THE PERFORMANCE OF CONCRETE GROUND FLOOR SLAB REINFORCEMENT TO CONTROL SHRINKAGE CRACKING D H Chisholm 1 ABSTRACT In a typical residential concrete floor slab, the cast in strip footings on the slab perimeter impose a high degree of restraint to slab drying shrinkage. This paper reports a test programme where the performance of such ground floor slabs incorporating different types of reinforcing was compared as they dried out. Six four metre long by one metre wide slabs were cast indoors, five of which were restrained from shortening by an external structural steel frame, the sixth slab was a control which was free to shorten. All slabs cracked to varying degrees in the six-month duration of the test. Surface crack widths in the slabs with plain and deformed mesh were maintained at less than 1mm, whilst final crack widths in the plain unreinforced slab and the slab with polypropylene fibre were over 6mm. 1. INTRODUCTION This test programme compared the performance of concrete ground floor slabs incorporating different types of reinforcement, under restraint, in their drying shrinkage movement in the plane of the slab. The extent, and type of the crack pattern which forms once the tensile strength of the concrete is exceeded, has been used as a basis for evaluating the influence of reinforcement type on shrinkage cracking. In a typical residential floor slab, a high degree of restraint to slab shrinkage is imposed by the continuous foundation wall on the slab perimeter. NZS 3604:1999 [1] requires that slabs be divided into bays not exceeding six metres between shrinkage control joints when 665 mesh is used in the slab. A shrinkage control joint, typically formed by sawcutting over the top of the reinforcing mesh, will have reinforcement passing through the joint. Free joints, where the reinforcement is cut or stopped on each side of the joint, are not required unless the concrete slab exceeds a dimension of 24 metres in any direction. This test programme was designed to simulate the restrained situation where there are no free joints in the slab. 2. SCOPE In establishing the scope, it was considered that only a full scale test undergoing natural drying shrinkage in the field would provide a valid comparison of reinforcement types. Smaller scale test samples evaluating shrinkage and the influence of various parameters would have been simpler, however it was considered that the combined effects of gradual development of shrinkage strain, increase of concrete tensile strength and the effects of creep were an essential part of the research. Other research using small samples such as a restrained ring test sample to evaluate post crack behaviour, does not allow for the release of strain on cracking which occurs in the field. Based on a literature search, the only full scale test previously carried out was by BRC Engineering UK in 1989. [2]. Six test slabs were cast on grade, each four metres long by one metre wide by 100mm thick. By choosing such long narrow slabs, it was considered that the drying shrinkage would be essentially one dimensional, and that uniaxial tensile stresses would be produced as a result of the restraint. For five of the slabs, each end was anchored to a structural steel hammerhead, which provided restraint to the slabs from shortening due to shrinkage of the concrete as it dried out. These slabs had four D16 starter bars approximately 600mm long at each end to connect the steel hammerheads. A diagrammatic view of the slab set-up is shown in Figure One. Restraint against concrete shrinkage was imposed by structural steel compression struts running lengthwise between the hammerheads on each side. Each slab acted independently with it s own restraint frame, and whilst the struts were used as edge formwork for the slabs, a polystyrene packing strip along the slab edges ensured that there was no significant frictional restraint between the steel struts and the concrete slabs (Figure Two). Ground friction was minimised by casting the slabs onto two layers of plastic sheet laid over sand. Two shrinkage control joints were placed in four of the restrained slabs using a crack inducer placed on the bottom of the slab, the details of which are given in Section Four. A sixth slab was cast as a control slab with no restraint to shrinkage imposed into the slab. A summary of the slab types, including the type of shrinkage control reinforcement used in each slab is shown in Table One. 1 Senior Concrete Engineer BRANZ Ltd.
