EVALUATION OF STRENGTH PERFORMANCE OF CONCRETE MADE FROM DIFFERENT TYPES OF

CEMENT FOR ROAD APPLICATIONS IN THE PHILIPPINES

Christian R. Orozco1 1Instructor, Institute of Civil Engineering, University of the Philippines Diliman, Quezon City, Philippines,

Telefax: +63 (02) 4343635, Email: christian.orozco@coe.upd.edu.ph

Received Date: September 30, 2013 Abstract This part was intentionally removed Keywords: Compressive strength; Flexural strength; Road applications; Rigid pavement; Philippine cements; Introduction Strength is the most commonly measured property of concrete and is often used as the basis for assessing concrete quality. In rigid pavement, concrete strength is one parameter that has significant impact on its performance. This is partly because strength measurements give a direct indication of concrete’s ability to resist loads and partly because strength tests are relatively easy to conduct [15]. In construction, the control of strength is achieved through close control of the mix proportions (aggregate, cement and water) and placement operations (ambient temperature, consolidation and curing). The standard method for evaluating the strength of concrete in pavement applications is to test molded specimens for compressive, flexural or tensile strength. The properties of concrete depend on the quantities and qualities of its constituents. Because cement is the most active component of concrete and usually has the greatest unit cost, its selection and proper use are important in obtaining most economically the balance of

 

 

properties desired for a particular concrete mixture. Most cement will provide adequate levels of strength and durability for general use. Generally, a concrete mix design is developed to satisfy minimum strength requirements corresponding to an accepted standard or specification. The age at which a given strength is required will vary depending on the need. In the Philippines, the required minimum flexural and compressive strengths are set by the Department of Public Works and Highways (DPWH). Most loads on pavements result in bending (or flexure), which introduces compressive stresses on one face of the pavement and tensile stresses on the other. Compressive Strength Compressive strength of concrete is the most common performance check of structure. In pavement, it is a very dominant stress because of heavy loads that pass on it. When a vehicle passes on the pavement, its large weight will cause the slab to compress. Compressive strength is measured by breaking a cylindrical specimen using compression-testing machine. The applied load to break the specimen divided by the cross-sectional area where the load is applied will give the compressive strength of concrete. Flexural Strength Flexural strength (sometimes called the modulus of rupture) is one measure of tensile strength of concrete. It is typically used in checking the quality of concrete pavement because it best simulates the slab flexural stresses when pavement is subjected to loading. During loading, pavement slabs experience large bending moment that usually causes its failure. While loads applied to concrete pavement produce both compressive and flexural stresses in the slab, the American Concrete Institute considers the flexural stresses to be more important because loads can induce flexural stresses that may exceed the flexural strength of the slab. In his paper, Taylor et al agreed with this claim saying that most slab failures are in flexure rather than in compression. Consequently, the flexural stress and the flexural strength (modulus of rupture) of the concrete are used in pavement design to determine required slab thickness. [15] Concrete pavement design focuses on limiting tensile stresses by properly selecting the characteristics of the concrete slab. The rigidity of concrete enables it to distribute loads over relatively large areas of support. For adequately designed pavements, the deflections under load are small and the pressures transmitted to the subgrade are not excessive. [3] Two kinds of flexural test can be performed in accordance to ASTM and AASHTO. These are ASTM C78/AASHTO T97 (third point loading) and ASTM C239/AASHTO T177 (center-point loading). In ASTM C78 [7] (third point loading), half the load is applied at each third of the span length. Maximum stress is distributed over the center 1/3 of the beam. The resulting modulus of rupture (MR) in this case is lower compared to center-point loading. For ASTM C239 (center-point loading), the load is applied at the center span. In this case, the center of the beam experienced the entire load which will results to greater MR. The variation in resulting MR is around 15%. Between these two tests, third-point loading is more preferred because, ideally, in the middle third of the span the sample is subjected to pure moment with zero shear. In the center-point test, the area of eventual failure contains not only moment-induced stresses but also shear stress and unknown areas of stress concentration. Also for AASHTO thickness design, it is important that the third-point loading, 28-day flexural strength be used in the AASHTO equation. Flexural strength of specimens is very sensitive to different factors, including

 

 

preparation, handling and storage. Beams can be easily damaged if not properly handled. During testing, the specimens should not be allowed to dry because it will result to lower strength. A short period of drying can cause a big drop in flexural strength. [13] Material and Methods In order to characterize the strength performance of concrete made from different types of cement different test cases were made. For the first case, strength performance for two different water-cement ratios will be investigated whereas the second was to investigate the strength development by varying the curing age as shown in Figure 1.

Figure 1. Test cases for the experiment

Cement

3 days 7 days 14 days 28 days 56 days 90 days

120 days

Type I 0.4 0.5

Type IP 0.4 0.5

Type P 0.4 0.5

For each set (i.e concrete made from Type I cement with 0.4 w/c cured for 3 days), four specimens were prepared. This is twice the minimum required number of specimens required by the American Concrete Institute for acceptance of strength test results of concrete. [2] Two sets of plain concrete samples were prepared. The first set includes four specimens of 6 inches diameter by 12 inches height cylindrical specimens and the second includes four specimens of 21 inches length by 6 inches width by 6 inches beam specimens for each type of cement, water cement ratio and curing age. In this case, a total of 168 plain concrete cylinders and 168 plain beam specimens will be prepared.

