Electricity: measuring and testing – Particle precession resonance
Reexamination Certificate
2000-03-02
2002-05-28
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
C324S301000, C324S306000
Reexamination Certificate
active
06396265
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatus for measuring and predicting parameters of capillary-porous chemically active materials while and after curing. More particularly, the present invention relates to methods and apparatus for non-destructive control and prediction of concrete strength.
As used herein throughout the specification and in the claims section below the phrase “capillary-porous chemically active material(s)” includes cementitious substances, such as, but not limited to, cement, concrete, lime, gypsum, clay and the like.
Concrete, which is used for construction, must be analyzed to determine the structural properties parameters, particularly strength and other physical-mechanical properties of the final cured product, such as its potential for shrinkage.
Concrete is mainly a mixture of water, cement, sand, and gravel. It is known that the fraction of water at all stages of its preparation (e.g., mixing, compaction, curing and hardening), is one of the most crucial parameters, responsible for the strength of the final cured cement stone and concrete.
The water content in concrete appears in two main states (stages) during curing: chemically bound water and physically bound water. The relative water content in formed structure during curing permanently decreases, as a portion of the physically bound water interacts chemically with other concrete components and transforms into a solid phase, whereas another portion is evaporating away from the surface.
It is known that the amount of water that is physically bound has an important influence on the compressive strength and other physical-mechanical properties of concrete at all stages of its formation and utilization.
The traditional prior art methods for testing the strength of concrete require 28 days to complete. The builder usually does not or cannot delay construction 28 days to receive the test results. Rather, the construction usually continues in the hope that the concrete is sound. If in the final analysis, the concrete does not meet the standards, the building may have to be reinforced or even torn down, perhaps incurring major additional costs.
Thus, a method of quick analysis of concrete properties which predicts its final strength or measures the structure strength in situ while hardening, is very desirable.
In addition, when concrete is utilized as a strength bearing member, it would be useful to know when “shrinkage” of the concrete has been completed so as not to load the member prematurely, as the premature addition of load to bearing members could lead to cracking of the concrete structure. Shrinkage of cement stone and concrete is correlated to their moisture content. However, to a larger extent, the shrinkage value of cement stone and concrete depends on the size of pores and capillaries in the formed structure, in other words, on the energy level of physically-bound water, interacting with the solid phase.
There is a known method for determination of cement stone and concrete strength according to its porosity. To this effect, see, Roy D. M., Gouda G. R. Porosity—strength relation in porous materials with very high strength” J. Amer. Ceramic Soc., 53, No. 10, 1973, pp. 549-550. According to this method, the strength, R, is determined by:
R
=
-
1
k
ln
π
π
0
(
1
)
where Π is the general porosity of the cement stone (concrete); Π
0
is the porosity at zero strength (R=0), which is approximately 60%; and k is a constant which equals 0.385×105 MPa.
There is an additional known method for determining cement stone and concrete strength, which is described by Powers T. C. “The physical structure and engineering properties of concrete” Port. Cem. Ass. Dept. Bul. 90, Chicago, July 1958.
According to this method, the volume of pores of the cement gel V
g
, and the general volume of capillary space V
k
of the concrete are experimentally measured; whereas the strength is determined according to the following equation:
R
=
A
(
V
g
V
g
+
V
k
)
n
(
2
)
where A and n are constants.
The change of the porosity parameters V
g
and V
k
of Equation 2 during process of hardening is well-supported by data from the literature. To this effect, see, for example, Sheikin A. E. “Structure durability and crack resistance of cement stone”. Moscow, Stroyizdat, 1974, p. 191.
Table 1 below, for example, provides porosity related data for a cement stone having a water/cement (W/C) ratio of 0.7 during hardening.
TABLE 1
The change of general (Π), capillary (Π
k
) and gel (Π
g
) porosity of a
cement stone at the process of hardening
Time of
hardening
(days)
(Π)
Π
k
Π
g
0
0.70
0.70
—
3
0.65
0.58
0.07
7
0.58
0.49
0.09
14
0.57
0.44
0.13
28
0.54
0.37
0.17
The difficulty and labor input associated with measuring the porosity of concrete and other capillary-porous chemically active materials (especially during hardening) are the shortcomings of the above mentioned methods.
This is due to the necessity to prepare a large quantity of twin-samples, each of which is tested at a particular stage of hardening.
Independent of the applied method for measuring porosity (e.g., nitric porometry, mercury porometry of low and high pressure, etc.), the tested sample should be completely dehydrated. This considerably complicates the testing method, increases its duration and considerably affects the properties of the tested material.
Since cement stone, concrete and other similar materials at any stage of hardening are poly-dispersed moist capillary-porous bodies, it is possible to avoid most of the above-mentioned shortcomings if concrete strength will be determined not by measuring its porosity, but rather by measuring the energy of physically bound water, which is contained in the pores and capillaries of its structure, which is indicative of its porosity and therefore of, for example, its strength.
Water (both in a liquid and gaseous form) is always in a state of thermodynamic equilibrium with the porous solid phase with which it interacts. Thus, the properties of water (viscosity, bounding energy, relaxation time, etc.) are changing in strict accordance with structure formation and, consequently, with the strength growth of the hardening material. To this effect, see, for example, Shtakelberg D. I. “Thermodynamics of water-silicate disperse materials structure-formation”. Riga, Zinatne, 1984, p.200; and Shtakelberg D. I., Sytchov M. M. “Self-organization in disperse systems”. Riga, Zinatne, 1990, p175; and Neville M. “Properties of concrete” Longman Scientific & Technical. NT., 1988, p779.
In a newly compressed cement paste, whose strength is minimal, e.g., in the order of 10
−1
Mpa, practically all the water is distributed between the grains of a non-hydrated cement. The average distance between the grains is approximately 5-10 &mgr;m. At this state, the bond energy of water molecules and the material constitutes only a few kDz/mol.
While hardening, a portion of the water becomes chemically bound, i.e., transforms into a solid state with bond energy in the order of 1000 kDz/mol. Another portion of the water is contained in the pores of the formed cement gel. The size of these pores is less than 10
−3
&mgr;m in diameter and the bond energy in this case is up to 50 kDz/mol. Another portion of the water occupies capillaries of a larger diameter (10
−2
-10
−1
&mgr;m) with bond energy of up to 10-20 kDz/mol.
T
2
relaxation time of physically-bound water, contained in capillary-porous structure of chemically-active material changes in a very wide range: from 30-40 &mgr;sec (liquid of thin surface layers) up to 3×106 &mgr;sec (bulk water).
During subsequent stages of concrete structure hardening and until its final formation, the water distribution reaches a steady state in which 45-50% of the water is chemically bound, 40% of the water occupies the smaller pores of the cement gel, whereas 10-15% of the water occupies larger capillaries of the concrete structure.
Thus, information pertaining to the energy le
Boyko Shimon
Shtakelberg David
Wilge Boris
Concrete Ltd.
Fetzner Tiffany A.
G. E. Ehrlich Ltd.
Lefkowitz Edward
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