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:
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:
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• 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.
• 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).
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.