ROAD RESEARCH LABORATORY Department of the Environment RRL REPORT LR 379 AN INVESTIGATION OF CRACKING IN SOIL-CEMENT BASES FOR ROADS by H. E. Bofinger and G. A. Sullivan Tropical Section Road Research Laboratory Crotiorne, Berkshire 1971 CONTENTS Page Abstract 1. Introduction 2. The causes of cracting 3. Experimental programme 4. Results 4.1 bboratory testing 4.1.1. Tende strength v time of curing 4.1.2 Shrtiage v time of curing 4.1.3 Unconfined compressive strength after 14 days of curing 4.2 Pflot-scde experiments 4.2.1. General comments 4.2.2 bngth 1 4.2.3 bngth 2 4.2.4 hngth 3 4.2.5 Subgrade restraint 5. Dscussion of results 6. Conclusions 7. References oC CROWN COPYMGHT 1971 1 1 2 3 4 4 4 6 6 6 6 6 8 9 10 10 12 12 Extracts from the text m~ be reproduced provided the source is actiowledged AN INVESTIGATION OF CRACKING IN SOIL-CEMENT BASES FOR ROADS ABSTRACT This report describes laboratory and pilot scale studies of some factors that influence the crack spacing in sod-cement bases. ‘Two lengths of pflot scale sod-cement base were cured for 7 and 11 days respectively before wheel loads were apphed. A third controllength of soil-cement base was left untrafficked. From measurements of tensile strength, shrinkage and subgrade restraint, the crack spacing that should occur in the untrafficked length was estimated. A simflar crack spacing was observed in the experiment. The cracking in the other two lengths was more extensive and was related to the time that had elapsed before the wheel loads were first applied. The crack spacing in these lengths was simtiar to the spacing which has been observed in this type of base in the field, and which is usudy attributed to shrinkage stresses. It is concluded that the crack spacing in sofl-cement bases is contro~ed by their strength at the time that traffic is first admitted. A knowledge of the shrinkage, strength, stiffness and subgrade restraint of soil-cement bases should enable crack size and spacing to be controlled during the construction operation. 1. INTRODUCTION When soil-cement is used in road bases the size and spacing of the cracks that develop in it are important factors in determining the performance of the whole pavement. If the cracks are closely spaced and sma~ there is less chance that they WN be reflected through the surfachg than when the cracks are more widely spaced and larger. It is the cracks which penetrate through the surfacing that most seriously affect the performance of the road since they permit the ingress of surface water to the sub-base and subgrade. In developing countries it is normal practice to surface dress cement-stabilized bases and the control of cracking in the base is much more critical than when thicker bituminous carpets are used as surface layers. It has often been suggested that shrinkage causes the cracking of sod-cement bases *-5 and it has been observed that for a given soil the higher the cement content the greater is the crack spacing. Some highway authorities restrict the maximum amount of cement that they Wow to be used in their soilcement bases so as to limit the size and spacing of the cracks. 6 We~-graded gravefly sofls that can be adequately stabilized with smafl additions of cement rarely exhibit “shrinkage” cracking, but it is quite a common phenomenon in the finer-grained materials which require more cement to achieve a specified strength, especially if they have a tendency towards uniform grading. 1 This Report describes experiments which have attempted to identify cement bases with the object of predicting and controfing its occurrence. 2. THE CAUSES OF CRACKING the causes of cracking in soflMan!