BS 6089:1981 download free.Guide to Assessment of concrete strength in existing structures.
1 Scope
BS 6089 gives information on tests that are available to determine strength of concrete in a structure. Relative merits of these tests are indicated and methods of carrying out such tests are given.
BS 6089 also contains guidelines to assist the engineer in interpreting results of tests, and outlines possible ways of comparing test results with the required strength for design purposes. The information given in BS 6089 amplifies sections of CP 110-1:1972 concerned with tests to measure and assess the strength of concrete in structures, including:
6.8.2.3 Action to be taken in the event of non-compliance with the testing plan.
9.2 Check tests on structural concrete.
2 References
The titles of the standards publications referred to in BS 6089 are listed on the inside back cover. A bibliography of some appropriate references is given in appendix A.
3 Definitions
For the purposes of BS 6089 the following definitions apply.
3.1
standard cube strength
the measured compressive strength of a cube made, cured and tested in accordance with BS 1881-1, BS 1881-3 and BS 1881-4
3.2
cylinder strength
the compressive strength of a cylinder with a lengthldiameter ratio (X) of 2 made and cured in accordance with clause 5 of BS 1881-3:1970, and tested in accordance with 3.2 of BS 1881-4:1970
3.3
core strength
the compressive strength of a core, cut, prepared and tested in accordance with the requirements of BS 1881-4, for a stated length/diameter ratio
3.4
estimated in-situ cube strength
the strength of concrete at a location in a structural member estimated from indirect means and expressed in terms of specimens of cubic shape
NOTE A direct measure of the in-situ cube strength cannot be obtained because it is not poss)ble to produce a cast cubic specimen from that location. However, it is possible to obtain an estimated in.situ cube strength by using one or more of the methods described in clause 5.
3.5
location
a region of concrete that, for practical purposes, is assumed to be of uniform quality
3.6
characteristic cube strength
the value of the standard cube strength (which in CP 110 is measured at 28 days) below which 5 % of the population of all possible strength measurements are expected to fall
3.7
design strength
the strength of concrete as used in calculations so that the allowable stress as defined by the relevant code of practice or other design basis employed is not exceeded under the loading conditions appropriate to that code or other design basis
3.8
design load capability of a structural member
a level of loading that a structural member is designed to sustain with the appropriate partial safety factors against collapse, deflection or local damage
NOTE Direct measurement of capacity of a member to withstand such a load will not destroy the member under test unless this is inadequate for its envisaged purpose. (See 9.6 of CP 110.1:1972 for details of test loads and assessment of results.)
3.9
ultimate strength of a structural member
a measure of the maximum load that a member is capable of sustaining, the loading pattern being that applied in service
NOTE Direct nurnsurcment of the ultimate strength of a member results in destruction of that member, but in some cases it may be necessary to undertake such a test to assess the loadbearing capacity of similar members. (A suitable test method applicable to individual precast units is described in 9.5.3 of
CP 110-1:1972.)
4 Planning an investigation
4.1 Information required from tests. A knowledge of in-situ strength of concrete in a structural member may be required for one or more of the following reasons.
a) Doubt concerning the strength of concrete in the structure as a result of non-compliance of standard cube test results carried out in accordance with a specified compliance plan.
b) Doubt concerning workmanship involved in batching, mixing, placing, compacting or curing of concrete.
a) Test location (see also 3.5). Factors to be considered include:
1) position of suspect concrete in the member;
2) position of highly stressed sections:
3) variation of strength through depth of lift;
4) position of reinforcement identified by the
use of drawings or cover meter;
5) need to avoid detrimental effect on
reinforcement;
6) presence of local defects that may influence
test results.
b) Effect of damage. The choice between destructive and non-destructive methods be influenced by the effect of:
1) testing on the surface appearance of
member;
2) drilling of holes (e.g. in small columns or
retaining walls); 3) cutting of reinforcement.
c) Testing accuracy required. This will depend upon the nature of the investigation and, often, upon the magnitude of the measured strength: if the measured strength is considerably higher than that required, precision may not be necessary. The level of accuracy that can be achieved will depend upon:
1) test method;
2) number of measurements;
3) accuracy and reliability of available
correlations (e.g. between pulse velocity and
strength).
4.4.3 Accuracy of estimates of in-situ strength. Confidence with which it is possible to assess in-situ strength of concrete will increase with the number of assessments made. In the case of some tests (e.g. ultrasonic pulse velocity, surface hardness) little extra cost is incurred by obtaining a large number of test results. In other cases (e.g. core and gamma-ray testing) the cost of each test is appreciable. The decision on the number and type of tests to be made will, therefore, be based upon an assessment of the cost of obt.aining a result of adequate reliability.
Benefit may be obtained by combining different testing techniques, e.g. combining pulse velocity measurements with core tests. Pulse velocity measurements on cores prior to crushing can increase the accuracy of strength estimates from pulse velocity measurements. The ease of taking a large number of pulse velocity measurements on structural components can provide a more comprehensive evaluation of the strength of a structure.
5 Test methods
5.1 Core test
5.1.1 General. The most direct method of obtaining a value of the estimated cube strength is generally to drill cylindrical cores and test these in compression. Whenever possible, the cores should be drilled, prepared and tested in accordance with section 3 of BS 1881-4:1970, although this standard recommends alternative methods of treating the results. Detailed advice for core testing procedure is given in Technical Report No. ii published by the Concrete Society (5).
5.1.2 Selection of drilling points. Each drilling point should be selected so that the core contains no steel parallel to its length and as little as possible perpendicular to its axis.
5.1.3 Accuracy of test and number of cores. The number of cores will depend upon the amount of information required, the required accuracy of strength estimates and the cost of drilling. preparing and testing the cores.
The accuracy of strength estimates depends upon the reproducibility of the test method and the number of cores tested. The strength estimated from a single core can be considered to lie (with 95 % confidence) within ± 12 % of the strength of the concrete at that location. The accuracy of the estimate is increased if more cores are taken at the same location. For n cores, the mean core strength can be considered to be accurate within ± 12/’In % of the strength at that location.
The degree of uncertainty that can be tolerated in the estimated in-situ strength will often depend upon the measured value of the in-situ strength when compared with the value that may be considered acceptable. If in-situ strength, based on the mean core strength, is found to be near the limit of acceptance. it may be necessary to drill further cores.
5.1.4 Size of cores. Before capping, a core should have a length at least 95 (y0 of its diameter. When prepared for test, it should preferably have a length at least equal to its diameter and not exceeding 1.2 times its diameter. Cores of both 100 mm and 150 mm nominal diameter may be tested provided the nominal maximum aggregate size does not exceed 20 mm and 40 mm respectively. Whenever possible, however, 150 mm diameter cores should be drilled as less variability due to drilling and more reliable results are obtained, with the following exceptions:
a) when reinforcement is congested, 100 mm diameter cores are less likely to contain pieces of steel;