Comprehensive Notes on Concrete Materials Engineering

Overview of Concrete as a Construction Material

• Concrete is recognized as the most widely used construction material globally.
• Primary factors for its widespread use include:
  • Excellent resistance to water.
  • Ability to be molded and formed into a vast variety of shapes and sizes.
  • Status as one of the most cost-effective and readily available building materials.
• Fundamental Definition: A hard, compact building material created when a mixture of cement, sand, gravel, and water undergoes the chemical process of hydration.

Major Components and Composition of Concrete

• Concrete consists of two primary parts:
  • Paste: Comprising Portland Cement, Water, Air voids, and Admixtures.
  • Aggregates: Comprising fine and coarse materials.
• Concrete Mixture Proportions:
  • Cement: Traditionally consists of $4$ parts calcined limestones and $1$ part clay. These ingredients are mixed, burned, and pulverized. It is commercially sold in $40\,kg$ bags.
  • Sand (Fine Aggregates): Can be natural sand or well-graded crushed stone.
  • Gravel (Coarse Aggregates): Consists of crushed stone or blast furnace slags.
  • Water: Must be clean, potable, and free from substances that could be harmful to the mix.
  • Admixtures (Additives): Ingredients added immediately before or after mixing to modify specific properties as required by environmental or structural situations.

Concrete Characteristics and Performance

• Structural Strength: Concrete is inherently strong in compression but weak in tension. To address tensile and shear stresses, steel reinforcement is required.
• Versatility: Capable of taking any shape with a diverse range of surface finishes, patterns, and textures.
• Fire Resistance: Naturally high resistance to fire and relatively low cost.
• Liability: A major disadvantage is its self-weight, typically estimated at $150\,lbs/cu.ft$.
• Requirement for Forms: Necessary provision for molding and forming must be established before placing, setting, and curing.

The Water-Cement Ratio and Abram's Law

• The water-cement ratio is the primary controller for strength, durability, and water-tightness in hardened concrete.
• Abram's Law (D.A. Abrams, 1919): States that the compressive strength of concrete is inversely proportional to the ratio of water to cement.
• Effects of Water Volume:
  • Excessive water: Weakens the concrete after curing and leads to bleeding and laitance.
  • Insufficient water: Results in a dense mix that is difficult to place and lacks workability.
• Average Ratio: Recommended at $6\,gallons$ of water per $40\,kg$ bag of cement.
• Bleeding: The emergence of excess mixing water on the surface of newly placed concrete due to the settlement of solids.
• Laitance: A milky deposit containing cement and fine aggregates on the surface of concrete, often caused by overworking the mix or improper finishing combined with bleeding.

Concrete Strength Categories and Classifications

• General Strength Categories:

  • Low-strength Concrete: $<20\,MPa$ ($<3,000\,psi$).
  • Moderate-Strength Concrete: $20 - 40\,MPa$ ($3,000 - 6,000\,psi$).
  • High-Strength Concrete: $>40\,MPa$ ($>6,000\,psi$).

• Specific Mix Classes and Uses:

  • Class AAA: Ratio $1:1:2$ (Cement:Sand:Gravel); $4,500\,psi$; Used for pre-stressed and post-tensioned structures.
  • Class AA: Ratio $1:1.5:3$; $4,000\,psi$; Used for underwater retaining walls, shear walls, elevator core walls.
  • Class A: Ratio $1:2:4$; $3,500\,psi$; Used for footings, columns, piers, beams, girders, joists, and slabs.
  • Class B: Ratio $1:2.5:5$; $3,000\,psi$; Used for slabs on fill and non-load bearing walls ($4\text{-inch}$ CHB).
  • Class C: Ratio $1:3:6$; $2,500\,psi$; Used for concrete plant boxes and parapet walls.
  • Class D: Ratio $1:3.5:7$; $2,000\,psi$; Used for plant boxes, footpaths, walkways, and lean concrete.

Concrete Categories by Aggregate Weight

1. Normal Weight Concrete:
   • Density: $2,200\,to\,2,400\,kg/m^3$ ($140\,to\,150\,lb/ft^3$).
   • Components: Sand, gravel, crushed stone, and air-cooled blast-furnace slag.
2. Lightweight Concrete:
   • Density: $1,350\,to\,1,850\,kg/m^3$ ($90\,to\,120\,lb/ft^3$).
   • Aggregates: Expanded shale, clay, slate, and slag.
3. Insulating Lightweight Concrete:
   • Density: $250\,to\,1,450\,kg/m^3$ ($15\,to\,90\,lb/ft^3$).
   • Aggregates: Pumice, scoria, perlite, vermiculite, and diatomite.
4. Heavyweight Concrete:
   • Density: $2,900\,to\,6,100\,kg/m^3$ ($180\,to\,380\,lb/ft^3$).
   • Aggregates: Barite, limonite, magnetite, ilmenite, hematite, iron, and steel shot/punchings.

