Caring for built concrete

by Darren
Caring for built concrete

Creative designs can showcase concrete’s design ability, but it is imperative to monitor and maintain concrete structures to prevent unnecessary failures.

Concrete is often used as the base material for buildings, but it is also creatively applied as an expressed material where its raw, natural appearance is exposed in finishes and features.

“It is a timeless material that doesn’t date,” notes architect Nico van der Meulen. “Aesthetically you can achieve so many different textures and looks by exploring various types of shutters. Concrete has the strength required for structural members and can also be moulded into the most intricate forms thanks to the advancement of self-compacting concrete.”

Wavy concrete roofs
At the foothills of Table Mountain, the rolling concrete roof of the Mountain House is an illustration of what can be achieved with innovative concrete design. A project of Van der Merwe Miszewski Architects (VDMMA), the house includes a series of terraces that are inserted into the landscape.

“The roof of the pavilion has an undulating concrete surface and is propped up on a very fine steel structure hovering over glass. The idea was to make the roof look as light as possible so that it seems as if the concrete is completely floating on top of the structure,” notes Lloyd Rubidge, director at VDMMA.

The rolling waves were achieved using a technique similar to that of making a framework for boat building, to shape the shutter work that the concrete was cast onto.

Precast concrete on 1 Silo
Another VDMMA project, 1 Silo showcases expressed precast concrete both externally and internally. According to Rubidge, the huge precast panels on the west facade were done in two forms – some being flat and smooth and the others ribbed.

“Most of these were done with a silicone form liner that was purposely made for this project to create the texture on the concrete. The image ‘1 Silo’ was also embossed into the liner, which left the lettering in the concrete,” he explains.

Internally, the concrete columns are exposed, extending all the way up to the exposed concrete roof and soffits.

Concrete garage doors
Pushing boundaries with the use of concrete in architecture, Nico van der Meulen Architects have designed four concrete garage doors to match the monolithic design of a concrete house. “Using ordinary garage doors would have detracted from the concept, so we tried something extraordinary to achieve our design vision,” he says.

The smaller doors measure approximately 2,4m high by 2,4m wide, each weighing 1,3 tons, while the larger doors are 4,5m high by 2,4m wide, weighing just over 2 tons per door. These were cast on site and hoisted into position using cranes.

The doors are only 75mm thick and framed with steel channels to prevent the corners from chipping. They are carried by linear motion sliders imported from Japan and guided by top rails. “These rollers are designed to carry excessive loads and make it possible for a person to open them singlehandedly. We will, however, be installing industrial motors for the large door,” Van der Meulen explains.

When working with concrete, depending on the application, Van der Meulen strongly recommends getting a specialist to specify the mixture of the concrete specific for the application.

Concrete mix quality
Peter McArdle, specification consultant at Mapei, confirms that the main issue with new concrete is poor quality control on site, with regards to the design of the concrete mix, but also in terms of the application and design of the formwork that encapsulates the concrete.

“Issues such as spalling tend to happen much quicker if the concrete isn’t of good quality or if there is a lack of adequate cover, which makes chemical attack likely at a much earlier stage than normal. The lack of concrete cover is a killer,” he states.

Ivor Boddington, technical manager at a.b.e Construction Chemicals Chryso Group, adds that this is particularly a problem in coastal regions and environments that pertain to aggressive environmental conditions such as chemical plants. “If the cover on the reinforcing is inadequate, corrosion is rapidly initiated through chloride or carbon dioxide ingress through the concrete,” he says.

“There is little solution for bad concrete but to pull it down and rebuild.”

Maintaining concrete structures
In addition, maintaining concrete structures and addressing problems as soon as they emerge, will avoid costly repairs later when the problem becomes advanced.

“It is an accepted fact that concrete is not maintenance free” states Wayne Smithers, head of technical services at Sika South Africa. “There is a constant attack on the reinforcement in concrete structures and if the concrete is not coated and regularly inspected, it can reach a stage where it is a huge cost to fix problems.

“It is like a cancer – while you might see some evidence of corrosion, you don’t always realise how bad it is until you start investigating. And one can do cosmetic cover-ups, but the problem will only get worse if not properly diagnosed and addressed,” Smithers warns.

Warning signs
When reinforced concrete starts corroding, the first signs are usually sections of concrete cracking, as well as weeping rust stains down the concrete surface.

“While a few cracks here and there might not look that bad, when recognising these signs, building owners should get a proper diagnostic survey done to determine exactly how bad the problem is in order for them to implement a strategy for repair,” Smithers advises.

Corroding reinforcement
Chemical attack is by far the biggest cause of concrete problems requiring repair. McArdle explains that when reinforcement is put into fresh concrete, it is naturally protected by the natural high alkalinity in the concrete. However, concrete loves to attract moisture.