4 m Structural steel compression member 1 m Concrete in tension Structural steel compression member Figure 1 Forces in Self Reacting Test Rig Figure Two: Slab Showing Set-up prior to Casting Slab Reference Reinforcement Type Crack Inducer? Restraint Details A None No No Restraint B None Yes Restrained C Deformed 665 mesh Yes Restrained D Plain 665 mesh Yes Restrained E 0.7 kg/m 3 Polypropylene Fibre Yes Restrained F Deformed 665 Mesh No Restrained Table One: Schedule of Slab Type
3. TEST MEASUREMENTS AND SLAB INSTRUMENTATION The slabs were cast indoors in an industrial warehouse which had no heating or humidity control. With the large amount of data to be measured during the drying out phase of the concrete slabs, the slabs and the environment were monitored with a data logger where readings were captured every hour. In all 60 data items were measured as follows: (a) Room temperature and humidity (b) Slab temperature and humidity (c) Restraining strut compressive loads (d) Concrete tensile strains (e) Reinforcing steel tensile strains (f) Slab surface movements (g) Slab shrinkage and curl of the unrestrained slab A. Apart from items (f) and (g) which were read weekly by dial gauge, readings were taken by the data logger. The slab surface movement readings (f) were based on preset Demek gauge points which were used to measure the development of crack widths in particular. The slabs were monitored for a period of approximately six months after which time they were dismantled. The restraining strut compressive load was measured via strain gauges wired as load cells fixed to the two vertical faces of the structural steel at approximately mid length of the struts. The reinforcing steel strains were measured by strain gauges fixed to the mesh steel prior to casting, and the concrete strains by concrete strain gauges placed into the concrete at mid depth, immediately after the screeding operation. Standard tests carried out on the concrete mix included compressive strength, flexural tensile strength, modulus of elasticity, and accelerated drying shrinkage. 4. SLAB CONFIGURATION AND CASTING The reinforcing mesh in slabs C, D and F was suspended off tie wires which were cut and removed after the concrete was finished. The mesh cover to the top of the slab was 30mm. Slabs B,C,D, and E had two shrinkage control joint formers positioned one metre from each end of the slab. These were a metal tee piece placed on the bottom of the slab projecting 30mm into the slab. The tee piece was taped with polythene to prevent bonding with the concrete. The purpose of the joint formers was to force the concrete to crack along these predetermined lines of weakness. For the two slabs C and D, the reinforcement strain gauges were placed over the control joints (Slabs B and E did not have reinforcement.) The concrete gauges were placed at various locations over the length of each slab. All the slabs were cast using readymix concrete specified as a 17.5 MPa - 13mm pump mix. This concrete specification was chosen so as to maximise drying shrinkage. The concrete had a measured slump of 180 mm and an initial air content of 4%. Concrete was placed directly from the truck into the slabs which were vibrated using a pencil vibrator, followed by hand screeding and then a finishing with a wood float. Slab E with polypropylene fibre was cast last after concrete for all the other slabs had been placed. Fibre was added to the truck based on the concrete volume remaining in the bowl. The slump was remeasured at 100mm after the addition of the fibre. The indoor conditions at the time of casting were 16 O C and 60 % Relative Humidity. On completion of finishing, the slabs were cured by covering with plastic sheet which was removed three days after casting. 5. RESULTS 5.1 Concrete Test Results The concrete was tested at specified ages to NZS 3112 [3] for strength testing, AS 1012.17 [4] for modulus of elasticity testing, and to AS 1012.13 [5] for accelerated shrinkage testing. The results are summarised in Table Two. The measurements taken at 43 days corresponded approximately with the time of first cracking to slabs B and E. The field cured compressive strength specimens were stored on site in plastic bags, until 48 hours prior to testing when they were uplifted and placed in the fog room. The target standard cured compressive strength for a 17.5 MPa concrete is 22.0 MPa. The lower 21 MPa strength achieved reflects the additional water required to achieve the high slump of this concrete. This has also increased the shrinkage twofold over that for a typical standard structural concrete. The lower field cured concrete strength reflects the lower temperature and humidity of the field cured specimens compared to standard cured specimens.
