CM 313: Concrete Ch.13-15 Lecture Notes

Welded wire reinforcing

Prefabricated, welded grids of reinforcing bars or wires • Especially common for concrete slab reinforcing.

reinforcing support

chairs or bolsters to properly position the steel

Reinforcing Special Coatings

galvanized or epoxy coated exposure to salts or sea water

Concrete

is made from cement, sand, and water. - Cement is composed of burned clay and silica, combined with slaked lime. - When mixed with sand and water it forms concrete which will set under water and is much stronger than mortar. - Concrete becomes stronger over time.

Reinforcing a continuous concrete beam

location of tension forces changes -- midspan: bottom in tension -- at beam supports: top in tension

Vapor Retarder/Moisture Barrier

• A heavy plastic sheet or other impervious material may be laid over the drainage layer. • This layer provides the slab with additional protection against subgrade moisture and is particularly important when the slab will be covered with moisture-sensitive finishes. • Vapor retarders are used only with interior slabs, not with exterior slabs. • Vapor retarders allow placed concrete to retain moisture during curing

Capillary Break or Drainage Layer

• A layer of crushed rock or gravel, usually 4 inches deep, is placed over the subgrade. • This rock material is well sorted (comprised of particles mostly uniform in size), that may range from approximately ¾-inch to 1½-inch in diameter. • This layer drains water easily and discourages moisture in the ground from rising up to the concrete slab through capillary action. • This layer also provides a structurally sound base for the concrete slab to follow.

Finishing the Concrete Wall

• A residential concrete foundation wall with the formwork stripped. • Without special efforts, the form panels and ties leave strong patterns on the wall surface. • A concrete foundation wall with blockouts to create openings in the wall for passage of building services. • Rough boards were used to line the inside of the wall forms, creating a board form finish on the cast wall.

Slab Edges

• Edge forms: Strips of wood or metal are placed to contain the concrete pour at the slab edges. • Forms are held in place by stakes of wood or metal rebar, or other bracing. • The tops of the forms are set level with the top the slab, to act as guides in later finishing operations.

Suspended or Structural concrete slab

a concrete slab that spans between intermediate lines or points of support

Slab on Grade

a concrete surface lying upon, and continuously supported by, the ground beneath

Concrete forces

column - compression beam - tension

Conventionally Reinforced Concrete

part of member in compression, part in tension. Over half of the concrete not carrying any load-- it's holding reinforcing in position and providing protective cover

Reinforcing

• A grid of reinforcing bars or sheets of welded wire reinforcing (WWF) is usually placed. • Reinforcing improves a slab's resistance to cracking due to: - concrete shrinkage during curing - effects of thermal expansion and contraction - concentrated stresses - differential settlement • This reinforcing is often called temperature steel, referring to its role in resisting cracking due to thermal stresses

One-Way Concrete Joist (Rib Slab)

• A system of beams and slender, closely-space ribs support a one-way slab. - Beams span between columns. - Joists (ribs) span from beam to beam. - The slab spans between the joists. • Greater spans are possible than with solid slab. • Modular, prefabricated form systems help to keep this system economical. • In one-way joist systems, the bottoms of the beams and joists are all on the same plane, simplifying formwork construction. • A simple, level surface is formed on site. Next, prefabricated metal or fiberglass joist pans are placed on this surface to form the system of beams and ribs.

One-Way Solid Sla

• A system of beams support a slab. The slab is reinforced to span in one direction only. - Girders (deeper beams) span between columns. - Beams (shallower) span from girder to girder. - The slab spans between the beams. • Depending on beam and column arrangements, this system can be designed for a wide range of load conditions. • Forming for girders and beams makes this system more expensive than many other systems.

Two-Way Solid Slab

• A system of beams supports a two-way slab. • Suitable for very heavy loads. • Forming of the beam systems tends to make this system more expensive than other two-way slab systems.

