Navigating concrete: How to ensure a structure stands the test of time

Concrete - the silent giant - promises a lifespan measured in decades, and steadily gaining strength year after year. Yet, this promise depends on a meticulous dance between proper management, timing, and meticulous attention to detail, from material selection to execution.

Material comes a bit later when an architect thinks of a design. The idea, context, concept, environment, ambiance, and objectives—such parameters are considered first.

When an architect chooses concrete as the primary material for the design, there are certain things s/he considers. Aspects such as the texture are considered, which could be plain, smooth, perforated, or rustic. It can even mimic textures such as the Bengal Shiplalay building in Dhanmondi, which gives you the impression it is made of wood because of its texture.

Even the colour of concrete can be an interesting aspect to explore. The Zebun Nessa Mosque, a monolithic pink concrete structure, incorporates the innovation of coloured concrete, instead of traditional grey concrete.

This thought-process is what made a regular concrete-made structure iconic.

The National Assembly building, the most iconic structure in Bangladesh, was completed in 1982 and its pristine precision makes it one of Louis Kahn's most notable works.

The complex is made out of poured in place concrete with inlaid white marble, which is a testament to local materials and values. The grand mass of the monumentally scaled National Assembly and the artificial lake surrounding the building act as natural insulators and cooling systems. It also creates interesting spatial and lighting conditions.

It was built at a time when modern technologies and materials, which are available today, were absent.

"Back in the 1960s, there was no concrete industry and no steel shutters in Bangladesh. The magnificence of the structure is that it is made with volume and surface; no beams or columns. It goes beyond frames and typical concrete ideology," architect Ador Yousuf said.

Even before then, the fine arts building (Charukala), built in 1953 right after the partition, was built by Mazharul Islam and the concrete structure defines modern architecture.

Both the Tin Netar Majar and the Kamalapur Railway Station (KRS) terminal buildings create the illusion of wavy surface, through the softness of the flow and their curves. The mausoleum was built in 1963 at the Doyel Chattar of Dhaka University by architects Masood Ahmed and S A Zahiruddin. KRS Terminal Building was designed by Daniel Dunham and Robert Boughey in the 1960s.

With the same typical aggregates, cement, and steel—these magnificent architectural structures were made way before modern architecture was established. They are strong enough to withstand climate change and the humid and hot weather of Bangladesh.

But nowadays, we are concerned about earthquake resilient structures, the safety of concrete buildings, and whether they will stand the test of time. No matter how beautiful and meticulously designed the structure is, from a civil engineer's perspective, there are certain things that contribute to the durability of an infrastructure.

Factors that hamper the strength of our concrete infrastructure

Having the best quality cement from well-known companies does not ensure the strongest structure. The harsh reality reveals itself once the process of making and casting concrete starts.

Negligence by the workers and the engineers, a lack of knowledge of building owners, and insufficient supervision may hamper the strength and durability of our infrastructure.

Timing is the cornerstone here. The casting of concrete within its initial setting time is the bedrock upon which proper placement, strength, and durability hinges.

In Bangladesh, we generally use Ordinary Portland Cement (OPC), which has an initial setting time of 30 minutes. After adding water to the cement, the countdown begins.

Concrete must be placed in the desired position within this time or it will begin to lose its plasticity and bonding ability, and the hardening process will start.

Once the process begins, it becomes difficult to achieve the desired shape and strength. This paves the way for structural flaws, uneven surfaces, and compromised strength. Each hour of delay beyond the casting time exacts a staggering 10%-20% decrease in concrete strength, studies suggest.

Well-graded aggregates

Another important thing that needs to be ensured is that the aggregates used in concrete are well-graded. Aggregates give body to the concrete and also reduce shrinkage, which occupies 70%-80% of the total volume of concrete. However, in some cases, poor-quality raw materials are used to save on costs.

Rounded aggregates result in the minimum percentage of voids (32%-33%), hence giving more workability. They require a lesser amount of water-cement ratio, but have poor interlocking behaviour and weak bond strength.

While irregular aggregates or angular aggregates provide less workability, they give 10%-20% more compressive strength due to the development of a stronger aggregate-mortar bond. So, these are useful in high strength concrete manufacturing.

Rounded stones are often selected due to cheap prices and used in large amounts instead of the high-quality coarse aggregates, which leads to weakened infrastructure.

Application of plaster

While the sight of the freshly completed projects marks the start of a new chapter, the honeymoon period of admiration can be short-lived, as subtle signs of structural compromise start to emerge merely two to three years down the line. Bubbles, cracks, and swelling become visible on the facade and walls.

Plastering serves to safeguard and elevate the aesthetic appeal of concrete structures, but its untimely application can spell disaster in the long run, undermining the structural integrity and overall durability of the construction.

With inadequate supervision, workers may haphazardly prepare excessive amounts of concrete mix, only to find themselves unable to place it within the initial setting time. This is because the plastering was done after the initial setting time, when the mortar had already expired.

There is a checklist for concreting work by the Public Works Department (PWD), where there is a clear mention of placing concrete within 30 minutes after mixing the water. However, this guideline is hardly followed in construction projects.

Its untimely application can undermine their durability and structural integrity. By the time it is noticed, the damage may be irreversible, leaving them with limited recourse beyond recurring cycles of re-colouring and patchwork.

Bridging the knowledge gap

The checklist for concreting work by the Public Works Department (PWD) includes guidelines that must be followed for a strong and durable concrete construction.

Aggregate for concrete material:

1. Stone chips must be made of crushed stone or boulder
2. Both coarse and fine aggregates should be free from all organic and deleterious materials.
3.  Aggregates should be well stored on brick solling platforms and properly soaked.
4. Percent wear of stone chips should be below 30%, as tested by Los Angeles Abrasion Test.
5. The coarse aggregates must be 19mm sieve size. 95-100% should pass the sieve.

Cement for concrete:

1. Cement is supposed to be well stored, not in a moist environment.
2. It should be laboratory tested.

Other materials for concrete:

1. The water should be free from salt and other impurities.
2. Spare vibrator machines, vibrator nozzles, levelling instruments, aggregate measuring boxes, sand filters, etc should be available on the site.
3. Water should be sprayed on the surface of the shutter/steel shutter.
4. Concrete is to be placed within 30 minutes after mixing the water.
5. Fresh concrete should be deposited against the concrete that is in position within 45 minutes.
6. Movement over concrete surfaces should be avoided for 24 hours.
7. The concrete surface is to be kept wet with water until the removal of the work.

Developer organisations, engineers, and building owners must strictly follow and be well aware of this checklist. Building owners should include skilled masons, site engineers, and supervisors for proper scheduling coordination of the process to ensure that workers cast concrete within the initial setting time.

The initiative by RAK Ceramics BD Limited and Housing and Building Research Institute (HBRI) for a year-long training workshop to develop skilled construction workers is commendable. RAJUK is currently prioritising earthquake and fire safety issues, but it is essential to conduct regular monitoring during the construction period.

By addressing these aspects, we can unleash concrete's enduring strength, ensuring structures of our long-cherished dream house that we build today stand tall not just for years, but for generations to come.

Dr. Sultan Ahmed is the Associate Director at Bangladesh Institute of Governance and Management (BIGM), Former Secretary of the Power Division, Former Chairman of Rajdhani Unnayan Kartripakkha (RAJUK), and Former Director-General of the Department of Environment.

Nafisa Binte Farid Prova is a BSc in Civil Engineering from Islamic University of Technology (IUT) and a Research Associate at Bangladesh Institute of Governance and Management (BIGM).