Building Technology 6.0: Concrete and Masonry Notes

Concrete and Concrete Products

Concrete Fundamentals

• Definition of Concrete: An artificial stone made from a mixture of cement, sand, gravel, and water.

• Definition of Mortar: A mixture consisting of cement, sand, and water.

Special Cements

• White Portland Cement: Identical to normal Portland cement in properties, but with a white color.

• Masonry Cement or Tile Adhesives: Specialized cements typically used for masonry work or as tile adhesives.

• Waterproofed Portland Cement: Designed to create watertight concrete, often containing a waterproofing compound.

Types of Aggregates

• Fine Aggregates: Primarily composed of sand.

• Coarse Aggregates: Typically composed of gravel.

Concrete Mixture Design

• Governing Principle: The design of concrete mixture is primarily based on the water-cement ratio.

• Water-Cement Ratio Impact:

  • Less water produces stronger concrete.

  • More water produces lesser strength in concrete.

• Measuring Box Sizes (for aggregates related to cement weight):

  • For $40 \text{ kg}$ cement: $300 \text{mm} \times 300 \text{mm} \times 300 \text{mm}$.

  • For $50 \text{ kg}$ cement: $320 \text{mm} \times 320 \text{mm} \times 330 \text{mm}$.

Concrete Mixtures (by Class)

• Ratios given as Cement : Sand : Gravel (for $0.50 \text{ cu.m}$ and $1.0 \text{ cu.m}$ volume of sand and gravel respectively):

  • Class "AA" (1:1.5:3): Requires $12.0 \text{ kg}$ cement (for $40 \text{ kg}$) or $9.5 \text{ kg}$ cement (for $50 \text{ kg}$) per $0.50 \text{ cu.m}$ sand and $1.0 \text{ cu.m}$ gravel.

  • Class "A" (1:2:4): Requires $9.0 \text{ kg}$ cement (for $40 \text{ kg}$) or $7.0 \text{ kg}$ cement (for $50 \text{ kg}$) per $0.50 \text{ cu.m}$ sand and $1.0 \text{ cu.m}$ gravel.

  • Class "B" (1:2.5:6): Requires $7.5 \text{ kg}$ cement (for $40 \text{ kg}$) or $6.0 \text{ kg}$ cement (for $50 \text{ kg}$) per $0.50 \text{ cu.m}$ sand and $1.0 \text{ cu.m}$ gravel.

  • Class "C" (1:3:6): Requires $6.0 \text{ kg}$ cement (for $40 \text{ kg}$) or $5.0 \text{ kg}$ cement (for $50 \text{ kg}$) per $0.50 \text{ cu.m}$ sand and $1.0 \text{ cu.m}$ gravel.

Mortar Mixtures (by Class)

• Ratios given as Cement : Sand (for $1.0 \text{ cu.m}$ volume of sand):

  • Class "A" (1:2): Requires $18 \text{ kg}$ cement (for $40 \text{ kg}$) or $14.5 \text{ kg}$ cement (for $50 \text{ kg}$) per $1.0 \text{ cu.m}$ sand.

  • Class "B" (1:3): Requires $12 \text{ kg}$ cement (for $40 \text{ kg}$) or $9.5 \text{ kg}$ cement (for $50 \text{ kg}$) per $1.0 \text{ cu.m}$ sand.

  • Class "C" (1:4): Requires $9 \text{ kg}$ cement (for $40 \text{ kg}$) or $7.0 \text{ kg}$ cement (for $50 \text{ kg}$) per $1.0 \text{ cu.m}$ sand.

  • Class "D" (1:5): Requires $7.5 \text{ kg}$ cement (for $40 \text{ kg}$) or $6.0 \text{ kg}$ cement (for $50 \text{ kg}$) per $1.0 \text{ cu.m}$ sand.

Admixtures and Formwork

Admixtures

• Definition: Various compounds, other than cement, water, and aggregates, added to a concrete mixture to modify its fresh or hardened properties.

• Function: They can accelerate or retard concrete setting.

• Types of Concrete Admixtures:

  • Air Entrainers: Primarily used to enhance freeze-thaw resistance of concrete by creating microscopic air bubbles.

