Concrete and Cement Types

Concrete Basics:

• Concrete consists of cement, aggregate, water, and sometimes admixtures.
• Concrete is valued for its strength, durability, stability, availability, adaptability and relatively low cost for construction and maintenance.

Methods of Concrete Placement:

• Traditional placement involves mixing in place, like soil cement or dry-casting.
• Newer methods include forming and molding (cast in place or precast), and spraying or air-blowing (shotcrete).

Cement

• Cement hardens through a chemical reaction with water called hydration, resulting in hardening of the cement paste.
• Cement is classified as either hydraulic or non-hydraulic based on its hardening mechanism.
• Common cement types include Ordinary Portland Cement (OPC), Portland Pozzolana Cement (PPC), Rapid Hardening Cement, and Quick Setting Cement.
• Other cement types are Low Heat Cement, Sulphate Resisting Cement, Blast Furnace Cement and High Alumina Cement.
• Additional cement types are White Cement, Colored Cement, Air Entraining Cement, Expansive Cement, and Hydrophobic Cement.

Aggregates

• Aggregates in concrete serve multiple functions and typically make up 60-75% of the volume of normal concrete.
• Common types of aggregate include Sand (Fine Aggregate), Gravel (Coarse Aggregate), and Crushed Stone.
• Various types of igneous, sedimentary, and metamorphic aggregate are used based on availability and intended use.

Water

• The water used in concrete should be clean, free of oils, alkalis, acids, organic materials, and deleterious substances, especially high concentrations of sulfates or salts.
• Potable water is generally suitable for concrete.
• The right water quantity is proportional to the amount of cement; too much water reduces strength.

Admixtures

• Admixtures are substances added before or during mixing to enhance concrete properties.
• Admixtures improve workability by minimizing separation of coarse and fine aggregate.
• They can reduce water requirements to increase strength.
• They can entrain air to improve durability and resistance to freeze/thaw damage and scaling from deicing chemicals.
• They also accelerate or retard hardening and increase flowability.

Preparation of Concrete:

• If available, ready-mixed concrete is preferable, unless the project is exceptionally large.
• Ready-mixed concrete should be delivered and placed within 90 minutes of adding cement.
• Concrete is tested by inspection personnel while fresh and in a hardened state.
• Common tests include slump, air content, and cylinder tests.

Concrete Innovations:

• Concrete 3D printers can quickly produce structures in customized ways not possible with traditional methods.
• Bendable concrete is made by adding silica sand and polyvinyl alcohol fibers to traditional mixes, making it significantly harder to crack, up to 500 times.
• Eco-friendly concrete reduces environmental impact by using magnesium sulfate or fly ash, which lowers the carbon footprint and water usage.
• Lightweight concrete is reinforced with carbon fiber, making it thinner, lighter, stronger, corrosion-resistant, and eliminates the need for sealers and barrier coatings.
• Concrete innovations are being developed to withstand the harsh Martian environment, resulting in concrete twice as strong as that typically used on Earth.
• Pervious concrete addresses urban flooding by allowing water to permeate into the ground.
• Self-healing concrete uses bacteria to generate a glue-like substance that seals cracks with limestone or calcite.
• Translucent concrete combines translucent fabric with fine-grained mixtures to allow light to shine through without compromising strength or durability.

Cast in Place and Pre-Cast Concrete:

• The following concrete descriptions describe floor and roof slabs

Flat Slab Construction:

• Flat slab construction is a reinforced concrete slab supported directly by concrete columns instead of beams.
• Flat slabs are one-sided or two-sided systems with sheer load targeting the square slab or supporting columns.

Flat Plate Construction:

• Flat plate construction is a two-way reinforced concrete framing system with a slab of uniform thickness, used for structural simplicity.
• Flat slabs are a two-way reinforced structural system with drop panels or column capitals at columns to resist heavier loads and permit longer spans.

Ribbed Floor Slab:

• Ribbed slabs are thin concrete floor products, like half slabs, used in precast preproduction and in-situ concrete at construction sites.

Waffle Slab:

• A waffle slab, or two-way joist slab, is a reinforced concrete slab with concrete ribs running in two directions on its underside forming a grid pattern.

Lift Slab:

• Lift slab is a technique where concrete floor slabs are poured on the ground, one on top of the other, and then lifted into place on top of columns by hydraulic jacks for tall multistory buildings, offering savings in formwork.

Span-Stress Floor System:

• Using span-stress prestressed T-joists with filler blocks, collapsible steel forms, or plywood forms, the system maintains a stiffness of a conventional slab because the concrete is poured monolithically and connected with the structure, providing connection reinforcement that transfers lateral forces to the load-resisting members.

Slip-Form Method System:

• Slipform construction is used to construct reinforced concrete walls for lift and stair core walls, and buildings taller than ten stories. -Formwork for walls is raised continuously as the wall concrete is poured.