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How to work with readymix concrete

Surface mining industry association, ASPASA, has launched a drive to inform readymix company operators on best practices when ordering aggregates to ensure consistency of supply and correct usage of the materials.

Aspasa is also making the construction/building industry aware to only purchase legally mined aggregate and sand. The  Assets Forfeiture unit could seize companies assets if illegal materials are used.

Quarries and readymix companies have a uniquely symbiotic relationship and are often co-located or situated nearby a quarry to ensure sustainable supply and drive down the cost of concrete production. By sharing best practices with readymix customers, ASPASA members hope to create a more sustainable readymix industry.

ASPASA director, Nico Pienaar

According to ASPASA director, Nico Pienaar, it has become abundantly clear that since the demise of the South African Readymix Association (SARMA) two years ago, the industry has been left floundering with little assistance and Aspasa is getting many calls for advice. New entrants often don’t understand the issues and the use of readymix concrete and how to order it. ASPASA released the following guidelines that may be useful:

Enquiries and tenders

As much information as possible should be provided to the readymix concrete company at the enquiry or tender stage. The contract specification may contain specific information pertaining to constituent material restrictions, minimum cement content, contents and maximum water cement ratios.

It is therefore essential that the readymix concrete supplier has sight of this order to determine if the locally held materials will be suitable or whether alternative materials need to be sourced. Please note that if you do not supply the full contract specification, this may lead to incorrect concrete being ordered.

Additionally, it is essential that the correct consistency and workability requirements are specified for the job. An estimate of the volume of concrete required is also essential. However, any additional information pertaining to “placement rates” would be useful, as forward planning will allow the readymix concrete producer to plan for the use of additional vehicles or late working etc.

Placing orders

To avoid confusion, always refer to the quotation and concrete reference number or letter when ordering rather than referring to the concrete description or part of it, as it is highly likely that a number of concretes will have the same compressive strength class but may then differ in terms of the specified maximum water cement ratio and or minimum cement content.

Your order and subsequent delivery of readymix concrete will be based upon the requested consistency and workability. Additional cost may be incurred if the original consistency or workability of the concrete is increased, as additional cementitious material will be required to maintain the strength and maximum water cement ratio requirements.

An important point to remember is to order the correct consistency for the job, rather than adding water on site. This is a bad practice and will adversely affect the quality of the final product. It will also render void any guarantee with regards to the concrete. It is also important to allow sufficient lead time for delivery when placing orders.

New readymix customers

Readymix concrete companies will usually advise and recommended concrete to be used (based upon relevant SABS 878 Standards) and will also be able to help with regard to calculating the quantities and volume of concrete required. It is important to also ask about the use of “self-compacting concrete” as this may be an ideal option with regard to ease of placement and reducing time and effort and also overall costs.

However, please be aware that this is only suitable for projects where the top surface is approximately level. Coloured and pattern imprinted concretes are also available and it pays to ask the readymix concrete company for more details.

Most readymix concrete companies have a selection of delivery vehicles, which vary in design and size to suit the type of work undertaken. Generally, delivery vehicles are available from 2m³ to 8m³, with the typical size being 6m³. The typical dimensions of a 6m³ trucks are as follows: approximately 9m long, 2.5m wide and 3.8m high. Weight: Approximately 10 tonnes when empty and 25 tonnes when fully loaded with normal weight ready mixed concrete.

Correct placement

On arrival at site, the readymix concrete is discharged in a controlled manner down a chute that extends approximately 7.7m from the back of the vehicle. The chute can be maneuvered from side to side, and to a limited extent vertically to assist in discharging the concrete as close as possible to its final position.

Some readymix concrete companies can offer specially designed delivery vehicles that incorporate either a “concrete pump” or “conveyor”. The readymix concrete company should be consulted with regard to availability of pump and conveyor trucks as well as their placement distance, height, weight, dimensions and extension capabilities.

Safe access for the delivery vehicle must also be provided, taking into consideration the size and weight of the truck when loaded with readymix concrete and the ground conditions, as well as access. Please note that the truck driver will maintain a safe distance from any excavation or area considered dangerous. Always consider how the concrete is to be moved from the point of discharge to its final position prior to placing an order.

Part loads

“Part load” charges may apply. The cost of this will be built in to the quotation at the enquiry stage. If a user under-orders and requires an additional delivery to complete the job (often quite small in volume terms), this may be relatively expensive due to the application of further part load charges.

Advice should be sought from the readymix concrete company. Also note that readymix concrete companies will need a period of time for adequate discharge, which is free of charge. However, please be aware that charges may be incurred following expiry of this period. Advice should be sought from the readymix concrete company.

Please be aware that if you have over-ordered and wish to return any readymix concrete which has not been discharged from the delivery vehicle, a charge may be incurred. Advice should be sought from the readymix concrete company. The production, ordering, delivering and handling of concrete ordered is not just a simple exercise. The process is complicated and needs to be supervised very carefully. Always ask a reputable readymix supplier.

Health and safety

Where skin is in contact with fresh concrete, skin irritations are likely to occur owing to the alkaline nature of cement. The abrasive effects of sand and aggregate in the concrete can aggravate the condition. Potential effects range from dry skin, irritant contact dermatitis, to severe burns in cases of prolonged exposure.

Users should take precautions to avoid dry cement entering the eyes, mouths and nose when mixing mortar or concrete by wearing suitable protective clothing. Take care to prevent fresh concrete from entering boots and use working methods that do not require personnel to kneel in fresh concrete. If cement or concrete enters the eye, immediately wash it out thoroughly with clean water and seek medical treatment without delay.

Wash wet concrete off the skin immediately. Barrier creams may be used to supplement protective clothing but are not an alternative means of protection. Readymix concrete is heavy, with a standard wheelbarrow load weighing over 100kg, so lifting just a small volume may cause physical injury. It is therefore essential that health and safety regulations be followed.

Use of equipment

Certain types of plant equipment create a significant vibration during use (for example pneumatic hammers, drills grinders and vibrating pokers. Prolonged exposure to vibration can cause hand arm vibration syndrome (HAVS). It is possible to reduce the effects of vibration by selecting plant with vibration dampers, by using antivibration gloves, taking regular breaks and keeping hands warm in cold weather.

ASPASA, Nico Pienaar, Tel: (011) 791 3327, Fax: 086 647 8034, Email: nico@aspasa.co.za, Web: www.aspasa.co.za     

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