TCM-123 CHapter 13

High-range water reducing (superplasticizers)

Organic compounds that transform a still concrete mix into one that flows freely into the forms. They are used either to facilitate placement of concrete under difficult circumstances or to reduce the water content of a concrete mix so as to increase its strength

Cement

Portland Cement: A fine gray powder. It may be manufactured from any of a number of raw materials, provided that they are combined to yield the necessary amounts of lime, iron, silica, and alumina. Lime is commonly furnished by limestone, marble, or marl, or seashells. Iron, silica, and alumina may be provided by clay or shale.

Making Concrete

Portland cement, water, aggregates. Keep in mind the water-cement ratio. For small jobs concrete will be mixed at the job site

Dry Batch

All ingredients are put in the truck individually and they are not mixed before they go into the truck

Water reducing

Allow a reduction in the amount of mixing water while retaining the same workability, which results in a higher-strength concrete

Temperature

At low temperatures, the curing reaction in concrete proceeds much more slowly. If concrete reaches sub-freezing temperatures the reaction stops completely. It is important to protect the concrete from server temperatures.

Formwork

is usually made of braced panels of wood, metal, or plastic. It is constructed as a negative of the shape intended for the concrete. Formwork for a beam or slab serves as a temporary working surface during the construction process and as the temporary means of support for reinforcing bars. It must be strong enough to support the considerable weight and fluid pressure of wet concrete without deflection.

Prestressing

Applying an initial compressive stress to a concrete structural member

Accelerating

Cause concrete to cure more rapidly

Silica Fume

Very finely divided silicon dioxide, a pozzolan, used as an admixture in the formulation of high-strength, low-permeability concrete; also called microsilica

Concrete Creep

When placed under sustained compressive stress from its own weight, the weight of other permanent building components, or the force of prestressing, concrete will gradually and permanently shorten over a period of months or years.

Supporting rebar in place prior to concrete placement (chairs, bolsters and standees)

When the simple bam is formed, it is supported by chairs or bolsters. Chairs and bolsters remain in the concrete after pouring, even though their work is finished. They help support the beams before concrete is poured.

Shrinkage Reducing

Reduce drying shrinkage and the cracking that results therefrom

Rebar marking system

Reinforcing bars are manufactured with identification marks, denoting the mill that produced the bars, bar size, type of steel, and steel grade.

Two-way slab

A concrete slab that is reinforced equally in both directions and share the bending forces equally between the two directions. They can be somewhat shallower, use less reinforcing steel, and cost less in comparison to one way slabs.

One-way slab

A concrete slab that spans across parallel beams or walls. They must be provided with shrinkage-temperature steel, a set of small diameter reinforcing bars set at right angles to, and on top of, the primary reinforcing in the slab. Their function is to prevent cracks.

Blast Furnace Slag

A hydraulic cementitious material formed as a byproduct of iron manufacture, used in mortar and concrete mixtures; also called slag cement

Concrete

A rocklike material produced by mixing coarse and fine aggregates, Portland cement, and water and allowing the mixture to harden. During the hardening of concrete, curing, the cement combines chemically with water to form strong crystals that bind the aggerates together, a process called hydration. During this process considerable heat, called heat of hydration is given off, the concrete will then shrink slightly, a phenomenon referred to as drying shrinkage. Concrete is normally considered cured and at full design strength after 28 days.

Pozzolans

A supplementary cementitious material that has few or no inherent cementitious properties, but that in the presence of moisture, can react with calcium hydroxide released by other cementitious materials to create a hydraulic cement product.

Concrete Curing

Because concrete cures by hydration, the chemical bonding of the water and cement, and not by simple drying, it is essential that it be kept moist until its required strength is achieved. The curing reaction takes place over a long period of time, but concrete is commonly designed on the basis of the strength that it reaches after 28 days. If it is allowed to dry out at any point the concretes strength will be reduced, and its surface hardness and durability can be adversely affected.

