Total width of section
(mm) Diameter of each main parallel
bar (mm) Total effective csa of main bars (mm2)
A range of high strength tendons is available for prestressing masonry, including stainless steel tendons. These are typically from 6−39 mm in diameter with nominal tensile strengths of over 1000 N/mm2 available and should comply with BS 448630 or BS 589631.
8. Damp-proof courses
Reference should be made to BS 5628: Part 31 to ensure that the damp-proof course is suitable. In reinforced masonry a damp-proof course may present a particular problem
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since in some applications it will not be possible to introduce a membrane which will not interfere with the structural behaviour of the wall. Even in more conventional applications, materials which might squeeze out in a highly loaded element should not be used. Care should also be taken to consider the effect of sliding at the damp-proof course as well as adhesion to the mortar when the masonry is acting in flexure12.
The absence of a damp-proof course in applications such as retaining walls may result in appearance and durability problems with certain facing units, and manufacturer’s advice should be sought. Materials such as engineering bricks can be employed as a dpc in some situations. In other applications, such as prestressed diaphragm walls, it will generally be possible to employ one of the more conventional dpcs13,14.
It may be necessary to provide a vertical membrane between the cross rib and outer face of a diaphragm wall. In this instance it is usual to employ a liquid dpc, either painted directly onto the outer leaf masonry or on the perpend of the cross rib. Further guidance is given in Section 37.
9. Wall ties
When the low lift grouting technique is employed in conjunction with cavity construction, the vertical twist type of tie complying with BS 124315 may be used. The requirements regarding length of tie in this Standard are not applicable to reinforced masonry but the designer should ensure that adequate embedment is possible. It is recommended that in situations where the masonry is likely to be wetted for prolonged periods, such as retaining walls, stainless steel ties be employed.
Where the high lift grouting technique is to be used with cavity construction then a more substantial tie should be used to resist the pressure exerted by the infilling concrete during placing. A suitable tie is described in Appendix B to the Code and, again care should be taken to ensure adequate protection against corrosion. Other forms of tie may be used providing they give adequate restraint against the pressure exerted by the concrete.
Whatever type of tie is employed it is clearly necessary to avoid filling the cavity until the leaves have achieved sufficient strength and sufficient bond strength has developed between the mortar and the tie. A minimum of three days is recommended in normal ambient conditions.
Wall ties for prestressed diaphragm wall construction where the cross ribs are not bonded into the outer leaf of the masonry will usually need to be obtained from a specialist supplier. A tie of substantial cross section is required to provide adequate shear resistance.
The spacing of ties is covered in Section 35.
10. Cements
The types of cement which may be used with reinforced masonry are as follows:
1. Ordinary and rapid-hardening Portland cement (BS 1216) 2. Portland blast-furnace cement (BS 14617)
3. Sulphate-resisting Portland cement (BS 402718)
Neither masonry cement nor high alumina cement are permitted. BS 5628: Part 3 still permits the use of supersulphated cement to BS 424819 but this does not seem to have been used in conjunction with reinforced masonry in the UK and has, therefore, been excluded.
Lime
Limes which may be non-hydraulic (calcium), semi-hydraulic (calcium) and magnesium, should meet the requirements of BS 89020.
11. Aggregates
The recommendations of BS 5628: Part 3 should be followed when considering the suitability of aggregates for mortar. Essentially this means that the fine aggregate should be free from deleterious substances and comply with BS 120021. Marine sands should be washed to remove chlorides. Sands for mortar should be well graded. Single size sands or those with an excess of fines should be avoided if possible, but where their use is unavoidable, trial mixes should be assessed for suitability. Sands to grade M of BS 882 may well be found to be suitable.
Aggregates for infill concrete should meet the requirements of CP 110, which are generally that they comply with BS 882 and 120122, BS 87723, BS 104724 or BS 379725.
Good mix design practice indicates in general that the largest possible maximum size of aggregate should be used in concrete. In the particular case of reinforced masonry, however, the need to produce a flowing concrete able to fill comparatively small sections without segregation will dictate the maximum size of aggregate which may be employed.
In any case the maximum size of aggregate should not be greater than the cover to the steel less 5 mm. The making of trial mixes is recommended to produce the best concrete from the materials available.
Attention is drawn to the limits on chlorides discussed in Section 15.
12. Mortar
12.1 General
The recommendations given in BS 5628: Part 3 and BS 5390 should be followed for the mixing and use of mortars. The mix proportions and mean compressive strengths at 28 days are provided in Table 2 of BS 5628: Part 2, which is reproduced here as Table 2.7.
The testing of mortars should be carried out in accordance with Appendix A1 of BS 5628: Part 1, which gives information on preliminary tests, the interpretation of test results and site tests. It should be noted that the compressive
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