The properties of concrete are a list of the qualities that concrete possesses. Defining these properties helps to clarify the concrete’s ability to provide structural integrity, durability, and functionality in various construction or design projects. Because concrete is such a widely used material, knowing its varied properties is essential for building professionals and homeowners who use it on a regular or periodic basis.
Understanding these concrete qualities will allow anyone to consider vital design and structural integrity, correct material selection, ensure performance and durability, and allow for innovation and optimization.
What Are The Properties of Concrete?
Concrete properties can be broken down into two main groups: those that it exhibits in its liquid state and those it exhibits in its hardened state.
Fresh Concrete Properties (Liquid Form)
Concrete exhibits certain qualities right after the hydration process which are the chemical changes that begin after the cement is mixed with water. The properties of concrete in its liquid or paste form are workability, setting time, bleeding, and segregation.
The property of workability describes the ease with which it can be mixed, transported, placed, and compacted without excessive segregation or bleeding. This property measures how easily the concrete can be molded and shaped during the construction process. A number of factors including the water content, the type and size of the aggregate, cement type and amount, admixtures, and mixing time and method influence the workability of concrete.
Diverse projects require different measures of workability. By assessing the workability of concrete, building professionals can decide which concrete is best for a particular project. Workability is addressed through various measures including a slump test, a Vee-bee test, or a compaction factor test depending on the type of concrete.
The setting time of concrete is the amount of time that it takes for the liquid concrete to form into a solid state. This is an important property because it determines the handling, placing, and finishing of a project. Many factors can impact the setting time of concrete including the concrete type, the water ratio, ambient temperature, and additional admixtures.
The setting time has two distinct phases: the initial setting time and the final setting time. The initial setting time is the time it takes concrete to go from its plastic form into a form that can no longer be changed. This is the initial phase where the concrete is gaining strength and rigidity. Manufacturers measure the initial setting time using a Vicat needle or other penetration resistance tests.
The final setting is the time that it takes for concrete to become completely rigid and when it is ready to withstand heavy loads. This time marks the end of the setting process and the beginning of other processes like curing or form removal. Concrete manufacturers measure this by penetration resistance tests.
The property of bleeding explains the phenomena in liquid or plastic concrete where water separates from the concrete mixture and rises to the surface. This creates an accumulation of water on the surface of the concrete. Bleeding in concrete is caused by the settling of solid particles because of gravity, leading to the displacement of the water. Excessive bleeding is not beneficial for concrete as it can cause surface deformation and reduced strength and durability.
Concrete manufacturers test their products to determine their bleeding qualities using particular tests like a bleeding cylinder test or a pressure plate test. Builders can determine and mitigate bleeding by the type of concrete, lower amount of water, use of a finer aggregate, and use of water-reducing admixtures.
Segregation in concrete refers to the non-uniform distribution of the ingredients throughout the concrete mixture. This happens when the solid particles in the mix, like the aggregate, separate from the other ingredients like the cement paste.
Multiple factors can impact segregation in concrete including aggregate size and density, the proportions of the concrete mix, improper handling and/or transportation, and inadequate consolidation or vibration. The consequences of segregation in concrete are non-uniformity of the mix which can impact structural integrity and durability. It can also cause surface defects. Some of the tests the manufacturers use to determine segregation in concrete are the sieve stability test and the flow table test.
Hardened Concrete Properties
Concrete manufacturers test hardened concrete at periodic intervals to test its specific properties, usually after the concrete has been allowed to cure.
The strength property of concrete measures its ability to withstand force while still maintaining its structural integrity. This is one of the most vital properties in determining its use and range of suitable projects. For standard concrete, the strength is measured after 28 days which is a sufficient amount of time to develop significant strength.
- Compressive Strength – The compressive strength property is the measure of a concrete’s ability to withstand compressive force without failing. It is the maximum load that concrete can withstand before it cracks, deforms, or collapses. Industry standards measure compressive strength in terms of megapascals (MPa) or pounds per square inch (psi).