/ factors contribute to the cracking and crack-spacing of sofl-cement bases. Probably the most important of these are:- (i) the tensile strength of the material (ii) its shrinkage characteristics (iii) the subgrade restraint fiv) external loadings such as those caused by traffic (v) stresses caused by volume, changes resulting from temperature or moisture variations. (vi) stiffness and creep properties of the material In this project the effects of the first four factors listed above were studied. GeneraHy, sofl-cement bases are constructed as continuous slabs urdess smafl tirnited areas are to be stabilized. When a sofl-cement base is cured under excellent conditions, i.e. well sealed and completely shaded, it can be assumed that there is no loss of water and that shrinkage within the base will be relatively uniform and independent of depth. When no external loads have been applied to the base, the spacing between the cracks which form should be a function of the subgrade restraint and the tensfle strength, the shrinkage, and the tensile modulus of deformation of the soil-cement. The latter three properties are time-dependent and their interaction is complicated. It is also necessary to consider the possible occurrence of stress-relaxation due to creep. Previous studies 7 have shown that when soil-cement specimens are cured for 7 days or more, stress-refief due to this factor is insi~ificant. It is appreciated that when tensfle strains are appfied to sofl-cement at an earlier age creep may significantly lower the tensile stresses that develop. Measured values of the seven-day tensfle strength and the subgrade restraint of other soti-cement bases 8 suggest that the spacing between shrinkage cracks could be quite large. Consider a simplified model of a 15 cm (6 in) thick soil-cement base. If a subgrade restraint of 5 kN/m2 (100 lb/ft2) has been mobfiiscd over the whole area and the tensile strength is 70 kN/m2 a slab of length 4.2 m (14 ft) should be stable and remain untracked. If the stress in the soil-cement is relaxed due to creep in the material during its early life, the stable slat) length would be greater than 4.2 m. When moisture dries from the surface of the base due to an inadequate curing technique, differential shrinkage could cause surface stress concentrations which would reduce the sp;~cing between the major cracks. When stresses induced by traffic exceed the ultimate strength of the material at the time they are apptied the fracture pattern would be expected to be dependent upon the rate of apphcation of the stresses and the magnitude of the overload. Many soil-cement bases appear to have been cracked by traffic loads, and approximate analyses g suggest that the normal 15 cm (6 in) - thick soil-cement pavement WN be heatiy overstressed under normal traffic loading. 2 3. EXPERIMENTAL PROGRAMME Harmondsworth brickearth wasused inthelaboratory andpilot-scde experimental testing. Its properties were as follows: Liquid limit = 53% Plastic hmit = 20% Plasticity index = 33% Specific gravity = 2.70 Particle Size: Sand Medium & Coarse Sdt Fine silt Clay ( <0.002mm) British Standard Compaction Test (BS = 7% = 55% = 10% = 28% 1377: 1967) Opt Moisture Content (with 8% cement) = 17% Max Dry Density (with 8% cement) = 1.78 M~m3 (111 lb/ft3) The cement used in the programme was “Typical Ordinary Portland Cement” supplied by the Associated Portland Cement Manufacturers Ltd. Several factors were investigated in the laboratory. The increase with time of the tensile strength of the soil-cement was determined together with the relation between its shrinkage and time. In addition, the unconfined compressive strength after 14 days of moist curing was measured. The shape of the specimens used in the tensile tests is shown in Fig. 1. The specimens were statically compacted in one layer in the mould which is shown in Plate 1. Tensile tests were performed after curing periods of 15 minutes, 30 minutes, 1% hours, 4 hours, 1 day, 2 days, 7 days and 14 days. The two groups of specimens that were tested at 15 min and 30 min respectively were cured in a damp atmosphere at 20°C. The remaining specimens were dipped in paraffin wax and cured at 20°C. M of the wax was removed immediately before they were tested. A photograph of the tensfle testing frame is shown in Plate 2. The specimen was gripped in the jaws and deformed at a rate of 0.38 mm/min (0.015 in/rein) until failure. For convenience, the same type of specimen was used in the shrinkage experiments. Steel bafls were attached to the centre of each end with quick-setting polyester resin and the specimens were sealed in wax. Within 15 minutes of being moulded they were placed in the measuring rig (see Plate 3) and the change in length was noted at regular intervals. The 10 cm x 5 cm diameter (4 in x 2 in) compression specimens were statically compacted, sealed in wax and cured for 14 days at 20°C before being tested at a deformation rate of 1.27 mm/min (0.05 in/rein). The pilot-scale investigations were conducted in a 28 m x 7 m (92 ft by 23 ft) pit which was enclosed in a building. The top 90 cm (3 ft) of the insitu clay were removed and replaced by 75 cm (2 ft 6 in) of compacted, unstabilized Harmondsworth brickearth and a nominal 15 cm (6 in) of cement-stabilized base. The moisture content of the Harmondsworth brickearth that was to be stabilized was raised to 14 per cent prior to being placed in a pugmiU mixer. 