Understanding the properties and characteristics of concrete

by Sally Bouorm | December 1, 2012 9:19 am

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By John Petrocelli

Structures surround the modern world. Buildings, bridges, roads, and tunnels are everywhere. By definition, a structure is something made up of parts that are held or put together in a particular way. In construction, structures are designed by architects and engineers, and built by contractors or builders using many different materials. The most common, however, are concrete, steel, timber, plastic, and glass.

Although a variety of materials can be used to create a structure, this article will focus on two primary materials used in the pool industry—concrete and steel. It will also discuss the characteristics and properties of these materials as well as look at how ‘hybrid’ materials incorporate the best of both to create a ‘super’ structural material.

Concrete facts

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In general, concrete is one of the most common materials used in the pool-building and construction industry in general. It comprises a heterogeneous mixture of the following main constituents:

• Cement (Portland);
• Water;
• Sand (minerals);
• Stone (rocks); and
• Air.

Together, sand and stone are referred to as ‘aggregates.’ Although there are many other ingredients used in modern day concrete mix (e.g. chemical admixtures), the elements above represent the most common. There are also many types of cement, including:

• Type 10: normal Portland cement;
• Type 20: moderate Portland cement;
• Type 30: high-early-strength Portland cement;
• Type 40: low-heat-of-hydration Portland cement; and
• Type 50: sulphate-resistant Portland cement.

Normal Portland cement (i.e. Type 10) is general-purpose cement and is the most popular cement powder used to create regular density concrete. This type of concrete is used in pavements, sidewalks, reinforced-concrete structures, bridges, and swimming pools.[3]

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The other cement varieties are used in special applications where specific properties are required. For instance, the use of low-heat-of-hydration cement (i.e. Type 40) in concrete mixes is often used to construct massive structures such as hydro-electric dams, where a tremendous amount of concrete is required.

If normal Portland concrete was used in such an application, the heat generated by the concrete as the cement hydrated would be so extreme it would actually destroy the structure.

Concrete for pools

Normal Portland cement is manufactured in a plant and comprises the following raw materials:

• Lime (CaO) from alkali waste, calcite, cement rock, chalk, clay, and seashells;
• Iron (Fe) from blast furnace flue dust, iron ore, mill scale, and ore washings;
• Silica (SiO₂) from fly ash, sand, shale, marl, quartzite, and rice hull ash;
• Alumina (Al₂O₃) from aluminum ore waste, bauxite, and copper slag;
• Gypsum (CaSO₄·2H₂O) from anhydrite, calcium sulphate, and gypsum; and
• Magnesia (MgO) from cement rock, limestone, and slag.

These raw materials are properly proportioned (with a maximum of five per cent limestone when creating normal Portland cement) and processed in a modern-day manufacturing plant to produce cement powder.

It is interesting to note, approximately 50 per cent of all industrial byproducts (from other types of manufacturing) have potential as raw materials for manufacturing cement powder. As such, concrete is an extremely ‘green’ building material with important environmental benefits.

The manufacturing process

The first step is the quarrying and blending of raw materials using drilling, blasting, and crushing operations. Once obtained, the materials are stored in separate silos and fed into a roller mill in the proper proportions where they are combined with hot gasses; dry mixed, and blended in additional silos. The mixed-and-blended material is then stored until being fed into a kiln system where it is preheated, burned, cooled, and stored as a ‘clinker’ (i.e. dark grey lumps or nodules), which are typically 3 to 25 mm (0.11 to 1 in.) in diameter.

At this stage, the ‘cement’ can be stored for long periods of time (i.e. months) with the right humidity conditions. It is often transported in this stage to other worldwide cement plants that do not have their own raw material processing operations.

In the final stages, five per cent gypsum is added to control the concrete’s setting properties and the ‘clinker’ is pulverized into the more familiar cement powder and kept in bulk storage silos until the material is ready to be shipped via tanker truck or rail, or packaged into 10- or 20-kg (22- or 44-lb) bags for distribution.

The role of aggregates

The aggregates used to manufacture concrete comprise a mix of sand and stone. By volume, aggregates makeup approximately 60 to 75 per cent of the concrete. Rocks compose most of the coarse aggregates, while minerals compose most of the fine aggregates. In comparison to cement powder, aggregates are inexpensive as there is little involved in the manufacturing process.

Essentially, aggregates are extracted from the earth via mechanical means (blasting, excavation, etc.), processed (crushed, washed, and sifted), and used in the concrete mix as per the proper proportions described by the particular mix design, depending on the strength required and application type.

