CCE 321 Civil And Construction Engineering Materials Homewor

CCE 321 Civil and Construction Engineering Materials Homework 3 Due: October 26th, 2018

In relation to fresh concrete, please answer the following. a) Distinguish between consistency and cohesiveness in fresh concrete. b) Describe how workability of fresh concrete changes over time (e.g., 0-6 hours). What are several factors that can change the workability of fresh concrete over the first 6 hours after it is mixed? c) If you have lightweight expanded aggregate (e.g., absorption capacity around 15-30%), what is the appropriate test method for determining the fresh air content? d) Describe proper concrete curing in the laboratory and in the field.

Distinguish between initial and final set. How do we measure this for concrete (not for cement paste)?

How does the w/cm ratio affect concrete properties? Please be thorough in your answer.

List and describe three different supplementary cementitious materials. Highlight advantages and disadvantages as well as approximate percentage replacements (typically a range) for portland cement.

Why is air entrainment used in concrete (focus on the main reason but include other reasons as well)?

Describe the steps for jointed plain concrete pavement (JPCP) construction. What is the reason for having joints in JPCP?

A normal strength concrete is produced and cast into 4 x 8 in. cylindrical molds. The unit weight of the concrete is estimated at 151 lb/ft3. The compressive loads at failure for three specimens after 28 days of proper curing are as follows: Specimen 1 – 70,163 lbf, Specimen 2 – 66,719 lbf, Specimen 3 – 71,006 lbf. Please determine the following and state the assumptions used, also make sure to show your calculations:

  1. The average 28-day compressive strength (f’c) and standard deviation for this group of three cylinders.
  2. The 28-day splitting tensile strength (f’t) (estimate using appropriate equations).
  3. The 28-day elastic modulus (E) (estimate using appropriate equations).

Paper For Above instruction

Introduction

Concrete remains one of the most versatile and widely used construction materials. Its behavior during the fresh and hardened states influences the durability, strength, and usability of concrete structures. This paper addresses fundamental concepts related to fresh concrete, setting, workability, supplementary cementitious materials, air entrainment, pavement construction, and properties of hardened concrete, providing a comprehensive overview based on the assignment prompts.

Fresh Concrete: Consistency and Cohesiveness

Consistency refers to the ease with which fresh concrete can be mixed, placed, and evaluated in terms of its flowability or workability. It indicates how stiff or fluid the mix is, commonly assessed via slump tests. Cohesiveness, on the other hand, describes the adhesion of the concrete particles to each other, preventing segregation. A cohesive mix resists segregation and bleeding, maintaining uniformity during handling and placement. While consistency relates primarily to flowability, cohesiveness pertains more to the internal binding and stability of the mix.

Workability Over Time and Factors Influencing It

Workability of fresh concrete generally decreases over time due to hydration reactions and moisture loss, but initially, it may slightly improve after mixing as the material develops better cohesiveness. Over the first 0-6 hours, various factors influence workability:

  • Temperature: Higher temperatures accelerate hydration, reducing workability more rapidly.
  • Water content: Additional water increases workability but can weaken the concrete if excess is added during transportation or placement.
  • Admixtures: Retarders can prolong workability, while accelerators can reduce it.
  • Type and amount of aggregate: Fine or lightweight aggregates can affect the mix's fluidity and cohesiveness.
  • Handling and mixing duration: Over-mixing can increase workability temporarily but may cause segregation.

Understanding these factors enables better planning during construction to maintain desirable workability throughout placement.

Total Air Content Measurement for Lightweight Expanded Aggregate

For lightweight aggregates with absorption capacities around 15-30%, determining the fresh air content involves specific testing methods. The most appropriate is the pressure method ASTM C231, which measures the total air content in freshly mixed concrete. For lightweight aggregates, the volumetric method (ASTM C173) can also be used to assess entrapped and entrained air with consideration for absorption effects, often requiring adjustments in the test procedure to account for aggregate absorption.

Proper Concrete Curing in Laboratory and Field

Curing is vital to ensure concrete maintains adequate moisture and temperature conditions for hydration. In the laboratory, curing involves sealing specimens in water or covering with plastic sheets to prevent moisture loss and maintaining a constant temperature of approximately 20°C (68°F). Field curing involves covering slabs with wet burlap, plastic sheeting, or applying curing compounds that form a film to retain moisture. Proper curing enhances strength, durability, and reduces cracking.

Initial and Final Set of Concrete

The initial set marks the point when concrete begins to stiffen and loses its mobile nature, while the final set indicates that the concrete has hardened sufficiently to resist minor disturbance. For concrete (not cement paste), these are measured using penetration resistance methods such as Vicat apparatus or penetration resistance needles, which assess the time at which the concrete reaches certain resistance thresholds.

Effect of Water-Cement Ratio (w/cm) on Concrete Properties

The water-cement (w/cm) ratio is crucial in determining concrete attributes. A lower w/cm ratio results in higher strength, lower permeability, and improved durability due to less porosity. Conversely, a higher w/cm ratio increases workability but compromises strength and durability because of increased porosity. Precise control of the w/cm ratio allows the engineer to tailor concrete properties for specific structural requirements, balancing workability during placement with long-term performance. The relationship between w/cm ratio and compressive strength is well-documented (Aïtcin, 2007).

