Characteristics and Performance of Concrete

Concrete is a fluid mixture of cement, aggregates, and water which can be formed into different shapes and cures to a hard and durable construction material. Masonry is construction of natural building stone or manufactured units such as brick or concrete block.

All building materials expand and contract. Concrete and other cement based products shrink permanently, and clay products expand permanently with changes in moisture content. Both materials (as well as wood, metal, glass, and plastics) expand and contract reversibly with changes in temperature. Since concrete and masonry are brittle, if construction does not accommodate this expansion and contraction, cracking and water penetration can result. Flexible anchorage and the installation of control joints in concrete and concrete masonry and expansion joints in clay masonry allow this natural expansion and contraction to occur without damage to the construction.

Concrete can be used as a structural and a finish material in slabs, walls, paving, and retaining walls. Masonry can be used as a structural system, as a veneer, or as a paving system and can be used to build fire places and retaining walls. Concrete and masonry are strong in compression but require the incorporation of reinforcing steel to resist tensile and bending stresses. Masonry veneers can be constructed over many types of structural frames and backing walls. Concrete and masonry also provide fire resistance, energy efficiency, and durability.

Fire Resistance: Concrete and masonry are noncombustible —they will not burn. This is a higher level of protection than mere fire resistance. Wood can be injected with chemicals to make it resistant to fire damage for a longer period of time than untreated wood, but ultimately wood becomes fuel for the fire. Steel is noncombustible, but it softens and bends when subjected to the high heat of a fire. In commercial construction, steel structural members must be protected from fire by sprayed on mineral coatings, layers of gypsum board, plaster, or masonry. The highest level of protection and the highest fire protection ratings are associated with concrete and masonry.

Durability: Concrete and masonry are durable against wear and abrasion and weather well for many years with little or no maintenance. Wood is highly susceptible to moisture damage and requires protective coatings to prolong service life. Properly designed and constructed concrete and masonry will provide many years of service to the home owner without any additional investment of time or money.

Energy Efficiency: For centuries the thermal performance characteristics of masonry have been effectively used in buildings. Large masonry fireplaces used during the day for heating and cooking were centrally located within a structure. At night, the heat stored in the masonry radiated warmth until dawn. In the desert South west of the United States, thick adobe masonry walls provided thermal stability. Buildings remained cool during the hot summer days, and heat stored in the walls was later radiated outward to the cooler night air. Until recently, however, there was no simple way of calculating this behavior.

Understanding Concrete

Understanding Concrete

Residential construction today nearly always includes concrete in some form and to some extent. Concrete is used in footings, foundation walls, floor slabs, retaining walls, sidewalks, driveways, and patios. Concrete is a strong, durable, and economical material whose appearance can be altered in many ways to make it decorative as well as functional. Concrete is a controlled mixture of cement, aggregates, and water. Because it is a fluid mix, concrete can be formed into almost any shape and finished with a variety of textures.

Concrete strength and durability are easier to achieve with an understanding of how concrete is mixed and cured, how strength develops, and how variations in materials and mix design can accommodate different seasonal weather conditions and project requirements. An understanding of concrete properties and ingredients will produce better projects with greater efficiency and economy, higher profits, and fewer callbacks.

Depth of Penetration of Water under Pressure

Depth of Penetration of Water under Pressure

Principle

Water is applied under pressure to the surface of hardened concrete. At the end of the test period the test specimen is split and the maximum depth of penetration of water is measured.

Test specimens

The specimens are cubes, cylinders or prisms with a minimum edge length or diameter of 150 mm.

The test area on the specimen is a circle with a 75 mm diameter (the water pressure may be applied from above or below).

Conditions during the test

• The water pressure must not be applied on a smoothed/finished surface of the specimen (preferably take a formed lateral area for thet est). The report must specify the direction of the water pressure in relation to the pouring direction when the specimens were made (at right angles or parallel).
• The concrete surface exposed to the water pressure must be roughened with a wire brush (preferably immediately after striking of the specimen).
• The specimens must be at least 28 days old at the time of the test.

Test

During 72 hours, a constant water pressure of 500 (±50) kPa (5 bar) must be applied.

The specimens must be regularly inspected for damp areas and measurable water loss.

After the test the specimens must be immediately removed and split in the direction of pressure. When splitting, the area exposed to the water pressure must be underneath.

If the split faces are slightly dry, the directional path of penetration of water should be marked on the specimen.

The maximum penetration under the test area should be measured and stated to the nearest 1 mm.