Cement and Concrete Essentials: Properties, Testing, and Application

Posted on Jul 16, 2025 in Civil Engineering, Transportation, and Urban Services

Understanding Cement and Concrete Fundamentals

Key Properties of Cement

Initial and Final Setting Times

Initial Setting Time: It is the time interval between the moment water is added to the cement and the time at which the paste starts losing its plasticity.

Final Setting Time: It is the time taken from the moment water is added to the cement until the paste has completely lost its plasticity and has attained sufficient firmness to resist certain pressure.

Fineness of Cement

Fineness of cement refers to the particle size of the cement and the specific surface area (total surface area of all the particles per unit weight).

Effects of Fineness of Portland Cement on Performance
  • Increases faster chemical reaction and early strength gain.
  • Higher early strength due to more reactive surface area.
  • Increasing fineness can be problematic in mass concreting due to higher heat of hydration.

Types and Grades of Cement

Portland Cement Explained

Portland Cement is the most commonly used type of cement in construction. It is a fine powder made by heating limestone and clay minerals in a kiln to form clinker, grinding the clinker, and adding a small amount of gypsum.

Ordinary Portland Cement (OPC) Grades

Ordinary Portland Cement (OPC) is the most commonly used type of cement in general civil construction. It is made by grinding clinker and gypsum and is known for its quick setting and high strength properties.

  • OPC 33: 33 Megapascals (MPa) for masonry, plastering, and non-structural work.
  • OPC 43: 43 MPa for general construction in residential and commercial projects.
  • OPC 53: 53 MPa for high-strength concrete applications.

OPC vs. Portland Pozzolana Cement (PPC)

  • OPC: Clinker + Gypsum
  • PPC: Clinker + Gypsum + Pozzolanic materials like fly ash, volcanic ash, silica fumes.
  • PPC is generally available as a single grade, equivalent to 33-43 grade OPC.
  • PPC exhibits slower strength gain initially but offers better long-term durability and resistance to chemical attacks.

Water-Cement Ratio in Concrete

The Water-Cement Ratio (w/c ratio) is the ratio of the weight of water to the weight of cement used in a concrete mix.

For example, if 50 kg of cement is mixed with 25 kg of water: w/c = 25/50 = 0.5

Impact on Concrete Strength

  • Lower w/c ratio → Higher strength (less water, denser concrete).
  • Higher w/c ratio → Lower strength (more water, more pores).

Impact on Workability

  • More water increases workability (ease of placing and finishing).
  • But excessive water reduces strength and durability.

Impact on Durability

  • High w/c ratio concrete is more porous → allows water, chemicals, and air to penetrate → reduces the life of the structure.

Admixtures in Concrete

Admixtures are materials added to concrete, mortar, or grout immediately before or during mixing to modify its properties.

Plasticizers (Water Reducers)

  • Purpose: Improve workability without increasing water content.
  • Effect:
    • Increases flowability of concrete.
    • Reduces water-cement ratio → improves strength.

Superplasticizers (High-Range Water Reducers)

  • Purpose: Greatly increase workability or significantly reduce water content.
  • Effect:
    • Used for highly workable concrete like in pumping or precast works.
    • Improves strength, finish, and compaction.

Retarders

  • Purpose: Delay the setting time of concrete.
  • Effect:
    • Useful in hot weather concreting.
    • Prevents cold joints in large pours.
  • Example: Gypsum (Calcium Sulfate) – commonly added during cement grinding.

Accelerating Admixtures

  • Definition: Accelerators increase the rate of hydration of cement, which leads to faster setting and early strength gain of concrete.
  • Example: Calcium Chloride (CaCl₂) – most common (used cautiously as it can corrode steel reinforcement).

Concrete Behavior and Characteristics

Shrinkage of Concrete

Shrinkage of concrete is the reduction in volume of concrete over time, mainly due to loss of moisture from the cement paste. It typically occurs after setting and during the hardening phase.

Factors Affecting Shrinkage

  • Water-Cement Ratio: Higher w/c ratio leads to more shrinkage due to more evaporation and larger pore spaces.
  • Cement Content: More cement content results in higher shrinkage because more paste means more volume change.
  • Aggregate Type and Content: More aggregate leads to less shrinkage as aggregates restrain movement of paste.

Segregation in Concrete

Segregation is the separation of concrete ingredients (cement paste, sand, and coarse aggregates) during handling, placing, or transportation. It leads to non-uniform concrete, affecting strength, durability, and appearance.

