mortar contained more paste (cement plus

water). It is known that hardened cement paste

The objective of this study was to develop a

has a specific surface area that is several orders of

relationship between the dielectric constant and

the moisture content of hardened concrete pro-

cement, sand, or coarse aggregate. Since adsorbed

portioned similarly to the concrete used for the

water has a lower dielectric constant than bulk

Denver International Airport pavement. Figure 3

water, a material with a higher surface area, such

shows that time-domain reflectometry can meas-

as the mortar, would be expected to have a lower

ure dielectric constants for concrete and mortar

dielectric constant than a material possessing a

that are predictably influenced by moisture con-

tentthe higher the moisture content the higher

lower surface area, such as the concrete, for equal

water contents. We see this trend in the Figure 3

the dielectric constant. However, neither concrete

data. This suggests that the dielectric constant of

or mortar results could be mathematically

concrete is a function of coarse aggregate and

described using Topp's equation for soils. When

paste content in addition to water content. We did

compared to Topp's predictions (Fig. 3), the test

not investigate these combinations of variables,

results show that TDR will predict a lower mois-

but the relation between dielectric constant and

ture content for concrete and for mortar than for a

water content might consist of a family of curves

soil of equal dielectric constant. This finding

dependent on paste, aggregate, and moisture

agrees with the problem experienced in the field

content.

where TDR, when related to Topp's equation,

Besides moisture content, the two variables

indicated that the Denver pavement was wetter

that we did investigate were the method used to

than it could possibly be. Thus, a separate rela-

embed a probe into concrete and the size of the

tion between dielectric constant and water con-

coarse aggregate. The findings (Fig. 3) show that

tent had to be developed for concrete.

it did not matter if the TDR probes were jiggled

There may not be a unique relationship for all

into the fresh mix or if the mix was carefully

types of concrete. We say this because the results

placed around the probes. The size of the coarse

for concrete were different from those for mortar.

aggregate did seem to have some effect on the

The data (Fig. 3) for mortar were closer to Topp's

results; Figure 3 shows that the smaller aggregate

equation predictions than were those for con-

produced different results from those of the

crete, with mortar having a lower dielectric con-

larger aggregate. However, the results from the

stant than concrete at any water content. The pri-

smaller aggregate bracketed those from the larger

mary differences between the mortar and the

aggregate; thus, we are uncertain what effect

concrete of this study (Table 2) were that the mor-

aggregate size has.

tar contained no coarse aggregate and that the

12

10

8

6

4

Y = 4.425 + 1.146X + 0.0001928X2

2

R2 = 0.775

0

4

6

8

10

12

14

Dielectric Constant (K)

6