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British Columbia Carpenter Apprenticeship Program

Level 2 Line G

Select Concrete Types, Materials, Admixtures and Treatments

7960003557

Competency G-1

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BC CARPENTER APPRENTICESHIP PROGRAM—LEVEL 2 1

Competency G-1Select Concrete Types, Materials, Admixtures and Treatments

ContentsObjectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Learning Task 1: Describe the Uses of Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Learning Task 1: Self-Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Learning Task 2: Describe the Three Basic Elements Of Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Learning Task 2: Self-Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Learning Task 3: Describe the Uses of Concrete Design Mixes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Learning Task 3: Self-Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Learning Task 4: Describe the Types of Admixtures for Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Learning Task 4: Self-Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2 BC CARPENTER APPRENTICESHIP PROGRAM—LEVEL 2

Competency G-1Select Concrete Types, Materials, Admixtures and Treatments

Concrete 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.

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

Competencies Written: "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.


Notes

COMPETENCy G-1 LEArNING TASk 1

Learning Task 1Describe the Uses of Concrete

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

Reinforced Concrete Concrete 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.

Notes


LEArNING TASk 1 COMPETENCy G-1

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 �⁄�� 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.


Notes


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.

Notes



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.


Notes


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.

Notes



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.


Notes


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.

Notes



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.


Notes


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 twenty 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.

Now complete Learning Task 1 Self-Test.



Learning Task 1 Self-Test

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.


Notes


Learning Task 2Describe the Three Basic Elements Of Concrete

Concrete 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.

Notes



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 30LH LHb Low heat of hydration hydraulic cement 40HS 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 General Use 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.


Notes


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 General Use 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 High Early Strength cement. Similarly, both retarders and a lower content ratio of GU cement will help to acquire Low Heat 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.

Notes



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.


Notes


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.

Notes



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 those and larger in diameter and fine aggregates are under (5 mm).


Notes


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.

Notes



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.





1. 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

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

a. Rockers

b. Portland stone

c. clinkers

d. limestone

3. 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

4. Normal Portland cement is now known as:

a. MH cement

b. GU cement

c. MS cement

d. HE cement

5. High-early strength hydraulic cement is:

a. MH cement

b. GU cement

c. MS cement

d. HE cement



6. 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

7. 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

8. Good aggregates should be shaped:

a. long and narrow

b. round or cubical

c. round and flat

d. cubical and long

9. 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

10. 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



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

a. cement

b. mortar

c. retarder

d. admixture

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

a. type N

b. type S

c. type M

d. type 10

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

a. CO2

b. H2O

c. O2

d. CO

14. 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

Notes



Learning Task 3Describe the Uses of Concrete Design Mixes

The design of the concrete mixture is dependent on the end use of the concrete. Exposed aggregate concrete used for exterior concrete walks requires air entrainment and the use of gap-graded aggregates. Concrete for residential footings will have to be strong enough to resist the load of the building. For pre-stressed beams, the concrete will need very high compressive strengths to resist the huge forces exerted by the post-tensioning cables.

Properly proportioned concrete mixtures will produce concrete that is economical, workable and provide a strong and durable finish.

Workability A workable concrete mixture can be easily placed, consolidated and finished without segregating. Segregation occurs when larger aggregates settle to the bottom of the mix. Mixtures with too much water will segregate easily.

Concrete that is designed with too little water will be stiff and difficult to work. Stiff concrete will take more time to place and finish. It will be hard to consolidate and may leave voids or honeycomb. Adding water to the mixture to increase the slump will change the water/cement ratio. This will reduce the strength and allow the coarse aggregates to segregate to the bottom of the mix.

If a low water/cement ratio is needed for strength, admixtures should be added to improve the workability. Water-reducers and plasticizing agents will increase the slump without adding water.

Doing a slump test checks the consistency and workability of concrete.

Slump Test The slump test is used to test the consistency of the concrete mixture (Figure 1). The concrete manufacturer should deliver concrete to the job site that has been mixed to the proportions specified by the engineer. The slump test will reveal variations in the consistency, which indicate changes in the proportions of the mix.

It is important for the proportions of the materials used in the concrete mixture to be kept the same. If the proportions vary, the strength and appearance of the finished concrete will also vary. Each load of concrete is tested and significant changes in the slump may require the load to be rejected and sent back to the redi-mix plant.


Notes


Measuring the Slump of Fresh Concrete A slump cone is used to measure the consistency of the fresh concrete. The slump cone is a tapered cylinder, measuring 300 mm high, 200 mm in diameter at the bottom and 100 mm in diameter at the top. It is filled with three layers of concrete of approximately equal volume.

