BC CARPENTER APPRENTICESHIP PROGRAM … CARPENTER APPRENTICESHIP PROGRAM — LEVEL 1 1 Program...

BC CARPENTER APPRENTICESHIP PROGRAMLEVEL 1

Line G: Concrete FormworkCompetency G-1: Use Concrete Types, Materials, Additives and Treatments

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Version 2, New, January 2017

ISBN 978-0-7726-7038-0

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AcknowledgmentsThe Industry Training Authority of British Columbia would like to acknowledge the Carpentry Articulation Committee and Open School BC, a division of the BC Ministry of Education, as well as the following individuals and organizations for their contributions in updating the BC Carpenter Apprenticeship Learning Guides:

Carpentry Articulation Curriculum Committee membersDennis Carlson, Tom Haag, Erik Hardin, Alf Leimert, Geoff Murray, Don Naidesh, Stephen Pelley, Al van AkkerWriters: Gary Backlund, Geoff MurrayReviewers: Trevor Feddersen, Roy Mironuck, Geoff Murray, Don Naidesh, Stephen Pelley

Open School BCChristina Teskey, project managementDennis Evans, photography, illustrationFarrah Patterson, print layout, illustrationShannon Sangster, copyright management, art coordinationBeverly Carstensen, print layout, illustrationMax Licht, illustrationGreg Aleknevicus, editingRobin Miller, editing

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ContentsProgram Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Learning Task 1: Describe Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Self Test 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Answer Key . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

DisclaimerThe materials in these Learning Guides are for use by students and instructional staff, and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the Carpentry Articulation Committee, the British Columbia Industry Training Authority or the Queen’s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for carpentry practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this Competency and that other or additional measures may not be required.

These materials contain information that has been derived from information originally made available by the Province of British Columbia at: http://www.bclaws.ca/, and this information is being used in accordance with the Queen’s Printer License – British Columbia available at: http://www.bclaws.ca/standards/2014/QP-License_1.0.html. They have not, however, been produced in affiliation with, or with the endorsement of, the Province of British Columbia, and THESE MATERIALS ARE NOT AN OFFICIAL VERSION.

Safety AdvisoryPlease note that it is always the responsibility of any person using these materials to inform him or herself about the Occupational Health and Safety Regulation pertaining to his or her work. The references to WorkSafeBC safety regulations contained within these materials may not reflect the most recent Occupational Health and Safety Regulation (the current standards and regulation in BC can be obtained on the following website: http://www.worksafebc.com).

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We want your feedback! Please go to the BC Trades Modules website (www.bctradesmodules.gov.bc.ca) to enter comments about specific sections that require correction or modification. All submissions will be reviewed and considered for inclusion in the next revision.

BC CARPENTER APPRENTICESHIP PROGRAM — LEVEL 1 1

Program Outline

Line A – Safe Work PracticesA-1 Apply Shop and Site Safety PracticesA-2 Apply Personal Safety Practices

Line B – Documentation and Organizational SkillsB-1 Use Construction Drawings and SpecificationsB-2 Interpret Building Codes and BylawsB-3 Plan and Organize WorkB-4 Perform Trade Math

Line C – Tools and EquipmentC-1 Use Hand ToolsC-2 Use Portable Power ToolsC-3 Use Stationary Power Tools

Line D – Survey Instruments and EquipmentD-1 Use Levelling Instruments and Equipment

Line E – Access, Rigging and Hoisting EquipmentE-1 Use Ladders, Scaffolds and Access EquipmentE-2 Use Rigging and Hoisting Equipment

Line F – Site LayoutF-1 Lay Out Building Locations

Line G – Concrete FormworkG-1 Use Concrete Types, Materials, Additives and TreatmentsG-2 Select Concrete Forming SystemsG-3 Build Footing and Vertical FormworkG-4 Build Slab-On-Grade Forms and Suspended Slab Forms

G-7 Place and Finish Concrete

Line H – Wood Frame ConstructionH-1 Describe Wood Frame ConstructionH-2 Select Framing MaterialsH-3 Build Floor SystemsH-5 Build Stair SystemsH-10 Build Decks and Exterior Structures

Line J – Building ScienceJ-1 Control the Forces Acting on a Building

2 BC CARPENTER APPRENTICESHIP PROGRAM — LEVEL 1

Competency G-1: Use Concrete Types, Materials, Additives and TreatmentsConcrete is one of the most widely used building materials in all types of construction. It is a simple mixture of cement, water and aggregates. The quality of the concrete is dependent on the proportions of the ingredients and how the fresh concrete is handled. Carpenters must know how to use this essential building material.

Learning ObjectivesWhen you have completed the Learning Tasks in this Competency, you will be able to:

• describe the uses of concrete

• describe three basic elements of concrete

• describe uses of concrete design mixes

• describe the types of admixtures and treatments for concrete

CompetenciesWritten: “Describe concrete types, materials, additives and treatments”

You will be tested on your knowledge of concrete types, materials used to make concrete, concrete additives and methods of concrete treatment.