  • Water Reducers: Neutralize surface charges on solid particles, causing them to repel each other. This reduces flocculation of cement particles, allowing for better dispersion and thus requiring less water for a given workability.

  • Set Retarders: Used to delay the initial setting time of concrete, sometimes up to an hour.

  • Set Accelerators: Admixtures that speed up the setting time of concrete, mortar, rendering, or screeds.

  • Superplasticizers (High Range Water Reducers): Additives used to make high-strength concrete by significantly reducing water content while maintaining or increasing workability.

  • Specialty Admixtures: A broad category including corrosion inhibitors, shrinkage control agents, alkali-silica reactivity inhibitors, and coloring agents.

Release Agents

• Definition: Materials (e.g., oil, silicone) used to prevent the bonding of concrete to a surface; also known as parting compound.

Properties and Testing of Concrete

Properties of Concrete (for Controlled Mixes)

• Consistency: Refers to the fluidity or stiffness of fresh concrete.

• Workability: The ease with which concrete can be mixed, placed, consolidated, and finished without segregation.

Control of Concrete Mixes (Testing)

• Slump Test: A quick method for determining the consistency of freshly mixed concrete.

• Compressive Test: A test performed to determine the compressive strength of a concrete batch, usually on cylinders or cubes of hardened concrete.

Types of Concrete Construction

Cast-in-place (Cast-in-concrete)

• Definition: Concrete that is deposited, formed, cured, and finished in its final position as an integral part of the structure.

Precast Concrete

• Definition: A construction product produced by casting concrete in a reusable mold or "form." It is then cured in a controlled environment, transported to the construction site, and lifted into place.

  • Contrast with Cast-in-place: Unlike cast-in-place concrete, which is poured into site-specific forms and cured on site, precast concrete is made off-site.

  • Precast Stone: Distinguished from precast concrete by using finer aggregates to achieve an appearance similar to natural rock or stone.

• Examples of Precast Concrete Applications:

  • Tilt-up: Large concrete panels cast horizontally on-site and then tilted into vertical position.

  • Lift Slab: A method where floor and roof slabs are cast on the ground, then lifted into position by jacks.

Concrete Placement and Finishing

Placement

• Definition: The process of depositing and consolidating freshly mixed concrete into a form or its final position where it will harden.

• Direct Placement: Discharging freshly mixed concrete directly into its final location.

• Segregation of Concrete: The separation of the concrete mixture's components (e.g., aggregates separating from cement paste) during transport or placement, leading to decreased quality.

• Methods of Placement:

  • Free Fall: Placing concrete by allowing it to fall directly into position, often in dry excavations.

  • Drop Chute: A device used to control the placement of concrete from a height, minimizing segregation.

  • Chute: An inclined trough used to convey concrete from one point to another.

Consolidation

• Definition: The process of removing entrapped air voids from freshly placed concrete, typically by vibration, to increase density and strength.

Finishing (Tools)

• Screed: A tool or board used to strike off freshly placed concrete to a proper elevation.

• Float (Bull Float, Hand Float): Tools used to smooth and level the surface of concrete after screeding.

• Trowel: A hand tool with a flat blade used for final smoothing and finishing of concrete surfaces.

• Darby: A long hand float used to flatten and level concrete surfaces.

• Pavement Saw: A specialized saw used to cut control joints in hardened concrete pavements.

Equipment for Concrete Work

• Agitator Truck: A specialized truck with a rotating drum to keep concrete agitated during transport, preventing segregation and premature setting.

• Concrete Mixer: Equipment used to uniformly mix cement, aggregates, and water.

• Buggy: A small, wheeled cart used for transporting small quantities of freshly mixed concrete on construction sites.

• Pneumatic Placement (Gunite): A method where concrete or mortar is conveyed through a hose and pneumatically projected at high velocity onto a surface.

Masonry

Definition of Masonry

• Refers to man-made units (like bricks, blocks) which are formed and hardened into modular building units.

Mortar Mixtures for Masonry

• Ratios given as Cement : Sand (for $1.0 \text{ cu.m}$ volume of sand):

  • Class "A" (1:2): Requires $18 \text{ kg}$ cement (for $40 \text{ kg}$) or $14.5 \text{ kg}$ cement (for $50 \text{ kg}$) per $1.0 \text{ cu.m}$ sand.

