Ready-Mix Concrete has become the most common method of concreting in the world’s industrial and housing sectors. Contractors now avoid the hassle of mixing concrete on job sites as they can order Ready-Mix Concrete from different manufacturers. It has reduced a lot of trouble for the workers and enhanced workability. Moreover, Ready-Mix Concrete offers higher precision and quality than concrete produced at job sites. Ready-Mix Concrete has become more prominent in Bangladesh as well.

Just like every coin has two sides, there is also a significant downside of Ready-Mix Concrete. Concrete is a sophisticated building material that starts the hardening process as soon as the water is mixed with cement. The setting time of Ready-Mix Concrete is a crucial factor to consider before starting construction. Ready-Mix Concrete is delivered through transit trucks that pick up the concrete from suppliers to the end consumer. It has to be delivered within a fixed period of time and revolutions of the drum. Otherwise, the concrete may lose its features and become unusable at job sites. As a result, the transit time of Ready-Mix Concrete is carefully monitored.

Time Constraint of Ready-Mix Concrete

Around 3% – 5% of Ready-Mix Concrete produced in the world gets returned to the concrete plants every year. A large portion of this concrete is washed out to recover the mix’s aggregates or thrown as garbage. Due to additional transportation and concrete manufacture, this waste leads to the landfilling of hardened concrete, contaminated washout water, significant disposal, cost, and increased greenhouse gas emissions. Reducing the amount of concrete sent back to the ready-mix factory will help the concrete industry become more environmentally friendly by minimizing wasteful landfill materials and lowering the need to wash aggregates recovered from used concrete with scarce water resources.

Source: The Constructor

One of the primary reasons concrete is regarded as waste after a certain period of time is the 90-minute rule. Many international associations and building codes support this rule. It states that the concrete should be discharged from the concrete mixing truck within 90 minutes after the mixing of the materials cement, water, and aggregates has begun. This rule is followed to protect the concrete mixture’s quality and ensure the structure’s durability and strength.

Due to increased traffic in urban areas and the relocation of ready-mix plants away from city centers, it takes longer for concrete trucks to deliver goods, raising the possibility that some of them may be returned to the facility after being rejected. This time constraint might not be essential due to developments in concrete technology, such as chemical admixtures that modify the properties of fresh concrete and lengthen setting times.

Different kinds of admixtures used in Ready-Mix Concrete increase the discharge time limit. Furthermore, it helps to enhance concrete properties that facilitate more robust build and unique architecture of buildings.

Effects of Transit Time on Ready-Mix Concrete

Ready-Mix Concrete is usually produced in large quantities and is transported to distant places for placement. Sometimes the distance can be many kilometers, and transit trucks may take a long to reach the desired job sites. Hence, the long duration of transit time can reduce the quality and integrity of the Ready-Mix Concrete. Here are some different effects of transit time on Ready-Mix Concrete:

Loss of Workability

In order to prevent the hardening and stiffening of the concrete, which would impair its workability, the concrete should always be placed in the desired location within a fixed amount of time. The workability of concrete will be impacted by a long transit time. This occurs as a result of the hydration reaction that occurs when cement and water are combined, the evaporation of the water-cement mixture in concrete, and the absorption of water by aggregates.

Even while the workability of concrete depends on a variety of elements, including the constituent material, mixed proportion, ambient temperature, humidity, manner of transport, etc., a reduction in workability may make placing concrete more challenging. Therefore, the factor of loss of workability in the expected travel time required should be considered while planning to use Ready-Mix Concrete. Loss of workability can be estimated by measuring the slump or compacting factor of concrete immediately the following mixing at the RMC plant and after transportation time. This factor can severely impact the workflow on a construction site. Hence, contractors might perform some tests to measure workability, especially when the job site is far from the supplier.

Setting of Concrete

The early concrete setting may occur when ready-mixed concrete transport times are long. Retarding admixtures can be used to delay concrete setting in order to prevent it from setting. When allowing the use of a retarder, it is essential to make sure that the dosage and suitability of the retarder have been determined following the requisite trials. It should be noted that generally, a retarder’s function is reduced at higher temperatures, and occasionally, a small number of retarders appear to be ineffective at extremely high temperatures. Therefore, it is preferable to maintain concrete at the lowest temperature possible.

Additionally, it’s crucial to test a retarding admixture dose by trial and error. A large amount of admixture or retarder may cause a negative delay in the setting of concrete or may completely prohibit it from setting. As a result, the concrete will ultimately go to waste. To guarantee appropriate workability, specific admixture dosing is done in phases. In these situations, the admixture should only be blended at the delivery site. Admixture addition shouldn’t be permitted to be mixed with the concrete inside the drum while a vehicle is en route.

