199-CUA : Concrete: use of additions
Technical Committee 199-CUA
Deputy Chair: Mr. Joseph PERXAS SERAS
The use of selected by-products is an increasing world-wide need specially by economical and ecological reasons. Some of them may be used as additions to concrete. Technical rules to use these additions as cementitious constituent of the concrete are detailed in several standards. Nevertheless in such standards the equivalence addition - cement is not very developed , and still less rules regarding durability of concrete with additions or production control of concrete with additions at which the use of additions is hard in practice . Equivalence between concretes with and without additions may be established by means of tests that prove equivalent performance. Some committees are working in this way. It is not the aim of this proposal. Equivalence between the cementitious constituents of the said concretes may also be established through the addition efficiency concept. The current system to evaluate the additions efficiency is the k value concept. In general this concept is only applied through compressive strength tests. The k value may be applied to all types of additions such as fly ash, silica fume, ground granulated blast furnace slags etc. The mix with additions shall be of the same compressive strength than the mix without additions. The k value at a given age depends on the properties of the addition and on the type and strength class of the cement used. The source of both materials influences the k value experimental result. To determine the value of k similar type of aggregates is to be used in mixes with or without additions. The k value depends on the water-cement ratio (so-called reference w/c) taken into account in its determination. The addition-cement ratio may also be considered in some cases. If the k value is determined in concrete, similar cementitious content in concretes with and without additions should be used. In any case a relationship of the type fc = f (w/c) is needed to evaluate the k-value being fc the concrete compressive strength, c the cement content and w the water content. The influence on the k value of the selected f (w/c) should be investigated. The k value may be determined through tests on mortar. As the k value is dramatically sensitive to the accuracy of the strength tests, testing mortar (with low variability) has some advantages instead of testing concrete. The variance of the results used to determine the k value should include only the variance attributed to the addition not that of the cement and aggregates used in the tests (in this case the k value results should be adjusted) . This type of adjustments should be investigated because influence the final characteristic value of k to be used. Only concrete with CEM I cement or combinations of CEM I and additions will be considered in this proposal . The addition content (a) of the combination is r.c (being c the cement content and r = a/c the addition - cement ratio) . Certain types of equivalence between cement and the said combinations are established in some national standards. Similar approaches are included in this proposal but through the experimental determination of the k value .The same durability is assumed if the concrete is produced with cement or with the equivalent combination if the durability requirements (specially maximum w/c and minimum cement content) are the same . Two types of equivalence CEM I - (CEM I + additions) may be established . Equivalence type A : the experimental determination of k-value permits additions to be taken into account in the calculation of the water-cement ratio and the equivalent cement content . These parameters may be compared with the main requirements that controls the concrete durability i.e. maximum water-cement ratio and minimum cement content. The cement content in the water-cement ratio calculation may be replaced by (c+k.a) because a relationship between w/c and fc is generally assumed , i.e. : fc= f (w/c) . Nevertheless the same replacement is not possible regarding the minimum cement content because there are not accepted relationships of the type fc = f (Y) , being Y the concrete property that controls its durability (e.g. frost resistance ) In addition more than one property should be taken into account for each exposure class . For the minimum cement content that part of the cement content that is possible to be replaced by addition (k.a) depends of the exposure class (and in some cases of the type of addition). It may be possible to replace the cement content by (c+q.k.a), being q<1 a constant dependent on the exposure class (values of q are to be established) . Some restrictions on the value of r according the exposure class (and in some cases of the type of addition) may be established . In fact the CEM I content is considered to be equal of (CEM I + k.a) regarding the maximum water-cement ratio, and to be equal of (CEM I + q. k.a) regarding the minimum cement content . Values of r and q for this type of equivalence should be investigated . Equivalence type B: (CEM I + addition) combination may be equivalent to some CEM cement (of the type II , III) of the certain strength class . The system involves the experimental determination of k and is to be based on the assumption that the w/(c+k.a) = 0,5 mortar with addition has an upper