Abstract:
An improved turbidimeter apparatus (10) is provided for accurate, rapid, field testing of fresh concrete mixes in order to determine the water-cement ratios thereof. The apparatus (10) includes an upright sieve chamber (12) adapted to receive a concrete sample (56), together with a turbidimeter (14); a recirculation system (16) is interposed between the chamber (12) and turbidimeter (14), and permits separation of the aggregate from the concrete sample (56) and formation of a dilute dispersion suitable for turbidimetric determinations. In use, dilute water-cement dispersions are created using the apparatus (10) and turbidimeter determinations are made via turbidimeter (14); these readings can then be used with standard, previously developed water-cement ratio versus turbidimeter reading curves to accurately measure the initial water-cement ratios of concrete mixes.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is broadly concerned with an improved method and apparatus for the rapid, accurate determination of the water-cement ratios of fresh, uncured concrete. More particularly, the invention pertains to a method and apparatus wherein a dilute dispersion or paste of the cement fraction of the concrete is passed through a turbidimeter to determine the turbidity of the dispersion; this value can then be compared with a known, standard turbidity curve to ascertain the water-cement ratio of the fresh concrete. 
     2. Description of the Prior Art 
     Concrete is a man-made composite, usually made up of cement, aggregate and water; various types and sizes of aggregates can be used, together with additives such as air-entraining agents. Concrete is one of the most widely used of construction materials. However, concrete is unique in that it is manufactured as used and generally cannot be tested for acceptance in advance. Such acceptance is sometimes based on a strength test at an advanced stage of curing, as concrete is known to gain strength over long periods of time. Acceptance may also be based on adherence to a specified formula, along with monitoring for strength. As a consequence, concrete may be placed which may or may not be acceptable at a later date. If found unacceptable, then the emplaced concrete must be removed and new concrete poured. 
     Accordingly, there is a real and unsatisfied--need in the art for a means of assessing the quality of concrete before placement, and for assuring that the concrete meets the specified standards. In this regard, the water-cement ratio of a plastic, uncured concrete is known to be a good candidate for assessing the quality of concrete delivered to a construction site. This is because in engineering practice, the strength and permeability of Portland cement concrete of a given age and cured at a prescribed temperature condition is assumed to depend primarily on two factors: the water-cement ratio of the fresh concrete and the degree of compaction. Thus, when full compaction is achieved, the only variable affecting the strength and permeability of any concrete mix is the water-cement ratio. For a fully compacted concrete, this strength is taken to be inversely proportional to the water-cement ratio, according to a relationship established by Duff Abrams in 1919: 
     