Test Type At 7 days At 28 days At 43 days Curing Compressive Strength 15.0 MPa 21.0 MPa 23.5 MPa Standard Cured Compressive Strength 16.0 MPa Field Cured Flexural Strength 3.8 MPa 3.8 MPa Standard Cured Modulus of Elasticity 16.2 GPa 18.4 GPa Standard Cured Standard Shrinkage 1650 Microstrain After 56 days Drying Table Two: Concrete Test Results 5.2 Unrestrained Slab Movements The horizontal shrinkage of the unrestrained slab, slab A, after six months was 3.7mm which is equivalent to a shrinkage strain of 920 microstrain. This is around 55% of the standard shrinkage strain which is in the range expected.the slab curled upwards at each end, reflecting the drying of the slab from the top surface. The maximum upward curl was 3.25mm which was measured at age two months. 5.3 Cracking of the Restrained Slabs The appearance of cracks in the restrained slabs was dependent on the rate of drying, the tensile strength development of the slabs, and the degree of restraint imposed by the struts. The first cracks occurred in both the unreinforced and polypropylene fibre reinforced slabs, almost simultaneously after 42 days. Details on the transverse internal cracks, as measured by Demek gauge on the surface of the slabs is shown in Table Three. Subsequent to the opening up of the internal cracks, gaps formed adjacent to the endplates on some of the slabs as shown in Table Four. Crack widths were estimated by feeler gauge at the surface of the slab at the end of the testing period. Slab No. Reinforcement Slab Age at Cracking Crack Location Initial Crack Width Final Crack Width B None 42 days At control joint 2.4mm 6.8mm C Deformed 665 75 days At control joint 0.8mm 0.7mm Mesh C Deformed 665 113 days At control joint 0.5mm 0.6mm Mesh D Plain 665 Mesh 81 days At control joint 0.7mm 0.7mm D Plain 665 Mesh 129 days At control joint 0.6mm 0.7mm E Polypropylene Fibre 42 days Between control joints 2.4mm 6.7mm F Deformed 665 6 months 330mm from 0.25mm 0.25mm Mesh end Table Three: Appearance of Internal Cracks Slab No. Reinforcement East End West End B None - 0.24mm C Deformed 665 Mesh 0.25mm 0.35mm D Plain 665 Mesh 0.25mm 0.34mm E Polypropylene Fibre 0.15mm 0.24mm F Deformed 665 Mesh 0.75mm - Table Four : Gaps Adjacent to Endplates The surface crack widths in slabs B and E without steel reinforcement were significantly higher than all slabs with reinforcement. For slabs B and E the final surface crack movement was nearly twice the shrinkage movement measured on the unrestrained slab A. Although no significant curl was apparent on slabs B and E, the surface crack movement would include the effect of both shrinkage and curl, whereas for slab A shrinkage at the slab ends was measured independently of curl.
Humidity and Temperature Shrinkage (Microstrain) The crack widths in all the reinforced slabs were less than 1mm. Joints less than 1mm in width are generally considered to transfer load across the joint by aggregate interlock. For slabs C and D the reinforcement passing through the control joint maintained the width of the crack and also transferred load so as to induce a subsequent crack at the second control joint. The narrow crack in slab F occurred within the region of the starter bars at one end. 5.4 Temperature and Humidity Measurements A chart of the air temperature and humidity readings is shown in Figure Three. Readings were also taken inside the concrete. The increase in shrinkage strain of the unrestrained slab A, with time is overlayed on the chart. 5.5 Restraining Strut Compressive Loads It was not expected that the load developed in the restraining struts would equate to the full restrained shrinkage load. This is because a small amount of slab shrinkage would still occur under elastic shortening of the restraining struts, and from any elastic bending in the steel restraining frames. In every instance when a crack occurred, this was accompanied by an instantaneous reduction in compressive load in the restraining frames. The maximum loads developed in each restraining frame and the load reduction coincident with cracking is given in Table Five. The two load cells on slab B failed soon after casting; however the load reduction on cracking could still be measured on one load cell. For slab F the crack occurred right at the end of the test period. The recording equipment was not operating at this stage. The maximum load developed in the struts was significantly less than expected. The maximum load in slab C equates to a direct tensile stress across the slab of 0.73 MPa (0.18 f c) or 1.05 MPa at the reduced crossection through the control joint. This is less than a third of the flexural tensile strength measured of 3.8 MPa (0.95 f c) measured in the standard cured beams at the time of first cracking. This difference can be explained in part at least by the lower field cured concrete strength and the localised stress raised around the control joint. 100 1000 90 900 80 800 70 700 60 600 50 500 40 400 30 300 20 200 10 100 0 0 0 14 28 42 56 70 84 98 112 126 140 154 168 182 196 210 224 238 Time (days) Mean Shrinkage 10 per. Mov. Avg. (Air Temperature) 10 per. Mov. Avg. (Humidity) Figure Three: Chart of Air Humidity and Temperature overlain with Unrestrained Slab drying Shrinkage Slab No. Reinforcement Maximum Load Age at crack Load Reduction on Cracking B None Not Recorded 42 days 69KN C Deformed 665 Mesh 73KN 75 days 20KN C 113 days 14KN D Plain 665 Mesh 34KN 81 days 19KN D 129 days 14KN E Polypropylene Fibre 47KN 42 days 56KN F Deformed 665 Mesh 55KN 6 months Not Recorded Table Five : Compression Load Developed in Restraint Frames