  • Class "B" (1:3): Requires $12 \text{ kg}$ cement (for $40 \text{ kg}$) or $9.5 \text{ kg}$ cement (for $50 \text{ kg}$) per $1.0 \text{ cu.m}$ sand.

  • Class "C" (1:4): Requires $9 \text{ kg}$ cement (for $40 \text{ kg}$) or $7.0 \text{ kg}$ cement (for $50 \text{ kg}$) per $1.0 \text{ cu.m}$ sand.

  • Class "D" (1:5): Requires $7.5 \text{ kg}$ cement (for $40 \text{ kg}$) or $6.0 \text{ kg}$ cement (for $50 \text{ kg}$) per $1.0 \text{ cu.m}$ sand.

Basic Brickwork Terminology

• Stretcher: A brick laid flat with its longest side parallel to the face of the wall.

• Header: A brick laid flat with its shortest side parallel to the face of the wall.

• Shiner: A brick laid on its longest narrow side with the header (end) visible.

• Rowlock: A brick laid on its longest narrow side with its stretcher (side) visible.

• Sailor: A brick laid on its shortest narrow side with its stretcher (side) visible.

• Soldier: A brick laid on its shortest narrow side with its header (end) visible.

• Course: A continuous horizontal layer of bricks or other masonry units in a wall.

• Wythe: A continuous vertical section of masonry one unit in thickness.

• Bed Joint: The horizontal layer of mortar on which masonry units are laid.

• Head Joint: The vertical layer of mortar between the ends of adjacent masonry units in a course.

Structural Bonds for Brick

• Running Bond: Consists of all stretchers, with vertical head joints staggered from course to course.

• Common Bond: A variation of running bond with a course of headers at regular intervals (typically every $5^{th}$ or $6^{th}$ course) to provide structural bonding.

• English Bond: Alternating courses of headers and stretchers.

• Flemish Bond: Each course consists of alternating headers and stretchers.

Types of Brick Bonds

• Pattern Bond: The method of arranging bricks in a wall to form a distinctive pattern or design.

• Structural Bond: The method by which individual units in a brick structure are tied together, either by overlapping bricks or using metal ties, to ensure stability.

• Mortar Bond: The adhesion of mortar to bricks or to steel reinforcement used in masonry construction.

Concrete Hollow Blocks (CHB)

• CHBs typically have three full cells and two half-cells at both ends, totaling four equivalent full cells (or often, three main cells and smaller end cells).

Sample Computation for Concrete Mortar Mixture

Problem: Calculate the cement and sand needed for a wall section.

Given: Area ($A = 35 \times 7.0$ = $275 \text{ sq.m}$) (Note: The calculation suggests the area used is $275 \text{ sq.m}$ not $35 \times 7.0$). Assume the values $1.013$ and $0.8438$ are material factors per square meter for Class "B" mixture.

**Solution (Using Class "B" Mortar Mixture, $40 \text{ kg}$ Cement reference):

1. Find the Area (Assumed from Calculation Context): $A = 275 \text{ sq.m}$ of surface to be covered with mortar.

2. For Cement:

   • Referring to the table for Class "B" Mortar Mixture ($40 \text{ kg}$ Cement): Assume a factor of $1.013 \text{ bags/sq.m}$ (This factor is derived from the table data in context of a real-world usage scenario and is not directly shown in the provided table for mortar quantities per $1.0 \text{ cu.m}$ sand; it represents a usage rate for a specific mortar application, possibly for plastering or block laying).

   • Cement needed = $275 \text{ sq.m} \times 1.013 \text{ bags/sq.m} = 278.575 \text{ bags}$, rounded up to $279 \text{ bags}$.

3. For Sand:

   • Referring to the table for Class "B" Mortar Mixture ($40 \text{ kg}$ Cement): Assume a factor of $0.8438 \text{ cu.m/sq.m}$ (Similarly, this factor represents a usage rate for a specific mortar application).

   • Sand needed = $275 \text{ sq.m} \times 0.8438 \text{ cu.m/sq.m} = 232.045 \text{ cu.m}$, rounded up to $24 \text{ cu.m}$.