mixtures for non-air-entrained concrete of medium consistency, 3 4 (75 mm 100 mm) slump. Two different maximum aggregate sizes are provided in order to be

consistent with the guidelines for coarse aggregate for ready-mixed concrete expressedabove. Guidelines for four (4) different target 28-day strengths are provided below.

Water:CementRatio

Targetfc,psi

MaximumSize of

Aggregatein.

Water,lb. per cu.

yd. ofconcrete(gallons)

Cement,lb. per cu.

yd. ofconcrete

Fine Aggregate,lb. per cu. yd. of

concrete

CoarseAggregate,lb. per cu.

yd. ofconcrete

0.80 2,000 340

(40.8)420 1,380 1,740

0.80 2,000 1 300

(36.0)375 1,270 2,050

0.70 2,500 340(40.8)

485 1,330 1,740

0.70 2,500 1 300

(36.0)430 1,210 2,050

0.60 3,000 340

(40.8)565 1,260 1,740

0.60 3,000 1 300

(36.0)500 1,150 2,050

0.55 4,000 340

(40.8)620 1,210 1,740

0.55 4,000 1 300

(36.0)545 1,120 2,050


14/44


The table above is replicated in metric units below.

Water:CementRatio

Targetfc,

MPa

MaximumSize of

Aggregatemm

Water,kg. per

1.0 m3of

concrete(liters)

Cement,kg. per cu

m. ofconcrete

Fine Aggregate,kg. per cu. m.

of concrete

CoarseAggregate,kg. per cu.

m. ofconcrete

0.80 13.8 19201.7

(201.7)249.2 818.7 1,740

0.80 13.8 38178.0

(178.0)222.5 753.5 2,050

0.70 17.2 19201.7

(201.7)287.7 789.1 1,740

0.70 17.2 38178.0

(178.0)255.1 717.9 2,050

0.60 20.7 19201.7

(201.7)335.2 747.5 1,740

0.60 20.7 38178.0

(178.0)296.6 682.3 2,050

0.55 27.6 19201.7

(201.7)367.8 717.9 1,740

0.55 27.6 38178.0

(178.0) 323.3 664.5 2,050

The two previous tables might be used as a guideline when arranging for ready-mixed

concrete to be provided by a supplier. During an assessment trip, the EWB chapter mightconsider meeting with one, or several ready-mixed suppliers to discuss their standard mixes.

A comparison with the tables above can then be made.


15/44


If site-mixed concrete is to be used, then the above tables could certainly be proportioned tofit the volume which might be mixed in either a hand mixing tub or portable powered mixer.

It might be useful to determine the mix proportions for a standard, 3.5 ft3

(100 l = 0.10 m3)

volume of concrete. Hand mixing plastic tubs and portable, powered mixers to fit thisvolume are quite common. If a different volume is to be used, then again, these quantities

can simply be proportioned, or ratiod down.

Water:CementRatio

Targetfc,psi

MaximumSize ofAggregate

in.

Water,

lb. per3.5 ft

3of

concrete(gallons)

Cement,lb. per3.5 ft

3of

concrete

FineAggregate,lb. per 3.5 ft

3

of concrete

CoarseAggregate,lb. per 3.5 ft

3

of concrete

0.80 2,000 44.1

(5.29)54.4 178.9 225.6

0.80 2,000 1 38.9

(4.67)48.6 164.6 265.7

0.70 2,500 44.1

(5.29)62.9 172.4 225.6

0.70 2,500 1 38.9

(4.67)55.7 156.9 265.7

0.60 3,000 44.1

(5.29) 73.2 163.3 225.6

0.60 3,000 1 38.9

(4.67)64.8 149.1 265.7

0.55 4,000 44.1

(5.29)80.4 156.9 225.6

0.55 4,000 1 38.9

(4.67)70.6 145.2 265.7

The table above is replicated in metric units below.

Water:CementRatio

Targetfc,

MPa

Maximum

Size ofAggregate

mm

Water,kg. per

0.10 m3

ofconcrete(liters)

Cement,

kg. per0.10 m

3of

concrete

Fine

Aggregate,kg. per 0.10 m

3of concrete

CoarseAggregate,kg. per 0.10

m3of

concrete

0.80 13.8 1920.0

(20.0)24.6 81.1 102.3

0.80 13.8 3817.6

(14.9)22.0 74.6 120.5

0.70 17.2 1920.0

(20.0)28.5 78.2 102.3

0.70 17.2 3817.6

(17.6)25.3 71.1 120.5

0.60 20.7 1920.0

(20.0)33.2 74.1 102.3

0.60 20.7 3817.6

(17.6)29.4 67.6 120.5

0.55 27.6 1920.0

(20.0)36.5 71.1 102.3

0.55 27.6 3817.6

(17.6)32.0 65.8 120.5


16/44


Since the small, site-mixed recipes are based upon the present-day weight and absolute-volume method, it might be difficult to attain unless one has a scale at the job site to use.

Obviously this will most likely not be the case. Since hand tools will generally be thedevice used to blend and mix the ingredients, it might be useful to provide some guidelines

as to how to alter the recipe with other units.

