Frequently asked Questions Readymix Concrete

Transcription

1 Frequently asked uestions Why should I use readymix concrete, i.e. why should I not mix it myself? If you use a Sarma member s readymix plant, then you will know the plant has been accredited. You are buying a product that is manufactured in a plant that has been accredited by Sarma s independent auditor. Ready mixed Concrete is manufactured under computer-controlled operations and transported and placed at site using sophisticated equipment and methods. RMC assures its customers numerous benefits. Unit 8 Coram Office Park, Ferero Road, Randpark Ridge, 2169 P O Box 1983, Ruimsig, 1732 Tel: office@sarma.co.za

2 Frequently asked uestions Ready mixed Concrete, or RMC as it is popularly called, refers to concrete that is specifically manufactured for delivery to the customer s construction site in a freshly mixed and plastic or unhardened state. Concrete itself is a mixture of portland cement, water and aggregates comprising sand and gravel or crushed stone. In traditional work sites, each of these materials is procured separately and mixed in specified proportions at site to make concrete. Ready mixed concrete is bought and sold by volume usually expressed in cubic meters. RMC can be custom-made to suite different applications. What are the benefits of readymix compared to the concrete I mix on site? Various advantages centralised batching plant can serve a wide area. The plants are located in areas zoned for industrial use, and yet the delivery trucks can service residential districts or inner cities. Better quality concrete is produced. Elimination of storage space for basic materials at site. Elimination of procurement / hiring of plant and machinery. Wastage of basic materials is avoided. Labour associated with production of concrete is eliminated. Time required is greatly reduced. Noise and dust pollution at site is reduced. Reduced costs. What is the difference between cement and concrete? lthough the terms cement and concrete are often used interchangeably, cement is actually an ingredient of concrete. Concrete is basically a mixture of aggregates and paste. The aggregates are sand and gravel or crushed stone; the paste is water and Portland cement. Concrete gets stronger as it gets older. Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver. Cement comprises from percent of the concrete mix, by volume. Through a process called hydration, the cement and water harden and bind the aggregates into a rocklike mass. The hardening process continues for years meaning that concrete gets stronger as it gets older. What does it mean to cure readymix concrete? Curing is one of the most important steps in concrete construction, because proper curing greatly increases concrete strength and durability. Concrete hardens as a result of hydration: the chemical reaction between cement and water. However, hydration occurs only if water is available and if the concrete s temperature stays within a suitable range. Page 2 of 11

3 Frequently asked uestions During the curing period-from five to seven days after placement for conventional concrete the concrete surface needs to be kept moist to permit the hydration process, new concrete can be wet with soaking hoses, sprinklers or covered with wet burlap, or can be coated with commercially available curing compounds, which seal in moisture. Can it be too hot or too cold to place new readymix concrete? Temperature extremes make it difficult to properly cure concrete. On hot days, too much water is lost by evaporation from newly placed concrete. If the temperature drops too close to freezing, hydration slows to nearly a standstill. Under these conditions, concrete ceases to gain strength and other desirable properties. Why does readymix concrete crack? Concrete like all other materials, will slightly change in volume when it dries out. In typical concrete this change amounts to about 500 millionths. Translated into dimensions this is about 4cm in 3 meters. The reason that contractors put joints in concrete pavements and floors is to allow the concrete to crack in a neat, straight line at the joint when the volume of the concrete changes due to shrinkage. Why test readymix concrete? Concrete is tested to ensure that the material that was specified and bought is the same material delivered to the job site. There are a dozen different test methods for freshly mixed concrete and at least another dozen tests for hardened concrete. What are the most common tests for fresh readymix concrete? Slump, air content, unit weight and compressive strength tests are the most common tests. Slump is a measure of consistency, or relative ability of the concrete to flow. If the concrete can t flow because the consistency or slump is too low, there are potential problems with proper consolidation. If the concrete won t stop flowing because the slump is too high, there are potential problems with mortar loss through the formwork, excessive formwork pressures, finishing delays and segregation. ir content measures the total air content is a sample of fresh concrete, but does not indicate what the final in-place air content will be, because a certain amount of air is lost in transportation, consolidating, placement and finishing. Page 3 of 11

4 Frequently asked uestions Unit weight measures the weight of a known volume of fresh concrete. Compressive strength is tested by pouring cylinders of fresh concrete and measuring the force needed to break the concrete cylinders at prescribed intervals as they harden. How can you tell if you re getting the amount of readymix concrete you re paying for? The real indicator is the yield, or the actual volume produced based on the actual batch quantities of cement, water and aggregates. The unit weight test can be used to determine the yield of a sample of the ready mixed concrete as delivered. It s a simple calculation that requires the unit weight of all material batched. The total weight information may be shown on the delivery ticket or it can be provided by the producer. Why do concrete surfaces flake? Concrete surfaces can flake for one or more of the following reasons: In areas of the country that are subjected to freezing and thawing the concrete should be air-entrained to resist flaking and scaling of the surface. If air-entrained concrete is not used, there will be subsequent damage to the surface. The water/cement ratio should be as low as possible to improve durability of the surface. Too much water in the mix will produce a weaker, less durable concrete that will contribute to early flaking and spalling of the surface. The finishing operations should not begin until the water sheen on the surface is gone and excess bleed water on the surface has had a chance to evaporate. If this excess water is worked into the concrete because the finishing operations are begun too soon, the concrete on the surface will have too high a water content and will be weaker and less durable. Will concrete harden under water? Portland cement is hydraulic cement which means that it sets and hardens due to a chemical reaction with water. Consequently, it will harden under water. Page 4 of 11

