Mix on-site concrete is the oldest way of preparing concrete for construction. Even though it’s an old method and ready mixed concrete is storming the construction field, we still practice mixed on-site concrete constructions due to the many plus points it has.
What is mixed on site concrete?
Mix on-site concrete is simply preparing the concrete mix at the site. Here we have to bring the material and mixing machinery to the construction location first. Then small batches of concrete are mixed following a volumetric mix proportion.
How to mix concrete at the site
Estimate the volume
First of all, you have to estimate the concrete volume. Calculate the required concrete volume of the structure and add at least a 5% margin for spillage and losses. After that, you can calculate the volume of each raw material according to your mix proportion. Here we usually go with volumetric mix proportions rather than weighted mix proportions. It is easy to measure volumes than weights at the site.
Store the raw material
Prepare the site locations to store the raw materials. Keep in mind that aggregates need to put under the sheds to protect from rain as well as heating under daylight. If they soak any water your consistency will vary and the strength will be loss.
For cement, use a store with controlled humidity conditions. Do not stack more than 10 bags in one column and keep them at least 2ft away from the roof and the walls. If the cement is correctly stored, you can keep them for 6 months at one place. Cement bags are not waterproofed. So keep them dry as much as you can.
Arrange the machinery
You must have a batch mixture to mix on-site concrete. There are concrete mixing machines with varying capacities. Manual mixing is possible for small works but the quality can vary from batch to batch. 2 cubic feet capacity wheelbarrow mixture will cost just over 300 dollars and after the work, you can re-sell to cover some of the cost.
Mixing process
First, you need to prepare a measuring box. Measure the material volumes according to your volumetric mix proportions and add them to the mixture. Recommended internal dimensions of the measuring box will be 400 mm × 350 mm × 250 mm. the total volume is 0.035 m3 which equal to the volume of cement in the 50 kg bag.
Common volumetric mix proportions to use
| Mix ratio | Strength (MPa) | Cement (50 kg bags) | Sand | Coarse aggregate | Water (Liters) | W/C |
| 1 : 1 : 2 | 30 | 1 | 0.035 m3 (1 box) | 0.07 m3 (2 boxes) | 20 | 0.4 |
| 1 : 1 ½ : 3 | 25 | 1 | 0.053 m3 (1 ½ box) | 0.105 m3 (3 boxes) | 22.5 | 0.45 |
| 1 : 2 : 4 | 20 | 1 | 0.07 m3 (2 boxes) | 0.14 m3 (4 boxes) | 25 | 0.5 |
| 1 : 3 : 6 | 10 | 1 | 0.105 m3 (3 boxes) | 0.21 m3 (6 boxes) | 35 | 0.7 |
Add the dry ingredients first and mix them for about 15 to 20 seconds to get an even mix. Then the water introduces to the mix before 25% of the mixing time has elapsed and continues mixing for not less than 2 mins time.
Manual mixing or machine mixing?
For mixing and handling works, you will need a minimum of 5 to 6 extra labors. The labor requirement changes with the scale of work and the transporting distances. On-site mixing, trucks are another option if you don’t want manual mixing. The truck comes with a mixing machine and hoppers for raw material. Mix the concrete at the site in small batches so that waste will be minimum.
If you mix the concrete at the site, your concrete comes in small batches compared to 7 to 10-meter cubes of concrete from ready mix plants. Then consider the batching rate and try to select a suitable concrete pump for your work.
You can use chutes to pass down the concrete to the construction of foundations. I did that multiple times for the foundation work because it was economical. Then move with the ready mix concrete for the remaining beams, columns, and slabs.
Price of mix on site concrete
The main factors affecting the mix on-site concrete price are the material price, machinery price or rental, and labor price. Since we have to follow volumetric mix proportions the raw material consumption is high. Roughly you will need 10% extra cement for the same strength grade compared to ready mix concrete.
For machinery, the cost will increase with the capacity. The cost of placing equipment and raw material transport should consider during pricing.