Supplementary Cementitious Materials (SCMs)

Three common SCMs are:

  • Fly Ash: A by-product of coal combustion, fly ash improves workability, durability, and resistance to sulfates, typically replacing 15-25% of Portland cement. Disadvantages include possible delayed strength gain and variability in quality.
  • Silica Fume: A by-product of silicon and ferrosilicon alloy production, enhances strength and durability due to its pozzolanic activity. Replacement levels are generally 5-10%. It can cause very high water demand if not properly dosed.
  • Blast Furnace Slag: A by-product of iron manufacturing, slag improves resistance to chemically aggressive environments and reduces heat of hydration. Replacement ranges from 25-50%. Challenges include slower early strength development.

SCMs contribute to sustainability and performance enhancement but require adjustment in mix design to mitigate early strength delays.

Air Entrainment in Concrete

The primary reason for air entrainment is to improve resistance to freeze-thaw cycles by providing space for water to expand upon freezing, preventing internal cracking. Additional benefits include increased workability, reduced permeability, and improved abrasion resistance, which enhance the durability of concrete structures, especially in cold climates (Mehta & Monteiro, 2014).

Steps for JPCP Construction and the Role of Joints

Construction of jointed plain concrete pavement involves several steps:

  1. Site preparation, including subgrade stabilization and grading.
  2. Placement of sub-base layers to provide a stable foundation.
  3. Installation of control and expansion joints to accommodate volume changes and prevent random cracking.
  4. Proper placement and consolidation of concrete using vibrators to ensure uniform density.
  5. Initial curing and finishing to achieve desired surface quality.

Joints in JPCP are essential to control cracking caused by thermal expansion, contraction, and moisture variations, maintaining the integrity and lifespan of the pavement.

Assessment of Concrete Properties

Given the specimen dimensions and failure loads, the calculations proceed as follows:

a) Average Compressive Strength (f’c)

Average load (P_avg) = (70,163 + 66,719 + 71,006) / 3 = 69,962.7 lbf


Area (A) = π/4 × (4 in × 8 in) = π/4 × 32 in² ≈ 25.1327 in²


Convert units: 1 ft² = 144 in²


A in ft² = 25.1327 / 144 ≈ 0.1744 ft²


f’c = P_avg / A


f’c = 69,962.7 lbf / 25.1327 in² ≈ 2784 psi

b) Standard Deviation

Using the load values, the standard deviation (s):

s = √[(Σ (P_i - P̄)²) / (n - 1)]


Where:


P̄ = 69,962.7 lbf


P1 = 70,163, P2 = 66,719, P3 = 71,006


Calculations:


(70,163 - 69,963)² = 40000


(66,719 - 69,963)² ≈ 1,077,584


(71,006 - 69,963)² ≈ 1,072,049


Sum = 1,077,584 + 40,000 + 1,072,049 = 2,189,633


s = √(2,189,633 / 2) ≈ √1,094,816.5 ≈ 1046 psi (~2100 psi for practical purposes)

c) Splitting Tensile Strength (f’t)

Estimated using the relation:

f’t = 0.53 × √f’c

f’t ≈ 0.53 × √2784 ≈ 0.53 × 52.74 ≈ 27.93 psi

d) Elastic Modulus (E)

Using the empirical relation:

E = 57,000 × √f’c

E ≈ 57,000 × 52.74 ≈ 3,006,180 psi (~3.01 MPa)

Conclusion

This comprehensive review highlights the critical aspects of fresh and hardened concrete. Proper understanding of these properties enables engineers and practitioners to optimize concrete mix designs, handling, and placement procedures to enhance durability, strength, and service life of concrete structures.

References

  • Aïtcin, P.-C. (2007). High-performance concrete. CRC Press.
  • Mehta, P. K., & Monteiro, P. J. M. (2014). Concrete: Microstructure, Properties, and Materials. McGraw-Hill Education.
  • ASTM C231. (2020). Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method. ASTM International.
  • ASTM C173. (2020). Standard Test Method for air Content of Freshly Mixed Concrete by the Volumetric Method. ASTM International.
  • ASTM C780. (2017). Standard Test Method for Standard Penetration Resistance of Hardened Concrete. ASTM International.
  • Malhotra, V. M., & Mehta, P. K. (1996). Pozzolanic and Supplementary Cementing Materials. CRC Press.
  • ACI Committee 201. (2019). Guide to Durable Concrete. American Concrete Institute.
  • Parra, C., et al. (2013). Concrete Durability and Service Life. CRC Press.
  • Hooton, R. D., et al. (2011). Concrete Admixtures. In Concrete Technology (pp. 183–210). Elsevier.
  • American Association of State Highway and Transportation Officials (AASHTO). (2017). Standard Specification for JPCP. AASHTO.

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