# Patent

## Title
Method for real-time on-line monitoring of concrete freeze-thaw damage

## Abstract

A method for real-time and in-situ monitoring of freeze-thaw damage to concrete, comprising embedding pairs of gradient electrical resistance probes into a surface of concrete being monitored at a depth of 0-50 mm, the paired gradient electrical resistance probes having an anti-corrosion metal bar, with the buried depth gradient of different pairs of electrical resistance probes being in the range of 1 mm to 20 mm, and the buried depth gradient of the pair of electrical resistance probes nearest to the surface being less than 5 mm; transmitting an impedance value of the concrete measured by the paired gradient electrical resistance probes to a monitoring center for processing; and evaluating a spalling status of freeze-thaw damage to the concrete based on changes in impedance value which may be abrupt. Real-time in-situ monitoring of the freeze-thaw damage of concrete can be achieved by the electrical resistance probes and monitoring center of this invention.

## Background
This application is the U.S. national phase of International Application No. PCT/CN2012/084198, filed 7 Nov. 2012, which designated the U.S. and claims priority to CN Application No. 201110375744.8 filed 23 Nov. 2011, the entire contents of each of which are hereby incorporated by reference.
FIELD OF THIS INVENTION
This invention refers to the technical field of nondestructive monitoring of concrete, specifically a real-time and online monitoring method for freeze-thaw damage of concrete.
BACKGROUND TECHNOLOGY
With constantly changing climate, freezing of seawater alongside Bohai Bay not only threatens the trips of fishing boats, but also results in serious freeze-thaw damage to concrete structure along the beach and at sea. In the cold area of northern China, in order to ensure traffic safety, deicing salt is used to melt snow in winter; the thermal shock of deicing salt leads to the freeze-thaw damage on the concrete bridge surface or the road surface. The typical characteristic of freeze-thaw damage of concrete is the spalling of surface concrete. Obviously, the surface spalling of land concrete can be observed by unaided eyes, while the structure concrete at the bottom of offshore engineering is just the opposite, which goes against the timely maintenance of structure due to the difficulty of observation.
So far, no method for real time on-line monitoring of concrete freeze-thaw damage is disclosed.
THE CONTENT OF INVENTION
In order to overcome the shortcomings of prior art listed above; this invention provides a method for real-time and online monitoring freeze-thaw damage of concrete, which could monitor the freezing and thawing damages of the concrete real-time and online.
To achieve the mentioned goals, the technical scheme of the invention is as follow:
Method for real-time and online monitoring the freeze-thaw damage of concrete consists of the following steps:
Step One, embed pairs of gradient electrical resistance probes into surface of the monitoring concrete at the depth of 0-50 mm, the paired gradient electrical resistance probes comprise an anti-corrosion metal bar, and the buried depth gradient of different electrical resistance probes is in the range of 1 mm to 20 mm, and the buried depth of the pair of electrical resistance probes nearest to the surface is less than 5 mm.
Step Two, transmit the impedance value of the concrete measured by the paired gradient electrical resistance probes to the monitor center for processing;
Step Three, fudge the spalling status of freeze-thaw damage of concrete through the abrupt change in impedance value
A. The electrical resistance probes with the same depth meet with abrupt changes for the concrete impedance value and there are three status:
a. On the curve of Impedance-Time, the impedance value changes from relatively stable to infinite, at the moment the concrete suffers spalling and there is no free water seeped into the concrete;
b. On the curve of Impedance-Time, the impedance value changes from relatively stable to almost zero, at the moment the concrete suffers spalling and free water seeped into the concrete;
c. On the curve of Impedance-Time, the impedance value presents a vertical change in trend, if the impedance value rises, the concrete suffers spalling and there is no free water seeped into the concrete, if the impedance value decreases, it suffers spalling and free water seeped into the concrete.
B. From the surface to the interior, as for the buried electrical resistance probe of one layer or deeper, if the impedance value of the concrete meets with the status as mentioned in A, refer to A and judge according to the standards.
Real-time online monitoring of the freeze-thaw damage of concrete can be achieved by the electrical resistance probes and monitoring center of this invention.



DESCRIPTION OF THE FIGURES
FIG. 1 is the principle diagram of monitoring freeze-thaw damage of concrete in the invention.
FIG. 2 is the schematic diagram of changes in the impedance value of the concrete surface with the time.
FIG. 3 is the diagram of the changes in impedance value of the concrete with time when embedding the electrical resistance probe in different depth.