         strength, f.sub.c =K.sub.1 /K.sub.2.sup.w/c 
    
     where w/c represents the water-cement ratio of the mix, and K 1  and K 2  are empirical constants. 
     Although the initial water-cement ratio is the predominant factor affecting strength of a hardened concrete, there is currently no reliable method available for measuring water-cement ratios of fresh concretes in the field or at a job site. An indirect evaluation of water-cement ratio can be made through consistency or workability evaluations using a &#34;slump&#34; test. Although this type of test is rapid and easy to employ, it often gives very inaccurate water-cement ratio results. 
     Accordingly, there is an unresolved need in the art for an improved method and apparatus for the determination of water-cement ratios in concrete mixtures, especially in the field or at job sites. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems outlined above and provides a greatly improved method and apparatus for the determination of water-cement ratios of uncured concrete containing water, cement and aggregate. The invention permits such water-cement ratio determinations quickly and easily using a portable field tester. 
     Broadly speaking, the apparatus of the invention includes means for separating the aggregate from the cement and water and for creating a dilute dispersion of the cement in added water. A turbidimeter is operably coupled with the separating and diluting means for determining the turbidity of the dilute dispersions as a measure of the initial water-cement ratio. Such involves use of standard turbidity/water-cement ratio curves which are consulted to determine water-cement ratios corresponding to determined turbidities. 
     In more detail, the preferred apparatus includes an upright tower or chamber having sieve means therein for receiving a quantity of the uncured concrete. A shower head or similar means is provided at the upper end of the chamber for directing dilution water over uncured concrete thereby causing separation of the aggregate from the cement and water, while also creating the desired dilute dispersion. In practice, a recirculation conduit is coupled between the bottom and top of the upright chamber so as to permit recirculation of the dilution water over time; to this end, a pump is interposed within this conduit. 
     The turbidimeter is advantageously coupled to the recirculation conduit of the apparatus, and appropriate valving is provided for directing the dilute water-cement dispersion through the turbidimeter for successive turbidity determinations. Although a variety of turbidimeters can be employed, DRT turbidimeters commercialized by H. F. Scientific, Inc. of Ft. Myers, Fla. have been found to be satisfactory. In this connection, turbidity is an expression of the optical properties that cause light to be scattered or passed through a liquid sample, and is largely a function of the refractive index, and the size and shape of particles in the liquid. Therefore, the particular turbidity results achieved may be dependent upon the instrument employed. 
     Generally, the water-cement ratios of the dilute dispersions tested for turbidity according to the invention should be in the range of from about 10-1000, and more preferably from about 50-250. Furthermore, it may be advisable to take multiple turbidity readings over time to insure that the most accurate results are obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of the preferred apparatus for determining water-cement ratios of fresh concrete mixtures; 
     FIG. 2 is a graph of average turbidities versus elapsed time for the non air-entrained concrete mixtures tested in Example 1; 
     FIG. 3 is a graph of average turbidities versus elapsed time for the concretes with air-entraining agent tested in Example 2; 
     FIG. 4 is a graph of average turbidities versus elapsed time for the concretes with air-entraining agent and polypropylene glycol tested in Example 3; 
     FIG. 5 is a graph of average turbidities versus elapsed time for non air-entrained concrete and concrete with air-entrainer and polypropylene glycol; and 
     FIG. 6 is a graph of water-cement ratios versus actual and predicted turbidimeter readings employing the regression analysis results described in Example 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings, and particularly FIG. 1, an apparatus 10 for determining the water-cement ratio of uncured concrete containing water, cement and aggregate is illustrated. Broadly speaking, the apparatus 10 includes an upright chamber or tower 12, a turbidimeter 14 and a recirculation system 16. 
     In more detail, the tower 12 is an upright metallic unit presenting a generally cylindrical main body 18 terminating in a lowermost conical bottom section 20, the latter presenting a tubular outlet 22. The upper end of body 18 has a top wall 24 which is apertured to receive the upper end of a recirculation conduit as will be described. The body 18 houses a plurality of transversely extending, vertically stacked sieves 26, 28, 30, 32. These sieves are preferably, in vertically descending order, #4, #10, #50 and #100 sieves. The lowermost conical section 20 of the chamber 12 includes an upright, central anti-vortex plate 34 adjacent outlet 22. 
     The turbidimeter 14 is a commercially available unit, specifically a DRT-100B turbidimeter sold by H. F. Scientific, Inc. of Ft. Myers, Fla.. The turbidimeter includes respective inlet and outlet ports 36, 38 in order to permit passage of liquid through the turbidimeter and turbidimeter determinations using the apparatus 14. 
     Recirculation system 16 includes a primary conduit 40 extending from outlet 22 of chamber 12 upwardly through top wall 24 and terminating in a dispersing outlet or showerhead 42 situated within the confines of chamber 12 above the stacked sieves 26-32. A pump 44 (in practice a 1/2 hp, 24 gpm pump) is interposed in main conduit 40, along with spaced valves 46, 48 and 50. The overall recirculation system 16 further includes a conduit 52 upstream of valve 50 and communicating with turbidimeter inlet 36; and a conduit 54 operatively coupled between turbidimeter outlet 38 and main conduit 40 downstream of valve 50. 
     In the use of apparatus 10, a known quantity of fresh concrete 56 is placed within chamber 12 (which is provided with an access door for this purpose) above the stacked sieves 26-32. Next, a known amount of water is charged to the chamber 12 to fill the lower portion 20 thereof. At this-point, valves 46, 48 and 50 are opened and pump 44 is actuated, thereby circulating water through main conduit 40 and showerhead 42, this serving to wash the concrete 56 through the sieve system to remove the aggregate from the concrete. After such recirculation has proceeded for a time sufficient to assure substantial separation of the aggregate, the valve 50 is closed, thereby diverting the recirculating, dilute stream of cement and water through turbidimeter 14 via conduits 52, 54 and corresponding inlet and outlet 36, 38. During passage of the dilute dispersion through the turbidimeter 14, readings may be taken at successive time intervals until a substantial steady state is achieved. Generally speaking, a measurement time period of up to about 300 seconds has been found to be sufficient. At this point, the resultant turbidimeter reading can be compared with a standard graph of turbidimeters readings versus water-cement ratios (similar to that shown in FIG. 6) in order to permit an accurate determination of the initial water-cement ratio of the starting concrete. 
     The following examples illustrate preferred embodiments of the invention and use thereof. It is to be understood, however, that these examples are presented by way of illustration only and nothing therein should be taken as a limitation upon the overall scope of the invention. 
     EXAMPLE 1 
     In this series of tests, non air-entrained concrete mix having water/cement ratios of 0.40, 0.44 and 0.48 were tested in the FIG. 1 apparatus to determine their turbidities over time. In each case, fresh concrete was made using a 1:2:3 mixture of cement, fine aggregate, and coarse aggregate (1 lb. cement, 2 lbs. fine aggregate, 3 lbs. coarse aggregate) with the appropriate quantity of water to achieve the respective water/cement ratio. 
     In each test, 20 lbs. of wash water was used by addition thereof to the conical base of the wash tower. The fresh concrete was allowed to stand for 19 minutes before the test began, and a 1 lb. sample of the concrete was placed on the uppermost sieve. The water was then circulated through the wash tower and turbidimeter, and readings were taken at specified elapsed times. 
     The following tables set forth the results of these tests, wherein two separate batches of concrete at each water/cement ratio were run. 
     
