FAST-TRACK CONCRETE PAVING

Transcription

1 FAST-TRACK CONCRETE PAVING Brecht E.P. Anckaert and Lucie A.R. Vandewalle Building Materials and Building Technology Division, Department of Civil Engineering, Catholic University of Leuven, Belgium Abstract fast-track paving, concrete roads, rehabilitation, workability, early-age concrete strength development Tests were carried out to study the effect of cement type, superplasticizer type and ambient temperature on the fresh concrete properties and compressive strength of concrete with 12-hours target compressive strength of 4MPa. The concrete was produced at different workability levels with target slump values of 1 to 4mm and 1 to 15mm respectively. Concrete considered in this paper was produced using on the one hand a Portland cement CEM I 52.5 R LA and on the other a blast-furnace slag cement CEM III/A 42.5 N LA. Superplasticizers used were a sulphonated melamine formaldehyde condensate and a polycarboxylic ether respectively. Concrete specimens were allowed to cure under different temperatures (5 C-1 C-15 C- C) simulating variable weather conditions. Test results show that the workability and early-age concrete strength development are influenced by the cement type, superplasticizer type and ambient temperature. It is recommended not to use cement type CEM III in order to achieve 4MPa compressive strength at ages varying between 12 and 24 hours even at higher ambient temperatures. Also different types of superplasticizer can influence the hydration process in a different way, which affects both workability and compressive strength development at early ages. 1. INTRODUCTION In Belgium, as well as in other countries in the world, the concrete road infrastructure suffers severely from the increasing traffic load. The Road and Traffic Administrations are forced to the expenditure of large amounts of money for repeated maintenance and rehabilitation of concrete pavements, causing inevitably considerable traffic grieves. Obviously, limiting the traffic disturbance during road reparations to a minimum has become a road engineering aspect of common interest since it can serve the community in both economical and social way. Thanks to a recently developed technique of rapidly hardening concrete known as fast-track concrete paving for the rehabilitation of both large and small road sections, cement concrete pavements can be repaired and reopened for the traffic within less than three days. Fast-track concrete mixtures aim at obtaining a compressive strength of

2 4MPa (cores, &113mm, h=1mm) after 12 to 24 hours at ambient temperatures ranging from 5 C to C. Fast-track mixtures are usually characterized by the use of elevated cement contents and a high range water reducer (superplasticizer), which allows a low W/C factor without loss in workability. The distance between the concrete plant combined with traffic jams force the mixture to maintain its workability during at least one hour. The study presented in this paper is a continuation of investigations on techniques to reduce the hardening time of concrete roads in Belgium to a minimum, which originate already from the early nineties. Recently concrete roads have already been rehabilitated within three days by using fast-track concrete which obtains 4MPa compressive strength after 36 hours. The reparation works were then executed during the weekend [1][2]. This paper reports the results of a study undertaken to investigate the effect of two different types of cement, two types of superplasticizer and variable ambient temperatures (5 C-1 C-15 C- C) on the fresh concrete properties and early-age compressive strength of concrete designed to achieve 12-hours compressive strength of 4MPa and two different slump values. 2. EXPERIMENTAL PROGRAM 2.1 Raw materials For the fast-track concrete mixtures two types of cement were used: a Portland cement type CEM I 52.5 R LA and a blast-furnace slag cement type CEM III/A 42.5 N LA. No mineral admixtures such as fly ash or silica fume were used for this investigation since they are not readily available in most concrete plants. Other materials used in this study were: natural river sand /4, crushed porphyry stones, a sulphonated melamine formaldehyde condensate type superplasticizer A and a polycarboxylic ether type superplasticizer B. 2.2 Mixture proportions Eight concrete mixtures were tested. They were designed to achieve 12-hours compressive strength of 4MPa at constant W/C factor. Different workability levels with target slump values of 1 to 4mm (S1 according to NBN EN 6-1) and 1 to 15mm (S3 according to NBN EN 6-1) were obtained by varying the superplasticizer dosage in the mixture. For the same workability level, the mixture proportions of all aggregates and paste volume were the same. Because it was envisaged that a Portland cement type CEM I 52.5 would become insufficiently workable at C, CEM III/A 42.5 was used instead. The concrete mix proportions are given in Table 1.

