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Shotcrete, gunite (/ ˈ ɡ ʌ n aɪ t /), or sprayed concrete is concrete or mortar conveyed through a hose and pneumatically projected at high velocity onto a surface. This construction technique was invented by Carl Akeley and first used in 1907. [1]: 7 The concrete is typically reinforced by conventional steel rods, steel mesh, or fibers.
Cracking and shrinkage. Concrete shrinks as it cures. It can shrink 1 ⁄ 16 inch (1.6 mm) over a 10-foot long area (3.05 meters). This causes stress internally on the concrete and must be accounted for by the engineers and finishers placing the concrete, and may require the use of steel reinforcement or pre-stressed concrete elements where ...
A variant of standard concrete transportation is the concrete (or, cement) mixing trailer. These small versions of transit-mix trucks are used to supply short loads of concrete. They have a concrete mixing drum with a capacity of between 0.76 and 1.34 cubic metres (1 and 1.75 cu yd).
A cement accelerator is an admixture for the use in concrete, mortar, rendering or screeds. The addition of an accelerator speeds the setting time and thus cure time starts earlier. [ 1 ] This allows concrete to be placed in winter with reduced risk of frost damage. [ 2 ]
Traditional rebars are generally used to improve the tensile strength of the concrete in a particular direction, whereas steel fibers are useful for multidirectional reinforcement. This is one of the reasons why steel fiber reinforced (shotcrete form) concrete successfully replaced weldmesh in lining tunnels. Less labour is required.
GGBS is used as a direct replacement for Portland cement, on a one-to-one basis by weight. Replacement levels for GGBS vary from 30% to up to 85%. Typically 40% to 50% is used in most instances. The use of GGBS in addition to Portland cement in concrete in Europe is covered in the concrete standard EN 206:2013.
Calcium silicate hydrate (also shown as C-S-H) is a result of the reaction between the silicate phases of Portland cement and water. This reaction typically is expressed as: 2 Ca 3 SiO 5 + 7 H 2 O → 3 CaO · 2 SiO 2 · 4 H 2 O + 3 Ca(OH) 2 + 173.6 kJ. also written in cement chemist notation, (CCN) as: 2 C 3 S + 7 H → C 3 S 2 H 4 + 3 CH + heat
All concrete structures will crack to some extent, due to shrinkage and tension. Concrete which is subjected to long-duration forces is prone to creep. The density of concrete varies, but is around 2,400 kilograms per cubic metre (150 lb/cu ft). [1] Reinforced concrete is the most common form of concrete.
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