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A lot can be done with site visits and visual inspections. However, it is often laboratory work that tells the tale of the tape. 

LABORATORY WORK

Petrographic exams use a branch of geology that deals with the descriptions and classifications of rocks. Hardened concrete is considered a synthetic sedimentary rock. Petrographic exams check for the following:

■ Aggregate condition

■ Pronounced cement-aggregate reactions

■ Deterioration of aggregate particles in place

■ Denseness of cement paste

■ Homogeneity of concrete

■ Settlement and bleeding of fresh concrete

■ Depth and extent of carbonation

■ Occurrence and distribution of fractures

■ Characteristics and distribution of voids

■ Presence of contaminating substances

CHEMICAL ANALYSIS

Chemical analysis of hardened concrete can be used to estimate the cement content, original water –cement ratio, and the presence and amount of chloride and other admixtures. This is another form of testing that is part of the larger puzzle in determining the qualities of concrete.

PHYSICAL ANALYSIS

Physical analysis is often done on core samples. This testing looks for nine elements:

■ Density

■ Compressive strength

■ Modulus of elasticity

■ Poisson’s ratio

■ Pulse velocity

■ Direct shear strength of concrete bonded to foundation rock

■ Friction sliding of concrete on foundation rock

■ Resistance of concrete to deterioration caused by freezing and thawing

■ Air content and parameters of the air-void system

NONDESTRUCTIVE TESTING

Nondestructive testing (NDT) is used to determine various relative properties of concrete. Strength, modulus of elasticity, homogeneity, and integrity of concrete can be calculated with NDT. There are many approaches to NDT, and they require inspectors to have expertise in the given approach to arrive at accurate data.

Rebound hammers

Earlier, we talked a little about rebound hammers. This is a form of NDT and a fast and simple way to test concrete. However, the test is imprecise and cannot accurately predict the strength of concrete. Some factors that can skew a test with a rebound hammer include the following:

■ Smoothness of a concrete surface

■ Moisture content

■ Type of course aggregate

■ Size, shape, and rigidity of specimen

Carbonation of concrete surface probes

Probes can be used to do NDT. The probe may use a powder cartridge to insert a high-strength steel probe into a section of concrete. The results of probe measurements can be converted to compressive strength values. There are reports, however, that probes can sometimes supply inaccurate data.

A probe is normally used to test density. A probe will embed deeper in concrete that is suffering from failure in density, subsurface hardness, and as the strength of concrete weakens. This type of testing is fine for on-site, general tests, but it is limited.

Precise measurements are not available from probe testing. The act of probing concrete will leave a hole in the concrete surface that must be repaired.

Ultrasonic pulse-velocity testing

Ultrasonic pulse-velocity testing is probably the most frequently used means of NDT. The results of this testing can be calculated. High velocities indicate good concrete, while low velocities reveal weak concrete. The system for this testing is portable and can penetrate about 35 linear feet of concrete. Testing of this type is fast. However, an inspector must have access to opposite sides of the section being tested, and this can present a problem.

Acoustic mapping

Acoustic mapping provides comprehensive evaluation of the top surface wear of concrete in such structures as aprons, sills, lock chamber floors, and so forth. Fast, accurate evaluations of horizontal sections below water can be done with acoustic mapping. Dewatering is not needed. Accuracy falls off at depths greater than 30 feet.

Ultrasonic pulse-echo testing

Ultrasonic pulse-echo testing is good for fl at surfaces. It can detect steel and plastic pipe that is embedded in concrete. Resolution is good with this type of testing equipment. Improvements in this form of testing continue to develop.

Radar

Radar is an NDT. It does not require contact with concrete. Resolution and penetration is somewhat limited. Some opinions favor signal testing over radar, but radar is a growing element in concrete evaluation.

Following might also be of interest:

☞Rebound hammer test

☞Comparison of Destructive and Non Destructive Testing of Concrete

Analyses of hardened concrete for the original water content of the fresh concrete, in order to calculate water-cement ratio (w/c), are often requested because w/c is the best single measure of potential concrete strength. However, there has not been any chemical method developed that provides reasonable accuracy unless the original concrete components are available as standards. If one considers what water is present originally and what becomes of it, the difficulty of the analysis
becomes clear:
 

The concrete mix contains sufficient water to render the aggregate “saturated surface-dry,” plus “free water.” Only the latter is the “w” of the w/c term. After casting:

• For most mixes, about half of the water evaporates, leaving holes of various sizes and shapes, some of which are connected to each other.
• Some of the water becomes loosely bound to the calcium silicates produced by the cement hydration.
• Some of the water becomes a component of crystalline compounds such as gypsum, ettringite, and calcium hydroxide.
• The crystalline water will not be driven off until the concrete is sufficiently heated, for example, to over 500°C (calcium hydroxide).
• Voids will also be present in the concrete due to incomplete compaction and to air-entrainment. Even in very good quality concrete, at least 1% entrapped voids is assumed in mix design. Concrete having 2 to 5% entrapped voids and 3 to 6% entrained air voids is not unusual. Free water did not occupy such voids when the concrete was placed.
• Some of the hydrate compounds, especially near surfaces, but to substantial depths in an old concrete structure, react with carbon dioxide in the air to convert the hydrates to carbonates. Thus calcium hydroxide converts to calcium carbonate within a few minutes at the concrete surface and even completely so over years or decades, depending upon the porosity of the concrete.

The water liberated may evaporate from the concrete. No significant voids are produced (the reaction
may involve a small volume change of the solids).

Therefore, published methods that determine original water content by resaturating the concrete with water or other liquid and by heating to drive off free and combined water, do not properly account for aggregate water, carbonation, and compaction voids, among the major sources of error.
 

Experienced petrographers can estimate original water-cement ratio, at least between w/c values of about 0.35 to 0.75, using such optical and physical features as color, crystal size, hardness, and porosity. Such values are usually significantly more accurate than those determined chemically. Exceptions may occur if the original aggregate is available to determine the amount of water required to produce a saturated surface-dry condition and, if correction is made for compaction voids as estimated from petrographic measurements, such as by using the linear traverse procedure of ASTM C457.

Procedure for Water Content of the As-Is Hardened Concrete
 

The free water content of hardened concrete may be determined by drying in a vacuum, by desiccation, or by heating to 110°C. The total water content is usually determined by heating to 600°C, which dehydrates most calcium silicate hydrates and calcium hydroxide, the major crystalline hydration product of portland cement. Some calcium silicate hydrates may not be dehydrated at this temperature. Ignition to 950°C will cause loss of carbon dioxide from carbonates and must be corrected for by determining carbon dioxide.

Sulfide sulfur will interfere by decreasing the ignition loss due to oxidation to sulfate. Sulfate determinations before and after ignition will provide a correction. Calculate sulfate in each determination as SO3 and add the difference to the loss percentage.
 

Procedure for Water Content of Original Concrete Mix

 

Determination of the free water content of the original concrete mix, as based on analyses of the hardened concrete, is generally unadvisable. A petrographic estimate is often useful, especially because it is recognized as an estimate whereas a chemically determined value is often assumed, falsely in most cases, to be accurate.