Early age strength assesement for high rise buildings Marijan SKAZLIĆ 1, Claus Germann PETERSEN 2, Zoran KRNJAK 3 1 Faculty of Civil Engineering, University of Zagreb, Zagreb, Croatia, marijan.skazlic@grad.hr 2 Germann Instruments A/S, Copenhagen, Denmark 3 Segrad, Djurdjevac, Croatia Abstract This paper presents a case study made on two high-rise buildings in Zagreb, the capital of Croatia. Two different non-destructive test (NDT) methods (pull-out testing with a Lok-test, and Schmidt hammer) were used for early age strength assessment. Parallel with NDT concrete strength tests cube testing was also done. An analysis of the results obtained shows a correlation between pull-out strength measured by Lok-test and concrete cubes strength. The said correlation is compared with a previous laboratory correlation. According to the results obtained, a comparison of different NDT methods for early age strength assessment was made. It is recommended that a pull-out Loktest should be used for early striking of formwork and thus for faster construction. Résumé Cet article présente une étude des cas concrets qui concernent deux constructions hautes à Zagreb, la capitale de la Croatie. Deux différentes méthodes d essais non destructifs (NDT) (essais d arrachement avec un Lok-test et le marteau Schmidt) ont été utilisées pour l évaluation de la résistance du béton aux jeunes âges. Des essais sur cubes, pour la détermination de la résistance du béton, ont été faits en parallèle avec les essais non destructifs. L analyse des résultats obtenus a montré une corrélation entre la résistance d arrachement mesurée par le Lok-test et la résistance des cubes de béton. Cette corrélation est comparée avec une corrélation précédente obtenue en laboratoire. D après les résultats obtenus, une comparaison des méthodes différentes non destructives pour l évaluation de résistance à ses débuts a été faite. On recommande l utilisation du Lok-test d arrachement pour le décoffrage tôt et ainsi la construction plus rapide. Keywords Concrete strength, non-destructive test methods, pull-out testing, Schmidt hammer 1 Introduction Compressive strength is a basic concrete property. Early age concrete compressive strength is determined mainly to achieve greater process efficiency and overall speed of construction. It is required that the concrete should have certain compressive strength necessary for striking of props and for pre-stressing ; additionally, compressive strength is used to define the quality of the protective layer and assess durability properties. When concrete is placed in extreme winter temperatures, knowledge of an actual increase in compressive strength is of crucial importance. Early age concrete strength in a structure can be determined by testing control concrete specimens, i.e. cubes or cylinders, or by using non-destructive (NDT) test methods. Testing of compressive strength on control specimens is performed using the same technique as is
applied for the proof determination of the class of the compressive strength at 28 days. The main disadvantage of this test method is the fact that compressive strength is tested on the ideal shape of a specimen and consequently does not indicate actual compressive strength of the concrete in the structure. Additionally, a large number of test specimens, apart from those necessary for the determination of the compressive strength class, put the contractor to an extra expense. [1, 2] Several different non-destructive test methods are available for testing early age concrete compressive strength in a structure; those most commonly used are Schmidt hammer and pull-out testing. Compressive strength testing with the Schmidt hammer represents a very fast and cheap testing method, but this method is not reliable enough in all cases. For this reason, pull-out test methods are generally used recently, and the Lok-test is the most often employed one. In such a test, the force required to extract an expanded metal disc (Fig.1) embedded in fresh concrete is measured. An insert is extracted by the load applied onto the stem on which a disk is placed. During the insert extraction, a conic portion of concrete is extracted together with the disk. The extraction force in the Lok-test has a producer correlation with the compressive strength measured on conventional concrete specimens. [1-4] This paper presents the test results of early compressive strength employing different test methods, that is to say parallel determinations of compressive strength using conventional specimens, the Schmidt hammer and the Lok-test method. Figure 1. Testing with pull-out test method the Lok-test (left) and the appearance of the concrete specimen and insert after testing (right) 2 Test results and analysis The experimental work was carried out on two high-rise buildings in Zagreb (Fig.2). Both high-rise buildings have eight storeys above ground. The testing performed is aimed at analysing reliability of various non-destructive test methods (Lok-test and Schmidt hammer) for assessment of early age compressive strength. [5] Early compressive strength was tested at different ages up to 28 days. All tests were carried out in the summer period, i.e. from April to July. Parallel with the tests mentioned above, compressive strength testing was performed with the Lok-test, Schmidt hammer and 150 mm cubes cast during concrete placement (Fig.3). Tests were carried out only on floor slabs of the structures mentioned above. A total number