Through experimentation, using a typical square-nosed garden shovel

(http://www.homegardenandpatio.com/cat.cgi?s=2625093&c=garden_tools_shovels ), it wasdetermined that an average scoop of the three basic ingredients (fine aggregate sand,

coarse aggregate, and cement) weigh approximately:

Fine Aggregate Sand: 6.6 7.0 lbs (2.99 3.17 kg)

Coarse Aggregate: 8.5 8.9 lbs (3.86 4.04 kg)Cement: 8.4 8.8 lbs. (3.81 3.99 kg)

Unit Weights of these three materials are:

Fine Aggregate Sand (dry): 100 lbs. / ft3

(1,602 kg / m3

)

Coarse Aggregate (dry): 105 lbs. / ft3

(1,682 kg / m3

)Cement (dry): 94 lbs. / ft 3 (1,506 kg / m 3 )

Using the shovel full weights, the recipe for the above standard, 3.5 ft3

(100 l = 0.10 m3) volume of concrete would be:

Water:CementRatio

Targetfc,psi

MaximumSize of

Aggregatein.

Water,lb. per

3.5 ft3of

concrete(gallons)

Cement,Approx. No.of Shovels

per3.5 ft

3of

concrete

FineAggregate,

Approx. No. ofShovelsper

3.5 ft3of

concrete

CoarseAggregate,Approx. No.of Shovels

per 3.5 ft3of

concrete

0.80 2,000 44.1

(5.29)6.3 26.3 25.9

0.80 2,000 1 38.9

(4.67)5.7 24.2 30.5

0.70 2,500 44.1

(5.29)7.3 25.3 25.9

0.70 2,500 1 38.9

(4.67)6.5 23.1 30.5

0.60 3,000 44.1

(5.29)8.5 24.0 26.0

0.60 3,000 1 38.9

(4.67)7.5 22.0 30.5

0.55 4,000 44.1(5.29) 9.3 23.1 25.9

0.55 4,000 1 38.9

(4.67)8.2 21.4 30.5


17/44


The table above is replicated in metric units below. But, since the volume of concrete mixedis the nearly the same, i.e. 3.5 ft 3 equals 0.10 m 3, it would stand to reason that the number

of shovels would be the same for each case, US or SI.

Water:CementRatio

Targetfc,

MPa

MaximumSize of

Aggregatemm

Water,kg. per

0.10 m3

ofconcrete(liters)

Cement,Approx. No.of Shovels

per 0.10 m3

of concrete

FineAggregate,Approx. No.of Shovels

per 0.10 m3

of concrete

CoarseAggregate,Approx. No.of Shovels

per 0.10 m3

of concrete

0.80 13.8 1920.0

(20.0)6.3 26.3 25.9

0.80 13.8 3817.6

(14.9)5.7 24.2 30.5

0.70 17.2 1920.0

(20.0)7.3 25.3 25.9

0.70 17.2 3817.6

(17.6)6.5 23.1 30.5

0.60 20.7 1920.0

(20.0)8.5 24.0 26.0

0.60 20.7 3817.6

(17.6)7.5 22.0 30.5

0.55 27.6 1920.0

(20.0)9.3 23.1 25.9

0.55 27.6 3817.6

(17.6)8.2 21.4 30.5

Concrete is always, in some way, formed, meaning that the wet plastic concrete is placed in a way

that retains a specific shape until the concrete hardens. The forms for the concrete must not interactin a manner that will injure or greatly degrade or lessen the quality of the concrete. Guidelines

about different kinds of concrete formwork are addressed as follows:

2.1.6 Earth-formed Concrete

It is very common to use the earth to form such items as footings, pole bases, equipment pad

foundations and the like. If the earth can be cut or shaped with relatively stable sides, then

by all means, it is allowed to use the earth as a form.

If the earth is used as a form, it is suggested that the overall dimensions of the concrete

element be increased slightly to account for the irregularities that will be inherent with earth

forming. For example, if a footing size of 9 thick x 16 wide is found to be necessary bycalculation, it would be suggested to earth form a footing that is 10 thick x 17 wide. The

added bulk will also allow for some degradation of the outside surface without lessening theneeded section for engineering purposes.

The earth should be relatively stable and able to withstand the placement (shoveling) of the

concrete without sluffing off into the placed concrete. It is suggested that the earth be moist,but not overly wet so as to cause it to be muddy in any way. The earth should most

definitely not be frozen.


18/44


2.1.7 Wood-formed Concrete

The most common approach to forming of concrete is to use wooden forms. This is true for

nearly all types of concrete structures. When wood forms are used, it must be recognizedthat the forming will add a significant amount to the cost of the concrete. In the third world,

cut dimension lumber boards or panelized lumber are considered expensive. The chapter

should make every effort to be able to re-use forms multiple times and if at all possible, finda final use for the lumber as opposed to simply discarding it as construction trash.

All wooden formwork should be coated in some fashion prior to placement of the concrete.

The concrete will tend to stick to the formwork and make removal difficult. If the forms aretoo difficult to remove, one may injure the green un-cured concrete by the removal

operation. This should be avoided. The coating is often referred to as form release agent,

but may also be referred to as a separator, parting agent, or parting compound. This agent isa special chemical in the U.S.A. but in the third world this might be many things which

could include; a thin layer of clay, a smearing of soft liquid soap, petroleum jelly, a thin oil,

kerosene or diesel fuel. For the obvious reasons, the flammable materials should beavoided. If there is a large amount of forming to be accomplished, you will find that a lot of

time and effort will be saved by using a specified form release agent or compound. A

company that is generally located throughout the world is Grace Construction Products. Acountry guide which lists distributors in several countries can be found at their web site

http://www.na.graceconstruction.com/sitemap.cfm.