- Tensile Strength – Tensile strength is the amount of stretching that concrete can withstand before incurring damage. Compression strength is the most important in concrete, but understanding tensile strength is vital for certain applications. The tensile strength of concrete is usually much lower than its compressive strength. Tensile strength is an important property in prestressed designs.
- Flexural Strength – Flexural strength is the ability of concrete to withstand bending or tensile forces. This property is dependent on the tensile strength of the concrete. The flexural strength is also called the “modulus of rupture”. This term refers to the maximum stress at failure in a bending test for a concrete beam or prism.
- Shear Strength – Shear strength is a property that measures the concrete’s ability to resist forces that cause a layer of the concrete to slide or deform parallel to another layer. This can happen when concrete beams, pillars, or slabs are subject to torsional stresses or bending.
Durability is the property that explains the concrete’s capacity to resist deterioration and maintain its aesthetic and performance qualities. This property is a critical component when designers and architects plan structures that will endure. The breakdown of concrete can occur due to causes that are physical, chemical, or mechanical. Multiple factors influence the durability of concrete including environmental exposure, the concrete mixture, the water-cement ratio, structural design, proper curing, and quality transportation and construction practices.
Concrete manufacturers test the durability of concrete through laboratory testing and field monitoring. They test the concrete’s permeability, chloride ion penetration resistance, alkali-silica reactivity, sulfate resistance, carbonation resistance, and resistance in freeze-thaw cycles.
The density property of concrete measures its mass per unit volume. It is a number which explains how much material is packed into a given space. Density impacts the strength, durability, thermal properties, and structural behavior of concrete. The components of a concrete mixture determine its density. This includes the type and quantity of aggregates, cement, water, and any supplementary materials.
The higher the density of concrete, the greater compressive strength and higher thermal mass. Density will affect a concrete’s workability. Higher-density concrete takes more expertise in mixing and handling.
Shrinkage refers to the property that describes the reduction in volume or dimension of hardened concrete due to the chemical reaction or the drying process. This is a natural occurrence that happens as the water evaporates from the concrete mixture. Shrinkage can lead to cracks and deformations in the concrete structure.
Concrete companies test their concrete to understand its potential shrinkage behavior. They are able to make recommendations to help builders reduce the shrinkage of their concrete through proper mix design and curing practices, vital joint placement, extra reinforcement, and admixtures that reduce the surface tension of the water within the concrete to allow more uniform drying.
The permeability property refers to the concrete’s ability to allow water and other liquids to pass through its pore structure. This quality directly affects the concrete’s durability and performance. The permeability of concrete is affected by the water-concrete ratio, aggregate gradation, cementitious materials, curing time, and certain additives.
Both high and low-permeability concrete is valuable in certain contexts, though concrete with low permeability is stronger and more durable. Concrete with high permeability is called porous or permeable concrete. Engineers use this type of concrete in water management applications.
The thermal properties refer to the concrete’s capacity to conduct, store, and resist the flow of heat. The property plays a significant role in the energy efficiency and thermal performance of a structure. There are several measures that determine the thermal properties of a particular type of concrete.
- Thermal Conductivity – Thermal conductivity measures how a material conducts heat. Concrete does not conduct heat well, but the denser the concrete, the better it conducts heat.
- Thermal Resistance – Thermal resistance, also the R-value, represents the concrete’s ability to resist the flow of heat. This is the opposite of conductivity.
- Thermal Mass – Concrete has a high thermal mass which refers to its ability to store and absorb heat. This is helpful in regulating the temperature from the indoors to the outdoors.
- Thermal Expansion and Contraction – Concrete can expand or contract with temperature changes. While these are relatively small changes for concrete, engineers must consider this in the design of the structure to accommodate these small movements.
Manufacturers of concrete use various tests to determine the thermal properties of concrete including thermal conductivity tests, differential scanning calorimetry (DSC), and thermal resistance calculations.