                                           TABLE 1__________________________________________________________________________Water/Cement Ratio = 0.40__________________________________________________________________________Trial No.             1    2__________________________________________________________________________Water-cement ratio    0.40 0.40Wt. of cement (g)     4268 4268Wt. of water (g)      1707 1707Wt. of fine aggregate (g)                 8537 8537Wt. of coarse aggregate (g)                 12805                      12805Total wt. of mix (g)  27317                      27317Wt. of water in the 1-lb. sample (g)                  28   28Wt. of wash water (g) 9072 9072Dilution (approximate)                 320  320__________________________________________________________________________       Elapsed Time (sec)                 Batch 1                      Batch 2                           Average                                Std. Dev.__________________________________________________________________________Turbidimeter Readings        0         4    4    4   0(NTU)*       60       322  325  324  2       120       223  222  223  1       150       208  213  211  4       180       195  201  198  4       210       188  197  193  6       225       187  198  193  8       240       183  198  191  11       255       183  198  191  11       270       182  201  192  13       285       184  204  194  14       300       178  205  192  19       315       178  208  193  21       330       179  183  181  3       345       181  180  181  1       360       179  174  177  4__________________________________________________________________________ *Nephelometric Turbidity Unit 
    
     
                                           TABLE 2__________________________________________________________________________Water/Cement Ratio = 0.44__________________________________________________________________________Trial No.             1    2__________________________________________________________________________Water-cement ratio    0.44 0.44Wt. of cement (g)     4268 4268Wt. of water (g)      1878 1878Wt. of fine aggregate (g)                 8537 8537Wt. of coarse aggregate (g)                 12805                      12805Total wt. of mix (g)  27488                      27488Wt. of water in the 1-lb. sample (g)                  31   31Wt. of wash water (g) 9072 9072Dilution (approximate)                 293  293__________________________________________________________________________       Elapsed Time (sec)                 Batch 1                      Batch 2                           Average                                Std. Dev.__________________________________________________________________________Turbidimeter Readings        0         5    4    5   1(NTU)*       60       348  342  345  4       120       245  255  250  7       150       235  228  232  5       180       221  220  221  1       210       217  205  211  8       225       215  205  210  7       240       219  202  211  12       255       205  198  202  5       270       203  203  203  0       285       202  205  204  2       300       208  210  209  1       315       208  209  209  1       330       202  211  207  6       345       205  194  200  8       360       207  197  202  7__________________________________________________________________________ *Nephelometric Turbidity Unit 
    
