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Federal Highway Administration Research and Technology
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|This report is an archived publication and may contain dated technical, contact, and link information|
Publication Number: FHWA-HRT-04-150
Date: July 2006
The w/cm, water to cementitious materials ratio, (here the cementitious materials include pozzolans such as fly ash and silica fume in addition to the hydraulic cements–portland cement and ground slag) is an exceedingly important parameter of the quality of the HCC. It is the main control of the compressive strength, abrasion resistance, and permeability. The w/cm 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 w/cm whenever concrete does not meet the compressive strength specifications. Whether requested or not, an estimate of this ratio should be a regularly scheduled portion of any general examination of the HCC. This chapter is primarily focused on the examination of hand specimens or polished surfaces. Thin-section examinations are covered in chapter 13 and SEM examinations are discussed in chapter 14.
The w/cm can be estimated by following a five-step procedure, as listed in table 19. Liu and Khan (2000) describe the use of these techniques and report that satisfactory estimations of w/cm of field concretes can be made if an appropriate suite of standard specimens with known w/cm is available.
Examine the tiny dark particles that are the remnants of the ferrite (iron-bearing interstitial) portions of the cement. Determine whether 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 of 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 w/cm is low. Careful observation of the texture of the paste at 80X to 100X magnification will indicate that the matrix of HCCs with a high w/cm is different (it is almost sugary (i.e., 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 w/cm is only moderately high, this feature is beyond the resolving power of the stereomicroscope; however, an experienced observer will be able to see that the paste texture is more open than desirable and there is not a compact tight structure as in HCC with a low w/cm.
HCC paste with a high w/cm 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 w/cm has a dense, solid appearance. In HCCs with a very low w/cm or when particular particulate admixtures are used, the paste appears very smooth and dense, almost like a plastic.
|Characteristic||Lower ←w/cm→ Higher|
|Color, tone||Darker Lighter|
|Density||Denser Less dense|
|Porosity (color intensity of dye-impregnated specimens)||Less Porous More porous|
Size of residual and relict grains
Abundance of residual and relict grains
|Degree of carbonation||Low High|
|Texture, polished surface||Porcelaneous Sugary|
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 cost 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 methods are discussed by Clemeña (1972) and Pistilli (1976). If the amount of water used is known (rare in field concrete), the w/cm 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 a mount of unhydrated cement, and paste with a sugary texture, the resultswill indicate a low cement content. The petrographer must use good judgment in selecting the portion of the specimen for chemical analysis to ensure that it is as representative as possible. There may be no representative portion of the specimen that is of sufficient size for chemical analysis. In such cases, the petrographer might inform the client about the data already obtained and recommend procuring additional specimens.
The result of the cement analysis is reported (usually to the client) in kilograms per cubic meter (kg/m3). If the reported amount of cement is significantly less than the amount of cement intended to have been used in the mixture, then the w/cm 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 w/cm, 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.