Concrete strength development

Surface concrete temperature t(°C) Concrete strength development (r = Scml/Scml ... 

Addressing Challenges of Concrete Strength Development of Mine Shaft Liner Cured Under Frost Exposure... 

Mixing of cement with water activates exothermic chemical reactions that release heat. The product, concrete, starts to harden and develop compressive strength. Previous studies indicate that this hydration process still occurs at temperatures below 0°C. However, exposure of hydrating concrete to low temperature will significantly slow down the hydration process, and hence the concrete strength development the process will stop when the concrete temperature drops below -10°C [Neville, 1996). As a result, a degradation of the strength and quality of the hardened concrete could occur. 

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. 

Beaudoin, J. J. Ramachandran, V. S. (1975). Strength development in magnesium oxychloride and other cements. Cement and Concrete, 5 (6), 617-30. 

If the admixture is added directly to a concrete as part of the gauging water with no other changes to the mix proportions, a concrete possessing similar strength development characteristics is obtained, yet having a greater workability than the control concrete. 

A concrete with similar workability and strength development characteristics can be obtained at lower cement contents than a control concrete without adversely affecting the durability or engineering properties of the concrete. 

Fig. 1.2 Compressive strength development of concrete containing various types of water-reducing admixture.

In general, superplasticizers will not have an adverse effect on strength, or strength development of concrete when added to produce highly workable concrete without reducing the water content. In fact several studies have indicated that an increase in strength may occur, and in some cases this can... 

Fig. 2.19 Strength development of high-strength flowing concrete containing melamine-based superplasticizer compared to concrete made with 400 kg of normal Portland cement per m in the stiff to low workability ranges (25-100 mm slump). 

Fig. 5.35 Calcium chloride does not affect the long-term strength development of dense concrete cured under laboratory conditions, or with outside exposure (Blenkinsop). 

Two types of calcium nitrite-based corrosion inhibitors are currently marketed, viz. a set- and strength-accelerating type and a normal-setting type. The former increases the early strength development in concrete. This effect increases with the dosage. Both admixtures are compatible with all types of Portland cements and... 

It is important to ensure that when using corrosion inhibitors with other conventional admixtures, they are added separately, at different times of the mix cycle. Corrosion-inhibiting admixtures which also accelerate the set of concrete may require the combination of a retarding admixture when ambient and mix temperatures exceed 35°C. In like manner, set-retarding corrosion inhibitors may require the addition of an accelerator to offset the retardation of early strength development (e.g. use of sodium nitrite in conjunction with sodium benzoate). 

The lower water-cement ratio afforded by the use of a superplasticizer may be used to increase existing compressive strengths or to reduce cement content. Thus the use of superplasticizers may enable the precast producer to use lower cement content without reduction in mix workability and rate of strength development. The actual amount of cement reduction achieved will depend on the cement type used and the mix proportion used in the concrete. Previous work [63] indicates that even with low cement content (306 kg m 3) a normal dose of superplasticizer can accelerate 3- and 28-day strengths by 90% and 55%, respectively, over levels attained with a plain mix. Cement reductions in the range of 11-20% have been achieved in mixes with a cement content of 415 kg m, while maintaining desired strength... 

The choice of a particular option or options is determined by the desired set time and rate strength development. Accelerated time of setting is usually achieved by increasing the heat of hydration of the concrete mixture. Selection of appropriate chemical admixtures enables the production of cold-weather concrete mixtures with both accelerated setting and early-age strength development characteristics, similar to that obtained with plain... 

A relationship exists between fineness and SAI. While 34 wt% fineness meets the ASTM C-618 specifications, finer ash samples have been shown to achieve higher compressive strengths. While sizing on a 325 mesh screen (>45 pm) can be indicative of the amount of fine materials present, it does not directly measure the reactive component of the ash, which is in the very finest fractions (e.g., <5 pm). The presence of this ultra-fine material has a profound impact on strength development of the concrete, as the surface area available for pozzo-lanic cementitious reactions is contained in the finest fractions. For example, the surface area of a typical Class F fly ash, with a mean particle size (D50) of 24 pm will have over 90% of the total surface area accounted for in the <5 pm... 

---