     
                                           TABLE 3__________________________________________________________________________Water/Cement Ratio = 0.48__________________________________________________________________________Trial No.             1    2__________________________________________________________________________Water-cement ratio    0.48 0.48Wt. of cement (g)     4268 4268Wt. of water (g)      2049 2049Wt. of fine aggregate (g)                 8537 8537Wt. of coarse aggregate (g)                 12805                      12805Total wt. of mix (g)  27659                      27659Wt. of water in the 1-lb. sample (g)                  34   34Wt. of wash water (g) 9072 9072Dilution (approximate)                 270  270__________________________________________________________________________       Elapsed time (sec)                 Batch 1                      Batch 2                           Average                                Std. Dev.__________________________________________________________________________Turbidimeter Readings        0         4    4    4   0(NTU)*       60       253  348  301  67       120       245  265  255  14       150       244  262  253  13       180       236  244  240  6       210       236  235  236  1       225       233  235  234  1       240       233  235  234  1       255       232  227  230  4       270       234  227  230  5       285       229  234  232  4       300       229  237  233  6       315       227  236  232  6       330       228  213  221  11       345       227  214  221  9       360       231  209  220  16__________________________________________________________________________ *Nephelometric Turbidity Unit 
    
     FIG. 2 is a graphical representation of the average turbidities versus elapsed time for the non air-entrained concrete mixtures described above. As can be seen, the average values are quite consistent, particularly above about 200 seconds elapsed time. 
     EXAMPLE 2 
     In another series of tests similar to those described above, concretes having water/cement ratios of 0.4, 0.44 and 0.48 were prepared which also included an amount of an air entrainment surfactant (Vinsol Resin sold by Fritz Chemical, Dallas, Tex.). Such air entrainment agents are quite conventional and are typically used with concretes which must be poured and cured in cold weather. 
     The tests were performed in the same manner as Example 1, save for the fact that the surfactant was added to the concrete during initial mixing thereof. 
     The following tables set forth the results of these tests. 
     
                                           TABLE 4__________________________________________________________________________Water/Cement Ratio = 0.40 + Surfactant__________________________________________________________________________Trial No.             1    2__________________________________________________________________________Water-cement ratio    0.40 0.40Wt. of cement (g)     4268 4268Wt. of water (g)      1707 1707Wt. of fine aggregate (g)                 8537 8537Wt. of coarse aggregate (g)                 12805                      12805Vol. of air-entrainer in concrete (ml)                 3.00 3.00Total wt. of mix (g)  27317                      27317Wt. of water in the 1-lb. sample (g)                  28   28Wt. of wash water (g) 9072 9072Dilution (approximate)                 320  320__________________________________________________________________________       Elapsed Time (sec)                 Batch 1                      Batch 2                           Average                                Std. Dev.__________________________________________________________________________Turbidimeter Readings        0         4    4    4    0(NTU)*       60       412  364  388  34       120       295  259  277  25       150       269  233  251  25       180       249  221  235  20       210       236  208  222  20       225       233  203  218  21       240       228  204  216  17       255       227  209  218  13       270       225  216  221   6       285       220  218  219   1       300       216  218  217   1       315       218  217  218   1       330       218  220  219   1       345       216  205  211   8       360       218  204  211  10__________________________________________________________________________ *Nephelometric Turbidity Unit 
    