3 Table 1: Concrete mixture proportions Materials [kg/m³] (5,S1) (15,S1) (15,S3) (1,S3) Porphyry 4/ Porphyry 6.3/ Sand / CEM I 52.5 R LA CEM III/A 42.5 N LA Superplasticizer A[%wt/wt] Superplasticizer B[%wt/wt] Water W/C [-] Preparation of specimens Before mixing, the materials were placed in a climate room having a regulated temperature equal to that of the curing temperature of the concrete specimens to ensure thermal equilibrium at the beginning of the test. First the dry materials (aggregates and cement) were mixed for one minute. Consequently, the water was added and mixed for another one minute. Finally, the superplasticizer was gradually added until the mixture obtained the appropriate slump value measured after 5 minutes of rest. Concrete cubes (15x15x15mm³) were cast in polystyrene foam molds. The top surface of the cubes stayed unisolated. Concrete specimens for 28-days compressive strength test were cast in plastic molds. A vibration table was used to ensure full compaction. After casting, the specimens were covered with a plastic sheet to avoid evaporation of the hydration water. 2.4 Curing process After casting, the isolated specimens were transferred to climate chambers maintained at variable curing temperatures until they were tested. The unisolated specimens were carefully brought to a chamber at the reference curing temperature ( C, RH 9%). For the investigation of the influence of curing temperature at early age on compressive strength gain, four different temperatures between 5 C and C were chosen simulating field and laboratory conditions respectively. Curing conditions were 5 C, 1 C, 15 C and C. The temperature-time history of a reference concrete specimen was measured by means of a thermocouple located at the center of the cube. 2.5 Fresh concrete testing Before casting the specimens, the fresh concrete properties were measured. The consistency and the air content of the fresh concrete mixture were determined according to EN , EN and pren respectively. Also the initial temperature and the density (EN ) were measured immediately after concrete preparation. 2.6 Compressive testing Compressive strength tests were performed at the age of 12h, 24h, 3h, 7d and 28d according to NBN EN First the cubes were stripped from their molds and the top

4 surfaces were shortly ground before testing. The average compressive strength was obtained from tests on 3 specimens. 3. TEST RESULTS AND DISCUSSION 3.1 Fresh concrete properties The fresh concrete properties are given in table 2. Superplasticizer type A dosages ranged from 1.25 to 3% by weight of cement. Figure 1 and 2 compare the slump and superplasticizer dosage of concrete containing superplasticizer type A and concrete containing superplasticizer type B at 1 C and C. To achieve similar slump values, it was necessary to increase the dosage of superplasticizer type A (see figure 1). Figure 2 shows the required superplasticizer dosage for concrete containing superplasticizer type A and type B. The concrete containing superplasticizer type B exhibited superplasticizer contents from.% to.25% to achieve similar slumps at a given temperature. The concrete containing superplasticizer type A required a 625 to 7% higher superplasticizer content than concrete with type B, depending on the curing temperature. Generally mixtures with superplasticizer type A showed a significant loss of workability in time. Table 2: Fresh concrete properties (5,S1) (15,S1) (15,S3) (1,S3) Slump [mm] VEBE [s] Temperature [ C] Air content Density [kg/m³]

5 6 5 SP dosage.25 SP dosage 1.25 SP dosage. superplasticizer type A superplasticizer type B 4 slump [mm] 3 SP dosage temperature [ C] Figure 1: Slump versus curing temperature 2 slump [mm] superplasticizer type A superplasticizer type B superplasticizer content [% wt/wt] 1,5 1,5 slump [mm] 5 slump [mm] 5 slump [mm] 45 1 temperature [ C] Figure 2: Superplasticizer dosage versus curing temperature

