ECONOMICAL TILT-UP CONSTRUCTION DESERVES MORE DESIGN CONSIDERATION, SAYS TCI

Tilt-up construction, often overlooked in the design of new concrete structures, can be an economical alternative to traditional building materials and does not negatively affect a structure’s looks and durability, says Gary Theodosiou, engineer and lecturer at The Concrete Institute.

Theodosiou says the economic viability of tilt-up/precast construction can be maximised by repetition: repeating identical panels and avoiding differentiation in panel sizing as much as possible.  Tilt-up offers cost-savings not only for warehouse construction but also for structural elements such as columns, beams, trusses, slabs and walls where there is sufficient repetition of the building elements.

“For tilt-up construction, the concrete elements are cast on site and lifted into final position. The elements are formed on a concrete slab – usually the building floor, but sometimes a temporary concrete casting surface, near the building footprint. After the concrete has cured, the elements are tilted from horizontal to vertical with a crane and braced into position until the remaining building structural components – such as roofs, intermediate floors and walls – are secured. By forming the concrete elements on the ground and not in their final position, tilt-up provides a significant reduction in man-hours and formwork,” Theodosiou states.

The tilt-up panels – set out according to a predetermined casting layout – span from base to roof diaphragm, act as beams and can carry roof and slab loads. They make up the building façade, structure and final finish in a single operation.

“Once the casting surface or floor slab has cured, forms are built on top. A high-quality plywood or fibre board with at least one smooth face is typically used, but steel forms are also suitable. Carpenters work off engineered drawings designed for each panel or element to construct on site, incorporating all door and window openings, as well as architectural features and other desired shapes that can be moulded into the concrete. Studs, gussets and attachment plates are located within the form for embedding in the concrete. The forms are usually anchored to the casting surface with masonry nails to prevent damage to the floor slab.

“Then, a chemically reactive bond-breaker is sprayed on the forms and casting surfaces to prevent the cast concrete from bonding with the slab. This allows the cast element to separate from the casting surface once it has cured. This is a critical step, as improper chemical selection or application will prevent the lifting of the panels, and may lead to costly demolition and rework,” Theodosiou adds.

A reinforcement grid is constructed inside the forms, after the form release is applied, and spaced off the casting surface to the desired distance with plastic “chairs”. Concrete is then placed, filling the desired thickness and surrounding all steel inserts, embedded features and reinforcement. The forms are removed when the concrete is cured, rigging is attached and a crane then tilts or lifts the panels into place.

“In situations where space is limited, concrete elements can be cast on top of each other, or stack cast. Quite often a separate casting pad is poured for this purpose and removed when the panels are erected.”

Theodosiou says tilt-up does not sacrifice quality for cost but provides a finished product with the strength and durability of reinforced concrete walls while being able to economically achieve aesthetic effects not possible or cost-effective with other methods of construction.

“Tilt-up walls furthermore do not produce substantial waste to dispose of which means lower environmental impact. The walls can be recycled if the building is taken down and reassembled at a new location which also adds to the building method’s sustainability,” Theodosiou comments.

MBA North and PRAWA partner to bring standards to the roofing industry

The Master Builders Association (MBA) North and the Professional Roof Repair and Waterproofing Association (PRAWA) have agreed to collaborate in order to accelerate the drive to introduce standards for the roofing and waterproofing industry.

“The start of the rainy season on the highveld is a good time to drive home the message that it does not pay to take short cuts when it comes to roofing and waterproofing. During tough economic times like the present, people can be tempted to go with the cheapest option, and then later find that sub-standard materials were used and there is no accountability,” says JJ Conradie, Executive Chair at PRAWA. “PRAWA was founded with the aim of helping the industry improve its skills and to provide a way to hold contractors accountable for the standard of the work they deliver to clients.”

“The MBA, and MBA North in particular, is doing a great job in helping the broader construction industry to upskill and to adhere to a proper professional code, so this alliance makes perfect sense.”

Conradie says thousands of people are active in the industry – participants range from handymen right up to industrial and commercial applicators dealing with sophisticated applications in the industrial, commercial and domestic sectors. While some contractors specialise in this area, others offer a range of construction services of which waterproofing and roof repair are just two.

Boitumelo Thipe, Marketing and Business Development Manager at MBA North says that the alliance with PRAWA is welcome because it brings an important sector within the construction industry into the fold. “Setting professional standards and providing training opportunities are essential ingredients of creating a professional industry that is able to gain the trust of the public, and can provide sustainable jobs,” she says. “Working with MBA North, PRAWA can play a massive role in improving the industry’s skills and thus opening up new opportunities, which is something our country desperately needs. Just as MBA-accredited builders are preferred by clients, in due course we will start to see PRAWA-accredited roof repairers and waterproofers identified as suppliers of choice.”