     
         __________________________________________________________________________Water/Cement Ratio = 0.44 + Surfactant__________________________________________________________________________Trial No.             1    2__________________________________________________________________________Water-cement ratio    0.44 0.44Wt. of cement (g)     4268 4268Wt. of water (g)      1878 1878Wt. of fine aggregate (g)                 8537 8537Wt. of coarse aggregate (g)                 12805                      12805Vol. of air-entrainer in concrete (ml)                 3.00 3.00Total wt. of mix (g)  27488                      27488Wt. of water in the 1-lb. sample (g)                  31   31Wt. of wash water (g) 9072 9072Dilution (approximate)                 293  293__________________________________________________________________________       Elapsed Time (sec)                 Batch 1                      Batch 2                           Average                                Std. Dev.__________________________________________________________________________Turbidimeter Readings        0         4    5    5   1(NTU)*       60       301  337  319  25       120       222  216  219  4       150       206  203  205  2       180       194  197  196  2       210       189  190  190  1       225       190  187  189  2       240       192  186  189  4       255       188  186  187  1       270       188  185  187  2       285       187  191  189  3       300       183  189  186  4       315       181  190  186  6       330       184  190  187  4       345       181  192  187  8       360       186  198  192  8__________________________________________________________________________ *Nephelometric Turbidity Unit 
    
     
                                           TABLE 6__________________________________________________________________________Water/Cement Ratio = 0.48 + Surfactant__________________________________________________________________________Trial No.             1    2__________________________________________________________________________Water-cement ratio    0.48 0.48Wt. of cement (g)     4268 4268Wt. of water (g)      2049 2049Wt. of fine aggregate (g)                 8537 8537Wt. of coarse aggregate (g)                 12805                      12805Vol. of air-entrainer in concrete (ml)                 3.00 3.00Total wt. of mix (g)  27659                      27659Wt. of water in the 1-lb. sample (g)                  34   34Wt. of wash water (g) 9072 9072Dilution (approximate)                 270  270__________________________________________________________________________       Elapsed Time (sec)                 Batch 1                      Batch 2                           Average                                Std. Dev.__________________________________________________________________________Turbidimeter Readings        0         5    5    5   0(NTU)*       60       233  256  245  16       120       194  176  185  13       150       189  180  185  6       180       183  179  181  3       210       183  178  181  4       225       185  181  183  3       240       185  177  181  6       255       186  180  183  4       270       183  180  182  2       285       189  180  185  6       300       187  177  182  7       315       192  180  186  8       330       200  182  191  13       345       207  185  196  16       360       211  198  205  9__________________________________________________________________________ *Nephelometric Turbidity Unit 
    
     FIG. 3 is a graph of the average turbidities obtained in these tests versus elapsed time. As can be seen from a comparative study of FIGS. 2 and 3, the average turbidities were lowered (and inconsisted) in the case of the air-entrained concretes. 
     EXAMPLE 3 
     In another series of tests, concretes identical with those tested in Example 2 were further supplemented with one or two air detraining agents, namely tributyl phosphate (TBP) at a level of 20 milliliters, and polypropylene glycol (PPG) at a level of about 2 milliliters. The PPG had an average molecular weight of about 2,000, and was obtained from BASF Corp. Chemical Div., Parsippany, N.J. under the trade designation &#34;PLURADYNE&#34; No. DB1021. The air-detraining agents were added to the 20 lbs. of wash water at the beginning of each test run. 
     The purpose of these tests was to determine whether an air detraining agent could be used with air-entrained concretes to obtain valid water/cement test results. 
     The following tables set forth the data recorded in this series of tests, and also include the average data from the corresponding tables of Examples 1 and 2. 
     
                                           TABLE 7__________________________________________________________________________Water/Cement Ratio = 0.40 + Surfactant + Air Detrainer__________________________________________________________________________                               Non Air-                               EntrainedTrial No.          1    2    Air Entrained                               mix__________________________________________________________________________Water-cement ratio 0.40 0.40 0.40   0.40Wt. of cement (g)  4268 4268 4268   4268Wt. of water (g)   1707 1707 1707   1707Wt. of fine aggregate (g)              8537 8537 8537   8537Wt. of coarse aggregate (g)              12805                   12805                        12805  12805Vol. of air-entrainer in concrete (ml)              3.00 3.00 3.00   3.00Total wt. of mix (g)              27317                   27317                        27317  27317Wt. of water in the 1-lb. sample (g)               28   28   28     28Wt. of wash water (g)              9072 9072 9072   9072Vol. of air-detrainer in wash water (ml)              20.00                   20.00                        0.00   0.00              (TBP)                   (PPG)Dilution (approximate)              320  320  320    320__________________________________________________________________________       Elapsed       Time (sec)              Batch 1                   Batch 2                        (Avg)  (Avg)__________________________________________________________________________Turbidimeter Readings        0      4    4    4      4(NTU)*       60    296  313  388    324       120    209  229  277    223       150    199  217  251    211       180    190  204  235    198       210    186  193  222    193       225    179  194  218    193       240    185  192  216    191       255    186  189  218    191       270    184  188  221    192       285    182  186  219    194       300    183  183  217    192       315    177  185  218    193       330    181  180  219    181       345    182  180  211    181       360    180  178  211    177__________________________________________________________________________ *Nephelometric Turbidity Unit 
    