COMPETENCy G-1: USE CONCRETE TyPES, MATERIALS, AddITIVES ANd TREATMENTS LEARNING TASk 1

LEARNING TASK 1

Describe ConcreteConcrete is a versatile building material. It is used for building all types of foundations, ranging from foundations for a one-story house to foundations for extremely tall freestanding structures such as the CN Tower in Toronto.

There are different types of concrete for different uses. They include:

• plain concrete • reinforced concrete • pre-stressed concrete • pre-cast concrete • concrete shells • architectural concrete • concrete masonry • concrete pipe • concrete paving • soil-cement concrete • shot-crete (gunnite) • grout • mortar

Plain ConcretePlain concrete contains no reinforcing steel or mesh. Simple footings and large gravity retaining walls are made of plain concrete.

Reinforced ConcreteConcrete has great strength in compression, and it can support heavy loads. Concrete has less strength when in tension. It can be pulled apart easier than it can be crushed.

Concrete is about ten times stronger in compression than in tension. When structural members such as beams or girders must resist large tensile stresses that tend to pull the concrete apart, steel bars or mesh are embedded in the concrete. The steel reinforcing is placed in a pattern that provides the best combination of tensile and compressive strength. This reinforcement makes it possible for concrete members to sustain a heavy load over a considerable span. It also reduces the cracking that occurs with the concrete’s shrinkage.



Three fundamental properties permit concrete and steel to work together as a composite material:

• thermal expansion is approximately equal • cement paste in concrete bonds strongly to steel reinforcing • concrete protects the steel against rust and corrosion

Thermal Expansion Most materials expand when heated and contract when cooled. Concrete and steel expand the same amount for a given increase in temperature. If they did not, concrete reinforced with steel would tear itself apart during large temperature changes. A sample of reinforced concrete, 100 feet long, will expand 9/16 of an inch when the temperature rises from –20°C to 20°C.

Bonding The bond between the cement paste and the reinforcing steel transfers the tension load from one steel reinforcing bar to the next (Figure 1). The bars are overlapped to allow the transfer to take place. The usual amount overlap is specified as 24 times the diameter of the bar.

24 BARDIAMETERS

Figure 1 — Overlap of reinforcing steel

Corrosion During the chemical reaction of cement with the water in the concrete mixture, the cement paste bonds to the reinforcing steel. This bond protects the steel from corrosion. Calcium chloride should not be used as an accelerator in concrete that is reinforced with steel.

In marine structures, and other locations where the chance of corrosion is great, reinforcing steel should be coated with an epoxy paint to protect it. The reinforcing can be made from stainless steel or be galvanized steel for protection from extremely corrosive conditions.



Uses Reinforced concrete is used in the following:

• footings • suspended slabs • piers • bridges (short-span) • walls • piles • columns • pile caps • pilasters • grade beams • corbels • panels • beams • tilt-up walls • girders

Pre-Stressed Concrete Concrete beams, slabs and girders can be pre-stressed to increase their ability to span distances and carry loads. The reinforcing members in the concrete are placed in a state of permanent tension before service loads are applied.

The induced stresses in the concrete member counteract the stresses developed by both the weight of the member itself and the additional service loads. When loads are applied to a pre-stressed member it deflects (sags) very little.

Pre-stressing concrete members requires special equipment and procedures. These procedures are costly and limit the use of pre-stressed concrete to construction requiring long clear spans or heavy loads. Standard reinforced concrete is used for most concrete construction.

There are two ways of inducing the pre-stressed forces into the concrete: pre-tensioning and post-tensioning.

Pre-Tensioning When concrete is pre-tensioned, high-strength steel cables are placed into the formwork for the member and tensioned before the concrete is placed. Most pre-tensioned concrete members are pre-cast.

Special pre-casting beds are used for the pre-tensioning process. At the end of each bed is a heavy concrete abutment that is used to support the cables during tensioning. The cables are tensioned using a hydraulic jack, which pulls the cable to the proper tension. When the concrete sets, the tension on the cables is released and the jacks are removed. Each cable is then cut off flush with the end of the member, and the member is removed from the casting bed ready for use.

The stress on the cable is transferred to the concrete member by the bond between the concrete and the cable.



Post-Tensioning When concrete is post-tensioned, high-strength steel cables are placed into the formwork for the member but not tensioned until after the concrete is placed. Post-tensioned concrete members may be pre-cast or cast-in-place. In either case, the tensioning is done at the job site.

Post-tensioning pre-cast concrete members involves pulling a number of cables through tubes that were cast into the member at the pre-casting plant. The cables are then tensioned using hydraulic jacks. The permanent transfer of the tension force to the concrete member is accomplished by pumping cement grout into the tubes to bond the cables to the concrete.

Post-tensioning cast-in-place concrete members involves placing a number of cables into the formwork. The cables have a plastic sheath that allows them to be tensioned, using hydraulic jacks, after the concrete has cured. Pairs of steel wedges grip the cable and permanently transfer the tension to the concrete member.

