Concrete Science: Materials, Safety, and Professional Practices

Posted on Jun 4, 2025 in Other subjects

Laboratory Safety and Chemical Management

Chemical Hazards and Safe Handling

• Shrinkage-Reducing Admixture: Product or product residue may explosively combust.
• Superplasticizer: Kills fish and ruins water; never pour down a drain!
• Colloidal Silver: Known to cause birth defects.
• Silver Nitrate Solution: A poison, oxidizer, and corrosive.
• Nitric Acid + Nitrobenzene: This combination results in a spontaneous explosion. Always ensure separate storage for acids versus bases.

Essential Laboratory Personal Protective Equipment (PPE)

• Disposable Safety Gloves:
  • One-time use.
  • Remove and discard when exiting the lab.
• Neoprene Gloves:
  • Designed for handling acids and bases.
  • Wash the outside thoroughly before removing so the next person can use them safely.
• Lab Coats:
  • Protect skin from chemicals, broken glass, and abrasions.
  • Protect your experiments from outside contaminants.

Effective Teamwork and Communication

Stages of Team Development: Bruce Tuckman’s Model

• Forming: Most members are positive and polite. Roles are not yet defined, and some anxiety may be present. Leaders play an important role.
• Storming: Team members may push against boundaries, express resentment at defined roles, and challenge authority. Different work styles can cause friction, and boundaries are tested.
• Norming: Differences are resolved, and members develop growing respect for each other. The team recommits to shared goals.
• Performing: The team is actively doing work and delegating tasks efficiently. Members joining or leaving cause few problems.
• Adjourning: This stage may be stressful for those who prefer structure, as the team disbands.

Understanding Communication Styles

• High-Context Culture:
  • Appropriate communication depends on decoding the situation, the relationship, and non-verbal behavior (the context).
  • Invest time in getting to know people.
• Low-Context Culture:
  • Appropriate communication relies on using concrete, logical, unambiguous, and task-oriented language.
  • Communication should be explicit and transparent; personal relationships, while beneficial, are not strictly necessary for effective communication.

Fostering Diverse and Effective Teams

Teams produce the best, most widely applicable results when they have “…the widest possible range of personalities, even though it takes longer for such psychologically diverse teams to achieve good cooperation. They must first cultivate an openness to opposing opinions and recognize the value of exploring a problem from various angles.”

Key Elements for Team Success

Consider these essential components for effective team function:

• Reflection
• Team Charter
• Task List (generic, with rotation)
• Problem Solving
• Communication Styles

Note: Chapter 8 often provides valuable insights on how to troubleshoot team problems.

Fundamentals of Concrete

The History of Cement and Concrete

• John Smeaton (1756): While seeking a new material for the third Eddystone Lighthouse, Smeaton discovered that limestone and clay, when burned and ground, harden underwater.
• “Roman Cement” (1780): Patented in 1780, leading to the emergence of competitors around 1810.
• Joseph Aspdin (1824): Created Portland cement on his stove in 1824.
• William Aspdin (1842): Joseph’s son, William, developed a better version in 1842.

Understanding Concrete Composition

• Clinker: The primary output from cement factories.
• Clinker + Gypsum: Forms Cement, the powder purchased in stores.
• Cement + Water: Creates Cement Paste.
• Cement Paste + Sand: Forms Mortar.
• Cement Paste + Sand + Gravel: Forms Concrete.
• Concrete also incorporates various admixtures.
• Note: The ‘p’ in Portland cement is not capitalized.

Global Impact and Properties of Concrete

• Annual Production:
  • Approximately 2,800 million tons (Mt) of Ordinary Portland Cement (OPC).
  • Approximately 6,600 Mt of concrete.
  • Roughly 1 cubic meter of concrete is produced per person per year globally.
• Environmental Impact:
  • OPC consumes 5% of the worldwide industrial energy supply.
  • A ton of cement produces at least 0.85 tons of CO2.
  • Concrete production is the 3rd largest CO2 producer, accounting for roughly 5% of global emissions.
• Typical Composition by Volume:
  • ~40 vol.% cement paste
  • ~60 vol.% aggregates
• Key Properties:
  • Good compressive strength.
  • Poor tensile strength, which is why reinforcing steel is used.
  • Utilizes universal raw materials.
  • Holds water without corroding.
  • Malleable (when fresh).
  • Very cost-effective.

Diverse Types of Concrete

• Normal Concrete (the focus of this discussion)
• Pre-cast Concrete
• High-Performance Concrete (HPC)
• Ultra-High Performance Concrete (UHPC), often featuring fiber reinforcement.
• Shotcrete (famously cited by Mike Rowe as his worst job).

Aggregates: Essential Concrete Components

Aggregate Quality and Characteristics

• Quality Impact: Aggregate quality significantly affects concrete performance.
  • Grading: Refers to particle size and distribution.
  • Nature: Includes shape, porosity, and texture.
• Economic Control: Aggregates control economics.
  • Use the largest possible aggregate size, depending on reinforcing steel, slab thickness, and other factors.
  • Maximum aggregate size should be no more than 1/5 the narrowest dimension between forms or 3/4 the space between reinforcing bars.
• Workability: Aggregate grading can influence concrete workability.

