Building Materials

Most used construction material in Cambodia

Cement

The most commonly used type of cement, Portland Cement, is a fine powdery grey substance usually made from grounded limestone, clay, iron ore and gypsum aggregates. Each of these components facilitates a different kind of reaction that altogether gives cement the properties that could be of use for different scenarios in construction work. For instance, gypsum in cement, balances out any reaction that cement might have with water; preventing it from solidifying too quickly. But as a whole, cement acts as a binding agent that glues aggregate together to create a strong and sturdy finished product.

Concrete

Concrete, on the other hand, is a composite material — a mixture of cement, sand, broken aggregates (usually gravel) and water — largely used in construction of structural infrastructure ranging from miniature sculptures to super tall skyscrapers and everything in between.

The terms cement and concrete can sometimes be used interchangeably (albeit incorrectly). One happens to be an ingredient to the other. Simply put, cement to concrete, is analogous to what flour is to a baked cake. You need the raw material of flour to make cake batter — the same way you need cement to cook up a batch of concrete.

Concrete, as the special mixture, is used to form the finished products such as structures for buildings, sculptures, roads, etc. Cement, however, isn't exclusively used for concrete, it can also be used as an ingredient in other finishing materials such as mortar and plaster.

Source: Cemnet, as of June 1, 2021

The key underlying raw material to cement is limestone. Large limestone deposits in Cambodia can easily be identified and characterized by those jagged karsts (rocky mountain hills) scattered throughout Cambodia. Currently, all domestic cement production plants, are found in two provinces — Battambang and Kampot — with Kampot being home to 4 out of the 5 producers. Once cement leaves its plant, it can either be sent out into the market to be sold for light purposes, such as mortar or plaster mixing, or be sent to a separate concrete batching plant where it is used in the production of concrete.

Concrete Reinforcement

Reinforced concrete refers to regular concrete whose strength has been reinforced by another material — most commonly steel — to absorb the tensile, shear, and sometimes the compressive stresses in a concrete structure.

Rebars are metal, typically steel, rods that are inserted into concrete for additional reinforcement. In the past, standard reinforced concrete utilized plain bars with smooth surfaces. Deformed rebar was introduced around the 1920s to replace the structurally inferior plain bar. Modern rebars are covered with patterned bumps called “lugs” which grips onto concrete to prevents any slippage when under intense stress.

Concrete holds up very well against compressive stress that's usually put on by the weight of people, furniture and appliances called “live loads". But its major flaw lies in how brittle it can get when that compressive stress converts to tensile stress — basically, when the concretes stretches and bends. Due to its non-elasticity, pro-longed stretching causes the concrete to crack from the bottom where tensile stress is most prevalent. This is where steel rebar comes in to counter all the tension and provide concrete the structural elasticity that it lacks.

Rebars are further classified based on their varying performances brought about by the chemical composition of the raw steel and its yield strength. Certain countries have their own industrial standard of steel classification (JIS-Japan, ASIC/ACI-America, etc.). Rebars found in Cambodia closely follow the JIS steel code. This classification helps identify the appropriate use case for each grade based on its yield strength — the strength in which a material can retain elasticity under until it deforms permanently.

The lowest performing of these grades, the SD295 (Steel Deformed with a 295N/mm2 yield strength) is sometimes used economically in less load-bearing areas such as roads or ground slabs while the mid-tier SD390 is often regarded as the national standard for buildings within Cambodia.

Known as “cover”, the distance between the rebar and the exterior of the concrete is also important as the closer the rebar gets to the surface, the higher the risk of it rusting — impacting the reinforcement strength. Before the castings of concrete, batches of pre-casted concrete spacers of around 20–70mm in height are placed in between the rebar and the formwork before the casting of concrete ensuring that the rebar cage is appropriately placed.

Brick usage throughout Cambodian history

Pre and early Angkorian religious temples and shrines — such as the ones found in Sambor Prei Kuk, Mount Kulen, and the Rolous region — can be characterized as being modestly-sized, with some intricate stucco carvings, and comprised entirely of stacks of ancient red clay bricks. However, as the Khmer empire grew richer, larger and more extravagant temples became the norm. Thus, large blocks of sandstone, easily extracted from the Kulen Mountain, became a more practical substitute to the old-fashioned red bricks, as they could provided more structural support to those newer monuments while also allowing for even more intricate carvings (i.e. Bas Reliefs of Angkor Wat). However, post-Angkor, the red bricks, which were more cost efficient, experienced a renaissance with the construction of Buddhist pagodas.

The archetype of residential buildings during these periods stayed relatively the same: wooden or thatched houses raised on stilts above the ground. This way of housing persisted for a long period of time due to the abundance of local/natural resources such as wood and palm leaves as well as the notion that masonry structures were reserved only for the holy.

It was only until the start of French colonial rule over Cambodia, did we start to see the steady shift towards residential buildings being built out of masonry. The French brought with them the concept of grounded masonry structures supported by solid foundations. That, along with the perks of it being more fire and termite-proof, really helped the trend to pick up among the locals — especially, after the great Phnom Penh fire of 1894 that pillaged most of its wooden structures. And as of today, as much as 80% of the country's development projects are still built from red bricks.