8 per cent of cement was blended with the partly wetted clay and the fmd mixing water 3 added togivea moisture content of 18 percent. After inking was completed, thematerial wasspread, screeded andcornpacte dwithin 1 hrofmifing bytwopasses ofa3%Mg (3%T)steel-tyred roUerfoflowed by 16 passes of a 17Mg(17T) pneumatic-tyred roller. No traffic of any kind was allowed on the lengths of sod-cement base after this time, except as specified in the testing programme. A plan of the pilot-scale area is shown in Fig. 2. Lengths 1 and 3 were 2.1 m (7 ft) wide while the centre length was 2.8 m (9 ft) wide. The pavement in length 1 was placed directly on top of the rolled, unstabilized clay subgrade. In Lengths 2 and 3, a layer of polythene was used to separate the rolled clay subgrade from the base. After the compaction was completed, 7.6 mm (0.3 in) wide transverse grooves, 50 mm (2 in ) deep, were cut by hand at approximately 3 m (10 ft) spacing for 12 m (40 ft) of lengths 1 and 2, and 9 m (30 ft) of length 3. These grooves were inclllded at one end of each length so that the maximum tensile stress during the initial shrinkage in the short slabs that were formed would be approximately 60 per cent of the tensile stress in the ungrooved section. Any deleterious effects of the higher maximum stresses during the early life of the ungrooved sections could then be detected. No tr:tffic loads were applied to Length 2. Lengths 1 and 3 were subjected to 50 passes of a 5 Mg (ST) fork-lift truck with wheel loads of 1910 kg (4210 lb) and 1170 kg (2574 lb) after a curing period of 11 days and 7 days respectively. Apprc)ximately four months after the construction was completed beams 15 cm wide (6 in) and 60 cm (24 in) long were taken from the pilot-scale lengths of base. The positions of the beams are shown in Fig. 2. The beams were cut out partly by a machine saw and partly by hand: a pavement saw was used to cut the top 7.5 to 10 cm (3 to 4 in) but the remaining material was carefully broken out by hand. All of the beams that were removed in one piece were tested to failure in flexure. They were loaded at the third points over a sl?an of 45.7cm(18 in), the bitumen surface being placed uppermost. Plate 4 shows a beam positioned in the testing rig. Subgrade-restraint tests were performed on square blocks approximately 45 cm x 45 cm(18 in x 18 in) from which the surrounding base material had been re moved. A jack applied a horizontal load to the block through a proving ring and the horizontal movement of the block was measured by a dial gauge. A photograph of a subgrade restraint test is shown in Plate 5. 4. RESULTS 4.1 LABORATORY TESTING 4.1.1 Tensile strength v Time of curing. The strengths of direct tensile briquettes that were cured for periods ranging from 15 minutes to 14 days are given in Table 1 and the results are plotted in Fig. 3. Each of the specimens was moulded at 18 per cent moisture content to a density of 1.78 Mg/m3 (111 lb/ft3) and cured at constant moisture content for up to 14 days at 20°C. 4 ,,..- .,-.. ..= .=, ---... IABLt 1 The direct tensile strength of the soil-cement briquettes at different ages Curing Strength (x) Average strength (~) Coefficient of Period variation kN/m2 lb/in2 kN/m2 lb/in2 percent + 15 min 69.6 10.1 >, 60.0 8.7 ,, 67.6 9.8 67.6 9.8 7.8 ,, 74.5 10.8 9, 66.2 9.6 30 min 71.0 10.3 ,, 67.6 9.8 ?9 71.0 10.3 69.1 10.0 4.1 79 64.8 9.4 99 71.0 10.3 1% hrs 101.4 14,7 ,, 106.2 15.4 >9 100.0 14.5 106.7 15.5 8.6 >> 122.7 17.8 97 103.4 15.0 4 hrs 203.4 29.5 ,, 217.2 31.5 ,, 237.9 34.5 218.6 31.7 8.8 99 196.5 28,5 99 237.9 34.5 1 day 284 41.2 ,, 329 47.7 ,, 313 45.4 311 45.1 5.3 $9 313 45.4 ,, 316 45.9 2 days 332 48.2 9> 316 45.9 99 332 48.2 332 48.1 2.7 9, 339 49.1 ,, 339 49.1 7 days 429 62.2 ,, 355 51.5 ,, 339 49.2 372 53.9 9.3 ,, 359 52.0 ,9 378 54.8 14days 429 62.3 ,3 390 56.5 >> 381 55.2 398 57.7 7.4 91 362 52.5 79 427 62.0 *s= standard deviation 5 4.1.2 Shrinkage v Time of curing. The relationship between the shrinkage and the period hr which four soil-cement specimens were cured is shown graphically in Fig.4. 