At the start of production, dry materials from the aggregate loading weigh bins, and cement from the cement weigh hopper, are conveyered into the stationary truck mixer. For each project the proportions of materials in the mix are custom-designed and computerised to ensure quality control. The customer typically determines characteristics such as aggregate size, slump, air content and strength based on the intended use of the end product.

Typical composition by volume is about 10-15% cement, 60-75% Mining & Aggregates and 15-20% water. Entrained air bubbles may also account for 2%. Using less water generally results in a higher quality concrete.

A readymix truck typically has a capacity of 6m³ - 9m³ and discharges the concrete from the rear. Because slump loss can occur during transport, it is required that the concrete be discharged on the job site within 45 minutes. Admixtures such as air-entraining agents, water-reducing agents and concrete-retarding agents may be added to the concrete at production stage in situations of extreme temperature or long delivery times.

The Putzmeister BSF 24Z.09 pump (2x BSF 24Z and 1x 32Z) was purchased directly from Germany and is equipped with four boom sections with a pumping capacity of 90m³/ hr. This plant can also withstand up to 71 bars of concrete pressure - a high tolerance for varying concrete densities.

The boom reach and pressure capacity benefits customers with challenging placement requirements such as high-rise, high gradient construction or simple efficiency and effectiveness. Other key benefits include:

• Quick setup and fast-pace;
• High-speed pinpoint placement capabilities;
• Long reaches for the toughest jobs;
• Minimum labour to place concrete;
• Cost-effectiveness;
• Optimised design reflecting experience;
• Flexibility with large accessories packet and use for longer-term operations; and
• Four-section boom technology enabling greater reach to the poorly accessible points of placement.
  

The division makes use of the TFM Mixer due to its durability. Other benefits include:

• Domex subframe
• Domex blades in the mixer drums
• Wear protection on top of the spiral
• The feed chute, vee chute, discharge chute and extension chute are manufactured from domex
• 150 litre compressed air water system
• Rexroth or Eaton pump
• ZF PLM 7 gearbox

1. Hot weather concreting

The challenge:

Hot weather accelerates the setting time of fresh concrete and increases the rate of slump loss.

Hot weather may be defined as any period of high temperature in which special precautions must be taken to ensure proper handling, placing, finishing and curing of concrete. Hot weather problems are most frequently encountered in the summer, but the associated climatic factors of high winds and dry air can occur at any time especially in arid climates such as South Africa. Generally, high relative humidity tends to reduce the effects of high temperature.

High temperatures alone cause increased water demand, which in turn will raise the water-cement ratio and yield lower potential strength. Concrete that is cured at high temperatures early will therefore not be as strong at 28 days as the same concrete cured at moderate (20ºC) temperatures.

Tips to be considered:

Before and during the pour:

• Schedule for cooler times of the day
• Thoroughly wet the sub-base and shuttering to help prevent water loss
• Have sufficient skilled workers onsite to quickly place and finish the concrete
• Notify Readymix of any delay in placing to re-schedule deliveries
• For large, slow or long-distance pours, use set-retarding admixture
• Use evaporation retardants or fog spraying
• Use crack inducers or grooving tools
• Erect wind and sun shades

Post pour:

• Never add water to concrete that is more than one and a half hours old
• Begin finishing as soon as the shine has left the surface of the concrete
• Begin curing as soon as finishing is completed
• Continue curing for at least three days
• Never use accelerators to accelerate the setting process

2. Cold weather concreting

The challenge:

Cold weather creates problems with fresh concrete because it extends both the setting time and strength growth. For every 5ºC drop in temperature, setting time is extended by one-third.

Tips to be considered:

Before and during the pour:

• Order concrete with a higher cement content (hydration creates heat which accelerates setting time)
• Use low-slump concrete
• Avoide adding water to the concrete
• Avoide pouring at night (cold ground will cool the concrete even further)
• Protect the concrete from cold winds to prevent wind chill and high evaporation rates

Post pour:

• Protect the concrete from cold winds
• Maximise exposure to sunlight and warm winds
• Cover the concrete with an insulating material such as polystyrene or plastic
• Cure the concrete properly
• If curing with water, keep the concrete surface constantly wet for a minimum of three days

3. Concrete testing

Frequently Asked Questions

What mix do I need?

Higher-grade or higher-strength mixes are used for flooring and external paving whereas lower-grade mixes are used for foundations. Adding water to a mix will:

• reduce the compressive strength of the concrete;
• increase the potential for shrinkage and cracking;
• increase the possibility of segregation; and
• increase the potential for future problems with the surface finish.

How much does a cubic metre of concrete weigh?

One cubic metre of concrete weighs around 2,4 tonnes.

Will the truck mixer be able to get to the job?

The truck mixer needs to have safe and reasonable access to the job and cannot be held responsible for damage caused to footpaths, driveways, kerbs, lawns, drains, manholes, etc. When loaded, truck mixers could weigh in excess of 20 tonnes.

What site preparation is required for a Readymix truck?

Before commencing any excavation an assessment of the site is always useful, including location of any services such as buried electricity cables or gas mains,
drainage systems, overhead cables for electricity or telephone, overhanging tree branches and tree roots.

What protective clothing is required?

It is important to be aware that concrete is both abrasive and alkaline, therefore it can cause skin irritation and burns. Eye protection, long-sleeve shirts and trousers should be worn along with rubber boots and gloves.

What should I expect when the concrete is delivered?

When the driver arrives onsite, he will assess the required delivery location and establish the most suitable means of discharging the concrete into place. If the driver cannot get the truck mixer close enough, off-loading directly into wheelbarrows will be possible.

What is the best way to handle and move the concrete?

Minimise the manual handling of concrete by ensuring that the truck mixer is as close as possible to its final location. The less you move it, the better. Start pouring the concrete in a corner and work away from it. Ensure the correct tools are used such as a square-nosed shovel or concrete rake - using the wrong tools can cause segregation.

How long before the concrete sets?

Once the concrete is delivered to site, there will be between one and two hours to place, level and compact the concrete and to finish the surface.

Why is compaction so important?

Compaction ensures that the optimum density of the mix is achieved, increasing compressive strength. Where reinforcement is used the bond between the concrete and redistribution bar is improved. Permeability of the concrete decreases and causes honeycombing entrapped air and segregation.

What curing techniques could I use?

Curing concrete enables it to reach its designed compressive strength, making the material more durable. To cure correctly concrete requires sufficient moisture content, a favourable temperature between 10ºC - 20ºC and time to reach its specified strength (a minimum of 7 days to reach 70%).