Plastic settlement

The stability of the concrete mix can be considered in terms of its cohesion , which is a subjective term used to describe its ability to maintain a homogeneous appearance when subjected to applied stress. Lack of cohesion leads to segregation of the mix components into layers relevant to their densities. A further term associated with mix stability is that of bleeding , which is the movement of water to the surface of the fresh concrete. This phenomenon can occur either in isolation or as a manifestation of segregation. Bleeding in excess is normally considered to be undesirable because of the dangers of water runs at the shutter/concrete interface and cracking due to plastic settlements, and there is also the possibility of adverse effect on the concrete-reinforcement bond due to the collection of water beneath the steel. 

Early load damage Plastic shrinkage Plastic settlement... 

Plastic settlement A Over reinforcement Deep sections Eccess bleeding Rapid early drying conditions 10 min to 3 h... 

Plastic settlement and plastic shrinkage cracks - occur within a few hours of casting. 

Granulated iron is oxidised by the water and oxygen in the concrete and expands. This expansion can continue beyond the plastic phase and can therefore, in theory, counteract both settlement and drying shrinkage. In practice, it is difficult to achieve such precise control of the rate and duration of oxidation. 

The plasticizers decreasing the intensity of bleeding influence simultaneously on the reduction of settlement of solid paste components, which leads to the decrease of shrinkage [109]. 

Hanehara and Yamada [360] studied the effect of PA admixture addition on the flow diameter and found an exact lelationship in the case of w/c = 0.4 however the results are not always good for w/c = 0.3 because of the effect of cement with water mixing procedure. This problem was resolved by addition of limestone powder. According to these authors there is no general method of cement-admixture compatibility establishment, but it is possible to settlement certain experimental conditions, which permit to determine it. It is moderate fineness of cement and suitable w/c. Moreover, the proper conformity criteria should be selected, because the plastic viscosity, yield stress value and flow diameter can give various results [400]. In Fig. 6.105 the effect of SP on the relative flow area, chosen as cement-SP compatibility determining parameter, after Yamada [400] is shown. SP does not affect the relative flow area below the threshold value, above this value the flow area is increasing proportionally with admixture, but above the saturation level this parameter is not influenced by an admixture content. 

Degradable plastics. Available http //www.environment.gov.au/settlements/... 

Fig. 14.4 (a) Tanks used to rear the larvae of Isostichopus fuscus (Ecuador), (b) Conditioning of settlement sheets for 7. fuscus in large tanks exposed to the sun (Mexico), (c) In some cases, post-settled juveniles are allowed to feed on the conditioned substrates (polypropylene cloth) for the first days or weeks of their growth. Juveniles of Apostichopus japonicus are visible as dark spots on the exposed cloth (China), (d) Undulated plastic sheets are also used to favor settlement of sea cucumber larvae (China, Ecuador, many countries of the Indo-Pacific). Photos courtesy of J.-F. Hamel and A. Mercier. 

According to XUtn (2004) the Chinese use fine flexible polyethylene or polypropylene cloth or sheets for the settlement of A. japonicus (Fig. 14.4b, c).This technique is also used for I. fuscus in Mexico. However, some hatcheries of A. japonicus also use corrugated plates affixed together to form stacks (Ito and Katimura, 1998) (Fig. 14.4d). For H. scabra, Agudo (2007) reported the use of plates made out of various plastic and fiberglass materials. For I. fuscus in Ecuador technicians also use undulated Plexiglas plates. 

It is important to remember that corrosion is not the only deterioration mechanism in reinforced concrete. Alkali-sillica reactivity (ASR), freeze-thaw, plastic shrinkage, thermal movement, settlement and other movement can all lead to cracking and spalling of concrete. 

Plastic Cracking. Cracking in concrete before it has set and hardened. Its cause may be differential settlement, or shrinkage. 

Elastic theory is often used to estimate the distribution of stress and settlement as well. Although soils are generally treated as elastic-plastic materials, the use of elastic theory for solving the problems is mainly due to the reasonable match between the boundary conditions for most footings and those of elastic solutions (Holtz 1990). Another reason is due to the lack of availability of acceptable alternatives. Observation and experience have shown that this practice provides satisfactory solutions (Scott 1981 Perloff 1975 Holtz 1990 Bowles 1996). 

Biodegradable Plastics - development and environmental impacts, Nolan-ITU Pty Ltd, prepared in associatiorr with ExcelPLas, Australia, October 2002, www.envirorunent.gov.au/settlements/ publicatiorrs/waste/degradables/biodegradable... 

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