Two-Way Flat Slab

• A system of beams supports a two-way slab. • Suitable for very heavy loads. • Forming of the beam systems tends to make this system more expensive than other two-way slab systems. • Between columns, more heavily-reinforced column strips act like shallow beams within the depth of the slab. • Between column strips, middle strips are reinforced for two-way slab action. • The extra structure around the column/slab juncture prevents the columns from "punching through" the slab.

Reinforcing

• Concrete has no useful tensile strength —ability to resist pulling forces. • Steel bars or wires placed @ lines of tension-provide resistance to these forces. • Steel and concrete work well together. - The two materials have similar rates of thermal expansion/contraction. - The alkaline chemistry of concrete protects the steel from corrosion. - The two materials bond well, work as single structural composite.

Portland Cement

• Primary ingredient: Calcium silicates • Lesser amounts of compounds of aluminum, iron, and magnesium • Raw materials can be extracted from a variety of sources depending on the locale, such as limestone, chalk, marl, marble, sea shells, clays, shales, industrial waste materials, iron ore, bauxite, and others. • Materials are crushed, ground, proportioned and blended into a very fine powder. • The blended materials are burn or sintered in a hightemperature, rotating kiln, creating clinker. • The clinker is cooled and ground to a fine powder. • Small quantities of gypsum are added to adjust the cement's setting rate. • In the U.S., a standard bag of cement contains 1 cubic foot of material and weighs 94 lb.

One-Way Solid Slab

• Principal slab reinforcing spans in only one direction, from beam to beam.

Cement Types

• Type I: General purpose • Types II and Type V: For concrete in contact with soils or water with high sulfate concentrations • Type III: For cold weather construction, concrete precast in plants, accelerated construction schedules • Type IV: For massive structures, such as dams, where heat generated during hydration must be limited to avoid excessive temperatures

Cement + water = hyrdation

When portland cement is mixed with water its chemical compound constituents undergo a series of chemical reactions that cause it to harden (or set). These chemical reactions all involve the addition of water to the basic chemical compounds listed in he previous table. This chemical reaction with water is called "hydration". Each one of these reactions occurs at a different time and rate. Together, the results of these reactions determine how portland cement hardens and gains strength

Formwork materials

- wood - metal - plastic/fiberglass - cardboard

Self-Consolidating Concrete SCC

MIX design • AGGREGATES: more fine, less large • ADMIXTURES - Superplasticizers, polycarboxylate-based - Viscosity modifying admixtures (VMAs), - Slump retaining products

Types of concrete elements

Slab on grade, Columns, Walls, Floors and Roofs, Other(stairs, panels, etc.)

Posttensioning

Steel strands are tensioned after concrete has been cast and reached adequate strength. Concrete cast on the construction site can be posttensioned.

Pretensioning

Steel strands are tensioned before concrete is cast. This requires heavy abutments to restrain the strands and is normally only done with concrete in precasting plants, but not on the construction site.

Steel Reinforcing Bars

The most common concrete reinforcing material. Rebar are hot-rolled steel, deformed with surface ridges so as to better bond with concrete. • Bar sizes are numbered, with numbers generally corresponding the diameter of the bar in 8ths of an inch. E.g., - #4 bar is 1/2 in. diameter - #8 bar is 1 in. diameter • Bars are made with steel grades of varying strength. • Higher strength bars are used to reduce rebar congestion, where reinforcing becomes crowded and concrete placement becomes more difficult. Use of higher strength steel allows for bars smaller in diameter or with greater spacing.

Isolation or Expansion Joints

Where slabs abut walls, columns, or other elements, compressible joint material is placed to create an isolation joint or expansion joint. • Isolation joints allow unrestrained expansion and contraction of the slab, as well as differential settling between the slab and the abutting elements.