4.1.3 Unconfined Compressive Strength after 14 days of curing. Five compression specimens tested after 14 (days of curing had strengths ranging from 2860 kN/m2 (41 5 lb/in2) to 3070 kN/m2 (445 lb/in2) with an average of 2970 kN/m2 (43 1 lb/in2). The Ministry of Transport’s Specification 3 for road and bridge works requires that soil-cement should have an unconfined compressive strength of 2760 kN/m2 (400 lb/in2 ) after 7 days of curing. Harmondsworth brickearth, with the addition of 8 per cent of cement, approximately complies with this specification. 4.2 PILOT-SCALE EXPERIMENTS 4.2.1 General comments. During the construction of the pilot-scale lengths of base the maximum time that was allowed to elapse between the initial mixing of the materials and their final compaction into the base was one hour. The capacity of the mixer was sma~ and the full 28 m (92 ft) length could not be mixed, laid and compacted within the time limitation. Hence the roller operated on small runs of up to 6 m (20 ft) in length. The surface profile of the compacted length of base was poor and depressions were often produced at the position where the rofler was stopped and reversed. A grader was not used to trim the surface as these construction loadings would have stressed the base after the one-hour time limitation, The average daily temperatures during the four months of the testing programme were 8.30C, 10.6OC, 15.50c and 15.70C , 4.2.2 Length 1. Prior to and after the application of the wheel loads at 11 days, no cracks were visible in the bituminous surfacing of any portion of this length. Beams were extracted after four months from the positions marked in Fig.2 and the average depth of the base was found to be 13.3 cm (5% in ). The moisture contents of the beams were determined and the average was 18.1 per cent. Men the side of a beam had been exposed (see Plate 6 ) it was examined for any signs of cracking and then carefully removed. Details of the extraction of the beams in Length 1 are given in Table 2. The beams that were successfu~y removed were tested in flexure and their faflure loads are included in Table 2. TABLE 2 Beam Tests on Specimens removed from hn~h 1 Beam Successfully Flexural Remarks removed load 1 NO — No cracks were visible in the sides of the beam but it separated across the centre when it was first moved. 2 NO — Two cracks visible; extracted in 3 pieces. 3 YES 1068 N — (240 lb) 4 NO — 1 crack visible; extracted in two pieces. 5 NO — 1 crack visible; extracted in two pieces. 6 YES 765 N — (172 lb) 7 YES 1272 N — (286 lb) 8 NO — 1 crack visible; extracted in two pieces. 9 YES 658 N — (148 lb) 10 NO — 1 crack visible; extracted in three pieces. 7 4.2.3 Ungth 2. Three major cracks appeared in the bituminous surfacing of the 16 m (52 ft) section of this length of base 15 days after it was laid. The positions of these cracks are shown in Fig.5. Each of the cr:icks formed at a transverse surface depression that was produced by the roller. No other cracks were visible in the surface. After four months, during which no loading was allowed on the base,beams were cut from the positions shown in Fig.2. The average depth of this length of base was 13 cm (5 1/16th in) and its moisture contt?nt after four months was 18.6 percent. A commentary on the removal of the beams from length 2, together with their test results, is included in Table 3. TABLE 3 Beam tests on specimens removed from Ungth 2 —— Beam 11 Successfully removed NO YES YES YES YES YES YES YES YES Removal not attempted Flexurd load — 600 N (135 lb) 560 N (126 lb) 916 N (206 lb) 743 N (167 lb) 1744 N (392 lb) 1383 N (31 1 lb) 387 N (87 lb) 418 N (94 lb) Remarks No cracks were visible but the beam broke as it was lifted from the excavation 1.8m (6 lineal feet) of beam was removed intact and then separated into 0.6m (2 ft) lengths for flexural testing 