In general, the aggregate should be good quality and must not contain any deleterious properties, which could weaken the concrete’s overall strength.

Aggregates are selected based on the right properties for producing a good quality concrete mix. They include:

• Abrasion resistance: contributes to the surface’s long-term wear and durability;
• Resistance to freeze/thaw: controls the concrete’s volume expansion in the winter;
• Particle shape and surface texture: contributes to the concrete’s shear strength;
• Grading: helps build a uniform heterogeneous mixture with consistent properties;
• Absorption and surface moisture: allows the mix water proportions to be controlled; and
• Alkali reactivity: aggregates used in concrete cannot react with alkali to cause abnormal expansion and map cracking.

ADVANTAGES OF USING CONCRETE AS A STRUCTURAL BUILDING MATERIAL

• Cost: In comparison to many other structural materials, concrete is an economical building material, both by weight and volume; • Availability: Concrete is readily available. There are dozens of concrete plants in most major metropolitan cities, which in most cases are minutes away from the jobsite; • Ease of use: In general, concrete is easy to work with and can be used to fill any form or space as required; • Strength: Concrete is extremely strong in compression and shear and is well suited for columns, walls, foundations, and slabs; • Permeability: Concrete is relatively impermeable to the flow of fluids. Although it is a porous structure, the pores are not continuous. This also makes it resistant to freeze/thaw damage; • Portability: Concrete does not require a ‘factory’ setting to work with. It is easily and effectively used outside, making it a versatile, economical material; • Durability: Properly mixed, placed, and cured concrete will last many years without much more than minor abrasion and cracking; • Time: Concrete construction is usually faster than building with other structural materials; • Weather resistance: Structures built from concrete can withstand tornadoes, hurricanes, and other extreme weather conditions; • Fire: Concrete is fairly resistant to the damaging effects of fire; and • Insects: Concrete structures are not subject to damage from insects as is the case with other structural materials (e.g. timber).

Turning cement into concrete

When cement, water, sand, and stone are mixed together in the proper proportions—usually according to a specific mix design (recipe)—the four materials combine to form concrete. Each material performs a specific function and plays an important role in the end product.

The cement powder is obviously the most important, since without it, the mixture would not harden into concrete. Once water is introduced to the cement, it reacts to form a cement paste, which is basically the glue that holds everything together. Mixing the two together (hydration) creates an exothermic reaction (i.e. a chemical reaction that produces heat). During this process, tiny microscopic hair-like structures develop and create a complex mesh of fibres that bind the aggregate matrix together. This calcium silicate hydrate is by far the most important cementing compound of concrete. Hydration will continue indefinitely; however, most will occur during the first few days and directly attributes to the concrete’s gain in strength.

As only a certain amount of water is required for the hydration process (according to the mix design and the volume of cement powder and aggregates), it is important not to add any excess water. Should excess water be added, it will weaken the concrete mix as the water will not be absorbed into the crystalline structure of the cement paste matrix, which binds the aggregates together.

This is the reason why adding water to the concrete mix on the job site should be avoided. Instead, chemical admixtures should be used to improve the workability of the concrete mix.

Finally, aggregate is added as filler. Essentially, it is used as an inexpensive way to create volume in the concrete. The cement paste and gel matrix basically coat the aggregate and adhere to its surface. As the gel hardens, it binds the aggregates together to produce hardened concrete; hence the importance of quality aggregate material. Aggregate must be clean, angular, hard, and somewhat porous, so the cement paste can adhere to the surface and form a strong mechanical bond. The overall strength of the concrete is directly related to the aggregate’s strength and physical properties.

Key properties of hardened concrete

The following physical properties of normal strength Portland cement concrete has led to its widespread use in the construction industry.

Density (2,240 to 2,400 kg/m³ [140 to 150 lb/cf])

Density is the concrete’s mass per unit volume. For example, 1 m³ (35 cf) of concrete weighs approximately 2,240 to 2,400 kg (4,900 to 5,300 lbs).

Compressive strength (20 to 40 MPa [3,000 to 6,000 psi])

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Compressive strength is one of the most common performance measures used by engineers in designing buildings and other structures. It is measured by breaking concrete cylinders in a testing machine by applying compressive force on the cylinder until failure. The failure load divided by the cross-sectional area of the cylinder is the compressive strength. This is an important property in the design of structural elements such as columns, piers, or footings.