Conradie says that upskilling is an important foundation for creating a credible profession. To that end, PRAWA has teamed up with Roofing Academy, which offers CETA accredited training for waterproofing up to NQF Level 2, thus providing an excellent starting point in the journey to upskill the industry. Experienced roofers will also be able to gain certification through recognition of prior learning.

Skilled, accredited staff are more motivated and take more of a pride in their work – a great bonus for both their employers and clients, he points out. They are also better positioned to build careers rather than simply do jobs.

PRAWA has arranged a sub-committee that will work towards setting up a minimum standard for the industry. This sub-committee is made up of current industry role players, including:

  • Browns Proofing
  • Blakmar
  • African Rope Access
  • Peche Roofing
  • WNS Waterproofing
  • JBC Roof Cover
  • Dusty Moon Construction

“By setting standards we will help to make the industry more accountable, more professional and ultimately more profitable,” Conradie concludes. “The final piece of the puzzle is to provide an interface between contractors and their clients. PRAWA offers clients the option of getting completed work inspected by a PRAWA-affiliated inspector, and inspectors can assist in resolving disputes.”

Metal Cladding: How To Get The Best From A Metal Roof

Published courtesy SAMCRA

In making the decision to have a metal clad roof one must remember that the most important functions of a roof cladding system (profiled cladding plus ancillary items and fasteners) is to provide a weatherproof membrane followed by aesthetic appeal, however, aesthetic appeal is invariably the governing consideration.

PROFILE:

The first step is to choose a profile (geometric shape) from the two main categories, pierce fix or concealed (secret) fix that best suites the aesthetic and service performance requirements of the building.

 Pierce fix profiles are those where the cladding is anchored to the supporting structure by a fastener that passes through the cladding, the head of which is permanently exposed. Corrugated and box rib are the most common forms of pierce fix profiles. These profiles are not suitable for flat roofs (less than 5°) generally it is recommended that corrugated be limited to a minimum slope of 10° and 7,5° for box rib.

The length of individual sheets is limited to between 10 to 13m due to transport constraints. This, however, eliminates the need to provide for thermal movement. We strongly recommend that both side and end laps be sealed with a reinforced butyl based sealer strip and that side laps are stitched as per manufacturer’s recommendations.

Concealed fix profiles have unique anchoring devices which are contained within the profile and are therefore not exposed to the elements. They also allow for unrestricted thermal movement of the cladding. These profiles can be used on slopes as low as 2°, moreover, they can be rolled on site thereby eliminating the need for end laps. These profiles should never be end lapped which means they are not suitable for in-plane rooflights.

An often overlooked component of cladding systems are the flashings, most leaks emanate from undersized and/or poorly fitted flashings. Whilst the basic design of flashings is universal to all profiles their dimensional proportions vary considerably and we recommend the cladding manufacturer’s standard designs for the respective profiles be adhered to. The two flashings that cause the most problems are the valley and counter flashings. Valley flashings are to have a return, similar to that on a counter flashing, along the full length of their longitudinal outside edges which are overlapped by the cladding, this is necessary to eliminate the formation of a capillary siphon.

Counter flashings have to be independent from head and sidewall flashings, the reason for this is the differential thermal movement between the flashing attached to the cladding and the counter flashing which is anchored into masonry. Under no circumstances is the counter flashing to be mechanically connected to any other flashing.

A similar situation arises with gable and barge flashings on concealed fix systems where sliding connections are required for the attachment of the flashing to the roof cladding. Sliding connections are also required for the attachment of headwall and ridge flashings. Most manufacturers of concealed fix cladding systems supply matching sliding connectors. Under no circumstances are paint-on membranes to be used as a substitute for metal flashings.

Fasteners are to be in accordance with the cladding system manufacturer’s specification and the requirements of SANS 1273 which requires that the durability of the coating on the fasteners and washers, together with that of the sealing gasket, is equal to or better than that of the cladding.

BASE MATERIAL:

The second step is to choose the material from which the cladding is to be roll-formed together with the finish, ie metallic or colour coated. Traditionally metal cladding is formed from metallic coated steel and less frequently from aluminium, stainless steel, titanium zinc or copper, all of which perform differently in a given environment. With coated steel there is a choice between galvanised and 55% aluminium / zinc (ZincAL, Zincalume, etc) both with a colour-coated option with finishes ranging from 30% gloss to matt and textured. Aluminium is also available with a 30% gloss painted finish.

Whilst galvanised is the cheapest option, it is, with a few exceptions, the most vulnerable to corrosion both from the environment and rainwater runoff from other metallic, painted or glazed (including glass and plastics) surfaces, whereas the 55% aluminium / zinc is vulnerable in highly acidic and alkaline environments (pH less than 4 and greater than 9), plus areas of intensive animal farming together with runoff from copper or lead.

