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Assessing Concrete Strength by Insitu Tests Ferreira, Almir P. MSc, Civil Eng., E mail: almir_ferreira@zipmail.com.br, Castro, Protasio F.PhD, Full Prof. Construction Technology, E mail: pcastro@civil.uff.br Universidade Federal Fluminense, R. Passo da Pátria, 156, Niterói, RJ, CEP 24 210 - 240, Brasil. Contact

ABSTRACT

The assessing of concrete strength in existing structures by NDT is of considerable interest to engineers. Several national and international standards recognize the various NDT methods as suitable. Basically, most test methods measure some others properties of concrete than its mechanical strength. However, the evaluation of insitu concrete strength is carried out by means of an established correlation of these properties with strength. Therefore, the reliability of assessing the concrete strength primarily depends upon the precision of the established calibration.

The relationship between the NDT result, mechanical or physical, and the strength is generally not unique. This arises from the diversity in the existing relations between the measured characteristic and the factors making up concrete strength, among others: cement contents, type of aggregates, bond between cement: paste and aggregates and water cement ratio.

In order to improve the significance of the NDT x Concrete Strength correlation the coarse aggregate strength may be introduced as an independent variable. Another way of improving the concrete strength estimation consists in combining two different non destructive tests.

The aim of the present paper is to check the validity of pulloff, pin penetration and ultrasonic pulse velocity for assessing concrete strength, that are widely used in structural field. The NDT are compared on basis of sensitivity of measurements and scatter of results. The advantage deriving from the aggregate strength used as a variable is also examined. An extensive series of tests was carried out to obtain a correlation of the NDT results with compressive strength. Nine concrete mixes for each three different coarse aggregate strength, of 19 mm max. size, were tested.
Keywords: Concrete; NDT; Pulloff Test; Penetration Test; UPV

INTRODUCTION

The reliability on the standard compressive test came as result of experience gained in extensive development work conducted throughout the world. However, concrete placed in a structures has different compaction and curing conditions to concrete tested using the standard compressive test.

Nondestructive tests (NDT) provide indirect data that can be empirically related to compressive strength by calibration with strength measurements from a number of cast specimens. NDT measurement technique has been used for more than two decades for concrete quality evaluation and assessing concrete compressive strength (f_c). During this period, the factors influencing the test results have been widely reported.

According to Castro[1] age, mix proportion, water/cement ratio, cement type, aggregate type have an influence on the pulses velocity test results and its relationship to f_c. Using different concrete mixes, but with the same materials Castro[2] showed a comparison between NDT through their relationship with compressive strength of cast cylinder specimens and f_c by extracted cores. Structural inspection to repair or reinforce structures is probably the main application for combine NDT. Measurement of NDT at points on a regular grid on the surface of concrete structures provides a reliable method for assessing the homogeneity of the concrete as shown by Ludwig and Castro[3].

This paper shows the results of an experimental program on NDT for assessing f_c. The variable assessed were:

1. concrete strength at different mix proportion with a range of 15 to 40 MPa to duplicate strengths found in practice; and
2. coarse aggregate of different crushing strength.
The effect of these parameters on the relation between compressive strength by cast specimens and the nondestructive tests results was measured.

PURPOSE OF THE INVESTIGATION AND EXPERIMENTAL PROGRAM

In most of the development of NDT methods for concrete attempts have been made to correlate the measurement with f_c measured on standardized test specimens under standard conditions of mechanical loading. This approach has often been linked to an assumption that compressive strength test results are of primary importance and represent a basic standard by which nondestructive tests may be judged. One results of this has been that limited confidence has emerged among engineers in the NDT results, since they are always correlated with standard compressive strength test results and the regression curves are disturbed by the influence of a large number of factors which are encountered in practice.

Portland cement (CP-II F40), complying with the ABNT - NBR 11578 / 91[4] , and natural sand were used in all mixes. Crushed coarse aggregate of three different origins were used in the research work. They were named after their crushed strength (kN) 95, 120 and 180. Nine series of mixes was carried out for each coarse aggregate origin. Therefore the mixes set used have the same:

1. Portland cement;
2. sand / aggregate ratio;
3. coarse aggregate maximum size of 19 mm;
4. and admixture proportion.
The mixes used for the test series are shown in Table 1.

