Concrete: Properties, Types, and Production

Posted on Dec 7, 2024 in Physics

Concrete

Material consisting of cement, water, and aggregates. Additives can be added to improve its characteristics. Concrete is a material with good compressive strength and poor tensile strength. The aggregates do not act directly on the setting of concrete; they make up approximately 80% of the mix. Aggregates provide increased strength, durability, abrasion resistance, and tensile strength. They also give more grip to the mix but make it less workable. Fresh concrete has to have plasticity to mold easily.

Consistency

This refers to the ease with which concrete can be deformed. It is measured by the Abrams Cone test. Depending on the slump formed by removing the cone, we can say that consistency is: dry (0-2), plastic (2-5), soft (5-9), fluid (10-15), and liquid (≥ 16).

Workability

This is the ability of fresh concrete to be placed on-site with the available equipment and normal compaction, with minimal loss of homogeneity.

Uniformity

This refers to the proper mixing of concrete components. Uniformity can be lost by segregation or exudation.

Additives

Additives are added to the mix before or during mixing, changing the characteristics of fresh or hardened concrete. Their aim is to improve its behavior. Types of additives include:

  • Air-entraining agents: Produce bubbles inside the concrete to improve workability and increase resistance to frost and aggressive water. If the percentage is excessive, a loss of strength may occur.
  • Water reducers: Allow for more workability with the same water content. Secondary functions include retarding the setting process, increasing mechanical strength, and decreasing permeability. Secondary effects can include increased shrinkage of the concrete.
  • Superplasticizers: Increase workability. More water results in less resistance and vice versa. A secondary function is to retard the setting. Secondary effects include an increase in shrinkage, and segregation can occur with an overdose.
  • Plasticizers: Increase workability. Secondary functions include slightly delaying the setting, improving water resistance, reducing segregation, and increasing durability. Secondary effects may include increased shrinkage.
  • Setting accelerators: Speed up the setting time. Secondary functions include increasing initial resistance and reducing the risk of segregation and the amount of water needed. Secondary effects include decreased final strength of the concrete, corrosion of reinforcement, and increased shrinkage.
  • Setting retarders: Slow the setting time. Secondary functions include enhancing long-term resistance. Secondary effects include lower initial resistance and increased shrinkage.
  • Waterproofing agents: Increase the compactness of the concrete. Secondary effects include decreased resistance and increased shrinkage.

Dosage

Dosage is the process of determining the proportions in which to mix the components to achieve a number of features. There are several methods, including Fuller, Bolomey, Faury, and De la Peña.

Manufacturing, Transportation, and Placement

Manufacturing is the kneading of the mixture to ensure the cement coats the aggregates. For transportation, two methods can be used: intermittent or continuous. The placement may be by pouring, pumping, injection, or using a Gamito.

Consolidation and Compaction

Compaction eliminates voids without segregation. To do this, we have three methods:

  • Tamping: Using a bar to compact the concrete.
  • Vibration: Using a vibrating bar inserted into the concrete to eliminate gaps.
  • Vibration with compression: Using vibrating rollers for large areas.

Curing

Curing prevents water loss and controls the temperature of the concrete. Its aim is to ensure that the space occupied by water is occupied by the hydrated products of the concrete. Curing is considered complete when the concrete reaches 70% of its strength.

Hardened Concrete Characteristics

Physical characteristics depend on the elements used and the conditions to which the concrete is subjected. Characteristics of concrete include:

  • Density: Higher density results in greater durability and strength.
  • Waterproofing: Depends on the percentage of pores and determines resistance to aggressive agents.
  • Compressive Strength: Concrete is very strong in compression. Cylinders are used for testing.
  • Characteristic strength: The value of resistance below which no more than 5% of breaks are expected. This follows a normal distribution (bell curve) and is used to determine the strength of concrete.
  • Tensile strength: Approximately 10% of compressive strength.
Factors Affecting Concrete Strength
  • Determination of components and additives, water-cement ratio.
  • Conditions during placement.
  • Placement methods, including segregation and compaction.
  • Curing process.
Durability

Durability is the ability of concrete to resist attacks from the environment, including physical, chemical, and biological factors. For good durability, proper mixing, compaction, and curing are essential. Physical attacks include ice-melting, cracking, surface wear, high temperatures, and chemical attacks from acids, sulfates, seawater, and corrosion of reinforcement.