All coated steel products must not be in direct contact with stainless steel, copper or lead. Please note that metallic coatings are available in a range of thicknesses and that in general durability is proportional to thickness, ie the thicker the coating, the longer the corrosion protection. When selecting the base material it is vitally important to consider both the macro (area or region as a whole) and micro (adjacent area surrounding a building) environments. Under no circumstances are solar heaters and PV panels to be installed on uncoated galvanised surfaces.

MAINTENANCE:

An important aspect that is regularly overlooked is maintenance, particularly in coastal and highly polluted areas where the regular washing, on a quarterly basis, of surfaces not washed by rainfall is necessary to preserve the protective coatings and compliance with warrantee conditions. We recommend an annual inspection of roofs to check the condition of the surface, attachment of flashings and in the case of pierce fix systems, the condition of the fasteners and their sealing gaskets. In all cases, the regular removal of debris is important as plastics and vegetative matter block drainage systems. In addition, accumulated vegetative matter accelerates corrosion of surface coatings, including painted ones.

INSTALLATION:

In order to ensure the best performance from a cladding system, we strongly recommend that it is installed by a competent, well-trained and experienced roofing contractor. A good starting point is to establish if a prospective contractor is approved by the manufacturer of the chosen system.

Could zinc be the secret to a more sustainable future?

By Simon Norton, International Zinc Association, Africa Desk

In the next decade, the United Nations projects, the world population will grow to a staggering 8.5 billion people. Supporting a global citizenry of this size will necessitate economic development and the expansion of the global economy at an unprecedented scale.

Meeting these needs will place enormous strain on our finite natural resources. As such, it’s crucial that we think in “circles” when it comes to the use of metals and minerals to ensure optimisation in its mining, production, product life cycle and, eventually, the recycling thereof.

Think zinc 

One resource that is particularly versatile and abundant is zinc, an essential element for all living things and presenting useful metallurgical and chemical properties. Zinc features in our daily lives with applications in everything from agriculture and building to wellness, X-ray machines and much more.

Adding to zinc’s overall appeal is the fact that zinc can be recycled without losing or compromising any of its metallurgical properties or overall value, meaning it can be used over and over again. Zinc is not only a sustainable option during use, but the recycling thereof also works to reduce concentrate demand, energy use, emissions and reducing waste disposal.

According to the American Galvanizers Association zinc has a reclamation rate of 80%, while some 30% of the zinc currently in use is from reclaimed zinc sources. There is however scope for this number to increase significantly as the technology used in zinc recycling is becoming more sophisticated all the time. As the recycling process advances, reclaimed zinc becomes an increasingly viable source for sustainable use across industries.

The circular solution

A circular approach to the use of metals and minerals will help to reduce the strain on natural resources and, ultimately, prevent the depletion of these life-giving materials.

It’s an ethos that also takes the question of climate change into consideration. Key to reducing our impact on the environment, and reducing an industry’s contribution to climate change, is a keen understanding of a particular material’s – zinc in this case – life cycle. Unpacking what happens at each stage, from mining to production, product life time and recycling, is the foundation for developing measures of optimization that reduce costs and protect resources.

Economy and environment 

We’re at a crucial point in history where all resources need to be understood to be finite. It’s to this point that the maximum efficiency and re-use of resources, through the cycle of production, consumption and recycling, needs to take priority. Beyond the environment, the economy sets to benefit from the strategic use of zinc.

In both the construction industry and civil engineering sector, zinc, in the form of hot dip galvanising, is used as corrosion protection. Hot dip galvanising of steel structures and steel items such as concrete reinforcing steel is a cost-effective way to prevent premature corrosion of steel and to ensure a long life for steel in corrosive environments. While hot dip galvanising may cost more initially, it provides a long-term maintenance-free service life, saving significant sums of money normally spent on repainting or coating maintenance over the prescribed life of the project. Hot dip galvanised steel structures can give a trouble-free life of more than 30 years in the right environment. Its maintenance-free longevity offers both an environmental and economic benefit, and in a developing economy like South Africa, solutions that cost less mean more capital for other projects, and with the more widespread use of zinc this is possible without compromising on quality.

As such, when long-term and sustainability enjoys priority over the short-terms gains allowed by taking shortcuts, both the economy and the environment benefit.

Full circle?

Currently, one of the biggest barriers to greater sustainability is the linear economic model of “take-make-dispose”. It’s key to rethink this model to keep waste out of the system. Zinc’s inherent features make it possible to completely eliminate the “dispose” aspect of the model.