Inland, the dissolved carbon dioxides in the moisture in the polluted atmosphere penetrate into concrete and react with the calcium that naturally occurs in concrete and cement. This chemical reaction creates a weak carbonic acid that is drawn towards the rebar.

“This reduces the pH of the concrete from 13 or 14 down to about 9, which makes the rebar vulnerable. And with the moisture and oxygen that are present, rust starts to form around the rebar and because concrete doesn’t have tensile strength, it starts to spall and break,” McArdle clarifies.

At the coast, water borne chloride ions enter the concrete and start pitting the rebar. According to Smithers, this is most prevalent within 5km of the coast, where salt deposits and wind-driven rain onto concrete structures amplify chloride attack.

“When steel corrodes, it expands up to eight times its original volume and puts a force on the covercrete, causing pieces of concrete to break off and exposing the rebar to the elements. Corrosion lessens the effective strength of the steel as the cross-sectional area of the rebar diminishes and eventually the structure may collapse,” he says.

Smithers advises that in order to repair, the contaminated area of concrete needs to be broken out, and the rebar needs to be cleaned and coated with a protective coating, after which the cavity should be filled with a proprietary branded mortar or grout.

“Quite often, sections of rebar also need to be reinstated. Where less than 10% of the rebar is left, the damaged section should be cut out and replaced,” says Smithers.

Guard against corrosion
Preventing corrosion starts with the design and construction of durable concrete for structures, according to Boddington. “There are additives available that can be mixed into the concrete, as well as protective coatings and pentrants for structures to reduce problems. Migrating corrosion inhibiters is one of the newer technologies that also assists in reducing the onset of corrosion on steel in concrete.

“However, these solutions are often overseen in the design phase, resulting in the need for repairs later on,” he states.

Smithers adds that galvanic anodes are another technology being used increasingly to reduce the effects of corrosion on the reinforcement. “It is tied to the rebar when local repairs are done, or by coring holes and embedding the anodes into the concrete,” he says.

Cement based coatings are now available for the protection of exposed concrete that will protect against carbonation and chloride attack, and, if applied by a competent sub-contractor may be warrantied for 20 years according to McArdle.

Alkaline aggregate reaction
Another potential problem occurring in concrete is alkali aggregate reaction, according to McArdle.

“A good example is the M1 double-decker bridge between Braamfontein and Newtown,” he says. “When it was constructed, quarries in and around Johannesburg couldn’t cope with the demand for aggregates due to major expansion in the city and aggregates were brought from Cape Town. However, the Cape aggregates reacted chemically with the local cements, causing a type of rust to form around large aggregate stones. This has the same effect as a rusting rebar since it eventually cracks the concrete,” he explains.

McArdle points out that in both the cases of rebar corrosion and alkaline aggregate reaction, if either the oxygen or moisture can be stopped, corrosion will also stop.

“Now it is very difficult to block oxygen since it is a gas that penetrates concrete easily, but moisture can be managed. To do this efficiently though, one needs a product that allows concrete to breath by allowing water vapour in and out. Since water vapour is such a small molecule, nothing will dissolve in it,” McArdle states.

According to Boddington, penetrating chemicals can render porous surfaces hydrophobic – not letting moisture in, but allowing vapour to expel, thus reducing the moisture content within the structure. “Once you reduce the transition rate of moisture ingress, you also reduce the transition ingress of deleterious soluble salts and compounds that are carried in the water,” he says.

Structural damage caused by impact or disaster
Mechanical damage happens when a truck collides into a motorway bridge, fire damage to concrete building elements, or when design or construct criteria is not quite adequate and elements move and break apart.

Before repair is affected on structural damage, Boddington stresses the importance of doing a thorough assessment to determine the cause, extent and the best way for repair. “There is a host of repair mortars available, from cementitious systems and epoxy systems for repairs, including related primers, and protective coatings, but it is also necessary to ensure that after substrate preparation the concrete is sound because effecting repairs on unsound or poor concrete is pointless,” he states.

Smithers points out that when doing structural repairs, mortars are mostly superficial and don’t reinstate the structural integrity of the concrete that has been taken out. Therefore each structure should be assessed on a case-to-case basis and it is wise to involve a structural engineer to determine whether the damaged concrete requires a structural or cosmetic repair solution.

Full thanks and acknowledgement go to Van Der Merwe Miszewski Architects, Nico van der Meulen Architects, Mapei South Africa (Pty) Ltd, a.b.e Construction Chemicals and Sika South Africa for the information given to write this article.

Common concrete problems:
–    Bad concrete mix quality.
–    Inadequate quality control on site.
–    Chemical attack causing corroding reinforcement.
–    Alkaline aggregate reaction.
–    Structural damage.

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