Porosity concrete

Control of characteristics of concrete (porosity, composition, thickness of concrete cover)... 

Increasing concrete porosity (i.e., raising the coefficient of diffusion) facilitating chloride ion ingress... 

Increases time for chlorides or carbonation to reach steel Reduces concrete porosity to increase time for corrodents to reach steel Reduces concrete porosity to increase time for corrodents to reach steel... 

A very good relationship is obtained between the porosity, from mercury poro-simetry measurements, and permeability of hardened pastes. However, this method caimot be recommended in the case of concretes, as it has been mentioned in Chap. 5. A representative concrete sample cannot be satisfactorily reduced to the small specimen for mercury porosimetiy measurements. Therefore different methods of concrete enrichment in paste component are used the last one being the concrete porosity controlling factor. The concretes with lightweight aggregate are an exception in this case. There is usually too much paste in these small samples of concrete and the methods of concrete enrichment in paste worsen additionally the situation. 

The determination of total concrete porosity is not sufficient. The pores stmcture should be taken into account The following types of pores can be distinguished ... 

The use of fly ash in concrete does not solve the disposal problem but only shifts it to another locale. If cement is composed of 30% fly ash, it has been estimated that the radon diffusing out of the concrete (porosity—5%) into a room (10 x 10 x 4 m) would be about 10 pCi/cm —100 times lower than the MFC. However, with higher porosity, concrete and lower ventilation rates the margin of safety decreases, and it means that concrete containing fly ash should not be used in structures for habitation though it would be permissible for use in foundations, bridges, and roads. 

Empirical relationships have been developed between material properties, snch as attemration and the speed of sound. Typical properties of interest include crushing strength (concrete), porosity (ceramics), and grain size (austenitic steel and cast iron). Experiments are nearly all performed in transmission, using pitch-and-catch, through a known distance in the test material. Many of these materials are heterogeneous, for which attenuation measnre-ments can be umeliable (see Section l.G). 

In concrete, porosity together with SSA values give information about its microstructure and permeability as well as durability. Mindess and Young (1981) established a relationship between the pore sizes and the main properties of concrete they affect, such as strength, permeability or shrinkage. 

Cathodic protection can be used to protect steel in concrete (see Chapter 19). There is no fear of damage by H2 evolution due to porosity of the mortar. Local corrosion attack can be observed under extreme conditions due to porosity (water/ cement ratio = 1) and polarization (f/jq = -0.98 V) with portland cement but not with blast furnace cement, corresponding to field IV in Fig. 2-2 [53]. However, such conditions do not occur in practice. 

The main difference between the two types are that the reaction products of the silico fluoride types are less soluble in water and are also harder, which may give better in-service performance but at a slightly higher material cost. However, with recent developments in floor-laying techniques, the concrete substrates for industrial floors are laid with much more dense low-porosity surfaces, so that neither silicate nor silico fluoride treatments are as effective as they used to be, when the concrete used had a slightly more open finish and hence was more receptive to these treatments. With modern concrete floors, it is imperative to wash any material not absorbed into the surface within a short period. Otherwise, unpleasant white alkaline deposits, which are difficult to remove, may occur. 

Test method for porosity in gold platings on metal substrates by gas exposures Test method for half-cell potentials of uncoated reinforcing steel in concrete Method for detection of copper corrosion from petroleum products by the copper strip tarnish test... 

Although the natural zeolites are widely used (around 4 million tpa) they are not particularly valuable as commercial catalysts. This is due to a number of factors including natural variations in crystal size and porosity as well as the actual small pore size, which limits their synthetic usefulness. Natural zeolites do, however, find widespread use in applications such as removal of heavy metals from water, odour removal and building materials e.g. cavity grouting and sprayed concrete). 

As a final application of the profiling technique, the sensor for large depth measurements described in Section 2.4.2.5 was used to resolve multi-layer polymer coatings on concrete samples. Such coatings are used to protect concrete from degradation and corrosion. They are applied to the concrete surface to reduce the porosity in the upper first millimeters to prevent the penetration of water and... 

