MMA Yearbook 2007 33 Where internal walls provide shared bearing to hollowcore or other full-width floor units and are less than 190mm wide, there must be temporary propping or other means of bearing enhancement until tie bars have been placed and the floor units have been concreted together over the support. For beam-and-block construction, the issue may not arise, because beams are placed alternately at the wall, giving each a full bearing. Another vital aspect for stability of the supporting walls is maturity of the mortar. A hardening period of three to seven days would normally be assumed for normal mortars and normal temperature conditions. Cold weather and/or retarded mortars will significantly extend these curing times. Checks should always be made on the condition of the mortar to ensure that it has hardened sufficiently prior to installing floor units. Narrow sections of wall Narrow sections of wall (less than 900mm) or piers should be given particular consideration and additional temporary support may be needed. Steel beams and lintels Often, steel sections are used to span openings in masonry construction and are placed without mechanical fixing onto padstones or the masonry with little “The PFF pioneered, some years ago, the adoption of crash-decks and netting systems.” consideration being given to stability during installation of precast floor units onto them. If the steel beam is loaded from one side only, it may rotate. PFF recommends that all such steelwork should be appropriately designed, fixed and/or braced to eliminate the possibility of movement. Pressed steel lintels Some proprietary steel lintels rely on the weight of the outer-leaf masonry to counteract loads applied on the inner face; however, this is rarely possible before installation of floor units and may mean that temporary support has to be given to the lintel. Such temporary support must be designed and installed by competent persons before the floor installer is required to carry out his work. Masonry workmanship Another important factor for stability at openings in masonry is the workmanship in brick/block laying. The recommendations of BS5628 “Code of Practice for the Use of Masonry” must be followed. In particular : . Full width blocks must be used at the bearing level of floors. . Lintels should always bear on full blocks. . Perpend joints should be staggered by not less than 75mm. With an excellent safety record during installation and unbeatable structural, acoustic and fire performance, allied with its thermal mass characteristics; precast flooring is the product of choice for all residential buildings, at both ground and upper levels. . Reader enquiries to: katherine.rowe@britishprecast.org Every year in England, approximately 285 people are killed and 9,000 people are injured in accidental house and apartment fires. Precast floors will delay a potentially lethal fire by anything between 30 minutes and 2 hours depending on the floor type installed. Where necessary, they can be upgraded simply by applying an appropriate finish. This is just one of the many significant benefits of precast floors which also include:  Soundproofing  Thermal insulation  Added security  Enhanced durability  Sustainability For more information please visit www.precastfloors.info today because, after all, every second counts. Precast concrete floors outperform timber at all levels. And here’s just one of them. Published with the support of A product group of British Precast

34 MMA Yearbook 2007 The Mortar Industry Association explains why it believes there is a growing trend towards the use of factory-produced mortar on sites. Factory-produced Use of factory-produced mortar has now risen to about 80% of the 2.5 million m3 of mortar used annually to help build a variety of projects, from national stadia to private homes, around the country. This growth reflects the increasing demand by specifiers for consistency in strength and colour in mortars and it has led to more and more major contractors changing to factory-made mortars without even considering the use of sitemixed material. The body that represents the manufacturers of factory-produced mortars, the Mortar Industry Association (MIA), believes this trend will continue and expects the use of factory-produced mortar to increase further over the next couple of years. Secretary of the Association Brian James said: “It is obvious that a man and a shovel cannot meet today’s high specification needs, nor do employers wish to bear the cost of mortar failures arising from poorly mixed mortar.” MIA has represented the interests of its industry for more than 30 years. It also provides independent technical advice and good practice information to mortar users and specifiers. Factory-produced mortars are manufactured or made under controlled conditions in purpose-built production units. All materials are either weigh-batched or dispersed through calibrated equipment to ensure that the mix proportions are accurate. All the mix ingredients meet the relevant British or European standards and are subjected to rigorous testing to ensure they comply with quality requirements. There are basically two different types of factory-produced mortar: ready-to-use and lime:sand. Ready-to-use mortar is available either wet or dry; wet ready-to-use mortar is delivered to site ready-to-use in every respect and requires no further mixing. It has guaranteed mix proportions and overcomes any quality and consistency problems associated with site mixing. Dry ready-to-use mortar is stored on site in silos complete with integral mixer. Once the site power and water supplies are connected, mortar can be produced to pre-programmed mixed proportions as required. This again overcomes all the problems of traditional site mixing. Lime:sand mortar is a blend of sand and lime to which cement and water are added on site. So why does the MIA recommend using factory-produced mortar so strongly? . The mortar mix is produced by a quality-controlled plant rather than with less accuracy, less rigour and a shovel on site. This ensures that the cement content can be accurately gauged and that the strength and colour will be consistent throughout the whole project. . When mortar is delivered readyto- use, mixing and labour costs can be reduced and there will less chance of wastage because each batch of mortar can be made to the exact specification and at the time it is required. . Bulk material delivered into silos or discharged into site containers eliminates the need for bags of cement on site, which can be a target for pilfering and wastage. . Factory-produced mortar allows for greater efficiency on-site while maintaining and improving quality. . The factory-produced mortar complies with the specification in every respect. But to appreciate fully the advantages of factory-produced mortar you have to listen to those who use it on-site. Rosedale Brickwork built the new Islamic centre in Oxford. Its Managing Director Lee Mackin said: “We had six 35t silos and it would have been impossible to batch that volume on A close-up of a section of the new Oxford University Sports and Social Centre