Light Aggregates

make lighter-weight concrete • Structural lightweight concrete: - Roughly 80 percent or less of the weight of ordinary concrete - Reduced structure weight saves costs - Lower thermal conductivity increases resistance to building fires • Nonstructural lightweight concrete: - Roughly 60 percent or less of the weight of ordinary concrete - Insulating roof toppings - Fill material • Expanded shale, clay, slate, slag: Minerals or industrial waste products thermally treated such that they expand and take on a less dense, cellular structure • Cinders and other volcanic rocks: Naturally occurring light-weight volcanic materials • Vermiculate: Thermally expanded mica • Perlite: Thermally expanded volcanic glass

Two-Way Framing Systems

• A two-way slab is reinforced so that it spans structurally in both directions. • Two-way slabs are more structurally efficient than one-way slabs of the same thickness. • Two-way slabs must span roughly the same distance in both directions. As the layout of slab supports becomes increasingly rectangular in proportion, the efficiency of the two-way slab decreases. • In contrast, one-way slab systems are more suitable for column and beam layouts that create rectangular bays.

Prestressing

• Applying an initial compressive stress to a concrete member, so as to improve its structural efficiency. • High-strength steel strands are stretched tightly and then restrained by the concrete, putting the concrete into initial compression. • When normal service loads are added, the concrete in prestressed members is subject to less tensile force. • Prestressed members are typically more slender and lighter than comparable conventionally reinforced members.

Placing Concrete

• Avoid delays, during which concrete can stiffen and become difficult to place. Depending on conditions, concrete can placed up to 90 minutes after mixing commences. • Concrete that does stiffen can have water added prior to placing, provided that: - Maximum w/c ratio is not exceeded - Maximum slump is not exceeded - Agitation limits are not exceeded • Concrete may be placed on site directly from the discharge chute of a transit-mix truck, or by combination of wheelbarrows, power buggies, crane-lifted buckets (right), conveyor belts, pumpers, or other devices. • Segregation, separation of large aggregate from the finer portions of the mix, must be avoided. • Place concrete as close to final position as possible. • Do not push concrete over large horizontal distances. • Avoid dropping concrete from high heights or discharging against obstacles (use drop chutes if needed).

Aggregates

• Coarse and fine aggregate make up 60 to 80 percent of the total concrete volume. • Most aggregate comes from natural sand and gravel deposits or is made from crushed rock. • Artificial aggregates may come from: - Blast furnace slag - Fly ash - Recycled concrete - Thermally treated clay, shale, and other minerals • Strength of concrete is heavily dependent on the quality of the aggregates: - Porosity - Size distribution - Moisture absorption - Shape and surface texture - Strength, elasticity, density, soundness - Contamination or detrimental substances

Casting Column Footings

• Columns may rest on isolated footings, pile caps, caissons, or enlarged portions of strip footings. • Vertical reinforcing bars increase the column's load carrying capacity and give it resistance to bending forces generated by lateral forces on the building structure or by connected beams. • Ties, lighter in weight, wrap around the vertical bars to resist outward buckling of the bars. • Ties also increase a column's resistance to extreme cyclical seismic loads. • Concrete is deposited into the column form by any number of means. • The concrete is vibrated or otherwise consolidated as needed as it is placed.

Insulating Concrete Forms

• Concrete forms made from rigid plastic foam blocks or other lightweight insulating materials are easy to erect. • The forms become a permanent part of the structure, creating a more energy efficient wall in comparison to conventional concrete construction.

Curing Concrete

• Concrete hardens by hydration, the chemical bonding of water and cement. • If concrete dries out prematurely, the hydration process stops and maximum strength is not achieved. • Hydration, along with increasing strength and durability, can continue for a very long time, even years. Concrete strength is normally specified at 28 days. • Exposed surfaces of newly poured concrete must be protected from evaporation and drying. Concrete may be regularly misted, covered with moisture-retaining materials, or treated with a chemical surface sealer. • In very hot or cold weather, steps may be taken to moderate the temperature of the concrete, such as pre-heating of ingredients, adding water as ice, or the use of retarders or accelerators to adjust cure rate.

