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ACHIEVING OPTIMAL EFFICIENCIES IN CONCRETE BLOCK PRODUCTION

September 12, 2023 1036 1036

The delivery of high-quality bricks and paving blocks to the market at the lowest possible input cost requires a fine balance in the composition of material inputs which must be monitored constantly to achieve optimal quality and cost efficiencies.

Over the years I have found that the three most important requirements of brick and block making to be aggregate quality, moisture control and production time cycles as measured in seconds, and these factors were paramount in the establishment of a new brick and paving block plant in Vryburg in 2020.

My initial exposure to the industry began in 1995 when I joined our family brick and paving block business in Bloemfontein. My first responsibilities centred on production systems and procedures, but even before I joined the company, my father told me of his excitement in 1974 when he bought his first brick and block-making machine from PMSA.

The business expanded in 2013 with the acquisition of a brick-making plant in Kuruman, and shortly thereafter, we acquired another plant in Kimberley.

Owing to my involvement in the Northern Cape market, I was presented with an opportunity in 2019 to become actively involved with Northwest Crushers in Vryburg, North West Province. The main purpose of this venture was the establishment of a brick-and-block manufacturing plant in a joint venture with the shareholders of the existing crusher and ready-mix plant. The new plant would source its aggregate from the plant thus forming a synergistic raw material to finished product relationship.

Planning, developing and erecting a brick making plant from the ground up posed a series of challenges, but that did not curb my excitement or enthusiasm and I relished the prospect of working together with Joe Deetlefs, a director and shareholder of Northwest Crushers, in bringing the project to fruition.

A unanimous decision was taken to acquire all the manufacturing machinery from PMSA in Johannesburg, a decision which was heavily motivated by past experience with PMSA equipment and the ready availability of back-up and spares.

Affordability also played a major role in our choice of equipment, long-term maintenance and product quality being the prime considerations, and we opted for a fully automated VB4X plant in combination with a finger car system, curing chamber and the newly developed Cuber (clamping and packing the bricks) packaging system.

The plant was ordered in December 2019 and the installation was especially challenging due to the fact that delivery and installation coincided with the COVID 19 pandemic and national lockdown. However, despite social distancing and other restrictions the first bricks were produced in September 2020 and we paid particular attention to the three main variables, aggregate quality, moisture control and production time cycles. It is critical that these three elements are continually monitored if the quality of the finished product is to be kept at a constant standard.

We are very privileged by being able to mine and crush our own aggregate because it enables us to ensure that our manufacturing plant is fed with a high-quality raw material. What’s more, our crusher dust is classed as sand with a grading from -2 to +0.5mm and our washed crusher dust has a grading of -6 to +2mm. Although unwashed crusher dust does not have a consistent grading, through the process of washing and splitting, the washed crusher dust and sand can be mixed back together in a regulated manner.

We also grade our aggregate weekly to ensure size consistency. This results in consistent compaction throughout the manufacturing process and is also beneficial to the overall consistency in the height or thickness of precast units.

Our material mixing is done by using a PMSA P1500 PAN Mixer with a capacity of 2.25 tons. It is equipped with a high-speed agitator, which is crucial for effective mixing, and our cycle time per mixture can be as low as 165 seconds.

Moisture is the most important element in brick strength. It is common knowledge that by adding the optimum amount of moisture during manufacturing, and retaining moisture during the curing process, it enhances strength.

We use moisture measuring probes in our material bins and the mixer. We do this because washed aggregate already contains high moisture levels which vary constantly. Rainfall is another contributing factor to moisture content. The moisture detected by the probes in the material bins is calculated as a percentage of the measured weight, and where necessary, the mix is adjusted and more material is added to compensate for the weight of the moisture.

For example, if a batch is measured as 1 000kg and has a 5% moisture content, then the moisture weighs 50kg. In this case 50kg of aggregate will be added, thereby upping the overall weight of the mixture to 1 050kg and increasing the solid material weight to 1 000kg. If moisture measurement is omitted, the ratio of moisture to aggregate content fluctuates and also affects the mixtures cement percentages.

Therefore in order to sustain consistent mixture ratios, it is of paramount importance that moisture content is measured. We acquired a moisture-measuring system at great expense, but have found that the benefit of product consistency, have made it well worth it.

Also, with moisture measuring equipment in the mixer, it allows for water addition adjustment to be done automatically to achieve optimal mix compositions. Moreover, this precludes the need for an extra person to estimate moisture content. And establishing consistent moisture to material ratios in the mix facilitates efficient production with little-to-no fine tuning of the brick making machines required.

The VB4X machine is capable of maintaining a cycle time between 12 to 14 seconds per pallet throughout the production cycle. According to the PMSA, the machine can manufacture 1 800 pallets of bricks per day. At an average of 13 seconds, it is theoretically possible for 1 800 brick pallets to be manufactured in six and a half hours.

The remaining three and a half hours of a normal shift is generally wasted due to chronic downtime. Experience has shown that if the mixture stays consistent, the manufacturing machine becomes the lowest contributor to downtime. In our experience, most downtime is caused by the packaging process which delays the return of production pallets to the production line.

We employ a closed system whereby the pallets are returned automatically to the production line via a conveyor and we have introduced several measures to mitigate any delays in this process.

One is the use of battery-driven strapping machines which are much faster than pneumatic machines and only require one machine and two operators to maintain a strapping cycle time of 13 seconds.

We also change machine brick moulds frequently because worn moulds causes the Cuber process, to drop the bricks, and cause downtime.

If we find more than two reject bricks on a pallet, we discard the entire pallet. It is immediately sent back to the mixer while the bricks are still wet and recycled. The net effect of this practice is that less time is wasted in the replacement of rejects during packing and sorting.

Storage also plays an important role in cost-cutting. We store our smaller bricks furthest from the plant and our larger bricks, such as Maxis, are stored closest to the plant. We have planned our storage yard in this manner due to the fact that a pack of maxi bricks is packaged every 78 seconds and a pack of 50mm pavers every 158 seconds. This correlates with the drive time of the forklift and allows the pallets to be moved timeously without causing unnecessary downtime.

These are a couple of simple measures we have taken, providing incredible results in terms of production efficiency. It is also worth noting that every plant is different and that these measures may not be beneficial to other plants.

Nevertheless, we are able to manufacture 2 000 pallets of bricks per day, calculated over an average of 21 days per month in a single shifts. We also stagger our lunch times so that we never have to power down during manufacturing shifts.

In conclusion therefore, it is more beneficial to address productivity in terms of moisture, aggregate and cycle times, rather than focusing on savings in cement. The fact remains that a 10% higher production rate carries greater financial benefits and savings potential than a 5% cement saving which can have a negative effect on product quality.

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