Flexural strength (3 to 5 MPa [400 to 700 psi])

The flexural strength (modulus of rupture) of unreinforced concrete is calculated by testing a rectangular ‘beam’ of concrete by applying loads at right angles to its axis. It is a measure of tensile strength. Concrete is weak in tension (approximately 10 to 20 per cent of the compressive strength). Flexural strength is important in the design of floor slabs, pavements, and beams. Due to its low flexural (tensile) strength, concrete is usually reinforced with steel in areas where flexural or tensile stresses are expected.

Tensile strength (2 to 5 MPa [(300 to 700 psi])

The tensile strength is similar to concrete’s flexural strength. It is determined by subjecting a concrete cylinder to forces that are perpendicular to its long axis in such a way that it splits in tension. The ultimate breaking force divided by the surface area is the tensile strength.

Modulus of elasticity (14,000 to 41,000 MPa [2,030,528 to 5,946,547 psi])

This is the ratio of the stress to strain of concrete. The modulus of elasticity can be used to predict how concrete will react (stretch/shrink) when subjected to forces. This is important to calculate a structure’s elongation under various loads.

Permeability

The permeability of concrete is the property that governs the rate of flow of fluid (e.g. water) through a porous solid. Water travels slowly through concrete (1 x 10^-10 cm/sec), thus making it a good barrier to water flow. It does not stop the flow of water, but slows it down considerably.

Shear strength (6,000 to 17,000 MPa [870,226 to 2,465,642 psi])

Concrete’s resistance to shearing stress is very high. This makes it fairly strong in applications with high-shear stresses. The shear strength of concrete is calculated as the force that tends to produce a sliding failure along a plane that is parallel to the direction of the force. Concrete is hundreds of times stronger in shear than in compression, and thousands of times stronger in shear than in tension and flexure.

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Of the above physical properties, the most important in the design and construction of common reinforced concrete structures are compressive, flexural or tensile, and shear strength.

Compressive strength is important in the design of load bearing members (e.g. columns), where the load is parallel and concentric with the structural member’s axis. As mentioned above, concrete is strong in compression, so it is an excellent material for designing and building load bearing structures such as walls, roads, runways, foundations, and footings. Wherever a large load must be supported axially (i.e. forming an axis), concrete is an excellent structural material.

On the other hand, as described above, concrete is extremely weak in tension or flexure (bending). Therefore, it is not ideal to use concrete in applications where it will be subjected to these stresses as it can easily fail. Concrete is never primarily considered for use in a tensile stress environment.

In some circumstances, however, and depending on the structure, concrete is often subjected to dual loading. This occurs when one side is subjected to compressive loads, for which concrete is ideally suited, and the other side to tensile stresses, which is the exact opposite to compressive stresses. This is due to the concrete’s modulus of elasticity and Poisson’s Ratio[7]. Dual loading is very common in structures; therefore, concrete must be coupled with structural steel, which behaves favourably in tensile stress environments.

Permeability is also important in the design and construction of concrete structures—especially swimming pools. Since concrete has a relatively low permeability, it is an ideal material for building structures meant to retain liquids (i.e. water). Concrete’s low permeability slows down or restricts the flow of liquids/water through the concrete wall, thus contributing to a watertight pool and making it an ideal construction material.

Reinforced concrete

If it were not for concrete’s weakness in tension and flexure, it would be the ideal building material for all structures. However, by integrating structural steel into a concrete element, it makes it extremely strong in tension and flexure (bending).

By proper structural analysis, engineers can calculate where concrete structural members will undergo tensile or flexural stresses when loaded as well as the magnitude of these stresses. By using the high-tensile strength of structural steel (i.e. rebar), a hybrid material known as reinforced concrete is created, which is strong in compression, tension, and shear.

 

Editor’s note: In the next issue, Petrocelli will discuss the ins and outs of structural steel and how it is used to create reinforced concrete. He will also look at various applications where reinforced concrete is used in swimming pool construction.

 

 

John Petrocelli, P.Eng., is the president of Spider Tie Canada Inc., a Canadian distributor of Spider Tie products. He holds a degree in civil engineering from the University of Toronto, specializing in concrete construction, structural engineering, soil mechanics, and project management. Petrocelli is also a licensed professional engineer in Ontario and a member of the Professional Engineers Ontario (PEO) and the Ontario Society of Professional Engineers (OSPE). He can be reached via e-mail at jpetrocelli@spidertie.ca[8] or by calling (416) 655-8171.

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