Ready-mixed concrete

Accelerator in Ready-Mix Concrete. Calcium chloride accelerates the set time of concrete giving it a high early strength development. It is not an antifreeze, but by using it duriag cold weather protection can proceed in a timely manner (31—34). In Russia, calcium chloride forms a component of several antifreeze admixtures (33). Reviews of the concerns and possible remedies of calcium chloride corrosion problems in concrete are available (21,35). There is no consensus on what the safe levels of calcium chloride in concrete are. 

In the past 30 years, the ready-mixed concrete industry became the primary customer for cement manufacturers. In 1990 more than 72% of the cement shipped was sold to the ready-mixed concrete industry, compared with 63% in 1975. The other primary uses are in building materials, concrete products, and highway constmction. 

Market and actors In Germany approximately 32 m. tons of cement are manufactured, of which about 16% are processed manually, 31% are used in factory manufacture of concrete parts and 53% are used in the manufacture of ready-mixed concrete. Around 5.5 m. tons are imported. The cement industry operates in a Fordist stmctured market for mass products (lengthy product cycles, dominated by price competition) with trends towards monopolies. Raw materials suppliers and downstream businesses (e.g. cement transportation enterprises) are also being controlled increasingly by the cement manufacturers. 

Edmeades, R.M. (1975). Proceedings of the Conference on Ready-Mixed Concrete, Dundee, Theme 3. 

Every year millions of cubic meters of ready-mixed concrete are returned for disposal in dump sites. There are now increasing environmental concerns and restrictions regulating the disposal of returned plastic concrete. In the last decade, environmental protection agencies in Europe and North America have classified both returned plastic concrete and truck wash water as hazardous waste. Consequently, the conventional methods of disposal in slurry tanks and landfill sites have become redundant, and disposal of plastic concrete is now a major problem for the ready-mixed concrete industry [102]. 

Table 7.5 Changes in standard deviations of a ready-mix concrete plant using a hydroxycarboxylic water-reducing agent...

Table 7.6 Seven-day and 28-day strengths and standard deviations for concrete containing a lignosulfonate water-reducing agent produced on a ready-mix concrete piant...

Another example of the effects produced by admixtures in ready-mixed concrete is also shown in Fig. 7.8, in which data from another ready-mix plant is presented. The slope lines show that a change in slump from 75 to 175 mm without a water-reducing admixture required an increase in water-cement ratio of 0.08. With the admixture, the same variation in slump required an increase in water-cement ratio of only 0.05, indicating that such concretes permitted variations in slump with less than the usual variation in water demand and water-cement ratio. 

Fig. 7.7 Frequency distribution of control tests from a typical ready-mixed concrete plant (Howard et al. [18])...

Ready-mixed concrete used for such purposes is of two types, structural and architectural. Architectural concrete, in addition to possessing structural properties, may require a specific mix parameter (e.g. exposed aggregate finish) to produce a variety of surface textures. 

As HPC is relatively new to most ready-mixed concrete plants and finishers that have not worked with the concrete before it is important that all parties involved in a construction project be informed about the production, placing, finishing and curing of HPC. The following critical aspects of HPC use in the field are presented as a reference to the practitioner. They cover three main topics ... 

The total volume of waste (including returned concrete plant mixer washout, washdown of truck, spillage of concrete and silt from yard washing) can exceed 140 t per year for a six-truck ready-mixed operation [87]. Environmental protection acts in Western Europe and North America now prohibit the disposal of concrete waste in landfill sites. Ready-mixed concrete operations have therefore altered procedures for disposal of alkaline waste and effluent. One such procedure where significant progress has been made is in the use of chemical admixture systems to eliminate washout from ready-mixed trucks. 

Other applications for the stabilizer and the recycling system include hot-weather concreting, the transportation of ready-mixed concrete over long distances (particularly in the Middle East countries) or through city traffic where unexpected delays can be encoimtered. Higher dosage of the admixture can be used to obtain the desired set extension and the appropriate dosage of the activator (carried on the truck) can then be added at the destination. 

In this section, information on the operational aspects pertaining to the use of admixtures in the field is presented. In addition to details concerned directly with the physical aspects of batching and dispensing equipment and their operation, considerable attention is directed to the material-related problems that arise in the use of admixtures both in mainstream and special applications. It also offers guidance in the selection, use and control of uniformity of the product. Most of the issues discussed here apply equally to the three major fields of concrete construction, namely, site-batched and -placed concrete, ready-mixed concrete, and precast concrete. 

Hester, T.W. (1979). Superplasticizers in Ready-Mixed Concrete. (A Practical Treatment for Everyday Operations), Publication No. 158, NRMCA, Silver Springs, Maryland, USA. 

NRMCA Publication No. 63, National Ready Mix Concrete Association, Silver Spring, MD, 9. 

Anon. (1976). Certification of Ready-Mix Production Facilities, National Ready-Mixed Concrete Association, Silver Springs, Maryland, USA, 18. 

Parsons, J.S. (1973). Report on Ready-Mixed Concrete Equipment Ministry of Transport and Communications, 36-112. 

Based on this experience and applied to a spread of ready-mix concrete cost of 40 to 80 a yard and extended to a range of quantity of sulfur concrete mixed per day, there is an indicated cubic yard comparison, materials only, of approximately 1% to 6 times the cost of Portland cement concrete for sulfur concrete. Greater volumes per day yield lower unit costs. Experience at Corpus Christi would put the premium cost of sulfur concrete approximately 15 pet more than the cost of Portland cement concrete. 

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