Placing the Concrete

• Concrete is placed by any of a number of methods, depending on the size of the pour and ease of access to the slab. • Concrete should be placed as close as possible to its final destination. • Pushing concrete along the ground can cause segregation of large and small particles in the concrete mix, leading to a lack of uniform density and uneven finish qualities in the completed slab. • If the reinforcing has not been set on bolsters or other supports, it must be lifted into approximately the middle depth of the slab as the concrete is poured.

Reinforcing Fabrication and Erection

• Concrete reinforcing requirements are shown on the structural drawings. • A reinforcing fabricator prepares shop drawings that are reviewed by the structural engineer. • Reinforcing is cut to length, bent as needed, and possibly partially assembled before being transported to the construction site. • Fabrication continues on site. Eventually reinforcing is assembled in its final configuration. • After inspection by the engineer, concrete may be poured.

Concrete Strength

• Concrete strength varies with design of the concrete mix. • Normal strength concrete - Up to 6000 psi compressive strength - Made with conventional ingredients • High-strength concrete - Greater than 6000 psi to roughly 20,000 psi - Supplementary cementitious materials are required to reach higher strengths. - Lower water content, required for higher strength, results in a stiff, unworkable mixture when wet. To compensate, water reducing admixtures or high-range water reducing admixtures (superplasticizers) are used to improve workability. • Specially formulated, ultra-high performance concretes have compressive strengths as high as 30,000 psi.

Wall Footing

• Concrete walls are most commonly cast over concrete strip footings. • The steel reinforcing projecting from the footing will overlap with reinforcing in the wall, to structurally tie the two

Fibrous Reinforcing

• Fibrous reinforcing: Short fibers of glass, steel, or polypropylene, added to the concrete mix • Microfiber reinforcing: Relatively low amounts of fibers, to aid concrete in resisting plastic shrinkage cracking that occurs during early curing • Macrofiber reinforcing: Greater concentrations of fibers, that also resist longer-term cracking due to drying and thermal stresses • Steel fiber reinforcing also increases the durability of the concrete surface

Concrete ingredients

• Fine aggregate (sand) • Coarse aggregate (gravel) Coarse and fine aggregate provide the structural mass of the concrete and constitute the majority of the concrete volume. • Portland cement - Cement binds the aggregate. • Water - Water is necessary for the chemical hydration of the cement and the hardening of the concrete.

Formwork

• Formwork: Construction, usually temporary, to hold freshly poured concrete in the desired shape until the concrete achieves sufficient strength to support itself. • Formwork also frequently helps to protect newly poured concrete from drying too quickly. • Formwork must be strong and stiff enough to support the weight and fluid pressure of the concrete. • In conventional concrete construction, formwork can account for half or more of total concrete construction costs. • Form release compounds are oils, waxes, or plastic coatings applied to formwork surfaces to prevent adhesion of the formwork to the concrete and ease formwork removal.

Maximum Aggregate Size

• Generally, with larger aggregates, less cement is required in the concrete mix. Reducing the quantity of cement reduces cost, since cement is the most expensive ingredient in the mix. • The largest aggregate must fit comfortably between reinforcing bars and within the overall thickness of the concrete. Largest aggregate size should not exceed: - 1/5 distance between form faces - 3/4 of the space between reinforcing bars - 1/3 the depth of a slab • Common maximum aggregate size for concrete used in buildings ranges from 3/8 to 1-1/2 in.

Air-entraining cement

• Ingredients added to the cement generate bubbles during concrete mixing that create small, distributed voids in the finished concrete. • Air content in the range of 2 to 8 percent of the total concrete volume is typical. • Air-entrained concrete has greater resistance to freezethaw damage. It is use for concrete exposed to wet, freezing conditions. • Air-entrained concrete also has improved workability when wet. • Air entraining reduces concrete strength, unless the proportions of other ingredients in the concrete mix are adjusted to compensate.

Wide-Module Concrete Joist System

• Like one-way joist, but ribs are spaced 4 to 6 feet rather than 20 to 30 inches. • A thicker slab is required to span the greater distance between ribs (useful when greater thickness is required for greater fire resistance). • Also called "skip-joist" system.

