Written by: Mr. Martin Gerard Joachim David
Concrete is perhaps the most important construction material other than steel for the construction fraternity. It is easy to develop a specific type of concrete at the university laboratory but the story is vastly different when the same concrete is placed at the construction site in a bigger volume. So it is important to understand the different components of concrete and how the different components interact within the concrete mix.
Traditionally, concrete is made of cement (Ordinary Portland Cement – OPC) + Water + Sand (Fine Aggregate) + granite stones (coarse Aggregate). For ready mix concrete, concrete admixtures such as retarder and superplasticiser are normally added to allow for transportation and maintain concrete specification.
Cement (OPC) is the binder or the glue that bonds the fine and coarse aggregates together when both are mixed together.
Water acts as the activator of the cement and help the transportation of the cement in a paste form around the aggregates so that a bond is created between the different ingredients inside the mix.
The sand (fine aggregates) fills the voids in between stones (coarse aggregates). When the cement paste is set and given enough time to cure, concrete in its hardened state is formed.
For the understanding of concrete enthusiasts, a basic design of concrete mixes is explained below:
First of all, concrete that uses Ordinary Portland Cement (OPC) as the binder, can achieve a design strength of up to 60 N/mm². Beyond this, special additives will have to be added.
Generally, concrete strength starts with grade 15 N/mm². For every increase of 5 N/mm², it is considered one grade higher.
Now certain “Rule of Thumb” needs to be known and they are briefly explain as follow: -
· 10kg/cement 1N/mm²
· Water to cement ratio (WC ratio) not more than 0.5 (unless otherwise prescribed).
· Approximately 20% of cement will not hydrate in the mix and will act as fillers.
· Sand to stone ratio (SA ratio): 0.4 to 0.45
· 1m³ of concrete is about 2400 kg/m³
Assuming a Grade 30N/mm² concrete is required the calculation for the mix per m³ is as follows: -
· Cement 30 x 10 = 300kg/m³
· Added 20% to compensate for the cement not hydrated.
· Therefore cement content is 360 kg.
· Water ( wc ratio) is 0.5 x 300kg = 150 kg/m³.
· The cement and water weight are added to become 510 kg.
· Since 1m3 of concrete weighs around 2400 kg, the weight of the stone + sand is 1,890 kg.
· Now how much sand and how much stone to be used?
· So we use the SA ratio of 0.45. We multiply 0.45 x 1890 kg (sand + stone)
· Therefore sand will be 850.5 kg and stone will be 1039.5 kg.
· In summary the theoretically grade 30 concrete mix will be
Cement : 360 kg
Water : 180 kg
Sand : 850.5 kg
Stone: 1039.5 kg
Now we have to look at the basic admixtures which are retarders and superplasticiser. Without them, commercial supply would be impossible.
Concrete without retarders start to set after 30 minutes once it is mixed and around 1½ to 2 hours for final set. This will make transportation and workability impossible unless the batching plant is located within the construction site. In most cases they are not. So retarders will have to be added into the concrete mix to prolong the initial set to approximately 2 hours and final set approximately 5 to 6 hours. The retarder works by lining the cement particles thereby not allowing the water to activate the cement. After the retarder time has become worn off, it will then allow the water to be in contact with cement and start the activation process. Keeping in mind that overdosing may cause issue with non-setting.
The superplasticiser is mainly acting as a water reducer where by you can maintain workability without increasing water content and compromising on the strength of the concrete. How this works is that the superplasticiser lines the cement particles with a negative charge thereby repelling particles from each other, thus avoiding clumps and releasing trap water in the cement clumps. Water can be reduced by 25% approximately.
In today’s technology, concrete is produced in automated or semi-automated batching plants. The pictures below give an idea on the production flow.
Once the concrete is place, a poker vibrator is used to vibrate to compact the concrete and to release the air trapped inside the concrete.
In order to work on the concrete, the concrete must not be too wet or too dry.
In order to check the concrete before placing to ascertain the design slump which indicates it has been produce according to design specification.
A slump check is done.
To ensure that concrete has met its design, a compressive test is carried out 28 day after production.
This article aims to provide a basic understanding of the concrete mixes. For more specific mix design, a more in depth understanding of concrete and others influencing factors are as important to achieve the intended outcomes. You may consult the professionals.