1 Approximately 0.6m (2 ft) of beam between 17 and 18 was also successfully extracted The area was badly damaged by the operation of the pavement saw. 8 4.2.4 bngth 3. No cracks were visible in the surface of this length of base before wheel loads were applied 7 days after it had been laid. Subsequently, surface cracks were visible in most sections of this length. Between the major cracks 14.3 and 20.lm (47 and 66 feet) from the northern end (see Fig.5), there was an area of pronounced surface cracking at spacings of 8 to 40 cm (3 to 15 in). At the southern end, cracks were spaced at approximately 30 cm (12 in); Plate 7 shows part of this section. ,Over the remainder of this length surface cracks were visible at approximately 45 cm (18 in) spacing. In an attempt to remove two beams successfully, Beams 23 and 24 were cut from two limited areas approximately 1m (3 ft) long which showed no surface cracking. The remainder were cut on a regular pattern without regard to the existence of surface cracks. Notes on the cracking in the beams that were removed from this length of base are included in Table 4. TABLE 4 Beam tests on specimens removed from hngth 3 Beam Successfully Flexural Remarks removed load 20 NO — 3 cracks were visible; beam was removed in 4 pieces 21 NO — Extensively cracked; Beam fractured into many pieces at it was removed 22 NO — As above 23 YES 1401 N 1 The positions from (315 lb) which these beams were taken were 24 YES 992 N specially selected (223 lb) 25 NO — 2 cracks were visible; beam was removed in 4 pieces 26 NO — Beam fractured into many pieces. Many cracks were visible 27 NO — 4 cracks were visible. Plate 6 is a photograph of this beam 4.2.5 Subgrade Restraint. The subgrade restraint was measured in Ungth 1, in which the pavement was ‘laid directly on the ro~ed subgrade, and Ungth 2 which had a layer of polythene included between the rolled subgrade and the base. In hngth 1 the restraint was 6.2 kN/m2 (130 lb/ft2 ) whereas the plastic sheeting reduced the restraint in hngth 2 to 3.4 kN/m2 (72 lb/ft2 ). 5. DISCUSSION OF RESULTS From the graphs in Figs. 3 and 4 it is seen that the rate of gain of tensfle strength and the shrinkage rate are both very high during the first few hours of curing. There is a practical difficulty in measuring the shrinkage which occurs during the first 15 minutes. After 15 minutes, the shrinkage rate decreases with increasing time of curing. Between 15 and 20 minutes the average shrinkage strain is 1.11 x 10 ‘4 whfle from 20 to 25 minutes and 25 to 30 minutes the average strain is 0.86 x 10-4 and 0.54 x 104 respectively. If it is assumed that the shrinkage strain during the first 20 minutes is linear, a strain of approximately 3.3 x 10-4 would have occurred before any strain measurements could be taken. The shrinkage strain in a continuous soil-cement slab that is untracked after 3% hours of curing is estimated to be 9.8 x 10-4. Curves relating tensile stress and strain 10 for specimens of Harmondsworth brickearth stabilised with cement and cured for 14 days indicate that a tensile stress of 128 kN/m2 (18.5 lb/in2) would be developed at this strain provided that there is no relaxation of stress due to creep. It would be expected that the value of the tensile secant modulus, Et, at 3% hours would be less than Et at 14 days. Hence the estimate of,% kN/m2 (18.5 lb/in2 ) is probably greater than the actual stress. At 3}i hours, the measured tensile strength of the material was approximately 207 kN/m2 (30 lb/in2). It is unlikely therefore that a continuous slab that was untracked after 3% hours of curing would subseqt~ently crack from shrinkage stresses alone. This conclusion can also be. drawn when curing periods of more than 3% hours are considered. After 30 minutes of curing, a continuous slab would have an estimated shrinks e strain of FO) is approximately 6.5 X 10-4. The stress that would be developed (from the previous stress-strain curves 83 kN/m2 (12.0 lb/in2). Since the measured tensile strength at 30 minutes was 69 kN/m2 (10 lb/in2 ) the cracking due to shrinkage alone in a base made from Harmondsworth brickearth stabilised with cement probably occurs within the first 30 minutes. Such early cracking was not observed in the pflot-scale