Plain concrete

Fig. 1.35 Concretes containing water-reducing admixtures tend to have more structure than a plain concrete at a given workability (after Hewlett).

Ligno sulfonate admixtures can be used to produce concrete of a required workability and strength characteristic at lower cement contents than the comparative plain concrete with no adverse effect on the durability of the concrete or total structure. The only exception to this rule would be in conditions where high-sulfate ground waters may be involved when the minimum cement contents of relevant codes of practice should be observed. 

Most of the studies in this area have indicated that superplasticized concrete has shrinkage and creep characteristics similar to plain concrete. Figure 2.21 illustrates the shrinkage results of plain and superplasticized concrete [66], while Table 2.6 shows similar results for creep [67]. 

Research into the susceptibility of superplasticized concrete to sulfate attack has concluded that there is no significant difference between plain concrete and the admixture-containing concrete [71, 72]. Figure 2.22 presents some data for concretes containing an SNF superplasticizer exposed to a 3% magnesium sulfate solution. 

Fig. 3.1 The effect of 300 freeze-thaw cycles (according to ASTM C666 Procedure B) on air-entrained (right) and plain concrete (left).

Table 3.14 Bleeding of plain concrete, air-entrained concrete and air-entrained concrete containing a water-reducing agent...

Recognized mix design procedures (or existing standard mixes) are used to produce a plain concrete meeting the requirements of workability, strength and cohesive character using available materials. 

This sequence is shown in Table 3.15, together with the resultant properties of the plastic and hardened concrete. The combined effects of sand and water reductions bring the cement content to approximately that of the comparative plain concrete so that 28-day strengths are similar. However, this is not consistent across all cement contents low-cement-content mixes tend to show an increase in strength, whilst higher cement content will show a slight decrease. This is illustrated by the data in Table 3.16 [31]. 

Total water-cement ratio Plain concrete Alr-entralned concrete (4%) ... 

The available data, therefore, suggest that by making the relevant mix design changes to compensate for the effect of entrained air on strength, the engineering properties of the concrete will be similar to a plain concrete of comparable strength. 

Table 3.24 Depth of carbonation of air-entrained and plain concrete...

Fig. 3.36 The increase in compressive strength of plain and air-entrained concrete up to a period of 1 year. Table 3.25 Drying shrinkage of air-entrained concrete is no greater than that of plain concrete...

Fig. 4.7The freeze-thaw resistance of plain concrete and concrete dampproofed with a wax emsulsion type of admixture. 

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

Curing temperature (°C) Increase in compressive strength at 28 days over plain concrete (%) ... 

Fig. 5.32 Potential-time curves for plain concretes and concretes containing calcium-chloride and calcium-formate-based accelerators (McCurrich).

Fig. 5.33 Bond strength against slip for plain concrete and concrete containing calcium chloride (Kondo).

The drying shrinkage of concrete containing calcium chloride is increased in comparison to plain concrete, even though the amount of moisture lost is less [22]. This is illustrated in Fig. 5.37 and it is thought that the reduced moisture loss will be due to the more advanced state of hydration in the specimens containing calcium chloride. The increased shrinkage must, therefore, be a characteristic of the type of cement hydration products formed. Under saturated conditions, such as total water immersion, the amount of expansion of the concrete is reduced when calcium chloride is present. 

Property Plain concrete Freeze-protection admixture... 

Table 7.2 Air-entrained concrete has a higher standard deviation that plain concrete, but this can be minimized by the use of an air-entraining, water-reducing agent...

Table 7.3 Comparative porperties of plain concrete and air-entrained concrete produced with neutralized wood resins and a water-reducing air entraining agent...

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