     
                                           TABLE 8__________________________________________________________________________Water/Cement Ratio = 0.44 + Surfactant + Air Detrainer__________________________________________________________________________                               Air-En-                                    Non Air-Trial No.          1    2           trained                                    Entrained Mix__________________________________________________________________________Water-cement ratio 0.44 0.44        0.44 0.44Wt. of cement (g)  4268 4268        4268 4268Wt. of water (g)   1878 1878        1878 1878Wt. of fine aggregate (g)              8537 8537        8537 8537Wt. of coarse aggregate (g)              12805                   12805       12805                                    12805Vol. of air-entrainer in concrete (ml)              3.00 3.00        3.00 0.00Total wt. of mix (g)              27488                   27488       27488                                    27488Wt. of water in the 1-lb. sample (g)               31   31          31   31Wt. of wash water (g)              9072 9072        9072 9072Vol. of air-detrainer in wash water (ml)              2.00 2.00        0.00 0.00              (PPG)                   (PPG)Dilution (approximate)              293  293         293  293__________________________________________________________________________        Elapsed         Aver-                            Std.        Time (sec)              Batch 1                   Batch 2                        age Dev.                               (Avg)                                    (Avg)__________________________________________________________________________Turbidimeter Readings         0     5    4    5  1   5    5(NTU)         60   366  325  346 29 319  345        120   267  257  262 7  219  250        150   239  244  242 4  205  232        180   232  233  233 1  196  221        210   216  226  221 7  190  211        225   216  222  219 4  189  210        240   218  219  219 1  189  211        255   217  218  218 1  187  202        270   204  214  209 7  187  203        285   207  217  212 7  189  204        300   209  213  211 3  186  209        315   208  212  210 3  186  209        330   203  209  206 4  187  207        345   206  208  207 1  187  200        360   298  208  203 7  192  202__________________________________________________________________________ 
    
     
                                           TABLE 9__________________________________________________________________________Water/Cement Ratio = 0.48 + Surfactant + Air Detrainer__________________________________________________________________________                             Air-En-                                  Non Air-Trial No.          1   2          trained                                  Entrained Mix__________________________________________________________________________Water-cement ratio 0.48                  0.48       0.48 0.48Wt. of cement (g)  4268                  4268       4268 4268Wt. of water (g)   2049                  2049       2049 2049Wt. of fine aggregate (g)              8537                  8537       8537 8537Wt. of coarse aggregate (g)              12805                  12805      12805                                  12805Vol. of air-entrainer in concrete (ml)              3.00                  3.00       3.00 0.00Total wt. of mix (g)              27659                  27659      27659                                  27659Wt. of water in the 1-lb. sample (g)               34  34         34   34Wt. of wash water (g)              9072                  9072       9072 9072Vol. of air-detrainer in wash water (ml)              2.00                  2.50       0.00 0.00              (PPG)                  (PPG)Dilution (approximate)              270 270        270  270__________________________________________________________________________        Elapsed              Batch                   Batch                      Aver-                          Std.                             11/25/93                                  12/2/93        Time (sec)              1   2   age Dev.                             (Avg)                                  (Avg)__________________________________________________________________________Turbidimeter Readings         0     5   4   5   1  5    4(NTU)*        60   408 300 354 76 245  301        120   312 246 279 47 185  255        150   295 243 269 37 185  253        180   285 243 264 30 181  240        210   276 232 254 31 181  236        225   264 240 252 17 183  234        240   258 236 247 16 181  234        255   252 228 240 17 183  230        270   248 232 240 11 182  231        285   238 231 235  5 185  232        300   231 231 231  0 182  233        315   236 228 232  6 186  232        330   224 225 225  1 191  221        345   222 227 225  4 196  221        360   227 226 227  1 205  220__________________________________________________________________________ *Nephelometric Turbidity Unit 
    
