Concrete Part 3: Strength, Reinforcement, and Finishing

Concrete Part 3: Strength, Reinforcement, and Finishing

Concrete Strength & Curing

• Maximum Strength Attainment:

  • Concrete's maximum strength is typically tested at $7$-day intervals.

  • It usually achieves its maximum strength at $28$ days.

• Curing Milestones:

  • Day 3: Formwork can often be removed.

  • Day 7: The concrete is strong enough for light footpath traffic.

  • Day 14: Cured to approximately $70\%$ of its strength, allowing construction and framing to begin.

Internal and External Forces in Structural Elements

• External Forces: Any structural element is subject to external forces acting upon it.

• Internal Forces: These external forces create internal forces within the element.

• Types of Internal Forces:

  • Compression: Forces that push materials together.

  • Tension: Forces that pull materials apart.

  • Tensile forces most often occur on the opposite face of where the external applied force is experienced.

Concrete Strength Measurement & Requirements

• Different Strengths: Concrete is available in various strengths tailored for specific applications.

• Canadian Measurement: Strength is measured in MPa (Mega Pascals).

  • $1$ MPa is approximately equivalent to $145$ psi (pounds per square inch).

• Minimum OBC Requirement: The Ontario Building Code (OBC) specifies minimum strength requirements for certain applications.

  • Specific strength requirements are also often dictated by 'Engineered' designs.

• Strength Determination: Concrete strength is determined by its design mix, specifically the ratio of cement-to-sand-to-aggregate.

Concrete Reinforcement (Rebar)

• Purpose: Reinforcing bar, commonly known as "Rebar," is made of steel and is used in \
  reinforced concrete to improve concrete's greatest weakness: tension.

• Mechanism:

  • Rebar is tied together to form a cage-like structure and then embedded into the freshly poured concrete.

  • "Reinforced Concrete" functions by transferring the tensional loads from the concrete (which is weak in tension) to the embedded steel rebar (which is strong in tension).

• Types of Reinforcing Bar:

  • Standard Rebar: Recognized by its distinctive "ribbed" finish, which helps create a strong bond with the concrete.

  • Smooth Rebar (Dowels):

    • Used when longitudinal movement along the dowel is desired, but reinforcement is required latitudinally across the dowel.

    • Applications include allowing for expansion/contraction while maintaining alignment.

  • Epoxy Coated Rebar:

    • Identifiable by its green epoxy coating.

    • Often used in environments exposed to highly corrosive elements, such as high salt or moisture areas (e.g., bridge decks where exposure to salt and water is substantial).

• Rebar Sizing Differences (US vs. Canada):

  • US Sizing: Bar size is based on the numerator of the size in $1/8"$. For example, a #$3$ bar is $3/8"$ in diameter.

  • Canadian (Metric) Sizing: Bar size is identified by its nominal bar diameter followed by an "M". For example, an $11.3$mm bar is referred to as $11.3$M.

• Placement & Coverage:

  • Rebar is typically positioned within the concrete structure closest to the face that is expected to experience the highest tensional forces.

  • Minimum Coverage: A minimum concrete cover of $25$mm (or double the bar diameter, whichever is greater) is required to protect the steel from corrosion and ensure proper bond.

Concrete Reinforcement in Slabs

• Scenario 1: High External Loading:

  • For concrete slabs expected to experience high external loading, standard rebar is used throughout the slab, often in both vertical and horizontal planes, to provide robust structural integrity.

• Scenario 2: Minimizing Cracking in Slabs:

  • For slabs that can carry the expected load but require reinforcement to minimize cracking (e.g., due to shrinkage or minor stresses), "Welded Wire Mesh" (WWM) is used.

  • WWM Characteristics:

    • Commonly available in $4\times8$ foot (approximately $1.2\text{m}\times2.4\text{m}$) sheets.

    • Common grid spacing combinations include $6\times6$ inches.

    • WWM provides distributed reinforcement to control crack propagation.

Concrete Floor Finishing Process

Once "Fresh Concrete" (also known as "Plastic Concrete") has been poured into the forms, the concrete finishing process begins. This process transitions the freshly poured concrete into a strong and smooth surface before it fully sets.

• Step 1: Spread the Wet Concrete:

  • Evenly spread the wet concrete to fill all parts of the form, moving from high to low spots.

  • This step may involve vibrating the concrete to remove air pockets and ensure full consolidation.

• Step 2: Screed the Wet Concrete:

  • Purpose: To make the concrete level.

  • Process: Use a screed board with a quick sawing motion.

  • Move excess concrete from high spots to low spots.

  • Screed $3$ to $4$ times to create a smoother surface.

• Step 3: Float the Surface:

  • Timing: Float the surface before bleed water appears.

  • Effect: This action embeds the larger aggregate particles below the surface and pulls excess water to the top.

  • Rest Period: A minimum of $15$ minutes rest period is typically observed before proceeding to the next step.

• Step 4: Edge the Concrete:

  • Timing: Edge the concrete after the bleed water has evaporated from the surface (following Step $3$).

  • Purpose: To create a rounded edge.

  • Benefits: Prevents breakage and chipping of the concrete edges, improving durability and appearance.

• Step 5: Control Joints:

  • Purpose: To prevent uncontrolled cracking, which can diminish durability and create a poor appearance.

  • Process: Grooves (joints) are added to the proper depth and spacing to create planned spaces for concrete expansion and contraction.

• Step 6: Surface Finish:

  • Broom Finish: Creates a textured, slip-resistant surface, often used for exterior applications or areas where traction is important.

  • Trowel Finish: Refines the surface with a trowel, creating a smooth finish. This is typically done if a flooring material will be applied on top.

• Step 7: Protect the Slab:

  • After finishing, the slab must be protected from:

    • Foot traffic

    • Debris

    • Rainfall

    • Adverse temperatures (e.g., extreme heat or cold)

  • Proper protection ensures the concrete cures correctly and achieves its intended strength and durability.

Slab-on-Grade (S.O.G.) Construction

• Definition: Slab-on-Grade (a.k.a. 'S.O.G.' or 'slab-on-ground') refers to a floor system that is located at ground level, regardless of the specific elevation of that ground level.

• Distinction from "Floor Construction": "Floor Construction" generally refers to a system located above ground level.

• Key Component: 6 MIL POLY Vapour Barrier:

  • "6 MIL": Refers to the thickness of the barrier, specifically $6/1000$ of an inch. A higher MIL rating indicates a stronger, more robust material.

  • "POLY": Stands for Polyethylene, the material from which the barrier is made.

  • "Vapour Barrier":

    • A material (typically a plastic or sheet) used for dampproofing.

    • Its primary function is to resist the diffusion of moisture through the item it is protecting, thereby preventing moisture from rising from the ground into the slab and subsequently into the building space above.