When concrete is placed in formwork, the forms must be able to adequately and safely retainand/or support the placed concrete. When a concrete stem wall or elevated wall is placed,

the retained concrete will tend to want to splay out or force the formwork apart through

hydrostatic pressure. This pressure is not to be overlooked and can be quite strong. Do notattempt to form walls without some means of holding the formwork together in a manner

that is safe, stable and secure. In general, the only way in which the forms for vertical wall-

like or stem wall-like elements can be restrained is by external bracing (kickers, whalers,

etc.) or internal form-ties. Both of these bracing methods require extensive planning andmaterials in order to accomplish. It can not be stressed enough that proper construction

practices and attention to formwork must be addressed. Forms that blow out on the job

site are costly, nearly impossible to repair, detrimental to schedule and could easily causeinjury or death in the worst case.

Internal form ties are generally a metal device or series of heavy gage wires which tie thetwo opposing forms together. The form tie resists the outward pressure via tension. After

the forms are stripped off of the concrete, the protruding portions of the form ties are

snipped off. Leaving them exposed can cause injury. If the concrete item is a wall of aliquid-retaining tank, it should be pointed out that the metal form ties may, over time,

corrode and contaminate the retained liquid or allow the retained liquid to leak. Specialattention must be given to these types of structures. Often in the developing world, specialform ties with cone nut ends are used which allow a sealant concrete to be placed over the

snapped off end of the form tie. The special concrete seals the form tie from attack. For an

example, see http://www.tpub.com/content/engineering/14069/css/14069_262.htm .

If elevated, overhead concrete elements are to be formed, then there is a high likelihood that

the formed element will require some sort of temporary shoring or vertical support. Do not

wait until you are forming an overhead element to discover this. Material for shoring mightbe significant and should be planned for.


19/44


2.1.8 Metal-formed Concrete

The only instance where metal-formed concrete might be considered is for an elevated slab

for a floor or a roof. Often a slab could be formed in place with wooden forms that willsimply be shored and then removed. The cost of the shoring and time to erect it and take it

down might be offset by the use of metal forming. This type of forming is called retained-

in-place metal form deck. This decking is corrugated and can come in a variety of depthsand thicknesses. Depth is referred to as the total depth of the panel and thickness is referred

to as the gage thickness of the sheeting that was used to make the corrugated panel.

Typically it is best to use metal form deck that is galvanized so that the deck has a long life.In general, the metal deck is only counted on for its strength to support the wet weight of the

concrete plus a small added load for the construction workers. Once the concrete hardens, it

is assumed that the concrete slab will support any additional superimposed loads. Of coursethere are a litany of different span conditions, metal deck types, thicknesses, grades of steel,

etc. All instances can not be addressed in these guidelines. If an EWB Chapter wishes to

use a retained-in-place metal form deck, they should investigate the types of deckavailable during an assessment trip. Other information that should be attained, if at all

possible, includes: Steel type, allowable stresses, thickness, cost, coating (galvanized),

standard lengths of sheets, standard widths of sheets, details for lapping sheets at sides andany load tables that might be offered by the manufacturer. A manufacturer of such decking

in the U.S.A. is called Vulcraft. Their Steel Roof and Floor Deck manual provides a lot of

useful information for this type of decking which they refer to as Non-Composite Floor

Deck Type (Type C) Conform. See pages 20-39 of their manual.http://www.vulcraft.com/downlds/catalogs/deckcat.pdf#search=%22Vulcraft%20Metal%20

Deck%22. Follow all of the manufacturers instructions for using this type of decking.

At the same time the forms are being erected, it may be necessary to place the reinforcing. The

following are general topics to be used as guidance with the placement of reinforcing.

2.1.9 Placing Concrete Reinforcing

Placing of the reinforcing carries a litany of specifications for typical U.S. type construction.

It is generally understood that a thorough presentation of these specifications will be of little

help in the developing world. Suffice to say that getting the rebars in the right place andkeeping them there during concrete placement is critical to the structure's performance. The

following bullet points are some of the most important items to consider when placing

reinforcing.

Placing reinforcement atop a layer of fresh concrete and then pouring more on top isnot an acceptable method for positioning. You must use reinforcing bar supports.

Generally these consist of specialized devices which are made of steel wire, precastconcrete, or plastic which act as chairs to support the rebars in a specific positions.

These devices are most common in footings. These devices may not be readily

available. It is most common to see the rebars sitting atop hard concrete rocks thatprovide the approximate specified concrete cover. Do not use overly large rocks, or

rounded rocks which will cause the bars to slip off.


20/44


In general, simply placing the bars on supports is not enough. Reinforcing steelshould be secured to prevent displacement during construction activities and

concrete placement. This is usually accomplished with tie wire. When tying bars,

there is no need to tie every intersection--every fourth or fifth is normally sufficient.

Rebars in walls can easily become displaced during concrete placement. Because thewall has forms, one may never see that the bars are dislodged. Care is required.

Welded wire mesh slab-on-grade reinforcing is usually laid directly on the groundand as the concrete placing operations proceed, the mesh is simply raised with hooks

to generally be at the middle of the slab. Welded wire mesh reinforcing which isallowed to lie on, and remain on the ground is useless. If you dont attempt to move

it to the middle of the slab, you may as well just leave it out and save the time and

money associated with buying it and placing it.