Two-Way Flat Plate

• Like the flat slab, a two-way reinforced slab is supported by columns without beams. • There are no column capitals or drop panels in the flat plate system. • Additional reinforcing within the depth of the slab at the column/slab junction is used to increase strength in that area. • Flat plate systems are best suited to lighter loads than flat slab systems. The simplified formwork also makes them less expensive to construct. • Two-way flat plate is one of the thinnest of floor framing systems available in any structural material, an economy that is compounded in multi-story construction

Supplementary Cementitious Materials (SCM)

• Materials added to concrete as a partial substitute for portland cement to achieve various benefits: - Increased concrete strength and durability - Higher early strength - Improved workability or wet concrete - Reduced concrete drying shrinkage - Reduced reliance on portland cement • Pozzolans: Materials that react with hydration byproducts in wet concrete to form additional hydraulic cementing compounds - Pozzolans require the presence of a other cementing ingredients with which to react. They are not, on their own, hydraulic cements. • Hydraulic cements: Materials with intrinsic hydraulic cementing properties - Hydraulic cements do not require the presence of other cementing materials with which to react so as to function as hydraulic cement. • Over half of all North American concrete includes SCMs in the mix. • SCMs can replace from 5 to 70 percent of the portland cement in the concrete mix (see below). • Use of SCMs reduces reliance on portland cement in concrete, lessening the high energy consumption and greenhouse gas emissions associated with portland cement manufacturing. • Use of SCMs can reduce the embodied energy of finished concrete by as much as one-third. • Use of industrial waste SCMs diverts these materials from landfills.

Mixing Concrete

• Most concrete is prepared at batch plants and delivered to the construction site in transit-mix (ready mix) trucks. • The concrete ingredients are mixed in the rotating drum of the truck so that the concrete is ready to pour on arrival at the construction site. • Smaller batches of concrete may be prepared on site by hand or with the aid of portable power mixers.

admixtures

• Other ingredients in the concrete mix used to alter or improve concrete properties in various ways: - Air-entraining - Water-reducing - Cure accelerating or retarding - Workability modifying - Shrinkage-reducing - Corrosion inhibiting - Freeze protecting (for cold weather concreting) - Coloring

Controlling Cracking

• Relatively thin, lightly reinforced concrete slabs on grade are especially prone to cracking, especially as the concrete shrinks during curing. • Control joint or contraction joint: A partial-depth joint or groove that creates a natural plane of weakness in the slab. • Control joints encourage shrinkage cracking to occur in an organized, visually acceptable manner. • Control joints may be saw-cut into a slab after the slab has partially hardened. • Control joints can also be formed during slab finishing operations using hand tools called groovers. • Control joints need to extend at least ¼ the depth the slab to be effective. • As a rule of thumb, control joints should be spaced from 24 to 30 times the depth of the slab. - Expansion joint or isolation joint: Expansion and isolation joints are full-depth separations between slab sections, that allow full freedom of movement between sections. • Reinforcing is also interrupted across expansion joints. (Across control joints, reinforcing is usually uninterrupted.) • Other means of crack control: - Concrete mixtures can be adjusted to reducing drying shrinkage. - Additional reinforcing or posttensioning can be added to a concrete slab to increase its tensile strength.

Two-Way Waffle Slab (Two-Way Concrete Joist)

• Similar to one-way joist, but with a two-way system of ribs • Constructed with prefabricated domes that simply formwork construction. • Around the columns, domes are filled solid to create heads that strengthen the column/slab connection, like drop panels in flat slab construction.

Subgrade Preparation

• Site is cleared and grubbed if necessary. • Organic top soil is removed. • Subsoil, or subgrade, is excavated to required depth. • Subgrade is graded level and compacted to the required density. • If the subsoil is too soft or unstable, it is over-excavated and replaced with more competent material.

