Part 13 - Strength

Strength may not be the most important characteristic of concrete; durability, volume stability, and impermeability may be equally significant. However, strength has become universally accepted as the most important indication of concrete quality. Factors that effect concrete strength may be divided into four categories: (1) constitute materials, (2) methods of preparation, (3) curing procedures, and (4) test conditions. We have already discussed methods of preparation and curing. Therefore, we will be concerned with the effects of constitute materials (water, cement, and aggregate) on concrete strength and mechanical properties.

Effect of Porosity on Strength

The primary factor that governs the strength of brittle materials, like concrete, is porosity. As the capillary porosity decreases compressive strength increases. Also there is data to indicate that large pores may be more effective than small pores in reliving stress concentrations at crack tips.

Gel/Space Ratio -- In 1946, Powers and Brownyard published a work that showed that the increase in compressive strength of Portland cement is directly proportional to the increase in gel/space ratio, regardless of age, w/c ratio, or type of cement. The gel/space ratio is the ratio of solid products of hydration to the space available for these hydration products, in other words, it is a measure of capillary pore space. Before hydration this space is occupied by mixing water, after hydration the space is the sum of the hydrated cement and the remaining capillary pore space. The basic trend of this graph will be the same if different cements or test shapes are used, however, the data points will change. Even thought is this an important quantity the gel/space ratio is difficult to determine.

Factors Affecting Strength

Water/Cement (w/c) Ratio -- The capillary porosity of a properly compacted concrete is determined by the w/c ratio. If concrete is not properly compacted it may contain voids which will contribute to its porosity. At low w/c ratios where full compaction is difficult to achieve, the relationship between w/c and strength is invalid.

There are some problems associated with using the w/c ratio as the primary indicator of strength. For example, if finer cements and admixtures are used, 7- and 28-day strengths may not represent the true quality of the cement. However, until some other field test is available, w/c ratio remains the best indictor of strength and durability.

Time -- The rate of strength gain depends of the w/c ratio, low w/c ratio mixes gain strength faster than high w/c ratio mixes. As a general rule the ratio of 28-day to 7-day compressive strengths lies between 1.3 and 1.7, and is generally less than 1.5. These ratios are not valid if accelerators or extreme curing temperatures are used.
The Maturity Concept -- The hydration of cement is greatly affected by both the time and the temperature of hydration, therefore, strength gain is controlled by these two factors. The concept of "maturity" is a function of the product of curing time and temperature. The assumption is that concrete of different mixes, curing times, and curing temperatures will have about the same strength at the same level of maturity. A datum or reference point below which no concrete will gain strength is commonly a value of -100C. The general trend is that as maturity increases compressive strength increases, especially at low maturity values.

There are a number of limitations on the use of maturity for predicting compressive strength. (1) humidity of curing is not considered, (2) only ambient temperature is considered; the contribution of heat of hydration is ignored, (3) maturity functions are not useful at low values (time should be calculated from when concrete actually begins to gain strength not at mixing and casting), (4) invalid over large curing temperature variations, (5) cement characteristics and w/c ratio affect strength, and (6) invalid for accelerated concretes.

*** Nevertheless, the maturity concept may be useful in establishing "after the fact" strength estimates of concrete.

Cement -- From our previous study we known that the chemical composition and fineness of cement affect the strength of concrete. Early strength comes from C3S and later strength from C2S. Cement that hydrates more slowly will have lower initial strength but higher ultimate strength. The degree of fineness also affects the strength; the rate of hydration increases with the increase of fineness.
Aggregate -- Second to w/c ratio, aggregate is an important factor affecting concrete strength. The most important properties of aggregate are shape and texture and the maximum aggregate size. Since aggregate is generally much stronger than cement paste the strength of the aggregate is less important. Texture affects both the bond and the stress level at micro-cracks. This type of behavior will affect the tensile strength but will not affect the compressive strength. Compressive strength depends on the strength of the aggregate itself.

Maximum aggregate size affects strength in several ways: larger particles reduce the specific surface area of the aggregate which leads to a reduction in bond strength; also, larger particles tend to restrain volume changes in the cement paste and therefore induce some internal stress which will weaken the concrete. These effects can be offset by reducing the water content, therefore the net effect of aggregate size is small. In general, at a constant w/c ratio, higher strength can be obtained by using a leaner mixture. If constant workability is maintained, strength will increase with cement content.

This website was originally developed by Charles Camp for his CIVL 1101 class.
This site is maintained by the Department of Civil Engineering at the University of Memphis.
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