How bricks are made in Cambodia

Source: Building and Wood Workers Trade Union Federation of Cambodia, 2019

There are currently over 400 active brick kilns found in Cambodia (predominantly located along the Mekong River and the Tonle Sap). Unlike the rocky and mountainous regions lining the borders of Cambodia, the geological make up of the central flood plain areas are dominated by sediments that had been broken down by rivers and streams over millions of years. These flood plains provide for a rich deposit of clay soil (ideal for the harvesting of brick making). Kilns can often be found in clusters around specific areas that are thought to have the best clay soil for higher quality bricks (i.e. Prek Anchchanh or Neak Loeung in Phnom Penh).

The modern method of brick making in Cambodia involves using simple ingredients such as clay soil, water and firewood. Here’s the step by step process:

Clay brick being pushed through a mold machine.

Credit: Goddard_Photography/iStock

Shaping the bricks

To achieve its desired rectangular form, a mixture of clay and water is pushed through a molding machine to form a continuous brick block that are then hand-cut into pieces of equal sizes using cutting wire.

Drying & Hardening

The dark and almost-solid yet-still-malleable pre-baked blocks are then piled onto a cart and are wheeled to a field where they are laid to sit atop of each other in rows. After being left in the open for 4-5 days to dry and harden, these blocks will start to become lighter in color.

Nicely organized rows of stacked pre-cook clay bricks.

To the kiln

After they've hardened enough, they are then transferred into a large kiln where they are packed and organized in a manner which would maximize the capacity of bricks being produced and is sealed using some pre-cooked bricks.

Baking

Torched firewood is thrown in as the main source of heat through small openings on the sides — creating temperature that can sometimes reach up to 650-degree Celsius.

A small opening beside a brick kiln surrounded by firewood.

Credit: Goddard_Photography/iStock

Additional Heat

Heaps of rice husks are sometimes also shoveled in from the side of the kiln to intensify the flame.

Completion

After the cooking process is complete, the now red clay bricks and the kiln itself are left to cool for approximately 10 days before workers can safely go in and unload the finished products — which would then be ready to be shipped off and sold.

A kiln opening with baked red clay brick stacked on top of another inside.

Soil types used in contruction work

The term “soil” loosely referred to the top layer of Earth — the result of millions of years of bedrock/mountain erosion. In addition to supporting terrestrial life, it can also lead to a plethora of uses in construction. Typically, soil is classified into 4 main categories: sand, loam, silt, and clay — all differentiated from one another by the sizes of their particles and their ability to retain water. Water retention is a major factor when choosing the appropriate type of soil for construction work. Smaller particle size makes it difficult for water to flow through since there's naturally less space in between each particle. Hence, finer soil types such as clay and silt are less permeable — meaning they have a tendency to hold onto more water for a longer period of time.

Source: DCS & Associates, 2019

Source: EduSmart.com

Sand has the largest particle size. When mixed with water and other substances, sand allows for greater flowability amongst all ingredients to evenly spread out and to adhere to each other. This makes sand an ideal filling material for mixtures like concrete, plaster, and mortar. Sand is also used in construction backfilling — where water retention must be minimized at all times to avoid damage on a building's overall structure (caused by poor substructure drainage).

Loam, a mixture of sand, clay and silt, has a slightly higher water retention capacity than sand. Sandy Loam can sometimes be substituted for construction backfill in areas where the pricing is more affordable. Sometimes, to further strengthen such backfill, rocks are also compacted alongside the sand or loam.

The finer grain soils (silt and clay) are also of use. As we've learnt before, clay-rich soil can be extracted to produce clay bricks used in masonry works as well as for clay floor/roof tiles (found throughout Cambodia/Southeast Asia). On the other hand, silt is often used as a nutritious base for gardening.

Clay soil's particles hold onto each other very strongly, thus, it doesn't easily crumble when wet or dry. This allows for moldability, a factor which many cultures around the world use to their advantage to produce meaningful clay products such as tiles, bricks, ceramics, etc.

How Cambodia got its abundance of river sand

Located along the lower-level flood plains of the Mekong River, Cambodia's share of the Mekong riverbed is blessed with an abundant deposit of highly-prized quality river sand — perfect for construction work. About half of the Mekong River sand that annually flows downstream into the South China Sea (crossing through the delta region into southern Vietnam) is fed by the erosions from the rocky upstream basins located further up in the Tibetan Plateau of China — the source from which the Mekong River starts. This high concentration of sand along with other sediments found in Cambodia's major rivers are what gives these rivers their signature yellow-ish hue.

Sand is further divided into 2 typical grades based on their grain sizes. A higher grade means that the grains are much finer and that the batches are much purer (free of other elements such as clay and silt). Since this higher-grade sand is required to go through an additional refinement process once it's been dredged out of the river, the price tag on it is also unsurprisingly heftier than its lower grade counterpart.

Generally, aesthetic appearance doesn't really matter in structural work (such as concrete, mortar mixing, and backfilling) — given that they'll eventually be covered up by other finishing work. Therefore, the coarser and inexpensive lower-grade sand is usually added as a raw filler material in those structural mixes. Meanwhile, finer and more expensive higher-grade sand is reserved for finishing works like plastering where it can help achieve smooth and undisrupted surfaces.

It is also important to note that although the coarser grain sand may in theory be more preferable in most backfilling scenarios for its affordability and its capability in providing better substructural drainage, the larger gaps in between each coarse particle can also make it harder to ensure sturdy compaction. To account for this, engineers must find a suitable mix of aggregates that strikes the right balance between these three obstacles. In some cases, sandy loam — which contains other finer soil types for compaction as well as a huge fraction of sand for good drainage — may be ideal.

002. Building Materials

The importance of knowing what you're working with