6 3.2 Temperature and compressive strength development Table 3 and figure 3 show for the different concrete mixtures used the development of compressive strength (cores, &113mm, h=1mm) with time. The temperature histories measured in the center of the representative specimens are shown in figure 4. From figure 4 it is observed that concrete temperature increases rapidly after 4 hours on average due to the hydration process. The maximum concrete temperature in the insulated cubes varies around 35 C for all mixtures. The concrete temperature equals the programmed temperature in the climate chamber after variable time periods. From a comparison of the compressive strength data in table 3 and the temperature profiles in figure 4 it is clear that an elevated concrete temperature at early ages results in higher compressive strength. The higher early compressive strength gain of specimens cured at 1 C as compared to 15 C prompts the idea that the superplasticizer type A has an accelerating effect due to higher contents in the mixture. However after 3 hours this discrepancy is almost eliminated. After 28 days mixtures with higher superplasticizer contents have the same compressive strength as the ones with lower contents. This supports the idea that the compressive strength of concrete is predominantly determined by the W/C factor rather than by the SP/C factor. Specimens cured at 5 C develop lower early-age compressive strengths. The so-called cross-over effect, which means that these specimens lead to at least the same later-age strength, is not yet observed after 28 days. In road engineering practice, the concrete must have a compressive strength of 4MPa before it can be exposed to traffic. Normal road concrete needs approximately 7 days to obtain this strength. This study aimed at reducing the hardening time of the concrete to a period of 12 to 24 hours only. From table 3, it is observed that all concrete mixtures obtained the target 4MPa compressive strength between 12 and 24 hours. Only and need more than 3 hours to reach this compressive strength level, even if cured at C. This is most likely due to the use of CEM III/A instead of CEM I which is a more rapidly hardening cement. From figure 5 one can derive that the use of superplasticizer type B results in a delayed and lower temperature development as compared to the use of superplasticizer type A. Therefore the concrete for which superplasticizer type A has been used, develops strength more rapidly. Opposite to superplasticizer type B concrete, superplasticizer type A concrete has reached more than 4MPa after 12 hours. However, this fact could explain why concrete with superplasticizer type B remains more workable for a longer period. and reach the same strength level after 7 days.

7 Table 3: Concrete compressive strength (in MPa) Time (5,S1) (15,S1) (15,S3) (1,S3) 12h h h d d compressive strength [MPa] (5,S1) (15,S1) (15,S3) (1,S3) age [h] Figure 3: Concrete compressive strength (in MPa)

8 temperature [ C] (15,S1) (15,S3) (1,S3) (5,S1) time [h] Figure 4: Temperature histories temperature [ C] time [h] Figure 5: Superplasticizer type dependency of temperature development

9 4. CONCLUSIONS - The fast-track concrete in this study obtained a compressive strength of 4MPa (cores, &113mm, h=1mm) between 12 and 24 hours. When blast-furnace slag cement was used, the concrete needed more than 3 hours to obtain this strength, even when cured at laboratory conditions. - Concrete specimens cured at lower temperature had lower early-age compressive strength. However, the so-called cross-over effect could not be observed within 28 days of curing at lower temperature. - The sulphonated melamine formaldehyde condensate type superplasticizer used in this study had an accelerating effect on the hydration process. Concrete specimens cured at lower temperatures and higher superplasticizer dosage developed early-age compressive strength more rapidly than specimens cured at higher temperatures and lower superplasticizer dosage. - Concrete mixtures with a sulphonated melamine formaldehyde condensate type superplasticizer presented a significant loss of workability in time, even at lower ambient temperatures. REFERENCES [1] VANDEWALLE, L., BEELDENS, A., CAESTECKER, C., LONNEUX, T., RENS, L., (Ultra)snelhardend beton voor wegen. Recent Belgisch onderzoek en toepassingen, Cement 7 (56) (4) [2] FOLENS, J. and VANFLETEREN, K., Ultrasnelhardend beton (Eng: Ultra fast-track concrete) (K.U.Leuven, Leuven, 3).