Wall forms

• Slender rods, called form ties, hold the forms in position and resist the outward pressure of the concrete when it is placed. • The plastic cones prevent the formwork for sliding along the ties, and form a neat, conical hole in the finished surface • After the wall is cast and the forms removed, the protruding ends of the metal ties are broken off and the plastic cones removed. • The remaining holes in the concrete may be left open, filled with mortar, or plugged with some other material. • If the broken end of a metal form tie rod is not covered, rust staining may result as the end of the tie gradually corrodes. • Fiberglass form tie rods may be used without plastic cones. The protruding ends of the rods are simply ground off flush with the face of the concrete, becoming virtually unnoticeable to the untrained eye. - On the outside, the form ties engage with slotted metal wedges. The wedges restrain the horizontal walers. • The walers (and sometimes vertical studs) brace the formwork panels • Wall forms must be constructed sufficiently stiff to resist the fluid pressures of the freshly poured concrete. • A proprietary, modular wall form system, that can be easily raised and reused as wall construction proceeds upwards. • Self-climbing formwork relies on hydraulic jacks to climb the concrete core structure as it is constructed.

Finishing Concrete

• Striking off or screeding: A wood plank or metal straightedge is drawn across the surface of the freshly poured concrete, using an end-to-end sawing motion. • A bulge of concrete is maintained in front of the screed, to fill low spots as the screed progresses. • Striking off establishes the elevation of the upper slab surface. • Floating: Immediately after screeding, floating is performed to consolidate and smooth the slab surface. • A darby is used to float areas of the slab that can be reached without the long arm of the bull float. • Further finishing operations depend on the type of final finish required for the slab. • Where a rough finish is acceptable, no further operations may be required. • When required, further finishing steps begin after the concrete has been allowed to stiffen, and free water that rises to the surface, called bleed water, has evaporated. • Edgers and groovers are used to create neatly formed, well-consolidated edges and joints in the slab surface. • Floating may be performed a second time, to further consolidate and densify the surface of the slab. • Floats are made of wood or metal with a slightly rough surface. The floating operation leaves the slab with a lightly textured surface. • Troweling: For a smoother finish, the slab is troweled immediately after floating. • Trowels are made of smooth-surfaced steel or other metal. Troweling may performed by hand or with power-operated machinery • Broom Finish: A stiff broom is drawn across the slab surface to create a striated, slip resistant texture • Restraightening: After each floating or troweling operation, a long straightedge may be drawn over the slab surface to reduce minor undulations • Shake-On Hardeners: Dry powders may be floated into the slab surface to create a harder, more durable surface • Laser Screeds: Laser-guided power screeds can be used to finish slabs to more precise flatness and levelness requirements. •Curing: To ensure proper curing of the concrete, freshly poured slabs must be kept damp for at least the first week. •Slabs are especially vulnerable to premature drying because of their relatively large exposed surface area: -Cover slab with impervious plastic sheets or with absorbent, dampened straw, sawdust, or burlap. -Coat slab with a liquid-applied curing compound, that dries to form a clear moisture barrier.

Strength Test Cylinders

• Test cylinders may be cast from each batch of concrete delivered to the construction site. - The most common cylinder size is 6 in. diameter x 12 in. high. - Concrete is placed in a cylindrical mold and consolidated to eliminate voids. • Cylinders are returned to the laboratory, cured under controlled conditions, and then strength-tested at appropriate times. Test results for laboratory-cured cylinders verify the quality of the concrete as delivered to the site. • Cylinders can also be cured on site in conditions similar to that for the cast concrete. Results from these tests can be used to determine when it is safe to remove formwork or subject cast concrete to construction loads.

