4.12.1 General
Low density aggregate concrete may generally be designed in accordance with the provisions given in clause 3 and 4.1 to 4.11. Subclauses 4.12.2 to 4.12.12 relate specifically to reinforced low density aggregate concrete of grade 20 or higher.
The structural use of concretes of grades lower than grade 20 should be limited to plain walls. In considering low density aggregate concrete, obtain specific data direct from the aggregate producer.
4.12.2 Durability and fire resistance
The maximum free cement/water ratios and minimum cement contents (with specified nominal cover to reinforcement) for concretes for use in specified conditions of exposure are given in SABS 0100-2.
When low density aggregate concrete of a grade lower than grade 20 is used, make the nominal cover to all reinforcement (including links) 25 mm for internal non-corrosive conditions. For fire resistance, see clause 7 unless appropriate test results are available.
4.12.3 Characteristic strength
Values of characteristic strength of low density aggregate concrete should be chosen correctly. When all aggregate in the concrete is fly ash, the related cube strength at other ages may be obtained from table 2. In the case of grade 15 concrete, reduce the values given in table 2 for grade 20 concrete by 25 %. These values apply to most other types of aggregate, but the manufacturer of the particular material under consideration should be consulted. With some aggregates used in rich mixes, there may be little increase in strength beyond that attained at 28 d.
4.12.4 Shear resistance of beams
Establish the shear resistance and shear reinforcement for low density aggregate concrete beams in accordance with 4.3.4.1 and 4.3.4.2, using table 26.
The shear stress v should never exceed the lesser of 0,55 fcu or 3,5 MPa, whatever reinforcement is provided.
Table 26 - Maximum design shear stress*) vc in low density aggregate concrete beams
1 2 3
The maximum shear stress should be taken as 0,8 times the appropriate value given in 4.3.4.1
*)Values of stress under maximum design (ultimate) loadings.
4.12.5 Torsional resistance of beams
Establish the torsional resistance and torsional reinforcement for low density aggregate concrete beams in accordance with 4.3.5, using table 27 in place of table 8.
Tablle 27 - Minimum and ultimate torsional shear stress in low density aggregate concrete beams
1 2 3
Concrete grade MPa
Minimum torsional shear stress, Vt
MPa
Ultimate torsional shear stress, Vtu
MPa
Deflection of low density aggregate concrete beams may be calculated using a value of Ec as described in 3.4.2.1. Alternatively, span/effective depth ratios may be obtained from 4.3.6.2 and 4.3.6.3 and multiplied by a factor of 0,85.
4.12.7 Shear resistance of slabs
Establish the shear resistance and shear reinforcement for lightweight aggregate concrete slabs in accordance with 4.4.5, 4.5.4 or 4.6.2, using table 26. The shear stress v should never exceed the value of the lesser of 0,55 fcu or 3,5 MPa, whatever shear reinforcement is provided (if any).
4.12.8 Deflection of slabs
Deflection of low density aggregate concrete slabs may be calculated using a value of Ec as described in 3.4.2.1. Alternatively, the provisions given in 4.4.6, 4.5.5 or 4.6.3 may be used for any slab subject to a nominal imposed load of 4 kN/m2 or less. For slabs supporting a higher nominal imposed load, multiply the span/effective depth ratios obtained from 4.4.6, 4.5.5 or 4.6.3 by a factor of 0,85.
4.12.9 Columns
The recommendations of 4.7 apply to lightweight aggregate concrete columns, subject to the following:
a) short columns: a column of reinforced low density aggregate concrete may be considered short when the ratios lex/h and ley/b (see 4.7.1.4) are less than 10; all other columns are slender.
b) slender columns: in 4.7.3.1, the divisor 2 000 in equation (12) should be replaced by the divisor 1 200. Values of βa in table 20 should be altered accordingly.
4.12.10 Walls
The recommendations of 4.8 and 6.5 apply to low density aggregate concrete walls, subject to the following:
a) short walls: a wall of low density aggregate concrete may be considered short when le/h (see 4.7.1.1) does not exceed 10; all other walls are slender;
b) slender walls: in 4.8.5, slender reinforced walls, when regarded as slender columns, require the use of the equations given in 4.7.3, modified as described in 4.12.9(b).