MixID	Mix Proportion	W/C Ratio	CACS* (kN)	fc** (MPa)	Prismatic Specimens (230x230x340 mm)
					Pulloff (N.m)	UPV (km/s)	Pin*** (mm)
1/95	1 : 2.53: 3.53	0.60	95	18.7	14.7	3.41	14.8
2/95	1 : 2.42: 3.64	0.60		26.3	16.3	3.71	19.0
6/95	1 : 1.58: 2.67	0.42		28.7	18.2	4.00	25.7
7/95	1 : 1.41: 2.50	0.42		32.6	23.3	4.12	26.6
8/95	1 : 1.24: 2.35	0.39		38.2	22.3	4.09	29.4
9/95	1 : 1.17: 2.21	0.37		31.8	23.2	4.07	27.9
2/120	1 : 2.42: 3.64	0.60	120	26.8	16.3	4.14	23.0
3/120	1 : 2.20: 3.14	0.54		24.1	11.7	4.12	25.0
4/120	1 : 1.95: 2.97	0.49		29.3	17.0	4.15	27.2
5/120	1 : 1.75: 2.75	0.45		26.3	17.8	4.15	24.6
7/120	1 : 1.41: 2.50	0.42		40.2	24.5	4.22	23.1
8/120	1 : 1.24: 2.35	0.39		40.6	22.3	4.25	25.7
9/120	1 : 1.17: 2.21	0.37		28.2	18.2	4.07	25.1
1/180	1 : 2.53: 3.53	0.60	180	29.5	20.7	4.22	23.7
2/180	1 : 2.42: 3.64	0.60		35.3	14.5	4.29	33.9
3/180	1 : 2.20: 3.14	0.54		24.7	14.8	4.19	28.2
4/180	1 : 1.95: 2.97	0.49		32.8	18.3	4.28	30.8
5/180	1 : 1.75 : 2.75	0.45		34.5	14.5	4.30	28.7
6/180	1 : 1.58: 2.67	0.42		34.8	22.0	4.36	24.5
7/180	1 : 1.41 : 2.50	0.42		41.9	19.7	4.30	35.5
8/180	1 : 1.24 : 2.35	0.39		26.3	19.7	4.39	26.7
Table 2: MIX CHARACTERISTICS AND TEST RESULTS

• Notes:
  (*)Coarse Aggregate Crushed Strength;
  (**)Compressive Strength from Cylindrical Specimens (150x300mm) at testing date;
  (***)Exposed Length.
• Outlier mixes due to test results: 3/95, 4/95, 5/95, 1/120, 6/120 and 9/180.

TEST RESULTS AND ANALYSIS

All test results were adopted as the mean of three individual test results, apart from compressive strength by cylinders which were mean of two. Also, all test results are 56 days of age, hence test date. NDT were carried out on prismatic specimens.

The development of the pulloff test device is described in Miranda[5]. The pulloff test is a partial NDT. The steel plate of 75 mm diameter is fixed to the concrete surface by epoxy. Applying an axial pulloff force to the bolt by means of a torque wrench and bearing system results in an vertical force on the plate, which eventually leads to the fracture of the concrete. The penetration test was introduced in Brazil by Pontes Vieira[6]. Thus, pin penetration tests were carried out according to the Brazilian tradition: smooth 55 mm long pin of 6.4 mm diameter and a short cartridge 22, medium powder potential. Test results were measured by using a caliber and referred to millimeter. UPV (54 kHz) measurements by direct transmission arrangement were used in the present research program.

The tests results of the present investigation were previously analyzed to figure out which were the outlier results for each test. Whenever a mix proportion has more than two outlier results from different non destructive tests, all NDT results for that mixes were excluded. Therefore, the analysis and conclusions are based on tests results, shown in Table 1.

Statistical tests on NDT results were carried out. The first test was to verify whether the NDT results and compressive test result fitted to a Gauss Distribution. The Henry straight line test shows all test fitted to Gauss Distribution. Just as a frequency distribution can be described diagrammatically by a frequency polygon, so can a cumulative frequency distribution be represented by a curve called an ogive. The ogive curves for the experimental test results are shown in Figure 1.

Fig 1: Ogive for the test results of the experimental program

The pulloff test results produced an ogive close to the concrete compressive test results. Actually the mechanical measurement of both tests results can be assumed as the main support for that close behavior. UPV and pin penetration test results produced an ogive significantly different from the compressive strength test results. This statement is supported by hypothesis test carried out to ogive area at 5% risk. The effect of the coarse aggregate crushed strength on the pin test result is assumed as a parameter for that behavior. The crush strength of the coarse aggregate is related to its density. Therefore, the effect of the coarse aggregate origin is assume to explain the UPV ogive behavior.