5 Frequently asked uestions What does 28-day strength mean? Concrete hardens and gains strength as it hydrates. The hydration process continues over a long period of time. It happens rapidly at first and slows down as time goes by. To measure the ultimate strength of concrete would require a wait of several years. This would be impractical, so a time period of 28 days was selected by specification writing authorities as the age that all concrete should be tested. t this age, a substantial percentage of the hydration has taken place (20.7 MPa) at 28 days. Concrete may be specified at other strengths as well. Conventional concrete has strengths of psi or less; concrete with strengths between and psi is considered high-strength concrete. How do you control the strength of readymix? The easiest way to add strength is to add cement. The factor that most predominantly influences concrete strength is the ratio of water to cement in the cement paste that binds the aggregates together. The higher this ratio is, the weaker the concrete will be and vice versa. Every desirable physical property that you can measure will be adversely affected by adding more water. Why should fresh concrete be properly cured? The surface of freshly placed concrete should be kept moist for at least 7 days. Contrary to popular belief, concrete should not be allowed to dry out. If the concrete is allowed to dry out, the ultimate strength gain will be considerably less that it s designed strength. Proper curing will also minimise the potential of cracking. The simplest method of moist curing is wetting the surface of the concrete, and then covering it with polyethylene. How is concrete strength determined? Concrete strength is determined by its water/cement ratio. Water/cement ratio is defined as pounds of water per pound of cement, or gallons of water per bag of cement in the mix. The higher the water content, the lower the strength of the mix. Page 5 of 11

6 Frequently asked uestions Why is concrete a more desirable driveway material than asphalt The reasons are many. First, a properly placed concrete driveway has a much longer life expectancy than an asphalt driveway. concrete driveway is much cooler in the summer months. Residue from an asphalt driveway can potentially be tracked into your home. This problem does not occur with a concrete driveway. There are many architectural applications for concrete. This allows for different colours, textures, etc. I ve heard the term slump pertaining to concrete. What is slump? The term slump simply refers to the consistency of the concrete in a plastic state (prior to hardening). Slump is a measure of how wet or stiff the concrete is. Obviously, the more water used in producing the concrete, the wetter (or higher) the slumps will be. gain, excessive water causes a dramatic loss of strength. pplications exist where higher slump (wetter) concrete is necessary because of difficulty in placement or specification requirements. These higher slumps can be attained through the use of water reducing admixtures (chemical additives). How long does concrete continue to gain strength after it is placed? Concrete s most rapid period of strength gain occurs in the first 7 days. The accepted time standard for measuring concrete strength is 28 days. However, concrete will continue to gain strength for long periods of time (months, even years!) s long as moisture is present to continue the chemical process of hydration What mix do I need? The type of concrete mix depends on the application you need it for. Different mix specifications are used for different purposes. Higher grade or higher strength mixes are used for flooring and external paving whereas lower grade mixes are used for foundations. Please note that 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 Increase the potential for future problems with the surface finish. Page 6 of 11

7 Frequently asked uestions How much does a cubic metre of concrete weigh? One cubic metre of concrete weighs around 2.4 tonnes. How much notice do I need to give when ordering concrete? Ideally a minimum of 24 hours notice is required for concrete orders, but the more notice you give, the greater the likelihood that it can be agreed what time delivery can be done. Will the readymix truck be able to get to the job? It is important that the readymix truck has safe and reasonable access to the job. The member company cannot be held responsible for damage caused to footpaths, driveways, kerbs, lawns, drains, manholes, etc. Be aware that truck mixers when loaded could weigh in excess of 30 tonnes. Please remember that it is the responsibility of the customer to provide safe and reasonable access, and if the driver is not certain that this is the case when he arrives on the site, he may ask you to sign an indemnity form. If you think this is insufficient for your project, we can use a concrete pump to place the concrete away from the readymix trucks. n extra charge for this service will apply. What preparation do I need to do? Before commencing any excavation an assessment of the site is essential, including location of any services such as: Buried electricity cables Draining systems Overhead cables for electricity or telephone Overhanging tree branches Tree roots ll organic matter should be removed before setting out the site. Using a combination of pegs, strings and spray line-marking paint, mark out the extent of the foundations. If laying a concrete base, floors or drives, dig out the area to twice the required depth of concrete (a minimum of 100mm of concrete is advised, so the area should be dug out to 200mm deep). You should also compact the soil. It is important to ensure that the foundation is level. Page 7 of 11