EMBODIMENTS
Detailed description of the invention with reference to the attached Figures and examples as follows.
According to FIG. 1, method for real-time and online monitoring freeze-thaw damage of concrete contains the following steps:
Step One, referring to FIG. 1 and FIG. 2, embed pairs of gradient electrical resistance probes R1-Rn into concrete T at the depth of 0-50 mm from a surface of the concrete, electrical resistance probes R1-Rn are connected to the impedance meter K, the paired gradient electrical resistance probes comprise an anti-corrosion metal bar, and the buried depth gradient of different electrical resistance probes is in the range of 1 mm to 20 mm, the buried depth gradient of the electrical resistance probes close to the surface shall be less than 5 mm, so as to identify the freeze-thaw damage to the concrete at an early stage of damage in the outermost surface.
Step Two, transmit the concrete impedance value tested by paired gradient electrical resistance probes to the monitoring center for processing.
Step Three, judge the condition of freeze-thaw spalling of concrete by the abrupt changes in concrete impedance value. There are two judging standards according to the conditions:
A. The electrical resistance probes with the same depth meet with abrupt changes for the concrete impedance value and there are three status: Refer to FIG. 2,
a. On the curve of Impedance-Time, the impedance value changes from relative stable to infinite, at the moment the concrete suffers spalling and there is no free water seeped into the concrete;
b. On the curve of Impedance-Time, the impedance value changes from relative stable to almost zero, at the moment the concrete suffers spalling and there is free water seeped into the concrete;
c. On the curve of Impedance-Time, the impedance value shows a vertical change in trend, if the value rises, the concrete suffers spalling with no free water immerses, while if the value decreases, it suffers spalling and free water seeped into the concrete.
B. From the exterior to the interior, as for the buried electrical resistance probe of one layer or deeper, if the impedance value of the concrete meets with the status as mentioned in A, refer to A and make conclusions according to the standards.
The technical principle of the method is that the electrical resistance probe shows open circuit, short circuit that leads to the dramatic changes in concrete impedance value when the concrete surface is spalling, thereby showing the freeze-thaw damage of the concrete; As for the regions that not suffer from freezing damage, the impedance value of concrete remains relatively stable, without any significant changes altogether.
EXAMPLES
Method for real-time and online monitoring freeze-thaw damage of concrete contains the following steps:
Step One, in the test specimen of concrete with coating in the tidal range zone, embed 6 pairs of gradient electrical resistance probes into surface of the concrete monitoring at the depth of 0 ̃50 mm, the paired gradient electrical resistance probe is brass(H60) with the diameter of 6 mm, the electrode distances of which are all 100 mm. The electrical resistance probe from the exterior to the interior is sequenced from No. 1 to No. 6, and the buried depths are respectively 0, 5, 10, 15, 20 and 40 mm.
Step Two, transmit the concrete impedance value tested by paired gradient electrical resistance probe to the monitoring center for processing.
Step Three, judge the condition of freeze-thaw spalling of concrete by the abrupt changes in concrete impedance value; according to FIG. 3, the concrete impedance at the depth of 5 mm from the coating is strongly affected by the external environment while at the depth of 10-40 mm it is essentially unchanged. In addition, because of the coating on the concrete test specimen surface, the impedance value of electrical resistance probe in the first layer varies in the range of 500 to 700 kOhm. When it suddenly decreases to dozens of ohm, the coating is broken off. When the value changes from dozens of ohm to infinite during receding tide while decreases to almost zero during rising tide, it can be concluded that the surface concrete suffers spalling; By that analogy, when concrete resistance between the electrical resistance probes from layer 2 to layer 6 changes dramatically, on condition that the corresponding concrete impedance value changes from dozens of ohm to infinite during receding tide and to almost zero during rising tide, it can be proved that the concrete suffers spalling at that depth.

## Claims

1. A method for real-time and in-situ monitoring the freeze-thaw damage of concrete, including the following steps:
(1)—embed pairs of gradient electrical resistance probes into a surface of the concrete at the depth of 0 ̃50 mm, each of the pairs of gradient electrical resistance probes comprising an anti-corrosion metal bar, the buried depth gradient between different pairs of electrical resistance probes being in the range of 1 mm to 20 mm, and the buried depth gradient of the pair of electrical resistance probes nearest to the surface being less than 5 mm;
(2)—transmit the impedance value of the concrete measured by each of the pairs of gradient electrical resistance probes to a monitor center for processing;
(3)—evaluate a spalling status of freeze-thaw damage to the concrete based on changes in impedance value as reflected in a curve of impedance versus time, there being two evaluation standards according to conditions:
(A). The electrical resistance probes with a same depth meet with abrupt changes for the concrete impedance value and there are three statuses:
i. on the curve of Impedance-Time, an impedance value changes from relatively stable to infinite at the moment the concrete suffers spalling and free water does not penetrate the concrete;
ii. on the curve of Impedance-Time, the impedance value changes from relatively stable to almost zero at the moment the concrete suffers spalling and free water penetrates into the concrete;
iii. on the curve of Impedance-Time, where the impedance value presents a vertical change in trend: if the impedance value rises, the concrete suffers spalling and free water does not penetrate the concrete, whereas if the impedance value decreases, it suffers spalling and free water penetrates into the concrete; and

(B). From the surface to the interior, for each pair of buried electrical resistance probes at each layer, if the impedance value of the concrete meets with the status as mentioned in A, refer to A and judge according to the standards discussed in A.