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36 BC2: 2008 5.1 Cover for Durability From the point of view of durability, tho

36 BC2: 2008 5.1 Cover for Durability From the point of view of durability, though it should be possible to reduce the covers to the steel in HSC from those which are specified in Table 3.4 of CP65 for relatively low strength concretes, it is recommended that the covers appropriate to C50 concrete should be used for higher grades. 5.2 Cover as Fire Protection No change to the current cover requirements is recommended. However, for concrete compressive strength greater than 60 N/mm2, the possible reduction of strength at elevated temperatures and the associated risk of spalling should be investigated, taking into account the relevant factors including moisture content, type of aggregate, permeability of concrete, possible heating rate and the silica fume content. Specialist literature and testing should be referenced for the fire resistance design of high strength concrete structures. 5.3 Flexural Design The principles of analysis in CP65 can be applied in design using high strength concrete. Design methodology based on the idealised short-term stress-strain (uniaxial) diagram in CP65 also applies. The principal significant difference between normal and high strength concrete is that ductility decreases as concrete strength increase. Hence, the ultimate strain in compression has to be reduced as strength increases and the compressive stress modified. In CP 65 Part 1, the ultimate concrete strain for flexural design is taken as a constant value of 0.0035. For HSC, the maximum ultimate strain limit of the diagram is modified by the following equations: For fcu ≤ 60 N/mm2 CP65 stress block is applicable For fcu > 60 N/mm2, εcu = 0.0035 - (fcu - 60)/50000 The ultimate strain, εcu , decreases with increase in the grade of concrete and if it is less than the strain at the tangent point then the stress is reduced accordingly. Hence, the design charts in BS 8110-3 should not be used for the design of beams higher than C60. The simplified design procedure based on a rectangular stress-block may also be acceptable with the same limitations on strain. Therefore, the simplified stress block in Figure 3.3 is also modified. Some longitudinal and transverse reinforcement should be provided in the compression zone. HSC concrete beams exhibit a lower flexural ductility than those of normal strength concrete for a similar tension steel ratio. In order to maintain a similar level of flexural ductility, the tension steel ratio needs to be lowered as concrete strength increases. To ensure a degree flexural ductility, CP 65 restricts the neutral axis depth to 0.50 of the effective depth ‘d’ where redistribution is less than 10%. This is to be modified as: fcu ≤ 60 N/mm2, x ≤ 0.5d 60 < fcu ≤ 75 N/mm2, x ≤ 0.4d 75 < fcu < 105 N/mm2, x ≤ 0.33d Section 5 Design Guideline for the Use of High Strength Concrete uploads/Voyage/ design-guideline-for-the-use-of-high-strength-concrete.pdf