All reinforcing should be free of dirt, mud or other laitenance that would reduce thebond between the reinforcing and the concrete. If possible, stress to the rebar

supplier that delivery of rusted reinforcing bars is unacceptable. If the rebars are

delivered in a clean condition, every attempt should be made to keep the bars fromrusting. It bears pointing out that if a rebars is cut or bent by the manufacturer or

provider, this area will be the first part of the bar that will experience rust. Earlier in

these guidelines, there was an operation that was referred to as applying form release

agent. Under no circumstances should you spray form release agent on theformworkafter the reinforcing bars are in place. This coating will reduce the bond

between the rebar and the concrete.

Splicing of reinforcing is a very critical issue that should be given proper attention inthe design and construction of concrete structures. Simply lapping the rebars willy-

nilly all over the place for different lengths is un-acceptable and should be avoided.

Splicing, in the context of these guidelines, means contact lap splices. These arerebars that actually contact each other. There are a litany of factors that affect the

length of lap for reinforcing bars; so many in fact that it is felt that this is beyond the

scope of these guidelines. With due respect to the general types of reinforced

structures that are common for EWB Chapters (tanks, walls, foundations, etc.) the

following rebar lap splices could be used:

Reinforcing Bar SizeImperial (Metric)

Contact Lap Splice Lengthin. (mm)

#3 (10) 12 in (305 mm)

#4 (12) 15 in (381 mm)

#5 (16) 18 in (457 mm)

#6 (19) 26 in (660 mm)

#7 (22) 37 in (940 mm)

#8 (25) 48 in (1,220 mm)

The length of contact lap splices can add significantly to a projects overall tonnageof reinforcing. Please be cognizant of this. All contact lap splices should be tied in

place.

In general, the splicing of welded wire fabric reinforcing is done by overlapping 1full mesh square, with wire tying.


21/44


2.1.10 Concrete Cover over Reinforcing

The reinforcing bars are supposed to be embedded in the concrete with sufficient concrete

cover so as to protect the bar from intrusion of debilitating elements such as water, sulfates,etc. The required, or recommended thickness of concrete cover should be as follows:

Minimum Concrete CoverInches (mm)

Concrete cast against and permanently exposedTo the earth (i.e. footings) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 in. (75 mm)

Concrete exposed to earth or weather, #6 through#18 bars (19 57 metric) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 in. (50 mm)

Concrete exposed to earth or weather, #5 bar, andsmaller (16 and smaller metric) . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 in. (38 mm)

For cast-in-place concrete slabs-on-grade, it is suggested that the welded wire mesh beplaced at the mid-depth of the slab. It is recommended that a minimum slab thickness, for

slabs that are not subjected to heavy loads (rolling or otherwise) is 4 in. (100 mm).

2.2.11 Placing Concrete

Placing of the actual concrete is also an act that is heavily and finely detailed in much of the

world for routine projects. A detailed dissertation on placing of concrete is beyond the

scope of these guidelines. There is a litany of factors that can have a direct affect on the

quality of the concrete in a structure. With due respect to the general types of reinforcedstructures that are common for EWB Chapters (tanks, walls, foundations, etc.) the following

bullet points should be used as guidance with regard to placing the concrete.

Placing concrete is generally facilitated by using the chute on the ready-mixed truck

or by transporting the concrete via buggys and wheel barrows from the truck to theformwork if the truck can not be positioned near to the formwork. If the truck can be

positioned to avoid buggys and wheel barrows, make sure that the truck does not hitthe formwork. If the concrete is site-mixed, then it is fairly obvious that the

transportation will be done with buggys and wheel barrows. Do not place more

concrete in a buggy or wheel barrow than can be easily handled by a single person.

When transporting and depositing concrete into the forms, avoid excessive bouncingor jostling of the concrete as it can lead to segregation of the aggregates.

When depositing concrete into the forms, avoid dropping the concrete from aposition higher than 4-0 (1.22 m) as it can lead to segregation of the aggregates.

If a project has a noticeable downhill or down slope or lower portions, place this

concrete first.


22/44


In the U.S.A., after the concrete is placed in forms (walls, footings, stem walls) it isthen consolidated. Consolidation compacts fresh concrete to mold it within the

forms and around reinforcement and to eliminate stone pockets, honeycomb, and

entrapped air. Vibration, either internal or external, is the most widely used method

for consolidating concrete. When concrete is vibrated, the internal friction betweenthe aggregate particles is temporarily destroyed and the concrete behaves like a

liquid; it settles in the forms under the action of gravity and the large entrapped air

voids rise more easily to the surface. Internal friction is reestablished as soon as

vibration stops. It is most likely not practical to have an internal concrete vibrator onthe job site in the developing third world on account of the fact that these devices

usually require electrical power or diesel powered motors. For an example, seehttp://www.toolfetch.com/Category/Concrete_Compaction/Vibrators/23582.htm . An

alternative to internal vibration is external vibration. A suitable method is simply to

tap (or slightly higher force) the forms on both sides with a hammer or stick in afashion that vibrates the formwork and retained concrete.

Handling of concrete should be carefully coordinated with placing and finishingoperations. Concrete should not be deposited more rapidly than it can be spread,

struck off, consolidated, and bull floated. Concrete should be deposited continuouslyas near as possible to its final position. In slab construction, placing should be

started along the perimeter at one end of the work with each batch placed againstpreviously dispatched concrete. Concrete should not be dumped in separate piles

and then leveled and worked together; nor should the concrete be deposited in largepiles and moved horizontally into final position.

