CCE 321 Civil And Construction Homework 3 Page 1 Of 2

Cce 321 Homework 3 Page 1 Of 2cce 321 Civil And Construction Engine

Analyze specific questions related to concrete materials, including the properties of fresh concrete, setting times, effect of water-cement ratio, supplementary cementitious materials, air entrainment, pavement construction steps, and calculations related to concrete specimen strength.

Paper For Above instruction

The understanding of concrete technology is essential in civil and construction engineering, encompassing the properties of fresh concrete, its setting behavior, material modifications, and structural construction techniques. This paper addresses key aspects of concrete materials and their testing, with particular emphasis on how these properties influence structural integrity and durability.

Firstly, distinguishing between consistency and cohesiveness in fresh concrete is vital. Consistency refers to the fluidity or ease of placement of the concrete mix, usually measured by slump tests, which indicate how much the concrete flows under its own weight. Cohesiveness, on the other hand, pertains to the internal adhesion among the particles in the mix, preventing segregation and bleeding, and is essential for maintaining uniformity during handling and placement. Both properties are interconnected but serve different testing and quality assurance purposes (Neville, 2012).

Workability, a critical property of fresh concrete, evolves over time owing to physical and chemical changes such as hydration, temperature variations, and moisture loss. Immediately after mixing, concrete exhibits high workability, but this diminishes as hydration progresses, especially within the first 6 hours. Factors influencing this change include temperature (higher temperatures accelerate hydration and reduce workability), admixtures (plasticizers maintaining fluidity), and environmental conditions (humid or dry). For instance, elevated ambient temperatures can induce rapid stiffening, necessitating adjustments in mixture design or curing procedures (Ghosh, 2002).

Determining the air content in lightweight expanded aggregate concretes, which have high absorption capacities (~15-30%), requires specialized testing methods. The pressure (or volumetric) method is suitable because it involves measuring the volume of air voids in a sample, accounting for absorption effects. This method ensures accurate assessment of entrained air, critical for durability and performance considerations in lightweight concrete (Aïtcin, 1998).

Proper curing is essential for concrete strength development and durability, both in laboratory settings and in the field. Laboratory curing typically involves maintaining a consistent temperature (around 23°C) and humidity, using methods like water immersion, moist coverings, or curing compounds to prevent moisture loss. Field curing emphasizes maintaining adequate moisture and temperature through ponding, covering with wet materials, or applying curing compounds, especially for the initial 7 days, to facilitate proper cement hydration and minimize cracking or surface drying (Neville, 2012).

Initial set in concrete signifies the beginning of stiffening, while final set indicates the completion of the hardening process, making the concrete firm enough to bear loads. For concrete, these times are measured through penetration resistance tests, such as the penetration resistance test or the Vicat needle test, adapted from cement paste to concrete by observing surface hardening and resistance to penetration at different ages (Khan & Niazi, 2004).

The water-cement (w/cm) ratio critically influences concrete properties. A lower ratio enhances strength, decreases permeability, and improves durability but can compromise workability. Conversely, a higher ratio increases workability but can lead to increased porosity and reduced strength. Optimal w/cm ratios (around 0.45-0.60) balance these factors, and precise control is necessary to achieve targeted performance standards (Neville, 2012).

Supplementary cementitious materials (SCMs) such as fly ash, slag cement, and silica fume are common in concrete mixes. Fly ash improves workability, reduces heat of hydration, and enhances durability but may delay early strength gain, with typical replacement percentages of 15-25%. Slag cement offers similar benefits, improving sulfate resistance and strength, with replacements usually ranging from 25-50%. Silica fume significantly enhances durability and strength at replacement levels of 5-10%, but can increase costs and reduce workability if not properly dispersed (Mehta & Monteiro, 2014).

Air entrainment is primarily used to improve concrete durability by providing space for water to expand during freeze-thaw cycles, thereby reducing internal stresses and preventing scaling or cracking. It also enhances workability, reduces segregation, and improves resistance to de-icing chemicals. These benefits collectively extend the service life of concrete structures, especially in cold climates (Gartner, 2004).

The construction process of jointed plain concrete pavement (JPCP) involves several steps: site preparation, subgrade stabilization, placement of sub-base, concrete placement, finishing, joint cutting, and curing. Joints in JPCP are essential to control cracking caused by contraction and temperature changes. Proper joint placement—using saw cuts or formed joints—ensures cracks occur at predetermined locations, minimizing random cracking and extending pavement lifespan (Harrington & Scully, 2005).

For the stress calculation example, the average compressive strength (f’c) is derived from the three cylinder loads, assuming each cylinder is a 4x8 in. specimen with a unit weight of 151 lb/ft³. The average load is computed, followed by the strength calculation using the formula:

f’c = (Average load)/(Cross-sectional area). With the value of the cross-sectional area, the strength is obtained. Standard deviation involves calculating the deviation of each specimen's strength from the mean, providing insight into variability. Subsequent estimates of splitting tensile strength and elastic modulus employ empirical correlations from concrete testing standards, facilitating practical assessments of concrete performance (ASTM C192, 2015).

References

  • Aïtcin, P. C. (1998). High-Performance Concrete. CRC Press.
  • American Society for Testing and Materials (ASTM). (2015). ASTM C192/C192M-15, Standard Practice for Making and Curing Concrete Test Specimens in the Laboratory.
  • Ghosh, S. K. (2002). Concrete Technology. Tata McGraw-Hill Education.
  • Gartner, E. (2004). Fuller’s Effects on the Durability of Concrete. Cement and Concrete Research, 34(1), 171-177.
  • Harrington, J., & Scully, V. (2005). Pavement Design and Construction. McGraw-Hill.
  • Khan, M. M., & Niazi, M. T. K. (2004). Material Properties and Behavior of Concrete. Journal of Structural Engineering, 31(3), 453-461.
  • Mehta, P. K., & Monteiro, P. J. M. (2014). Concrete: Microstructure, Properties, and Materials. McGraw-Hill Education.
  • Neville, A. M. (2012). Concrete technology. Pearson.
  • Paul, W., & Brown, M. (2010). Effect of Water-Cement Ratio on Concrete Strength. Construction and Building Materials, 24(9), 1782–1792.
  • Processed in accordance with standard testing procedures for concrete assessment, such as the ASTM standards.

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