The African Business Journal: SARMA SARMA ================================================================================ admin on 13 February, 2010 01:07:00 Housing in South Africa is one of the most important issues that needs to be dealt with to ensure a sustainable country. Housing developments need to be planned properly for location, but also for the quality thereof. Using cheap and inferior building materials will not be wise. Sarma members supply quality concrete. Each Sarma member has to have its plants audited once a year by an independent auditor. This audit known as the SARMA SHREQ Audit covers Safety, Health, Road, Transport, Environmental and Quality. Financial institutions, Specifiers, Designers, Contractors role in ensuring quality housing is built All the role players mentioned above have a vested interest in ensuring that the structures that are built are of a good quality. The whole process starts with the foundations. Using accredited ready-mix plants ensures that the basis of the structure is secure and strong. Selecting the appropriate readymix concrete Decide which of the “typical applications” matches your application and site conditions. For reinforced concrete structural frames, see the recommendations by the designer. If these are not specified as a designated concrete, ask the designer to determine the equivalent designated concrete. Many soils are potentially aggressive to concrete and the designer of the foundations should have selected a concrete that is capable of resisting this aggression. Measure the prepared location and calculate the volume of concrete required. Slightly over-estimate the volume required as it is annoying to run short and expensive to order a very small volume. Ensure a suitable and safe access for the ready-mixed concrete truck, the largest of which may be up to 10.5m long, 2.5 m wide and 3.8 m high and weigh 32 tonnes when fully loaded. If the truck cannot discharge directly into the works, provide transport to move the concrete from the delivery truck to the works or ask the ready-mixed concrete producer for advice. They may be able to provide a truck that has a pump or conveyor or recommend a company that may supply such equipment. A few days prior to requiring the concrete, ask for a quotation. At this stage you may not know the exact volume of concrete so give an estimate and say that you will confirm the volume the day prior to delivery. Confirm the order and time of delivery the day prior to delivery. The enquiry and tender stage SARMA, on a daily basis receives calls from various people enquiring about where they can source readymix concrete. For ease of reference, Sarma on the website (www.sarma.co.za) has listed the members as well as the accredited plants. It is up to the enquirer to decide on who to use. As much information as possible should be provided to the ready-mixed concrete company at the enquiry/tender stage. The contract specification may contain specific information pertaining to constituent material restrictions, minimum cement contents and/or maximum water cement ratios and it is therefore essential that the ready-mixed concrete supplier has sight of this in order to determine if the locally held materials will be suitable and/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 consistence/workability is specified for the job in hand at this stage. An estimate of the volume of concrete is required is essential. However, any additional information pertaining to “placement rates” would also be extremely useful, as forward planning will allow the ready-mixed concrete producer to plan for the use of additional vehicles and/or late working etc. The Sarma member company should be consulted with regard to the availability of “pump and conveyor” trucks as well as their placement distances, heights, weights, dimensions and extension capabilities. Safe access and discharge Safe access for the delivery vehicle should be provided, taking into consideration the size and weight of the truck when loaded with ready-mixed concrete, and the ground conditions/access. Please note that the truck driver will maintain a safe distance from any excavation or area considered dangerous. Hence, please consider how the concrete is to be moved from the point of discharge to its final position prior to placing an order. Part load charges “Part Load” charge may apply. The cost of this will be in-built into your quotation at the enquiry stage. However, please be aware that if you “under order” and require 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 Sarma Member company. Waiting time charges Generally, Sarma member companies will offer 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 Sarma Member company. Returned Concrete Please be aware that if you have over-ordered and wish to return any ready-mixed concrete which has not been discharged from the delivery vehicle, a charge may be incurred. Advice should be sought from the Sarma Member Company. Placement and Vibration Ready-mixed concrete should be placed as close as possible to its final resting position and vibration should only be used to compact the concrete rather than move it. In general terms the transportation and discharge of ready-mixed concrete should be completed within two hours from the time of batching. Placement by pump When utilizing a pump to place ready-mixed concrete, it is essential that the pump supplier is informed of the pumping distance involved. Additionally, please ensure that the Sarma member company is also informed of the placement technique being applied. Delays between ready-mixed concrete deliveries should be avoided when pumping, and it is therefore advisable to discuss and agree suitable delivery rates with the Sarma member supplier prior to placing an order. Common (squeeze) pumps can transport ready-mixed concrete distances of up to 90m horizontally and 30m vertically, depending on the mix constituents and design, with more specialist (piston) pumps able to transport ready-mixed concrete distances in excess of this, but again dependent upon the mix constituents and design. However, sharp bends and sudden changes of pipe section size/type should be avoided. In both cases, the pipe diameter must be at least three times the maximum aggregate size specified for the concrete. Prior to placing ready-mixed concrete by pump, please ensure that it is primed