lengths; if it did occur it may have been concealed by the bituminous membrane. In length 2, widely spaced reflected cracks in the surface were noted after 15 days. Men they appeared thek widths ranged from 4.6 mm to 6.4 mm (O.18 to 0.25 in). These cracks had undoubtedly formed in the base at an earlier age and the sudden appearance of cracks of this width was probably due to the breakdown of the thick bituminous membrane over the cracks. No cracking could. be seen in th~surface of length 1 during four months of observations. men beams were extracted from this length a number of fine cracks were observed in the base material but none of these cracks could be detected in the curing membrane. Ungth 3 showed extensive surface cracking after wheel loads had been applied. men a continuous base slab first cracks its crack spacing is contro~ed by the subgrade restraint. During the first 30 minutes of curing the tensile strength of the soil-cement was 69 kN/m2 (1 O lb/in2 ). For a 13 cm (!j in) thick base which has a subgrade restraint of 3.4 kN/m2 (72 lb/ft2 ) (the measured value for hngth 2), an untracked lengthof5.1m(16 ft 8 in) should be stable. The spacing of the cracks that 10 .. formed in hngth 2 ranged from 3.6 m (9 ft 6 in) to 5.0 m (16 ft 3 in). me three pilot-scale lengths of base were cured under conditions that minimised the possibility that vertical differential shrinkage would occur in the pavement. Length No.2 was subjected only to shrinkage stresses. The thick bitumen seal and the polythene sheeting restricted the loss of water from the pavement and the shrinkage stresses could be expected to be unifom from the top to the bottom. Under these ideal curing conditions, no cracks were detected in the 3m (1 O ft) slabs at the end of Ungth 2. In the uncut section 16 m (52 ft) long, three major cracks formed at sections that had been weakened by the rolling and it was concluded that these points of weakness partly contro~ed the crack spacing. The cracks divided the length into slabs 3.6 m(11 ft 9 in), 5.0 m (16 ft 3 in), 4.4 m (14 ft 6 in) and 2.9 m (9 ft 6 in) long, as is shcwn in Fig 5. Wfle the possibility that other cracks existed in Length 2 cannot be excluded, the recovery of intact beams shows that these should be rare. Eight out of 9 beams were successfully removed and, in two instances, 1.8 m (6 ft) lengths of pavement were proved to be untracked. Only Beam 11 was damaged on removal and, as no cracks were visible in the sides of the beam, tfis damage may have” been caused by the handting stresses. No attempt has been made to estimate the flexurd stresses in the beams at failure. The flexural tests were performed simply to indicate the degree of cracking that had occurred in the base slab. Previous work 10 has shown that the calculation of stresses is complicated for a homogeneous beam with an irregular cross-section. The beams extracted from the base had a density gradient from top to bottom and stress estimations would have little value. It could be expected that the crack spacing in Length 2 would be reduced proportionately by any increase in the subgrade restraint. Subgrade restraints ranging from 5.2 kN/m2 (109 lb/ft2) to 9.6 kN/m2 (200 lb/ft2) have been measured for soil-cement bases that were constructed by the “mix-in-place” methods. No values for premixed soil-cement bases could be found. Both Lengths 1 and 3 were much more extensively cracked than Length 2. A comparison of Tables 2 and 4 shows that Length 3, which was loaded 7 days after construction, had cracks at a closer spacing than Length 1, which was loaded after 11 days. The fadure loads of beams that were successfully removed could not be correlated with the difference in crack spacing. . It is concluded that sofl-cement bases with properties similar to Harmondsworth brickearth stabdized with 8 per cent of cement and which have a minimal loss of moisture during curing wfll have a crack spacing due to shrinkage rdone of the order of 3m (1 O ft) or more. More closely spaced cracks that are normafly attributed to shrinkage may have been initiated by stress caused by traffic or by poor curing conditions. If a soil-cement base can be loaded with carefully controlled wheel loads after a short curing period it may be possible