Dependence of the relation R with the number of layers is given in Table 1. The best type of catalyst is the Sn-organic compound. The relation R for concrete increases slowly after the first layers and very rapidly after the 4th and 5th layers. This is explained by the high porosity of concrete. In the case of cement and gypsum R increases rapidly after the 2nd layer. The total consumption of chemicals is about 0.4 L/m2 for concrete (but it depends on type of concrete), 0.3-0.4 L/m2 and 0.2-0.3 L/m2 for cement and gypsum, respectively. There is no a great difference between air, Ar and radon permeabilities. 

L.4.3 Approximately 10 to 25 mm (0.5 to 1 in.) of the top of the cementitious foundation material should be scarified with a chipping hammer before the grout is applied. This procedure is recommended to remove low-strength, high-porosity concrete in this area. The concrete foundation should be allowed to cure for at at least 7 days prior to this surface preparation. 

The three most important properties of concrete used in calculations for load-bearing applications are the compressive strength, the tensile strength and the modulus. However, for certain applications, e.g. water-retaining structures, the permeability or porosity of the concrete will be a relevant design criterion and this is also considered here. 

The permeability of concrete is a guide to its durability (Section 1.5.2) but it can also be relevant to the design of structures which are intended to withstand a hydraulic head of water or other liquid. Extreme porosity is usually... 

The permeability and porosity of concrete containing calcium chloride in relation to a plain concrete depends on two conflicting variables ... 

The porosity of the concrete at an advanced state of maturity is increased in the presence of calcium chloride and, therefore, will allow a greater opportunity for air and moisture to come into contact with the steel reinforcement, encouraging corrosive effects. In practice, with reinforcement cover meeting the relevant codes of practice, this effect is regarded as of minimal significance. 

The presence of an excessive number of voids in the concrete [38] or porosity due to poor compaction or a deficiency of fine aggregate [39] leads to an increase in the amount of reinforcement corrosion. 

The reduction in porosity, decreased water content, and air entrainment that results when latexes are used in mortar and concrete mixes make them much more resistant to freezing and thawing conditions than conventional mortar and concrete. Figure 6.17 presents the freeze-thaw durability in water (-18 to 4°C) of combined water-and dry-cured SBR-, PAE- and EVA-modified mortars [98], The frost resistance of mortars made with these latexes is markedly improved even at polymer-cement ratios of 5%. However, an increase in the polymer-cement ratio does not necessarily produce further improvement in freeze-thaw resistance. EMM and EMC, when exposed to outdoor conditions involving freeze-thaw, UV radiation and carbonation show better weatherability when compared with conventional mortar and concrete. 

Durability most proprietary accelerating admixtures adversely affect the concrete s resistance to freezing and thawing [114, 118]. More recently, the widespread use of silica fume has enabled the use of considerably low dosages of accelerators and also contributed to the improvement of porosity values, thereby dramatically improving durability of such concretes. 

When added at dosage of 2% by weight of cement to a concrete mixture with 460 kg m of cement without adjustment for the volume of the water introduced by the admixture, the concrete s slump and porosity are increased. However, when substituted for an equal volume of water, the SRA has little or no effect on concrete slump. It does have a slight retarding effect on the rate of hydration and may extend the setting time up to about an hour. The admixture also affects the air content of fresh concrete and therefore when used in air-entrained concrete, the air-entraining admixture dosage must be increased to achieve a specified air content. 

Durable concrete is typically characterized by low porosity because the fundamental porosity of concrete influences all of its material properties. For this reason, most of our standard practices for the construction of concrete structures have as their objective the minimization of paste porosity, which consequently increases both strength and durability. Although low water-cement ratio (W/C) is responsible for improved mechanical properties and enhanced durability, attaining a low W/C necessitates either a sacrifice in workability, or the use of high cement content, neither a desirable consequence. A more advantagious alternative is the use of water-reducing admixtures (WRAs). 

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