variations.txt

al aggregate.   Tests to date have shown the material capable of producing a concrete  with compressive strengths as high as 50 MPa (7000 psi) at 28 days. There will,  obviously, be problems with variations in the composition of the raw ash, and the long-term  durability characteristics of the material have yet to be determined, although results  
istributions or grading curves. The fineness modulus  cannot, therefore, be used as a description of the grading of an aggregate, but it is valuable  for measuring slight variations in the aggregate from the same source, i.e. as a day-to-day  check. Nevertheless, within certain limitations, the fineness modulus gives an indication of  the probable
quivalent  standard cube strengths from core strengths is no longer considered to be valid, because of large  variations in the relationship between the two strengths arising from variations in site conditions  including percentage of reinforcement, dimensions of members and the methods of compaction and  curing. The effect on the measured strengt
h of the variations in the height - diameter ratio of drilled  cores is taken into account by using the correction factors given in table 14.3. The effect of the shape  of the test specimen is taken into account by multiplying the cylinder (core) strength, for a cylinder  (core) having a height - diameter ratio of 2, by 1.25 to obtain the equivale
h-strength mature concretes cured in water are most sensitive to loading rate and  particularly so for loading rates greater than 600 N mm-2 min-1. BS 1881: Part 116 requires concrete  in compression tests to be loaded at 12 to 24 N mm-2 min-1 and within this small range of loading  rates variations in the measured strength of concrete will be ins
ught about by the disruptive action of alternate  freezing and thawing of free water within the concrete and expansion and contraction of the concrete,  under restraint, resulting from variations in temperature and alternate wetting and drying.   Damage to concrete from freezing and thawing arises from the expansion of  porewater during freezing; 
 concrete, variations in temperature and alternate wetting and drying.  When a change in volume is resisted by internal or external forces this can produce cracking. The  greater the imposed restraint, the more severe the cracking. The presence of cracks in concrete  reduces its resistance to the action of leaching, corrosion of reinforcement, att
e, mix proportions and curing  conditions. However, for a given concrete the variation in dynamic modulus of elasticity can give a  good indication of variations in strength and static modulus of elasticity, and this test is particularly  useful for assessing the progressive change in strength and durability as affected by various factors  such as
In a properly  proportioned and compacted concrete the voids are usually less than 2 per cent. The  properties of concrete in its fresh and hardened state can show large variations depending  on the type, quality and proportions of the constituents, and from the discussion to follow  students should endeavour to appreciate the significance of thos
 for this test because of the unacceptably large variations of strength thus  obtained. Standard aggregates are used for making prescribed mortar or concrete test mixes to  eliminate aggregate effects from the measured strength of the cement.    TABLE 12.7 British Standard requirements for strength of the principal Portland cements    Soundness   
me owing to the large variations  that can occur in their moisture content. Their bulk density normally ranges from 350 to 850 kg m-3  for coarse aggregates and from 750 to 1100 kg m-3 for fine aggregates. The methods for testing  lightweight aggregates are described in BS 3681.      12.3 Water    Water used in concrete, in addition to reacting wi
dinary drinking water.  A tolerance of about 10 per cent is usually  permitted to allow for the chance  variations in strength; an appendix to BS 3148:1980 also suggests 10 per cent. Such  tests are recommended when water for which no service record is available contains  dissolved solids in  excess of 2000 ppm or, in the case of alkali carbonate 
ut is very useful in detecting variations in the  uniformity of a mix of given nominal proportions.   There are some slight differences in the details of procedure used in different countries,  but these are not significant. The prescriptions of ASTM C 143-78 are summarized  below.   The mould for the slump test is a frustum of a cone, 305 mm (12 
: true, shear, and collapse     Mixes of stiff consistence have a zero slump, so that in the rather dry range no  variation can be detected between mixes of different workability. Rich mixes behave  satisfactorily, their slump being sensitive to variations in workability. However, in a  lean mix with a tendency to harshness a true slump can easily
somewhat higher value of the compacting factor than the small apparatus.  Unfortunately, the compacting factor apparatus is not often used outside precast  concrete works and large sites.   Table 4.2 lists values of the compacting factor for different workabilities,  as given in Road Note No. 4. Unlike the slump test, variations in the workability
ed to  measure variations in the mix, such as those due to a variation in the moisture content  of the aggregate.    Fig. 4.8. Relation between Kelly ball penetration and slump    Nasser's K-probe    Among the various recent attempts to devise a simple workability test, the probe test of  Nasser deserves mention. Here, a probe is inserted vertical
tively simple, methods have been developed.  These methods, by and large, give a helpful indication of  "workability" and are certainly  adequately sensitive to variations in the water content of  the mix.    Comparison of Tests    It should be said at the outset that no comparison is really possible as each test  measures the behaviour of concret
variations in the sample  and 15 kg / m3 (25 lb / yd3) to variations in the machine. Thus, for a mix with the cement  content of 370 kg / m3 (620 lb / yd3), the 95 per cent confidence limits would be +/-43 kg  / m3 (+/-72 lb / yd3). The use of duplicate sub-samples would reduce the variability but,  according to Cooper and Barber, the accuracy of 
tor, Vebe time and flow results for different concretes. In the  following sections the salient features of these tests together with their merits and  limitations are discussed.     TABLE 13.1 Permissible variations in concrete workability    Figure 13.1 Apparatus for workability measurement: (a) slump cone, (b) compacting  factor and (c) Vebe co
 zero or near-zero slump, moderate variations in workability do not result in measurable  changes in slump. For wet mixes, complete collapse of the concrete produces unreliable  values of slump.   The three types of slump usually observed are true slump, shear slump and  collapse slump, as illustrated in figure 13.2. A true slump is observed with 
cohesive  and rich mixes for which the slump is generally sensitive to variations in workability. A  collapse slump is usually associated with very wet mixes and is generally indicative of  poor quality concrete and most frequently results from segregation of its constituent  materials. Shear slump occurs more often in leaner mixes than in rich on
and therefore a reasonable correlation between the test  results and site assessment of workability can be expected.   The test is suitable for a wide range of mixes and, unlike the slump and compacting factor tests, it is  sensitive to variations in workability of very dry and also air-entrained concretes. It is also more  sensitive to variation 
the constituent materials. Workability is relatively  insensitive to changes in only the cement content and, for practical purposes, may be considered  dependent on only the water content for variations in cement content of up to 10 to 20 per cent  depending on the cement content, the effect of cement content being greater for the richer mixes.  F
splaced and rises to the surface, and some may also leak through  the joints of the formwork. Separation of water from a mix in this manner is known as  bleeding. While some of the water reaches the top surface some may become trapped  under the larger particles and under the reinforcing bars. The resulting  variations in the effective water conte