The addition of water to the mixed concrete at the site is strictly forbidden.

Concrete shall not be placed on frozen ground, nor shall concrete be placed inunfavorable conditions which may be detrimental to the quality and finish of theconcrete in the structure unless adequate precautions have been taken. Unfavorable

conditions shall be deemed to include low temperatures (below 41o

F {5o

C} with

temperatures descending, or below 36oF {2

oC} with temperatures ascending).

Hot weather conditions above approximately 77 F (25 C) can adversely impact the

quality of concrete. In general if the temperature at the time of concrete placementwill exceed 77 F (25 C), a plan should be developed to negate the effects of high

temperatures. The precautions may include some or all of the following:

o Moisten subgrade, steel reinforcement, and form work prior to concreteplacement.

o Erect temporary wind breaks to limit wind velocities and sunshades to reduceconcrete surface temperatures.

o Cool aggregates and mixing water (site-mixed concrete) added to theconcrete mixture to reduce its initial temperature.

o Provide sufficient labor to minimize the time required to place and finish the

concrete, as hot weather conditions substantially shorten the times to initialand final set.


23/44


2.2.12 Construction Joints

When a concrete item can not be placed in a continuous operation, there will be a need to

put construction joints in the item being constructed. Construction joints are where theconstruction stops at one point and then will start again. It is beyond the scope of these

guidelines to provide a detailed presentation of the intricate details of construction joints.

The EWB chapter must give proper attention to this concern and a professional structural

engineer or suitable faculty advisor or suitable mentor should provide a proposed detailshould it become necessary to install a construction joint in any concrete item, including

slabs-on-grade.

Some guidance about construction joints can be seen at;

http://www.cement.ca/cement.nsf/0/F84E52F8C61D94F4852568AA006D5ED8?OpenDocument.

2.2.13 Finishing Concrete

There is usually no need to pay attention to finishing concrete that is totally formed. Theonly surface that is exposed is the top surface and special attention need not be placed on

this surface. A simple striking off of the concrete at the top of the form is all that is needed.

For slabs-on-grade, it is a completely different story. As you place the concrete in theforms, use a rake and 2x4s to smooth out the concrete, so it is flush with the top of the

forms. This is called screeding the concrete.

Once the concrete is screeded off, either in total or a section, you must work the top of the

concrete with a wood float

(http://www.concretenetwork.com/concrete/concrete_tools/hand_floats.htm ) or a

magnesium float trowel(http://www.concretenetwork.com/concrete/concrete_tools/trowels.htm ). You swirl these

tools on the partially stiff concrete. This motion drives the coarse aggregates down into the

slab and brings the fine sand and cement component to the top. Ideally, you would like tohave the stones about 1/4 inch (6 mm) below the finished surface. This act is called

toweling the concrete. You should avoid overworking the top surface of the concrete as this

can lead to segregation of the fine aggregate and the cement. This will leave only cement atthe top surface of the slab and upon completion (days later) you will most likely notice that

this top layer (referred to as wedding cake icing) is coming off and delaminating. This is

because the cement is all by itself and has no fines or coarse aggregate to bind into a matrix.There is little you can do about this if it occurs. If the concrete surface is to be exposed to

walking and if it is desired to have a non-slip type finish, you can use a stiff bristle pushbroom to create a nice, finished texture. Simply pull the broom lightly across the smoothconcrete to get the desired look. This type of non-slip finish should always be applied to

exterior walks. The striations should be perpendicular to the direction of the foot traffic.

Concrete slabs shrink notoriously as they cure and harden. They shrink 1/16th inch (1.6mm) for every 10 linear feet (3.04 linear meters) each way. You must create control joints

at even intervals so that you do not get random cracks across the new sidewalk. These

cracks will always occur, there is very little that you can do to avoid them. The reasons aremany; such as; friction between the underlying soil and the slab is different at the bottom


24/44


than at the top, evaporation which is generally more off of the top of the slab, un-evendrying. Control-joints are generally saw cut into slabs-on-grade in the improved world.

This is usually done within 18 hours of the slab being finished. In the developing third

world, it is highly doubtful that a slab saw cutting device is available. In lieu of sawn joints,hand tooled in joints can be worked into the nearly set concrete. The tool for gouging these

control joints into the slab is called a groover

(http://www.concretenetwork.com/concrete/concrete_tools/groovers.htm ).

For a slab-on-grade that is 4 thick, the sawn joint or grooved joint should generally be1/8 in wide x in deep (3.2 mm x 19 mm). The mesh reinforcing is not required to be

interrupted at these control joint lines. When the concrete slab-on-grade cracks, and it will,it will simply crack in the control joint, hence the name control. This joint is a weakened

plane through the concrete and more times than not, the concrete will find it and crack there.

The wood float, trowel and groover tools should be readily available or could be easily

transported to the job site from the U.S.A.

The layout of the control joint lines requires some thought. In general, you would wish to

have all slabs saw cut or grooved into square shapes that are generally no more than 15 ft.

(4.6 m) on center each way. The square shape is the best because it shrinks the same in eachdirection. Square shapes are not always possible. Rectangular shapes are acceptable but it

is preferred to have the ratio of the long side to the short side not be any greater than 2:1

with the long dimension no longer than 24 ft. (7.3 m).