For plain slender walls in 6.5.3, take the additional eccentricity due to deflection ea used in equation (21) as l2e /1 700.
4.12.11 Local bond, anchorage bond and laps
4.12.11.1 Establish local bond stress, anchorage bond stress, and lap lengths in reinforcement for low density aggregate concrete elements in accordance with 4.11.6, except that the bond stresses shall not exceed 80 % of those given in 4.11.6.2.
4.12.11.2 For foamed slag or similar aggregates, it may be necessary both to ensure that bond stresses are kept well below the above maximum values for reinforcement that is in a horizontal position during casting and to obtain the advice of the manufacturer.
4.12.12 Bearing stress inside bends
The recommendations of 4.11.6.9 apply to low density aggregate concrete, except that the bearing stress shall not exceed 4fcu
3(1 % 2Φ ab )
5 Prestressed concrete (design and detailing) 5.1 General
This subclause gives methods of analysis and design that will in general ensure that, for prestressed concrete structures of classes 1, 2, and 3 as given in 3.2.3.3.1.2, the criteria set out in clause 3 are met. Other methods may be used provided that they can be shown to be satisfactory for the type of structure or element under consideration. In certain cases, the assumptions made in this clause may be inappropriate and the engineer will have to adopt a more suitable method, having regard to the nature of the structure in question.
For low density aggregate concrete, the prestress losses will in general exceed those for dense aggregate concrete, and specialist literature should be consulted.
When structures are to be erected in seismic areas, the effect of adverse bending of the prestressed elements should be considered.
5.1.1 Basis of design
5.1.1.1 This subclause follows the limit states philosophy set out in clause 3, but since it is not possible to assume that a particular limit state will always be the critical one, design methods are given for the ultimate limit state and the serviceability limit states.
5.1.1.2 In general, the design of class 1 and class 2 elements is determined by the concrete tension limitations for service load conditions, but check the ultimate strength in flexure, shear and torsion.
5.1.1.3 The design of class 3 elements is usually determined by ultimate strength conditions, or by deflection or by cracking or by both.
5.1.2 Durability and fire resistance
For guidance on the minimum cover to reinforcement and prestressing tendons that have to be provided to ensure durability, see 5.9.3. Use the results of fire tests or other evidence to ascertain the fire resistance of an element or, alternatively, refer to clause 7.
5.1.3 Stability and other considerations
For recommendations concerning such considerations as vibration and stability, refer to the general provisions of clauses 3 and 4.
5.1.4 Loads
5.1.4.1 Values of loads
The values of the design ultimate loads are those given in 3.3.3.1 and 3.3.4.1. The design loads to be used for the serviceability limit states are given in 3.3.4. (See also 5.3.2.)
5.1.4.2 Design load arrangements
In general, when assessing any particular effect of loading, ensure that the arrangement of loads is the one that causes the most severe effect. Consider the secondary effects due to both the construction sequence and the prestress, particularly for the serviceability limit states.
5.1.5 Strength of materials
5.1.5.1 Characteristic strength of concrete
The characteristic strengths of concrete that may be specified for prestressed concrete are given in table 28 together with their required strengths at other ages. The minimum grades recommended are those that have characteristic compression strengths of 30 MPa and 40 MPa for post-tensioning and pre-tensioning, respectively. The concrete strength at transfer should be at least 18 MPa for unbonded systems and 25 MPa for bonded systems.
Table 28 — Strength of concrete fcu
1 2 3 4 5
*)These increased strengths due to age should only be used if it has been demonstrated to the satisfaction of the engineer that the materials to be used are capable of producing these higher strengths.
The design should be based on the 28 d characteristic strength or, if appropriate, on the required strength given in table 28 for the age at loading.
5.1.5.2 Characteristic strength of steel
The specified characteristics of prestressing tendons and wires are not covered by this code. The characteristic strengths of reinforcement are those given in 4.1.5.2.