Physical and Mechanical Properties

• Creep: Permanent deformation resulting from long-duration stress over time.
• Shrinkage: Volumetric reduction during the curing process, controlled via expansion joints and steel reinforcement.
• Hardness: Relates to durability and aesthetics; improved by troweled or floated finishes that pull paste to the surface.
• Porosity: Presence of pores left by hydration or excessive evaporation/cracks; negatively impacts water-tightness.
• Durability: Often measured via the "Rapid Chloride Permeability Test," which assesses the flow of electrical current as a surrogate for permeability.
• Workability: Determined by the fresh concrete's rheological properties:
  • Yield Stress: Effort needed to initiate movement; correlates with slump test results.
  • Plastic Viscosity: Flow characteristics during movement.
• Self-Compacting Concrete (SCC): Offers improved strength, density, and volume stability; highly effective in confined areas where vibration is difficult. Schedule reduction is limited due to formwork time requirements.

Steel Reinforcement in Concrete

• Reinforcement (bars, strands, wires) is essential to absorb tensile and shear stresses.
• Functions: Provides tensile strength for bending members, resistance to shrinkage, and overall ductility.
• Design must be investigated by a competent structural engineer.
• Bar Size Designations:
  • #3: $3/8\text{-inch}$ ($10\,mm$)
  • #4: $1/2\text{-inch}$ ($12\,mm$)
  • #5: $5/8\text{-inch}$ ($16\,mm$)
  • #6: $3/4\text{-inch}$ ($20\,mm$)
  • #7: $7/8\text{-inch}$ ($22\,mm$)
  • #8: $1\text{-inch}$ ($25\,mm$)
  • #9: $1\text{-}1/8\text{-inch}$ ($28\,mm$)
  • #10: $1\text{-}1/4\text{-inch}$ ($32\,mm$)

Mortar and Grout Varieties

• Mortar: A mixture of cement, sand, and water used for masonry, plastering, and painting.
• Plaster: Composed of cement, lime, sand, and water.
• Grout: A thin liquid version of mortar with a considerable amount of water, used for filling joints.
• Paste: A simple mixture of cement and water.
• Types of Mortar:
  • Cement Mortar: Proportion of $1$ part cement to $2 - 8$ parts sand; must be used within one hour (before initial set).
  • Lime Mortar: Proportion of lime and sand; inferior in strength; sets with carbon dioxide and should not be used in damp/underground conditions.
  • Lightweight Mortar: Mixing wood powder/sawdust with cement or lime; used for heat and soundproofing.
  • Fire-resistant Mortar: Proportion of $1$ part aluminous cement and $2$ parts crushed fire bricks; used for ovens/fireplaces.
  • Mud Mortar: Cheapest and weakest; clayey soil mixed with water, prepared from soil free of grass and pebbles.

Cement: History, Manufacturing, and Chemistry

• Definition: A material with adhesive and cohesive properties used to bind aggregates and fill voids.
• History: Portland cement was patented in $1824$ by Joseph Aspdin, an English mason. He named it after the limestone on the Isle of Portland in the English Channel.
• Cement Types:
  • Pozzolan Cement: Derived from Pozzouli, Italy; hardens under water (hydraulic).
  • Portland Cement: Hydraulic cement primarily composed of hydraulic calcium silicates.
• Chemical Composition:
  • Calcium Oxide ($CaO$): $60\% - 65\%$
  • Silica ($SiO_2$): $20\% - 25\%$
  • Aluminum Oxide ($Al_2O_3$): $4\% - 8\%$
  • Ferrous Oxide ($Fe_2O_3$): $2\% - 4\%$
  • Magnesium Oxide ($MgO$): $1\% - 3\%$
• Major Compounds in Portland Cement:
  • Tri-calcium Silicate ($C_3S$): Responsible for initial set and early strength.
  • Di-calcium Silicate ($C_2S$): Hardens slowly; contributes to strength after $1\,week$.
  • Tricalcium Aluminate ($C_3A$): Liberates high heat during first few days; minor early strength developer.
  • Tetracalcium Aluminoferrite ($C_4AF$): Reduces clinkering temperature during manufacturing; contributes little to strength.