Concrete Wall Reinforcing

• The size, spacing, and arrangement of reinforcing bars varies with the structural requirements of the wall. • Typically, reinforcing is placed in one or two layers of vertical and horizontal reinforcing bars. • Reinforcing for a concrete shear wall (designed to resist lateral forces such as wind or earthquake), consisting of two layers of vertical and horizontal bars. • The wall is reinforced more heavily at either end, where it must resist greater stresses. • Heavy reinforcing at the base of concrete shear walls for a large, multistory building

Slump Test

• The slump test provides a rough measure of the workability of concrete while wet. • Concrete is placed into a conical cylinder; the cylinder is removed, and the loss in height of the concrete mass is measured. - Concrete with too low slump may be difficult to place. - Concrete with too high slump may have had too much water added. • Specified maximum slump is usually in the range of 3 to 5 inches. • Slump tests are performed on batches of concrete as the arrive on the concrete site. • Slump that varies excessively from one batch to the next may indicate quality control problems in the concrete mixes.

Water-Cement Ratio

• The water-cement ratio (w/c ratio) is the most important determinant of concrete strength. Lowering the proportion of water to cement: - Increases concrete strength and durability - Decreases workability - Increases cost • W/C ratio is measured by weight, not volume. • When supplementary cementitious materials are added to the mix, the ratio is measured as the water-cementitious materials ratio. • Why use higher-strength concrete: - Reduce column dimensions in tall buildings. - Achieve higher earlier strength, allowing construction to proceed more quickly. - Satisfy more stringent structural requirements.

Reinforcing a Simple Beam

• Top: In a simply supported beam, the greatest tension forces occur at the bottom middle of the beam. • Where tension forces cross compression forces closer to the beam ends, shear forces also occur. • Bottom: The placement of reinforcing in a concrete beam approximates the lines of tension, but is simplified to reduce fabrication costs.

Reinforcing a Concrete column

• Vertical or column bars: Larger diameter bars placed vertically in the column • Ties or spirals: Wrap around the vertical bars • The vertical bars add to the strength of the column in compression, and resist tensile forces that are introduced from wind or seismic forces, or from connections to beams. • Ties prevent the vertical bars from buckling outward. • Note how, where the vertical bars extend beyond the ties, they are bent slightly inward toward the center of the column. As the column height is extended further, the next section of reinforcing will nest and overlap with these ends.

Water

• Water is an essential ingredient in concrete, that combines chemically with the cement as the concrete hardens. • Water must be free of contaminants. • Water that is potable is acceptable for use in making concrete. • ASTM C 1602 permits the use of other sources such as waste water from washing out concrete trucks or storm water runoff at concrete production facilities that may contain limited amounts of concrete waste material. • The quantity of water in the concrete mix must be controlled as closely as any other ingredient: Adding unneeded water dilutes the cement paste, weakening the hardened concrete.

Concrete Quality

• Workability: Ease of placing, consolidating, and finishing wet concrete • Structural properties when hardened: Strength, stiffness, durability • Many other important properties: - Ease of placement - Rate of early strength gain - Degree shrinkage during curing - Flatness, for slabs and paving - Surface hardness-industrial slabs - Porosity - Density - Surface appearance, for architectural concrete - Resistance to freeze/thaw and weather, for exterior concrete - Watertightness, for dams, tanks, exterior walls

Pouring/Placing Concrete Wall

•Concrete is placed in the wall. •It is consolidated by vibrating or hammering on the sides of the formwork. •The top of the wall is covered to limit water loss, and the wall is left to cure. •After several days, the formwork may be stripped. Curing should continue for at least one week.

Tilt-Up Construction

•Concrete wall panels are poured lying flat, much like a slab on grade. •Once the panels have gained sufficient strength, they are lifted into final position. •Tilt-up construction significantly reduces formwork costs, which can account for 50 per cent or more of the cost of conventional concrete construction.

Consolidating Concrete

•Consolidation (compaction): elimination of voids & air pockets within the concrete pour. - Hand rodding or tamping - Screeding (top) - Internal vibration (bottom) - External vibration • Consolidation is especially critical with stiff concrete mixes or when concrete is placed around densely packed reinforcing arrays. • Over-consolidation must be avoided, as it can lead to segregation of aggregate as larger particles descend and finer components rise to the surface.