of 70 parallel compressive strength tests were done on cubes, and with the Lok-test and with Schmidt hammer. Figure 2. The experimental work was carried out on two high-rise buildings in Zagorska Street (left) and Radnička Street (right) in Zagreb, Croatia Figure 3. Testing of concrete compressive strength in the floor slab with the Lok-test, the Schmidt hammer, and on 150 mm cubes To perform testing with the Lok-test, plastic inserts were installed in each about 15 m 3 of the concrete slab (Fig.4 left). During installation, care was taken to avoid the reinforcement and the formation of possible bubbles beneath the insert, which could affect the test results. On the plastic inserts a small quantity of concrete was placed to avoid the action of buoyancy forces that could eject them. The inserts were inclined at 10 o to 15 to the vertical (Fig.4 right). General correlation for the Lok-test defined by the manufacturer of the Lok-test system Germann Instruments is used (Eq.1) [3] f C = 0.76 F 1.16 (1) where is f c (N/mm 2 ) concrete cube compressive strength, and F (kn) Lok-test pull-out force. Estimated compressive strength based on the Lok-test general correlation is within ± 6 % of the strength measured from standard specimen tests for a maximum aggregate size of 38 mm. The coefficient of variation of individual Lok test results is about 8 % for normal density concrete. [3]
Figure 4. The appearance of the floor slab just after concrete placement (left) and that of a plastic insert placed in fresh concrete for the Lok-test (right) Schmidt hammer testing is carried out on clean, plane and dry concrete surfaces. Estimated in-situ compressive strength is an average value of 12 test results. Concrete compressive strength obtained by Schmidt hammer was computed from a Schmidt hammer rebound index based on an expression obtained from previous tests. Considering that the test results obtained by Schmidt hammer are most affected by the type of aggregate incorporated in the concrete mix, previous tests were carried out on concrete containing the aggregate from the same part of the country. 35 30 Compressive strength (MPa) 25 20 15 10 5 0 1-2 days 3-7 days 8-28 days Age (days) Schmidt hammer Lok test Concrete cubes Figure 5. Testing results of parallel determination of compressive strength using concrete cubes, Lok-test and Schmidt hammer on the floor slabs in the high-rise buildings
The results illustrated in Fig. 5 show the comparison between different methods for testing concrete compressive strength. It is well known that compressive strength in concrete structure is mostly because of curing condition approximately 15 % lower than concrete cube compressive strength. Results present in Fig. 5 show that Lok-test strength results are 21 %, 19 % and 18 % lower than concrete cubes compressive strength results. Testing results from Fig. 5 show that Schmidt hammer strengths are 69 %, 9 % and 5 % higher than concrete cubes compressive stength at the concrete age of 1-2 days, 3-7 days and 8-28 days. Getting results show that Lok-test pull-out method is much more accurate for determining concrete compressive strength than Schmidt hammer method. Beside that, Lok-test pull-out method is a good indicator of actual compressive strength of the concrete in structure. Experience from tests show that general correlation for the Lok-test (Eq. 1) defined by the manufacturer could be used for testing of concrete strength in structure. The Lok-test proved to be a very time saving testing method for estimating the concrete compressive strength gain in the structure. Knowledge of in-situ concrete compressive strength at an early age may in high-rise buildings affect the construction procedures by early striking of formwork and props. In this manner the contractor can directly contribute to construction process efficiency and potential savings in the work schedule. In the Lok-test, it is recommended that the number of inserts and location of tests should be well-planned whereby the type and importance of the structure should be taken into account since this affects the reliability of testing. 3 Conclusions In this paper the results are presented of testing early compressive strength carried out on the floor slabs in two high-rise buildings. Parallel testing with the Lok-test, Schmidt hammer and 150 mm cubes in the laboratory was performed. On the basis of the test results obtained, the conclusions can be made as follows: - Prior to starting in-situ testing with a Lok-test, correlation curves obtained from the manufacturer should be verified. - The reliability of testing can be influenced by planning the number and location of the tests carried out using the Lok-test method depending on the type and importance of the structure. - The results of concrete compressive strength estimation with the Lok-test are more reliable than those obtained with the Schmidt hammer. - Lok-test pull-out method is a good indicator of actual compressive strength of the concrete in structure. - In building construction, early compressive strength can be estimated by using the nondestructive test methods. In this way, early striking of props and formwork can contribute to faster and more economical construction. Acknowledgements The authors would like to express their thanks to the Ministry of Science, Education and Sports of the Republic of Croatia for their support. The experimental research described in this paper has been carried out as part of a research project (Modern methods of engineering materials testing, 082-0822161-2996, Project Leader Marijan Skazlic, PhD, Assistant Professor) supported by the above Ministry.
References 1. Bungey, J.H., Millard, S.G. (1996) "Testing of concrete in structures", Blackie Academic & Professional, Glasgow 2. Malhotra, V.M., Carino N.J. (2004) "Handbook on Nondestructive testing of concrete", CRC Press, Boca Raton, London, New York,Washington 3. Petersen, C.G., "'LOK-TEST and CAPO-TEST pullout testing, twenty years experience", http:// www.germann.org 4. Bungey, J.H., Long, A.E., Soutsos, M.N., Henderson, G.D. (2000) "Early age acceptance of concrete", BRE Report 387, CRC Ltd. 5. Krnjak, Z. (2007) "NDT testing of concrete strength" (in croatian), Diploma Thesis, Faculty of Civil Engineering, University of Zagreb, Zagreb, Croatia