All concrete is made of cement, water and aggregate, rocks varying in size from sand to gravel. The cement and water make a paste that bonds the aggregates into a solid material. Both concrete blocks and precast concrete are made by pouring concrete into prepared forms and allowing it to dry or cure. Why would there be any difference? It's because concrete varies according to use; different types can have different strengths.
Blocks Are Hollow
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The standard concrete block is nominally 8 inches by 8 inches by 16 inches; actual dimensions will vary slightly because the nominal size allows for mortar between blocks. Blocks may be solid, but most have hollow openings on each side to reduce their weight. They may be installed with mortar between the joints to secure them or "dry-stacked" without mortar and covered with a surface bonding cement to make them solid.
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Precast Is Solid
Precast concrete walls are solid concrete that is poured into forms to a specific size. Most precast concrete is manufactured in plants and delivered on large trucks, but there are techniques to allow concrete walls to be formed and poured at a job site and then raised into place as a complete wall. Precast concrete walls are usually thinner than blocks, but are made with reinforcing steel inside them. Precast concrete uses larger aggregate than blocks.
Strengths Can Be Equal
Concrete blocks usually are poured with finer sand and smaller aggregate so they'll better conform to the molding required. A concrete block and a precast concrete section of the same size will have about the same compressive strength, or the ability to support a load. Neither block nor precast has much tensile strength — the ability to withstand stretching or bending — without steel reinforcing.
Reinforcing Blocks
Concrete blocks can be strengthened after construction with the addition of reinforcing bars through the hollows, secured by grout or cement poured into the openings. Such a reinforced concrete block wall, whether dry-stacked or laid with mortar, will have roughly the same strength as a precast concrete wall with similar size and reinforcing.
Precast Can Be Faster
Precast concrete walls go up faster because they install as a single large unit, as opposed to being built of individual components. They use less on-site labor but require a crane or similar heavy equipment for installation, which increases costs.They usually are set into a compacted gravel base, while concrete blocks are laid on a solid concrete footing. Local building codes will provide regulations and specifications for use of either block or precast concrete.