The cone, after being placed on a flat surface, is filled with a first layer of concrete to a depth of 65 mm, then rodded 25 times. The second layer goes to about half the height of the cone, and the third layer overfills it. Each layer is rodded separately for 25 strokes. The rod must be 600 mm long and 15 mm in diameter with round ends.

Note: When rodding between layers, be sure the rod only just penetrates the previous layer.

After the cone is rodded, the top of the concrete is struck off flush with the top of the cone. The cone is lifted off and placed beside the cone of concrete. The distance from the top of the cone to the average of the top of the concrete sample is the slump. A 3 to 4" slump is common for good quality concrete.

Figure 1. Taking a slump test

The slump of the concrete is a measure of the consistency of the concrete. A high slump concrete does not necessarily contain a higher water cement ratio. Water reducing admixtures can produce concrete with a high slump but still maintain acceptable water cement ratios.

Concrete made using a “superplasticizer” will have a very high slump, as much as 8 to 10 inches, without increasing the water cement ratio.

Notes



Strength and Durability The properties of hardened concrete, such as durability, freeze-thaw resistance, water tightness, wear resistance and strength is determined by the quality of the aggregates and the cement paste. The cement paste should have a low ratio of water to cement and contain entrained air.

Water/Cement Ratio and Strength Compressive strength is the universal measure of concrete’s quality. It is inversely proportional to the water/cement ratio. The more water, the less strength.

The water/cement ratio is the ratio of the weight of cement to the weight of water used in the mix. The compressive strength of the concrete must be sufficient to provide support for the building or to allow horizontal members to span their required distances.

The 28-day compressive strength for increasing water/cement ratios is shown below.

W/C Ratio Strength in MPa

Comments

0.25 > 50

0.25 Kg of water per Kg of cement is the minimum amount of water needed to allow the hydration of the cement. This mixture would not produce a workable concrete without the use of water-reducing admixtures.

0.4 42 Very stiff mixture, water-reducing admixtures are required.

0.5 33 Stiff mixture, water-reducing admixtures should be used to provide a workable mix.

0.6 26 Stiff mixture that is just barely workable without the use of admixtures.

0.7 20

Workable mixture that will consolidate well with vibration. 60% of the mix water is not used for the hydration and will have to evaporate from the mix. Excessive bleed water on the surface of concrete slabs.

0.8 15 Workable mixture but the strength is at the absolute minimum. 70% of the mix water is not needed for hydration. Excessive shrinkage could occur due to water evaporation.

Part 9 of the Building Code requires a minimum of 15 MPa concrete for footings and walls, a minimum of 20 MPa for floors slabs, and a minimum of 32 MPa for garage and carport floors and for exterior steps (note: 1 MPa = 150 psi). The Building Code also sets limits for the amount of water used in mixing concrete.


Notes


Water in the Aggregates When mixing concrete, the water that is contained in the aggregates must be considered. Fine sands can hold large amounts of water. If not allowed for, this water will greatly increase the water/cement ratio.

Controlling the water/cement ratio for concrete mixed at the job site is done by reserving some of the mix water until the very end of the mixing process. The last of the mix water is added slowly until the required consistency is obtained. All of the mix water will not be used if the aggregates contained excess water.

Concrete Exposed to the WeatherConcrete that will be exposed to freezing and thawing must be able to withstand the expansion of the freezing water without cracking. Adding air entrainment admixtures will increase the resistance to freeze-thaw cycles. Using a low water/cement ratio is as important as adding entrained air. The lower the water/cement ratio, the more watertight the concrete will be. Concrete that is watertight will not allow water to be absorbed into it. Without water there is no damage from freezing.

Table 1 shows the maximum water-cement ratios suggested by the Canadian Standards Association (CSA).

Table 1. Water/cement ratios for concrete exposed to the weather

Exposure class Exposure W/C ratio by mass, maximum

A Frequent freeze-thaw when saturated in seawater or subjected to de-icers

0.45

B

Frequent freeze-thaw when saturated in fresh water or infrequent wetting by seawater or complete continuous immersion in seawater

0.50

C Frequent freeze-thaw when not saturated 0.55

The lowest ratio is for concrete that is exposed to the worst case and the highest is for the least damaging exposure.

Notes



Minimum Cement Content Each particle of aggregate must be fully covered with cement paste. Minimum amounts of cement are required to accomplish this. The amounts vary depending upon the size of the aggregate. Smaller aggregates require more cement because there is more surface area to cover.

Adding air-entraining admixtures to the concrete reduces its 28-day compressive strength. To obtain the same strength as non air-entrained concrete, extra cement is added to the mix.