Pre-Cast Concrete Using pre-cast concrete components for buildings provides many advantages over using cast-in-place components for certain building designs. Pre-cast components are:

• economical • high quality • fast to use • available in standard sizes and shapes

The following components are able to be pre-cast:

• beams • wall panels• girders • curtain wall panels• single and double floor joists • stairs • floor slabs • landings piles • roof slabs • bridge decking

Pre-cast concrete is not suitable for buildings that include many different concrete shapes that are not duplicated in the building design.

Concrete Shells The first concrete shell roof was constructed in 1923. Today there is a shell-roof size, type or shape for every kind of building.

A shell is a long, continuous beam of curved cross section, which combines the advantages of trusses, purlins and wind bracing through the interaction of its parts. This mutual action of all parts creates high lateral stability, which in turn provides a capacity to carry unbalanced roof loads.



Engineers have successfully applied reinforcing and pre-stressing force to the structural capabilities of shells. By introducing a pre-stressing force in the edge beam of the shell, or in the shell itself, or in both the edge beam and the shell, it has been possible to extend spans and considerably increase load-carrying capacities.

Shell roofs, for all their strength, have been built as thin as 50 mm. In many cases, even this thin cross section is more than the thickness needed for required strength.

There are four common shapes of concrete shell roofs:

• barrels • domes • hyperbolic paraboloids • folded plates

Figure 2 — Concrete shell roof

Architectural Concrete Contemporary styles of architecture make use of concrete as a finished building material. Because of its plasticity, fresh concrete can be molded into practically any shape or form. This gives the architect great freedom in building design.

All elements of a concrete building, ornamental as well as structural, may be cast in a single operation at a substantial saving of cost. Fluting, rustications, relief patterns and other ornamental devices are easily added with concrete.

Because of its durability and architectural flexibility, pre-cast curtain-wall panels are being used extensively on many low- and high-rise buildings.

Numerous texture and color variations are available to enhance both interiors and exteriors. The textures range from ruggedness to ceramic-like smoothness.

A simple way of obtaining texture is to reproduce the texture of the forming material. Timber-marked textures, for example, can reveal grain impressions and the joint lines of rough or dressed lumber. Form liners also impart pattern directly to a concrete surface. The most commonly used liners are made of rubber or plastic.



Other methods of giving texture to concrete surfaces include bush-hammering, sand embedment, and exposing surface aggregates by removing the surrounding mortar with a jet of water, acid etching or sandblasting. Exposed aggregate panels may be ground smooth to take on a terrazzo like appearance.

Because of its plastic nature, concrete is especially well suited to patterning. In creating a design in a wall panel, there are three basic approaches:

• high and low relief • colored aggregates • contrasting textures

Designers have a wide choice of color for concrete wall panels. They may use exposed colored aggregates, or they may add mineral oxide pigments to the matrix to produce any number of attractive colors, including pure white.

Concrete Masonry The term “concrete masonry” is applied to pre-cast block and brick building units molded of concrete and used in masonry construction for homes and buildings of all types. Concrete masonry units are made in several sizes and shapes to fit different construction needs.

Units are produced to comply with the required standards set by associations and local Building Codes. Compressive strength requirements provide a measure of concrete masonry’s capacity to carry loads and withstand structural stress with an adequate margin of safety. Absorption requirements provide a measure of the resistance of concrete to water penetration.

Either normal or lightweight aggregates are used for making concrete masonry units. In manufacturing plants, high-speed, high-producing machinery can produce several thousand blocks a day. Autoclaving (high-pressure steam curing) speeds up the curing process. This allows blocks to be shipped to a construction site the day after they are produced.

Concrete units are used for all types of masonry construction, including:

• load-bearing walls • chimneys • partition walls • landscaping fences • piers • walkways • firewalls• roads • backup walls for brick, stone or stucco facings



Many plants also make lintels and concrete floor filler units. The kinds of block available include shadow-wall block, slump block, split block (which has an attractive, stone-like surface) and screen block (which provides privacy when laid into a wall, yet permits air to circulate and light to enter).

Concrete Pipe This product is manufactured throughout the world for irrigation and drainage systems, sewers, culverts and water supply mains. It may be made of reinforced or pre-stressed concrete and cast in forms of the desired design.

Concrete Paving Concrete is recognized as a leading paving material. Concrete roads can be accurately designed to carry any specific volume and weight of traffic. They cost less to build than other equal load-carrying pavements and have a low maintenance cost. The use of sliding or slip forms has greatly increased the speed at which pavements can be laid. Concrete paving can be made skid-resistant by using a finishing method that leaves a rough surface.

Air Entrainment The use of air entrainment helps the concrete to resist the damage caused by numerous freeze-thaw cycles. One cubic meter of concrete with 4% to 6% air entrainment can contain as many as 650 billion microscopic air bubbles.

The air bubbles provide tiny chambers for water to expand into as it freezes. This relieves the internal pressure on the concrete during freezing and thawing. Air entrainment is also effective in preventing concrete surfaces from scaling when they are exposed to de-icing chemicals.