Advantages and Disadvantages of Concrete

Advantages of Concrete

  • High Compressive Strength: Concrete can withstand heavy loads, making it ideal for structural elements.
  • Durability: Resistant to weathering, fire, and decay, lasting long under proper conditions.
  • Versatility: Can be molded into any shape before it sets, used in beams, slabs, columns, etc.

Disadvantages of Concrete

  • Low Tensile Strength: Cracks easily under tension; needs reinforcement like steel.
  • Prone to Cracking: Shrinkage, temperature changes, or poor workmanship can lead to cracks.
  • Heavy Weight: High self-weight requires strong support and foundations.

Quality Control and Best Practices

Field Tests for Cement Quality

These tests help to quickly assess the quality of cement on site.

Color Test

  • Objective: To check the uniformity and color of cement.
  • Method: Good cement should be uniform in color, typically greenish-grey. Variations in color may indicate poor quality or mixed materials.

Touch Test (Fineness)

  • Objective: To feel the texture and fineness.
  • Method: Rub a pinch of cement between your fingers. It should feel smooth and powdery, not gritty. A gritty feel means it may contain impurities or be coarse.

Float Test

  • Objective: To check for the presence of unburnt lime or other impurities.
  • Method: Throw a small quantity of cement into a bucket of water. Good cement should float for a few seconds before sinking, indicating proper fineness and absence of large impurities.

Hand Insertion Test

  • Objective: To detect any setting or reaction with moisture.
  • Method: Insert your hand into a bag of cement. It should feel cool, which means there’s no hydration (reaction with moisture) taking place.

Laboratory Tests for Cement

Vicat Apparatus Test

The Vicat Apparatus Test is used to determine the initial and final setting times of cement. This helps ensure that the cement sets neither too quickly (which makes working difficult) nor too slowly (which delays construction).

Apparatus Required
  • Vicat apparatus (with plunger, needle, and mold)
  • Measuring scale
  • Weighing balance
  • Stopwatch
  • Mixing tools (trowel, glass plate, etc.)
  • Water
  • Cement

Curing of Concrete

Curing is the process of maintaining satisfactory moisture content and temperature in concrete for a definite period immediately following placing and finishing, to ensure proper hydration of cement and hardening of concrete.

Key Objectives of Curing

  1. Ensure complete hydration of cement for strength gain.
  2. Prevent moisture loss to avoid surface cracking.
  3. Improve durability by reducing permeability.
  4. Reduce shrinkage and prevent cracking.
  5. Enhance long-term performance of concrete.

Methods of Curing

Various methods are employed for curing concrete, including water curing, membrane curing, and steam curing.

Water Curing: Ponding Method

Explanation of One Method: Water Curing (e.g., Ponding)

  • Process: A small “pond” of water is formed on flat concrete surfaces like slabs using clay or sand barriers. The surface is kept fully submerged in water.
  • Duration: Typically done for 7–14 days, depending on the type of cement and environmental conditions.

Precautions in Concrete Handling

During Transportation of Concrete

  1. Avoid Segregation: Use proper containers (wheelbarrows, buckets, transit mixers). Avoid dropping concrete from heights or using long chutes without control.
  2. Prevent Delay: Transport concrete quickly to avoid setting before placement. Maximum time between mixing and placing should be within 30–45 minutes, depending on weather.
  3. Protect from Weather: In hot weather: Cover concrete to avoid moisture loss. In rainy weather: Use waterproof covers to protect concrete.

During Placement of Concrete in Formwork

  1. Ensure Clean Formwork: Remove dust, debris, and water from formwork before placing concrete.
  2. No Free Fall from Height: Avoid free fall of concrete from more than 1.5 meters to prevent segregation. Use hoppers, chutes, or tremie pipes for controlled placement.
  3. Proper Compaction: Use vibrators (needle or surface) to compact concrete properly and remove air voids.

Field Tests for Fresh Concrete

Slump Test

  • Purpose: Most common field test for medium-workability concrete.
  • Apparatus:
    • Slump cone (300 mm high, 200 mm bottom dia, 100 mm top dia)
    • Tamping rod
    • Base plate
  • Procedure:
    1. Fill the cone with fresh concrete in 3 equal layers.
    2. Each layer is tamped 25 times with the rod.
    3. Cone is lifted vertically.
    4. Measure how much the concrete slumps (settles) from the original height.