Measuring and testing – Liquid analysis or analysis of the suspension of solids in a... – Viscosity
Reexamination Certificate
1999-05-03
2001-05-08
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid analysis or analysis of the suspension of solids in a...
Viscosity
C073S054030, C073S053040
Reexamination Certificate
active
06227039
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to concrete in general and more particularly to an improved system and method for the design and control of concrete production.
BACKGROUND OF THE INVENTION
Concrete is generally prepared according to a mix design which specifies the proportions of the various material constituents used to produce the concrete. For example, a concrete mix commonly described as a “1:2:4” mix refers to mix proportions of one part of cement to 2 parts of fine aggregate (such as sand) to 4 parts of coarse aggregate (such as gravel). The proportion between cement and water is also usually specified. The design of the mix may depend on many factors and specific engineering requirements.
Some of these requirements relate to the rheological properties required from the mix in its fresh state during its transportation, placement and compaction. The requirements are determined to suit specific conditions of application, for example use of pumping in transportation, the measure of vibration employed during compaction, the measure of cohesion and mobility needed during placing, etc.
Whilst the mechanical properties of hardened concrete are mainly influenced by the water/cement ratio of the mix, the rheological properties of the fresh mix are strongly influenced by the water content and also strongly by the particle size distribution of the solid particles in the mix and by the relative content of fine particles, in the aggregate system.
The particle size distribution in the mix depends on the mix design as well as on the particle size distribution within each of the constituents from which the mix is produced. A change in the character or size distribution of one of the aggregate constituents of a mix having a given design, results in changes in its rheological properties.
To offset such changes, the mix design can be modified. Such modifications are commonly conducted under the supervision of a concrete technician. The suitability of the mix in its fresh state to a given application requirements is commonly measured through the shear deformation of the mix. This is commonly determined by measuring “workability” vis a known test method. A common test method, known as the “Slump Test”, is especially suited for the measurement of the workability of mixes having a soft consistency. A supporting frame in which a mass of concrete has been cast is lifted, allowing the concrete body to freely slump under gravity. The drop in height of the concrete mass is measured.
A test method commonly used for the definition of the “workability” of dry mixes measures the time, in seconds, it takes a body of concrete to change its shape from a truncated cone to a cylindrical shaped body under the effect of standard vibration.
Adjustments in workability are commonly achieved during production of concrete mixes by changing the quantity of water in the mix.
Also common in the monitoring of workability of a concrete mix whilst in a mixing drum is the measurement of the force required to mix the concrete by rotation of the drum or the paddles in the drum. A measurement of the force can be obtained by monitoring the hydraulic pressure or current needed to operate the mixer motor.
The resistance to deformation of a mix can be regulated by changing the amount of water added to it. It is known how to effect an automatic adjustment to the amount of water in the mix so that a constant workability is maintained. For example, European Patent Publication 0 126 573 A1 to Durant proposed a method of controlling the quality of the concrete mix in mobile mixers by measuring the workability of the concrete mix and selectively adding water to achieve the required consistency for the concrete mix.
Different concrete mixes can exhibit equal workabilities when measured by different techniques and yet can possess totally different rheological properties relating to their suitability for commonly required applications. For example, when a concrete mix is designed, for pumping, its rheology is especially adjusted to be pumpable under the set of conditions.
A standard mix design “assumes” that the characteristics of the various constituents are constant. However, in practice, the characteristics of a specific aggregate may vary in time. It may become necessary to change the mix proportions in order to maintain the desired mix characteristics. Unfortunately, present technology does not have any objective procedure of accurately defining the rheological requirements and methodically making changes to the mix proportions so as to ensure that the final product has the required characteristics. The changes, when made, are according to a subjective and qualitative assessment of a concrete technician. It is known to alter the amount of water within a concrete mix in order to change the workability of the mix.
SUMMARY OF THE INVENTION
Commonly used workability tests for concrete are conventions chosen to generate deformation in a concrete mix under set conditions for the purpose of providing a measure of the resistance of the concrete mix to deformation. Applicant has recognized that the resistance to deformation of a concrete mix is influenced by the stress state acting on the shear planes during the deformation. Furthermore, Applicant has realized that different mixes are differently affected in terms of their resistance to deformation following a change in the stress states acting on the shear planes in a deformational regime.
Since, in reality, concrete is applied under various deformational modes and regimes producing different stress states, it is important to provide a parameter which measures the change in the resistance to deformation of a given mix under changing stress states. Such a parameter reflects the internal friction of the mix governing its behavior between the liquid and rigid aggregate phases.
The provision of such a characteristic in the definition of the behavior of concrete provides an important addition to the presently available definition of resistance to deformation which is restricted to a particular workability test method conducted under a given deformational mode. A vector consisting of at least two such variables is hereinafter called the “Rheological Profile” of the mix. In order to obtain readings of two such variables, at least two measurements of resistance to deformation, or related values, are conducted under at least two “measurement environments”. A measurement environment is a domain within a concrete mass having a characteristic stress configuration (state and level) under a given condition in a deformational mode in the concrete mass.
Two measurement environments could be provided, for example, by taking measurements under at least two deformational regimes under changing stress states. The invention offers methods by which a concrete mix undergoes shear in a sequence of at least two changing deformational modes during which the resistance to deformation or related values, are measured.
In this way, a scale is formed by which it is possible to measure and define variables of the rheological profile of a mix. By testing a sequence of a few mixes having controlled changes in the mix design, the mix required to produce any particular rheological profile is established.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a measuring system which includes a surface, a shearing unit and at least one sensor. The surface is in contact with a part of a mass of the concrete mix. The shearing unit effects shear deformation in the mass. The sensor senses a measure of the force which is transferred to the surface by the concrete mass as a result of the shear deformation.
Additionally, in accordance with a preferred embodiment of the present invention, the surface is part of a confining envelope and the sensor is mounted on the confining envelope. There can be two sensors located on two different places of the confining envelope. Alternatively, the two sensors can be located in two different measurement environments along the confirmi
Foley & Lardner
Wiggins J. David
Williams Hezron
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