Concrete is used not only for paving highways and roads, but also for constructing sidewalks, streets, parking areas and airport runways. Using slip forms, a crew in one day can lay as much as 1372 m of runway, 7.6 m wide and 380 mm thick. Concrete airport pavement has added greatly to flying safety because of its high visibility, skid-resistance and freedom from loose particles that damage aircraft. As well, the pavement is not damaged by jet fuel, jet heat and jet blasts.

Railroads also depend on concrete. There are well over 150 railroad uses for Portland cement and concrete. They range from huge trestles several miles in length to linings for railway tunnels and small pre cast ties and signposts.

Soil-Cement Soil-cement is a simple, highly compacted mixture of soil, Portland cement and water. As the cement hydrates, the mixture becomes hard and durable.

Soil-cement is used mainly for paving roads, streets and airports. To complete the pavement, a thin bituminous sealing course is placed over it.



Soil-cement can be used for widening roads, for building shoulders and parking areas, and for laying sub-bases for pavement. It is also used for facing earth dams and lining reservoirs, ditches and canals.

Shot-Crete (Gunnite) Shot-crete is Portland cement concrete that is applied by a pneumatically operated gun. It is used in the construction of walls, roofs, floor slabs, reservoir linings, boat hulls and landslide control facings for cliffs.

Shotcrete can also be used for restorative work on concrete structures damaged by the weather or fire.

Grout Portland cement grout is placed under pressure and used for a variety of purposes, including stabilizing foundations, making rock foundations under dams watertight and structurally sound, filling cracks in concrete work and sealing oil wells. Non-shrink grout is used for supporting machine bases and columns.

Mortar Mortar is a mixture of cement, water and sand. It is used in masonry construction and for patching and repairing concrete surfaces. Mortar for masonry must bond the units into a strong, durable wall. At the same time, it must provide a watertight joint, low shrinkage and resistance to efflorescence.

Fibre-Reinforced Concrete Concrete can be reinforced with glass or metal fibres, which are mixed with the concrete at the time of batching. Fibreglass reinforcing is useful when there is a danger of chemicals corroding steel reinforcing. It is used in tanks, pipes and boat hulls.

Self-leveling Gypsum A special gypsum concrete is used on floors in wood-frame apartment buildings and condominiums. The concrete reduces noise and acts as a fire separation between floors. Gypsum concrete is self-leveling and needs no troweling or finishing. The mix is spread over the floor and raked. The tines on the rake give a uniform thickness, and the concrete forms a smooth level surface after hardening. Finish flooring can be placed over the concrete.

Pervious ConcretePervious concrete is a concrete mix that is used for specific purposes. Most of its uses at present are of the slab-on-ground type. For example, driveways, parking lots and landscaped areas are common places to use pervious concrete. This type of concrete is relatively new. We have been using it in North America for just over 20 years.



Pervious concrete is similar to regular concrete in that it is made with cement, aggregates and water. What makes it different, though, is that the finished product is porous, that is it has voids throughout its area and cross-section. That means when we use it as a sidewalk or parking lot, water or rain will pass through it.

Advantages of Using Pervious ConcreteIf you have ever stepped outside on a rainy day and had to walk around a water puddle, you could probably appreciate a sidewalk that didn’t trap water. On a bigger scale, a lot of material and labor is required to install sub-surface drainage to accommodate storm water that would fall over a large uncovered parking lot. With a Pervious concrete pavement, the water simply flows right though it and percolates naturally into the soils below without affecting its stability.

Pervious concrete pavements are environmentally friendly. Conventional paving around shrubs and trees can reduce their water supply, but pervious concrete allows rain water to easily make its way to the root system.

Describe the Three Basic Elements Of ConcreteConcrete is used in all types of building construction and over the years, builders have found more and more imaginative ways to include it in their designs. Concrete is a mixture of cement, water and aggregates. The cement, which is Portland cement, and the water form a paste, which binds materials such as sand and gravel or crushed stone into a rocklike mass. This happens as the paste hardens through a chemical reaction between the cement and the water.

You can make good concrete or bad concrete with good materials, but you cannot make good concrete with bad materials.

Cement Portland cement is “hydraulic cement”, which means that it sets and hardens by reacting with water. This reaction, called hydration, combines cement and water to form a stone-like mass. When combined with aggregates we get concrete.

Portland cement was invented in 1824 in England and is called Portland cement because the concrete that it produces looks like the color of the natural stone quarried on the Isle of Portland. It is manufactured by blending limestone, iron, silica and alumina. This blended material is ground to a fine powder and then burned in a kiln to fuse it into small rocklike substances called clinkers. The clinkers are crushed, gypsum is added, and the combined product is ground to a very fine powder.

Portland cement is manufactured to meet different physical and chemical requirements by using varying percentages of limestone, iron, silica and alumina. Previously there were five types of Portland cement; Type 10, Type 20, Type 30, Type 40 and Type 50. In 2004, the Canadian Standards Association published a new standard called CSA A3000-03. This gave each type of cement a new two-letter designation. Table 1 shows the new name designations for the different types of cements.