Types of Aggregates

• Coarse Aggregates:
  • Approximately 80% are sedimentary rocks, typically found near the surface.
  • Generally range between 3/8 and 1.5 inches in diameter.
  • Commonly gravels or crushed stone.
• Fine Aggregates:
  • Natural sand or crushed stone.
  • Must pass through a 3/8-inch sieve.
  • Often sourced from riverbeds or beaches, but chlorides must be washed off first.
  • Consideration: Why don’t we grind quarried materials for fine aggregates?

Critical Aggregate Properties

• Cleanliness: Essential to prevent anything from hindering a good bond with the cement paste.
• Shape: Aggregates that are too flat can lead to poor mixing and anisotropy.
• Texture:
  • Rough texture promotes a better bond.
  • Smooth texture improves mixability and flow.
• Isotropic: “Flakiness” is undesirable as it indicates anisotropy.
• Non-Reactive: Aggregates should contain no impurities or substances that will react with aggressive media.

Common Aggregate Reactions and Issues

• Alkali-Silica Reaction (ASR): Cement reacts with silica in aggregates to form an expansive gel, causing cracking and deterioration.
• Alkali-Carbonate Reaction (ACR): Cement reacts with carbonates in aggregates to form different carbonates, also leading to expansion and damage.
• Sulfate Attack: Occurs in many different forms. It turns cement paste into mush, while aggregates typically remain unaffected.

Mastering Technical Communication

This section is prepared with materials provided by Lorraine Higgins, WPI Writing Center.

Principles of Effective Writing

Proper Preparation Prevents Poor Performance

Before you begin writing, ask yourself these critical questions:

• What am I trying to say?
• Who is my target audience?
• What do I aim to achieve by writing this?

Ensure you have:

• Enough time.
• Enough sleep.
• A quiet place to work.

Structuring Your Research Document

Crafting a Compelling Introduction

What is this lab about? Why should anyone care?

• Introduce the topic you are investigating.
• Explain the opportunity or problem that calls for this research.

Presenting Background and Previous Work

What technical concepts must I know to understand this work? How does this build on the work of others?

• Define essential background concepts.
• Review and evaluate previous studies.
• Cite sources appropriately.
• Provide an overview of the objectives and parameters of the current study.

Detailing Materials and Methods

What did you do? How did you do it?

• Explain materials and methods clearly, using the past tense.
• Include visuals (e.g., diagrams, photos) as necessary.
• Show formulas and refer to relevant standards as needed.

Reporting Your Findings (Results)

What happened? Where’s the evidence?

• Use clear section headings as appropriate.
• Briefly summarize each key finding.
• Support findings by referring to figures and tables or discussing key measures.
• Refer to appendices for raw data.
• Remind your reader how you arrived at key findings.

Interpreting Results and Implications (Discussion)

What does it all mean? What are the implications?

• Summarize key findings and describe patterns and relationships.
• Interpret findings and provide context.
• Discuss any anomalies (though not shown in this example).

Remember: Your results are your CLAIM, and your discussion is your SUPPORT.

Drawing Meaningful Conclusions

How has your research changed anything?

• Briefly summarize the entire paper.
• Point out implications and recommendations for future work.

Building Credibility and Ethical Practices

According to Williams and Ireton, credibility is built through:

• Credibility of self.
• Credibility of authorship.
• Credibility of words.
• Credibility of structures.
• Credibility of ideas.

Concrete Mix Design and Proportioning

Principles of Mix Design

• Mix Design: Determining the required characteristics of your concrete, reflecting the final usage of the structure.
• Mixture Proportioning: Selecting the specific ingredients for your concrete.

Properly proportioned concrete should:

• Have acceptable workability.
• Exhibit good durability, strength, and appearance.
• Be economically feasible.

Keep it simple! Overly complex designs are difficult to control.

Factors Influencing Concrete Strength and Durability

• Concrete is usually selected based on strength, though durability and permeability are becoming increasingly important.
• Minimum cement contents are crucial for protecting finishability and durability.
• Water-to-Cement (w/c) Ratio:
  • Determines workability.
  • Strength is inversely proportional to water content (more water leads to more porosity and less strength).
  • The w/c ratio should be the lowest value required for anticipated conditions.
• Curing: Concrete gains strength over time as hydration reactions continue. Temperature and humidity are the most important factors in curing.
• Specified Compressive Strength (f’c):
  • f’c is the specified compressive strength at 28 days (average of 3 samples).
  • ACI 318 requires a minimum f’c of 17.5 MPa (2500 psi). No single sample can be 3.5 MPa below the average.
  • Always aim slightly higher than the required strength to incorporate a safety factor. The required average compressive strength (f’cr) is f’c plus a specified margin.
• Entrained Air:
  • Reduces freeze-thaw damage.
  • Applicable for mild, moderate, or severe exposure categories.
  • The amount depends on aggregate properties.
  • Can reduce the amount of water needed.
  • Targets can be challenging to hit; for example, for a 6% target, aim for a range of 5–8%.