5.6 Concrete and Reinforcing Steel Strains There was an instantaneous change in the concrete and reinforcing steel strains when the slabs cracked, simultaneous with the reduction in the restraining frame loads. The maximum change in concrete and reinforcing steel stresses on cracking are given in Table Six. Theoretically, before the concrete cracks, there is no strain in the concrete slab as it is prevented from shortening by the external frame. On cracking the strain taken up by the concrete is uniform over its length and proportional to the crack width. The maximum concrete stress change was measured on the plain and polypropylene fibre reinforced slabs which reflects the large initial crack width on these slabs. This correlates with the higher load reduction measured in the restraint frames for slabs B & E on cracking (Table Five). The maximum tensile stress measured in the mesh reflects the precise location of the strain gauge in relation to the control joint. Based on conventional theory, on cracking of the concrete, the reinforcement passing through the crack takes up the full tensile load. As the reinforcement bonds with the concrete on each side of the crack, the steel tensile stress will reduce over the bond length. This explains the variation in mesh steel stress for slabs C and D given in Table Six. Slab No. Reinforcement Age at crack Concrete Stress Mesh Steel Stress B None 42 days 0.7 MPa NA C Deformed 665 Mesh 75 days 0.2 MPa 400 MPa C 113 days 0.26 MPa 68 MPa D Plain 665 Mesh 81 days 0.35 MPa 81 MPa D 129 days 0.28 MPa 316 MPa E Polypropylene Fibre 42 days 0.7 MPa NA Table Six : Change in Concrete and Reinforcement Stresses on Cracking In all cases the measured stress in the mesh reinforcement was below the specified proof stress of the steel of 485 MPa. The purpose of control joints is to ensure that the concrete cracks thorough the weakened plane at a lower tensile strength, so that the steel across the crack will not yield when the load is transferred from the concrete to the steel on cracking. This appears to be the case for slabs C and D. The crack 330mm from the end of slab F after six months, in the end region through the 16mm starter bars cannot be easily explained. With this high reinforcement volume, only micro cracking not visible to the naked eye would be expected to occur. Full transfer of the restraint forces should have been transferred to the concrete assuming a maximum bond development length of 300mm. The maximum load recorded in the slab F restraint frame was the highest of all the mesh slabs, however this load reduced significantly after the first month of monitoring. One possible scenario is that the restraint frame failed at some point. the starter bars, there is an increased likelihood that the tensile force at first crack would exceed the mesh yield strength resulting in a crack width greater than for slabs C or D. 6. CONCLUSIONS A test programme set up to compare the performance of different slab reinforcement types under restraint to drying shrinkage has illustrated significant differences in crack formation. Six four metre long slabs, one metre wide by 100mm thick were cast and restrained from shrinkage movement by an external structural steel frame. Four slabs had shrinkage control joints one metre in from each end. The following conclusions can be made from the programme - (a) The plain slab with no reinforcement, and the polypropylene fibre reinforced slab were the first slabs to crack. The initial crack surface width of 2.4mm opened up to 6.8mm after the six month test duration. No further cracks occurred. Theoretically, with no control joints in slab F, it would be expected that the initial crack would occur subsequent to cracks appearing slabs C and D, as these have a weak plane through the control joints. It is considered that had the initial crack occurred in the middle section of the slab between (b) The slab with plain 665 mesh cracked first at one control joint with an initial crack width of 0.7mm. The reinforcement across the crack maintained this narrow crack width. In time a crack appeared at the second control joint of the same width. Both cracks remained at
(c) (d) (e) (f) their original width for the six month test duration. The slab with deformed 665 mesh behaved in a similar manner to the plain mesh slab but with the initial crack width of 0.8mm appearing at an earlier age. The mesh reinforced slabs were effective in limiting crack widths at the control joints below 1 mm, thus maintaining aggregate interlock across the crack. The slab without control joints cracked in an unpredictable manner. Control joints induce cracking at an early age, before the proof stress of the steel bridging the crack is reached. This test programme showed that the use of plain or deformed 665 mesh in conjunction with control joints is an effective way of controlling shrinkage cracking and maintaining aggregate interlock across the joints in ground floor slabs. Plain unreinforced concrete or polypropylene fibre concrete is not effective in ensuring that the control joints open evenly, and resulted in unacceptable crack widths occurring at one location only. 7. REFERENCES 1. New Zealand Standards NZS 3604:1999 Timber Framed Buildings 2. C Deacon Welded Steel Fabric in Industrial Floor Construction UK Concrete Nov Dec 1991 pp 41 44 3. New Zealand Standards NZS 3112:1986 Methods of Test for Concrete 4. Australian Standards AS 1012.13 Determination of the Static Chord Modulus of Elasticity of Concrete 5. Australian Standards AS 1012.17 Determination of Drying Shrinkage of Concrete