Table 1

New type designations

Descriptions Old designation

Portland cement

Blended hydrolic cement

Portland cement type

GU GUb General-use hydraulic cement 10

MS MSb Moderate sulphate-resistant hydraulic cement 20

MH MHb Moderate heat of hydration hydraulic cement 20

HE HEb High early-strength hydraulic cement 30

LH LHb Low heat of hydration hydraulic cement 40

HS HSb High sulphate-resistant hydraulic cement 50

General Use Hydraulic Cement (GU)GU cement is general-use, commonly referred to as Portland Normal cement. It is suitable for all uses where the special properties of other types of cement are not required or where the special properties are obtained by the use of admixtures. It is used in all types of general purpose construction.

Moderate Sulphate Resistant Hydraulic Cement (MS)MS cement is used where precaution against moderate sulphate attack is important. It is used in any structure that is exposed to soil and ground waters where sulphate concentrations are higher than normal but not severe.

When sulphates in moist soil or water enter concrete, they cause chemical reactions leading to expansion, scaling and cracking. Because seawater contains sulfates, concrete exposed to seawater is often made with MS cement.

Moderate Heat of Hydration Hydraulic Cement (MH)MH cement is manufactured to generate less heat, at a lower rate than GU cement. Heat of hydration is the heat generated by the chemical reaction when cement is mixed with water. This type of cement is used in mass structures such as large piers and thick retaining walls and bulk foundations. Since MH cement reduces temperature rise and possible temperature related cracking. MH should be considered when concrete is placed during hot weather.

High Early Strength Hydraulic Cement (HE)HE cement provides high strength very quickly, usually in a week or less. Some uses of HE cement are when forms need to be removed quickly, when the structure must be put into service in a short period of time, or in cold weather when short curing times are required. High rise construction and cold weather conditions are good uses for HE cement.



Low Heat of Hydration Hydraulic Cement (LH)LH cement is used when expansion caused by the heat of hydration must be minimized. It develops strength at a much slower rate than GU cement and is intended for massive concrete structures such as large gravity dams.

High Sulphate Resistant Hydraulic Cement (HS)HS cement is used in concrete exposed to severe sulphate action. It is used where soil or groundwater has very high sulphate content. It gains strength more slowly than GU cement.

Using AdmixturesIt is not very practical or economical for concrete producers to have all the above-mentioned cement types on hand at all times. With a wide variety of Admixtures available today, blending other ingredients with different ratios of GU cement can create most of the characteristics of the various cements. For example, both accelerators and a higher content ratio of GU cement can be used to help obtain HE cement. Similarly, both retarders and a lower content ratio of GU cement will help to acquire LH cement.

Bulk Cement Storage Concrete manufacturers store the cement in large silos. These silos are filled from bulk tanker trucks. A second silo is used for storing fly ash. Up to 20% of the cement can be replaced with fly ash to reduce the cost of the concrete mixture. Fly ash is a pozzolanic admixture that used alone does not provide any cementitious qualities. By reducing the cement content, the heat of hydration is also reduced.

Other Cements White Cement White Portland cement cures to a white colour as opposed to normal Portland cement that cures to a grey colour. It is manufactured under controlled conditions by selecting raw materials that produce a white product. It is used primarily for architectural purposes such as pre-cast curtain walls and facing panels, terrazzo surfaces, stucco, cement paint, tile grout and decorative concrete. It is recommended when white or colored concrete or mortar is desired.

Masonry Cement Masonry cement is a mixture of Portland cement, air entraining additions and supplemental materials selected for their workability, plasticity and water retention. Because of manufacturing controls, the workability, strength and color of masonry cements are maintained at a uniform level.

There are three basic types of masonry cement manufactured in Canada: Type N, Type S and Type M. Type N is mostly used for non-load bearing situations, such as brick veneer. Type S can be used for load bearing situations requiring a mid-range compressive strength and walls that require some lateral strength such as parapet walls. Type M has a higher compressive strength



than the other two types and can be used at or below grade. Type M is suitable cement for mortars used in foundations and with bricks designed for use below grade. All three masonry cements are mixed with sand and water to create masonry mortars.

Blended Hydraulic Cement Blended hydraulic cement is a mixture of either Portland cement and pozzolan, or Portland cement and ground, granulated, blast furnace slag. These ingredients are combined either during the grinding operation at the mill or during a blending process after grinding. The blended mixture contains from 25% to 70% granulated blast-furnace slag and up to a maximum of 40% pozzolan. Blended hydraulic cement sets at a slower rate than Type 10 cement and generates less heat during hydration. Pozzolan or blast furnace slag improves the properties of hardened concrete.

Special Cements Air-Entrained Portland Cements Small quantities of air-entraining materials are inter-ground with the clinker during manufacture. They are added to the Type 10, 20 and 30 cements. These cements produce concrete with an improved resistance to freeze-thaw conditions and to scaling caused by chemicals applied for snow and ice removal. Such concrete contains minute, well-distributed and completely separated air bubbles. The Canadian practice is to achieve the necessary levels of air entrainment in concrete by using air-entraining admixtures during concrete mixing.