Table 2. Minimum cement contents for varying aggregate sizes

Specified compressive strength, MPa

Minimum cement content (kg/m3) for indicated nominal size aggregate (mm) Non-air-entrained concrete

Minimum cement content (kg/m3) for indicated nominal size aggregate (mm) Air-entrained concrete

10 20 40 10 20 40

15

20

25

285

325

365

250

290

320

225

260

290

290

335

390

255

300

340

235

270

315

Canadian Standards Association (CSA) standard (A23.1)

The minimum cement content for flat work, such as sidewalks, roadways and floors is shown in Table 3. If the floor is subject to abrasive wear, stronger mixes must be used.

Table 3. Minimum cement for slabs

Maximum size of aggregate (mm) Cement (kg/m3)

40 270

28 300

20 320

14 340

10 360

Aggregates Aggregates make up the bulk of concrete, between 60% and 80%. They should be well graded for particle size and distribution so that the smaller sizes fill most of the voids between the larger sizes. It is economical to use the largest sized aggregate possible for the job because less cement paste is required.


Notes


The maximum size of aggregate depends to a large degree on the form shape and the amount and the distribution of the reinforcing steel. The size also depends on the method of placement and the finish required.

The maximum size of aggregate should not exceed one-fifth the minimum dimensions of the member, nor three-fourths of the clear space between the reinforcing steel, or between the forms and the steel. When slabs on grade are not reinforced, the maximum size should not be more than one-third of the slab thickness.

Entrained Air Concrete that will be exposed to freezing and thawing and de-icing salts should contain entrained air. Entrained air prevents concrete from scaling during freeze-thaw cycles by producing billions of minute air bubbles in the concrete, which provide space for the expansion of freezing water.

Entrained air also makes concrete easier to work, reduces the amount of bleed water, and reduces the amount of mixing water required for achieving the correct slump. The result is concrete stronger and more watertight than concrete without entrained air. Table 4 shows a range of air content for various exposures of concrete.

Table 4. Range of air content required for various exposures Class of

exposure Condition of exposure Range of total air content (percentage)

required for concretes with indicated size of coarse aggregate

10 mm 14 mm 20 mm 40 mm

A

Frequent cycles of freezing in a saturated condition and 1. subject to de-icing chemicals 2. not subject to de-icing chemicals

7 to 10 6 to 9

6 to 9 5 to 8

5 to 8 4 to 7

4 to 7 3 to 6

B

Frequent cycles of freezing and thawing in a saturated condition in fresh water; infrequent wetting by seawater; or complete and continuous immersion in seawater

6 to 9 5 to 8 4 to 7 3 to 6

C Frequent cycles of freezing and thawing in a unsaturated condition 5 to 8 4 to 7 3 to 6 3 to 6

D No exposure to freezing and thawing or to the application of de-icing chemicals < 5 < 4 < 3 < 3

Approximate amount of entrapped air in non-air-entrained concrete 3 2.5 2 1

Canadian Standards Association (CSA) standard (A23.1)

Notes



Pump Mixes Weaker strength, large aggregate or low slump concrete placed by pump is prone to clogging the hoses, so the method of placement may affect the mix requirements. Previously it was common to add more water, but that weakens the mix. Adding more cement helps flowability but also increases cost. Instead, the use of an admixture such as a superplasticizer is now more appropriate. The aggregates should be round and smooth to facilitate a smooth flow through the pipeline. Before ordering pump mix concrete for a particular job, check with the pump operator for the largest size aggregate and the design mix the pump can handle.





1. Name three qualities of properly proportioned concrete mix.

2. The slump cone should be filled up in four layers and rodded when full.

a. true

b. false

3. Which of the following is the proper size of a slump cone?

Diameter top Diameter bottom Height

a. 200 mm 200 mm 100 mm

b. 300 mm 200 mm 200 mm

c. 100 mm 200 mm 300 mm

d. 200 mm 300 mm 200 mm

4. How is the slump measured?

a. from the bottom to the top of the concrete

b. across the width of the concrete at its largest dimension

c. according to how high the concrete stands after removing cone

d. the distance from the top of the cone to the top of the concrete

5. To make strong, durable concrete, the water-cement ratio should be:

a. high

b. low

c. equal

d. 5:1



6. What is the importance of water-cement ratio and minimum cement content?

7. In concrete, the aggregates make up:

a. 40% to 55%

b. 60% to 80%

c. 80% to 88%

d. 89% to 95%

8. Except for slabs-on-grade, the size of aggregates should be of the width or thickness of the member.

a. one-quarter

b. one-fifth

c. one-third

d. one-tenth

9. The addition of entrained air to a concrete mix results in:

a. billions of minute bubbles

b. thousands of small bubbles

c. billions of large bubbles

d. thousands of large bubbles

10. Concrete that is being pumped should have:

a. the same mix as for crane buckets

b. a superplasticizer added

c. less cement paste

d. a stronger mix design


Notes


Learning Task 4Describe the Types of Admixtures for Concrete

Admixtures for concrete are materials added to the concrete’s basic ingredients: Portland cement, water and aggregates.

