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MATURITY TESTING OF CONCRETE
This paper deals with one of the non-destructive testing method known as maturity method, which is used for the determination of concrete strength. This method accounts for the effects of time and temperature on strength development of in-place structures. Some of the new techniques in this approach have also been discussed. Various benefits on using the maturity method in various civil engineering fields are also included in this paper.
INTRODUCTION
Testing of hardened concrete plays an important role in controlling and confirming the quality of cement concrete works. Systematic testing of raw materials, fresh concrete and hardened concrete are inseparable part of any quality control programme for concrete, which helps to achieve higher efficiency of the material used and greater assurance of performance of concrete with regard to both strength and durability. The test methods should be simple, direct and convenient to apply.
One of the purposes of testing hardened concrete is to confirm that concrete used at site has developed the required strength. The compressive strength of concrete is one of the most important and useful properties of concrete. In most structural applications, concrete is primarily employed to resist compressive stresses. In those cases where strength in tension or in shear is of primary importance, compressive strength is used as a measure of these properties.
Strength of concrete depends on various factors such as water-cement ratio, maximum size of aggregates, grading, surface texture and shape of aggregates etc. Lower water/cement ratio will yield high strength. The larger maximum size of aggregate gives lower surface area for development of gel bonds, which is responsible for lower strength of concrete. Quantum of increase in strength also depends upon the grade and type of cement, curing and environmental conditions.
3.0 METHODS USED TO DETERMINE CONCRETE STRENGTH
The compressive strength of concrete is generally determined using three testing methods: They include:-
(i) Cylinder or cube testing (compression testing)
(ii) Core testing
(iii) Non-destructive testing
(i) Compression test:
Compression test is the most common test conducted on hardened concrete. It is carried out on specimens cubical or cylindrical in shape. The cube specimen is of size (150x150x150) mm. If largest nominal size of aggregate does not exceed 20mm,then 100mm size cubes may also be used as an alternative. Cylindrical specimens are 150mm in diameter and 300 mm long.
The test specimens are made as soon as practicable after mixing in such a way as to produce full compaction of concrete with neither segregation nor excessive laitance. They are then stored in place free from vibration, in moist air and a temperature of 27O+ 2OC for 24 hours. After this period, specimens are marked and removed from moulds and are submerged in clean fresh water until taken out just prior to test. Then they are being tested in standard compression testing machine and noted down the strength at various ages.
Comparison between cube and cylinder strength.
It is difficult to say whether cube strength or cylinder strength is more accurate. However, cylinder is less affected by end restrains caused by steel platens on both of its ends and hence it seems to give more uniform results than cube. Moreover cylinders are cast and tested in same position unlike cubes, which are cast in one direction and tested from other direction. In actual structures in field, casting and loading is similar to that of cylinder and not like the cube. Point in favour of cubical specimens is that shape of cube resembles shape of structural members often met with on ground and also that cubes does not require capping, whereas cylinder requires capping.
(ii) Core test:
Most of the members cannot be tested; we test parallel concrete by making cubes or cylinders. But strength of these specimens cannot be same as that of member due to differences in degree of compaction, curing standard, uniformity of concrete, evaporation, loss of mixing water etc.
To arrive at a better picture of strength of actual member, attempts are made to cut cores from parent concrete and test these cores for strength. The major disadvantage in this method is that the diameter to height ratio may be other than that of standard cylinder which may affect the strength of concrete.
(iii) Non-destructive testing:
In this method of testing, specimens are not loaded to failure. They are used to obtain estimation of properties of concrete in the structure. The methods adopted include ultrasonic pulse velocity, rebound hammer, pull out and maturity. These tests provide alternative to core tests for estimating strength of concrete in a structure or can supplement data obtained from a limited number of cores.
4.0 NEED FOR MATURITY METHOD
Concrete is usually tested at the age of 7 days and 28 days. At the work site, concrete is poured daily over the previously laid concrete as in the case of columns, retaining walls etc. and no one has time to wait for 7 or 28 days to know the actual strength using conventional testing. Moreover it will rather too late for remedial measures if the result of these test specimens at 28 days is too low and the structure will be uneconomical if it is too high. It is of tremendous advantage if we could predict this strength within a short period of casting. Thus came the importance of maturity method.
Conventional testing do not represent the actual in place strength of concrete due to environmental conditions, geometry of the structure etc. Conventional testing is also expensive and more time consuming. Concrete gain strength with age due to continued hydration., But in conventional testing this increase in strength beyond 28 days is not accounted for. But in maturity method this gain in strength is also being included.Handling of test specimens in the lab may also affect the test result a lot. Improper preparation, handling and testing of these specimens may lead to low cylinder breaks that affect the strength determination. All these disadvantages in conventional testing paid way for the development of a new method for determining concrete strength.
5.0 DEVELOPMENT OF MATURITY METHOD
These methods have been in existence for over 60 years, but they are not widely used since older equipments used for it were cumbersome and difficult to employ. Interest in this concept significantly revived in the mid 1980s, as concrete industry and others began looking for ways to accelerate pavement construction and implement high-speed construction. FHWA (Federal Highway Administration) and MCL (Mobile Concrete Laboratory)has played a major role in sharing the success stories and working with state highway agencies to implement maturity method.
Over the past 5 years, use of this method has increased dramatically as state highway agencies and industries have become more aware of the technology and equipment has become more advanced. Maturity method is a proven strength determination technique (ASTM C 1074) for predicting the 28 days strength within a short period of casting. Thus maturity is a reliable and accurate means of estimating the strength of new concrete in place and in real time. This method was also recognized and approved by ACI, FHWA.
6.0 MATURITY TECHINIQUE
6.1 Definition
Strength is expressed as a function of summation of product of time and temperature. This summation is called maturity of concrete.
6.2 Principle
Chemical processes involved in concrete strength gain liberate heat. The amount of heat generated over time-maturity is unique to each concrete mix. By correlating time and temperature history of concrete mix to laboratory strength tests, a dependable strength estimate can be made in field by monitoring time and temperature of in-place concrete.
6.3 Mathematical Expression
Maturity method is based on the mathematical formula used for calculating “maturity index”. This index is associated with relationship between concrete mixture’s temperature changes, its curing rate and rate of strength development One of the most common equation used to find maturity index is the Nurse- Saul equation.
Nurse-Saul equation is stated as:
M (t) = (Ta - To) t
Where, M (t) = Maturity (TTF) at age t
t = time interval in days or hours
Ta = Average concrete temperature in oC during time interval and
To = datum temperature in oC the lowest temperature at which strength gain is observed.
Temperature is reckoned from an origin lying between -12 oC and -10 oC.
It was experimentally found that hydration of concrete continues to take place upto about
- 11o C. Therefore - 11o C is taken as a datum line for computing maturity. For more accurate results ASTM C 1074 gives a suggested value for To.