Oil-Well Cements Oil-well cements, used for sealing oil and gas wells during drilling, are usually made from Portland cement clinker or blended hydraulic cements. Generally, they must be slow setting and resist high temperatures and pressures.

Waterproofed Portland Cement Adding a small amount of stearate (calcium, aluminum, or other) to Portland cement clinker during final grinding will produce a waterproof Portland cement. The cured concrete will be either white or grey in colour.

Plastic Cement Plastic cement is made by adding plasticizing agents, up to 12% by total volume, to Type 10 or Type 20 Portland cement during the milling operation. Plastic cement is often used for making mortar, plaster and stucco.

Expansive Cement Expansive cement is hydraulic cement that expands during the early hardening period after setting. Normal types of cement shrink during the early hardening period after setting. Expansive cements are used for patching cracks and holes in concrete work to help prevent



water leaks. Expansive cements can also be used to compensate for the effects of a decrease in concrete volume due to shrinkage through drying.

Cement and the EnvironmentOne of the biggest environmental issues today is the “Greenhouse Effect.” The burning of fossil fuels is a major contributor to this problem. The major greenhouse gas that is responsible for global warming is carbon dioxide or CO2.

The current milling process heats clinker to very high temperatures which creates large volumes of CO2. Studies have shown that for every tonne of Portland cement produced, approximately one tonne of CO2 is released into the atmosphere. It is thought that the cement production industry is responsible for as much as seven percent of the total CO2 emitted worldwide.

The main focus is to capture and dispose of the CO2 during various stages of production. Different methods to do this are being explored. The two most popular concepts are to discharge the CO2 into underground aquifers and natural gas reservoirs or deep into the ocean.

Eco-Friendly CementThe construction industry knows cement production is not environmentally friendly, so some companies have created new formulas for cement. One, High Volume Mineral Additive (HVMA), replaces some of the clinker in the production process with inexpensive mineral additives, creating an economical cement.

Another high-tech product is High Performance or HP Cement. Both HVMA and HP cement use a new reactive silica-based admixture called Supersilica. Each type of cement can be custom ordered to achieve either very strong superior cement, or inexpensive, eco-friendly cement.

Other companies are developing cement that absorbs more CO2 than it releases. As of December 2010, Ontario, Quebec and British Columbia have adopted in their building codes the use of Portland-limestone cement (PLC). This cement reduces carbon dioxide emissions by as much as 10%.

Aggregates Aggregates occupy 60% to 80% of concrete’s volume. The type, size, and quality of the aggregates greatly influence the quality of the concrete.

Naturally occurring aggregates are a mixture of rocks and sand. The rocks compose most of the coarse aggregates; the sand most of the fine aggregates. Coarse aggregates may be natural gravels or crushed rocks.

Coarse aggregates are larger in diameter, while fine aggregates are under 5 mm.



Quality of Aggregates Aggregates must be clean, hard, strong and durable. They must be free of absorbed chemicals, coatings of clay and other fine materials in amounts that could affect the hydration and bond of cement paste.

Aggregate particles that are friable (weak and easily crumbled or broken) or capable of being split are undesirable. Aggregates containing appreciable amounts of shale or other shaley rocks should also be avoided. Shale may contain cherts that expand when exposed to water and will cause pop-outs in the surface of the finished concrete.

The abrasion resistance of an aggregate is often used as an index of its quality. This resistance is essential when the aggregate is used in concrete for floors or pavements.

Grading Aggregates The size and grading of the aggregates will affect the cost of the concrete mixture. Each particle of aggregate is covered with cement paste in the concrete mixture. A large number of small pieces of aggregate will require more cement paste than a few large pieces of aggregate. Using the largest aggregate practical will reduce the cost of the mix.

Using large aggregates alone will not result in a saving of cement paste. The gaps between the larger aggregates need to be filled with smaller aggregates and the gaps between those need to be filled with even smaller pieces. The entire amount of aggregate needs to be a mixture of evenly graded pieces, not too many small and not too many large.

Gap-Graded Aggregates Concrete used for exposed aggregate work uses gap-graded aggregates. The large aggregate is 3/8" in diameter with the next largest size being sand. This gap in the sizes provides an even pebbled surface to the concrete.

Concrete for exposed aggregate work is expensive.

Water Water that is drinkable, with no pronounced taste or odor, is satisfactory as mixing water for concrete. Water that is not fit for drinking may also be used if mortar cubes made with this water have strengths that are equal to at least 90% of companion specimens made with water known to be of acceptable drinking quality.

Excessive impurities in mixing water may not only affect the setting time, concrete strength and volume stability (change in length), but they may cause efflorescence, staining or corrosion of reinforcing steel.

If the quality of mixing water is in doubt, have it tested by a laboratory that specializes in this work.