to use this method to control the crack spacing. There is some evidence that this method is feasible. In Japan 11, when roads with soil-cement bases were opened to traffic immediately after construction, there were no detrimental effects on their performance. A report from Sweden 12 describes the use of vibrating roflers to apply loads to a stabdised sofl base during the early curing period to minimise the size and spacing of cracks. At the present time the cement content in stabilized-sod road bases is often limited to ensure that the base is sufficiently weak to crack at close spacings but a possible disadvantage of this a~proach could be that the resulting “blocks” are also weakened and the pavement is less durable. 11 6. CONCLUSIONS 1. When Harmondsworth brickearth is stabilized with 8 per cent of cement, the shrinkage and rate of gain of tensile strength are high during the first few hours. Approximately 50 per cent of the 14 day tensile strength was attained after only 3% hours of curing. During the same period approximately 75 per cent of the 14-day shrinkage had occurred. From an examination of the shrinkage and tensde characteristics it was concluded that cracks due to shrinkage alone probably develop in soil-cement bases of this type within the first half-hour t~fter final compaction has been completed. 2. It is concluded that for soil-cement bases of the type investigated that are compacted within 1 hour of mixing and :~reproperly cured: (a) If no external loads are applied after find compaction the spacing of cracks caused by shrinkage alone is governed by the subgrade restraint that is mobilised and by the tensile stress-strain characteristics of the material. (In the pilot-scale length of base that was examined, a shrinkage crack spacing of up to 5.0 m (16 ft 3 in) was measured and this is much greater than the crack spacing that is normally found in sod -cement roads). (b) The crack spacing in such a base depends primarily on the length of time that elapses between the final compaction and the passage of traffic over it. As this length of time increases there is a corresponding increase in the strength of the material and an increase in the crack spacing. 3. Itma)r be feasible to control the spacing of cracks, and hence the size of “blocks” in soil-cement bases, by the al)plication of external loads during the initial curing period. The results of investigations in Sweden and Japan support this conclusion. Further studies are therefore to be made aiming at defining and producing an acceptable pattern of base-cracking in stabilised soils as used in tropical countries, i.e. with running surfaces consisting of single or double surface dressings. 1. 2. 3. 4. 12 7. REFERENCES HIGHWAY RESEARCH BOARD. Soil stabilization with Portland cement. Bulletin 292. Washington. DC, 1961 @ational Research Councfl), pp 13-15, 133. NAKAYM~H.and R L. HANDY. Factors influencing shrinkage of soil cement. Highway Research Record No. 86. Washington DC, 1965 (Highway Research Board), pp 15-27. GEOItGE, K.P. and D.T. DAVIDSON. Development of a freeze-thaw test for the design of soil-cement. Highway Research Record No. 36. Washington DC, 1963 (Highway Research Board), pp 77-96. GEOI{GE, K.P. Cracking in cement-treated bases and means for minimizing it. Highway Research Record No. 255. Washington DC, 1968 (Highway Research Board), pp 59-71. 5. 6. 7. 8. 9. 10. 11. 12. 13. GEORGE, K. P. Cracking in pavement influenced by the visco-elastic properties of soil-cement. Highway Research Record No.263. Washington DC, 1969 (Highway Research Board), pp 47-59. CALIFORNIA DIVISION OF HIGHWAYS. Method of calculating the design thickness of pavement sections based on the stabilometer and expansion pressure. Sacramento 1960. (California Division of Highways). BOFINGER, H. E. The creep of clay-cement under steady tensile stress. AusWalian Road Research, 1970,4 (3), 80-5. BOFINGER, H. E. Soil-cement as a rigid pavement material. Unpublished Ph D Thesis, University of Queensland, 1968. MACLEAN , D,J. and P.J.M. ROBINSON. Methods of soil-stabilisation and their application to the construction of airfield pavements. fioc. lnsm. Civ. Enfls. Part 11, 1953 2 (2), 447-86; Discussion, 487-502. BOFINGER, H. E. The measurements of the tensile properties of soil-cement. Road Research hboratory Report LR 365. YAMONOUCHI, T. and M. ISHIDO. Laboratory and in-situ experiments on the problem of immediate opening of soil-cement base to general traffic. Fourth Austrdi~New Zealand Conference of the International Society of Soil Mechanics & Foundation Engineering, Adelaide, 1963. MOLLER, G. and R. LINDELL. Rolling of cement-stabilised bearing courses. Gullkornet, 1964,14 (2), 14-17. MINISTRY OF TRANSPORT. Specification for Road and Bridge Works. HM Stationery Office, 1969. 