Know your Concrete...

We have seen few customers uncomfortable with the concept of Ready Mix Concrete though it is a widely proven standard to give precise product compared to regular hand mix. We know how much those customers are losing-out in terms of handling the whole complexities at the construction site rather than simply procuring ready Mix Concrete. But on the other side of the equation, we also know the fears of the builders which are faced by the market and their previous history. The problem boils down to bad concrete quality even though they procured from RMC plant. The complains were as common as improper water content to as bad as concrete not setting for days together which is a debacle. This newsletter shows the relation between the concrete quality and the plant-to-site distance.

The choice made by buyer for supplier finalization has to be done very carefully by giving enough importance to the distance between the plant and the site. Most of the times, Builders choose suppliers just based on the price quotation. When the distance between the plant and the site is very far and considering the traffic conditions of the Tier-1 cities in India, the probability of getting good mix comes down significantly. The following properties of concrete gets affected due to long haul. 

1. Workability: Due to excess time taken for transportation, the water content will get decreased because of evaporation, absorption by aggregates and hydration reaction between water and cement. This less workability will lead to many imperfections like pumping problems, segregation, etc.

This problem can be sorted our by measuring the slump for every vehicle. Generally suppliers address this problem to make sure the pumping goes well. However it is advisable to have a resource to make sure this issue is not triggered.

2. Setting of Concrete: When the transit time is high, initial setting of concrete takes place which is not an ideal thing to happen. Using plasticizers(retarders) is the obvious solution for this situation. But this is the case where the market has seen the worse cases. Improper usage of retarders can prolong the setting of concrete to more than the desired time or sometimes the concrete doesn’t set at all.

3. Transit Time: In order to maintain the workability and setting of concrete, the concrete should be delivered completely to the site within 90 mins (when the atmospheric temperature is above 200C) and within two hours ( when the atmospheric temperature is at or below 200C)of adding the mixing water to the dry mix of cement and aggregate or adding the cement to the aggregate whichever is earlier. This is very tough in the cases where the batching plant is very far away.

So in order to address all the problems, there is one single and simple remedy. It is always better to have the plant nearby. Builders shouldn’t get tempted for the minimum rates quoted by the suppliers who are far away yet quotes very handy price. This practice keeps the whole concrete quality at stake.

It is often observed in Indian market that the GGBS(Ground Granulated Blast Furnace Slag) is not widely accepted by buyers though it is a promising replacement of cement upto a considerable proportion. Many builders including some big scale developers are often demanding 100% cement in the concrete they purchase. Let’s discuss now some ill affects of this decision.

Now, everybody understands that cement is the primary binding material in concrete. It is the substance which holds all the other solid and graded material together. And then GGBS was identified. GGBS not being a direct non-cementaceous substance, there is no wonder why normal builders generally reject this though commercial batching plant suppliers actually insist in going for it. No body wants to take the risk in experimenting their structure with some new material and this decision is often backed-up by few rare news from fellow buyers with bad experiences from low grade suppliers.

But here’s the fact. GGBS is not new anymore and there are enough researches and evidences to state that using it is totally safe. Not just safe, it is actually recommended! There are benefits in every aspect of it which are listed below. But before going to technicality, lets discuss the costing part. GGBS is roughly half the price of cement. Therefore you can work on the costing part to replace almost 50% of the cement value(strictly depends on supplier) so you can avail that cost benefit. But at the same time, you just cannot believe every supplier who is giving you 100% cement mix at a very minimum cost. There all the chances that your concrete is being with mixed with GGBS already and you are just not being informed.

Looking at the benefits of the GGBS, there are speculations that GGBS mixing is going to be made mandatory by few governments to increase the life span of the structures and also lower the carbon footprints on the world. The durability of the concrete is healthily increased by GGBS and here are some points:

Creep and Shrinkage of Concrete with GGBS: The measure of deformation of concrete under sustained load with age is named as a creep. Shrinkage can be either drying, chemical or autogenous shrinkage. The prominent one is drying shrinkage that is mainly caused due to the evaporation of water from the voids of the concrete structure. Blending concrete with GGBS given prominent results to curtail the amount of shrinkage happening over a period of time thus increasing the life of the structure.

Chloride Resistance of Concrete with GGBS: A study conducted by Dhir et al. (1996) showed that the chloride resistance of ordinary portland cement concrete in increased with the addition of ground granulated blast furnace slag.

Sulphate Resistance of Concrete with GGBS: The sulphate attack is an undesirable factor to which most of the hardened concrete are subjected to. It results in deterioration of the respective concrete structure. GGBS role is that it keeps the concrete highly impermeable, thus limiting the impact of sulphate.

Freezing and Thawing of Concrete with GGBS: The variation of concrete properties with the change in temperatures is an important factor questioning the durability of the structure. Both freezing and thawing are primarily caused due to the entrainment of water with the concrete. The GGBS particles help in reducing the porosity which is core reason of water entrainment. This itself give an entirely different mineralogy for the concrete. 

Corrosion of Reinforcement in Concrete with GGBS: Studies have shown that there are a great relation and dependence between the porosity and the corrosion activity of a concrete. The replacement of GGBS with silica fume with the concrete conventional mix have proved better performance that OPC concrete under corrosion resistance.

Often during the concreting process in the construction sites, there is a common concern and complaint of all the parties. The water content in the concrete. One important equation which should be engraved in all the mind before the process is that “Water content is directly proportional to the pumpability/ease of placing of the concrete and inversely proportional to the strength of the concrete”. This is the very reason why the water content is very crucial and should be calculated carefully in the mix design. Although everyone understands the concept of slump, it is generally challenging to track the exact measurement of every batch or every lot of mix. Therefore, following some measures and practices at the time of concreting to maintain a healthy and desired slump is beneficial. Here are some of the important aspects which cover most of the challenges.

Batch Slip:

Check the water content in the batch slip and judge it according to the convenience of placing and pumping ability. At the beginning when you feel the slump is not according to your necessity, try to change the water content by coordinating with the batching plant team until you get the required slump or visible viscosity. It is imperative that your combination should be cross-checked by the slump cone test at concreting site. Once you achieve the needed slump, all you have to do is keep an eye on the water content in every batch slip and maintain its unanimity throughout the process.

In the sites where batching is done manually or by small millers, the quantity of water should be calculated and should be maintained every-time.

Monitoring Pumping & Placing Teams:

Many times the labor teams who deal with the pumping and placing process are taken on lumpsum basis. So often they mind their comfort and try to add water in the concrete so that the process becomes fast and easy yet diminishing the concrete quality. To avoid this, it is advisable to deploy a dedicated engineer to mind the water content while pumping the concrete. Generally, this water is mixed near the location of pump or transit mixer. The appointed engineer would be more productive of he has a good knowledge about the water levers and hoses inside the transit mixer and the pump.