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Publication Number: FHWA-RD-97-146
Date: NOVEMBER 1997
The water-cement ratio is an exceedingly important parameter of the quality of HCC. It is the main control of the compressive strength, abrasion resistance, and permeability. The water-cement ratio can be estimated by petrographic means, or the cement content can be determined by chemical analysis and compared with the cement content specified. The petrographer may be requested to estimate the water-cement ratio whenever concrete does not meet compressive strength specifications. Whether requested or not, an estimate of this ratio should be a regularly scheduled portion of any general examination of HCC.
The water-cement ratio can be estimated by following a five-step procedure, as listed in Table 9-1.
1. Consider the air-void parameters. Determine if there is any indication of an excess amount of air voids or excess size of air voids (see Figs. 6-2 and 6-3). Usually, the air-void system is the first-noticed indication of a high water-cement ratio. The changes in the air-void parameters are because (1) more air is generated and stabilized in mixtures where there is more water, and (2) air-entraining admixtures are diluted (the surface tension inside the air voids is less effective). The dilution causes irregular voids and excessively large voids to be more prevalent. In concrete with a low-to-medium water-cement ratio, the air-void system appears normal and the specific surface is high. There are few (usually less than 1.5% by volume) large and irregular voids.
2. Consider the quantity of the paste. In HCCs with a high water-cement ratio that has occurred because excess water was added to the mixture, the paste content appears high. That is, the aggregate content may appear low. This is due to the fact that water is a component of the paste and excess water expands the paste volume relative to the aggregate volume (see Fig. 7-2A).
3. Consider the appearance of the paste. Study a finely lapped slice of the specimen with the stereomicroscope at 100X. Compare the appearance of the finely
1. Consider the
2. Consider the quantity of the paste.
3. Consider the appearance of the paste.
4. Study the reaction to needles and picks.
5. Estimate the absorption and permeability.
Examine the tiny dark particles that are the remnants of the ferrite (iron-bearing interstitial) portions of the cement. Decide if these particles are more or less prevalent in the specimen under study than in the specimens in the reference collection of normal HCC fabricated with the same type of cement. If the cement type is not known, study the concretes of all the cement types in the collection.
Study the texture of the paste, and compare it with that of specimens in the reference collection. If the paste has a very smooth uniform surface resembling a plastic or ceramic, the water-cement ratio is low. Careful observation of the texture of the paste at 80X to lOOX magnification will indicate that the matrix of HCCs with a high water-cement ratio is different; it is almost sugary. That is, it appears to be composed of a mass of individual particles, as is a sugar cube. This is why it fragments so readily. In extreme cases, these particles may seem to be equant individuals just barely stuck together. When the water-cement ratio is only moderately high, this feature is beyond the resolving power of the stereomicroscope but an experienced observer will be able to see that the paste texture is more open than desired and there is not a compact tight structure as in HCC with a low water-cement ratio.
HCC paste with a high water-cement ratio looks and is more fragile than paste in normal HCC. It is typically lighter in color than would normally be expected.
The average HCC with a medium-to-low water-cement ratio has a dense, solid appearance. In HCCs with a very low water-cement ratio or when certain particulate admixtures are used, the paste appears very smooth and dense, almost like a plastic.
4. Study the reaction to needles and picks. Scratch and pick at the paste while observing the reactions of the paste with the stereomicroscope. If the water- cement ratio is high, edges will seem to come off by fragmentation rather than by the cracking or bending that is common in HCCs with a low water-cement ratio.
5. Estimate the absorption and permeability. Place a drop of water on the finely lapped (oil free) slice, and observe the drop. If the water beads up from surface tension (looks as if it could roll around) before it sinks in, the water-cement ratio is normal or low. If the drop spreads out with a feathery edge and sinks into the concrete quickly, the water-cement ratio is high. The more slowly the water drop disappears into the concrete, the lower the water-cement ratio and the permeability.
If the compressive strength of the concrete is low, the chloride ion permeability is high, the microstructure of the paste appears to be sugary, an aggregate with a known high water demand was used, or more quantitative data are required, a chemically determined cement content of the hardened concrete may be indicated. The costs of this analysis and the arrangements with the chemist or testing laboratory are usually the responsibility of the client. The methods used over the years for this chemical determination are discussed by Hime (1978) and in ASTM C 1084. Other modern methods are discussed by Clemena (1972) and Pistilli (1976). If the amount of water used is known (rare in field concrete), the water-cement ratio can be calculated from this determination.
The proper chemical determination of the cement content requires the use of a method appropriate for the type of aggregate present. A chemical determination of the cement content gives a result that is an average of the cement content of the specimen and provides no information on the extreme conditions that may exist in local zones in the specimen and whose extent and continuity may be critical to the strength and durability of the subject concrete. The selection of the particular portion of a specimen for analysis will affect the results. The chemical method cannot distinguish between cement that has been tied up in only partially hydrated rims and balls and cement that has dispersed and hydrated and thus contributes to the strength of the HCC. If the portion selected has a large proportion of knots of cement or cement rims on aggregates or both, the results will indicate sufficient cement content. If the portion selected is a light-colored portion containing excess air voids, less than a normal amount of unhydrated cement, and paste with a sugary texture, the results will indicate a low cement content. The petrographer must use good judgment and a sense of justice in selecting the portion of the specimen for chemical analysis. There may be no representative portion of the specimen that is of sufficient size for chemical analysis. In such a case, the petrographer might inform the client of the data already obtained and recommend procuring additional specimens.
The result of the cement analysis is reported (usually to the client) in pounds per cubic yard. If the reported amount of cement is significantly less than the amount of cement intended to have been used in the mixture, the water-cement ratio is high and either the volume of the concrete increased (usually because of excess (water) or less than the prescribed amount of cement was used. Because the analysis is not performed on specimens of HCC suspected of having a normal or low water-cement ratio, we have never had to report a case where the cement content indicated that a significant excess of cement was added or that a significant amount of water was omitted.
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