ASTM Standard Types of Portland Cement (C150)

• Type I/IA: General purpose (pavements, bridges, buildings).
• Type II/IIA: Moderate sulfate resistance; used when groundwater/soil has higher acid content; generates heat slower than Type I.
• Type III/IIIA: High early strength; used for quick form stripping or cold weather.
• Type IV: Low heat of hydration; used for massive structures like dams.
• Type V: High sulfate resistance; used for structures exposed to coal mine drainage.
• Blended Hydraulic Cements (ASTM C 595): Includes Type IS (Portland Blast Furnace Slag) and Type IP (Portland Pozzolan).

Setting, Hardening, and Laboratory Tests for Cement

• Setting Stages:
  • Initial Set: Paste begins to stiffen; measured as the time until a $1\text{-mm}$ needle penetrates to a depth of $25\,mm$ in a Vicat apparatus.
  • Final Set: Paste begins to harden and sustain loads.
  • Flash Set: Rapid development of rigidity with high heat and slow strength development.
  • False Set: Rapid rigidity without high heat; allows further strength development.
• Properties for Quality Control:
  • Fineness: Measured by the Blaine Air Permeability Test; affecting hydration rate.
  • Soundness: Ability to retain volume; checked for hard-burned free magnesia.
  • Consistency: Mobility of fresh paste; regulated via normal consistency method or flow test.
  • Loss of Ignition: Determined at $900^{\circ}C\,to\,1,000^{\circ}C$; indicates pre-hydration or carbonation.
  • Specific Gravity: Generally around $3.15$.
• Compressive Strength Test (Cement):
  • Ratio $1:3$ (Cement:Sand); Water-cement ratio $0.4:1$.
  • Specimens are $76\,mm$ cubes.
  • Standard average strengths: $115\,kg/cm^2$ ($11.5\,N/mm^2$) at $3\,days$ and $175\,kg/cm^2$ ($17.5\,N/mm^2$) at $7\,days$.
• Tensile Strength Test (Cement):
  • Uses briquette moulds with $8\%$ water and $1:3$ cement-sand ratio.
  • Requirement: $>20\,kg/cm^2$ at $3\,days$ and $>25\,kg/cm^2$ at $7\,days$.

Air Content and Chemical Admixtures

• Air Voids in Concrete:
  • Entrapped Air: Visible to the eye, non-uniform distribution due to insufficient compaction; not desirable.
  • Entrained Air: Barely visible bubbles, uniform distribution; desirable for durability.
  • Total Air: Sum of both; Typically $6\% \pm 2\%$ in concrete.
• Chemical Admixtures (AASHTO M 194):
  • Type A (Water-reducing): Reduces water needed by $5\% - 15\%$.
  • Type B (Retarding): Slows setting up to $4\,hours$; useful in hot weather.
  • Type C (Accelerating): Speeds up setting/hydration; commonly uses Calcium Chloride ($1\% - 2\%$ of cement mass).
  • Type D: Water-reducing and retarding.
  • Type E: Water-reducing and accelerating.
  • Type F: High Range Water-reducing (Superplasticizer); reduces water by $12\%\,or\,greater$.
  • Type G: High Range Water-reducing and retarding. Superplasticizers can reduce water by $25\% - 35\%$ and increase 24-hour strength by $50\% - 70\%$.

Transportation and Placement of Concrete

• The goal in transportation is to prevent modification of water-cement ratio, slump, and homogeneity while preventing segregation and bleeding.
• Transport Methods:
  • Mortar Pan: For small quantities; high exposure to drying.
  • Wheelbarrow: For ground level; pneumatic wheels used for rough ground to avoid vibration.
  • Crane and Bucket: Fast; for vertical and horizontal transport in high-rise construction.
  • Chute: For lower levels (trenches); slope should not be flatter than $1\,vertical\,to\,2.5\,horizontal$.
  • Transit Mixer: Capacity of $4\,to\,7\,m^3$; agitates at $2\,to\,6\text{-RPM}$. Wet mix must reach the site within $1\,to\,1.5\,hours$.
  • Pumps/Pipelines: Universal and reliable.
  • Tremie: Used for underwater transport; pipe diameter of $20\,cm$. Bottom end is closed with a plug before lowering into water.