Handling ConcreteDepending upon the requirements of the construction, the concrete may be made on the job site or purchased from a concrete manufacturer. Site-made concrete is used if the job site is a long distance from urban centres and the delivery of ready-mix concrete is not feasible.

For very large construction projects, a concrete batch plant can be constructed on the construction site to reduce transportation costs.

TransportFor small jobs, concrete is often delivered directly to the formwork from the concrete truck; this is referred to as a “tailgate placement”. Tailgate placement of concrete is only possible if the chute will reach the formwork or the concrete can be wheeled using wheelbarrows.

For residential and commercial construction, a concrete crane mounted pump is used to place the concrete into the forms. Concrete pumps enable low-slump concrete to be placed with ease; this allows the water/cement ratio to be kept low and the concrete strength high. In some situations a concrete line pump is used instead of a crane mounted pump.

Figure 3 — Pump truck with boom

Moving materials during the construction of taller buildings is usually done with a crane. Climbing cranes can allow material movement to extremely tall buildings. A concrete bucket is used when moving concrete with a crane.

Conveying systems are sometimes used to transport concrete from an on-site batch plant to the point of placement. They are only cost effective for massive concrete construction projects.



PlacementThe following is a list of guidelines for placing concrete:

1. Double-check the integrity of the formwork and falsework prior to beginning the placement of the concrete.

2. Place the concrete into wall forms in uniform layers, not exceeding the designed rate of placement.

3. Begin placement at the corners of formwork and continue placing concrete without leaving gaps.

4. Vibrate each layer of concrete fully, particularly around inserts, blockouts, and window and door bucks.

5. Do not use the vibrator to move concrete as this can cause segregation of the aggregates (and get the vibrator stuck).

6. When placing concrete in multiple layers, vibrate through each layer and about 150 mm (6") into the previous layer.

7. Do not drop concrete through reinforcing steel as this can cause segregation.

8. Place the concrete close to its final location. When placing concrete on sloping surfaces, start at the bottom of the slope and proceed upwards.

9. Place all of the concrete into the formwork within 120 minutes from when the water was added to the mix.

10. If the formwork shifts or deflects excessively, stop the placement of concrete immediately and do not continue until the formwork has been inspected and repaired.

FinishingThe top surfaces of cast-in-place concrete must be finished to a smooth uniform surface for appearance and durability.

Once the concrete has been placed and vibrated, it is then struck off at the correct height. This process is known as screeding or striking off.



Figure 4 — Using a strike off bar

After the concrete has been placed and screeded level, the surface is then floated. The floating pushes any large aggregate below the surface of the concrete slab. This can be done with knee boards and hand floats, bull floats, or with a power float.

If the edges of the finished concrete are going to be rounded over, the edger should be used just after the floating has been completed. If control joints are to be tooled in, the first pass should be done immediately after floating and then again when finishing.

Figure 5 — Magnesium bull float



Floats are normally wooden or made from magnesium. They smooth the surface without sealing it and trapping bleed water. Bleed water is extra water in the concrete mixture that is not needed for hydration.

Once all bleed water has come to the surface and evaporated and the surface is firm enough, a steel trowel, fresno, or power trowel is used to give the concrete its final finishing. In some cases a broom is used to create a non-slippery surface or mats are used to create a stamped surface. All edging and control joint tooling is repeated after trowelling.

Figure 6 — Using kneeboards and hand trowels

CuringHydration is the chemical reaction that takes place when the water reacts with the cement in the concrete mixture. This chemical reaction grows the crystals that lock the grains of sand and rock together to form hardened concrete.

The moment the water is added to the cement hydration process begins and the concrete starts to gain strength. Concrete must be kept moist to allow the hydration process to continue. If the concrete dries out, hydration will stop.

The process of keeping the concrete moist is called “curing”. The concrete mixture is designed to provide a specific strength of concrete and “green” concrete must be cured until it gains strength. When a 20 MPa mix is ordered from a ready-mix concrete plant, that concrete is designed to reach 20 MPa if it’s able to cure (i.e. continue hydration) for 28 days.

1. Concrete that is not cured at all will only attain approximately 50% of its 28-day strength.

2. If the concrete is cured for 3 days, it will reach approximately 70% of its 28 day strength.

3. If cured for 7 days, the concrete will attain over 90% of its 28-day strength.

4. If concrete is kept moist continuously, it will continue to gain strength for many years.

There are several ways to cure concrete. The most common technique is to coat the surface of the concrete with a spray-on liquid curing compound, or to use an air barrier such as a plastic sheet to keep the water in the concrete from evaporating.



Concrete can be cured by:

• water curing (such as ponding)• water retaining (wet coverings)• mechanical barriers (plastics, papers)• chemical membranes (curing compounds)

Now complete Self Test 1 and check your answers.



