Chapter 9. Water-Cementitious Materials Ratio

Previous | Table of Contents | Next

9.1 OVERVIEW

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.

9.2 PROCEDURES

9.2.1 Estimation

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.

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 figures 53 and 54). Usually, the air-void system is the first-noticed indication of a high w/cm. 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 w/cm, the air-void system appears normal and the specific surface is high. There are few large and irregular voids (usually less than 1.5 percent by volume).
2. Consider the quantity of the paste: In HCCs with a high w/cm that has occurred because excess water was added to the mixture, the paste content appears high (i.e., the aggregate content may appear low). This is because water is a component of the paste and excess water expands the paste volume relative to the aggregate volume (see figure 63).
3. Consider the appearance of the paste: Study a finely lapped slice of the specimen with the stereomicroscope at 100X for the characteristics listed in table 20. Compare the appearance of the finely lapped surface with that of specimens in the reference collection that have a known w/cm.

   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.

   Table 20. Paste characteristics reflective of w/cm.

   Characteristic	Lower ←w/cm→ Higher
   Color, tone	Darker                         Lighter
   Hardness	Harder                         Softer
   Density	Denser                         Less dense
   Porosity (color intensity of dye-impregnated specimens)	Less Porous         More porous
   Cementitous materials: Size of residual and relict grains Abundance of residual and relict grains	Larger                          Smaller
   	More                            Less
   Calcium hydroxide: Size Abundance Morphology	Smaller                          Larger
   	Less                               More
   	Anhedral                        Euhedral
   Degree of carbonation	Low                               High
   Texture, polished surface	Porcelaneous                 Sugary

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 w/cm 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 w/cm.
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 w/cm is normal or low. If the drop spreads out with a feathery edge and sinks into the concrete quickly, the w/cm is high. The more slowly the water drop disappears into the concrete, the lower the w/cm and the permeability.

9.2.2 Chemical Determination

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.