Concrete Field Tests and Procedures

• Slump Test:
  • Apparatus: Cone with $12\text{-inch}$ height, $4\text{-inch}$ top diameter, and $8\text{-inch}$ bottom diameter.
  • Molding: $3\,layers$ with $25\,strokes$ per layer using a $16\,mm$ rod.
  • Allowable Slumps: Beams/Columns ($3\text{-inch}$); Slabs/Parapets ($2\text{-inch}$); Side walls/Arches ($4\text{-inch}$).
• Compressive Cylinder Test:
  • Uses metal cast cylinder ($12\text{-inch}$ high, $6\text{-inch}$ diameter).
  • Tested at $7\,to\,28\,days$.
• Flexural Strength Test:
  • Uses beam molds ($6 \times 6 \times 21\,inches$).
  • Molding: $2\,equal\,layers$, rodded once for every $2\,in^2$ of surface area.
  • Modulus of Rupture ($R$) Formulas:
    • Fracture in middle third: $R = \frac{PL}{Bd^2}$
    • Fracture outside middle third by $<5\%$: $R = \frac{3Pa}{Bd^2}$
    • Center-Point Loading: $R = \frac{3PL}{2Bd^2}$

DPWH Item 311: Portland Cement Concrete Pavement (PCCP)

• Material Requirements:
  • Cement: Type I Portland Cement unless otherwise specified. Type IP is allowed if trial mixes meet strength. Minimum content: $364\,kg$ per cubic meter.
  • Fine Aggregate: Natural sand or stone screenings; max $3\%\,passing\,0.075\,mm$ sieve.
  • Coarse Aggregate: Max $40\%\,wear$ loss; max $1\%\,passing\,0.075\,mm$ sieve.
  • Steel: Tie bars must be deformed; Dowels must be plain round bars (half length painted).
• Pavement Quality Requirements:
  • Slump: $40 - 75\,mm$ (not vibrated) or $10 - 40\,mm$ (vibrated).
  • Flexural Strength: Min $3.8\,MPa$ ($550\,psi$) by third-point method at $14\,days$.
  • Compressive Strength: Min $24.1\,MPa$ ($3,500\,psi$) for cores at $14\,days$.
• Construction Operations:
  • Mixing Time: $50\,to\,90\,seconds$ for site or central mixing.
  • Temperature Limit: Mixed concrete must not exceed $32^{\circ}C$ ($90^{\circ}F$).
  • Haul Time: Max $45\,minutes$ (non-agitator trucks); max $120\,minutes$ (truck mixers).
  • Joints: Longitudinal joints must be sawed before the end of the curing period. Construction joints require an interruption of $>30\,minutes$. No construction joint within $1.50\,m$ of an expansion/contraction joint.
  • Curing: Concrete must not be left exposed for more than half an hour between stages. Curing materials include burlap ($72\,hours$ saturation for straw curing), waterproof paper, white polyethylene, and liquid membrane (rate: $1\,L\,per\,3.5\,m^2$ or $4\,L\,per\,14\,m^2$).

Acceptance and Deficiency Penalties for Pavements

• Strength Acceptance: Satisfactory if the average of three sets of three consecutive tests equals or exceeds specified strength ($f_c'$) and no single test is deficient by more than $15\%$.
• Core Strength Criteria: Concrete is adequate if average core strength is $85\%$ of specified value, and no single core is less than $75\%$.
• Strength Deficiency Price Adjustment:
  • $<5\%$ deficiency: $100\%$ payment.
  • $5\%\,to\,<10\%$: $80\%$ payment.
  • $10\%\,to\,<15\%$: $70\%$ payment.
  • $20\%\,to\,<25\%$: $50\%$ payment.
  • $25\%\,or\,more$: $0\%$ payment.
• Thickness Deficiency Price Adjustment (Lot basis: $1000\,m$ single lane or $500\,m$ double lane):
  • $0-5\,mm$: $100\%$ payment.
  • $6-10\,mm$: $95\%$ payment.
  • $11-15\,mm$: $85\%$ payment.
  • $21-25\,mm$: $50\%$ payment.
  • $>25\,mm$: Remove/Replace (No payment).

Specialized Paving Techniques and Details

• Slip Form Method: Pavement constructed without fixed forms. Machine must vibrate full width and depth. Edge slump must not exceed $6\,mm$.
• Opening to Traffic: Opening occurs when test specimens reach minimum strength requirements. If no tests are conducted, opening occurs after $14\,days$.
• Steel Bar Retests: Permitted on two random specimens if original failure is within tight tolerances: within $14\,MPa$ for tensile, $7\,MPa$ for yield, or $2\%$ elongation.

Questions & Discussion

• What if a core is deficient by more than $25\,mm$?: Additional cores must be taken at no less than $5\,m$ intervals parallel to the centerline in each direction until a core within the $25\,mm$ deficiency limit is found.
• When is hand finishing allowed?: Only in events of mechanical breakdown (for concrete already on the grade) or in narrow/irregular dimensions where machines are impractical.