If it is deemed necessary to seal the control joint, after the concrete has cracked, it is

suggested that the joints be filled with a polyurethane joint sealant. This is not necessary,

but only suggested if the floor finish over the top of the slab is sensitive to moisture that maywick up and migrate through the crack due to capillary action. An example can be seen at;

http://www.sealantsandcoatings.com/frameset.cfm?sheetLink=http%3A%2F%2Fwww%2Ec

hemrex%2Ecom%2Fdocuments%2Fnp1%5Ftdg%2EPDF&compID=90&CSICode=102&pr

odID=611&pageID=other&clickType=datasheet .

This wicking and moisture migration can also be controlled by using a polyethylene sheet

below the slab-on-grade as it is cast. However, one must recognize that if a polyethylenesheet is used below the slab, the water in the concrete mix only has 1 avenue to escape the

concrete. All of the water must go upward. This lag between having the top dry first and

then have lower level water migrate through the upper zones of a slab can cause a lot ofdistortion (curling) in the slab. If a polyethylene sheet is used, an example can be seen at;

http://www.acehardware.com/product/index.jsp?productId=1308347&cp=1254881.1306647

&parentPage=family&searchId=1306647 . It is suggested to use a clear sheet to reduce solargain which could cause problems with the freshly placed concrete.

2.2.14 Curing Concrete

After concrete is placed, a satisfactory moisture content and temperature (between 50F and

75F, 10C and 24C) must be maintained. This process is called curing. Adequate curing isvital to quality concrete. Curing has a strong influence on the properties of hardened

concrete such as durability, strength, and water tightness. Exposed slab surfaces are

especially sensitive to curing.


25/44


Curing the concrete aids the chemical reaction called hydration. Most freshly mixedconcrete contains considerably more water than is required for complete hydration of the

cement; however, any appreciable loss of water by evaporation or leaking will delay or

prevent hydration. If temperatures are favorable, hydration is relatively rapid and occurs inthe first day after concrete is placed. Retaining water during this period is important. Good

curing means evaporation should be prevented or reduced.

In general, formed surfaces such as walls, stem wall, footings (not earth-formed) and the like

can be cured by simply leaving the forms in place for a moderate amount of time. The mostimportant curing time period is in the first day or two after forming. If the design of the wall

element requires a high strength such as 3,000 psi (20.7 MPa) or higher, then the EWB TACmay wish to have the EWB Chapter pay particular attention to forming. For items which are

utilitarian in nature and do not require high strength, then it might be suitable to simply

leave the forms in place for 24-36 hours maximum. In no case should the forms be removedsooner than 12-16 hours after casting.

Unformed surfaces, such as slabs, are cured in a much different manner. Items such as thesehave a tremendous amount of surface area that will allow a tremendous amount of

evaporation of mix water. If curing of the slab is not given proper attention, then the slab

will be subjected to a litany of debilitating effects.

The most popular and least expensive method of curing a slab is to spray membrane-forming

curing compounds on the surface and edges of freshly finished concrete. This method

doesn't prevent moisture from leaving the slab, but it does retard moisture loss. Amembrane-forming curing compound is a specialized item and may not always be available.

This compound is also manufactured by Grace Construction Products. A country guide

which lists distributors in several countries can be found at their web sitehttp://www.na.graceconstruction.com/sitemap.cfm. This product locks in the water that was

used to mix the concrete. The concrete uses this water to complete the chemical reaction

that continues for many months. This hydration reaction is what allows the concrete to

reach its final design strength.

In general, simple floor slabs and the like are not critical and key concrete elements and they

are usually utilitarian in nature. This is not to say that proper attention should not be givento curing. But, it is to say that the curing process not be held to a very rigid and controlled

standard. On the other hand, slabs that form the bottom of a water-retaining tank structure

are very critical. Improper curing could lead to cracking which is very detrimental to such astructure.

For critical elements such as tank bases/slabs, the goal is to maintain 80% relative humidityin the concrete for a minimum 7-day period after placement. Other curing methods include

wrapping a slab (including the edges) with polyethylene film, ponding it with water, placingdirt or sand on the slab and keeping it moist, and using wet coverings like burlap and burlap-polyethylene sheeting. Any of these methods, if entertained, should be evaluated during a

site assessment trip. Material availability, cost and waste should be evaluated.


26/44


2.2.15 Concrete Safety

Anyone handling concrete should understand and practice a number of basic safety tips

concerning protection, prevention, and common sense precautions. Precautions not limitedto the following tips concerning protecting your head, back, skin, and eyes, should be

considered.

Wear a hard hat for head protection!

Be careful how you move heavy materials. Working with the normal materials thatare required to make and pour concrete, such as Portland cement, aggregate, sand,

and water can be very strenuous to the average person's back.

Take care to lift properly keeping your back straight and your legs bent to avoidserious back strain. Carry materials, if you have to, keeping them waist high and

centered between your legs to lessen the chance for injury. Ask a co-worker for help

if you need to lift heavy materials.

Push the concrete to its final position with a shovel, rake or similar tool. Do not liftthe concrete mix.

Protect your self from skin irritation and chemical burns when working with freshconcrete. Severe burns can result with on-going contact between fresh concrete and

skin surfaces, eyes, and clothing.

Don't handle wet cement directly since it is basic so it will be injurious to your skin.Don't handle dry Portland cement without protection since it will draw moisture from

your skin.

Wear protective clothing, such as waterproof gloves, long-sleeved shirts, and longpants to keep the concrete from making contact with your skin.

Wear rubber boots if you must stand in the fresh concrete while it is being placed,screeded, or floated to prevent concrete from flowing into them and making contact

with your lower legs, ankles and feet.