     FIG. 4 is a graph of the average turbidities versus elapsed time for the PPG tests of this example. This graph illustrates that the use of an air detraining agent such as PPG substantially eliminates the effect of an air entrainment agent in the concrete mixes, thereby permitting accurate water/cement determinations. This effect is further confirmed by the graph of FIG. 5, which illustrates average turbidities versus elapsed time for both the non air-entrained concrete and air entrained plus PPG concrete mixes. As can be seen, the data points for all of these mixes are sufficiently close to permit essentially accurate water/cement ratio determinations. Thus, use of PPG or a similar air detraining agent results in accurate water/cement ratio determinations even when the concrete being tested includes an air entrainment agent. 
     EXAMPLE 4 
     In another series of tests, turbidimeter readings were taken for water-cement pastes having initial water-cement ratios varying from 0.35-0.50, with dilution ratios of 1:10 (for each gram of water in the paste, an additional 10 grams of water was added), 1:100 and 1:1000. The turbidimeter used was a DRT-100B unit commercialized by H. F. Scientific, Inc. of Ft. Myers, Fla. (an instruction manual entitled &#34;DRT-100B &amp; DRT-100B RESEARCH TURBIDIMETERS&#34; provided by the manufacturer is incorporated by reference herein and explains the construction and operation of this preferred turbidimeter). In each case, the known water-cement dilution was prepared by mixing cement and water in a beaker, followed by stirring using an automatic dispersion mixer for 30 seconds. The dilution was then transferred to the turbidimeter cuvette as quickly as possible, and three readings were taken. All readings were taken within 60 seconds after the dilution was placed in the cuvette, as it was observed that any further delay resulted in a lower reading. 
     Table 10 sets forth the results of these tests and indicates that the 1:100 dilution ratio provided the most consistent turbidimeter readings in the range 0 to 100 NTU (Nephelometric Turbidity Unit). Based upon these results, 0-100 NTU and 1:100 dilution ratios were selected for further investigations. 
     
                       TABLE 10______________________________________Turbidimeter Readings of Cement for DifferentRanges of Turbidimeter for Different Dilution RatiosSample  w/c      Turbidimeter reading (range)No.     ratio    1      10       100  1000______________________________________Dilution 1:101       0.35     0.85   0.76     0.7  12       0.40     0.85   0.81     0.8  03       0.45     0.82   0.68     0.8  04       0.50     0.84   0.57     0.5  0Dilution 1:1001       0.35     1.20   2.24     2.1   22       0.40     1.73   5.21     5.3   53       0.45     1.00   9.45     10.2 104       0.50     1.00   11.11    15.4 14Dilution 1:10001       0.35     1.00   1.00     1.0  4852       0.40     1.00   1.00     1.0  5023       0.45     1.00   1.00     1.0  4444       0.50     1.00   1.00     1.0  42.5______________________________________ 
    
     Next, an experiment was designed to determine any statistically valid correlation which might be present between the water-cement ratio of the cement dilutions and turbidimeter readings. Ten water-cement ratios were chosen ranging from 0.36-0.54 at increments of 0.02. The pastes were first diluted to 1:100, mixed and turbidimeter readings were taken as set forth above. The turbidimeter readings from these ten samples showed a consistent trend. Table 11 sets forth the turbidimeter readings as well as a summary of the relevant statistics for the readings. Average turbidimeter readings varied from 5.5 for a water-cement ratio of 0.36 to 17.4 for a water-cement of 0.54. Low values of coefficient of variation for each measurement indicate the consistency of the test procedure. 
     