13 76 mm o (3”5 in) 25”4 mm ~(1 in q.) Fig.1. TENSILE SPECIMEN ,. ,,, ,,. . ,.., !8m 3m 3m 3m 3m ‘6m + 7m Fig.2. LAYOUTOFTRIAL LENGTHSOF STABILIZED BASE ANO POSITIONS FROM WHICH BEAMS WERE CUT 400 - 350 -~ 300 – 40 250 – -m 200 –.: 30 150 – 20 100 – 10 50 – o_ o 10 102 103 104 1 Curing timQ (rein) Fig.3. VARIATION IN THE AVERAGE TENSILE STRENGTHWITH THE LENGTHOF CURING TIME I I I + I & ‘.\. \ , \ (,-oLx) u!DJls a6D~U!Jqs m c 4- m c : (47j 14.3m — [66’) G I “Surface cracks spaced at appmx. 45cm(18 in) \ Pronounced surfacQ cracking at spacings usually varyin bQtW~Qn 8 to 4%cm (3 to 15in) Surface crocks at appmximatQly 30cm (12 in)spacing Fig. 5. CRACKINGVISIBLE IN SURFACE OF LENGTHS2 & 3 OISTANCES ARE MEASURED FROM NORTHERNEND . — - =.-.—. -— —.----- = _.. ---.s. -—_ —. —_. ~—-”’ “’-”” ““’ ”-* ‘“- – x - —- — \ -.— ‘1 PLATE 3 Neg NoH 1522/70 Measuring rig for shrinkage tests ‘! I . Neg No H 411/68 PLATE 4 Testing a beam -.. —- ---?, (12) Dd635272 4M. 1/71 H. P. Ltd., G191S PRINTED IN ENGLAND. ABSTRACT An investigation of cracking in soil-cement bases for roads: H. E. BOFINGER and G. A. SULLIVAN: Department of the Environment, RRL Report LR 379: Crowthorne, 1971 (Road Research Laboratory). This report describes laboratory and pilot scale studies of some factors that influence the crack spacing in soil-cement bases. Two lengths of pilot scale soil-cement base were cured for 7 and 11 days respectively before wheel loads were applied. A third control-length of soil-cement base was left untrafficked. From measurements of tensile strength, shrinkage and subgrade restraint, the crack spacing that should occur in the untrafficked length was estimated. A similar crack spacing was observed in the experiment. The cracking in the other two lengths was more extensive and was related to the time that had elapsed before the wheel loads were first applied. The crack spacing in these lengths was similar to the spacing which has been observed in this type of base in the field, and which is usually attributed to shrinkage stresses. It is concluded that the crack spacing in soil-cement bases is controlled by their strength at the time that traffic is first admitted. A knowledge of the shrinkage, strength, stiffness and subgrade restraint of soil-cement bases should enable crack size and spacing to be controlled during the construction operation. o ABSTRACT An investigation of cracking in soil-cement bases for roads: H. E. BOFINGER and G. A. SULLIVAN: Department of the Environment, RRL Report LR 379: Crowthorne, 1971 (Road Research Laboratory). This report describes laboratory and pilot scale studies of some factors that influence the crack spacing in soil-cement bases. Two lengths of pilot scale soil-cement base were cured for 7 and 11 days respectively before wheel loads were applied. A third control-length of soil-cement base was left untrafficked. From measurements of tensile strength, shrinkage and subgrade restraint, the crack spacing that should occur in the untrafficked length was estimated. A similar crack spacing was observed in the experiment. The cracking in the other two lengths was more extensive and was related to the time that had elapsed before the wheel loads were first applied. The crack spacing in these lengths was similar to the spacing which has been observed in this type of base in the field, and which is usually attributed to shrinkage stresses. It is concluded that the crack spacing in soil-cement bases is controlled by their strength at the time that traffic is first admitted. A knowledge of the shrinkage, strength, stiffness and subgrade restraint of soil-cement bases should enable crack size and spacing to be controlled during the construction operation.