Post-tensioning

Compressing the concrete in a structural member by tensioning high-strength steel tendons against it after the concrete has cured

Concrete Reinforcing steel (types and sizes and purpose)

Concrete has no useful tensile strength. So steel bars or wires are laid into concrete along lines of tension, to provide resistance to these forces. - Steel reinforcing bars for concrete construction are hot-rolled in much the same way as structural shapes. They are round in cross section and deformed with surface ribs for better bonding to concrete. Typical length is 60ft. Bars are specified by a number that corresponds to eigths of an inch of bar diameter. - Weld wire reinforcing: Prefabricated, welded grids of reinforcing bars or wires. Especially common for concrete slab reinforcing.

Pre-Mix

Concrete that is all mixed together before arriving on site

Air Content

Conducted to determine if delivered concrete is within the air content specifications established by the engineer. The field technician fills a circular metal base with three lifts of concrete, which are tamped using a metal rod. Once the base is full of concrete, a metal lid with a pressure gauge attached is placed on top and the two parts are locked together. A hand pump is used to pressurize the device to a calibration point and then it is allowed to stabilize. After stabilization, the pressure is released, and the technician can read the concrete air content from a dial attached to the device.

Air-entraining

Contain ingredients that cause microscopic air bubbles to form in the concrete during mixing. These bubbles improve workability during placement of the concrete, and more importantly, greatly increase the resistance of the finished concrete to damage caused by repeated cycles of freezing and thawing

Fly Ash

Dust collected in the stacks of coal-fired power plants, used as a supplementary cementitious material in concrete and mortar

Coloring

Dyes and pigments used to alter and control the color of concrete for building components whose appearance is important

Type IA

Normal, air entraining

Type IIIA

High early strength, air entraining

Type III

High early strength, hardens more quickly than the others, and is employed in situations where a reduced curing period is desired (in cold weather, in pre-casting of concrete structural elements, or when the construction schedule must be altered

Type V

High resistance to sulfate attack, used when the concrete will be in contact with water that has a high concentration of sulfates

Workability Agents

Improve the plasticity of wet concrete to make it easier to place in forms and finish. They include pozzolans and air-entraining admixtures, along with certain fly ashes and organic compounds

Concrete Admixtures

Ingredients other than cement and other cementitious materials, aggregates, and water, broadly referred to as admixtures, are often added to concrete to alter its properties in various ways

Type IV

Low heat of hydration, used in massive structures such as dams, where the heat emitted during curing may raise the temperature of the concrete to damaging levels

Type I

Normal, used for most purposes in construction

Supplementary Cementitious Materials (SCMs)

May be added to concrete mixtures as a substitute for some portion of the portland cement to achieve a range of benefits.

Secondary reinforcment

Mid-bars calculated as temperature reinforcements to prevent cracks due to changes in temperature

Type II

Moderate resistance to sulfate attack, used when the concrete will be in contact with water that has a high concentration of sulfates

Type IIA

Moderate sulfate resistance, air entraining

Aggregates

Must be strong, clean, resistant to freeze-thaw deterioration, chemically stable, and properly graded for size distribution. Lightweight aggregates are used for various special types of concrete. Structural lightweight aggregates are made from minerals such as shale. Coarse aggregate is normally gravel or crushed stone, and fine aggregate is sand.

Concrete Testing

Testing of concrete is vital to ensuring the strength and resilience of built structures

Compressive Strength Test

Tests if the concrete delivered meets the strength requirements as established by the engineer. Test cylinders may be cast from each batch of concrete delivered to the site. Cylinders are returned to the laboratory, cured under controlled conditions, and then strength tested. The results verify the quality of the concrete.

Slump

The hollow metal cone is filled with concrete and tamped with the rod according to a standard procedure. The cone is carefully lifted off, allowing the wt concrete to sag, or slump, under its own weight. The slump is then measured. The higher the slump the more fluid the concrete is, the lower the slump the slump the stiffer the concrete is.

Unit Weight

Unit weight determination of concrete is relatively easy. Fresh concrete is placed inside a container of known volume and weighed to provide the unit weight or density of the concrete.

Water for Concrete

Water must be free of harmful substances, especially organic material, clay, and salts such as chlorides and sulfates. Forms a paste that binds the aggregate together. The water causes the hardening of the concrete

Retarding

slow its curing to allow more time for working with the wet concrete