Time Period for Discharging of Ready-Mix Concrete

If adding water to the dry mix of cement and aggregate or adding the cement to the aggregate occurs earlier, the concrete should be delivered entirely to the work site within one and a half hours (when the atmospheric temperature is above 20 degrees C) and within two hours (when the atmospheric temperature is at or below 20 degrees C) of doing so.

It is important to verify that the time frame for concrete delivery is enough. Suppose the job site’s location makes this time frame insufficient. In that case, the delivery time period should be extended and clearly indicated while still allowing for any roadblocks (such as level crossings, checkpoints, areas with significant traffic congestion, etc.).

Make sure to prepare the job site so trucks can enter the premises. Sometimes stockpiles of other construction materials obstruct the free movement of the trucks. The workers should prepare themselves at least half an hour before the trucks arrive. It can increase work efficiency and reduce complications in work.

Suitability of Admixtures

Discuss with your Ready-Mix Concrete manufacturer before incorporating admixtures in the mix. It is an excellent practice to prepare with and without admixture concrete mix using the same raw materials for the work. The latter can be treated as a reference or controlled concrete mix. Be careful before adding different types of admixtures, as some can increase or nullify the effects of one.

How to Prevent Reduction of Workability of Ready-Mix Concrete

A water reduction agent or a superplasticizer is typically used to slow the rate of workability loss. After ensuring that the planned concrete mix is suitable for admixtures, the use of water-reducing agents and admixtures should be approved.

Trials should be used to determine the water reduction agent/superplasticizer dose. Segregation may result in an excessive dose. Retardation is another possibility.

Engineers approve re-tempering or adding water or an additive to concrete on site to make it more workable if the uniformity requirements are met. Properties including air content, slump, compressive strength, and density, among others, have variation limits specified by uniformity requirements. There is no way to determine whether the stated uniformity requirements are satisfied on-site. Additionally, the addition of water impacts the strength and durability of concrete. Re-tempering or adding water after mixing is therefore advised not to be permitted at all, either during transit or at the construction site.

It is essential to discuss with the supplier about the transportation procedure. The two primary goals when transporting Ready-Mix Concrete from the mixing plant to the Jobsite are to prevent segregation and not to reduce the workability of the mix. The transportation route should be selected beforehand, and you should check if there are any roadblocks or other issues on the route. As a rule of thumb, thirty to sixty minutes of transportation are acceptable for small jobs. Concrete from a truck mixer or agitator truck at a central or mobile ready-mix plant must be discharged within two hours. This duration is shortened to one hour if non-agitating transportation machinery is utilized. In order to avoid cold joints or honeycombing, any delays must be avoided.

Several criteria determine the most appropriate mode of transportation. When choosing a method of transport for your concrete, factors such as the type and composition of the concrete mix, the size and type of the construction, topography, weather conditions (such as temperature, humidity, and wind speed), the location of the batch plant, and cost are all taken into account. Your concrete could be segregated if you select the incorrect transportation means, rendering it unusable. Hence, you should carefully consider the Ready-Mix Concrete truck capacity and select the best mode of transportation according to your requirements.

Summary

The successful mixing and transportation of concrete depend on various crucial factors. Once these factors are carefully sorted out, you can move on to the next important step of placing concrete. The effect of transit time is a critical factor in Bangladesh. Big cities like Dhaka and Chittagong suffer from terrible traffic. As a result, delivering concrete within the appropriate time limit gets pretty tricky. Hence, you will require admixtures to delay the setting of Ready-Mix Concrete to account for the extra time. You can plan to ask the supplier to deliver the concrete on weekly holidays to minimize the amount of traffic your delivery truck has to go through. Awareness of the effects of transit time on concrete facilitates better workability of concrete and ensures the durability of the structure.

Frequently Asked Questions (FAQs)

How long can concrete be placed after mixing?

Ans: It is better to place the concrete within 90 minutes of mixing, but it can change depending on the temperature and admixtures used in the preparation process.

How long does it take to empty a Ready-Mix Concrete truck?

Ans: Unloading a truck takes an average of 6 minutes per yard. So, a 5-yard truck will take 30 minutes to unload the concrete thoroughly.

What happens if the concrete hardens inside the drum of the truck?

Ans: Hardened cement can reduce the amount of space inside the drum. So, the hardened concrete should be cleaned out properly.

Should I order Ready-Mix Concrete for small jobs?

Ans: You can order Ready-Mix Concrete for small jobs, but it may cost you more than mixing concrete on job sites.

How to order Ready-Mix Concrete in remote locations?

Ans: Sometimes suppliers establish temporary mixing plants inside or near the construction site if ample space is available to deliver Ready-Mix Concrete in remote locations.