The admixtures are used to modify the properties of the concrete. All concrete delivered to the site will include some admixtures.

Type of Admixture Desired Effect

Air entraining Improve durability

Water reducer Improves strength by reducing water required for a given consistency

Retarder Retard setting time

Accelerator Accelerate setting and early-strength development

Water reducer and retarder Reduce water and retard set using only one admixture

Water reducer and accelerator

Reduce water and accelerate set using only one admixture

Bonding admixtures Increase bond strength

Colouring agents Colour concrete

Corrosion inhibitors Reduce steel corrosion activity

Gas former Cause expansion during setting.

Damp proofing and waterproofing agent

Decrease permeability

Pumping aids Improve pumping

Air detrainer Decrease air content to produce high density concrete

Superplasticizer High-range water reducer to greatly increase the slump of the concrete mixture

Notes



Air-Entraining Admixtures Air-entraining admixtures put microscopic air bubbles into concrete. The bubbles improve the durability of concrete during freezing and thawing cycles. Air entrainment also enhances the workability of concrete and reduces the amount of water required for a given slump.

Water-Reducing Admixtures Water-reducing admixtures are added to the concrete to reduce the amount of water required, yet still maintaining the required slump. They may also retard the setting time. By reducing the amount of water needed for a given slump, the strength of the concrete is increased.

Retarding Admixtures When the temperature is 30°C or more, the rate of setting increases, which makes placing and finishing more difficult. Retarding admixtures are added to the concrete to slow its setting rate, allowing the carpenters to place and finish the concrete properly.

One such product is DELVO® STABILIZER, a retarding admixture that can extend the working and setting times anywhere from one to five hours.

The concrete can also be cooled by using cold mixing water or by adding some of the mixing water as shaved ice. Cooling the aggregates with a mist of water can also reduce the temperature of the concrete mixture, thus reducing the rate of set.

Accelerating Admixtures Accelerators are added to the concrete mixture to increase the rate of strength gain. Rapid strength gain is needed to: allow early stripping, protect the concrete from freezing and to generally speed up the construction process.

Calcium chloride is used to accelerate the setting and increase strength gain. Being a salt, it will corrode steel and should not be used for reinforced concrete. The use of calcium chloride will increase the drying shrinkage of the concrete, which may cause cracking.

Non-chloride accelerators are available in liquid form for use in reinforced concrete.


Notes


Workability Agents The workability of concrete can be improved by adding certain admixtures. Air entraining admixtures will improve the workability of fresh concrete and the freeze thaw resistance.

All water reducing agents will improve the workability of the concrete by allowing the same final compressive strength for a concrete that has a higher slump during placement.

Superplasticizers Superplasticizers are added to concrete to increase the slump of the mixture. They are active for a short time, usually not more than 60 minutes. Liquid plasticizing agents are added at the job site just prior to placement. Flowing concrete is produced using normal water-cement ratios. It will easily consolidate around very dense reinforcement with little or no vibration.

Using very low water/cement ratios produces high-strength concrete. These mixes are too stiff to be used without the addition of superplasticizers. Adding the plasticizer makes very stiff mixes workable, allowing consolidation by internal vibration.

Superplasticizers are generally more expensive than other water-reducing admixtures, and are only used if their effects are needed.

Dampproofing and Permeability-Reducing Agents When concrete is sound and dense and made with a water-cement ratio of 0.50 or less, it is generally watertight if properly placed and cured.

Dampproofing and permeability-reducing agents are included when low cement content or a high water-cement ratio is used in concrete production. As well, dampproofing admixtures are sometimes used to reduce the transmission of moisture through concrete that is in contact with damp earth or water.

XYPEX® company produces a line of high performance cement additives. One of their products, XYPEX ADMIX C-500, is a waterproofing agent that is mixed into the concrete at the time of batching. A reaction takes place that forms microscopic crystals within the matrix of the concrete, leaving it permanently sealed against the penetration of water.