and lower compressive strength that fulfils the upper and lower compressive strength limits of the strength class of the cement different of type I . The consumers risk shall be the same in both cases . These cements will have the same constituents than the combination because the addition shall fulfil also the specifications of this material as cement constituent . The sameness of other properties (e.g. the setting time) of both materials may be also checked if any , to strengthen the equivalence . The margin of values of r of the combination (that may be derived from the k value) shall be within that of the equivalent cement . In this way the former constant q amount 1 and (c+a) may be used as cement content in the verification of the main durability requirements of the concrete produced with these equivalent cements because k=1 if the addition becomes a cement constituent . In fact CEM I content is considered to be equal of CEM different of I ( type II , III ) content . Equivalent CEM cement types to (CEM I + additions) combinations should be investigated. Regarding the production of (CEM I + addition) concrete , specific control rules should be established . Concerning initial production enough number of values of k are to be obtained to determine its characteristic value . Adjustments related with the solely addition variability (that influence the said characteristic value ) are to be determined . The actual compressive strength of each concrete type is evaluated with regard to fck through two criteria . The first criterion is based on the average of n individual results (Xia) and the estimate of the standard deviation (d) i.e. Xia equal or greater than (fck + t .d) . Value of t is usually comprised within 1,4 and 1,6 and that of (t . d) is the so-called strength margin . The second criterion is based on the individual test results (Xi) that are evaluated against fck minus a constant (e.g. 4 N/mm2) i.e. Xi equal or greater than (fck - 4) . The figure has been established bearing in mind populations derived from strength test results of concrete without additions . As the actual compressive strength is important in any exposure class any refinement in its evaluation is interesting . On that score the second criteria should also be related to d . It is possible in certain conditions to estimate a value b = f(d) that allows to establish the inequality (b . Xi) equal or greater than fck . Being A = 1/b , a new coefficient may be used as an alternative to second criterion . These assumptions allow to investigate the alternative criterion Xi equal or greater than A . fck . In summary the objective of the TC-CUA is to develop RILEM recommendations allowing to refine the experimental determination of the k value based on compressive strength tests and to use these results in the verification of the main concrete durability requirements (maximum water-cement ratio and minimum cement content) and in the production control of concrete, in particular of the concrete with additions.
Terms of reference
The estimated time needed to achieve the TC-CUA objective is 4 years. The following time schedule is proposed : - First year : inventory and evaluation of existing rules regarding the experimental determination of k value . To agree new proposals. - Second year : inventory and evaluation of technical rules to establish the equivalence addition - cement . To agree new proposals. - Third year : inventory and evaluation of existing rules regarding the use of k value in the production control of concrete . To agree new proposals. - Fourth year: to agree TC Report "Rules to take into account the additions efficiency". RILEM recommendations.
Detailed working programme
For the TC objective the following steps are envisaged: - Task 1 : survey of established rules - Task 2 : evaluation of the said rules - Task 3 : proposals to improve the said rules - Task 4 : to agree RILEM recommendations for the rules taken into account and if any , round robin tests related with these rules . - Task 5 : to write the TC Report taken into account relevant contributions to any workshop promoted by the TC . TC-CUA will work by correspondence and meetings. The necessity of any workshop or round robin tests will be evaluated between tasks 2 and 3.
TC-CUA is related to several committees, such as: */ CEN / TC 104 / WG 4 concerning fly ash standardisation */ CEN / TC 104 / WG 9 concerning silica fume standardisation */ CEN / TC 104 / SC 1 concerning revision of EN 206-1 */ Any other international committee developing some of the items described in this proposal. Liaison will be established with these committees.
The TC is expected to produce a Report about RILEM recommendations concerning the use of the additions and if any, round robin tests results and proceedings of workshops promoted by TC-CUA.
Group of users
RILEM recommendations agreed by the TC will be of interest of concrete and additions specifies, producers and users.
Specific use of the results
RILEM recommendations agreed by the TC may be used in developing provisions and experimental methods to apply the additions efficiency concept (k value) in the verification of the main durability requirements (maximum water-cement ratio and minimum cement content) of concrete with additions, and in the production control of the concrete in particular of the concrete with additions .