Regression analysis is a useful tool to evaluate the relationship between NDT and concrete tests. Statistical parameter are first used to evaluate the fit of the chosen regression curve. However, when comparing models obtained for different test types, the regression curves parameters should be carefully used. Table 2 shows regression models, correlation coefficient and standard errors obtained for the experimental program. The standard errors of these correlation is the statistical parameter used as basis of comparison between tests: it has the units of MPa. Although the correlation coefficient shows the extend to which the NDT results and the compressive strengths are related, the effectiveness of f_c evaluation cannot be obtained from this parameter. The residuals standard deviation is the measure of the accuracy in evaluating the f_c hence the measure of the model's effectiveness. Gross model violations, when present, are often exposed by an appropriate graph of the residual. The standardized residuals were plotted as the ordinate against the model value. The normal pattern fits into two parallel straight lines.

NDT	CACS(kN)	Regression Curves	Correlation Coefficient	Standard Erroro
Pulloff Test
	95	fc = 1.101 B1.100	R2 = 0.792	3.660
	120	fc = 13.005 e0.046.B	R2 = 0.856	3.021
	180	fc = 1.763 B	R2 = 0.694	5.934
	All results	fc = 0.049 B2 - 0.776 B + 27.849	R2 = 0.379	4.961
	All with C	fc = 1.346 B 0.667 C 0.242	R2 = 0.466	0.067
UPV
	95	fc = 0.435 V3.083	R2 = 0.896	2.936
	120	fc = 94.644 V - 362.647	R2 = 0.705	4.018
	180	fc = 0.437 V2.948	R2 = 0.069	5.804
	All results	fc = 1.304 V2.222	R2 = 0.394	5.138
	All with C	fc = 1.415 V3.070 C-0.263	R2= 0.463	0.068
Pin Penetration Test
	95	fc = 1.825 P0.876	R2= 0.910	2.486
	120	fc = 0.674 P + 10.112	R2= 0.257	1.994
	180	fc = 0.819 P + 8.713	R2= 0.386	4.689
	All results	fc = 12.782e0.032 P	R2= 0.635	3.493
	All with C	fc = 3.876 P0.771 C-0.091	R2= 0.437	0.069
Table 2: The regression models

Note: fc - Concrete Strength; B - Pulloff Test Results; V - UPV Test Results; P - Pin Penetration Test Results; C - Coarse Aggregate Strength

Factorial test or Experimental Test Design was carried out, so that the influence of the coarse aggregate crushed strength on the NDT results could be assessed. Therefore, according to the factorial analysis the coarse aggregate crushed strength has an influence on the NDT results at 5% significant level.

CONCLUSIONS

The main aim of this research was to analyze the effect of crushed coarse aggregate on NDT relationship to concrete compressive strength. The approach for assessing that effect was to introduce aggregate crushed strength as an independent variable to the correlation models. A laboratory program was developed using data collected from different mixes and three coarse aggregate of different origin. A set of variables (age, aggregate grading and size, curing procedure, etc) considered to influence the NDT - compressive strength relationship was excluded.

The procedure used for analyzing data included one variable statistic and regression analysis by least squares method. An analysis of residual variation was undertaken to evaluate the effect of the aggregate origin on the tests relationship.

The present NDT test results confirms that a crushed strength effect is present, i. e., coarse aggregate crushed strength has an influence on NDT - concrete strength relationship and test results.

REFERÊNCES

1. CASTRO, P. F.(1985). "An Expandable Sleeve Test for Assessing Concrete Strength". University College London, England. PhD Thesis.
2. CASTRO, P. F. (1987). "Concrete Strength - Comparison Between Non-Destructive Tests". Fourth International Conference on Durability of Building Materials & Components, pp. 885-890. Singapore.
3. LUDWIG FILHO, U. and CASTRO, P. F. (1991)."A Homogeneidade do Concreto na Estrutura de Edificios". XXV Jornadas Sul-Americanas de Engenharia Estrutural, Porto Alegre, pp. 221-231.
4. ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS - NBR 11578/91. "Cimento Portland composto".
5. MIRANDA, Heleno C.(1990). "Equipamento Portátil para Ensaio de Aderência". 97 pp. MSc. Dissertation - Universidade Federal Fluminense, Niterói - RJ.
6. PONTES VIEIRA, Domingos.(1978). "Método Brasileiro de PenetraÇÃo de Pinos". - XIX Jornadas Sul-americanas de Engenharia Estrutural, Santiago, Chile.

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