Notes



Bonding Agents Bonding agents are used when fresh concrete is to be applied to the face of old existing concrete. They may be added to the new concrete mix or applied to the face of the existing concrete. Bonding agents tend to increase the air content of the concrete.

Grouting Agents Cement grout is used to: stabilize foundations, fill cracks and joints in concrete, stabilize machine bases and fill post-tensioning conduits. The grout is a mixture of cement, water and sand. To make this mixture able to be worked into small cavities grouting agents are added to the mix. Grouting agents include: air-entraining, accelerators, retarders, workability and non-shrink agents.

Gas-Forming Agents A gas-forming agent expands grout as it cures. A gas-forming agent would be used in a grout designed to prevent water leaks through wall cavities and cracks.

PozzolansThe materials used in the manufacture of concrete alter the properties of the finished concrete. Pozzolanic materials are primarily used in the mixture to reduce the cost of the finished concrete. Pozzolans, sometimes called cement replacers, decrease the amount of Portland cement needed to produce the desired strength.

The pozzolan itself possesses little or no cementing properties, but when mixed with cement, it will help to bond concrete together.

In British Columbia, the most commonly used pozzolanic admixture is “fly ash.” Fly ash is collected at coal-fired electrical generating plants around North America and shipped to the various concrete manufacturers in bulk tankers. The bulk pozzolan is stored at the manufacturer’s site in a silo similar to those used to store the Portland cement.

The amount of pozzolan that is used in the making of concrete is significant, usually 20% of the cementious materials. For example, if 100 Kg of cementious materials were to be used in a concrete mixture, 20 Kg would be pozzolan and 80 Kg would be Portland cement. In some cases much higher proportions of pozzolan are used, up to 50% of the cementious materials.


Notes


Other pozzolanic materials are available including ground blast-furnace slag, silica fumes and naturally occurring materials like volcanic ash. In Roman times, volcanic ash was mixed with lime to create the concrete materials to build the Coliseum and other historic structures. These buildings have survived for over 2000 years. The fact that historic buildings were built using pozzolan concrete adds weight to the current thought that the use of pozzolan also increases the durability of the concrete.

When using pozzolan in the mixture, less Portland cement is needed. Using less cement decreases the total heat of hydration for a given volume of concrete. Construction of hydroelectric dams and other massive concrete structures use larger percentages of pozzolanic materials to reduce the heat generated by the curing concrete.

Presently, GU cement is the most widely used cement across the industry. Since using one or more of the various admixtures and pozzolans can duplicate the specific properties, the other five types of cement are rarely used. For example, type HS, high sulphate resistant cement can be replaced by using GU cement mixed with the pozzolan fly ash.

There are environmental benefits when using pozzolans in place of cement. Most pozzolans will add to the service life of the concrete and require less energy to produce than cement.

Pozzolans: Supplimentary Cementing Materials (SCMs) or Mineral Admixtures

Pozzolan Desired Effect

Fly ash Cement replacement and improves durability and workability of concrete

Blast furnace slagCement replacement and enhances the service life of the concrete

Silica fumeCement replacement and used where high impermeability or high strength concrete is required

Notes



Natural Pozzolans

Table 1. Admixtures By Classification Calcined clay Cement replacement and resistant to sulphate attack

Calcined shale Cement replacement

Metakaolin

More of an additive than a replacement for cement.

Used where low permeability and high strength is required.

Volcanic ash Cement replacement-low cost

LEED® and Cement UseAs the cement industry shifts to more eco-friendly methods and materials, it follows suit with other leaders in environmental awareness such as LEED®. The LEED® (Leadership in Energy and Environmental Design) green building system recognizes the use of pozzolans as beneficial to reducing greenhouse gas emissions. It encourages energy conservation, the use of natural materials and minimal environmental impact. The more eco-friendly, energy efficient cements we incorporate in new construction, the more LEED® points will be achieved for any given project.





1. Materials added to basic concrete are called:

a. additives

b. additions

c. admixtures

d. extras

2. What is the effect at adding of an air-entraining agent?

a. It removes air from a concrete mix.

b. It adds minute microscopic bubbles to a concrete mix.

c. It speeds up the setting of concrete.

d. It retards the setting of concrete.

3. Retarding admixtures are used to:

a. speed up the setting time of concrete

b. reduce the water required for concrete

c. increase the water required for concrete

d. slow down the setting time of concrete

4. What is the purpose of an accelerating admixture?

a. It speeds up the setting time of concrete.

b. It reduces the water-cement ratio in concrete.

c. It improves the workability of concrete.

d. It reduces the cement content of concrete.

5. Superplasticizers are used to produce flowing concrete and high-early strength concrete.

a. true

b. false

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