Self Test 1

1. Plain concrete is:

a. concrete with reinforcing steel in it

b. a form of pre-stressed concrete

c. concrete without any reinforcing

d. a fibreglass concrete

2. Thermal expansion of concrete or steel:

a. is the same in both

b. is larger for steel than for concrete

c. is larger for concrete than for steel

d. cannot be determined

3. Cement paste bonds only to:

a. aggregates

b. reinforcing steel

c. concrete forms

d. both steel and aggregates

4. Pre-stressed concrete is:

a. under tension at all times

b. in a state of pre-compression

c. plain concrete

d. known as reinforced concrete

5. Pre-tensioning is:

a. done after concrete has been placed

b. before concrete is placed

c. when concrete is being placed

d. at any time before or after concrete has been placed

6. Which of the following shapes is NOT a type of concrete shell roof?

a. barrel roof

b. dome roof

c. flat roof

d. folded plate roof



7. Which of the following is used to reproduce the texture of forming materials?

a. bush-hammering

b. sandblasting

c. form lining

d. acid etching

8. The term “concrete masonry” means construction from:

a. bricks or blocks

b. pre-cast panels

c. pre-stressed concrete

d. reinforced concrete

9. Concrete with air entrainment has:

a. very large air bubbles

b. microscopic air bubbles

c. no air bubbles

d. air placed in it after it has set

10. A cement mixture applied with compressed air is called:

a. grout

b. mortar

c. air entrainment

d. shotcrete

11. What makes pervious concrete different from other concrete types:

a. It contains air entrainment.

b. It is porous.

c. It is self-leveling.

d. It is not used for slab-on-ground.

12. Advantages of pervious concrete:

a. Water percolates through into soil below.

b. It is environmentally friendly.

c. May save materials and labour.

d. All of the above.



13. The three basic elements of concrete are:

a. limestone, aggregates and water

b. limestone, iron and alumina

c. aggregates, limestone and silica

d. cement, water and aggregates

14. The rocklike substance produced by a kiln is called:

a. rockers

b. Portland stone

c. clinkers

d. limestone

15. The four basic ingredients used in the manufacture of cement are:

a. limestone, gypsum, alumina and water

b. gypsum, silica, aggregates and cement

c. alumina, silica, iron and gypsum

d. iron, silica, alumina and limestone

16. Normal Portland cement is now known as:

a. MH cement

b. GU cement

c. MS cement

d. HE cement

17. High-early strength hydraulic cement is:

a. MH cement

b. GU cement

c. MS cement

d. HE cement

18. LH cement is used for:

a. high-early-strength concrete

b. low heat-of-hydration concrete

c. normal general purpose concrete

d. sulphate-resistant concrete



19. Expansive cements cause concrete to:

a. shrink during setting

b. expand during setting

c. be very expensive to place

d. be used extensively for all work

20. Good aggregates should be shaped:

a. long and narrow

b. round or cubical

c. round and flat

d. cubical and long

21. Good aggregates should:

a. be coated with fine materials

b. have absorbed chemicals

c. be easily split or broken

d. be clean, hard and strong

22. Fine aggregates are usually considered to be:

a. 1.2 mm or larger

b. less than 5 mm

c. 1.2 mm or less

d. over 10 mm

23. An accelerator is an example of a/an:

a. cement

b. mortar

c. retarder

d. admixture

24. The masonry cement best suited for foundation construction is:

a. type N

b. type S

c. type M

d. type 10



25. A major greenhouse gas produced by the cement industry is:

a. CO2

b. H2O

c. O2

d. CO

26. One way of reducing CO2 emissions during cement production is to:

a. increase the amount of clinker used

b. decrease the amount of clinker used

c. avoid using mineral additives

d. increase the amount of water

27. Describe the steps to finish concrete once it has been placed in the forms.

28. What is bleed water?

29. How does concrete harden and why is curing important?


COMPETENCy G-1: USE CONCRETE TyPES, MATERIALS, AddITIVES ANd TREATMENTS ANSwER kEy

Answer Key

Self Test 11. c) concrete without any reinforcing

2. a) is the same in both

3. d) both steel and aggregates

4. b) in a state of pre-compression

5. b) before concrete is placed

6. c) flat roof

7. c) form lining

8. a) bricks or blocks

9. b) microscopic air bubbles

10. d) shotcrete

11. b) it is porous

12. d) all of the above

13. d) cement, water and aggregates

14. c) clinkers

15. d) iron, silica, alumina and limestone

16. b) GU cement

17. d) HE cement

18. b) low heat-of-hydration concrete

19. b) expand during setting

20. b) round or cubical

21. d) be clean, hard and strong

22. b) less than 5 mm

23. d) admixture

24. c) type M

25. a) CO2

26. b) decrease the amount of clinker used

27. vibrate, strike-off, float, wait for bleed water to evaporate, steel trowel

28. Excess water in the concrete mix not needed for hydration.

29. Concrete hardens through a chemical reaction between water and cement. Curing is making sure that there is enough moisture to maintain this chemical reaction. 28 days of curing will allow concrete to reach its design strength.

7960003735

ISBN 978-0-7726-7038-0

9 780772 670380

BC CARPENTER APPRENTICESHIP PROGRAM … CARPENTER APPRENTICESHIP PROGRAM — LEVEL 1 1 Program...

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