8 Frequently asked uestions Then place a sub-base layer of hardcore approximately 100mm deep and compact it to give the concrete a solid base and prevent sinking. Sprinkling a thin layer of sand on top of the hardcore will provide a smooth surface to work with. Laying a sheet of polythene damp proof membrane between the sub-base and concrete will help to stop the concrete from drying out too fast, and will also help to prevent rising damp on the finishing project. What tools do I need? It is likely that, if you are a gardener or diy-er, you will already have the tools required for the job, but its worth double-checking before the delivery arrives to make sure you have everything you need: Tape measure to measure your site and calculate how much concrete is required. String and pegs for marking out the area you need to fill with concrete Timber to make pegs and formwork, and to use as a tamping beam Wheelbarrow handy for moving the concrete around the site Shovel to help move the concrete Rake to help move the concrete Spirit level to ensure the job is level and maintain accuracy Float or trowel for surface finishing Brush to help you clean up after the job What protective clothing is required? Gloves wet concrete or mortar is trapped for long period on skin surfaces and cause serious skin burns. Shirt, pants & boots Long sleeved shirts for reducing skin exposure to caustic materials such as concrete. Gloves to prevent wet concrete seeping into exposed skin. Long pants or coveralls are recommended. Waterproof boots should be slip-resistant, high enough to protect ankles and lower legs. Other accessories Waterproof knee pads or dry knee boards should be used by employees working around wet concrete. Protective goggles and masks are recommended for employees exposed to dry or wet concrete. Page 8 of 11

9 Frequently asked uestions What should be expected when the readymix is delivered? The first thing the driver will do when he arrives on the site will be to assess the required delivery location, and establish the most suitable means of discharging the concrete into place. If the driver cannot get the readymix truck close enough, off-loading directly into wheelbarrows will be possible. How long will the readymix truck wait on site before a waiting charge is made? If the readymix truck is required on site for more than 20 minutes, a small additional charge may be applied. What s the best way to handle and move the concrete? Try to minimise the manual handling of concrete by ensuring that the readymix truck is as close as possible to its final location. The less it is moved, the better. Start pouring the concrete in a corner and work away from it. Ensure that the correct tools, such as a square-nosed shovel or concrete rake are used. Using the wrong tools can cause segregation. How long does it take before the concrete sets? Once the concrete arrives on site, normally there is between 1 and 2 hours to place, level and compact the concrete and to finish the surface. Why is compaction so important? Compaction ensures the optimum density of the mix is achieved, increasing compressive strength. Where reinforcing is used, the bond between concrete and redistribution bar is improved. Permeability of the concrete decreases, decreasing cold joint, 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 degrees centigrade and time to reach its specified strength (a minimum of 7 days to reach 70%). Page 9 of 11

10 Frequently asked uestions Methods of curing: Sprinkling or fog spraying, keeping the surface continuously damp (alternative wetting and drying can cause cracking) Covering the concrete with wet hessian, straw, sawdust or sand (avoid materials that may cause discolour in the concrete) Covering the concrete with plastic sheeting (while in hot weather, black in cold weather) Use curing compounds (avoid using if the surface is to be painted or will have other surface cover) Drying winds can cause severe cracking, begin curing as soon as the finished surface of the concrete has been laid. re there quality standards for readymix concrete? Yes, concrete must comply with SBS 878. sk you supplier if he is a member of SRM and if his plant has been audited and accredited. What is fly ash? Fly ash is a by product of coal combustion. Most commonly the material is produced by coal fired electrical generating facilities. Fly ash is a cementious material, meaning it has certain properties that cause it to harden upon exposure to water. Typically, fly ash does not develop much compressive strength on its own. However, in the presence of Portland Cement, fly ash can develop strength characteristics very similar to cement. The fly ash reacts with a chemical by-product of the cement hydration reaction called calcium hydroxide CaOH-. CaOH can cause deleterious effects in concrete such as increased porosity and efflorescence the formation of calcium carbonate crystals on the concrete or mortar surface. Because fly ash reacts with the calcium hydroxide to form more calcium silicate hydrate the binder derivative of cement fly ash actually adds strength to the concrete and helps to remove an agent that may be harmful. Fly ash is also particularly beneficial for use in hot weather as it tends to slow the generation of heat in the concrete. Page 10 of 11

11 Frequently asked uestions How do I stain/paint my concrete floor? ny paint store will have concrete stain or paint. Discuss how you want the floor to look and what function it will perform with the paint store manager, and he/she should be able to recommend the appropriate stain/paint. Is there an average going price per cubic metres of readymix concrete? No. The reason for this is that in S we have strong competition law, which makes it illegal for companies or Sarma to get involved in any form of collusion on price or market share. Each member prices each project and quotes on it as the situation requires. It also depends on how big the job is, how busy the plant is, how eager the supplier is to get work, etc. Sarma therefore cannot give any prices to anybody simply we do not know what prices are!! re all readymix companies members of Sarma? NO Sarma is a voluntary association that has set itself up to do self governing in the industry. Some companies do not want to be audited and therefore choose not be members. udits are compulsory for members. This makes one think. Tel: office@sarma.co.za Page 11 of 11