Passage I

Concrete is a mixture of cement, water, and aggregate (usually sand or gravel). The cement and water react chemically to form a hard paste, which binds the aggregate together.

The strength of a cylindrical sample of concrete is measured by subjecting the sample to increasing vertical compression until it breaks (see Figure 1). The vertical compression needed to break the sample is, called the compressive strength and is measured in kg/cm².

An engineer conducted the following studies to determine factors that affect the compressive strength of concrete.

Study 1

Eight groups of cylindrical concrete samples were prepared—4 using Type I cement (Groups A-D) and 4 using Type II cement (Groups W-Z). All samples had the same dimensions. The water content, by weight, was the same in each sample but the cement content, by weight, was varied to create different water to cement ratios in each group. Each sample contained the same type and amount of aggregate. After the samples had been aged (allowed to sit undisturbed) under identical conditions for 28 days, their compressive strengths were measured. The results are shown in Table 1.

Study 2

Cylindrical concrete samples were prepared as in Study I using Type II cement. Groups of samples with varying water:cement ratios were aged under identical conditions for 1, 3, 7, or 28 days and were then tested for compressive strength. The results are shown in Figure 2.

Figure adapted from Steven Kosmatka and William Panarese, Design and Control of Concrete Mixtures. ©1979 by the Portland Cement Association.