Table 1. Number of specimens per type of cement per test case

Age w/c

3 days

7 days

14 days

28 days

56 days

90 days

120 days

0.40 4 beams 4 cylin.

4 beams 4 cylin.

4 beams 4 cylin.

4 beams 4 cylin.

4 beams 4 cylin.

4 beams 4 cylin.

4 beams 4 cylin.

0.50 4 beams 4 cylin.

4 beams 4 cylin.

4 beams 4 cylin.

4 beams 4 cylin.

4 beams 4 cylin.

4 beams 4 cylin.

4 beams 4 cylin.

Total number of beam specimens/cement type = 56 Total number of cylinder specimens/cement type = 56 Materials Characterization and Preparations The materials used in this study include cement, fine aggregates and coarse aggregates

Effect of cement type, water to cement ratio

Effect of curing age

Interaction of the effects

 

 

sourced out from local suppliers. Three different types of cements were used including: Type IP, Type I and Type P. The chemical and physical properties of these cements were provided by the manufacturer. Both fine and coarse aggregates were sieved following ASTM C136 in accordance to ASTM C33/C33M-11a. Also, both the coarse and fine aggregates used were in compliant with the grading requirements set in the DPWH Standard Specifications for Public Works and Highways, 2004 Edition: Volume II, Highways, Bridges and Airports. Prior to fabrication of test specimens, the aggregates were first washed with clean water to remove impurities and then air-dried prior to their use. The physical properties of the aggregates were tested in accordance to applicable ASTM standards. The properties determined were absorption, initial moisture content, maximum dry density and fineness modulus.

Table 2. Summary of fine and coarse aggregate properties AGGREGATE PROPERTIES

Coarse Fine Absorption, % 0.42 0.81 Initial Moisture Content, % 1.69 3.25 Maximum Aggregate Size, mm 19 Dry Rodded Density, g/cc 1557 Fineness Modulus 2.26

Specimen Preparation The required amount of coarse and fine aggregates, water and cement for each concrete mix design was determined using the absolute volume method prescribed by ACI 211.1-91R02. The computed amounts are shown in Table 3.

Table 3 Concrete mix proportion per cubic meter W/C Mass (kg)

0.4 0.5

Cement 462.50 370 Water 145.87 145.87 Fine Aggregates 679.72 772.22 Coarse Aggregates 1066.90 1066.90

Before actual mixing, molds, concrete components, and other materials were prepared. Cylindrical molds were cleaned and greased inside so that concrete won’t stick to it after it hardens. Coarse aggregates are sieved to obtain the maximum aggregate size of ¾”. This will also ensures that particles present with size lesser than 0.075 mm is not greater than 1% of the total mass of coarse aggregate. After sieving, coarse aggregate is then washed and dried to remove unnecessary materials present. The mixer was cleaned before mixing. Inside surface of mixer was wiped with wet cloth to avoid absorption of water during actual mixing. Concrete components were then measured according to the proportion computed.

 

 

Compressive and Flexural Strength Tests Third point flexural test (ASTM C78/C78M) was made as soon as after removal from moist storage. The specimens were capped with plaster of Paris to eliminate gaps. Using Instrom UTM, the specimen was loaded continuously without a shock until rupture occurs. The specimen was loaded to failure to determine actual compressive strength. To make sure that the faces of the specimens are leveled, grinding was performed prior to testing.

Figure 2. Flexural strength test based on ASTM C78

Figure 3. Beam specimens after flexural loading

Figure 4. Compressive strength test of concrete in universal testing machine

Figure 5. Plain cylinder specimens after loading to failure

 

 

Method of Analysis This section was intentionally removed Results and Discussion This section was intentionally removed Conclusions This section was intentionally removed Recommendations This section was intentionally removed References

[1] ACI 211.1-91 Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete

[2] ACI 214 Evaluation of Strength Test Results of Concrete [3] ACI 360R. Guide for Construction of Slabs-on-Grade [4] ACI 228 In Place Methods for Determination of Strength of Concrete. [5] ASTM C192 / C192M - 07 Standard Practice for Making and Curing Concrete

Test Specimens in the Laboratory [6] ASTM C39 / C39M - 12 Standard Test Method for Compressive Strength of

Cylindrical Concrete Specimens [7] ASTM C78 / C78M - 10 Standard Test Method for Flexural Strength of

Concrete (Using Simple Beam with Third-Point Loading) [8] ASTM Manual 7 on Presentation Data and Control Chart Analysis, 6th Edition [9] ASTM E178 Practice for Dealing with Outlying Observations [10] ASTM E2586 Standard Practice forCalculating and Using Basic Statistics [11] DPWH Standard Specifications for Public Works and Highways, 2004 Edition:

Volume II, Highways, Bridges and Airports. [12] Kosmatka, S. (March 1998), Compressive Versus Flexural Strength for Quality

Control of Pavements, Concrete Products, pp. 14-15. [13] National Ready-Mixed Concrete Association (1992), What, Why, and How?

Flexural Strength of Concrete in Practice 16. NRMCA, Silver Spring, Md. [14] Navidi, William Cyrus. Statistics for engineers and scientists. McGraw-Hill,

2011. [15] Taylor et al. Integrated Materials and Construction Practices for Concrete

Pavement: A State-of-the-Practice Manual. Federal Highway Administration Office of Pavement Technology. October 2007