Concrete Fundamentals: Hardened Concrete

Hardened Concrete

• Hardened concrete is made with siliceous rounded gravel which provides better workability, or crushed limestone.
• Curing is the process of maintaining a satisfactory moisture content and temperature in concrete for a suitable period of time immediately following placing and finishing.
• Factors that affect the strength of concrete include:
  • Unhydrated cement still present
  • Moisture content above 80%
  • Favorable temperature
  • Sufficient space for hydration products to form
• Curing has a significant effect on strength development in concrete.

Compressive Strength

• Compressive strength is defined as the measured maximum resistance of a concrete or mortar specimen to an axial load, usually expressed in MPa at an age of 28 days.
• General use concrete typically has a compressive strength of 20 to 35 MPa, while high-strength concrete has a compressive strength of 70 MPa or greater.
• Testing of compressive strength involves the use of specific test specimen sizes, such as 50 mm cubes for mortar and 100 x 200 mm high cylinders for concrete.
• Approximations of concrete strengths include:
  • 7-day strength is approximately 75% of 28-day strength
  • 56 and 90-day strength is approximately 10-15% greater than 28-day strength

Density

• Density of concrete can vary depending on the type of concrete, with normal concrete typically having a density of 2200 to 2400 kg/m3, reinforced concrete having a density of 2400 kg/m3, and low-density insulating concrete having a density of 240 kg/m3.
• High-density concrete can have a density of up to 6000 kg/m3, and is often used in applications such as radiation shielding and counterweights.

Watertightness and Permeability

• Watertightness refers to the ability of concrete to hold back or retain water without visible leakage.
• Permeability refers to the amount of water migration through concrete when the water is under pressure or the ability of concrete to resist penetration by water or other substances.
• The relationship between hydraulic permeability, w/c ratio, and initial curing is important to understand.

Abrasion Resistance

• Abrasion resistance is critical in applications such as floors, pavements, and hydraulic structures.
• Concrete must have high resistance to abrasion in these applications.
• Abrasion resistance is closely related to the compressive strength of concrete.

Volume Stability and Crack Control

• Concrete expands slightly when kept continuously moist, and shrinks when permitted to dry.
• Shrinkage is influenced by factors such as water content, aggregate properties, concrete mass size, relative humidity, temperature, curing method, and degree of hydration.
• Joints are an effective method for controlling unsightly cracking, and the three main types of joints are:
  • Contraction joints
  • Isolation joints
  • Construction joints

Durability

• Freezing and thawing while the concrete is wet is the most destructive weathering factor.
• Resistance to freezing and thawing can be improved with air-entrained concrete and low w/c ratio.

Alkali-Aggregate Reactivity (AAR)

• AAR is a reaction between active mineral constituents of some aggregates and the sodium and potassium alkali hydroxides and calcium hydroxide in the concrete.
• Alkali-Silica Reaction (ASR) and Alkali-Carbonate Reaction (ACR) are types of AAR.
• Methods to mitigate ASR include using non-reactive aggregates, SCMs, blended cements, and limiting concrete alkali content.

Carbonation

• Carbonation can lead to corrosion of embedded steel.

Chloride Resistance and Steel Corrosion

• Chlorides can corrode embedded steel in concrete.
• Methods to reduce corrosion include using low w/cm ratio concrete, moist curing, reducing permeability with SCMs, increasing concrete cover, corrosion inhibitors, epoxy-coated reinforcing steel, concrete overlays, surface treatments, and cathodic protection.

Sulphate Attack

• Sulphate attack can be mitigated by using low w/cm ratio concrete and sulphate-resistant cement.

Concrete Exposed to Seawater

• Concrete exposed to seawater is most vulnerable in the tidal or splash zone where there are repeated cycles of wetting and drying.

Properties of Concrete

• Permeability is the ability of concrete to resist penetration by water or other substances, and is influenced by the water pressure or the ability of water to migrate through the concrete.
• The relationship between hydraulic permeability, W/C-ratio, and initial curing is crucial in concrete design.

Abrasion Resistance

• Floors, pavement, and hydraulic structures are subjected to abrasion, and therefore require high resistance to abrasion.
• Abrasion resistance is closely related to the compressive strength of concrete.
• The effect of compressive strength and aggregate type on the abrasion resistance of concrete is significant.

Volume Stability and Crack Control

• Concrete expands slightly when kept continuously moist, but shrinks when permitted to dry.
• Shrinkage is mostly influenced by the water content of the freshly mixed concrete, but other factors such as aggregate amount, properties, size, and shape of the concrete mass, relative humidity, temperature, curing method, degree of hydration, and time also play a role.
• Joints are an effective method for controlling unsightly cracking, and there are three main types: contraction joints, alkali-silica reaction (ASR) joints, and alkali-carbonate reaction (ACR) joints.

Alkali-Silica Reactivity (ASR)

• ASR can be mitigated by using non-reactive aggregates, supplementary cementing materials (SCMs), blended cements, and limiting concrete alkali content.
• Testing for ASR effectiveness is crucial.

Durability

• Durability is a critical property of concrete, and it can be affected by various factors such as carbonation, chloride resistance, sulphate attack, and exposure to seawater.

Carbonation

• Corroded steel can occur due to carbonation, which can lead to structural damage.

Chloride Resistance and Steel Corrosion

• Methods to reduce corrosion and embedded steel by chlorides include using low w/cm ratio concrete, moist curing, reducing permeability with SCMs, increasing concrete cover, using corrosion inhibitors, epoxy-coated reinforcing steel, concrete overlays, surface treatments, and cathodic protection.

Sulphate Attack

• Sulphate resistance can be increased by using low w/cm ratio concrete (e.g., 0.4) and sulphate-resistant cement (Type MS and Type HS).

Concrete Exposed to Seawater

• Concrete structures exposed to seawater are most vulnerable in the tidal or splash zone, where repeated cycles of wetting and drying occur.
• Isolation joints and construction joints are used to separate a slab from other parts of a structure and to permit horizontal or vertical movement of the slab.

Resistance to Freezing and Thawing

• Freezing and thawing while the concrete is wet is the most destructive weathering factor.
• Air-entrained concrete with low w/c ratio is more resistant to freezing and thawing than non-air-entrained concrete with high w/c ratio.

Alkali-Aggregate Reactivity (AAR)

• AAR is a reaction between the active mineral constituents of some aggregates and the sodium and potassium alkali hydroxides and calcium hydroxide in the concrete.