Wear proper eye protection when working with cement or concrete. Splatteringconcrete and blowing dust can easily enter your eyes during a concrete placement.

Wear safety glasses with side shields, depending on the conditions at your project.

2.2.16 Wrap-up on Concrete

There is a wide variety of information on the internet for concrete, mixes, placement,reinforcing, joints, etc. One of the best guides available can be seen at;

http://www.concrete.net.au/pdf/concretebasics.pdf#search=%22concrete%20construction%2

0joints%22


27/44


2.2 Reinforcing

Nearly all concrete is reinforced. Concrete is extremely strong in compression and very weak in

tension. The reinforcing in concrete is intended to provide the tension strength in concrete bendingelements. In concrete compression elements, the reinforcing does add some strength as well.

Reinforcing generally means steel wire mesh or steel bars referred to as rebars.

In the structural engineering design of reinforced concrete elements, the rebars are a critical part of

the design. One of the most important items that an EWB assessment team needs to determine isthe type of reinforcing steel that is readily available for the project. In the U.S.A., and quite

possibly throughout the world, there is a type of reinforcing bar that we refer to as ASTM A615,Grade 60. This type of rebar has a yield strength of 60,000 psi (413.7 MPa). However, it is not

uncommon to also find a lower grade steel that is referred to in the U.S.A. as ASTM A615, Grade

40. This type of rebar has a yield strength of 40,000 psi (275.8 MPa). In the U.S.A., this lowergrade steel was normally only used for very small diameter rebars, 3/8 (#3). In the developing

world, there is no telling what sort of reinforcing is readily available. If an implementation design

has assumed a high strength reinforcing (60,000 psi or 413.7 MPa), there is a high likelihood thatthe EWB TAC will require the EWB Chapter to obtain evidence that this steel has been provided

and installed.

In cast-in-place concrete slabs-on-grade, it is common to use a wire mesh instead of reinforcing

bars. Generally in the design of concrete slabs that are not subjected to heavy wheeled or point

loads, the mesh reinforcing is simply installed as a means to resist the stresses in the slab due to

plastic shrinkage and temperature. In the U.S.A., this welded wire steel mesh is generally a latticeof smooth steel wires that are spaced at either 4 or 6 on center. The gage (diameter) of the wires

can vary from very small to quite large. For the most part, the EWB projects that utilize a cast-in-

place concrete slab-on-grade will be adequately reinforced by the minimal size wire mesh. Ingeneral, these meshes come in either rolls or sheets. Rolls are more common. It is not imperative,

but if it is possible at all, the EWB Chapter should try to obtain any specifications for the mesh that

they are to use. For pure comparison, in the U.S.A., common mesh reinforcing that is suitable for

cast-in-place concrete slabs-on-grade is referred to as 6x6-W1.4xW1.4 welded wire fabric. A guidefor converting from U.S. nomenclature to metric for welded wire fabric can be examined at

http://www.wirereinforcementinstitute.org/HTDocs/PDFs/TF%20206-R-03.pdf#search=%226x6-

W1.4xW1.4%22.

With regard to concrete reinforcing, there are certain criteria that should be made known and

followed. It is not the intent of these guidelines to provide an exhaustive presentation on each ofthese criteria. A discussion of the main criteria follows:

2.2.1 Metric Sizes for Reinforcing

Nearly every country on earth, except the U.S.A. specifies their reinforcing bars in metricsizes. The bar sizes are very similar to those used in the U.S.A. In the structuralengineering of concrete elements, it is suggested that the metric bars sizes be used in the

calculations because they represent what is actually going to be placed. Again, for higher

strength rebars, substantiate the material strength. There are many on-line guides to show

the side by side comparison between normal U.S. size rebars (sometimes referred to asimperial) and generally available metric size rebars. One example is provided by the

Georgia Department of Transportation. This comparison chart can be seen at the following:

http://www.dot.state.ga.us/preconstruction/bridgedesign/brinfo/rbarmet.shtml .


28/44


2.2.2 Reinforcing Safety

Anyone working around or with concrete reinforcing should be aware of the hazards and

safety precautions related to this operation. For example, working more than 6 feetabove any adjacent working surface, placing and securing reinforcing steel in walls,

piers, columns, etc., should be done while use a safety belt, full body harness or other

equivalent form of fall arrest or restraint protection.

Do not work above vertically protruding rebar unless it has been protected to eliminatethe hazard of impalement. The top of the rebar should be covered with a rebar cap,

wood board or similar device. See http://www.versarebarcap.com .

Reinforcing bars and welded wire mesh reinforcing are nearly always rusty. As such,

there is a high likelihood of being scratched by these items. This can lead to tetanus.All EWB Chapter members must consider this risk and protect themselves. See

http://www.cdc.gov/nip/vaccine/tetanus/default.htm .

2.3 Masonry

Masonry construction is one of mans oldest construction methods. Most early applications

used natural stone assembled in a dry stack process. Later a cement, sand and water

mixture (mortar) was introduced to create a bond between units. The desire for uniformity

in size and shape, and the availability of clay led to the development of brick. Brick wasfirst fabricated from mud and baked in the sun. It is now made from clay and fired in high-

temperature kilns. More recently, concrete masonry units (CMU) made from cement, sand,

small gravel, water and a series of admixtures have been developed to provide a larger, lessexpensive building block. Each of these and many other types of masonry are prevalent

throughout the world. The EWB team should make every effort to investigate and

determine the type of masonry that is common to the community. Pay particular attention

to the notion that a particular trade or skill should still exist in a community and to not justbe led to believe that if one sees a particular type of masonry that the present villagers are

capable of construction with that type of material. Investigate thoroughly.