When compared to the option of ready mix concrete, transportation is less with mix on-site concrete. Concrete agitator trucks have less capacity than dump trucks. Nearly 3 times the traveling increases with ready mix concrete.
Mix on-site vs RMC
Yes, for most of the construction jobs, ready-mix concrete suits best. But I have faced some situations the mix of on-site concrete did better in many aspects. If you compare both first, you can see there are many advantages in both mixes on-site and ready-mix concretes.
Advantages of mix on site concrete
More independent!
The number one advantage of mixed on-site concrete is it gives you independence in the construction. Traffic, access road conditions, and breakdowns will not affect the process of mix on-site concrete. Once you transported the material and stored it in the covers, you don’t have to depend on outside matters. This is important when you have bad road traffic around the area.
No delay!
Also if the ready mix plant is located far away, your delivery time will vary from truck to truck. This can be bad for the construction. Environmental temperature and wind conditions can reduce the setting time of the concrete. This can make the concrete harden before the next concrete truck leaving unwanted joints in the concrete. Actually, these joints weak the structure, and treating them is costly.
More control in the mixing process
The third advantage is, here you can control all the mixing process with mix on-site concrete. If the material is low on quality, you see them before you start the work. You can be sure about the mix proportions that follow in the mixing. Since the batches are small in volume, even adjustments are possible without wasting too much concrete.
Easy quality control
Quality control is easy when you mix the concrete at the site. As you might know, all the batches from ready mix plants are not going through a slump and temperature checks. Typically it’s 2 times for a shift of a few for a pour. This gives some advantage to the supplier to deliver some poor batches in between. These trucks have a small water tank for cooling the drum. So, truck operators can add water to the concrete in case of low consistency due to traffic delays without knowing any party.
Adjustable rate of pour
Mix onsite concrete offers better flexibility with the pouring process. Imagine sudden rainfalls during the pouring for few hours. If the ready-mix truck is on the way you will only get few hours to place the concrete. Otherwise, this concrete will be a waste. With mix on-site concrete, it is just a matter of slow down the pouring rate and keeps the progressing end fresh until the weather clears out.
Even with the ready mix, this method is possible. However, it is easy to control the concrete rate when you mixing it at the site.
More fresh concrete at site
When concrete is mixed at the site, you have the whole setting time to pour and work the concrete. Mixing is quick and delivery time is minimum. Then workers have enough time to lay it, vibrate it, and even finish the concrete without a hurry.
Less retarders
Sometimes, ready mix concrete will need retarding admixtures to allow the transporting time. Since the mix of on-site concrete is batched at the site, you don’t need retarding admixtures due to no traveling delay. This affects the price.
Might be less expensive and convenient
Mix on-site concrete is cheaper in most cases. However, this is not due to the material cost or machinery cost is less. Ready-mix concrete is well optimized and therefore less expensive. However, suppliers demand higher profit margins. Sometimes this can be 30% and beyond. It is beret to roughly calculate the cost before you select the concrete type.
Also for smaller constructions, ready mix concrete cost will be higher than the mix on-site concrete because of plant and delivery cost. Mixing plants always target large constructions. So small orders of few meter cubes will not interest them.
Advantages of Ready-mix concrete
less work!
Ready-mix concrete is more convenient for nowadays construction because you have less work. All the raw material ordering, stocking, and handling is free from your responsibility. You just order the concrete and pays for it. No need to measure and mix the concrete batch by batch, an automated plant will mix the concrete and deliver it to your doorstep by trucks.
Less retarders!
Also, the ready-mix plants use weighted mix proportions. These are well optimized and you can alter some characteristics even. If you need higher setting time in your concrete they will adjust using admixtures. No need to handle these chemicals by yourself.
Good for large jobs
Ready-mix concrete is more convenient for larger jobs. If you don’t have enough space for material stockpiles, ready mix concrete is one solution. High delivery rates are more possible with ready-mix concretes which is more suitable for large concreting jobs.
You can place the ready mix concrete quickly with less labor involvement. The supplier will come up with a stationary pump line or a pump car to place the concrete. It’s easy and quick!. Vibrate and compact with suitable vibrators.