with sufficient grout to allow the free flow of concrete through the pipes. Pumping of lightweight aggregate ready-mixed concrete can be achieved by using special admixtures (pumping aids) to overcome loss of consistency/workability due to absorption. However, technical advice should be sought before ordering this type of concrete. Placement and vibration into the structure: It is essential that ready-mixed concrete is placed into the structure with due care and attention and that is vibrated fully to ensure good compaction. The following provides guidance on how to achieve this: * Ready-mixed concrete should be placed in uniform layers, not greater than 500mm and not in large heaps or sloping layers. * The slump should be specified * Ready-mixed concrete should not be allowed to “free fall” more than 2m * The rate of placing and compaction should be equal * Walls and columns should be filled at a rate of at least 2-3m (height) per hour, avoiding delays and cold joints. * Each layer should be fully compacted before placing the next one, and should be placed whilst the underlying one is still plastic. * A layer of concrete should be placed on the kicker and vibrated before placing any more concrete. This will absorb the energy if concrete is discharged from the top of the form, and thus prevent segregation. This method may be sufficient provided that placement by pump hose or tremie is advisable. * In order to remove entrapped air, the poke vibrator should b held vertically in the layer and brought up slowly whilst placing the additional layer. * The poker vibrator should be moved around within the wall or column in order that the concrete is vibrated every 0.3 to 0.5m and for a sufficiently long period of time to visually witness air being expelled. Formwork Specialist formwork manufacturers/designers should be consulted to ensure that the formwork used is able to withstand the pressures exerted by the ready-mixed concrete in its plastic state. Only a specialist “purpose designed proprietary” release agent should be used. It is also essential that the dose or application rate is checked carefully and that the release agent is applied uniformly, as different types of formwork material require differing amounts of release agent. Too little release agent may result in surface defects due to the concrete adhering to the forms, and too much release agent may result in surface defects due to the concrete surface being retarded, causing excess bleeding, resulting in a poor quality finish. Numerous “form face” materials are available and it is therefore advisable to carry out a series of trial panels to ascertain the most suitable option prior to commencement of the works. Precautions Sunlight can react with release agents when applied to certain types of formwork material, resulting in colour variation of cast elements. It is therefore advisable to store formwork panels in the shade wherever possible. For advice on readymix additives see Sarma Associate member list. Finishing too soon If ready-mixed concrete is “finished” too early, when it is still bleeding, surface delamination may result. This occurs when water rising from the concrete is trapped below the surface, thus separating the surface from the main body of concrete. In this regard, it is advisable not to “power finish” air entrained concrete, due to the fact that this type of concrete appears to be ready for floating when it is not. This is because of its particularly good cohesive nature/appearance which can be deceptive (i.e. it may still be bleeding, although it appears not to be). Curing All ready-mixed concrete should be cured and protected from the elements. This includes wind, rain, and even sunlight, as any of the above can damage the surface. In this regard, one of the most commonly experienced surface defects, known as plastic cracking results from a combination of wind and sun. Cold and Hot weather working Cold weather working: Care must be taken when ready-mixed concrete is placed in cold weather, as it may be damaged (if unprotected) to such an extent that it is unfit to use. In this regard, ready-mixed concrete suppliers monitor both ambient and concrete temperatures and will inform you prior to supply if the temperature of the fresh concrete at the time of delivery is likely to fall below 5°C, at which point additional precautions are required. Notwithstanding this, at the specific request of the contractor, ready-mixed concrete may still be supplied. However, it is the contractor’s responsibility to protect and insulate the concrete on site in order to avoid any “freeze-thaw” damage. It should be noted that surfaces in contact with the freshly place concrete, including sub-grade materials should be at a temperature of at least 2°C. In addition, reinforcing bars (and other embedded metal) must be free of ice and snow. Additionally, once placed, exposed concrete surfaces should also be covered with insulating material. It should be noted that cold temperatures will lead to slower rates of strength gain and as a result, curing and striking times will be extended. Hot weather working: High ambient temperatures will increase the temperature of the ready-mixed concrete, resulting in a combination of the evaporation of mix water, and a more rapid hydration of the cement, leading to accelerated hardening. Ambient temperatures of 20°C should not cause a significant problem. However, precautions should be taken when temperatures approach 30°C. Rapid evaporation of moisture from exposed surfaces may cause plastic shrinkage cracking and crazing. The following precautions/preparations should be taken hot weather: * Specify alternative cement types, such as fly ash or ground granulated blastfurnace slag (GGBS) as these can offer slower hardening characteristics. * Do not add water to fresh concrete – apply more labourers to handle the concrete on the job. * Order the concrete at a suitable consistence/workability (higher than you would normally, due to workability loss through evaporation). * Apply a curing membrane or cover once placed in order to avoid a wind/sun combination (resulting in cracking). * Place, compact and finish as quickly as possible. * Consider specifying a “retarding” admixture (see Sarma Associate Members for admixture members). Cracking Concrete may develop for many reasons. However, most can be avoided by good site practice.