For the purposes of these guidelines, masonry is the building of structures from individual

units laid in and bound together by mortar. The common materials of masonry construction

are bricks and concrete blocks. Masonry is generally a highly durable form of construction.However, the materials used, the quality of the mortar and workmanship, and the pattern the

units are laid in can strongly affect the durability of the overall masonry construction.

Advantages. The use of materials such as brick and concrete block can increase the thermal

mass of a building, giving increased comfort in the heat of summer and the cold of winterand can be ideal for passive solar applications. Brick typically will not require painting andso can provide a structure with reduced life-cycle costs, although sealing appropriately will

reduce potential spalling due to frost damage. Concrete block of the non-decorative variety

is generally painted or stuccoed if exposed. Concrete block should be waterproofed it if is

for a below grade application. Masonry needs no formwork.

Disadvantages. Extreme weather may cause degradation of the surface due to frost damage.

This type of damage is common with certain types of brick, though relatively rare withconcrete block. If non-concrete (clay-based) brick is to be used, care should be taken to


29/44


select bricks suitable for the climate in question. Masonry must be built upon a firmfoundation (usually reinforced concrete) to avoid potential settling and cracking. If

expansive soils (such as adobe clay) are present, this foundation may need to be quite

elaborate and the services of a qualified structural engineer may be required. The highweight increases structural requirements, especially in earthquake prone areas.

Structural limitations. Masonry boasts an impressive compressive strength (vertical loads)but is much lower in tensile strength (twisting or stretching) unless it is reinforced. The

tensile strength of masonry walls can be strengthened by thickening the wall, or by buildingmasonry "piers" (vertical columns or ribs) at intervals. Where practical, steel reinforcement

also can be introduced vertically and/or horizontally to greatly increase tensile strength. Forthe most part, many of the EWB Chapter projects that use cast-in-place concrete should be

examined for suitability and an alternate method, masonry, should be considered.

2.3.1 - Concrete Masonry Units

CMU is used extensively in many developing countries, which means that labor skilled in

placing the block should be easily found. The major question is whether it will meet

structural requirements. If the CMU is likely to be a material used in the project, the teamshould attempt to make a visit to a local block factory. Document the standard sizes that

are available. Investigate how much cement is used in each unit or batch and the kind of

aggregate that is used. Discuss with the owner the possibility of increasing the amount of

cement for an added cost.

The basic components of CMU blocks are cement, water, sand and gravel. CMU blocks may

be steam-cured for 1-2 days, which gives it a relatively low embodied energy. CMU blocks,like brick, have beneficial qualities such as durability, compressive strength, acoustical

performance, low R-value, chemical make-up and fire resistance. In general, CMU blocks

often cost less than brick. However, CMU blocks are not as water resistant as fired brick.

CMU blocks can be sprayed with a water repellent coating or it is sometimes manufacturedwith an admixture which gives it water repellence. The spray-applied coating might last

about 10 years, while the admixture lasts for the lifetime of the block. The admixture is

generally more expensive but quickly gains advantage over the sprays as the building ages.

Generally CMU structures are reinforced with reinforcing bars that can be in either the

vertical (more common) or horizontal (less common) directions. The general guidelines forplacing the reinforcing steel are very similar to those that are described above in the

Concrete section. There is an additional reinforcing item that is common to CMU

construction and this is called joint reinforcing. Since the joints of CMU are generallyonly 3/8 (9 mm) wide, it is difficult to place deformed reinforcing bars in the joints unless

they are very small. In many countries, there exists a specialized joint reinforcing whichconsists of longitudinal steel wires and varying types of steel cross wires. It is common toutilize this type of reinforcing at every other horizontal course in CMU construction. There

are special corner pieces to assure continuity. There are different widths to fit into different

thicknesses of CMU. There are different sizes where some units offer more area of steel.

One of the most common manufacturers of this type of joint reinforcing is Dur-O-Walllocated at http://www.dur-o-wal.com. For specific examples of a common variety of joint

reinforcing, see http://www.dur-o-wal.com/prod/singlewythe.html .


30/44


2.3.2 Mortar in Concrete Masonry Units

Individual CMU units are bound together with mortar. Mortar is a material used in masonry

to fill the gaps between blocks in construction and bind the blocks together. Mortar is a

mixture of sand, a binder such as cement or lime, and water. Mortar is applied as a pastewhich then sets and hardens. There are generally three (3) families of mortars which are as

follows:

Portland Cement Mortar. This is created by mixing Portland cement with sand andwater. It was invented in the mid nineteenth century, as part of scientific efforts to

develop stronger mortars than existed at the time. It was popularized during the

nineteenth century and it had superseded lime mortar by the 1930s. The mainreason for this was that it sets hard and quickly, allowing a faster pace of

construction.

Lime mortar is created by mixing sand, slaked lime and water. The earliest knownuse of lime mortar dates to about 4000 BC in Ancient Egypt. Lime mortars havebeen used throughout the world, notably in Roman Empire buildings throughout

Europe and Africa. The vast majority of pre-1900 masonry buildings in Europe andAsia are built from lime mortar. The process of making lime mortar is simple.

Documents

Structural Guidelines