Admixtures for Concrete Performance

• Water-Reducing Admixtures (WRA): Improve workability, allowing a reduction of water up to 12%.
• High-Range Water-Reducing Admixtures (Superplasticizers): Reduce water content by 12-30%.
• Other Common Admixtures:
  • Anticorrosion agents
  • Accelerators (to speed up setting)
  • Set retarders (to slow down setting)
  • Colorants

Important: Multiple admixtures should always be tested together beforehand to ensure compatibility.

Practical Steps for Mix Proportioning

When starting, consider using a previous mix design with an f’c within 7 MPa (1000 psi) of your requirements, assuming materials and characteristics are similar.

Determine if f’cr is met based on the number of trials:

• For f’c < 35 MPa, typically 2 trials are sufficient.
• For f’c > 35 MPa, typically 3 trials are sufficient.

Example: Calculating Mix Proportions

Step 1: Determine Required Strength (f’cr)

Based on Table 9-1, if f’c is 35 MPa (greater than 31 MPa), we proceed with 35 MPa.

Using Table 9.11, f’cr = f’c + 8.5 MPa.

Calculation: 35 MPa + 8.5 MPa = 43.5 MPa.

Step 2: Determine Water-to-Cement (w/c) Ratio

(Implicitly derived from f’cr and other factors, leading to a value like 0.31 in the example below.)

Step 3: Determine Air Content

(This step would involve consulting tables based on exposure conditions and aggregate type.)

Step 4: Determine Slump

(This step involves selecting the desired workability.)

Step 5: Determine Water Content

(This step involves calculating the total water needed based on slump and aggregate type.)

Step 6: Calculate Cement Content

Using simple math: 135 kg of water per m³ divided by a w/c ratio of 0.31 yields 435 kg of cement per m³.

Note: This calculated value (435 kg/m³) is greater than the minimum 310 kg/m³ from Table 9.7, which is acceptable.

Step 7: Calculate Coarse Aggregate Content

From Figure 9.3, assume a volume fraction of 0.67.

Given a unit weight of 1600 kg/m³: 1600 kg/m³ × 0.67 = 1072 kg (dry) of coarse aggregate.

Step 8: Calculate Admixture Content

• From the Material Safety Data Sheet (MSDS), assume 8% air content requires 0.5g of air-entraining admixture (AEA) per kg of cement: 0.5 g/kg × 435 kg = 0.218 kg AEA.
• Water-reducing admixture (WRA) is used at 3g per kg of cement: 3 g/kg × 435 kg = 1.305 kg WRA.

Step 9: Calculate Fine Aggregate Content

First, sum the volumes of all other known materials per m³:

• Water: 135 kg / (1 × 1000 kg/m³) = 0.135 m³
• Cement: 435 kg / (3 × 1000 kg/m³) = 0.145 m³
• Coarse Aggregate: 1072 kg / (2.68 × 1000 kg/m³) = 0.4 m³
• Air (as a volume percentage): 8/100 = 0.08 m³

Total volume of known materials: 0.135 + 0.145 + 0.4 + 0.08 = 0.76 m³.

Therefore, the volume of fine aggregate (FA) is: 1 m³ – 0.76 m³ = 0.24 m³.

To convert to mass: 0.24 m³ × 2.64 (relative density) × 1000 kg/m³ = 634 kg (dry) of fine aggregate.

Summary of Dry Mix Ingredients (per m³)

• Water: 135 kg
• Cement: 435 kg
• Coarse Aggregate (dry): 1072 kg
• Fine Aggregate (dry): 634 kg
• Air-Entraining Admixture (AEA): 0.218 kg
• Water-Reducing Admixture (WRA): 1.305 kg

Note: The volume of these admixtures is typically very low and can often be ignored in volume calculations. However, other admixtures, such as corrosion inhibitors, can be very large and must be accounted for in the w/c ratio and volume calculations.

Adjusting for Aggregate Moisture

Aggregates often contain surface moisture that contributes to the total water content of the mix. We need to adjust the added water based on the aggregate’s moisture content and absorption.

• Coarse Aggregate (CA) adjusted mass: 1072 kg × 1.02 (assuming 2% surface moisture) = 1093 kg.
• Fine Aggregate (FA) adjusted mass: 634 kg × 1.06 (assuming 6% surface moisture) = 672 kg.

Calculate the effective surface water contributed by aggregates:

• CA surface water: 2% (total moisture) – 0.5% (absorption) = 1.5% effective surface water.
• FA surface water: 6% (total moisture) – 0.7% (absorption) = 5.3% effective surface water.

Therefore, the total water to be added to the mix is:

135 kg (initial water) – (1072 kg CA × 0.015) – (634 kg FA × 0.053) = 85 kg.

Final Adjusted Mix Ingredients (per m³)

• Water: 85 kg
• Cement: 435 kg
• Coarse Aggregate (moist): 1093 kg
• Fine Aggregate (moist): 672 kg
• Air-Entraining Admixture (AEA): 0.218 kg
• Water-Reducing Admixture (WRA): 1.305 kg