                       TABLE 11______________________________________Turbidimeter Readings ofWater-Cement Paste with 1:100 Dilution       Turbidimeter ReadingSam-        (range 100)       Summary Statisticsple  w/c    Read-   Reading                      Reading      Std. Coeff.No.  ratio  ing 1   2      3      Mean  Dev. of Var.______________________________________1    0.36   5.4     5.5    5.7    5.5   0.15 2.762    0.38   6.4     6.2    6.6    6.4   0.20 3.123    0.40   7.9     7.8    8.2    8.0   0.21 2.614    0.42   8.3     8.3    8.1    8.2   0.12 1.405    0.44   9.8     9.6    9.9    9.8   0.15 1.566    0.46   11.0    10.6   11.2   10.9  0.31 2.797    0.48   11.3    10.8   10.5   10.9  0.40 3.728    0.50   12.6    12.8   13.0   12.8  0.20 1.569    0.52   15.8    15.6   15.3   15.6  0.25 1.6210   0.54   17.2    17.3   17.6   17.4  0.21 1.20______________________________________ 
    
     A linear regression analysis was performed between the water-cement ratios (dependent variable) and corresponding turbidimeter readings (independent variable). The results yielded the following relationship between these variables: ##EQU1## 
     The high value of coefficient of determination, R 2 , indicates that there is a significant correlation between the water-cement ratio (with 1:100 dilution) and turbidimeter reading. This experiment was repeated to find whether the relationship described by the above equation was valid. Table 12 shows the results of the replicate experiment. The replicate equation is as below: 
     
         Water-cement Ratio=0.27+1.75×10.sup.-2 * Turbidimeter Reading (R.sup.2 =0.95, n=10)                                     (2) 
    
     
                       TABLE 12______________________________________Turbidimeter Readings ofWater-Cement Paste with 1:100 Dilution       Turbidimeter ReadingSam-        (range 100)       Summary Statisticsple  w/c    Read-   Reading                      Reading      Std. Coeff.No.  ratio  ing 1   2      3      Mean  Dev. of Var.______________________________________1    0.36   5.5     5.7    5.7    5.4   0.15 2.762    0.38   7.1     6.9    6.8    6.9   0.15 2.203    0.40   7.8     7.9    7.8    7.8   0.06 0.744    0.42   7.6     7.8    8.0    7.8   0.20 2.565    0.44   9.7     9.3    9.6    9.5   0.21 21.86    0.46   10.9    10.6   10.5   10.7  0.21 1.957    0.48   11.1    11.1   11.4   11.2  0.17 1.558    0.50   13.2    12.9   13.1   13.1  0.15 1.179    0.52   14.8    14.6   14.4   14.6  0.20 1.3710   0.54   16.4    15.9   15.4   15.9  0.50 3.14______________________________________ Standard error of estimate (SEE) = 9.51 × 10.sup.-3 Standard error of coefficient = 9.26 × 10.sup.-4 
    
     The new equation appeared to describe a better correlated relationship between turbidimeter reading and water-cement ratio. FIG. 6 shows the two equations graphically. The discrepancy in readings for the two trials increases for water-cement ratio higher than 0.42. 
     EXAMPLE 5 
     A turbidimeter test was performed on a concrete mix having a mix proportion 1:2:3 by weight (cement:fine aggregate:coarse aggregate) and a density of 639 lb/cu.yd., with water-cement ratios of 0.49 and 0.45. The cement, water, sand and coarse aggregate were first mixed and one pound of the mix was washed successively through #4, #10 and #100 sieves using 5 lb. of water. This was repeated ten times using the same water to create water-cement dilutions. These dilutions were mixed with an automatic mixer for 30 seconds and three turbidimeter readings were taken with each dilution. 
     Table 13 sets forth the turbidimeter readings for this experiment. The readings are consistently lower than those of Example 4, which is to be expected since the dilution is different, approximately 1:60. Any fine particles attached to the aggregates and the remaining cement particles not completely washed out may also have contributed to the differences in turbidity values. 
     
                       TABLE 13______________________________________       Turbidimeter ReadingSam-        (range 100)       Statistical Dataple  w/c    Read-   Reading                      Reading      Std. Coeff.No.  ratio  ing 1   2      3      Mean  Dev. of Var.______________________________________1    0.45   6.4     6.5    6.4    6.4   0.06 0.902    0.49   7.2     6.9    6.8    7.0   0.21 2.99______________________________________